EPA 600
Apni 1 98
Research and Development
&EPA Control of Organic
Substances in
Water and
Wastewater
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EPA-600/ 8-83-011
April 1983
Control of Organic Substances
in Water and Wastewater
Technical Editor
Bernard B. Berger
Professor Emeritus - Civil Engineering
University of Massachusetts
Amherst, Massachusetts 01003
U.S. Environmental Protection Agency
Region V, Library
230 South Dearborrf- Street
Chicago, Illinois 60604
Office of Research and Development
U.S. Environmental Protection Agency
Washington, D.C. 20460
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DISCLAIMER
Mention of trade names or commercial products does not
constitute endorsement or recommendation for use.
»
Eiwtocnnwffltal Protection Agenctf f
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FOREWORD
The formation of the U.S. Environmental Protection Agency in 1970 ushered in
the first decade of environmental awareness as a total national phenomenon. It was a
decade punctuated by major Congressional mandates to restore the nation's waters,
to reduce air pollution, and to find a comprehensive approach to other environmental
problems—those associated with pesticide use, hazardous waste disposal and toxic
substances. It was a decade underscored by the demand for new technology and
better science to answer environmental questions and to solve environmental
problems.
As the scientific and technical arm of the Agency, The Office of Research and
Development is responsible for advancing the state of knowledge about the environ-
ment such that critical issues and questions can be addressed and answered
effectively, based on the application of state-of-the-art science and technology. In the
years since 1970, The Office of Research and Development has produced manifold
increases in the data base from which environmental decisions are made and in the
sophistication of the understanding which has provided the basis for decisions.
This volume represents our effort to take stock of scientific advances in research
pertaining to organic substances since the inception of the Agency and to gauge
what progress has been made and what remains to be accomplished. The essays in
this volume present a range of perspectives on the subject, from the vantage points of
the scientific and technical disciplines which have been carrying out relevant
research. The points of view represented are varied and sometimes conflicting. But
scientific progress depends on just such diversity The authors at times have
speculated about emerging problems and research needs. Such attempts require
extrapolation based upon informed scientific judgment. The outcome of that
process must, in the final analysis, be recognized as opinion and not fact.
•r:.i:*!
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PREFACE
INTRODUCTION
The presence of organic substances of industrial origin in wastewaters, storm run-
off and in surface and groundwaters may not always be an unmitigated evil—but, it is
safe to say, it never is good. Our concern is not new. Reports of federal efforts to
identify and contend with such pollutants go back 30 years. In the last decade, and
with good reason, this fear has flared up. In 1976, EPA was required to give special
emphasis to 129 "priority pollutants" that present probable hazard in the water re-
source. Of these 129 priority pollutants, 114 are organics. The number will surely be
increased with additional knowledge and experience.
We have made a national commitment to eliminate or reduce these priority or-
ganic pollutants. But it is difficult and costly to do so, and before we can proceed
effectively, we must have additional information concerning these substances. First,
of course, we must know they are indeed present; it is hard to take protective action
against a specific organic compound whose presence is not known or even suspected.
This is important, but by itself it is not enough. We must know what maximum con-
centration of that substance may be tolerated. Such information gives point and
purpose to our approach to control. And then we must have the technology to effect
the reduction. Each "we must " represents a challenge to those engaged in water
and wastewater research.
Detection of the presence of a hazardous pollutant in water or municipal waste-
water may result simply from knowledge of its use in the community or on land. Its
presence in industrial wastewaters may be a logical deduction from knowledge of
materials contributed by the industrial plant. Maintaining such intelligence is a
routine but important activity of pollution control agencies. Knowledge so gained
must be supplemented by information obtained by actual examination of the
process waters and plant discharges.
Unfortunately, knowledge derived from plant discharge monitoring may not be
complete. This is particularly true if the pollutant gives no physical, chemical or bio-
logical clue to its presence. We became aware of this problem in the early 1950's
when municipal wastewater treatment plants throughout the nation generated enor-
mous, even dramatic, quantities of foam. This was quickly traced to the alkyl ben-
zene sulfonate (ABS) component of household detergent which had only recently
been placed on the market. There was no difficulty in identifying ABS; the physical
evidence was clear to the point of gross nuisance. The ABS episode stimulated us to
ask what other chemical pollutants might be present that unlike ABS were not de-
tectable and that conceivably might be much more important from the standpoint of
public health.
Detecting and measuring the unknown pollutant is one major challenge. Here we
look to the chemists. Another is determining the effect of the pollutant, and based on
this, recommending a tolerable concentration in the effluent and water resource. For
this information we look to our co-workers in epidemiology, toxicology and in the
ecologic sciences. Their conclusions present a third and major challenge to the
engineer: developing procedures for treating water and wastewater to meet stringent
requirements for water quality. This document is a status report on how this chal-
lenge is being met.
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The document is comprised of 14 chapters, each prepared by a specialist. Chapter
1, Fundamental Considerations in the Removal of Organic Substances in Water is in
essence an examination of a wide spectrum of physical chemical principles that are
being used, are under study, or may in the future be adapted for use in the separation
of organic pollutants in waters.
Chapters 2, 3, 4, and 5 review the status of techniques for the separation or
destruction of organic impurities in drinking water. Chapter 2, Coagulation-
Sedimentation-Filtration Processes for Removing Organic Substances from
Drinking Water, assesses the state-of-the-art for the long used, conventional
processes in respect to the new challenges posed by organic pollutants in general and
trihalomethanes in particular, and discusses ways in which the full potential utility of
these processes may be employed. Chapter 3, Adsorption of Organic Substances in
Drinking Water, reviews the theory of adsorption phenomena, experience in the use
of adsorption in removal of trace organics in drinking water, and problems still to be
overcome in assuring maximum efficiency of the process in water purification.
Chapter 4, Removal of Organic Substances from Water by Air Stripping, describes
the theory of gas exchange, analyzes recent experience in air stripping as a treatment
process for drinking water, and shows how this process may be efficiently designed
to remove trace organics in the raw water. Chapter 5, Oxidation of Organic
Substances in Drinking Water, reviews the theory of chemical oxidation and
assesses the potential of a number of oxidation processes, alone and in combination
with U V, in the destruction or modification of organic pollutants present in drinking
water in trace concentrations.
Chapters 6, 7, 8 and 9 review current methods, old and new, and assess their
potential role in the future control of the priority organic pollutants contained in
municipal wastewaters. Chapter 6, Reduction of Organic Substances in Municipal
Wastewater by Biological Processes, examines long familiar processes from a new
perspective which provides a deeper understanding of basic physical-chemical and
microbiological phenomena, and in so doing, shows how such processes, including
their latest adaptations, may be employed most efficiently in removing priority
organic pollutants. Chapter 7, Reduction of Organic Substances in Municipal
Wastewaters by Physical-Chemical Processes, describes experience in the USA with
such processes, including recent applications and the future potential based on this
experience, and focuses on the theory and applications of adsorption as that
physical-chemical principal having the greatest potential in removing trace priority
pollutants. Chapter 8, Treatment of Municipal Wastewaters for Recycle/Reuse,
discusses current policy and practices, obstacles to be overcome, particularly those
presented by trace organic compounds, and the probable future role of reclamation
of municipal effluents. Chapter 9, Management of Organic Residuals Separated
from Municipal Wastewaters, describes the probable behavior and fate of trace
priority organic pollutants in sewage, a difficult and uncertain effort because of very
scanty data, and discusses possible interactions between such pollutants and
processes of sewage sludge conditioning and treatment for disposal.
Chapters 10, 11, 12, 13 and 14 examine existing and proposed new methods for
control of organic substances in industrial wastewaters. Chapter 10, Reduction of
Organics in Industrial Wastewaters by Biological Treatment, represents a thorough
review of experience with existing biological treatment systems, evaluates the
potential effectiveness of such systems in controlling trace organics, and describes
design and operating factors that will improve treatment efficiency. Chapter 11,
Removal of Organic Compounds from Industrial Wastewaters Using Adsorption
Processes With Activated Carbon, reviews industrial experience in use of adsorption
focusing particularly on practical design and operational factors for adsorption and
carbon regeneration systems. Chapter 12, Separation of Organic Substances in
Industrial Wastewaters by Membrane Processes, presents a timely analysis of the
state-of-the-art, problems still to be solved, and the probable future role of
membrane processes in controlling trace organic substances in industrial effluents.
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Chapter 13, Treatment of Industrial Wastewaters for Recycle/ Reuse, discusses the
potential role of such recycle/reuse in reclaiming industrial wastewaters, describing
recent experience, the problems to be solved, proposed approaches to such
recycle/reuse, and the substantial advantages to industry in reclamation of waste-
water. Chapter 14, Management of Organic Residuals Separated from Industrial
Wastewaters, describes the character and quantity of residuals produced by various
industrial categories, and the methods recommended in handling, treating and
disposing of these residuals.
The foregoing chapters focus on the priority organic pollutants that are known to
us, on ways to make maximum efficient use of our treatment technologies, and on
new processes that may be used in the future. It is evident, of course, that new prob-
lems resulting from generation and wide use of the organic products of industrial
operations will surely emerge during the decade to come. Many will be similar to
those we now experience. Others will challenge our perceptivity, our foresight and
our judgment. The research now underway should help in meeting the tasks of the
future. The status report represented by this MONOGRAPH could well be viewed
as an audit of 1980 concepts in contending with synthetic organic pollutants. They
tend to indicate, also, the probable technologies of the future.
Bernard B. Berger
Technical Editor
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CONTENTS
Page
Foreword iii
Preface iv
Figures viii
Tables xv
Fundamental Considerations in the Removal of Organic Substances
from Water-a General Overview, Louis Koenig 1
Coagulation-Sedimentation-Filtration Processes for Removing Organic
Substances from Drinking Water, James K. Edzwald 26
Adsorption of Organic Substances in Drinking Water
Francis A. DiGiano 65
Removal of Organic Substances from Water by Air Stripping
Perry L. McCarty 119
Oxidation of Organic Substances in Drinking Water, William H. Glaze ... 148
Reduction of Organic Substances in Municipal Wastewater by
Biological Processes, Robert L. Irvine 175
Removal of Organic Substances from Municipal Wastewaters by
Physicochemical Processes, Walter J. Weber, Jr. and
Frederick E. Bernardin, Jr 203
The Impact of Organic Substances on Municipal Wastewater Reuse,
J.Carrell Morris and John F. Donovan 255
Management of Residuals Separated from Municipal Wastewater,
Richard I. Dick 278
Reduction of Organics by Biological Treatment, Raymond C. Loehr 305
Removal of Organic Compounds from Industrial Wastewaters Using
Granular Carbon Column Processes, Robert P. O'Brien, Joseph L. Rizzo
and Wayne G. Schuliger 337
Separation of Organic Substances in Industrial Wastewaters by
Membrane Processes, Ronald F. Probstein, Calvin Calmon and
R. Edwin Hicks 353
The Impact of Organic Substances on Wastewater Recycling in Industry,
Robert H. Culver and John F. Donovan 392
Management of Organic Residuals Separated from Industrial Wastes,
C. Lue-Hing, T.B.S. Prakasam and D. Zenz 408
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FIGURES
Number Page
Edzwald
1 - Possible parallel competitive reactions occurring when chlorine
is added to water 28
2 - Trihalomethane terminology 31
3 - Chloroform formation curves for the Grasse River (Canton, NY).
Samples collected in February and April; chlorination conditions:
C12 dose 15-20 mg/L, pH 7.5, 20°C 32
4 - Conventional waste treatment plant 36
5 - Direct filtration 36
6 - Isoparticle number concentrations (N) in particles per cm3 38
7 - Turbidity vs. solids concentration for humic acid and kaolinite 39
8 - Solubility of Al(OH)3(s) in water 42
9 - Single collector efficiency as a function of suspended particle diameter 45
10 - Comparison of jar test results with filter performance 49
11 - Model humic compound 50
12 - Effect of pH on the coagulation of humic acid with alum 52
13 - Effect of pH on the coagulation of humic acid using alum and high
molecular weight polymers 53
14 - Schematic representation of the destabilization and aggregation of
humic acid by polyethylenimine 54
15 - Turbidity and head loss data for spiked humic acid-gravel pit water,
direct filtration 56
16 - Apparent color and TTHMFP data for spiked humic acid-gravel pit
water, direct filtration 57
17 - Monitoring of the Canton,NY water treatment plant in April 59
18 - Monitoring of the Canton, NY water treatment plant in April 60
DiGiano
1 - Modification of conventional water treatment practice to include
the adsorption process 68
2 - Schematic representation of the movement of the adsorption zone
and the resulting breakthrough curve 69
3 - Passage of pesticides through carbon beds 70
4 - Effect of time on removal of carbon chloroform-extractable
materials 71
viii
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Number Page
5 - Effect of contact time on length of operation 71
6 - Concentration-iime profile for a flow rate of 2 gpm/sq ft (81.4
I/ mini sq m) in a 250-g fluid bed of 0.7111 -mm carbon 73
7 - Specific adsorption isotherms 75
8 - Relationship between the amount of compound adsorbed by
activated carbon (at an equilibrium concentration of 10~3 mol/L)
and the net adsorption energy 77
9 - Distribution of net adsorption energy calculated for organic
compounds identified in Philadelphia's water supply source 78
10 - Plot of the logarithm of the extent of adsorption versus
hydrocarbonaceous surface area for aliphatic alcohols, ketones,
aldehydes, and acids as per the simplified solvophobic model 80
11 - Chloroform breakthrough curve for filter-adsorber with EBCT = 15
min. at Philadelphia 81
12 - Total THM breakthrough curve with EBCT = 10 min. at Cincinnati .. 82
13 - 1,2 DCE breakthrough curve for post filter adsorber (EBCT = 20
min) at Jefferson Parish, LA 82
14 - Relationship between bed service time and EBCT for several
poorly adsorbed specific compounds and for background TOC 83
15 - Adsorption of various types of humic substances 84
16 - Adsorption of molecular weight fractions of soil fulvic acid 84
17 - Adsorption constant of humic substances as a function of pore
volume within a certain range of pore radii of carbon 85
18 - Effect of inorganic ions present in tap-water on the breakthrough
profile for humic acids in granular carbon adsorption columns 85
19 - Model predictions of breakthrough curves for peat fulvic acid on
a,b different brands of activated carbon (a) before, and (b) after alum
coagulation 86
20 - Performance of terminal TTHM through post filter adsorber
(EBCT = 24 min) at Jefferson Parish, LA 87
21 - Calculated and experimental breakthrough curves for PNP and
a,b PCP, respectively, in the PNP/ PCP system at different bed depths .. 88
22 - Experimental breakthrough, desorption, and displacement profiles
for carbon tetrachloride 90
23 - Bed service time vs ratio of trace to TOC background selectivities .... 92
24 - Removal of trihalomethanes by Ambersorb® XE-340 (EBCT = 10 min) 94
25 - Commercial humic acid isotherms as a function of pH weak base
resins, strong base resin and polymeric resin 95
26 - Adsorption isotherms after repeated regeneration 96
27 - Dynamic testing of regeneration effectiveness in full-scale testing of
GAC bed performance in Dusseldorf, West Germany 97
28 - Depiction of three zones of activity in activated carbon adsorbers
when microbial activity is significant 99
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Number Page
29 - Predicted effluent concentration as a function of service time with
the GAC bed 100
30 - Influence of ozonation prior to adsorption on trihalomethane
formation potential removal 101
31 - Adsorption of Bis(2-chloroisopropyl) ether on various activated
carbons at different filter depths 105
32 - Cumulative adsorption of chloroform by seven brands of granular
activated carbon 106
33 - Total production cost versus reactivation frequency in months for
380-ML/day (100-mgd) post-filter adsorption 108
34 - Jefferson Parish adsorber TOX profiles 109
Me Cany
1 - Transfer of volatile components from liquid phase to gas phase in
response to a concentration gradient 122
2 - Difference in concentration gradients between liquid and gas phases
for liquid phase and gas phase controlled processes 124
3 - Configurations for different air-stripping processes 127
4 - Flow scheme and concentration profiles for a countercurrent flow
air stripping tower 129
5 - Computed effect of air to water (G/Q) ratio and tower height (z) on
removal of chloroform by decarbonator with countercurrent flow ... 132
6 - Computed effect of air to water ratio and Henry's law constant (H,)
on percentage removal for countercurrent flow stripping tower or
decarbonator at Water Factory 21 133
7 - Relationship between the air to water ratio (G/Q) and the water flow
rate per unit cross-sectioned area of tower (Q/A)r which produces
flooding in countercurrent flow towers for packing media with
various F values 135
8 - Flow scheme and concentration profiles for a cross flow air stripping
tower 137
9 - Calculated effect of air to water ratio (G/Q) on efficiency of ammonia
removed by Water Factory 21 cross-flow stripping tower 139
10 - Calculated effect of air to water ratio and Henry's law constant on
removal efficiency by Water Factory 21 cross-flow stripping tower .. 140
11 - Flow schemes for surface aerator and diffused air-stripping basins .. 141
12 - Computed effect of air to water ratio (G/Q) and water detention time
(V/Q) on chloroform removal by the Palo Alto Reclamation plant
mechanical aerator stripping basin 144
Glaze
1 - Formation of trihalomethanes and non-volatile organohalides by
chlorination of filtered water from a surface source in southern
United States 153
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Number Page
2 - Schematic diagram of typical ozone generating and contacting
facility for water treatment 154
3 - Effect of low doses of ozone on trihalomethane precursors (THMFP)
in a pilot plant 157
4 - Effect of ozone dose on the destruction of trihalomethane precursors
in water from the same source as Figure 3 158
5 - Schematic diagram of typical facility for generation of chlorine
dioxide from sodium chlorite and hydrochloric acid 162
6 - Schematic diagram of typical facility for generation of chlorine
dioxide from chlorine and sodium chlorite 163
7 - Trihalomethane formation by CIO2 and excess free available
chlorine, ERC pilot plant settled water 164
8 - Formation of haloforms from combinations of chlorine and chlorine
dioxide 165
9 - Normalized THM formation potential; ozone destruction of THM
precursors-Caddo Lake water 168
Irvine
1 - Transformation of initial state into goal state 177
2 - The sequence of operators 179
3 - The biological utilization of organics 187
4 - The 5-day BOD is an arbitrary measure 190
5 - The flow of electrons during biodegradation of organics 192
6 - The relative amount of filamentous organisms 193
7 - The reactor-clarifier and activated bio-filter 198
8 - The deep shaft and sequencing batch reactor 199
Weber and Bernardm
1 - Applications of activated carbon in municipal wastewater treatment . 206
2 - Typical clanfier configurations 210
3 - Tube-settlers for sedimentation 211
4 - Filtration systems (from Environmental Pollution Control
Alternatives: Municipal Wastewater, USEPA Technology Transfer,
EPA-625/5-76-012) 212
5 - Schematic representation of a membrane process using pressure as
a driving force for separation 214
6 - Conceptual depiction of the adsorption process 214
7 - Distribution of an organic substance between solid and liquid phases 217
8 - Conceptual drawing of carbon granule 218
9 - Significant mass transport steps in adsorption 222
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Number
Page
10 - Completely-mixed reactors 223
11 - Plug-flow reactor 224
12 - Typical GAC reactor configurations 225
13 - Idealized breakthrough curve 226
14 - Adsorber breakthrough curves for p-chlorophenol and various
adsorbents 228
15 - Dodecyl benzene sulfonate influent and effluent concentration
(C2/C20) profiles; phenol - dodecyl benzene sulfonate mixture 229
16 - A scanning electron microscope picture of carbon surface with
attached microorganisms 230
17 - PACT® flow diagram 232
18a - Schematic diagram of a multiple-hearth regeneration furnace 234
18b - Schematic diagram of a rotary tube regeneration furnace 234
18c - Schematic diagram of a fluidized-bed regeneration furnace 235
18d - Schematic diagram of an electric-belt regeneration furnace 235
19 - Schematic diagram of a wet air regeneration system 236
20 - Tertiary treatment schematic South Lake Tahoe, CA 241
21 - Tertiary treatment schematic Orange County, CA 243
22 - Physical chemical treatment schematic Niagara Falls, NY 244
23 - Physical chemical treatment schematic Vallejo, CA 246
24 - PACT® treatment schematic Vernon, CT 247
Morris and Donovan
1 - Distribution of existing water reuse projects in the United States .... 258
2 - Schematic diagram of pilot testing systems for potable reuse,
Dordrecht, The Netherlands 260
3 - Chromatogram of Rhine River water and secondary effluent -
Dordrecht, The Netherlands 261
Die k
1 - Relative quantities of treated effluent and sludge and the
comparative costs of wastewater treatment and sludge management 281
Loehr
1 - Typical components of a wastewater treatment system 306
2 - A waste treatment system is designed and operated to achieve
specific goals 308
3 - Components of an activated sludge system 316
4 - Schematic top view of an aerated lagoon 316
5 - Schematic cross-section of an oxidation pond 317
xii
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Number Page
6 - Components of a trickling filter system 319
7 - Schematic of the PACT process in which activated carbon is added
to an activated sludge unit 319
8 - Mechanisms for pollutant removal in biological treatment processes . 321
O'Brien, Rizzo, and Schuliger
1 - Isotherm plot of wastewater containing multi-components 348
2 - Pilot system flow diagram for evaluating the feasibility of carbon
adsorption 349
3 - Typical breakthrough curves from pilot carbon adsorbers 350
4 - Superficial contact time vs. exhaustion rate 351
5 - Effluent quality vs. adsorber changeover 353
6 - Effluent quality vs. pulse frequency 353
7 - Effluent quality in continuous countercurrent system 353
8 - Fixed bed system configurations 355
9 - Installed costs for fixed bed adsorption systems 358
10 - Installed costs for reactivation systems 359
Probstein, Calmon, and Hicks
1 - Outline of simple reverse osmosis arrangement 366
2 - Effect of solution flow rate past membrane on ultrafiltration flux ... 370
3 - Hollow fiber flow configuration 373
4 - Packaging of spiral wound module 375
5 - DuPont hollow fiber module 376
6 - Tubular membrane element 377
7 - Plate and frame membrane configuration showing cross section of
membrane unit and flow in module 378
8 - Rejection of carboxylic acids as a function of pH 379
9 - Rejection of phenols by cellulose acetate membranes as a function
of pH 380
10 - Rejection of two amines as a function of pH 381
11 - Semi-batch system for treatment of oily wastes 384
12 - Water treatment by liquid membrane emulsions 386
13 - The mechanism of removal of phenol by liquid membranes 387
Culver and Donovan
1 - Countercurrent rinse system commonly used in the textile industry .. 395
2 - Example of photographic processing recycling system 396
3 - Flow diagram of advanced waste treatment plant at FMC
Corporation's chemical research and development facility 400
xiii
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Number Page
4 - Rough schematic program of proposed diatomaceous earth filter
system to treat recycled poultry chilling wastewater 403
5 - Initial milking parlor recycle system 404
6 - Modified milking parlor recycle system 404
xiv
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TABLES
Number Page
Koenig
1 - The physical states in which organics occur in water 4
2 - Concentrations of organics in municipal sewage 7
3 - Distribution of organics (in %) in municipal sewage according to
size fractions 7
4 - Approximate rate of fall in water of spheres, density, I.I gm/cc 19
Edzwald
1 - Drinking water standards for turbidity and organics 27
2 - TOC and chloroform yield data 34
3 - Design and operating parameters: conventional treatment and
direct filtration 37
4 - Examples of polymers 43
5 - Elemental analysis of humic and fulvic acids 51
DiGiano
1 - Average annual CCE concentration in Missouri River water at
Omaha 67
2 - Parts per million of activated carbon required to reduce the pesticide
level in distilled water and in Little Miami River water 70
3 - Comparison of Langmuir equilibrium constants for organic
pesticides and selected adsorbates 72
4 - Summary of carbon adsorption capacities measured for suspected
chemical carcinogens 76
5 - European water treatment plants employing GAC 103
6 - Design criteria for GAC filters in Germany 104
7 - Amortized capital and O&M costs for GAC systems 107
McCarty
1 - Calculated Henry's Law constants and water diffusion coefficients
at 20°C for selected organic and inorganic compounds 125
2 - Removal of trihalomethanes by countercurrent flow packed tower
decarbonators at Water Factory 21 131
3 - Packing factors F for a variety of packing materials 136
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Number Page
4 - Removal of volatile compounds by cross-flow packed stripping
tower at Water Factory 21 138
5 - Removal of organic compounds by spray aeration stripping basins
at Palo Alto Wastewater Reclamation Plant 143
6 - Summary of operational characteristics and power requirements
for various stripping processes 145
Glaze
\ - Potentials of selected oxidants at 298° K 150
2 - Effect of treatment methods on selected organic halides in River Rhine 159
3 - Design parameters and total costs for ozonation 159
4 - Estimated costs for an ozonation facility for removal of toxic
organic compounds 160
5 - Advantages and disadvantages of ozone in drinking-water
treatment 160
6 - Removal of organic micropollutants with UV radiation and
hydrogen peroxide 168
7 - Pilot scale oxidation of model compounds with ozone-hydrogen
peroxide 169
8 - Research issues related to the use of oxidants in drinking water
treatment 170
Irvine
1 - Typical initial and goal state 180
2 - Selected actions influencing goal state limits 182
3 - The improved performance resulting from CCP 183
4 - The list of organic priority pollutants 183
Weber and Bernardin
1 - Approximate performance characteristics of different municipal
wastewater treatment processes 207
2a - Properties of selected commercial granular carbons 219
2b - Properties of selected commercial powdered activated carbons 220
3 - Comparison of representative PCT applications 238
4 - Design, operating parameters, and performance summary
(South Tahoe, CA Type AWT) 240
5 - Design, operating parameters, and performance summary (Orange
County, CA, Type AWT/ RO) 242
6 - Design, operating parameters, and performance summary (Niagara
Falls, NY, Type 1PCT) 245
7 - Design, operating parameters, and performance summary (Vallejo,
CA, Type 1PCT) 245
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Number Page
8 - Design, operating parameters, and performance summary (Vernon,
CT, Type PACT®) 248
Morris and Donovan
1 - Reuse projects in the United States 259
2 - Removal of organic micropollutants in the reverse-osmosis system .. 262
3 - Removal of trace organic substances by lime treatment at Water
Factory 21 269
4 - Removal of organic materials by granular activated carbon at
Water Factory 21 270
5 - Removal of trace organic substances by air stripping at Water
Factory 21 271
6 - Removal of contaminants by pilot reverse osmosis systems at
Water Factory 21 273
Dick
1 - Major conventional organic constituents in municipal sludge 283
Loehr
1 - Categories of the priority pollutants 309
2 - Methods for the identification of organics in wastewater 310
3 - Common biological treatment processes 315
4 - Range of liquid detention times in different biological treatment
units 318
5 - Typical range of BOD removals and effluent BOD and suspended
solids concentrations for biological treatment systems 320
6 - Approaches to minimize effluent variability 321
7 - Occurrence of organic priority pollutants in POTW influent
samples 323
8 - Occurrence of organic priority pollutants in POTW effluent
samples 323
9 - Occurrence of organic priority pollutants in untreated POTW
sludges 325
10 - Removals of organic compounds achieved by POTW secondary
treatment 326
11 - Wastewater treatment system performance at five organic chemical
industry plants — conventional and non-conventional parameters ... 327
12 - Wastewater treatment system performance at five organic chemical
industry plants — volatile organic priority pollutants 328
13 - Wastewater treatment system performance at five organic chemical
industry plants — base/neutral organic priority pollutants 329
14 - Wastewater treatment system performance at five organic chemical
industry plants — acid organic priority pollutants 329
xvii
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Number Page
15 - Organic priority pollutants in the influent and effluent wastewaters
of an organic and inorganic chemicals manufacturer 330
16 - Performance of industrial waste treatment systems treating leather
tanning and finishing wastewaters 332
17 - Biodegradability of specific organic compounds 333
O'Brien, Rizzo, and Schuliger
1 - Classes of organic compounds amenable to adsorption on activated
carbon 339
2 - Equilibrium removal capability of activated carbon for selected
toxic compounds 339
3 - Summary of isotherm studies of wastewaters from major industries . 340
4 - Review of operating adsorption system parameters 341
5 - Operating costs ($l,000/yr) for reactivating spent granular carbon .. 359
6 - Comparative consumption of energy for treatment systems of
similar capacity studied by the U.S. Corps of Engineers 360
Probstein, Calmon, and Hicks
1 - Useful particle size ranges of reverse osmosis and ultrafiltration and
reference particle sizes 365
2 - Typical osmotic pressure data at standard temperature 368
3 - Amicon Corp. ultrafiltration membranes 372
4 - Rejections of some organic priority pollutants by reverse osmosis ... 382
5 - Reverse osmosis treatment of oil shale retort water 386
Culver and Donovan
1 - Use of freshwater by industry, 1975 394
2 - Water quality limitations on water use 397
3 - Selected effluent characteristics at FMC Corporation's chemical
research and development facility 402
4 - Construction cost for zero discharge treatment plant at FMC
Corporation's chemical research and development facility 402
5 - Suspended solids and total bacterial count comparison of
conventional system and system using recycled wastewater to chill
poultry 403
6 - Comparison of total bacterial counts of dairy equipment cleaned
with recycled and with conventional sanitary solutions 406
7 - Cost comparison of system using various degrees of recycled
wastewater 406
xvm
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Number Page
Lue-Hing, Prakasam, and Zenz
1 - Data extracted from the Metropolitan Sanitary District of Greater
Chicago's sludge manifest reporting system for February -
December, 1979 410
2 - Estimates of total solid and hazardous industrial waste generated
and organics contained in it in USA and the state of Illinois 411
3 - Estimated quantities of waste residuals generated in the textile
industry 412
4 - Total quantity of potentially hazardous wastewater treatment
sludges, dry kkg/yr generated in the textile industry 413
5 - Number of plants employing current and best technology for the
disposal of waste treatment plant sludges in the textile industry 414
6 - Atypical waste residuals generated in the textile industry 4J5
7 - Waste generation factors as percent of total production in various
polymer production categories 417
8 - Concentration of toxic organic pollutants in sludge from paint
formulating industry 419
9 - Quantities of hazardous waste residuals and mode of disposal in
paints and coatings industry , 420
10 - Types of refineries and the wastes they generate 422
11 - Production of potentially hazardous constituents in wastewater
treatment sludges and all waste residuals from petroleum refining
industry in 1974, 1977, 1983 422
12 - Concentration of organics in influent, effluent, and sludges from
a POTW receiving refinery wastewater 424
13 - Percent estimates of refinery sludge disposal methodologies for
1973 and 1983 425
14 - Quantities of hazardous components and methods of disposal of
residuals from organic chemical industry 426
15 - Hazardous waste disposal practices in the organic chemcials
industry 429
16 - Characterization of explosive propellent and warfare material wastes 430
17 - Type and quantity of hazardous waste residuals and waste disposal
practices in industrial inorganic chemical, pharmaceutical, and
petroleum rerefining industries 432
18 - Summary of wastes and disposal practices in drum reconditioning .. 434
19 - Annual waste oil quantities produced in the automotive service
industry 434
20 - Sludges thickening and dewatering equipment 436
21 - Advantages and disadvantages of various types of incinerators 438
22 - Destruction of industrial sludges by thermal processes 440
23 - Residuals generated from non-hazardous and hazardous industrial
wastes and their treatment and disposal 443
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Number Page
24 - Quantities of residuals disposed of by different methods in on-site
and off-site facilities by various industries 445
25 - Present and projected hazardous waste management methods 449
26 - On-site disposal of hazardous wastes in various industries 449
27 - Degradation products formed by microwave detoxification of
selected wastes 454
28 - Net unit costs of disposal of industrial and municipal residuals
Allegheny County, Pa 456
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FUNDAMENTAL CONSIDERATIONS IN THE
REMOVAL OF ORGANIC SUBSTANCES
FROM WATER - A GENERAL OVERVIEW
Louis Koenig, PhD
INTRODUCTION
The Nation has become increasingly aware of the importance of water cleansing
processes. Enormous sums are spent on wastewater treatment works and municipal
plants for the treatment of drinking water. These processes, onc.e mysterious—even
arcane—to the general public, are now described in books available even to school
children. Most are now familiar with the conventional terminology—grit removal,
clarifiers, trickling filters, activated sludge, sludge digestion, coagulation and floc-
culation, settling, filtration and disinfection.
Ironically, at a time when the public has shown it has a general appreciation of
works designed to separate pollutants from water and has expressed its willingness
to approve huge appropriations for that purpose, it learns that many potentially
dangerous pollutants are not in fact removed. The bulk of these pollutants, organic
chemicals of industrial origin, are, in a sense, new to nature.
It is appropriate and timely that we re-examine the pollutant separation processes,
not in terms of efficiency, as important as this surely is, but rather to assess our un-
derstanding of the fundamental physical, chemical and biochemical mechanisms
involved. More than that, we should extend our inquiry to identify principles to be
potentially applied in meeting the challenges presented by the new, often complex,
synthetic organic chemicals reaching our water resources. A preliminary re-exam-
ination is the objective of this chapter.
THE STATES OF AGGREGATION OF ORGANICS IN WATER
Organics in water may be in dissolved, suspended, settled, or floating states. Or-
ganic substances that are not dissolved in water can exist in solid, liquid or gaseous
phases as either continuous or disperse.
If an organic gas exists in a continuous phase, its lower surface must be the upper
surface of the liquid. Thus, the organic substance is already separated from the
water. The same is true if the organic substance is in a continuous liquid phase which
differs in density from that of water.
The Author Dr. Louis Koenig is the principal in the research firm of Louis Koenig - Research,
San Antonio, founded in 1956. Prior to that he was a research executive with several non-profit
research institutes, with the Atomic Energy Commission, and a research chemist for chemical
manufacturing companies In recent years he has been concerned with both the natural and the
man-made processes involved in water supply and pollution control He was Advisor to Secre-
taries of the Interior on the establishment and performance of the Saline Water Conversion
Program, and to the U.S Public Health Service on the Advanced Waste Treatment Research
Program.
1
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Organics as a Disperse Phase
In the disperse phase, particles are separated from each other and surrounded by
water molecules. The particles may be solid, liquid, or gas.
Dispersions are classified according to particle size, the size being thought of as
the diameter of a sphere of the volume equal to the volume of the particle. The classi-
fication distinguishes between coarse dispersions and colloidal dispersions or col-
loids. The properties which distinguish colloids from coarse dispersions are particle
size and surface area. Since these properties vary continuously with those param-
eters, there cannot be a precise boundary between the classes.
Colloidal Particles—
The properties peculiar to colloidal particles become evident at particle sizes
below 500 nanometers (nm, 10 9 meters), which is about the lower limit of visibility
with a standard microscope; thus, colloidal particles cannot be studied effectively
with a standard microscope but only with the ultramicroscope, an instrument which
passes a beam of light through the dispersion and observes the particles by means of
the flashes of light reflected from them. Such an instrument can detect particles of
about 5 nanometers (10 meters), the approximate diameter of some complex mole-
cules of high molecular weight such as proteins and starches which exhibit colloidal
properties even though the "particles"are single molecules. The size of 5 nanometers
is therefore used to define the boundary betwen colloidal materials and true solu-
tions.
So far we have presented colloidal particles as simple spheres; however, the forms
that colloids actually take are more complicated and this affects their properties. For
individual, or primary, particles, the shape may be spherical, ellipsoidal, plate-like
or rod-like, or filamentous.
An aggregation of many simple particles may also be referred to as a particle. The
forces which hold the aggregate together are either coulombic, arising from net
positive or negative charges on the primary particles or "van der Waals" forces.
Coulombic forces may be either attractive or repulsive, their magnitude decreasing
with the square of the distance between the particles. The van der Waals forces are
generated by a shifting of the electrical charges within the neutral particle so as to
concentrate the positive charge toward one end of the particle and the negative to the
other, a phenomenon called polarization. In this way, attractive forces occur be-
tween particles. The magnitude of the attraction decreases with the cube of the dis-
tance, and therefore van der Waals forces are generally significant only at very small
distances between particles.
It is obvious that such aggregation increases the diversity of particle sizes so
much that it may make suspensions out of colloids, again one of the mechanisms for
separation of organics.
A third modification to the simple spherical particle concept is that of hydration, a
phenomenon in which molecules of water become associated with a colloidal par-
ticle. Some of this water may be tightly bound around the surface of the particle in a
layer about one molecule thick. Frequently, larger quantities of water molecules are
associated, but these are more loosely bound than the initial layer and may be
merely entrapped in the coils and folds of an aggregate particle. This association of
water molecules increases the volume of the particle and thus the diameter, but it
also makes the specific gravity of the particle closer to that of the water in which it is
suspended. Since the difference in density between the particle and the water is im-
portant for settling and floating, the extent of hydration becomes important in the
separation of the particles from the water phase
Adsorbed Organics—
Organics have thus far been discussed as if they were the only materials in the
dispersed phase. However, if inorganic particulates are present, they may adsorb
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organic molecules or particles. Conversely, the organic particulates might aggregate
not only with other organic molecules and particulates but also with inorganic par-
ticulates. This is important because physical separation techniques that depend only
on particle size—sedimetation, for example—remove both the organics and the in-
organics. Furthermore, some of the removal techniques involve adding inorganic
particulates to adsorb the organics and make them amenable to removal.
The Embodiments of the Physical States
Table 1, shows the physical states in which organics occur in water as a disperse
phase and the corresponding embodiment for solutions. The continuous phase, that
is the dispersing medium, is, of course, the water solution. The table shows that
organic solids, liquids, and gases can occur as coarse suspensions, or as colloids, in
which case the names applied are sols, emulsions, and foams. Most emulsions have
particle sizes in the upper portion of the colloid range. There is no special terminol-
ogy for the coarse suspensions. Also, organic compounds which, in the pure state,
are solids, liquids or gases can occur as solutes under ambient conditions in the water
phase. In this case, they are liquids in the sense of being part of the homogeneous
liquid phases and have lost their characteristics as solids or gases.
THE CHEMICAL NATURE OF ORGANICS
The distinction between organic compounds produced by the cellular processes of
living organisms and organic compounds produced by man in factory or laboratory
becomes important in the subject we are addressing. Before the development of the
chemical manufacturing industry, all organic compounds in natural waters and in
wastes reaching them did come from the cellular processes of living organisms.
Other organisms evolved which were adapted to utilizing these compounds in their
own metabolic processes, excreting in turn simpler compounds with the eventual
end products being inorganic compounds such as carbon dioxide and water. By such
adaptation, certain organisms, especially the microorganisms, served to remove
organic compounds from soils and waters, and, with respect to the latter, provide
purification. Hence, they were harnessed by man under artificial conditions to do so,
as in sewage treatment plants.
In the past half century, the situation has become complicated because a host of
synthetic organic compounds has been produced. These compounds differ from
those which certain organisms had adapted to metabolizing. Indeed, some of the
manufactured compounds, e.g. the pesticides, have been produced specifically to
thwart the established metabolic processes. These new products, termed "exotics,"
thus defy conventional treatment processes. They pass through conventional treat-
ment plants unchanged and are termed "refractory,""recalcitrant,""biorefractory,"
and "non-biodegradable," the latter two terms reflecting the dominance of biolog-
ical processes in man's approach to the problem.
The Multiplicity of Organic Compounds
An advanced textbook intended for the education of organic chemists easily in-
dexes 4000 compounds. And that, as with all textbooks, is only the beginning.
Chemical Abstracts has in its file some 4,600,000 organic compounds which have
been mentioned in the literature since 1965, to be added to the 1,200,000 which were
already prepared or identified before 1965 but not yet filed. And the file is being ex-
panded at the rate of about 330,000 per year.
The number of organic compounds that can theoretically exist is obviously enor-
mous. For example, among the paraffin hydrocarbons, compounds of carbon and
hydrogen only, CH4, methane, C2H6, ethane, those with 20 carbon atoms in the
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Table 1. The Physical States in Which Organics Occur in Water
Coarse Suspensions Colloids Solute
Number of phases
Particle size, nm
Physical states
Solid
Liquid
Gas
More than one
Greater than 200-500
X
X
X
More than one
200-500
Sols
Emulsions
Foams
One
Less than 10
Liquid
Liquid
Liquid
molecule number approximately 366,000 possible compounds or about 3.5 million,
if one counts stereoisomers, compounds of the same structure but with different
spatial orientations among the parts. Among the simple aliphatic alcohols such as
methyl CH3OH and ethyl CzHsOH, there are 5.6 million compounds possible with
20 carbon atoms, and 82 million if stereoisomers are counted.
The Classes of Organic Compound!
Some general classes of organic compounds differ in their roles as pollutants.
These classes are distinguished by their essential carbon-carbon bonds.
In aliphatic compounds, the carbons or other elements are linked in either linear or
branched simple chains. Some are saturated (i.e., each carbon is linked to its adja-
cent carbon by a single valence bond). Others are unsaturated (i.e., the carbon bonds
are double or triple). The unsaturated carbon double or triple bond is more easily
attacked by chemical reagents than is the single bond. In alicyclic (nonaromatic)
compounds, the carbon chains have closed upon themselves forming a ring structure.
Heterocvclic (nonaromatic) compounds, contain an element other than carbon in
the ring. Aromatic compounds are ring compounds that have special properties. In
most, the ring is made of carbons, with a particular linkage of alternating single and
double bonds, but heterocyclic compounds may also be aromatic. The closure of the
ring in this manner produces, despite the double bond, a structure that is very stable
against attack.
Of course, large molecules that occur as organic pollutants in water may be com-
posed of several of the foregoing types of structures, and so may have the properties
of the aliphatics at one end of the molecule and of aromatics at the other end.
Many aromatic compounds are toxic or carcinogenic. Two particular classes that
are prominent in current environmental literature are polychlorinated biphenyls
(PCBs), and polynuclear aromatic hydrocarbons (PAHs).
And finally while describing compounds of major current concern, one should in-
clude the trihalomethanes (THMs). These are aliphatic compounds composed of
methane, Cm, in which three of the hydrogens have been replaced with a halogen,
the most common being chlorine, producing chloroform, CHCh.
"Natural and Unnatural" Compounds
The reason for this sketchy introduction to organic compounds is to emphasize
that by far the greater part of the Earth's mass of living matter is in the form of ali-
phatics than in theformof cyclics or aromatics. The bulk of living substance consists
of proteins (made up of amino acids), carbohydrates, and fats, all of which are ali-
phatics. The non-aromatic compounds, alicyclic and heterocyclic, occur in much
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smaller amounts, and are typically found in the minor components of living orga-
nisms including bark, nuts, essential oils, pigments, hormones, and vitamins.
The aromatics are even more rare and are found in living organisms in much
smaller amounts in the form of hydroxybenzene, phenol, cresol and relatives in the
tannins, lignins, resins, gums and balsams of vegetable material.
Because of the amount and ubiquity of natural aliphatics the organisms adapted
to "cleaning-up" the natural environment are well suited to handling this class of
compounds; they are less well adapted to metabolizing cyclic compounds, aromatic
or non-aromatic.
Organic compounds occur not only in living organisms but also in such fossilized
remains as coal and petroleum. In these materials, which have made such an impact
on our environment, aromatic compounds occur more frequently and more plenti-
fully.
In the sweep of evolution, therefore, one sees the development of communities of
organisms adapted to use (and thus destroy) the organic metabolic products and
wastes of other organisms; this symbiosis is highly effective for aliphatic compounds,
but much less so for the largely refractory aromatics and other cyclics. However, a
rather felicitous order in nature has been put in disarray. We are now producing
large quantities of aromatics from fossil materials that previously were not available;
more than that, we are now synthesizing organic compounds, aromatics and others,
never before known to nature. These materials are often released in concentrated
form, overwhelming nature's defensive mechanism. The immense challenge their
control presents cannot be exaggerated. With this view it is interesting to recognize
that among the organics and pesticides listed by EPA as priority pollutants most are
aromatics and most of the residue of non-aromatics are aliphatic compounds which
are found nowhere in nature. Many of the latter are more resistant to destruction
than the aromatics.
THE ORGANICS IN WATER AND WASTEWATER
It is reasonable to assume that an examination of organics removed will contain
some description of the specific organic compounds and their concentration actually
found in water and wastewater. However, a mere mention will have to suffice; to list
the organics found in water and wastewaters would require all of the remaining
pages of this monograph. Over 1300 have been identified in the United States. Even
in treated drinking waters, supposedly largely free of organic matter present in the
water source, about 300 synthetic organic compounds have been found occurring in
two or more systems. Thousands of organic compounds may be found in single-
celled microorganisms routinely recovered from wastewaters. Many more will
surely be identified in the future.
Organics in Natural Waters
While most of the unresolved problems associated with organics in waters are
man-induced, the notion that "natural" waters, that is, waters untouched by man,
are "pure" has long since been dispelled. When the Advanced Waste Treatment Pro-
gram was being established by the U.S. Public Health Service in 1960, an effort was
made to set the maximum allowable concentration goal for nitrogen compounds at a
low value. However, that goal was abandoned when it was pointed out that water in
a pristine headwater stream running over the granite in the remote regions of the
Rocky Mountains had a nitrogen content higher than that goal.
Indeed, compared with some of the goals being set today, even rainwater before
reaching the ground may be contaminated. The organics in some rains studied
throughout the world typically lie in the span 5 mg/L to 25 mg/L.
The unpolluted surface waters and groundwaters in North America show organic
concentrations averaging about 8 mg/L, and the estuaries about 12 mg/L. However,
5
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a survey of the water supplies of 80 cities conducted in connection with the search for
trihalomethanes showed the median organic concentration of raw water supplies to
be about 1.0 mg/L for ground water and about 6 mg/L for surface water.
The organics in natural unpolluted waters are derived from the products and de-
gradation of plant materials in and on the soil. The major types of compounds are
"humic materials," tannins, lignins, phenolics, amino acids, hydrocarbons, and fatty
acids. The humic materials are currently of particular interest because of their
reported role as the major precursors of trihalomethanes upon chlorination.
It is not uncommon that after organic compounds produced by industrial pro-
cesses are found to be toxic or carcinogenic, intensified research stimulated by that
revelation shows that these compounds may also be produced by natural processes.
For example, the source of PAHs, especially benzo(a)pyrene (B(a)P), which was
found to be carcinogenic was first attributed to industrial high temperature pro-
cessing; however, further research revealed that significant amounts of PAHs and
specifically B(a)P were produced by bacteria, algae and plants.
Organics in Drinking Water
As already stated, some 300 organic compounds have been identified in the drink-
ing waters being supplied to United States consumers at present. Water, if contami-
nated, is subjected to appropriate treatment; the finished water has generally been
found to meet the established criteria. These criteria and the prescribed treatment
methods had been largely concerned with (1) obvious nuisance materials, (2) turbid-
ity, (3) pathogenic organisms, (4) color, (5) taste and odor, and (6) certain chemicals
associated with public health hazard. The last comprised for the most part the inor-
ganic impurities. Thus, in the sequence of drinking water standards promulgated
passim between the issuance of the initial U.S. Treasury Drinking Water Standards
in 1914 and the National Interim Primary Drinking Water Regulations called for by
the 1974 Safe Drinking Water Act, little attention was given to the organic com-
pounds present which while present in small concentrations would be consumed
over long periods.
Three movements over the past two decades account for the current interest in
residual organics in drinking water and for organics in waters in general. These are:
(1) increased capability to identify and measure small concentrations of organics, (2)
increased public awareness of environmental threats, and (3) increased concern over
chronic effects associated with repeated exposures to low concentration of contam-
inants over prolonged periods.
The main stimulus for the study of organics in drinking waters came from the dis-
covery in 1974 of chloroform and other trihalomethanes (TH Ms) in the New Orleans
drinking water. Since chloroform is alleged to be carcinogenic the EPA responded
with a prompt, widespread and penetrating investigation of the occurrence of such
compounds in other drinking waters and of the causes and remedies for the threat.
Because TH Ms were not present in the raw waters it was reasoned that they must be
produced in the treatment processes; these were soon narrowed down to the com-
monly practiced chlorination disinfection process. The chlorine, reacting with or-
ganic precursors in the raw water, mostly the humic materials, produced THMs,
mostly chloroform.
Organics in Municipal Wastewaters
Not only are organic compounds many and diverse but also the waters and waste-
waters bearing them are many and diverse in their organic makeup. Typical charac-
teristics of municipal sewage are presented in Tables 2 and 3. It should be recognized
that the compositions vary from day to day and from city to city and therefore the
values are to be taken as only rough averages.
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Table 2. Concentrations of Organics in Municipal Sewage
Strong 600mg/L*
Median 350mg/L
Weak 175mg/L
mg/L = milligrams per liter, equivalent to parts per million by weight.
Table 3. Distribution of Organics (in %) in Municipal Sewage According to
Size Fractions
Non-filterable 51 Settleable or floating 30
Not dissolved 63 Not settleable 21
Colloidal 12 12
Dissolved 37 37 37
Total orgamcs 100 100 100
Table 2 shows that the strength of sewage, measured as total organic matter con-
centration, varies over at least a four-fold range and is some hundred-fold greater
than the organics in natural waters.
Table 3 shows how the organics are distributed among the various size fractions.
The non-filterable is that retained on a laboratory filter paper, the not settleable
being that portion of it that will not settle or rise in one hour. One worker distin-
guished between these as settleable meaning settleable or floatable, and supracol-
loidal (larger than colloidal) which he measured by centnfuging. Note that only
about one-third of the organics are in the dissolved state and about one-half are re-
tained on a laboratory filter.
Among the three major groups of organics in sewage the fats comprise about 10%
and the remainder is split about equally between proteins and carbohydrates. Other
compounds contribute only a few percent at the most to the total organic matter, but
may be important as pollutants.
The importance of these distinctions is that with two-thirds of the organics in the
nondissolved phase, efficiency in removal requires attention to the removal of par-
ticulates. Since such removal methods operate via the physical properties of the par-
ticles, they cannot distinguish between the organics and the inorganics (except via
gross differences, e.g. settling out the dense inorganics while floating off the less
dense organics). Thus processes of this kind for removing the organics also have to
handle the inorganics. Furthermore the inorganics may either enhance or hinder the
removal of the organics.
The subject should not be left without a reminder that the major components—the
proteins, carbohydrates, and fats-are relatively easy to remove by processes histor-
ically known and applied; but normal municipal sewage will also contain small
amounts of such exotic chemicals as detergents, waxes, insecticides, and other house-
hold chemicals. It is these components that may be harmful to man and ecosystems
-------
and more difficult to remove. These components of sewage are, however, the pro-
ducts of industry and as previously stated, their sheer multiplicity precludes any
overview of them here.
Organics in Industrial Wastewaters
The problem of industrial wastewaters has been intensified by the enormous
growth of the petrochemical industries and their ancillary chemical industries. Their
products pervade our society and reach our waters either directly through discharges
from industrial plants or indirectly through consumer discharges to municipal
sewers or on land.
We have learned to cope with such traditional industrial wastes as those from food
processing, textile manufacture, pulp and paper manufacture, Pharmaceuticals
manufacture, the metal industries, and conventional chemical plants. The best avail-
able technology appears adequate for handling wastewaters from these industries.
The challenge we face comes primarily, but not solely, from the synthetic organic
chemicals. This challenge has been defined in terms of specific priority pollutants or
families of pollutants that are of special concern because of feared carcinogenic,
mutagenic or teratogenic effects. Every industrial plant producing wastes containing
these pollutants must meet stringent requirements for waste treatment whether the
plant effluent is discharged to the receiving water or the municipal sewer. Develop-
ment of ways to separate these difficult pollutants is a major task facing science and
engineering.
PHYSICAL MECHANISMS FOR THE REMOVAL OF ORGANICS
The two general approaches to removing organics from water are to destroy them
or to move them somewhere else. The first involves chemical transformations of the
organic substances, the essential process being a cleavage of the chemical bonds in
the organic molecule. This may be accomplished by electrical forces, heat, or
chemical agents either added as such or generated in the water phase by radiation or
electrical means. The necessary action is that the molecule change into one or more
other molecules. It is most desirable that the new molecules not themselves be
organics; the ultimate objective is complete destruction and removal of organics by
the transformation into CO2, H2O, nitrogen and other easily removed or harmless
inorganic substances. Less desirable is the transformation of the original organics
into organics of lower molecular weight which are generally more easily removed
and have lesser potential for harm than the original compounds. This does not repre-
sent destruction of the organics, because for every original molecule one or more
different molecules, also organic, are produced. However, this is still removal of the
original molecule species and can be a desirable goal, such as the breaking up of a
toxic molecule into small non-toxic molecules.
The second approach uses physical forces to move pollutants from the main water
phase to another location where they can be separated from the main water body.
The physical forces used may include a quasi-chemical bonding, for example, ad-
sorption on surfaces. Also, to be effective, the physical force used may have to be
preceded by a chemical reaction, for example, one that changes a soluble organic
compound to an insoluble solid that precipitates and can be removed by physical
means But the essential step is a displacement from the main water phase.
Fugacity and Activity
Fugacity and activity are thermodynamic properties involved in a number of
mechanisms for removing organics from water. Fugacity measures the escaping
tendency, i.e. the tendency to relocate, and activity measures the tendency to enter
into chemical reactions.
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In a closed system of a water layer (phase) and a gas layer (phase), if a small amount
of a volatile organic compound is injected into the water, the concentration of the
organic will be high in the water phase and zero in the gas phase. However, the mole-
cules of the organic solute are in random motion in the liquid and some of these
random motions propel them through the water surface into the gas phase. The num-
ber of molecules per second passing into the gas will be proportional to their concen-
tration in the water phase. But as the molecules build up in the gas phase, they are
also in motion, and some of them pass back through the surface into the water phase.
The rate at which these do so is also proportional to their concentration in the gas
phase. Thus, the concentration in the water phase will decrease and that in the gas
phase will increase until the number per second passing out of the water equals the
number per second passing out of the gas. Thereafter, no further changes in concen-
tration or in net transport will occur. The system is said to be in equilibrium, some-
times expanded to dynamic equilibrium, which refers to the exchange transport that
is still going on.
The molecules of the gas and water and the organic compound that are in the gas
phase are so far apart that each species acts as if the others were not present. The total
pressure in the gas is the sum of the separate pressures of the separate species, and
these separate pressures, called partial pressures, are proportional to the concen-
tration of the particular molecule species. That being so, the partial pressure
becomes a measure of the concentration.
In the water phase, the molecules which are much closer together interfere with
each other's movements. Consequently their velocities are lower than the velocities
in the gas phase. For that reason, the concentration in molecules per cubic centi-
meter at equilibrium has to be much greater than that in the gas phase, but, as in-
dicated, the rate of passage through the surface is proportional to the concentration.
When that concentration is expressed in terms of mol fraction (i.e. the ratio of the
number of moles of compound to the number of moles of water and other com-
pounds in the water phase), then it is found that for any given compound the partial
pressure in the gas phase is proportional to the mol fraction in the water phase. This
is called Raoult's Law.
The same mechanism, namely the equalization of the rates of transfer in the two
directions, applies even if there is no boundary. For instance if a dye is injected at one
point into a body of water, it will distribute itself throughout the body until the con-
centrations are the same everywhere.
Such an explanation would be made by a kineticist (a physical chemist who studies
the rates at which processes occur and who therefore thinks of events in terms of
rates). His fellow physical chemist, the thermodynamicist, thinks of events, or more
properly of conditions, in terms of energy.
There is a thermodynamic property of systems called "free energy." Any sponta-
neous change occurring in a system does so in such a direction that the free energy
(content) of the system decreases. When a system is at equilibrium the free energy
content is at a minimum; therefore, all systems not at equilibrium and not under
any external force move in the direction of equilibrium, that is toward a lower free
energy state and never in the opposite direction. Thus in the dye example the free
energy, per mole of dye, in the concentrated dye injected is greater than that in the
bulk of the solution. The dye moves so as to decrease its free energy, that is, from the
concentrated portion toward the dilute portion and never in the opposite direction.
Thermodynamicists have developed a body of knowledge by which free energies
can be calculated from concentrations or pressures, and this is extremely useful in
predicting in which direction a process will proceed and how far it will go. But it has
been found that these relations hold only in dilute solutions and at low gas pressures.
When they are applied at higher concentrations or pressures they do not give the cor-
rect answers.
One of the giants of physical chemistry, G.N. Lewis, proposed around 1900 a
means to handle this divergency which is immensely useful and widely applicable, al-
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though it sounds like a trick. Lewis created an ideal pressure called fugacity and a
corresponding ideal concentration called activity which had magnitudes such as to
make the thermodynamic equations for free energy give the correct answers when
they were substituted for the actual pressures and concentrations. Fugacity measures
the escaping tendency, i.e. the tendency to relocate. Activity measures the tendency
to engage in chemical reactions.
As fugacity and activity are companion entities, it was necessary to invent fugacity
coefficients and activity coefficients, which are the ratios of the fugacity or the ac-
tivity to the pressure or the concentration. These, as well as the fugacity and activity,
vary with pressure and concentration. Again this simple trick made possible the cor-
relation of a wide number of properties and processes and a prediction of the be-
havior of systems, including prediction of the freezing points and boiling points of
solutions, the solubilities of substances, the electromotive force(voltage) of electrical
cell reactions, the rate and extent of chemical reactions, the direction of physical and
chemical changes, and the distribution of components at equilibrium, e.g., the
partial vapor pressures over solutions. As to the latter the law is that at equilibrium
the activities, or the fugacities, in the various phases are equal and systems will react
so as to attain that condition.
So far this discussion of fugacity has dealt with bulk bodies and surfaces. A sur-
prising fact resulting from free energy considerations is that the fugacity of sub-
stances in small bodies is greater than in large. Thus, the fugacity in droplets is great-
er than in the bulk liquid, and the free energy is in fact inversely proportional to the
diameter of the droplet. Therefore, the escaping tendency in small droplets is greater
than in large ones, and material from small droplets will be transferred to the larger
ones even without any contact or coalescence. For the same reason, the escaping
tendency is greater for finely divided solids than for coarse grains, and as a result the
solubility of finely ground solids is greater than that of coarse grains; the vapor
pressure is also greater.
Because most of the organics in water occur in finely divided form, including as
solute molecules, such considerations are of importance in an understanding of the
removal of organics and in developing or improving processes for doing so.
Because free energy and fugacity are involved in and control all transport separa-
tion processes, even those based on gravitational, centrifugal, magnetic and electri-
cal mechanisms, and chemical change processes, they are particularly applicable to
processes that the chemical engineer would call mass transfer processes.
Physical Relocation Concepts
Several distinctions must be made in a logical system for organizing the concepts
involved in separation by physical motion.
First, distinction must be made between motion in general and the more specific
form that directly results in separation. There are motions experienced by molecules
and particles that are either random, undirected, or Compensated; in neither case
does bulk mass transfer in any particular direction occur. Examples on the micro-
scale are the thermal motions of molecules and the Brownian motions of particles re-
sulting from these random movements of molecules. An example on the macroscale
is thermal convection in which bulk transfer occurs upwards in response to the lower
density but is exactly counterbalanced by a corresponding downward flow. For
want of a better term, we use "relocation" as excluding motions which do not result
in gross mass transfer toward a target. We exclude the nondirected motions from the
concept of relocation, but we are aware of the fact that such motions still contribute
to the ultimate relocation. They do so in ways to be described subsequently.
Secondly, a distinction must be made between directed motion on a macroscale
and that on a microscale. The motion of a charged ion or particle toward the oppo-
sitely charged surface of an adsorbent is not random, since there is a net displace-
ment toward the adsorbent whether it be fixed in place, as in a packed column, or
10
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mobile in the liquid in the form of an added coagulant. I n that sense, such motion is
not among the random undirected motions excluded earlier. However, with the
mobile adsorbents, such motion does not immediately result in physical separation,
since that occurs only when the mobile adsorbent is itself separated from the liquid
by some subsequent mechanism.
The Driving Forces and Mechanisms In Physical Relocation
The mechanisms that are operative in relocation of solids in water are:
Gravity
Centrifugation
Magnetism
Surface Tension
Foam Fractionation
Froth Flotation
Solvent Extraction
Stripping
Distillation
Osmosis
Reverse Osmosis
Adsorption
Electrokinetics
Electrophoresis
Electroosmosis
Electrodialysis
Freezing
Hydration
Filtration
Microfiltration
Flocculation and Sedimentation
Forces classed as macro are those in which the driving force operates at a distance
from the target location and the motion occurs over a long pathway; those identified
as micro are forces in which the object and the target have to be close together and
the motion occurs over only a short pathway. It is obvious that any force that oper-
ates on the macroscale must also operate on the microscale. There follows a de-
scription of each of the forces or mechanisms listed.
Gravity—
Gravity, the gravitational force between the earth and body near its surface, pulls
downward both the liquid and the particle immersed in it with a force proportional
to their respective weights, i.e. densities. Since natural systems tend toward the con-
dition of lowest energy state the result is that the dense bodies gravitate to the bot-
tom, the light bodies to the top.
Centrifugation—
Centrifugation is a force quite similar to gravity in its effect but brought about by
rotation of the liquid to generate a force outward along the radius of rotation. This
may be done by rotating a vessel holding the liquid or by causing the liquid to flow in
a circular path in a fixed vessel.
Magnetism—
Magnetism is that force exerted on a material by a magnetic field. All substances
experience a force in a magnetic field, a paramagnetic material being drawn to a
magnet, a diamagnetic one being repelled from the magnet. Most materials
11
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experience only very small forces in a magnetic field but a special class of paramag-
netics, called ferromagnetic, experiences forces 106 to 108 times greater. Thus, though
organic substances are not magnetic, finely divided ferromagnetic substances can be
added, which, adsorbing the organics or being adsorbed or bound in them, allow the
separation by a magnetic field
Surface Tension—
Surface tension is a force existing at the interface between a liquid and another
liquid, a solid, or a gas. It exists because while the molecules in the body of the liquid
are fully surrounded by other molecules and thus attracted equally in all directions,
those on the surface are attracted only in the direction of the bulk of the liquid. As a
consequence the surface tends to contract to the smallest possible area. In order to
extend the surface it is necessary to bring a molecule to the surface against the inward
force. This work is measured as a force, dynes per centimeter, and the force required
to pull the surface apart on a line one centimeter long is called the surface tension.
Because of surface tension, liquids that wet a solid experience a force driving them
into small pores of the solid, the force being proportional to the surface tension and
inversely proportional to the radius of the pore. Thus a blotter, a solid body having
small pores, will, against the force of gravity, soak up water which wets it.
In the other direction, if the liquid does not wet the solid a force is experienced
which in effect keeps the liquid out of the pores. Thus by proper choice of solid, one
may separate wetting liquids from non-wetting liquids, for example, water from oils.
Solids wetted by water are called hydrophilic; those not wetted by water are called
hydrophobic.
These phenomena are identified as capillary and absorption, and the liquid pene-
trating the pores is said to be absorbed. This is to be distinguished from the phenom-
enon of adsorption to be discussed. Absorption is an effect associated primarily with
the surface properties of the liquid. Adsorption is associated with the surface prop-
erties of the solid.
Certain organic compounds have properties that cause them to preferentially ac-
cumulate at water-air or water-solid interfaces. Their attraction for each other and
for the water molcules is much weaker than the attraction of water molecules for
each other. Also the chemical grouping at one end of the molecule may be attracted
to water and at the other end repelled. As a result forces are created that produce a
concentration of the compound at the water-air surface or at the water-solid surface.
Because its presence reduces the interfacial tension of the water, it is termed a surface
active agent. The surface activity of soap and detergents, which are surface active
agents, is responsible for their detergency and their foaming tendency.
Foam fractionation—is a process that takes advantage of the foaming tendency as
a means of removing surface active agents from water. Since they move to a surface,
it follows that their concentration at the surface of a submerged bubble (of air) is
greater than in the bulk of the liquid, and that when such bubbles become airborne
(i.e. a foam), the ratio of compound to water in the foam will be greater than in the
bulk liquid. This is the basis of foam fractionation in which a foam is artificially pro-
duced and removed, carrying the surface active agents with it
Froth flotation—is also based on interfacial tension and is related to but should
not be confused with foam fractionation. Froth flotation has been extensively
studied and applied in the separation of the constituents of ores. It is used to separate
participate matter, not dissolved matter. When the surface of a particle suspension
or colloid is hydrophobic, i.e. repelled by water, the particle will attach itself to the
air-water interface of a bubble and will float to the surface. Since most natural min-
erals have a hydrophilic surface it is the function of an added "collector"to make the
hydrophobic surface. The actual molecular operation is amazingly selective. It has
been extensively studied but is still a subject of controversy. To confound the picture,
secondary additives, called conditioners, can promote or inhibit collection. Those
12
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that promote collection are called activators. Whether they act by altering the sur-
face of the particle or by influencing the action of the collector is also a matter of con-
troversy.
Solvent Extraction—
Solvent extraction is a process in which the water is contacted with an immiscible
liquid solvent. The organic to be removed is more soluble in the solvent than in the
water, that is, its fugacity in the water phase is greater than in the solvent phase, so
the escaping tendency is from the water to the solvent until equilibrium is reached.
The organic compound of interest transfers from the water to the solvent until the
fugacities are equal in the two phases.
Solvent extraction is usually carried out in the sense just indicated, namely, the
solute is extracted by the solvent. However, it is quite possible that organic
pollutants are very insoluble in the solvent and that the water is very soluble in the
solvent. In that case the water would transfer to the solvent phase and the organic
pollutants would remain. A considerable amount of experimental research and
ingenious theoretical research on this mechanism employing physical-organic elec-
tronic theory and the quantum theory of valence was carried out under the Saline
Water Conversion Program. It appears that the extraction effect, i.e. the transfer of
the water, as well as the selection effect, i.e. the rejection of the solute, depends on
groups in the solvent molecule capable of hydrogen bonding, i.e. of the water.
Whether this rationale would also apply in the case of organic compounds, which
themselves have bondable hydrogen, is an intriguing theoretical and experimental
question.
By solvent extraction one ordinarily means extraction of the solute. No special
name has been coined for solvent extraction of the water.
Stripping—
Stripping is a process in which the water is contacted with a gas, air being the obvi-
ous choice. The more volatile (higher fugacity) organic contaminants transfer to the
air phase and are removed.
Distillation—
Distillation is a process for removing organics that are much less volatile than
water. Clearly, from the standpoint of thermodynamics, if more volatile compounds
can be removed from water by being transferred to the air by stripping, then water
can be removed from the less volatile substances by the same means. Distillation is a
version of stripping in which the water vapor itself, rather than added air, is the
carrier. However, in order to transfer water into the vapor and transport it to the
condenser, it is necessary to heat the water. While much of the heat can be recovered,
the efficiency of heat transfer and utilization is low, with the result that large amounts
of heat are required, and for that reason this process is impractical for removing
organics on the scale involved
Osmosis—
Osmosis is a process in which the driving force also results from a difference in
fugacities, that between water and a solution. In the description of the diffusion of a
dye into the bulk of a water body only the escaping tendency of the dye, that is, the
tendency to move from more concentrated regions to the less concentrated regions,
was mentioned. H owever, a strict reading of the laws shows that in that case the water
also moves from the region where it is more concentrated (i.e. the bulk of the liquid),
to that region where it is less concentrated (i.e. the injected dye element). The mol
fraction of water in the original water is 1.0 and that in the injected dye element is less
than 1.0. Thus both the water and the solute will relocate until the fugacity of solute,
and also of water, is the same throughout.
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If the dye element is separated from the water body by a membrane that allows
the passage of water but not of dye, then the water will pass through the membrane
but the dye cannot although it has the escaping tendency to do so. Therefore, the
water side of the membrane retains its original fugacity and continues to move indef-
initely into the dye side until the fugacity (of the water) is the same on the dye side as
on the water side (i.e. to infinite dilution). Now, however, if the dye side is a confined
space, the incoming water will cause the liquid level on the dye side to rise, thus cre-
ating a pressure difference. The process continues until the pressure increases the
fugacity of the water on the dye side to the point at which it just equals the fugacity of
the water on the water side. Such a membrane is called an osmotic membrane, the
process, osmosis, and the pressure difference between the two sides at equilibrium,
the osmotic pressure.
Reverse Osmosis—
Reverse osmosis involves an external pressure (on the dye side) equal to the
osmotic pressure. With any further increase in the applied pressure, a movement of
water will occur in the reverse direction, from the concentrated side to the dilute side.
Reverse osmosis is now a commercial process for water purification.
Of interest is that when reverse osmosis was first proposed, as a saline water con-
version process about 30 years ago, the general scientific and engineering community
gave it a low priority and little chance for success, dubbing it almost a laboratory
curiosity. One reason was that process engineers are often repelled by high pressure
proposals because of the expensive equipment and the energy required to compress
materials. However, most high pressure processing is carried out with gases and the
concept of high energy requirements comes from that technology. The fact is that the
energy required to raise the pressure on a material is proportional to the pressure
times the change in volume. Gases undergo a great change in volume with an
increase in pressure, so both terms in the multiplication are large and consequently
the energy requirement is large. However, water is highly incompressible, that is, it
undergoes only a small change in volume with pressure; therefore, the volume
change factor is small and the energy requirement is not so great as it would first
appear to be.
A second obstacle to early acceptance of reverse osmosis was the fact that the orig-
inal membranes did not transfer water at rates that made the process economical.
Furthermore, the molecular mechanism by which the water was transferred in either
direction had been under dispute for decades and there seemed to be no rationale by
which this property could be improved. Fortunately, by a combination of
legerdemain, dogged scientific experimentation, and pure luck, two young men in
California developed a membrane that had good transfer properties. This showed
the way to widespread investigation and improvement of the molecular process.
Adsorption—
Adsorption is the process by which molecules or particles in the liquid or gas phase
become attached to a solid. In adsorption, the thermodynamic driving force is the
difference in free energy, in activity, between material dissolved or suspended in a
liquid and material attached to the surface of a solid. The solid is called the ad-
sorbent, the attached material, the adsorbate.
In general, the whole surface of the adsorbent is not active in the attachment.
Some spots are more strongly attractive than others. The theory of adsorption, de-
scribed in later chapters, therefore involves the number, strength, and distribution of
these active spots. The actual forces involved are electrostatic bonding, covalent
bonding, or van der Waals bonding. In electrostatic bonding the adsorbed particle
bears a net electrical charge, either positive or negative and is attracted to and held at
a spot on the surface of the adsorbent which bears the opposite local net charge.
Since excess electrons are mobile on continuous surfaces, such localized net excess
of electrons does not often occur. A covalent bond is one in which pairs of electrons
14
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are shared between two atoms, thus completing the shells of the atom. The van der
Waals bond arises as essentially an electrostatic attraction between objects, mole-
cules, particles, or surfaces in which there is no net excess or deficiency of electrons.
Instead, the electrons in one or both are mobile so that a positive charge on an ad-
sorbent attracts the electrons in a particle thus building up an excess on the side of
the particle facing the adsorbent. Since these electrons are closer to the adsorbent
than the positive charges on the other side, the net effect is an overall attraction. The
range of effectiveness in attraction is of the order of two molecular diameters.
Adsorption through van der Waals forces is called physical adsorption, distin-
guishing it from adsorption through covalent bonding, which is called
chemisorption. Physical adsorption is accompanied by a heat production called the
heat of adsorption, which is 5 to 10 kilocalories (kcal) per mole adsorbed. Chemi-
sorption, a stronger bond, has a heat of adsorption of 10 to 100 kcal/mole. In some
cases the covalent bond between adsorbent and adsorbate is stronger than the or-
iginal bonding in the adsorbent itself. Thus, when oxygen is adsorbed on charcoal
and then is heated slightly to desorb, the resulting compound is not oxygen but CO
or CC>2, indicating that the carbon-oxygen bond is stronger than the carbon-carbon
bond of the charcoal.
As has been mentioned, in electrostatic attraction the force field falls off as the
square of the distance between the objects, and in van der Waals attraction as the
cube of the distance. The force field of covalent bonds falls off even more rapidly
with distance, and as a result it is doubtful that covalent adsorbates lie more than one
molecule thick on the active surface. The van der Waals forces, however, by their
very nature polarized, are exerted over greater distances. Therefore, layers of several
molecules thickness can be adsorbed in van der Waals adsorption.
Since adsorption occurs at the solid-liquid interface, it follows that, other things
being equal, adsorbents with large surface areas per gram will adsorb large amounts,
in grams adsorbate per gram of adsorbent. Therefore, although all surfaces show
adsorption powers, the most effective adsorbents have the largest amount of surface
area. Thus, the specific surface areas of commercial activated carbons are in the
range of hundreds or even thousands of square meters per gram. For the same basic
reason, particles of colloidal size are especially active as adsorbents.
As can be predicted from free energy and activity concepts the amount adsorbed
per gram of adsorbent increases as the concentration of adsorbate in the liquid
increases, up to the level at which the active surface is saturated, that is, completely
covered. Thereafter further increases in concentration do not result in any increase
in adsorption.
Solute compounds differ in their adsorbability. A number of conditions are of
special interest in this regard. Large molecules are more strongly adsorbed than
small ones. Polar groups such as -OH, -NH2 decrease adsorption because they are
attracted to the water and the adsorbent must overcome this attraction. For a similar
reason, the more soluble compounds are the less well-adsorbed. The spatial ar-
rangement of the groups in the molecule affects adsorbability; for example,
branched chain acids are less adsorbed than straight chain ones. In the case of phys-
ical adsorption the difference in solute concentration between the bulk of the
solution and the adsorbing surface is proportional to the change in the interfacial
(surface) tension with concentration of adsorbate. Compounds that strongly de-
crease the surface tension of the liquid are adsorbed more strongly than those that do
not. (Most substances decrease the interfacial tension of water.)
All of these considerations lead to the fact that in multicomponent systems ad-
sorption is competitive among the various adsorbates. Even in a system which in-
cludes only water, a solute, and the adsorbent, there is competition between the ad-
sorbate and the water, although water is only very weakly adsorbed on most adsorb-
ents for organics. Therefore, the process of removing orgamcs from waters, complex
enough with single solutes, becomes even more complicated and intractable in
practice.
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Adsorbents, like collectors in froth flotation, are amazingly selective. For ex-
ample, bone charcoal adsorbs the molecules imparting color to sugar liquors in the
presence of a concentration of sugar molecules several hundred thousand times as
great, even though sugar itself is a complex molecule presenting a variety of bonding
opportunities including hydrogens, hydroxyls, carbonyls, etc.
Electrokinetics—
Electrokinetics refers to the motion of ions or molecules or particles along a gradi-
ent of electrical potential. Strictly, electrolysis involves the migration of ions, but
since this is associated with a chemical change at the electrode, it is discussed later
with the chemical mechanisms.
Particles, colloidal or suspended, in water usually bear a negative electrical
charge. This electrical charge induces a positive charge in the immediately adjacent
liquid layer. "Electrical double layer" is the term applied to this phenomenon. Thus,
a difference in potential exists between the particle and the bulk of the solution be-
yond the region of influence of the induction, that is, across the electrical double
layer. This difference, known as the electrokinetic potential, is also called the "zeta
potential." For most colloids the electrokinetic potential is in the range of 30 to 60
millivolts.
Electrophoresis—refers to the movement of suspended charged particles that
occurs when an electromotive force (i.e. a potential difference), is externally applied;
the negatively charged particle is propelled toward the cathode, and the positively
charged layer toward the anode. This constitutes a method for relocating organic
particles in water. The velocity at which the movement occurs depends upon the
gradient of the applied field. The rate standardized at a gradient of one volt per centi-
meter is called electrophoretic mobility. Most colloids have an electrophoretic
mobility in the range of 2 to 4 * 10 "4 cm/sec., which is only about one-half that of
ordinary ions (except hydrogen and hydroxyl).
Electroosmosis—refers to the movement of water that occurs under an electro-
motive force when the particles are restrained from movement, as by a permeable
membrane. This water movement is a method for relocating the organics. Much
more important for the removal of organics than these macro movements (electro-
phoresis and electroosmosis) is the effect of electrical double layer in the micro
movements involved in producing settleable particles from nonsettleable ones, as
will be explained later under sedimentation.
Electrodialysis—is the final electrokinetic mechanism discussed here. An electro-
dialysis membrane is one which allows the selective electrokinetic passage of ions. In
a cell made up of pairs of these membranes and subjected to electrodialysis, the
anions in a compartment move through one membrane out of the compartment but
the cations on the other side of this membrane cannot move into the compartment.
The reverse is happening at the other membrane. As a result the solution in the com-
partment is depleted of ions. The primary use of electrodialysis in the water field is
for removal of inorganic ions as in saline water conversion.
Freezing—
Freezing permits the separation of water from its impurities. When water con-
taining a solute is frozen, the ice that precipitates contains none of the solute except
that in microdroplets of a solution occluded as the ice forms. Thus freezing is a
mechanism for producing pure water and a residual solution in which the solute is
more concentrated than in the original. Considerable work has been done in this
mechanism as a means of sea water conversion. The initial impetus was the fact that
the heat required for freezing is only I/7th of that required for distillation. However,
when all factors are considered, the freezing process has only slight advantage ther-
modynamically over distillation for a process operating at a finite rate on sea water,
containing 35,000 ppm of solute.
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However, for wastewaters of approximately I/ 100th this concentration, one of
the disadvantages in sea water freezing, the lowering of the freezing point, practically
disappears. Furthermore, the viscosity of pure water at its freezing point is about
30% lower than of sea water conversion brine at its boiling point and the thermal
conductivity is higher. These factors might improve the washing and heat transfer as
compared with the washing and heat transfer in saline water conversion.
To sum up, freezing is a mechanism for removing organics (and inorganics) but we
are not sure how it will compare economically with distillation, a process which has
already been ruled out on the basis of cost.
Hydration—
Hydration, like freezing, separates water from its impurities. Just as ice is a form
of water in which the free energy is less than that of liquid water, so there are sub-
stances which will form ice-like solid compounds with water (hydrates) in which the
free energy is also lower. Some of these are thermodynamically superior to ice
because the phase change can be accomplished well above the freezing point of
water. Hydrates typically contain 1 to 10 moles of water per mole of compound, but
there are certain metallic salts and tetra alkylammonium salts that have 50 to 100 or
more moles of water, and they are fairly insoluble in water. The impurities remain in
the supernatant and the water is regenerated by warming the hydrate to its melting
point. Low solubility and easy removal of the hydrating compound is a requirement.
Some gases that form hydrates have been studied as a means of saline water con-
version. The lower hydrocarbons, methane and butane, form hydrates with up to 15
moles of water per mole of hydrocarbon (which brought them to the attention of the
petroleum industry because they cause freeze-ups in pipelines). The water is regen-
erated by heating the hydrate to recover the hydrocarbon gas and water containing a
small amount of hydrocarbon.
Such gas hydrates are clathrate (cagelike) compounds. The water molecules form
a cage around the hydrocarbon molecule. The lattice structure of the resulting
clathrate is different from that of ice. As an example of tactics based on theory, the
large cage space in the lattice is about 70 nm in diameter. Therefore, molecules larger
than this can be excluded as hydrate candidates.
Filtration—
Filtration is a seemingly simple but actually complex process for the separation of
solids from water. It is one of the mechanisms for separation of organics that does
not easily lend itself to categorization under a fundamental force. The mechanisms
involved in filtration are many and they have been extensively studied and debated.
The process itself may be described simply: under a hydrostatic pressure the filter
medium will allow the hydrodynamic flow of water and dissolved matter but will re-
strict or prevent the concurrent transfer of particles and larger molecules. The filter
medium may be a bed of sand or it may be a membrane including a simple screen.
With a simple screen the separation process is clear and not subject to dispute. It is
obvious that particles too big to pass through the openings in the screen will be re-
tained while the water passes through both the screen and the body of retained
particles.
The picture gets more complex as the size of the openings gets smaller. In such
cases other forces come into play. Some filters will retain particles definitely known
to be smaller than the pore sizes. The retention may involve electrostatic forces. It
may be that the first layers of particles retained comprise a body in which the pores
between particles are smaller than those in the medium so that the particles them-
selves become the filter medium and the medium itself merely their support. Or there
may be a bridging effect whereby several particles arriving at a pore at the same time
form a bridge which retains both the particles and subsequent arrivals even though
the pore size of the medium could pass any of them individually. The goal of the de-
sign of such a filter is to retain the particles while maintaining a pore structure per-
meable enough to pass the water at economical rates.
17
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Microfiltralion or Ultrafiltration—
Microfiltration or ultrafiltration refers to a process which employs membranes
with pores fine and uniform enough to retain specific size particles and pass smaller
ones, and even to separate different sizes of molecules. The term microfiltration is
sometimes applied erroneously to reverse osmosis, which also employs a membrane
and a hydrostatic pressure, and provides for the retention of some components and
the passage of others.
Microfilters are ordinarily the last step in removing organic particulates from
water. From the previous description it can be seen that to obtain the maximum
throughput of water, the feed water should have as few particles as possible, the bulk
being removed by less expensive methods, and the particles shoud be as large as pos-
sible. Since the bulk of the organic pollutants in waters are in paniculate form, e.g.
municipal wastes, the removal of particulates is of primary concern. An additional
consideration is the fact that the particulates and especially the colloids and their
aggregates have the ability to adsorb dissolved organics and thus removal of these
particulates results in removal of some of the dissolved organics.
Flocculation* and Sedimentation—
Flocculation and sedimentation subsume many of the fundamental mechanisms
previously discussed. Though centrifuging is a form of sedimentation (and can
generate forces hundreds and thousands times the force of gravity) our discussion
here is directed to gravity sedimentation because the force and energy involved in
this process are generally free of cost.
Table 4 shows the approximate rate of gravity fall of spheres of density 1.1 gm/cc
in completely quiescent water. At these rates of fall, the slightest movement of the
water or the slightest temperature differences resulting in a convection current
would keep the colloidal particles in suspension forever. Also, it shows why it is de-
sirable to agglomerate the particles into the largest size possible. Not shown is the
fact that since the velocity is proportional to the difference in density between water
and the particle, a particle of 1.2 gm/cc density instead of 1.1 gm/cc would fall at
twice the velocity of the smaller. The 1.1 gm/cc density has been chosen as repre-
sentative of an organic particle. Since the density of inorganic particles would be 2
gm/cc or even 3 gm/cc, it can be seen that the presence of inorganic particles in-
corporating or incorporated with the organics is very important for sedimentation
rates.
The velocity is also inversely proportional to the viscosity of the water. Since vis-
cosity is reduced steeply with temperature increases, even slight increases in temper-
ature as from solar heating will increase the particles' settling velocity. However,
such temperature increases also will generate convection currents which act to re-
suspend the particles. It is clear that the greatest effect on velocity is to be achieved by
agglomerating the particles. In order to do this, it is necessary that the particles be
brought into contact with each other. Three mechanisms accomplish this: Brownian
motion, shear, and differential settling.
Brownian motion is the random displacement of small particles due to the random
and unbalanced impacts on the particles of the water molecules in thermal motion.
These random micromotions bring particles into contact with one another.
Shear occurs along the juncture of two volume elements of water moving relative
to each other. Particles in or near the juncture are dragged past each other and con-
tact occurs. Such shear forces may be natural, as with convection currents, or may be
*(Note. There is ambiguity in the common use of the term fiocculation Some use flocculation to mean
agglomeration, i e. the coming together of particles to form larger aggregates This no doubt comes about
because the aggregates are called floes. But the strict meaning of flocculation is as we here use it. namely the
operation of slow stirring to induce agglomeration.)
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Table 4. Approximate Rate of Fall in Water of Spheres, Density, 1.1 gm/cc
Diameter
fjm
100
10
1*
0.1*
0.01*
cm/hr
200
2
.02
—
—
cm/yr
_
—
200
2
2 x 102
0.001* — 2x10"
* = colloidal dimensions.
artificially induced by an operation called flocculation. In the latter, the liquid is
stirred very gently by slowly moving paddles so as to allow contact opportunities
without breaking up the delicate floe already formed.
Brownian motions are constant and cannot be controlled, but they do not inter-
fere with settling because averaged over time, displacement is equal in all directions.
Agglomeration by shear forces can be controlled, but such shear movements inter-
fere with sedimentation and therefore flocculation is carried out separately from the
sedimentation process.
Agglomeration through differential settling operates only in the sedimentation
process itself. If the particles in a suspension are monodisperse, then each particle
settles at the same rate and no particle can settle faster than another. Therefore, there
are no contact opportunities and very little agglomeration will occur. However, if
the system is polydisperse, the larger particles are settling faster than the smaller, and
as a consequence, there are many collisions.
One force that keeps particles from agglomerating is their charge; because they
carry the same charge, they repel each other and thus cannot come into contact and
agglomerate. However, if a colloid bearing the opposite charge is added or if ions of
opposite sign that can be adsorbed by the particle are added, the particle charge is
neutralized and the particles can come into contact and agglomerate. That the
mechanism is essentially electrostatic is shown by the fact that the required amount
of oppositely charged ion to be added is reduced if that ion has a high valence.
Water has a high dielectric constant, that is, the strength of an electrostatic field is
strongly attenuated with distance in water. Charged colloidal particles that are
highly hydrated, that is, surrounded with a sheath of water molecules, tend to resist
coagulation by added ions simply because the attraction between the ion and the
particle is much reduced. Some highly hydrated colloids remain stable even though
their electrokinetic potential is reduced to zero. The water sheath prevents contact. If
a dehydrating agent, such as acetone or alcohol, is added, the sheath is destroyed and
the colloid is again subject to coagulation by added ions.
Agglomeration does more than merely gather up the particulate material. By ad-
sorption, the floes can pick up dissolved molecules and carry them down in the sedi-
mentation process. Floes can be artificially formed by adding chemicals which them-
selves form colloidal, particularly gelatinous, masses which can be generated in sub-
stantial quantities. Floes so formed are very effective in removing suspended
colloidal and dissolved organics. Iron and aluminum salts are commonly used to
form the the gelatinous hydrous oxide floes which are insoluble in water and
relatively easily settled.
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The process of flocculation and sedimentation, involving as it does so many of the
fundamental forces for relocation, is of primary importance in organics removal,
and indeed by far the major portion of organics removal is accomplished by these
processes. A first step in organics removal is to remove as much as possible through
sedimentation, leaving the residual compounds in the supernatant for additional
treatment.
CHEMICAL MECHANISMS FOR THE REMOVAL OF ORGANICS
Role of Free Energy
The free energy change determines whether an action can occur and in which
direction it will proceed. It follows logically that the free energy change will also de-
termine how far an action will go before it comes to a stop at an equilibrium con-
dition. No reaction can occur unless the free energy of the products is less than the
free energy of the reactants. And if the reverse reaction is possible, the larger the free
energy change, the higher the ratio of concentration of products to concentration of
reactants at equilibrium. If the reverse reaction is not possible the reaction goes to
completion.
This does not mean that every reaction with a high free energy change can readily
occur. With chemical reactions and with some physical processes, another energy
condition must be satisfied. A reaction between two molecules is visualized as a
coming together of the two, forming an ephemeral compound called the activated
complex which then, as a result of rearrangement of atoms, breaks up into the re-
action products. This will occur only if the collision of thetwo reactants has given the
activated complex a certain minimum energy content. Otherwise, the two reactants
simply fail to complete the reaction and will retain their individual charges. Even if
the free energy content of the reactants is higher than that of the products, no reac-
tion occurs unless there is additional energy available to raise the level of the reac-
tants to the activated state. Thus, the rate of the reaction depends not simply on the
rate of collisions between the reactants but upon the rate of collisions that have the
necessary energy.
It is possible, by use of a discipline called statistical mechanics, which is not con-
nected with thermodynamics, to calculate the frequency of collisions and also the
frequency of collisions having an energy greater than that of the activated complex
and thereby to explain and predict for a wide variety of reactions just how various
forces, concentration, heat, light, catalysts affect the rate of reactions.
All the chemical reactions discussed in this section must follow those thermo-
dynamic and kinetic laws. But we shall use them particularly in explaining the chem-
ical mechanism of oxidation.
Chemical Oxidation
How far in the direction of complete destruction the oxidation reaction will go (or
whether it will go at all) depends on the magnitude by which the free energy of the
reactants exceeds that of the products. There is nothing that can be done about the
free energy contribution of the organic materials, but that of the oxidizer reactant
can be increased by changing the oxidant.
The free energy level can be measured as the voltage of a galvanic cell in which the
reaction is or can theoretically be carried out. For an oxidant, it is expressed as an
oxidation-reduction potential at a specified standard state. (Pressure, temperature,
concentration). The higher the redox potential the higher the free energy contribu-
tion, and the more powerful the oxidant, i.e. oxidations that will not occur at all with
oxygen as the oxidant can be carried out if a more powerful oxidant is used.
20
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Since the end products, CO: and FhO, have the same free energies regardless of
how formed, this free energy contribution from a more powerful oxidant increases
the free energy change upon reaction, but that does not affect the rate. But if the
oxidant plus organic reactants have a higher free energy than the oxygen plus or-
ganic reactants, then the difference between the free energy of the reactants and that
of the activated complex is reduced, and so a reaction can occur which cannot occur
with oxygen.
The standard redox potential for dissolved oxygen is 1.23 volts. This measures the
free energy that enzymes have to work with. With more powerful oxidants, chemical
oxidations can be carried out even without enzymes or other catalysts. Chlorine,
also an oxidizing agent, has a redox potential of 1.36 volts and accordingly can effect
oxidations that even oxygen cannot bring about. (A small difference in voltage
corresponds to a very large difference in free energy.)
Some other oxidants with high redox potentials are permanganate (1.52 and
1.67), ozone (2.07), and fluorine (2.85). The first two are now used for organic
destruction in water. The last, fluorine, is the most powerful oxidizer known. A
number of modern compounds should have high potential as oxidizers, including
perchlorofluoride, FC104 nitrogen trifluoride, NFj, alkali ferrates, Na4FeO4. One
advantage of ozone is that, being composed solely of oxygen atoms, it leaves no
deleterious residue in the water.
It is not necessary that the oxidation be complete in order to be useful. I ncomplete
oxidation can produce organic compounds more amenable to other removal or de-
struction methods. For example, the alcohol groupings, C-OH, can be oxidized to
acid groupings, ClQ . The hydrogen in an alcohol molecule grouping is not ion-
izable, but that of acid is, leaving an anion of the organic molecule which might be
subject to removal by ion exchange.
Electrolytic Oxidation
Oxidizing reactions of interest in pollutant removal may occur at the anode where
Hie migrating ions surrender electrons. The anion oxidized, i.e. neutralized, may not
be the one that has migrated. The anion discharged, concentrations being compar-
able, will be that which has the lowest redox potential.
That often is the hydroxyl ion, OH~:
4 OH"- O2 + 2H2O + 4 electrons
which has a redox potential of 0.4 volts. (The discharge generates oxygen, formerly
called "nascent oxygen" from the concept that being new-born, it was especially
active.) Modern experimentation and thermodynamic and kinetic theory, however,
have shown that the electrode reactions generate very active groups called "free
radicals," which are chemical groupings with very high energy content. In the
hydroxyl case the free radical is OH', a hydroxyl ion that has lost its electron but yet is
not combined with anything else. These free radicals can react with each other, with
other ions or compounds, or other free radicals near the electrode including our or-
ganic pollutant molecules. Or they can generate other free radicals, oxygen itself for
instance, which having high energy content can react with organics that molecular
oxygen itself cannot attack.
The problem is to get the proper ion to be discharged so that the productive free
radicals are formed. Manipulation of this is possible through the phenomenon of
overvoltage—the difference due to polarization and other effects, between the theo-
retical redox potential and the actual voltage at which discharge at a given current
density occurs. The overvoltage varies with the nature of the anode material. On gold
anodes the oxygen overvoltage can be as high as 0.9 volts, so one has a range up to
1.3 volts in which to maneuver.
21
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Manipulation is also possible if the concentration of the competitive ion is ad-
justed. For instance, the redox potential of the hydroxyl ion, OH", the main com-
petitor, is only 0.4 volts at the standard state, a concentration of approximately 1
mole per liter. While the concentration of pollutants is to be very low, in the parts per
billion region, the concentration of the hydroxyl ion at pH6 is only about 2 parts per
billion.
However, electrolytic oxidation as detailed above is not confined to the organic
ions. The free radicals can oxidize equally well nonionized organic molecules in the
near electrode region.
In some oxidation-reduction systems which are slow to reach equilibrium at the
electrode, it has been found that the reaction rate may be increased by the addition of
a very small quantity of a more active redox compound. The mechanism by which
this is accomplished has yet to be explained. The material added is called a "potential
mediator."
Electrolytic oxidation, a near-electrode process, should not be confused with
chemical oxidation in which the oxidant is regenerated by electrolysis. That process,
which takes place in the body of the liquid, is simply a chemical oxidation process, as
already discussed. There have been a number of sewage electrolysis plants in the
United States, wherein the chlorides in the sewage are converted to chlorine at the
anode. The chlorine dispersed through the sewage acts as a chemical oxidizing agent
(and disinfectant), producing chloride and the process is repeated. Peroxides can be
similarly used.
Role of Radiation
Free radicals can also be generated in aqueous solutions by various kinds of
electromagnetic radiation, and chemical reactions can be induced either by the
action of the radiation directly, or by its effects on other chemical agents.
It is interesting to note that a reaction of the latter type, photosensitization, which
as photosynthesis, is responsible for the production of carbohydrates, the most
common organic compound produced. The formation of carbohydrates from CCh
and HaO involves a complex sequence of oxidation-reduction reactions, all enzyme
catalysed. The main step is catalysis by chlorophyll. The chlorophyll absorbs the
sunlight and transfers it to an unidentified organic compound which undergoes an
oxidation-reduction reaction with water. Note, however, that the main reaction in
this process is a reduction and an organic compound is synthesized rather than
broken down.
Sunlight can also break down organic compounds (i.e. without an oxidant), by
photolysis. Benzo(a)pyrene, for instance, has a half-life of less than a day in water ex-
posed to sunlight. The half-life of para-cresol in similiar conditions is several hun-
dred days. However, in the presence of humic acid this half-life is reduced more than
ten fold, indicating that humic acids are photosensitizers for para-cresol.
Chemical Precipitation
Chemical precipitation is a common method for separating inorganic compounds
from water. It has two forms. In one, a chemical is added that converts the compound
into a less soluble one that precipitates and is separated. In the other, the added
chemical does not produce a chemical change but merely reduces the solubility so
that the compound precipitates. This is called salting out. Both forms are also
possible for organic chemicals in general.
But while it is true that polluted waters containing organic paniculate matter
must also have some of this organic material in solution, the levels to which reduc-
tion is desired in toxic organics, pesticides, carcinogens, etc. are much below their
solubility levels. Accordingly, there is little chance of precipitating them by merely
reducing the solubility slightly. The only possibility is to change them into insoluble
22
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compounds by chemical reactions. Unfortunately, there is little chance of finding
any chemicals that would produce insoluble compounds from the pollutants of
importance, the solubilities of which are already in the /ug/L and ng/L range. It is pre-
sented here as a theoretical possibility.
Ion Exchange
Ion exchange is a form of chemical precipitation that actually accomplishes what
we have just stated to be unlikely. Although some would consider it a form of
adsorption, ion exchange is a true chemical reaction in which the pollutant ion dis-
places an ion on the solid ion exchange medium. Chemical bonds are broken and
formed, and the reaction can be written just as for any other chemical reaction, for
example:
_R - OH + P~ ~ R - P + OH",
and
R - H + P+ - R - P + H+
where:
R is the main portion of the ion exchange medium (R = resin)
-OH is a hydroxyl attached by ionic bonding to the resin
P" is a pollutant anion
-H is a hydrogen attached by ionic bonding to the resin
P* is a pollutant cation
OH" and H+ are hydroxyl and hydrogen ions, or other anions or
cations.
Note that the reaction is reversible. It therefore has an equilibrium position and is
subject to all the thermodynamic laws governing equilibria. But this is a hetero-
geneous equilibrium, involving two phases, for the resin is a solid. By this re-
action, the pollutant is fastened to the solid ion exchange resin, leaving the solution
depleted in pollutant. Then, the solution removed, the R-P is regenerated to the
hydroxyl or hydrogen form by contacting it with a small amount of alkali solution,
acid or other ion and the attached pollutant moves into the liquid and is disposed of.
One of the difficulties is that waters also contain inorganic anions and cations which
will compete with the organic ions, and, since they are present in higher concentra-
tions, will dominate that competition.
Ion exchange resins will also adsorb organic molecules, but this is a physical re-
moval mechanism although the processing steps are identical with ion exchange: ad-
sorption on the solid resin, removal of the water depleted of adsorbed material, and
regeneration of resin to its original form.
BEYOND THE STATE-OF-THE-ART
The history of the interaction between science and technology has been that
between the urge to know and the urge to do. For many centuries technology, the
urge to do, was the prime mover in invention and the development of processes,
much of which was done without an understanding of the fundamental theory. In the
18th century scientists began experimenting with and theorizing about natural
phenomena without, in general, much thought about, or motivation by, their
practical application. They or others took these fundamental discoveries and laws
and used them in the invention and development of practical processes (and
machines and techniques, but here we are concerned with processes). Thus science
stimulated technology.
In the late 19th century something of a reversal occurred. The needs of technol-
ogy and the zeal of inventors outdistanced the investigations of pure science and
there was not only a great expansion in applying scientific discoveries to practical
23
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needs, but the technological needs began to stimulate new scientific investigation
into fundamentals and theory. (There were, of course, technologists engaged in non-
scientific based invention and pure scientists who followed where curiosity led.) Thus
technology stimulated science.
When in the 1980s one reviews the subject matter and accomplishments of the
numerous specialized scientific disciplines involved in removing organics from
water, a surprising revelation emerges. To a large extent the work being done in each
discipline as applied to organics removal is not being done by the outstanding
specialists in that discipline. As a rough illustration, one can search the monographs
on colloid chemistry by the acknowledged masters of colloid chemistry and find no
mention of municipal wastewater. There is no mention in treatises on surface chem-
istry of the adsorption of water organics. Works on enzymology give little attention
to enzyme catalysed oxidation of sewage organics. Solid state physicists are busy
with transistors and are not intrigued by how loose electrons might be made to
oxidize organics.
The bulk of the current work in applying fundamental principles to organics re-
moval is being done by sanitary engineers, sanitary chemists, and others closely
allied with the technology, who have been forced by that urge to grasp at competence
in fields in which others are masters.
This does not mean to imply that creditable research is not done by such technol-
ogically oriented workers. Indeed, the best work in physical chemistry at present is
being done by the chemical engineers. And certainly some understanding of the
technological needs and possibilities is required to harness existing knowledge for
the invention of superior removal processes or to create new knowledge to that end.
For example, il one wanted to explore molecular orbital theory for possible removal
of organic substances in water, one would not begin by contacting a sanitary en-
gineer, as he would have to familiarize himself with molecular orbitals. Instead, a
specialist in molecular orbitals should be contacted to undertake the fundamental
research.
The original Office of Saline Water had the mission of finding ways to make
potable water out of salt water, a task exactly corresponding to that of current inter-
est, to free water from its organic impurities. A major undisputed accomplishment
was its successful program of basic research into the properties and characteristics of
water and its solutions. This was accomplished by seeking out the country's experts
in the various disciplines and paying them to think about water. Money is a powerful
stimulus to research.
It is not quite correct to say that this activity was undisputed. There are some who
look down upon basic and theoretical research as an anomolous activity from which
no forseeable and certainly no guaranteed result is forthcoming. Consider thatjust
in the area of ihermodynamics, there are certain individuals who believe unimagin-
able pipe dreams, such as frictionless pistons, quantities for which only the changes
and not the actual amounts can be measured, processes that operate without any
driving force and so slowly that nothing ever goes on at all, waves in a substance that
does not exist, or barriers that have convenient tunnels through them. It is no
wonder that in the non-scientific world and even to some extent in the engineering
world, much that is termed theoretical or that whic ' penmental in nature is dis-
missed as having no bearing on the real world. What we aid their reaction be if they
knew that one of the world's greatest thermodynamicists, Joshia Willard Gibbs,
never carried out an experiment in his entire adult career. He was a professor of
mathematical physics, (as was Einstein). Yet students of physical chemistry can
hardly avoid running into one or more of Gibbs' many deductions or equations,
upon which whole industries are based.
A few decades ago the science of biology underwent a revolution. A group of
biologists turned away from studying biology on a macro scale. They turned instead
to biology on a micro scale -- how the cells worked, what the molecules were doing,
how they responded to external challenge, how the atoms were put together in bio-
24
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logical materials. They gave it a very descriptive name, molecular biology. The sub-
sequent accomplishments of molecular biology are among the triumphs of science
and technology in the second half of this century, now verging upon the actual chem-
ical synthesis of life materials including the basic material of all life, DNA.
There is a distinguished professor who from his earliest days has had an unusual
attitude toward the microorganisms he studies and uses. He considers them his
friends, even his relatives. Anthropomorphic is the word for it. He calls them "bugs,"
and when he is thinking and talking about them he refers to an individual bug. He
says: "the bug does this,"or even more: "HE does this,"and "HE doesn't like it there
and wants to go here." In other words he has an intimacy with the bugs he studies.
With regard to molecules, atoms, and electrons, this is the sort of intellectual
intimacy that must be the habit of researchers who will next advance the technology
of removing organics from water.
My assignment was to review the state-of-the-art as to fundamentals and not to
project and certainly not to command the future. I find it necessary to say, however,
that I believe that achievement in the future in the removal of organics from water
and in water purification generally will come through directing the highest level of
competence in a number of specialized fields, few of which have previously been con-
cerned with water and wastewater, to an examination of the fundamental behavior
of the molecules, atoms and forces involved in water cleansing. The following are
some of the fields and disciplines that seem promising:
Thermodynamics of the molecules and reactions
Thermodynamic data on organics
Chemistry of surfaces, especially small size surfaces
Colloid chemistry
Solid state physics, of catalysts and of organics
The nature of chemical bonding
Enzyme catalysis and enzyme synthesis and tailoring
Electrokinetics, and electrophoresis
Exotic high energy oxidants
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COAGULATION-SEDIMENTATION-FILTRATION
PROCESSES FOR REMOVING ORGANIC
SUBSTANCES FROM DRINKING WATER
James K. Edzwald
INTRODUCTION
"Among the principal failings of chlorination were odors and tastes produced by
chlorine residuals, by plankton killed by chlorine, and by compounds formed with,
or accentuated in odor or taste by, chlorine. Of special concern were the medicinal or
iodoform-like tastes caused by traces (/ag/ L) of phenolic substances through the for-
mation of chlorophenols." The above passage is from a paper1 by Gordon M. Fair
entitled "Fifty Years of Progress in Water Purification, 1913-1963." Formation of
chlorinated organics from chlorinating water supplies containing certain organics
was recognized as a problem in the early 1960's as indicated by Fair. In the past our
analytical techniques were crude so that we measured the effects by tastes and odors,
or we were able to screen water supplies for organic content only by using a gross,
non-specific test—the carbon chloroform extract. In addition to tastes and odors our
concerns regarding organics have intensified over the years. This is illustrated in
Table 1. With the development of gas chromatographic techniques for measurement
of specific organics in drinking water during the 1970's the problem of trihalometh-
anes was clearly identified.
Trihalomethanes (THMs) are produced within the water treatment system as by-
products of chlorination. A wide variety of organics, including dissolved, colloidal,
and particulate forms, act as THM precursors. This chapter's goal is to describe the
present state of knowledge regarding the removal of trihalomethane precursors from
drinking water using the conventional treatment processes of coagulation, sedimen-
tation, and filtration. Emphasis is placed on the removal of naturally occurring or-
ganic substances, the so-called humic substances, since they constitute the major
fraction of THM precursors and their removal by the above processes is feasible.
The removal of synthetic organic chemicals including pesticides derived from indus-
trial sources or runoff is not considered here since these pollutants are removed more
effectively by other processes such as adsorption, which is covered in another
chapter.
The A uthor. Dr. Edzwald is Professor of Civil and Environmental Engineering, Clarkson Col-
lege of Technology, Potsdam, New York He was graduated with a Ph. D. in environmental en-
gineering in 1972 from the University of North Carolina - Chapel Hill. Dr. Edzwald also holds
B.S. and M.S. degrees from the University of Maryland in civil engineering and sanitary engi-
neering, respectively. Dr. Edzwald has 18 years professional experience in water supply and
treatment. He is a registered professional engineer and has served as a consulant to private in-
dustry, consulting engineering firms and EPA. He has held sanitary engineering positions with
the Water Supply Agency for Washington, D.C. (Washington Aqueduct Division, U.S. Army
Corps of Engineers), and the Federal Water Pollution Control Administration (predecessor
agency of EPA) and has taught at the University of Missouri - Columbia. Dr. Edzwald is Vice
Chairman of the Research Committee on Coagulation, American Water Works Association.
26
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Table 1. Drinking Water Standards for Turbidity and Organics
Dates Turbidity
1 962-1 976 (USPHS) 5 units
1977 (EPA) 1 unit
Color Organics
15 units Alkyl benzene
sulfonate
Carbon chloroform
extract
15 units* Endrin
Linda ne
Methoxychlor
Toxaphene
2,4-D
2,4,5-TP
0.5 mg/L
220 ug/L
0.2 fjg/L
4/^g/L
100/ug/L
5fjg/L
100A
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Pathogenic Organisms = Disinfection
NH3 = Formation of Chloramines
H2S, Fe+2, Mn+2 = Oxidation of Reduced
Inorganic Chemicals
Organics = Trihalomethanes and Other
Chlorinated Organics
TRIHALOMETHANES
CHCI3 Chloroform, CHBrCI2 Bromodichloromethane
CHBr2CI Dibromochloromethane, CHBr3 Bromoform
Figure 1. Possible parallel competitive reactions occurring when chlorine is added to
water
1908. As aptly stated in a report on "Drinking Water and Health"from the National
Academy of Sciences3, "The introduction of chlorination after 1908 provided a
cheap, reproducible method of ensuring the bacteriological quality of water. Chlo-
rination has come down to us today as one of the major factors ensuring safety of our
drinking water."
In Fig. I we see that chlorine in the presence of ammonia results in the formation
of chloramines (combined chlorine). Chloramines can also provide disinfection,
although less efficient than free chlorine (HOCl and OCl), and have been used in the
United States for protection of distribution systems. Chlorine is also used to oxidize
reduced inorganic materials (Fef , Mn+", H?S) whose presence is often troublesome.
The last reaction is undesirable and was not recognized until the mid-1970's, al-
though THMs have obviously been produced in water treatment plants as long as
chlorination has been practiced. The detection of these compounds had to await the
development and application of gas chromatographic techniques to drinking water.
The major forms of the trihalomethanes are indicated at the bottom of Fig. I.
The presence of THMs in drinking water was first reported by Rook . In studies
of the Berenplaat plant in the Netherlands, Rook showed that THMs were being
produced during water treatment. He attributed the THM formation to the chlorin-
ation of waters containing humic substances which serve as TH M precursors. About
this time Bellar, et al.5 reported that chloroform and other trihalogenated methanes
were being produced as a result of the chlorination process during water treatment.
In 1975 the U.S. Environmental Protection Agency (EPA) published the results of
28
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its study known as the National Organics Reconnaissance Survey (NORS)6. This 80-
city survey showed the widespread occurrence of THMs in chlorinated drinking
waters in the United States, and demonstrated that they result from chlorination. In
general, total THM concentrations were related to the organic content of the water.
It was also observed that higher concentrations of THMs were found when surface
water was used as the water supply source, pre-chlorination was practiced, and more
than 0.4 mg/L free residual chlorine was being used. The National Organics Moni-
toring Survey (NOMS) expanded upon the NORS study - 113 cities throughout the
United States - and confirmed the occurrence of THMs in finished drinking water
due to chlorination7'8.
On February 9, 1978 EPA published7 "proposed regulations" for the control of or-
ganic chemical contaminants in drinking water. These proposed regulations con-
tained two parts. The first part set a Maximum Contaminant Level (MCL) of 100
jug/L for total trihalomethanes (TTHMs) in finished drinking water, and was pro-
posed as being initially applicable to community water systems serving populations
of greater than 75,000. The second part proposed granular activated carbon treat-
ment for control of synthetic organic chemicals derived from industrial and
agricultural sources, and urban runoff. After public hearings and comments, EPA
issued a final rule8 for control of THMs on November 29, 1979. EPA maintained the
MCL of 100 jug/L for total trihalomethanes, but extended the coverage to include
communities with populations less than 75,000 as indicated in Table 1. EPA also
separated the granular activated carbon requirement from the THM regulations,
and stated that regulations for the control of synthetic organic chemicals would be
reproposed in the future.
Protection of the public health is a primary concern in setting drinking water regu-
lations. Chloroform has been shown to be an animal carcinogen, and is suspected of
being a human carcinogen3'8. Several factors were listed by EPA in setting a MCL
for trihalomethanes. In summary they are 8: "the potential human health risks of
chloroform and other THMs; the fact that drinking water is the major source of hu-
man exposure to THMs; the fact that THMs are the most ubiquitous synthetic or-
ganic chemicals found in drinking water in the United States and are generally found
at the highest concentrations of any such chemicals; the fact that THMs are intro-
duced in the course of water treatment as by-products of the chlorination process
and thus are readily controllable; that low cost and feasible means have been gen-
erally available since 1974 to reduce their concentrations in drinking water; that
monitoring is feasible; and that the THMs are also indicative of the presence of a
host of other halogenated and oxidized, potentially harmful by-products of the chlo-
rination process that are concurrently formed in even larger quantities but which
cannot be readily characterized chemically."
TRIHALOMETHANE FORMATION
Trihalomethane Measurements
THMs result from a series of complex reactions between chlorine and precursors.
The precursors are largely humic macromolecules. Reaction pathways are generally
unknown9. For the purpose of this discussion the THM formation reaction is ex-
amined through Equation (1).
Organics + Free Chlorine + (Br~ or F) = THMs + Other Halogenated
(Precursors) Organics (1)
It is important to define three THM measurements10. All three measurements are
useful; however, the selection of these measurements depends upon the intended use
of the data.
29
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Instantaneous THMs—
Instantaneous THMs (Inst THMs) is the concentration of THMs in a water
sample at the time of sampling. At the time of sampling, any free chlorine present is
quenched by adding a reducing chemical such as sodium thiosulfate or sodium sul-
fite. This halts the formation of THMs. The Inst THM measurement is applicable to
the 100 Mg/L TTHM MCL established for municipal water supply systems. Four
samples are collected from the distribution system on a quarterly basis. One sample
each quarter must be collected from the far end of the distribution system. Compli-
ance is based on a running annual average of the quarterly samples. Fig. 2 illustrates
Inst THM development in a treatment plant and in a distribution system. Note the
higher Inst THM concentration in the distribution system compared to the treat-
ment plant. Variables which affect TH M formation in the distribution system - e.g.,
hydraulic residence time - are discussed in a subsequent section.
Terminal THMs—
Terminal THMs (Term THMs) is the concentration of THMs after storing an un-
quenched sample for a specified reaction period. As long as free chlorine is present
the THM reaction will continue, and additional THMs will be formed. Depending
on the precursor concentration this reaction may continue for periods of 7 to 10 days.
The selection of the storage period depends on the intended use of the results. For ex-
ample, if one is interested in the THMs which might be formed in the distribution
system, then a storage period equivalent to the hydraulic residence time of the dis-
tribution system should be selected. Temperature affects the THM formation re-
action, so storage of samples under controlled temperature conditions is important.
One choice is to store the sample at a temperature similar to that of the water temper-
ature in the distribution system. At the end of the storage period an aliquot of the
sample should be used for measurement of free chlorine - the remainder of the
sample is quenched and then used for THM analysis. An absence of free chlorine
would indicate the THM reaction stopped, and therefore the THM concentration
measured may not indicate the THM levels that could develop in the distribution
system. The difference between the Term THM level in the treatment plant (e.g.,
water sample from the sedimentation tank) and the Term THM for water entering
the distribution system as shown in Fig. 2 would indicate the reduction in THM
precursors accomplished by treatment processes following sedimentation - e.g.,
filtration.
Trihalomethane Formation Potential—
Trihalomethane Formation Potential (THMFP) is the increase in the concentra-
tion of THMs which occurs during the storage period for determination of Term
TH Ms - i.e., the difference between the Term THM concentration and the Inst THM
concentration. It is a measure of the precursor level remaining in a sample at a point
in the treatment plant or in the distribution system. For raw waters, prior to chlorin-
ation, one can determine its TH M FP by chlorinating a sample in the laboratory with
sufficient chlorine to maintain a free chlorine residual throughout the storage
period. The sample is stored under controlled conditions of temperature and pH.
THMFPs are illustrated in Fig. 2.
TH M formation curves for raw water samples collected from the Grasse River are
shown in Fig. 3. The Grasse River is a colored, low turbidity water supply in North-
ern New York and is typical of many surface supplies in that region as well as New
England and other areas of the United States. The two curves illustrate differences in
precursor levels for different times of the year. The lower THM formation curve is
for water collected in February when the river was ice-covered while the other curve
is for a sample collected in April during the spring runoff period. As illustrated the
THMFP is time dependent and, of course, dependent on the precursor
concentration. A higher THMFP resulted for the spring runoff sample which con-
tained a nonvolatile total organic carbon (NVTOC) concentration of 6.3 mg/L.
30
-------
c
o
to
g
o
c
o
u
c
to
x:
s
5
1
H-
Inst
•wo^vi
T
HMI
!s
I
1
|
(0
.0
LJ_
5
i-
:P
••
£
c
_o
o
0>
a
E
to
(/>
i
V7/A
*fr (Cone, at time of sampling)
wm
THMFP + Inst = Term THM
nit/** (Sample stored
Y/yft at desired
conditions)
d.
P
"D
(D
•£
9)
'*=
0)
^
2
0)
X
Q.
0.
I
I
t
Term
THM
"T"
Inst
^ i
a.
u.
2
1
-
i
Term
THM
t
Inst
I ,
f
Source
Treatment
t
\
Distribution
System
> CI2
CI2
Figure 2. Trihalomethane terminology.
Before discussing the factors which influence THM formation it is instructive to
make an analogy with the widely used BOD (biochemical oxygen demand) test. The
BOD test is used to measure the oxygen demand of water and wastewater samples
due to the presence of biodegradable organics.
Organ ics + 02
CO2+ H20+ NH3
(2)
We do not measure the organic matter (oxygen demanding precursors) directly, but
rather we measure the oxygen consumed as a result of the biochemical oxidation of
the organic matter by microorganisms. In essence the BOD test is a bioassay, which
is time dependent and requires standard conditions. The test is influenced by a num-
ber of factors including: type of organic substrate, organic concentration, tempera-
ture, pH, and presence of trace elements. Dilution water, saturated with oxygen and
containing trace elements and a phosphate buffer, is seeded with a bacterial culture.
Standard conditions of a 5-day incubation period at 20°C are used.
31
-------
600
500
400
5
jf 300
I
o
200
100
TOC (mg/l) 6.3
UV(254, cm-') 0.243
Color (Pt-Co) 97
Feb.
TOC (mg/l) 2 75
UV (254, cm-') 0.113
Color (Pt-Co) 67
40
80 120
Reaction Time, hours
160
200
Figure 3. Chloroform formation curves for the Grasse River (Canton, NY) Samples
collected in February and April, chlormation conditions CI2 dose 15-20
mg/L, pH 7 5, 20°C
The TH M FP determination may also be thought of as a measure of organic con-
tamination. It is an indirect measurement of THM precursors since we are mea-
suring the effect of the precursor material. In order to determine the formation po-
tential of a raw water sample, chlorine is added to the sample in the laboratory.
While conditions covering storage (reaction) period, temperature, and pH have not
been standardized, they influence THM formation and therefore in reporting results
these conditions should be specified. The formation curves in Fig. 3 were developed
at pH 7.5 and at 20°C with free chlorine available throughout the reaction period.
The 7-day chloroform formation potentials (CHChFPs) of the April and February
samples were 532 ^g/ L and 237 /ug/L, respectively. THM species other than CHCh
were insignificant. Note that the THM reaction for both samples was still proceeding
after seven days.
32
-------
THMFP measurements are useful for two reasons. First, they provide a measure
of the precursor level in raw waters as previously discussed. Second, they provide a
measure of the efficiency of treatment processes in removing THM precursors as
shown schematically in Fig. 2. This is particularly useful in pilot plant studies or
treatment plant evaluation.
Factors Affecting THM Formation
Factors influencing THM formation have been discussed in several publica-
tions10 ~'4. These factors include nature and concentration of precursors, pH, temper-
ature, chlorine dose, and concentration of bromide ion. The reader is referred to the
publications for detailed discussion of these factors; here, some summary comments
are presented.
A wide variety of organic substrates can act as THM precursors. Humic sub-
stances are considered a major source of precursor material. However, "humic sub-
stances" is a broad classification as will be noted in a subsequent discussion. Morris
and Baum9 have presented data showing that naturally occurring soluble organics
other than humics are also THM precursors. These organics included pyrrolic-type
compounds (found in chlorophyll) and acetogenins (natural pigments). Rook15 has
investigated the chlorination of natural glycosides including hesperidin, heperetin,
rutin, and phlorizin. He used these compounds because they contain hydroxylated
aromatic rings with two free meta-positioned OH-groups which he proposes as the
active sites for trihalomethane formation. Of interest to water utilities obtaining
their drinking water from reservoirs is that algal cells and algal cell by-products are
also THM precursors 16~18.
A convenient way to examine THM formation data is by expressing it in terms of
CHCh yields defined here as /jg CHCh produced per mg total organic carbon (TOC)
for extended chlorine reaction periods. The data in Table 2 are limited, but do show
CHCh yields for humic acid, algal biomass and extracellular products, and two
natural waters. It is interesting to note that the CHCh yields for the Grasse River are
approximately equal even though the THM precursor level in the river during spring
runoff (April sample) was higher than that in the winter (February sample), as pre-
viously shown in Fig. 3. There can be a large variation in yields due to differences in
the nature of the THM precursors, pH conditions, temperature, and chlorine contact
period.
Seasonal variation in temperature will influence THM formation. In general,
greater THM formation occurs with increasing temperature. Stevens and Symons10
have reported that approximately twice as much CHCh was produced from Ohio
River water at 25° C as at 3° C. Using these data Kavanaugh et al. '4 have determined
third-order kinetic rate constants for THM formation in which the rate constant
doubles for every 10 degree increase in temperature between 0° and 30°C.
In general, an increase in THM formation is found at higher pH. This is due to the
final hydrolysis reaction which requires alkaline conditions. Morris and Baum9 have
presented data which show that TH M formation can continue after the free chlorine
has been quenched if the pH is raised. This suggests that if chlorination is practiced
at pH 6 to 7, additional THM formation can occur when the pH of the water is raised
even if no free residual chlorine is present. This indicates the presence of chlorinated
intermediates. The effect of pH on the CHCh yield of humic acid is shown in Table 2.
The data of Stevens et al." show twice the yield at pH 9.2 as at pH 6.7. Clearly, pH is
an important parameter in evaluating THM data for distribution systems, treatment
plants, and pilot plants and should be measured and reported.
Bromide and iodide ions are oxidized by aqueous chlorine to species capable of
organic substitution reactions resulting in formation of brominated (or iodinated
forms) forms of the trihalomethanes . For fresh waters, chloroform is the dominant
THM produced. In the presence of Br~ which can occur in water supplies subject to
salt water intrusion, there is a shift to the brominated forms12'11'" .
33
-------
Table 2. TOC and Chloroform Yield Data
Water
Humic acid
Humic acid
Humic acid
Humic acid
Grasse River
Grasse River
Stonelick Lake
Algal biomass
Algal extracel-
lular products
TOC
mg/L
231
385
~0.6
-06
275
63
7.27
2 1-75.8
-
pH
8.1
81
6.7
9.2
75
7.5
8.0
7 5,9.3
7 5,93
CI2Contact
Period
hrs
135
135
96
96
191
168
141
Not specified
Not specified
Yield
fjg CHCI3/mg/
TOC Comments
128
128
70
140
873
84.4
83.0
199-398.4
3.98-498
Humic acid extracted
from Mich, peat
(Edzwald19)
Commercial humic acid
(Stevens et al ")
See Figure 3
Northeast of Cine ,
Ohio (Edzwald19)
Four algal species
(Hoehn et al.17)
-------
COAGULATION, SEDIMENTATION, AND FILTRATION IN WATER
TREATMENT PRACTICE: PRINCIPLES
Introduction
The coagulation process involves both the chemical destabilization of particulates
and the physical transport of destabilized particles resulting in particle collisions to
form larger aggregates called floe. The latter step - particle transport - is the rate
limiting one, and is referred to as flocculation.
Particulates in water supplies are normally removed through a series of processes
consisting of coagulation-flocculation, sedimentation, and filtration. This is illus-
trated in Fig. 4 for a conventional water treatment plant. An objective is to remove
most solids by sedimentation leaving the filtration step as a polishing process. In the
conventional water plant one is interested in using chemical coagulants in such a way
that raw water particulates are flocculated into aggregates large enough for settling.
Particulates can also be separated without sedimentation in which all of the solids
are removed in the filters-direct filtration. A schematic of direct filtration is shown
in Fig. 5. Direct filtration may include a flocculation tank or one can bypass this pro-
cess and go directly to the filters (in-line filtration). In either case it is necessary to
add chemical coagulants before filtration to destabilize the particles in the raw water.
A listing of each process used in conve ntional treatment and direct filtration plants is
presented in Table 3 with appropriate design and operating criteria.
In this section general principles regarding the nature of particles in water supplies
and water treatment practice are presented. These depict the state of our knowledge
and emphasize developments in the last 10 to 20 years.
Particles
The type, size, and concentration of particles affect their behavior in water treat-
ment systems. Raw water supplies may contain inorganic particulates such as
asbestos fibers, clays, and silts. Within the treatment plant, coagulant precipitates
(e.g., Al (OH)3 (s)) may be produced, adding to the particle mass and concentration.
Raw waters may also contain organic particulates such as humic substances, viruses,
bacteria, and plankton. These particles cover a broad range of sizes from the nano-
meter end for humic substances and viruses to tens of micrometers for plankton and
coagulant precipitates. It is significant that the bulk of these particles are classified as
colloids. Although colloids are generally considered as ranging in size from 1 nmto 1
/j.m, no exact boundaries exist. Colloidal systems may contain small particles and
large molecules (e.g., humic substances). Colloidal particles are distinguished by
their large surface areas per unit volume and by their light scattering properties.
Colloidal systems scatter light (Tyndall effect) whereas true solutions scatter very
little light. Turbidity, which is used in controlling the operation of water treatment
plants, involves measuring the light scattering properties of a water sample.
A few considerations regarding turbidity measurements follow. The amount of
light scattered (turbidity) depends on the number, size, shape, and refractive index of
the particles as well as the wavelength of the incident light, and the geometry and
detection characteristics of the turbidimeter. In water treatment, turbidimeters have
been standardized only to the extent that the light scattered at an angle of 90° to an
incandescent lamp source is measured. Even this is a recent development as prior to
the mid-1970's a variety of instruments measuring scattered and transmitted light were
used, including the Jackson Candle Turbidimeter.
It is important to emphasize a few points about the information turbidity measure-
ments do or do not provide. Much of the material presented here was taken from
Drinking Water and Health3, O'Melia25,and Friedlander26. Turbidity measurements
are extremely useful as an indication of water quality and as an operating control
35
-------
Chemicals
Sludge
Backwash
Water
Raw
Water"
Rapid
Miv
Tank
_^ Slow Mixing
*"" (Flocculator)
Coagulation Process
Chemicals
Clarifier
Che
(Op
Soli
Sep
T
mic
tion
J-Li
ara
Filter
a Is
al)
quid
tion
Treated
Water
Figure 4. Conventional water treatment plant
Chemicals
Raw
Water"
Chemicals
(Optional)
1 '
Rapid
Mix Tar
In-Line
r~ *"
i
1
ik *
Slow Mixing
(Flocculator)
Coagulation Process
1
1 Ch
Backwash
Water
Filter
Solid-Liquid
Separation
Chemicals
Treated
- Water
Figure 5. Direct filtration.
parameter in water treatment plants. The measurements can be made easily and
rapidly, and the instrumentation is relatively inexpensive. Turbidity measurements do
not give complete information on the size, number, mass, or type of particles. Small
particles (e.g., maximum dimension less than about 0.1 ^m) do not scatter much vis-
ible light. Therefore a water containing asbestos fibers, viruses, or humic substances
may have a large particle number concentration, but a low turbidity. Larger
particles such as clays or plankton which have particle diameters approximately the
length of visible light scatter light more effectively and thus would yield higher
turbidities. This is illustrated by the examples below and use of Figs. 6 and 7.
36
-------
Table 3. Design and Operating Parameters: Conventional Treatment and
Direct Filtration (From Refs. 22-24)
Process/Parameters Conventional Direct Filtration
Rapid mix tank
Detention time, sec 30-300 Optional*
Velocity gradient, sec ~1 700-1000 (pilot plant studies)
Flocculation tank
Detention time, min 30-60 Optional*
Velocity gradient, sec "1 10-100 (pilot plant studies)
GT 10"-105
Sedimentation tank
Detention time, hr 4 Not applicable
Overflow rate,
m3/m2-day 37-49
Filtration
Sand filters Not applicable
Filtration rate, m/hr 4.9
Effective size, mm 0.45-0.55
Media depth, m 0.6-0.76
Dual media filters
Filtration rate, m/hr 4.9-19.5 4.9-19.5
Effective size, mm
Anthracite 0.9-1.2 0.9-1.2
Sand 0.45-0.55 0.45-0.55
Media depth
Anthracite, m 0.46-0.61 0.38-0.90
Sand, m 0.38-0.46 0.20-0.30
'Mixing including flocculation: detention times of less than 1 min. to 40 min.
Case 1: Raw water containing 5 mg/L humic material
Assume: particle density (p) of 1.01 g/cm3 and
particle diameter of 0.021 fj,m.
Particle number: 1012 particles/cm3
This water would scatter very little light so that its turbidity would be approximately
1 NTU as shown in Fig. 7.
Case 2: Raw water containing 5 mg/L clay
Assume: p of 2.5 g/cm3 and parti
Particle number: 106 particles/cm3
Assume: p of 2.5 g/cm3 and particle diameter of 1.6 /urn.
This water would contain a particle number concentration six orders of magnitude
less than that of Case 1, but its turbidity would be higher than Case 1 - e.g., 5 mg/L
kaolin clay would have turbidity of 7 to 8 NTU.
37
-------
10~3
10~2
10-'
10
102
1000
500
10~3
10-' 1
Particle Diameter, fj m
10
Figure 6. Isoparticle number concentrations (N) in particles per cm3 (Solid lines, particle density 2 5 g/cm3, dashed
lines, particle density 1.01 g/cm3. Cases refer to examples in text.}.
-------
Humic Acid
Kaolinite
I
200
160
120
80
40
20 40 60 80
Concentration, mg/l
100
Figure 7. Turbidity vs. solids concentration for humic acid and kaolimte.
Coagulation-Flocculation
As mentioned earlier coagulation in water treatment involves two separate and
distinct steps. First, stable particles are rendered unstable or "sticky" by the addition
of chemicals (coagulants). Second, collisions among particles are induced resulting
in the formation of particle aggregates or floe. A schematic flow diagram of the co-
agulation process in a conventional water treatment plant is shown in Fig. 4. The first
step is fast, occurs in the rapid mix tank, and although a simplification, is referred to
as a chemical step. The second step involves physical transport of particles and is the
rate limiting step. This step is carried out in flocculation tanks with gentle mixing for
periods of 30 to 60 minutes.
Particle Destabilizatwn—
Theories of colloid stability are described in a number of references27"29.
Applications to the water treatment field were made by a number of investigators in-
cluding: A.P. Black, C.R. O'Melia, J.J. Morgan, W. Stumm, and their students.
Particle destabilization may be accomplished by four mechanisms: (1) compression
of the electrical double layer by addition of electrolyte, (2) adsorption/charge neu-
tralization, (3) interparticle bridging, and (4) enmeshment of colloids in a precipitate
of a metal hydroxide. Compression of the electrical double layer occurs naturally in
saline waters (estuaries and oceans)30 but the salt concentration required for effective
destabilization is too high for water treatment applications. Aluminum (III) and
iron (III) salts can accomplish particle destabilization by both charge neutralization
and enmeshment of colloids in a precipitate ("sweep floe"). Polyelectrolytes, de-
39
-------
pending on their charge and molecular weight, can accomplish destabilization by
charge neutralization or by interparticle bridging. Specific coagulants are discussed
later in greater detail.
Panicle Transport —
Collisions between particles can occur by. (1) Brownian diffusion (perikinetic
flocculation), (2) bulk fluid motion (orthokinetic flocculation), and (3) differential
settling in which a particle that is settling rapidly overtakes and collides with a
particle settling at a slower rate. The aggregation of particles depends upon the rate
of collisions between particles (particle transport), and whether the particles "stick"
together (particle destabilization) upon colliding. Theories of particle transport for
coagulation were presented by Smoluchowski31 in 1917. Application of Smoluchowski's
theories to design of flocculation basins was presented by Camp12'" and later by
TT J ^ 4 l*i
Hudson • .
It is instructive to present some simple equations for flocculation and discuss the
influence of particle size on particle collisions. The concepts have been developed by
O'Melia2*'29 and Friedlander26. For a suspension of particles of uniform size (mono-
dispersed or homogeneous), the rate of change in the total concentration of particles
for perikinetic flocculation (Brownian diffusion) is:
d.N _ _4_ kTN2 (3)
dt 3 M
in which N is the total number concentration of particles at time t, k is Boltzmann's
constant, T is the absolute temperature, and /j. is the fluid viscosity. For a homo-
geneous suspension subjected to agitation causing velocity gradients (orthokinetic
flocculation), the equation is:
dN 2
dt ~ 3
(4)
in which G is a mean velocity gradient (time ') and d is the diameter of the particles.
Both equations assume destabilization is complete; that is, the particles are sticky
so that each collision is successful. For monodispersed suspensions flocculation due
to differential settling cannot take place. The ratio of the rate of orthokinetic floc-
culation to perikinetic flocculation is:
Ortho. Rate 1/2 G d3 n (5)
Pen. Rate ~ kT
For water at 25°C, l-/jm diameter particles, and a velocity gradient of 10 sec this
ratio is unity. In other words particle transport by Brownian diffusion equals that
caused by gentle stirring. It can also be shown that particle transport efficiency is at a
minimum for 1-jum diameter particles. For particle diameters less than 1/xm,
Brownian diffusion becomes predominant and orthokinetic flocculation becomes
negligible even for high values of G. For particles greater than 1/um, particle trans-
port by stirring becomes predominant.
Equations for heterogeneous suspensions are cumbersome. O'Melia* has dis-
cussed the effect of particle sizes on flocculation kinetics for collisions between
particles of two different sizes. The following conditions were considered: temper-
ature of 20° C, particle specific gravity of 1.02, and velocity gradient of 10 sec"1. Col-
lision rates were calculated for particles with diameter of 10/jm undergoing
collisions with particles varying in diameter from 0.01 urn to 1000 /urn. For this heter-
ogeneous system a minimum in contact efficiency is observed, but the minimum ef-
ficiency occurs for a broad particle size range - approximately, 0.1/^m to 1 /urn.
40
-------
Brownian diffusion is predominant only for particle diameters less than 0.01 /j.m.
Collisions by shear (stirring) predominate over a wide range - particle diameters of
0.01 nm to 100 pm. Differential settling becomes significant as particle diameter
approaches 100 /j.m. O'Melia25 has pointed out that flocculation tanks can be effec-
tive if the suspension is heterogeneous and contains larger particles. In water treat-
ment practice, operators produce heterogeneous suspensions by adding particles
such as clays or by practicing "sweep floe" coagulation.
Chemical Coagulants—
Salts of aluminum (Al(III)) and iron (Fe(III)) are widely used as coagulants. How
they function as coagulants requires an understanding of the chemistry of the hydro-
lysis of these metal ions. Alum is used here as an example, although iron behaves in
an analogous fashion. Much of our knowledge regarding the chemistry of alum with
respect to coagulation was developed in the 1960's. Alum (A12(SO4)3-14H2O) dis-
solves readily in water releasing SOt ions to solution. The Al'+ (actually a shorthand
since it exists as A1(H2O)6+) undergoes hydrolysis reactions, some of which follow:
Al3* + H2O = A1OH2+ + H+ (6)
A13+ + 2H2O = Al (OH)* + 2H+ (?)
A13+ + 3H2O = Al(OH)3(s) + 3H+ (8)
A13+ + 4H20 = Al(OH)j + 4H+ (9)
8A13+ + 20H20 = A18 (OH)4/0 + 20H* (10)
Protons are liberated making alum an acid. The aluminum species formed depend
on the pH of the water and the alum dosage. Fig. 8 shows the solubility of Al(OH)3(s)
in water with the distribution of major dissolved species as a function of pH.
Due to its chemistry, alum can act as a coagulant in two ways. In many applica-
tions, sufficient alum is added so that Al (OH)3 (s) is precipitated. The shaded area in
Fig. 8 illustrates conditions of pH and alum dosage where this is favored. Colloids in
the raw water become enmeshed in the Al(OH)3(s) precipitate. The gelatinous pre-
cipitate that is formed is sticky enough that aggregation of floe is achieved. The
precipitation of aluminum hydroxide generates additional particles which improves
the kinetics of particle aggregation. This is important in the coagulation of low tur-
bidity waters, since additional particles are needed for particle aggregation given the
flocculation period typically found in water treatment plants. This method of co-
agulation is often termed "sweep floe."
Alum can also accomplish coagulation by the adsorption of positively charged
monomers (e.g., A1OH+ ) and polymers (e.g., A18 (OH)2o ) on negatively charged
colloids. This adsorption is strong and can result in particle destabilization by charge
neutralization. The availability of these positively charged aluminum species de-
pends on pH and the alum dosage. Turning our attention to flocculation kinetics,
particle destabilization by charge neutralization would have application in treating
raw waters containing high concentrations of particles. For low turbidity waters,
particle destabilization can also be achieved; however, floe formation would be lim-
ited due to insufficient numbers of particles to produce a high rate of collisions
among particles. It is noted that polymeric forms of aluminum exist as intermediates
in the precipitation process, and that they play a role in coagulation even when
"sweep floe" is practiced, particularly at slightly acid pH conditions.
The use of synthetic organic polymers to improve solids-liquid separation has
been one of the most significant developments in water treatment technology in the
last 25 years. Increasing use of polymers in water treatment has taken place during
41
-------
-2
-4
o
J -6
8
-8
-10
-12
100 mg/l Al, (S04)3.14H,0
10
12
pH
Figure 8. Solubility of AI(OH)3(s) in water (Shaded area is the concentration-pH
range often used for alum in water treatment).
the last 10 years. Many commercial polymers have been approved by EPA for use in
potable water treatment36.
A polymer is a chemical structure characterized by a series of repeating chemical
units (monomer units) held together by covalent bonds. A summary of polymer ex-
amples is listed in Table 4. Cationic (positive charged) polymers are used as primary
coagulants and coagulant aids. They are also used as filter aids and in sludge de-
watering as chemical conditioners. Most cationic polymers being produced for treat-
ment applications have molecular weights less than 105. These polymers tend to ac-
complish particle destabilization by adsorbing on negatively charged colloids thus
producing charge neutralization at the proper dosage. At low turbidities particle de-
stabilization can be achieved, but the kinetics of flocculation would be slow due to
poor particle contact opportunities.
42
-------
Table 4. Examples of Polymers
Charge
Cationic
Nonionic
Structural Type/Functional Group Molecular Weight
Polyethylenimme[-CH2-CH2-N+H2-]n 600 to 50,000-100,000
Polydiallyldi methyl -
ammonium chloride
("Cat-Floe")
" CH, ""
i_ i 1 2 .
~~OH (jH— OHo—
II
CH2 /;H2
"-A
CH3 CH3
Variety of Cat-Floe
types available,
M.W. 10" to 105
Polyacrylamide p -i -J n ~106
"OHj ~C»H"~1
i i
do
Anionic
Polyacrylic acid
Hydrolyzed
polyacrylamide
~106
~106
Nonionic (no charge) and anionic (negative charged) polymers are produced with
molecular weights usually in excess of 106. These polymers are used as coagulant
aids, filter aids, and in sludge conditioning. These polymers function by forming
particle-polymer-particle bridges - i.e., interparticle bridging. Size of the polymer is
important. Molecular weights of 106 or more are necessary for the polymers to
bridge colloidal particles to form floe. Earlier comments regarding the influence of
particle numbers (turbidity) on flocculation kinetics are applicable.
Filtration
As shown in Figs. 4 and 5 filtration is used in conventional water treatment plants
and direct filtration plants for the purpose of solids removal. The subject of filtration
is extensive, and the body of knowledge represents an accumulation over the last 90
years. Thus no attempt is made to review this subject here. An excellent review of
deep bed filtration with applications to water treatment can be found in a publica-
tion of Baumann37. What is stressed here is the importance of particle destabilization
(pretreatment) for filtration and some analogous features of filtration coagula-
tion38-40
Filtration is a two step process: particle transport and particle attachment.
Particle transport involves the transport of destabilized (sticky) particles from water
to the surface of the collector. A single spherical grain of filter media - single col-
lector - is considered in which under laminar flow conditions particles may be trans-
ported from the flowing fluid to the collector by three mechanisms: Brownian dif-
fusion, interception, and sedimentation. The removal efficiency (77) of this single col-
lector is defined as follows:
Rate at which particles strike collector
Rate at which particles approach collector
(11)
43
-------
Equations for rj for each transport mechanism follow:
2/3
kT \
dp dc v0 /
*?s - (pp - p) g dp
(13)
(14)
18 M V0
r/Totai = rjo + rji + rjs (15)
Here D,I, and S stand for diffusion, interception, and sedimentation, respectively. In
addition, k is Boltzmann's constant, T is the absolute temperature, n is the fluid
viscosity, dp is the diameter of the particles being applied to the filter bed, dc is the
collector diameter (filter media), v0 is the filtration rate, pp and p are the densities of
the suspended particles and water, respectively, and g is the gravitational constant.
It is noted that coagulation and filtration are analogous in that both are two step
processes involving particle transport and particle sticking (destabilization). As in
coagulation, there is also a suspended particle size, or size range, in which there is a
minimum in particle transport efficiency. Equations (12) - (15) are used to illustrate
this for the following conditions: temperature of 25°C, particle density of 1.01
g/cm3, collector diameter of 0.5 mm, and filtration rate of 2gpm/ft2(4.9m/hr). The
results are plotted in Fig. 9. As in coagulation, small particles (say 1 /urn or less) are
removed by transport from the water to the filter media by Brownian diffusion.
Particles greater than about 5 ^im are transported effectively by settling and inter-
ception. There is a size range, in this case about 1 jum to 5 /urn, in which there is a min-
imum in transport efficiency. Note that particle straining has not been discussed.
Many believe straining is the main way in which filters function. Particle straining is
not a dominant mechanism in deep bed filtration unless the particles being applied to
the filter are quite large. It would also be characterized by very shallow penetration
into the filter and rapid clogging requiring frequent backwashing. Straining occurs
when the suspended particle size is greater than 0.2-dc (0.2 times the collector diam-
eter). Thus for 0.5-mm filter media, straining would be important for suspended
particles greater than 100 /urn. The bulk of the particles being applied to filters in
water treatment are smaller than 100 jitm.
Recent work in filtration has demonstrated the role of retained particles or "dirty"
bed filtration40''". Suspended particles are initially removed by the clean bed; how-
ever, these retained particles then act as additional collectors so that one gets particles
"sticking" to previously retained particles as well as the original filter media. This in-
crease in collectors improves filtration and provides an explanation for the observed
phenomenon of filter bed ripening in which filtration performance improves with
time immediately after backwashing. Sticking of particles to surfaces including
particle-particle sticking can be quite strong when polymers have been used for
particle destabilization. Particles sticking^ to presumably clean surfaces and to each
other occurs in nature. Costerton, et al. showed that dental plaque is the result of
bacteria sticking to the tooth surface and to each other due to a glycocalyx (polymer)
coating.
44
-------
10-'
c
0>
'5
it
LLJ
O
O
a
"5
o
10-3
10~4
10-5
T = 25°C
P p = 1.01 g/cm3
10-3
10-'
10-1
10'
102
Figure 9. Single collector efficiency as a function of suspended particle diameter
Process Selection
It is recognized that high turbidity waters are readily treated in a conventional
water treatment plant (Fig. 4) while direct filtration (Fig. 5) is potentially applicable
to the treatment of low turbidity waters. The analysis that follows describes a dimen-
sionless product called the coagulation effectiveness factor. This coagulation effec-
tiveness factor will be used to establish guidelines for the design and performance of
coagulation-flocculation systems.
Let us examine the design of flocculation basins in conventional plants. Camp33
introduced the dimensionless product, GF, and indicated that good flocculation can
be expected in a basin when this product is in the range of 104 to 10s. This product
reflects the influence of two physical parameters - velocity gradients provided by
mechanical mixing to induce particle collisions and detention time to permit floe
formation. We assume that chemical pretreatment has been successful, and that
there are sufficient particle numbers either in the raw water or introduced by the co-
agulant for flocculation to be efficient. Clearly, if insufficient coagulant has been
added for particle destabilization or if there are insufficient particle numbers for col-
lisions, then poor floe formation will result with poor separation of solids in the sedi-
mentation tank. O'Melia43 has indicated that we must consider other parameters in
45
-------
addition to Camp's dimensionless product to account for particle destabilization
and particle number concentration.
Coagulation Effectiveness Factor—
Using the orthokinetic flocculation equation (Equation 4) and including a para-
meter a, we get:
JiL =- -1 « G d3N2 (16)
The parameter a is a colloid stability factor which reflects then the success we have in
adding coagulant to destabilize particles. It may also be viewed as a collision effi-
ciency factor or sticking factor, since it represents the fraction of the total number of
particle collisions that are successful in producing aggregation. Particles which are
completely stable (no coagulant addition) would have a values approaching zero
while complete particle destabilization (all particles stick upon colliding) would cor-
respond to a value of 1.0.
Introducing the expression for the floe volume fraction (), which is defined as the
volume of colloidal particles per unit volume of suspension, yields:
(17)
Since we can assume particle volume is conserved in a flocculation tank we can
substitute Equation (17) into Equation (16) which yields:
dN 4arf>GN
-57-= -—•e
dt n
Integration of Equation (18) for the boundary conditions N=N0 at t=0, and N, at
any time t results in:
- 4aGt
N = N0e (19)
Setting t as the detention time (t) of the flocculation process shows that the perfor-
mance of the coagulation-flocculation process should be evaluated from the dimen-
sionless product (aGl) - coagulation effectiveness factor.
Of course this is a simplification, since we assumed a monodispersed particle sys-
tem; nonetheless, coagulation effectiveness will be considered using this product,
since it considers not only GtXCamp's product), but also a (particle destabilization)
and (/> (particle volume fraction). Three cases are presented to illustrate the signifi-
cance of the coagulation effectiveness factor.
Case I '• Raw water containing humic particles at a concentration of 5 mg/ L. This
suspension was previously described as Case 1. It will be assumed that the particles
have been destabilized with either a cationic organic polyelectrolyte or by positively
charged dissolved aluminum hydrolysis products. Characteristics of the particles
follow:
p =1.01 g/cm3
do = 0.021 /urn (initial diameter)
No = 1011 particles/cmj (initial number)
46
-------
For these particles to flocculate to a size (say, 2.1 ^m) in which orthokinetic floccu-
lation is applicable requires a decrease in particle number concentration. Fig. 6
shows that an increase in a particle size of two orders of magnitude decreases particle
number concentration by six orders of magnitude (N is proportional to 1 /d3). At this
point 4> would be 4.95 X 1(T6. We will assume a equal to 0.5 meaning half the colli-
sions are successful in particles sticking. Using typical values for G and Tfor conven-
tional flocculation basins of 20 sec ' and 45 min, respectively, results in:
aGt = 0.5 (4.95 X 10~6) (20) (45) (60) = 0.13
Note that Camp's product is 54000, which would be considered an acceptable design.
Case 2: Low turbidity water containing clay-type particles which has also been de-
stabilized in the same way and to the same degree as Case 1 - i.e., a equal to 0.5. The
suspension previously presented as Case 2 is used.
d = 1.6 pm and p = 2.5 g/cm3
Solids concentration = 5 mg/L (turbidity of 7-8 NTU)
N = 10" particles/cm3
0 = 2 X 10"6
ac/>Gt = 0.5 (2 X 10"") (20) (45) (60) = 0.05
Again note that Camp's product has been maintained at a value of 54000.
Case 3: High turbidity water containing clay-type particles which has been destabi-
lized in the same manner and degree as the previous cases. Characteristics of the
particles follow:
d = 1.6 /urn and p = 2.5 g/cm3
Solids concentration = 100 mg/L (turbidity of say 160 NTU)
N = 1.87 X 107 particles/cm'
= 4 X 10"5
a, takes into account coagulant selection and dosage (a) and
particle concentration ($). Both factors are influenced by raw water characteristics
and may be altered by plant operation. A water plant operator can treat the waters
described in Cases 1 and 2 by adding particles (increasing 4>) through the addition of
a coagulant aid (clays) or by using metal salts as coagulants under conditions in
which a metal precipitate is produced ("sweep floe"). In addition, choice of coagulant
and dosage may improve a. The product, a<£, may be increased by plant operation,
and therefore influences operating costs.
Another option is the use of solids contact flocculation-sedimentation basins (up-
flow tanks with a sludge blanket), which produces a high floe volume fraction as well
as long solids detention times.
47
-------
Jar Tests and Direct Filtration—
Jar tests are used to evaluate the destabilizing ability of various coagulants, to
choose dosages, and to evaluate the effects of coagulant aids. In spite of their wide-
spread use, jar test experiments are often criticized as being inadequate in predicting
coagulant dosages for actual water treatment plants. Perhaps much of the criticism
is due to a lack of understanding of what information jar tests provide.
Examining the coagulation effectiveness factor (atjiGi), we see that in conventional
treatment a number of parameters influence overall performance including: coagu-
lant type and dosage which affect a, floc volume fraction (0), mixing conditions of
flocculation (G), and detention time (t). The information obtained from jar tests is
limited by the fact that they are batch tests and therefore cannot be expected to du-
plicate the continuous flow mixing conditions or fluid flow patterns of an actual
plant. Thus, jar test conditions are standardized with respect to mixing intensity and
times of flocculation and settling.
The jar test provides us with information about the destabilizing ability of a
coagulant. In other words we can compare coagulants, and for a particular coagu-
lant rhoose a dosage at which the particles have been destabilized to the greatest de-
gree (a— 1) - i.e., coagulant dosage at which the particles are most sticky. The kinetics
of flocculation as affected by $, G, and T must be taken into account in applying jar
test n. suits to actual practice.
Consider the jar test data from Glaser and Edzwald44 in the top of Fig. 10. The raw
water contained 5 mg/ L humic acid and the humic matter was destabilized with the
cation c polyelectrolyte PEI-1000, polyethylenimine with a number average molecu-
lar we ght of 50,000 to 100,000. The jar test data indicate that 3 mg/L is the best
destab lization dosage, or the dosage at which the particles are most sticky. Dosages
below and above this optimum dosage indicate underdosing and overdosing, respec-
tively. The results of three filter experiments are shown at the bottom of Fig. 10, in
which t icre was continuous feed of polymer at the dosages indicated. A polymer pre-
coat of the sand media was used prior to filtration. Using the optimum jar test dos-
age of 3 mg/ L resulted in almost complete removal of the humic acid (based on ab-
sorbance measurements at 420 nm). Dosages which the jar test predicted to be an
underdose (0.5 mg/L) or an overdose (10 mg/L) resulted in unsatisfactory filtration
performance.
If we consider that a filter operates, after an initial period, by collection of par-
ticles on previously retained particles, then the jar test must be able to select coag-
ulant do ages for direct filtration. This is because the jar test tells us what dosage the
particles will best stick together regardless of whether the particles stick together in a
beaker (j.ir test) or stick to each other in a filter bed.
ORGANICS REMOVAL: CONCEPTS
This section is organized as follows. First, a summary of the physical and chemical
properties of humic substances is presented. Second, knowledge on the coagulation
of color oi humic substances is examined with a view to describing the factors affect-
ing the stability of humic materials and identifying the physical and chemical para-
meters which affect their removal from water supplies. Finally, recent work on the
application of direct filtration to removing color and THM precursors is reviewed.
Humic Substances
Humic substances are amorphous, acidic, predominantly aromatic, hydrophilic,
chemically complex macromolecules consisting of a complex mixture of naturally
occurring organics, and are responsible for the natural color imparted to waters. A
model humic compound, presented in Fig. 11, shows that part of the molecule has
an amorphous mass of polyhetero condensate associated with certain functional
48
-------
11
a>
.2 ir
E c
3 O
1.0
0.8
0.6
0.4
0.2
PEI-1000
True Color
10
15
20
25
1.0
Polymer Dose, mg/l
A A - -
PEI-1000
• 0.5mg/l
• 3.0 mg/l
A 10.0 mg/l
30 min. floe, time
120 180
Filtration Time, min.
240
300
Figure 10. Comparison of jar test results with filter performance(Filtration conditions:
5 mg/L humic acid in raw water, pH 6, filtration rate 2 gpm/ft2, 0.6 mm
sand, bed depth of 14 cm) (After Glaser and Edzwald44).
groups. The terminology and much of our knowledge pertaining to humic substances
comes from the field of soil science. Classification of the fractions of humic material
has traditionally been tfased on the solubilities of three main fractions as follows: (1)
humic acid (HA), which is soluble in alkali but insoluble in acid; (2) fulvic acid (FA),
which is soluble in both alkali and acid; and (3) humin, which is insoluble in both
49
-------
Polyhetero Condensate
of Organic Moieties
Figure 11. Model humic compound (From Trussell and Umphres'2)
alkali and acid. Humic matter in natural waters may be soil derived or may be pro-
duced within the water body. The composition of aquatic humic matter would de-
pend on the local biota. The color that we observe is due to the absorbance of light by
these substances.
The three humic fractions are structurally similar, but differ in elemental analysis,
functional group content, and molecular weight distribution45. A summary of the
major elements typically found in humic and fulvic acids is presented in Table 5.
Fulvic acids contain more oxygen but less carbon and hydrogen, and have a higher
content of oxygen containing functional groups (COOH, OH, C=0) per unit weight.
Nitrogen may be an impurity, although degradation of protein matter would explain
its presence.
Humic substances contain the following functional groups in varying amounts:
carboxyl, phenolic, alcoholic, methoxyl, carbonyl, ether, and ester. The molecular
weight of humic substances is normally considered to range from the hundreds to
tens of thousands. The molecular weight distribution of fulvic acids is much less and
may be considered as being from about 200 to 10,000. Humic acids are considered as
a higher molecular weight fraction. For the pH conditions of most natural waters,
humic materials are negatively charged macromolecules or anionic polyelectrolytes
in which the carboxyl and phenolic functional groups affect their colloidal stability.
As the pH increases above say about 5, the negative charge and therefore stability of
humic materials increase due to dissociation of the functional groups. In addition,
the configuration of the humic molecule in water will depend upon ionic strength
and pH. An extended configuration would be expected with increasing pH as a result
of repulsion between negatively charged functional groups.
Coagulation of Color
Work m the 1960's highlighted key water treatment operational parameters affect-
ing the coagulation of humic substances or color with alum and iron salts47"49. These
parameters include: the concentration of humic material in the raw water, coagulant
dosage, pH, and the influence of particulates (turbidity). Recent work50"52 has sum-
marized the state-of-the-art and examined mechanisms of coagulation.
50
-------
Table 5. Elemental Analysis of Humic and Fulvic Acids
(From Ref.46)
Percentage, dry ash-free weight
Element
C
H
0
N
S
Humic acid
50-60
4-6
30-35
2-6
0-2
Fulvic acid
40-50
4-6
44-50
<1-3
0-2
In summary, the coagulation of humic substances with inorganic coagulants such
as alum can be accomplished through two mechanisms of destabilization - charge
neutralization or precipitation. Destabilization may be accomplished by charge neu-
tralization resulting from specific chemical interaction between positively charged
aluminum species and the negatively charged groups on the humic colloids. Destabi-
lization by this mechanism would be accomplished over a narrow pH range (pH 4 to
6) and a stoichiometric relationship between the raw water humic concentration and
the optimum coagulant dosage would be observed. Humic substances can form
water-soluble and water-insoluble complexes with metal ions. As the alum dosage is
increased precipitation may occur; however, destabilization via this mechanism may
include incorporation of humic material within aluminum hydroxide floe or co-pre-
cipitation as aluminum humate. The effect of pH on the coagulation of humic acid
with alum is shown in Fig. 12. High alum dosages are required above pH 6.
The stability of color colloids differs from that of particles such as clays. Conse-
quently, it has been found that the optimum pH for removal of natural organics is
also different52'53. Coagulation of humic substances with hydrolyzing metal ions is
favored at acidic pH conditions, pH 5 to 6 with alum and pH 4 to 5 with ferric chlo-
ride. Coagulation at these pH conditions, which relies principally on charge neutral-
ization, would mean that greater coagulant dosages would be needed due to the pre-
sence of turbidity. Another alternative is to coagulate at higher pH conditions
with enough coagulant added to achieve a "sweep floe" effect in which the humic
material is incorporated in the floe.
An effective method for the removal of humics involves the use of high molecular
weight polymers as coagulant aids with alum51"54. Excellent color removal is achieved
over the pH range of 4.5 to 6 as shown in Fig. 13. For best results, the alum is added
first followed shortly by polymer. Destabilization mechanisms are charge neutrali-
zation by positively charged aluminum species and interparticle bridging of the floe
by the polymers. Excellent solids-liquid separation is achieved as the floe settles
rapidly.
Cationic polyelectrolytes used as the sole coagulant can destabilize humic sub-
stances44'55 '6. Glaser and Edzwald44 reported on the coagulation of humic acid using
a homologous series of cationic polyelectrolytes. A stoichiometry was observed be-
tween the raw water humic acid concentration and the optimum polymer dosage for
destabilization. They proposed a model for destabilization and aggregation which is
illustrated schematically in Fig. 14. The destabilization of the humic acid is highly
charge dependent, and is accomplished by neutralization of the anionic functional
groups on the humic macromolecule by cationic polymer functional groups. Aggre-
gation can occur as humic and cationic polymer molecules are cross-linked in a
random configuration to form floe.
51
-------
u
E
D
X
ID
DC
I I I I I I
50 100 150
Alum Dose, mg/l
200
Figure 12. Effect of pH on the coagulation of humic acid with alum (After Edzwald51).
52
-------
E
X
"a
3
;g
'to
to
DC
Cationic 0.5 mg/l
Alum 10 mg/l
H.A. 5 mg/l
Anionic 1 mg/l
Alum 10 mg/l
H.A. 5 mg/l
Nonionic 1 mg/l
Alum 10 mg/l
H.A. 5 mg/l
pH
Figure 13. Effect of pH on the coagulation of humic acid using alum and high
molecular weight polymers (After Edzwald15).
Direct Filtration
Direct filtration offers advantages over conventional treatment in lower capital
costs and possibly, lower operating costs. In spite of the potential advantages, there
is very little information on the use of direct filtration for treating low turbidity,
colored water supplies. Some results of recent pilot plant research by the author are
presented here.
53
-------
Initial Chemical Interaction
Humic Acid
Cationic Polymer
Destabilized Particle
Floe Formation
Destabilized Particles
Cross-Linked Floe Particle
Figure 14. Schematic representation of the destabilization and aggregation of
humic acid by polyethylenimine (After Glaser and Edzwald44).
Glaser and Edzwald44 investigated the use of direct filtration for color removal
using both a low turbidity, colored river water and a synthetic water containing
humic acid. Significant findings were:
• Excellent color removal was achieved as illustrated in Fig. 10; also, low filter
effluent turbidities were observed.
• Polymer dosages for filtration can be selected from jar test experiments. As
shown in Fig. 10, underdosing and overdosing in the jar test produce the
same phenomena in filtration.
• For low turbidity waters containing natural color in which the particles are sub-
micron in size, a brief flocculation period prior to filtration reduces the head
loss across the filter bed when compared to direct filtration without flocculation.
The limited data on THM precursor reduction by direct filtration in treating
colored waters are based on two pilot plant studies19'57. In both studies a cationic
polyelectrolyte was the only destabilizing chemical used. Scheuch and Edzwald57
found reductions in TH M precursors (measured by the reduction achieved in CHCU
54
-------
formation potential between raw and filtered waters) of 50 to 60% for humic acid
waters and approximately 40% for a low turbidity, colored river water.
The EPA's pilot plant facilities were used inastudy'9employingdual media filters
and filter rates of 4.9 m/ hr (2 gpm/ft2) and 12.2 m/ hr (5 gpm/ ft2). Filter experiments
utilized a low turbidity water spiked with humic acid to produce color. Excellent
results were achieved in terms of classical measures of filtration performance: ef-
fluent turbidities less than 0.3 NTU, effluent apparent color less than 5 Pt-Co units,
and moderate head loss development. Total trihalomethane formation potentials
were reduced by approximately 50 percent. Typical results are shown in Figs. 15 and
16.
ORGANICS REMOVAL AND THM REDUCTION: WATER TREAT-
MENT PRACTICE
In recent years there have been numerous seminars and publications on control-
ling THM formation and removing organics, particularly THM precursors. Each
month additional articles are published in scientific journals and additional EPA
publications appear. What may be gleaned from the voluminous information al-
ready available to assist water treatment professionals in solving the problem with-
out increasing capital costs of new or existing plants? There are several considera-
tions. The practice of prechlorination should be evaluated at many water treatment
plants. Common practice is to add chlorine at high dosages so that free residual chlo-
rine is measured at the end of the plant. For many waters which contain humic sub-
stances, most of the chlorine demand is due to the humic material so that chlorine is
being wasted by converting the humics to chlorinated organics. It would be better to
remove the humic material prior to chlorination, thus reducing the chlorine demand
and THM formation as well as realizing the cost savings resulting from using less
chlorine. The need for prechlorination of water supplies on a continuous basis to
control slime growths in sedimentation basins and filters should be re-examined.
This practice may be necessary only on an intermittent basis; during the winter
season it might be discontinued altogether. Another option is to use an alternative
predisinfectant such as chlorine dioxide. The latter does not produce THMs
unless chlorine resulting from generation of the chlorine dioxide is present. Chlorine
dioxide has other advantages in that it can oxidize reduced iron and manganese,
control slime growths, and destroy taste and odor producing compounds. Oxidants
such as chlorine dioxide are discussed in another chapter. In summary, some eco-
nomical options are: reduce the prechlorination dosage, omit prechlorination, and
optimize the removal of organics by coagulation and sedimentation prior to chlo-
rination.
Two cases from water treatment plant practice are presented below to illustrate
some simple principles regarding THM formation and control. The first case in-
volves moving the point of chlorination at the Durham, N.C. water treatment plant.
This case is taken from the published work of Young and Singer58. The second case
involves work of the author at the Canton, N.Y. water treatment plant.
Durham, N.C.
The raw water source for Durham is an impounded reservoir, Lake Michie. The
water treatment plant is a conventional plant employing liquid alum addition to the
rapid mix basins for coagulation. A nonionic polymer is added ahead of the dual
media filters as a filter aid. The plant was sampled thirty times between July 1976 and
January 1977 while prechlorination was being practiced by addition of chlorine to
the rapid mix basins. Over this period, the average raw water non-volatile total
organic carbon (NVTOC) was 5.1 mg/ L, the average chlorine dosage was 5.8 mg/ L,
and the average CHC13 concentration of the finished water was 129 jug/ L. In January
55
-------
1.6
1.2
0.8
0.4
0
40
30
O
N
"o
c
8 20
o
CD
0)
10
T
T
Fill Rate = 5 gpm/ft2
RawTurb =09-66 NTU
Control (pH 8.3)
5.25 mg/l Polymer (pH~6.3)
5.25 mg/l Polymer (pH 6.3)
-A A A A-
I
Control (pH 8.3)
i
4 6
Filtration Time, hr.
10
Figure 15. Turbidity and head loss data for spiked humic acid-gravel pit water, direct
filtration (Pilot plant operated at pH conditions indicated)
1977, prechlorination was stopped and the point of chlorination was moved to a
point following sedimentation. Over the next few months, the average raw water
NVTOC was 5.7 mg/L, but the chlorine dosage was reduced to 3.5 mg/L and the
average CHCh concentration of the finished water was 77 /j.g/ L. Temperature effects
56
-------
60-
_o
o
o
1 40|-
<
c
20-
0
400
5.25 mg/l Polymer, (pH 6.3)
300
5
2- 200\~
Q.
I
100-
Raw Color = 50-100
Fill Rate = 5 gpm/ft2
Raw Water: 342 fjg/\
Control (pH 8.3)
5.25 mg/l Polymer (pH 6.3)
I
I
4 6
Filtration Time, hours
10
Figure 16. Apparent color and TTHMFP data for spiked humic acid-gravel pit water,
direct filtration (Pilot plant operated at pH conditions indicated, TTHMFP
data for pH 8.3).
were discounted so that the 40 percent reduction in CHCb was attributed to the
change in the location of chlorination, and by the fact that the plant removed 35 per-
cent of the total organic carbon. An important point is that not only was the CHCU
reduced to below 100 jug/ L, the reduction in the amount of chlorine added resulted in
a savings of $50/day in plant operation.
57
-------
Canton, N.Y.
The Canton, N.Y. water treatment plant is a conventional plant employing co-
agulation, flocculation, tube settlers, and filtration. Alum and a nonionic polymer
are used in coagulation-flocculation. The water source is the Grasse River, a raw
water supply of low turbidity, but high color. The plant was monitored during a
period of spring runoff (April). In the monitoring program, total trihalomethane
formation potential (TTHMFP@ pH 7.5, 20°C) of the raw water as well as Inst
TTHMs and Term TTHMs throughout the plant were measured to evaluate process
performance. The 7-day TTHMFP of the raw water was 537 jug/L. Its formation
curve was shown earlier in Fig. 3. Samples for Term TTHM were buffered at pH
7.5, spiked with 15 to 20 mg/L chlorine, and held 7 days at 20°C. These chlorine
spiked Term THM samples served as a basis for evaluation of precursor removal.
Fig. 17 shows THMs through the plant and various surrogate parameters for
THM precursors. The water temperature through the plant was 5°C to 6.5° C and a
free chlorine residual was maintained as shown in Fig. 18 with a prechlorination
dosage of only 1 mg/ L. The total organic carbon was reduced from 6.3 mg/ L to 2.1
mg/L, a 67 percent reduction. The Inst THM of the finished water was 51 jUg/Ldue
to the low temperature, low chlorine dosages, and brief chlorine contact period in
the plant. Comparing the 7-day raw water TTHMFP (537 /ug/L) with the 7-day
finished water Term TTHM (168 //g/L) shows good reduction of THM precursors—
69 percent reduction. Ultraviolet (UV) absorbance was reduced by 83 percent
through the plant.
Turbidity data for the plant showed, as did the TOC and Term TH M data, that the
filters were removing most of the floe and organics. In effect, the plant was being
operated much like a direct filtration plant. Other data indicate that when the water
temperature is warmer, the efficiency of the sedimentation tank improves for re-
moval of both turbidity and organics".
FINAL COMMENTS
Much is known about the coagulation of turbidity. During the 1970's the problem
of organics in drinking water stimulated research on the coagulation and removal of
organics from drinking water supplies, particularly naturally occurring organics and
THM precursors. Additional research is needed. A description of research needs
follows.
Fundamental studies on the coagulation of organics are needed. These studies
should include well-characterized organic materials - e.g., humic and fulvic acids of
known molecular weight distributions - as well as research with actual waters. It is
important, however, that the organic material be characterized so that mechanisms
of coagulation can be elucidated and so that the results could be interpreted for their
applicability to appropriate water supplies. The research should examine the use of
alum, iron salts, and polymers in combination with the inorganic coagulants. Opti-
mum coagulation conditions should be identified such as pH and coagulant dos-
ages as a function of the nature of the organic material in the raw water and the organ-
ic concentration. Mixing conditions for the addition of the coagulants should be
studied, in particular when polymers are added as coagulant aids. These studies
should determine the removals of organics and TH M precursors that can be accom-
plished by coagulation.
Many water utilities obtain their water supplies from sources of low turbidity and
currently practice only disinfection. These utilities are faced with the problem of
adding additional treatment to ensure that they meet the one NTU turbidity stan-
dard, but must now also consider the ability of the proposed treatment to remove
THM precursors. Typical options would be a conventional water treatment plant or
direct filtration. While there have been some pilot scale direct filtration studies
which have investigated the ability of direct filtration to remove organics and THM
58
-------
1.1 min
Process Detention Times
48.7 min 15.0 min
2
T
\
o>
O
E
o
600
400
200
6.0
4.0
2.0
0.20
0.10
100
o 80
£ 60
£ 40
20
n
r7/"\ Inst. i — i 7-Day | | Term
5370 tZdTTHM I I TTHMFP k52TTHM -
485.0 pH 7.5, 20°C
5
3
95
0
*°?j5 168.0.
i — i
H K&
6.3 6.2 6.3
~
-
2.3 21
n n
0.243
_
0.140 0.134
n
II °n «*
97
—
-
55
53
-
£ 4T
Source
Rapid
Mix
L
L 1
'
L
Samp
Flocculation
Sedimentation
lin{
1
j Points
Filtration
0.6 ppm Polymer
.0 ppm CI2
I
Clear
Well
t
• 2.0 ppm
Distri-
bution
CI2
•— 35 ppm Liquid Alum, 15 ppm Na2C03
Figure 17. Monitoring of the Canton, NY water treatment plant in April (flow 2.8 MGD,
water temperature through plant 5 to 6.5°C).
precursors, additional research is needed with surface waters on both a larger pilot
scale and full plant scale. There are many direct filtration plants in the United States
which could be examined for their ability to remove THM precursors.
Finally, surrogate measurements are needed for characterizing the problem of
organics in raw water supplies and for use as process control parameters in water
treatment plants. The total organic carbon content of a water may be measured, but
this is a collective measure as it measures both THM precursors and non-precursors.
However, we can definitely expect, as pointed out in this chapter, a relationship be-
tween the organic content of a water and its trihalomethane formation potential.
Two observations about total organic carbon measurements are important. First,
there is a scarcity of data on the total organic carbon (TOC) content of untreated and
59
-------
Process Detention Times
5
i
a
600
400
200
50
40
30
20
10
0
2.0
1 0
0
7
6
5
537.0
-
-
r 45
-
-
-
~
-
-
Source
6.9
Rapid
Mix
L
T \
P-T-, Inst ( — , 7-Day , — i Term
\22JTTHM \ | TTHMFP E23 TTHM _
485 0 ph 7.5, 20°C
5
395.0
208.5
0168.0
V7\
_
-
. , , !-, 24.5
n
I I FT
-
0^ o.4 0.3 °-4
n n i—i r~i
6.8 6.8 6.9 7.0
L
.0
1 t 1
Sampling Points | |
Flocculation
Sedimentation
-1 Filtration \ Jlear 1
Distri- 1
bution 1
- 0.6 Polymer L 2.0 ppm CI2
ppm CI2
*— 35 ppm Liquid Alum, 15 ppm
NA2C03
Figure 18. Monitoring of the Canton, NY water treatment plant in April (Flow 2 8 MGD,
water temperature through plant 5 to 6 5°C).
especially treated waters. Most natural waters have TOC levels in the I to 20 mg/ L
range, although a particular water supply can vary considerably over the year. For
example, the Grasse River which was mentioned earlier (Fig. 3) has low TOC con-
centrations (2.75 mg/L) in February during ice-cover, higher values (6.3 mg/L) in
April during spring runoff, while the highest TOC concentrations (12.6 mg/L) occur
in the summer following heavy rains in the watershed. Data are needed on the
seasonal variation of TOC in raw water supplies and the ability of water treatment
plants to remove organic matter. Second, instrumentation for measuring organic
carbon is expensive. Small water utilities will be unable to afford such instrumenta-
tion. Thus there is a need for a surrogate measurement of organics in water. UV ab-
sorbance is a promising surrogate measurement60'61. The measurement can be made
60
-------
easily, rapidly, and inexpensively, and is thus advantageous in water treatment
processes for monitoring dissolved organics. Sontheimer et al.61 have reported on
using UV absorbance at 254 nm in water treatment practice for this purpose. For a
particular water supply, correlations could be developed between not only total
organic carbon and trihalomethane formation potential (THMFP) or Term THMs,
but also between UV (254 nm) absorbance and THMs. This might be done for both
raw and treated waters.
Traditionally, color has been measured in water treatment plants by comparing
samples visually with platinum-cobalt standards. The measurement is rapid and
easy, but has many drawbacks. Particulates must be removed from the sample to get
so-called true color readings. In addition color intensity is affected by pH with
greater intensity at higher pH. The lack of accuracy with the visual comparison
method and the lack of standardizing conditions such as pH makes it difficult to
compare color readings of one water treatment plant or laboratory with that of
another. Since color could serve as a surrogate measure of TOC, and thus TH M pre-
cursors, simple color measurements could serve as a rough monitoring parameter
in water treatment. There is a need, however, to replace the visual comparison
method with a spectrophotometric measurement at a standard wavelength and pH.
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5. Bellar, T.A., J.J. Lichtenburg, and R.C. Kroner. "The Occurrence of Organo-
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615-620, 1976.
12. Trussell, R.R. and M.D. Umphres. "The Formation of Trihalomethanes,"
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13. Bird, J.C. and R.A. Minear. "The Effect of Bromide on Trihalomethane For-
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61
-------
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14. Kavanaugh, M.C., A.R. Trussell, J. Cromer, and R.R. Trussell. "Empirical
Kinetic Model of Trihalomethane (THM) Formation: Applications To Meet
the Proposed THM Standard," Proceedings of the 1979 Annual Conference of
the American Water Works Association, pp. 465-473, 1979.
15. Rook, J.J. "Chlorination Reactions of Fulvic Acids in Natural Waters," Env.
Sci. and Technol., Vol. 11, pp. 478-482, 1977.
16. Hoehn, R.C., C.W. Randall, R.P. Goode, and P.T.B. Shaffer. "Chlorination
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Ann Arbor, MI, pp. 519-535, 1978.
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P.T.B. Shaffer. "Algae as Sources of Trihalomethane Precursors," Jour.
Amer. Water Works Assoc., Vol. 72, pp. 344-350, 1980.
18. Briley, K.F., R.F. Williams, K.E. Longley,and C.A. Sorber."Trihalomethane
Production from Algal Precursors," Paper presented at the Third Conference
on Water Chlorination Environmental Impact and Health Effects, Colorado
Springs, CO., 1979.
19. Edzwaid, J.K. "A Preliminary Feasibility Study of the Removal of Trihalo-
methane Precursors by Direct Filtration," Report of the Drinking Water
Research Div., EPA, Cincinnati, OH, Feb. 1979.
20. Stevens, A.A. and J.M. Symons. "Formation and Measurement of Trihalo-
methanes in Drinking Water," Proceedings - Control of Organic Chemical
Contaminants in Drinking Water, Office of Drinking Water, U.S. EPA,
Cincinnati, OH, Jan. 1980.
21. Lange, A.L. and E. Kawczynski. "Controlling Organics: The Contra Costa
County Water District Experience," Jour. Amer. Water Works Assoc., Vol. 70,
pp. 653-660, 1978.
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of State Sanitary Engrs., Water Treatment Plant Design, Published by the
Amer. Water Works Assoc., New York, 1969.
23. A Report of the Comm. of the Great Lakes-Upper Miss. River Board of State
Sanitary Engineers, "Recommended Standards for Water Works," 1976 ed.,
New York State Dept. of Health, Albany, NY, 1976.
24. Committee Report by the Direct Filtration Subcomm. of the AWWA Filtra-
tion Comm. "The Status of Direct Filtration," Jour. Amer. Water Works
Assoc., Vol. 72, pp. 405-411, 1980.
25. O'Melia, C.R. "Coagulation in Wastewater Treatment," Chapter in The
Scientific Basis of Flocculation, edited by K.J. Ives. Sijthoff and Noordhoff
Int. Publishers, Alphen aan den Rijn, The Netherlands, 1978.
26. Friedlander, S.K. Smoke, Dust, and Haze, John Wiley and Sons, New York,
NY, 1977.
27. Verwey, E.J.W. and J.Th.G. Overbeek. Theory of the Stability of Lyophobic
Colloids, Elsevier Publ. Co., New York, NY, 1948.
28. van Olphen, H. An Introduction to Clay Colloid Chemistry, 2nd ed., John Wiley
and Sons, New York, NY, 1977.
29. O'Melia, C.R. "Coagulation and Flocculation," Chapter in Physicochemical
Processes for Water Quality Control, edited by W.J. Weber, Jr. John Wiley
and Sons, New York, NY/1972.
30. Edzwaid, J.K., J.B. Upchurch, and C.R. O'Melia. "Coagulation in Esturanes,"
Env. Sci. and Technol., Vol. 8, pp. 58-63, 1974.
31. Smoluchowski, M." Versuch einer mathematischen Theorie der Koagulations-
kinetik kolloider Losungen," Z. Physic. Chem., Vol. 92, pp. 129-168, 1917.
32. Camp, T.R. and P.C. Stein. "Velocity Gradients and Internal Work in Fluid
Motion," Jour. Boston Soc. Civ. Eng., Vol. 30, pp. 219-237, 1943.
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33. Camp, T.R. "Flocculation and Flocculation Basins," Trans. American Society
of Civ. Eng., Vol. 120, pp. 1-16, 1955.
34. Hudson, H.E., Jr. "Physical Aspects of Coagulation," Jour. Amer. Water
Works Assoc., Vol. 57, pp. 885-892, 1965.
35. Hudson, H.E., Jr. and J.P. Wolfner. "Design of Mixing and Flocculation
Basins," Jour. Amer. Water Works Assoc., Vol. 59, pp. 1257-1267, 1967.
36. "Report on Coagulant Aids for Water Treatment," EPA, Technical Support
Div., Office of Drinking Water, Cincinnati, OH, Apr. 1979.
37. Baumann, E.R. "Granular-Media Deep-Bed Filtration," Chapter 12 in Water
Treatment Plant Design, R.L. Sanks ed. Ann Arbor Science Publ. Ann
Arbor, Ml, 1978.
38. Yao, K.M..M.T. Habibian and C.R. O'Melia. "Water and Waste Water Filtra-
tion: Concepts and Applications," Env. Sci. and Techno!., Vol. 5, pp. 1105-
1112, 1971.
39. Habiban, M.T. and C.R. O'Melia. "Particles, Polymers, and Performance in
Filtration," Proc. Amer. Soc. of Civil Engrgs., Envir. Engr. Div., Vol. 101, pp.
567-583, 1975.
40. O'Melia, C.R. and W. Ali. "The Role of Retained Particles in Deep Bed Filtra-
tion," Progress in Water Technology, Vol. 10, pp. 167-182, 1978.
41. Tien, C., C. Wang, and D.T. Barot. "Chamlike Formation of Particle Deposits
in Fluid-Particle Separation," Science, Vol. 196, pp. 983-985, 1977.
42. Costerton, J.W., G.G. Geesey, and K.J. Cheng. "How Bacteria Stick," Scien-
tific American, Vol. 238, pp. 86-95, 1978.
43. O'Melia, C.R. "Coagulation," Chapter 4, In Water Treatment Plant Design,
R.L. Sanks ed., Ann Arbor Science Publ., Ann Arbor, MI, 1978.
44. Glaser, H.T. and J.K. Edzwald. "Coagulation and Direct Filtration of Humic
Substances with Polyethylenimine," Env. Sci. and Technol. Vol. 13, pp. 299-
305, 1979.
45. Schnitzer, M. and S.U. Kahn. Humic Substances in the Environment, Marcel
Dekker, New York, NY, 1972.
46. Schnitzer, M. "The Chemistry of Humic Substances," in Environmental Bio-
geochemistrv, Vol. 1, edited by J.O. Nriagu, Ann Arbor Science Publ., Ann
Arbor, MI, 1976.
47. Black, A.P. and D.G. Willems. "Electrophoretic Studies of Coagulation for
Removal of Organic Color," Jour. Amer. Water Works Assoc., Vol. 53, pp.
589-604, 1961.
48. Black, A.P., J.E. Singley, G.P. Whittle, and J.S. Maulding/'Stoichiometry of
the Coagulation of Color-Causing Organic Compounds with Ferric Sulfate,"
Jour. Amer. Water Works Assoc., Vol. 55, pp. 1347-1366, 1963.
49. Hall, E.S. and R.F. Packham. "Coagulation of Organic Color with Hydro-
lyzing Coagulants," Jour. Amer. Water Works Assoc., Vol. 57, pp. 1149-1166,
1965.
50. Mangravite, F.J., Jr.,T.D. Buzzell, E.A. Cassell, E. Matijevicand G.B. Saxton.
"Removal of Humic Acid by Coagulation and Microflotation," Jour. Amer.
Water Works Assoc., Vol. 67, pp. 88-94, 1975.
51. Edzwald, J.K. "Coagulation of Humic Substances," AIChE Svmposium
Series, Water 1978, Vol. 75, pp. 54-62, 1979.
52. Committee Report, Prepared by a subcommittee of the AWWA Research
Committee on Coagulation, (J.K. Edzwald, J. Mclntyre, R.L. Sanks, M.J.
Semmens, and J.S. Taylor.) "Organics Removal by Coagulation: A Review
and Research Needs, "Jour. Amer. Water Works Assoc., Vol. 71, pp. 588-603,
1979.
53. Kavanaugh, M.C. "Modified Coagulation for Improved Removal of Trihalo-
methane Precursors, "Jour. Amer. Water Works Assoc., Vol. 70, pp. 613-620,
1978.
54. Edzwald, J.K., J.D. Haff, and J.W. Boak. "Polymer Coagulation of Humic
63
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Acid Waters," Journal of the Environmental Engineering Divison, ASCE,
Vol. 103, pp. 989-1000, 1977.
55. Narkis, N. and M. Rebhun. "The Mechanisms of Flocculation Processes in the
Presence of Humic Substances, "Jour. Amer. Water Works Assoc:, Vol. 67, pp.
101-108, 1975.
56. Narkis, N. and M. Rebhun. "Stoichiometric Relationship between Humic and
Fulvic Acids," Jour. Amer. Water Works Assoc., Vol. 69, pp. 325-328, 1977.
57. Scheuch, L.E. and J.K.. Edzwald. "Removal of Color and Chloroform Pre-
cursors from Low Turbidity Waters by Direct Filtration, "Jour. Amer. Water
Works Assoc., Vol. 73, pp. 497-502, 1980.
58. Young, J.S and P.C. Singer. "Chloroform Formation in Public Water Sup-
plies: A Case Study," Jour. Amer. Water Works Assoc., Vol. 71, pp. 87-95,
1979.
59. Gong, B. "Evaluation of Trihalomethane Precursor and Organics Removal
from theCirasse River by Direct Filtration and Conventional Water Treatment
Methods,' Master's thesis. Department of Civil and Environmental Engineer-
ing, Clarkson College of Technology, Potsdam, NY, 1980.
60. Dobbs, R A., R.H. Wise and R.B. Dean. "The use of Ultra-Violet Absorbance
for Monitoring the Total Organic Content of Water and Wastewater," Water
Research. Vol. 6, pp. 1173-1180, 1972.
61. Sontheimer, H S , E. Heilker, M.K. Jekel, H. Nolle, and F.H. Vollmer. "The
Mulheim Process," Jour Amer. Water Works Assoc:, Vol. 70, pp. 393-396,
1978.
64
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ADSORPTION OF ORGANIC SUBSTANCES IN
DRINKING WATER
Francis A. DiGiano, PhD
INTRODUCTION
Early History
Throughout the modern history of water treatment, adsorption has been used for
removal of taste and odor causing compounds. Compounds associated with taste
and odor derive from chlorinated phenols and by-products of algal and vegetable
matter decay. In the case of chlorinated phenols, their precursor, phenol, may be ad-
sorbed prior to chlorination. Adsorption has also found application following chlo-
rination for removal of chlorinated aromatic compounds and excess free or
combined chlorine which may be present, for example, after treatment of more high-
ly contaminated surface waters.
The adsorbent of choice has traditionally been powdered activated carbon (PAC).
This is so because it does not require significant capital investment and because it is
used only when taste and odor problems arise, e.g. seasonally. The point of addition
of PAC is usually in the chemical coagulation step of the conventional water treat-
ment plant.
As an historical note, Rosenau1, in his classic text Preventive Hygiene and Med-
icine (Sixth Edition, 1935) heralded the addition of PAC as a great advance in water
treatment. He pointed out that PAC was the accepted adsorbent despite the fact that
Baylis2, a well-known researcher, had earlier demonstrated the effectiveness of gran-
ular activated carbon (G AC) for removal of taste and odor and for dechlorination in
large-scale experiments at the Chicago water works. Rosenau stated that GAC is not
the adsorbent of choice because "such a practice would involve considerable initial
outlay for material and its frequent removal or regeneration when it became charged
with adsorbed impurities." Today, this same argument is being used to oppose pro-
posals to control synthetic organic contaminants by GAC. This contemporary argu-
ment will be discussed later in greater detail.
The 1960's: A Growing Interest
Although the historical functionof PAC as an adsorbent of taste and odor com-
pounds remains important today, the early 1960's brought recognition of the more
general role of adsorption in removing organic contaminants. The specific classes of
The A ulhor. Dr. Francis A. DiGiano is Professor of Environmental Sciences and Engineering at the Univer-
sity of North Carolina at Chapel Hill He has been actively engaged in adsorption research for 15 years, in-
cluding a stay at Water Chemistry Institute at the U niversity of Karlsruhe in Germany. He recently served as
Chairman of the Organic Contaminants Committee of the American Water Works Association Research
Division He also served on the sub-committee of the Safe Drinking Water Committee of the National Acad-
emy of Sciences which prepared a report entitled "An Evaluation of Activated Carbon for Drinking Water
Treatment" for Vol. 2 of Drinking Water and Health.
65
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compounds of concern were the pesticides and the detergents. An attempt was made
to measure trace organic contaminants collectively by passing the water sample
through GAC, desorbing adsorbed compounds by treating the GAC with either
chloroform or chloroform and ethyl alcohol, and finally weighing the extract to
measure the organic compounds collected. This procedure takes advantage of the
ability of granular activated carbon to adsorb a wide spectrum of organic contam-
inants, particularly those that are non-polar and slightly polar. Similarly, extraction
in either chloroform or alcohol would selectively concentrate non-polar contam-
inants.
The ineffectiveness of conventional water treatment in removing organic contam-
inants can be illustrated by carbon-chloroform extractions (CCE) of raw and treated
Missouri River water at Omaha, Nebraska during the period 1958-1962 (Table 1).
These measurements showed that conventional treatment, consisting of coagulation
(with PAC addition), sedimentation and filtration, was not removing compounds
concentrated by the CCE procedure3. It must be emphasized, too, that not all organ-
ic compounds were being detected. Later surveys by the Organic Contaminants
Committee of the American Water Works Association (AWWA) revealed that
organic contamination, as measured by several different procedures, was a wide-
spread problem4. Despite these findings, conventional United States treatment prac-
tice was not to be changed easily.
The 1970's: Evolving Public Policy
The American Water Works Association was concerned enough about organic
contaminants in the early 1970's to expand the responsibility of its Research and
Technical Practice Committee on Taste and Odor to include organic matter in gen-
eral. The Committee cosponsored a special conference with the University of Illinois
in 1973 to examine the growing problem of organic contamination . By this time,
approximately 100 organic contaminants, some thought to be toxic, had been iden-
tified in public drinking water supplies. The importance of controlling organic con-
tamination by the adsorption process was stressed at this conference by Middleton
of the U.S. Environmental Protection Agency (EPA). He recognized rather prophet-
ically, however, that setting of "realistic and practical standards that can be justified
and enforced" would be a problem. Nevertheless, Middleton questioned the wide-
spread practice of delivering water to consumers from a polluted source without
treatment which included activated carbon.
By the mid 1970's, a concern with organic contamination of drinking waters had
blossomed into one of the leading issues of the water works industry. It was fueled by
the National Organics Reconnaissance Survey (NORS)5 and the National Organics
Monitoring Survey (NOMS)6, both conducted by the EPA. These surveys showed
the ubiquitous nature of organic contamination and confirmed in factual detail the
apprehensions expressed by environmental engineers and scientists in the 1960's. In
addition to contaminants traced to industrial sources, special concern arose over the
production of trihalomethanes (i.e., chloroform, bromodichloromethane, chloro-
dibromomethane and bromoform) in water treatment by the practice of chlorin-
ation.
The Safe Drinking Water Act of 1974 mandated the regulation of drinking water
contaminants which "may have an adverse effect on the health of persons;" how to
control these contaminants became a major question of the 1970's. In this regard, the
control of trihalomethanes as a by-product of disinfection became a separate, identi-
fiable problem. The EPA reported to Congress7 in 1975 that although adsorption was
a possible solution, its effective use for removal of trihalomethanes (THMs) and
their precursors was limited by inadequate experience in use of the adsorption pro-
cess. A large effort was therefore mounted to test and demonstrate the effectiveness
of activated carbon, as well as alternative adsorbents, in controlling specific and
66
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Table 1. Average Annual CCE* Concentration in Missouri River Water at
Omaha3
Year
1958
1959
1960
1961
1962
Raw
Water
CCE-ppb
35.7
383
37.7
83.5
40.2
Tap
Water
45.2
27.3
24.5
27.6
33.5
ppb = parts per billion
*CCE is carbon-chloroform extract as measured by the high-flow carbon adsorption
method (CAM-hf)
non-specific organic contaminants, the latter being measured by some overall para-
meter, such as, for example, non-purgeable total organic carbon (NPTOC).
As a result, emphasis on adsorption research shifted greatly. In the 1960's and early
1970's, interest was directed mainly to removal of residual organic contaminants in
wastewater treatment. Now, the major focus is clearly on the problem of removing
quite specific contaminants which were being found in very low concentrations in
water supply sources. Promulgation of the Interim Primary Drinking Water Regu-
lations in 1977 and the attempt in 1978 by EPA to promulgate yet another, moreen-
compassing regulation8 did much to shape development of the adsorption process as
a part of water treatment. GAC adsorption was the recommended method of con-
trolling the six pesticides specified in the 1977 regulation9. This was not nearly as
controversial as the proposed 1978 regulation which would have required that GAC,
or its equivalent, be used on water supplies serving populations greater than 75,000
and believed to be subject to contamination by synthetic organic chemicals of in-
dustrial origin.
Observers may find it difficult to understand why the water works industry itself
took the official position of opposing this regulation which seemed to offer greater
protection to the public10 12. However, important questions had been raised con-
cerning costs and effectiveness, and these needed to be answered if the process was to
be accepted. The AWWA aired these issues at its national conferences and in its
Journal. As a result, technical and management personnel have become far better in-
formed than a decade ago about the details of the adsorption process and the experi-
ence with its use in several Western European countries.
Although the 1978 regulation requiring installation of the adsorption process was
not promulgated, it heightened the interest of water utilities in adopting some
method of controlling organic contamination. Should GAC adsorption be a recom-
mended treatment method, its inclusion with coagulation, sedimentation, filtration,
and disinfection could expand the concept of conventional water treatment practice.
The most likely point of introduction of the adsorption process as a separate unit
operation in the treatment scheme is after filtration, as shown in Fig. 1. At the
same time, however, it should be noted that some plants have simply replaced the
filter media (normally a mixture of sand and anthracite coal) by GAC and extended
the bed depth so as to lengthen the time of contact with the water for better removal
of the organic contaminants.
67
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Disinfection
Rapid
Raw Mix
Water
Supply
Multi-media
Filtration
Adsorption
Figure 1. Modification of conventional water treatment practice to include
the adsorption process
PRINCIPLES OF ADSORPTION
General Considerations
Treatment by GAC, in contrast to PAC, requires a specific unit operation,
commonly referred to as a contactor system. Fig. 2 depicts schematically the pro-
gression of the active adsorption zone down the length of the contactor and the re-
sultant profile of concentration to be expected in the effluent. The capacity of the
adsorbent to remove a contaminant is limited by physical and chemical factors
which are related to the properties of both the adsorbent and the contaminant. As
sorptive capacity becomes exhausted at any location in the bed, the active adsorp-
tion zone migrates further down. Eventually, there is very little sorptive capacity
remaining and the contaminant begins to leak through the adsorbent bed. More and
more leakage occurs as the final remnants of sorptive capacity are used. Finally, the
bed becomes completely exhausted and no treatment is achieved. This pattern of in-
creasing effluent concentration with operating time, or volume passed, is termed the
breakthrough curve. Its shape and position are determined by principles of adsorp-
tion equilibrium and adsorption rate.
At some point in time along the breakthrough curve, the effluent quality deterio-
rates to an unacceptable level. This is termed the breakpoint concentration. The time
of bed operation to breakpoint is commonly referred to as the service time.
Empty bed contact time (EBCT) is a commonly used design parameter. It refers to
the hold-up time of water in the empty contactor volume, i.e. under the hypothetical
condition of not being filled with the adsorbent. Because the contactor actually con-
tains the adsorbent particles, which themselves occupy volume, the actual contact
time is considerably shorter. The EBCT is calculated by dividing the volume of the
contactor by the flowrate of water being treated. As EBCT increases, more water can
be treated before breakthrough occurs because the volume of contactor, and thus the
amount of adsorbent contained in the contactor, increases in proportion to the
EBCT. The longer the EBCT then, the longer is the service time, or bed life.
M uch of the adsorption process research of the early 1960's was stimulated by the
U.S. Public Health Service's (USPHS) Advanced Wastewater Treatment Research
(AWTR) Program. The emphasis here was on treating wastewater by adsorption to
the extent that it could be reused. A classic example is the early research of Weber
and co-workers on removal of detergents and pesticides14'15. This work resulted in a
fundamental understanding of the adsorption process and provided the impetus for
an expanded program of research on adsorption phenomena. Although the research
was initially justified by a concern for reclaiming wastewater, the knowledge gained
was applicable to removal of organics from any water source.
The early 1960's also saw practical evaluation of the adsorption process in water
treatment. Workers at the USPHS's Robert A. Taft Sanitary Engineering Center
were at the forefront of this research. Particularly significant was the work of
Robeck, et al.16 on adsorption of pesticides (lindane, dieldrin, DDT, parathion and
68
-------
o
o
S 1 .0
-
c
o
(J
c
0>
D
c
0)
_
s
o •
(Co/ Co)
(C4/C0)-
Adsorption
Zone
(C3/C0)
(C2/C0)
(C,/C0)—
Breakpoint
Time, or Volume of Water Treated
Figure 2. Schematic representation of the movement of the adsorption zone and the
resulting breakthrough curve13.
2,4,5-T ester) in the very low concentration range of interest (ppb levels) and under
the practical conditions imposed by pilot plant treatment of the Little Miami River
water. Some of their most significant findings dealt with the effect of competitive ad-
sorption between trace contaminants and naturally present background organics
compounds. As may be seen from Table 2, they noted that much larger dosages of
PAC were required when pesticides were to be adsorbed from the Little Miami River
water than from distilled water. Their findings gave practical significance to the
theory of competitive adsorption as first discussed by Weberand Morris17. Compet-
itive adsorption is also relevant to today's problem of evaluating the effectiveness of
the adsorption process for removal of a wide variety of organic contaminants.
Another phase of research at the Robert A. Taft Center provided the first practical
evaluation of GAC beds installed following conventional water treatment, i.e. after
coagulation-sedimentation-sand filtration 6 (Fig. 3). It is clear from these results
that GAC was very effective in removing pesticides. However, competitive adsorp-
tion was shown by Woodward, etal.18 to cause displacement of organic compounds,
these being measured by the CCE test. Occasional desorption actually yielded "nega-
tive" removals across GAC beds (denoted by black regions in Fig. 4), i.e. the ef-
fluent exceeded the influent concentration. These investigators noted that desorp-
tion corresponded to periods when the influent CCE levels were low. Again, the his-
torical perspective gained by reviewing these observations of the early 1960's is use-
ful in light of the fact that similar adsorption research questions were to be addressed
in the 1970's.
Interest in adsorption led to GAC replacing sand in full-scale filters at the Nitro,
West Virginia plant" in the mid-1960's. This is noteworthy because it marked the
first time that specific design parameters of the adsorption system had actually been
explored in pilot phase and at full-scale at an operating water treatment plant. The
Nitro plant tests confirmed the intuitively obvious conclusion that the life of the
adsorbent bed, i.e. its service time, should increase in direct proportion to the contact
time provided by the bed (Fig. 5).
69
-------
Table 2. Parts per Million of Activated Carbon Required to Reduce the
Pesticide Level in Distilled Water and in Little Miami River Water16
Pesticide
Method
10ppb*
1 0 ppb*
Parathion
2,4,5-T ester
Endrin
Lindane
Dieldrin
JTJ
P§
JT
P
JT
P
JT
P
JT
P
10f
2.5
5
2.5
11
1.8
11
2
29
3
18
o.it
5
10
17
126
14
126
12
70
12
85
o.it
0.5
0.9
1.5
11
1.3
11
1.1
6
1.1
7
0.05t
0.6
1.1
3
23
2.5
23
2
9
1.7
12
Initial level of pesticide.
tPesticide level after treatment.
JJar test, in which pesticide is removed from distilled water by activated carbon alone,
with a contact time of an hour
§Plant treatment, iruwhich pesticide is removed from river water by conventional
treatment and activated carbon.
3.0
I 2.0
0
0
8 i.o
Q.
0
A 1
D
n»
Fli
r>--u •_
n. fin.
n_ Hn fin
Linda ne^
7.7-3
[
61 1 A
1 '
•* 2,4,5-T L
""(BE)
Parathion
DDT
1. L
-
„ -
R
60 120 180
Water Throughput, 1000 gal
240
Figure 3. Passage of pesticides through carbon beds. The clear bars represent the
initial stage, the cross-hatched bars, the result after passage through
one bed; and the solid bars, the result after passage through the second
bed16.
Other design parameters were investigated at the Nitro plant. It was found that de-
creasing the particle diameter of the carbon increased bed service time, but that
changing the velocity of the water through the bed, i.e. the application rate, had little
effect (provided that the bed is lengthened so that contact time remains constant).
Once again, sorption principles, which were being studied at the time by Weber and
Morris ° and Weber and Rumer21, explained these observations. Their research
showed that sorption rate is often controlled by diffusion of the contaminant within
70
-------
10 15 20
Duration of Operation, weeks
25
30
Figure 4. Effect of time on removal of carbon chloroform-extractable materials
The stippled area denotes removal by adsorption; the black, passage by
desorption18
10
c
E
c
o
O
o>
o>
CO
I
D Pilot Plant Experiment 1
A Pilot Plant Experiment 3
Large Carbon Filters:
O Bed No. 9
• Bed No. 10
All Have Standard
8 x 30 Mesh Carbon
O
8 12 16 20 24
Length of Operation before Threshold
Odor Number Exceeded 2 Days
28
32
Figure 5. Effect of contact time on length of operation.19
the extensive internal pore structure of carbon particles. Internal diffusion, as it is
termed, is therefore related to particle diameter. A smaller particle will promote
faster adsorption, all else being equal, because the diffusional path length is shorter.
Application rate, on the other hand, is unimportant because it only affects the rate at
which the contaminant arrives at the external carbon surface and this in not the rate
limiting step.
There were also differences in bed service time observed for each of the contam-
inants being removed by G AC at the Nitro plant. This suggested differences in sorp-
tive behavior among these contaminants. The work of Weber and Gould15 in the
mid-1960's focused on quantifying such effects for related organic pesticides,
detergents and phenol. As Table 3 indicates, they found notable differences in ad-
sorption equilibrium. In this case the very well-known equilibrium model developed
by Langmuir was used to quantify the maximum adsorption capacity, Xm, which
71
-------
Table 3. Comparison of Langmuir Equilibrium Constants for Organic
Pesticides and Selected Adsorbates16
Compound
2,4-D*
2,4,5-Tf
Silvex
DNOSBPJ
DNOCHP§
Parathion
Phenol
Sulfonated 2-dodecylbenzene
1 -Chloro-4-nitrobenzene
*m
mg/gram
387
448
464
444
500
530
103
139
400
b
liters/mg
0.43
0.58
0.54
072
0.55
4.17
1.15
12 82
4.54
*2,4 dichlorophenoxyacetic acid
f2,4,5 trichlorophenoxyacetic acid
tdmitro-o-sec-butylphenol
§dmitro-o-cyclo-hexylphenol
could be obtained and the partitioning of the contaminant between the solid adsor-
bent and the aqueous solution. The higher the value of the constant b in Table 3, the
more strongly is the contaminant held to the adsorbent, even when it is present in
very low concentrations.
A bridge, weak to be sure, was being built in the 1960's between adsorption theory
and practice. Many questions were being raised simply by observing GAC bed per-
formance at the Nitro plant and at other pilot plants. These necessitated a return to
more fundamental studies to gain a better understanding of the process itself.
Use of Mathematical Models
The need for a mathematical model derived from principles underlying the
adsorption process became increasingly apparent. However, the use of this device to
predict the effluent concentration-time profile, i.e. the breakthrough curve, was vir-
tually unknown in water treatment studies until Keinath and Weber22 in 1968 pre-
sented their adaptation of principles hitherto applied in the chemical engineering
field. The model serves several purposes: it facilitates convenient investigation of the
effects which different parameters may have on the process without resorting to time
consuming and expensive pilot plant tests; it also provides a guide to system design.
As Fig. 6 illustrates, Keinath and Weber22 were actually quite successful in verify-
ing their model using laboratory data obtained from GAC bed operation. Their
model required that input data for adsorption equilibrium and rate be described by
submodels. The output of the model could be used to show the effects of application
rate , particle diameter, empty-bed contact time and feed concentration on the shape
of the breakthrough curve (in this model, application rate is important because ac-
count is taken of both external and internal diffusion). Such knowledge provides a
basis for design of the most efficient contactor system.
72
-------
1.0
0.9
-2 0.8
n
QC
6 0.7
c
o
~ 0.6
o>
0.5
* 0.3
0.2
0.1
O O Experimental Breakthrough Data
Predicted Profiles
J I L
0 500 1000 1500 2000 2500 3000
Time, hours
Figure 6. Concentration-time profile for a flow rate of 2 gpm/sq ft (81 41 /min/sq m)
in a 250-g fluid bed of 0.71 11 -mm carbon Sorbate is dmitro-
o-secbutylphenol (DNOSBP)22
Another form of mathematical modeling was proposed by Dostal, et al.23 to ob-
tain the best process control strategy and the design to achieve it. With only limited
information concerning sorption equilibrium and rate, their model predicted the
seasonal demand, i.e. the amount of GAC required as a function of time of year,
given knowledge of the seasonal pattern of taste and odor encountered. The model
also predicted the required rate of GAC regeneration which determined the size of
regeneration system required. Estimates of process costs, while possible, were of
limited use in the absence of reliable information on how taste and odor causing
compounds are removed by GAC.
More complete descriptions of adsorption process dynamics presented in the
1970's covered the effects of contaminants competing for sorption sites, time depen-
dent inlet concentrations and, simultaneous microbial degradation on the sorbent
surface. These will be discussed later in more detail.
Factors Influencing Adsorbability
The most common way of describing adsorption of specific contaminants is by
measuring the equilibrium distribution between the solid (adsorbent) and liquid
(aqueous) phases. This distribution, or partitioning, is most often referred to as the
adsorption isotherm, to acknowledge the fact that equilibrium adsorption is
temperature dependent, increasing with decreasing temperature. Many such iso-
therms were developed in the 1960's for the alkylbenzene-sulfonates14 (the essential
component of detergents) and the organic pesticides15. These early isotherms were the
forerunners for many more which were prepared in the 1970's as the catalog of
organic contaminants continued to grow.
73
-------
In its study of adsorption technology, which was supported by contract with the
EPA, a subcommittee of the Safe Drinking Water Committee of the National
Academy of Sciences (NAS) emphasized the wide range of adsorbabilities which
have been measured24. As is shown in Fig. 7, the higher the surface (adsorbent phase)
concentration, the better adsorbed is a given contaminant. It is also apparent that
some of the notable halogenated aliphatic, trace contaminants of concern such as
chloroform, bromodichloromethane, carbon tetrachlonde, and trichloroethylene
are poorly adsorbed in comparison to the aromatics, such as 2,4,6 trichlorophenol,
which are more typical of pesticide derivatives. The widespread use of gas chro-
matographic analysis in the 1970's did much to expand the library of adsorption iso-
therms and to extend adsorption data down to the very low concentration of interest,
i.e. 10~6 to 10~9 moles per liter (see Fig. 7). The lack of adsorption data in the low con-
centration range for most organic pollutants, however, was still of concern to EPA
when it published the 1978 guideline report on GAC treatment25.
Measuring the adsorbability of potential chemical carcinogens was a high priority
of the 1970's. Adsorption tests, however, could be conducted only at the relatively
few laboratories which were equipped to store and handle chemical carcinogens. A
large fraction of such adsorption data was collected in an EPA-sponsored project at
the Illinois Institute of Technology Research Institute26. Although there were ex-
ceptions, most compounds were found to be adsorbable, even at very low
concentrations. The extent of adsorption, however, was quite variable, as might be
expected from the large differences in physical and chemical properties among
various classes of carcinogens; this variability is illustrated by the data presented in
Table 4.
With most of the emphasis placed on data collection, there was a lag in sorting out
general rules which could account for the many physical and chemical factors af-
fecting adsorbability. As the list of organic contaminants grew, it was apparent that
a predictive capability derived from such general rules could be of very practical
use. The recent contributions by McGuire and Suffet27, Manes28, Belfort29 and
Weber and van Vliet , are quite significant in this respect. Their work returned at-
tention to the fundamental physical and chemical principles which govern the inter-
actions between the adsorbent and the contaminant, the adsorbent and water, and
the contaminant and water.
The approach taken by McGuire and Suffet27 derives from the principles of liquid-
solid chromatography and provides for calculation of a thermodynamically based
term referred to as net adsorption energy. This term is obtained from the solubility
parameter which accounts for interactions among adsorbent, contaminant and
water. The intent is to calculate the energy associated with transferring the contami-
nant from the polar solvent, water, to what is described as the non-polar surface of
activated carbon; there is, however, considerable debate as to the importance of
polar functional groups on GAC30. Aqueous solubility (which depends upon hydro-
gen bonding) and refractive index (which depends upon non-polar dispersive forces)
are both important.
To test the net adsorption energy concept, McGuire and Suffet plotted experi-
mental adsorption equilibrium data for 13 contaminants against their corresponding
calculated net adsorption energies (Fig. 8). These results were encouraging in that a
positive correlation seemed to emerge upon which the adsorption of other contami-
nants could be based. The distribution of calculated net adsorption energies for over
70 compounds identified in the Philadelphia raw water supply31 is shown in Fig. 9.
Highest energies were associated with the aromatics, e.g. substituted phenols and the
lowest with the low molecular weight, chlorinated aliphatics, e.g. chloroform. If the
concept put forth by McGuire and Suffet holds, the expected adsorbability of these
organic compounds should range rather widely. Acceptance of this concept, how-
ever, must await further testing.
A group of researchers led by Manes28 revived the very old concept proposed orig-
inally by Polanyi to explain the adsorption of gases. Known as the adsorption
74
-------
sz.
Surface Concentration, moles/g
(Q
c
-D
— 6
T3 3
2" 5
IT o
SL5
ffl CO
Q. w
Q. 01
O •=:
li
Z £
o •?
O 13
3 =•
.
0} CU
* »
-------
Table 4. Summary of Carbon Adsorption Capacities Measured for
Suspected Chemical Carcinogens26
Compound
bis(2-Ethylhexyl)
phthalate
Butylbenzyl phthalate
Heptachlor
Heptachlor epoxide
Endosulfan sulfate
Endrm
Fluoranthene
Aldrin
PCB-1232
beta-Endosulfan
Dieldrin
Hexachlorobenzene
Anthracene
4-Nitrobiphenyl
Fluorene
DDT
2-Acetylammofluorene
alpha-BHC
Anethole*
3,3-Dichlorobenzidine
2-Chloronaphthalene
Phenylmercunc acetate
Hexachlorobutadiene
gamma-BHC (Imdane)
p-Nonylphenol
4-Dimethylaminoazobenzene
Chlordane
PCB-1221
DDE
Acndme yellow*
Benzidme dihydrochlonde
beta-BHC
N-Butylphthalate
N-Nitrosodiphenylamme
Adsorption*
Capacity,
mg/g
1 1 ,300
1,520
1,220
1,038
686
666
664
651
630
615
606
450
376
370
330
322
318
303
300
300
280
270
258
256
250
249
245
242
232
230
220
220
220
220
Adsorption*
Capacity,
Compound mg/g
Phenanthrene
Dimethylphenylcarbinol*
4-Aminobiphenyl
beta-Naphthol*
alpha-Endosulfan
Acenaphthene
4,4'Methylene-bis-
(2-chlroroanilme)
Benzo(k)fluoranthene
Acridine orange*
alpha-Naphthol
4,6-Dmitro-o-cresol
alpha- Naphthy la mine
2,4-Dichlorophenol
1 ,2,4-Tnchlorobenzene
2,4,6-Tnchlorophenol
beta-Naphthylamme
Pentachlorophenol
2,4-Dmitrotoluene
2,6-Dmitrotoluene
p-Nitroaniline*
1,1-Diphenylhydrazme
Naphthalene
1 -Chloro-2-mtrobenzene
1 ,2-Dichlorobenzene
p-Chlorometacresol
1 ,4-Dichlorobenzene
Benzothiazole*
Diphenylamme
Guanine*
Styrene
1 ,3-Dichlorobenzene
Acenaphthylene
4-Chlorophenyl phenyl
ether
Diethyl phthalate
215
210
200
200
194
190
190
181
180
180
169
160
157
157
155
150
150
146
145
140
135
132
130
129
124
121
120
120
120
120
118
115
111
110
'Adsorption capacities when equilibrium fluid phase concentration is 1 mg/L.
potential theory, it assumes a force of attraction to the adsorbent phase which is in-
versely proportional to the available unfilled adsorption space. The model considers
only London forces of attraction, which are physical in nature. As such, it does not
consider the interaction between sorbate, i.e. the contaminant being adsorbed, and
specific functional groups on the adsorbent surface; this interaction is referred to as
chemi-sorption. In reviving the Polanyi adsorption potential theory, then, Manes
rekindled the great debate over the importance of physical adsorption forces vis a'
vis chemical interactions with the surface32.
76
-------
TO'3
3
13
C
3
O
Q.
E
<3
10-"
10-6
10-6
Methyl Ethyl Ketone-
O Nitromethane
1,4 Dioxane
Propionitrile
2-Propanol
p-Chlorophenol
p-Nitrophenol
p-Chlorophenol
p-Cresol
Acetone
OUrea
Note Different symbols
denote different studies
compiled by author.
5 678910
Net Adsorbtion Energy, kcal/mol
Figure 8. Relationship between the amount of compound adsorbed by activated
carbon (at an equilibrium concentration of 10"3 mol/L) and the net adsorption
energy27.
-------
30
i
o
c
o
a
o
O
15
09
.a
3
10
2.5 5.0 7.5 10.0 12.5 15.0
Net Adsorption Energy, kcal/mole
Figure 9. Distribution of net adsorption energy calculated for organic compounds
identified in Philadelphia's water supply source. Such a wide distribution
seems reasonable based on the wide range of organic compounds present.
Highest energies were associated with aromatic compounds and lowest
with low molecular weight, halogenated aliphatics(After McGuire)31.
To adapt the theory for describing adsorption from the aqueous as opposed to
gaseous phases, Manes took account of solubility (which replaces vapor pressure in
the Polanyi model), molar volume (already included by Polanyi) and refractive in-
dex (a measure related to polarizability of the sorbate). This last factor represents the
truly original contribution of Manes to the theory. Together, these three factors can
be used to "scale" the adsorbability of any given contaminant (on a specific adsorb-
ent) once the adsorption isotherm of just one reference sorbate is measured. Put in
other words, it should be theoretically possible to "collapse" adsorption isotherms of
different sorbates into one single "characteristic curve" by adjusting for the three
scale factors.
The modified Polanyi model is similar in some respects to the net adsorption ener-
gy concept applied by McGuire and Suffet in that solubility and refractive index are
included. However, the modified Polanyi model describes the entire shape of the
adsorption isotherm, not just a single adsorption point. Its drawback is that calcula-
tions become highly involved when the sorbate is readily miscible in water and solu-
bility no longer influences adsorption. An attempt was made to make practical use of
the model in predicting adsorbability of trace organic contaminants in EPA-spon-
sored pilot plant studies at the Preston Water Treatment Plant in Hialeah (Greater
Miami), Florida, hereafter referred to as the Miami, Florida Water Treatment
Plant33. In this study, however, it was shown that the observed sorptive capacity for
each of five volatile, chlorinated organic compounds was much less than predicted
78
-------
from the Polanyi model. Both competitive adsorption and errors in the Polanyi
predictions were likely to have caused the noted discrepancies.
Belfort29 has taken still another thermodynamic approach to predicting adsorb-
ability which in this case derives from the field of reverse phase chromatography.
The basis for his approach is the solvophobic theory which is used to describe the
effect of the solvent (water) on the association reaction between the solute and the
sorbent. This theory assumes the formation of a cavity in the solvent in order to ac-
commodate the solute and the interaction of the solute with the solvent and the sor-
bent. In work sponsored by the EPA, he has explained how the theory may be simpli-
fied to allow its practical application. Using adsorption equilibrium data available
from the literature, Belfort was able to use the theory to show the importance of
both polar (caused by hydrogen bonding) and steric (caused by structural branching
of the sorbate molecule) effects. Most interesting is his attempt to correlate adsorb-
ability with just one factor, molecular surface area34. Molecular surface area relates
to the free energy associated with the formation of a "cavity" adjacent to the sorbent
surface. As is shown in Fig. 10, Belfort achieved reasonable correlations between
amount adsorbed (Qm) and the hydrocarbonaceous surface area (HSA) for several
families of polar organic contaminants, e.g. acids, alcohols, aldehydes and ketones.
Most recently, Weber and van Vliet30 took another approach which also draws
upon the adsorption potential theory but in a different way than that described by
Manes. In their research, the adsorption potential theory permits both the adsorbent
and adsorbate characteristics to be examined quantitatively. Manes had been
interested only in the latter.
Using molecular polarity of the test organic compounds as a comparative para-
meter, Weber and van Vliet were able to show that specificity of sorption sites was
important for activated carbons but not for polymeric resin adsorbents. A much
stronger adsorbent-adsorbate affinity is therefore produced in activated carbon ad-
sorption. This is especially important in the very low concentration range of interest
in water treatment where low solid phase loadings suggest intimate surface inter-
actions. This affinity makes activated carbon a better adsorbent than polymeric
resins; however, so-called carbonaceous resins (to be discussed later) were shown to
have sorption affinities somewhat similar to activated carbon.
PROCESS DEVELOPMENT
The Case of THM's and Humic Substances
Much attention was given in the 1970's to the removal of low molecular weight,
halogenated, aliphatic compounds such as members of the trihalomethane (THM)
family, carbon tetrachloride, dichloroethane, tri-and tetrachloroethylene, and vinyl
chloride. Pilot plant studies showed that many of these compounds began breaking
through GAC beds after only one to two months of operation: illustrative break-
through curves of chloroform at Philadelphia, Pennsylvania35, total THM's at
Cincinnati25 and dichloroethane at Jefferson Parish, Louisiana36 are given in Figs.
11-13, respectively. In removal of these contaminants, the service times are very
short in comparison to those experienced when removing taste and odor or pesti-
cides. This result might be predicted by examining adsorption equilibrium capacities
given in Fig. 7.
Practical considerations limit the size of GAC contactors and thus, the empty bed
contact time. Normally, replacement of a filter media with GAC gives the minimum
acceptable EBCT of about five minutes. Construction of post filter adsorbers implies
extension of EBCT to perhaps 20 to 25 minutes (with two beds in series). EPA pilot
plant studies in Miami, Florida33 provided experimental evidence of the sorption
principle which states that longer service times are obtained as EBCT is increased
(Fig. 14). In this figure service time is defined as the time for the concentration of
79
-------
Functional Group, R"
CHJ C2Hs
C4H9
30
20
6
TJ
0)
_a
5
c
3
O
E
O
10
Alcohols
• R'-OH (linear)
A R-OH (branched)
I
80 100
150
200
250
300
HSA, A 2
Figure 10. Plot of the logarithm of the extent of adsorption versus hydrocarbonaceous
surface area for aliphatic alcohols, ketones, aldehydes, and acids as
per the simplified solvophobic model34.
each THM in the G AC bed effluent to reach 0.5 jug/ L- Extending the EBCT from five
minutes to 25 minutes provides a service time that is still less than three months in the
case of chloroform removal. A longer service time would be the case if the full ad-
sorption capacity of the bed were to be used. Regardless of the definition of service
time employed, a much greater frequency of regeneration would be required for
THM removal than for removal of taste and odors or pesticides. These findings
prompted research on the effectiveness of alternative adsorbents, including synthetic
resin materials; these will be discussed later.
Humic substances are naturally occurring, amorphous, macromolecules ranging
in molecular weight from a few thousand to greater than 100,000. Their exact iden-
tity is still a matter of some speculation although certain functional groups are very
well known. It is now well established that these humic substances react with chlorine
to produce THM's and other chlorinated compounds. Therefore, one obvious func-
tion of adsorption would be to remove them before chlorination.
Because humic substances are formed in nature, they reflect the organic matter in-
digenous to a given location. Therefore, differences in their composition, reactivity
with chlorine, and adsorbability can be expected. This was shown in EPA-sponsored
work of McCreary and Snoeyink37. Wide differences in the adsorbability of a com-
mercially supplied humic, fulvic acid extracted from leaves, and fulvic and humic
acid fractions extracted from soils were apparent (Fig. 15). In each case, total organic
carbon (TOC) was used as the surrogate parameter to measure the humic substances.
Upon chlorination, these humic substances also produced different amounts of
80
-------
120
100
80
o>
5 60
o
40
20
10 20 30 40 50 60 70 80 90 100
Time in Operation, days
Figure 11. Chloroform breakthrough curve for filter-adsorber with EBCT = 15 min.
at Philadelphia35.
chloroform. Another interesting finding was the variability in adsorption of the var-
ious molecular weight fractions derived from a soil fulvic acid (Fig. 16). The larger
molecular weight fractions were clearly less easily adsorbed (as measured by adsorp-
tion of TOC associated with each fraction). This was explained as being due to either
molecular exclusion from pores or limitations in adsorption rate which prevented
attainment of equilibrium in the time allotted. The results of research on adsorption
of large molecular weight substances suggested the importance of pore size distri-
bution of the adsorbent. This was proven nicely in follow-up studies by Lee and
Snoeyink38 and Lee, et al.39. Activated carbons varying widely in pore volume and
pore size distribution were used to adsorb different molecular weight fractions of
humic substances (Fig. 17). The results confirmed that assumption that adsorbability
increased linearly with the pore volume contained in selected pore size ranges. Ad-
sorption of the larger molecularjveight fractions of the humic substances was related
specifically to pores 100 to 500 A in size, wheiyas adsorption of the smallest fraction
was related to the micropores (less than 70 A in size).
Lee and Snoeyink38 also stressed the importance of coagulation as a pretreatment
for GAC where humic substances are to be adsorbed: with such pretreatment, they
noted an increase of fivefold in adsorption capacity and an increase in adsorption
rate. Removal by coagulation of some poorly adsorbed fractions is only partly re-
sponsible for this improvement; there is also an adsorptive interaction between alu-
minum ions, used as the coagulant, and the humic substances. This conclusion was
reached independently by Weber, et al.40 and by Randkte and Jepsen41. Weber, et al.
suggested the formation of an ion-humate-carbon complex. The impact of inorganic
ions on the removal of a humic acid by GAC was illustrated vividly by comparison of
the breakthrough curves obtained when tap water was used instead of distilled de-
ionized water to prepare the feed solutions (Fig. 18). The tap water solution, which
contained significant amounts of Ca, Me, Fe, and OC1 , produced much better re-
moval of the humic acid. Weber, et al.42' 3 also showed that the adsorption of some
organic molecules, such as the PCB's and dieldrin, may be enhanced by association
with humic substances.
81
-------
150
100-
x
CD
=1
1
I
W
10 20 30 40 50 60 70 80 90 100 110
Time in Operation, days
Figure 12. Total THM breakthrough curve with EBCT = 10 min at Cincinnati25
_ 30 -O-O Adsorber Influent (Sand Filter Effluent)
20 40 60 80 100 120 140 160 180
Time in Operation, days
Figure 13. 1,2 DCE breakthrough curve for post filter adsorber (EBCT = 20 mm)
at Jefferson Parish, LA36.
82
-------
130
120
110
100
90
-g 80
70
60
50
40
30
20
10
0
1
Dibromochloromethane
Cis 1,2-dichloroethene
Bromodichloromethane
Chloroform
TOC
10
15
EBCT, min.
20
25
30
Figure 14. Relationship between bed service time and EBCT for several poorly
adsorbed specific compounds and for background TOC33.
Lee, et al.39 have been successful in formulating and verifying a mathematical
model of breakthrough. Their studies revealed that the diffusion of humic sub-
stances within the carbon's internal surface is very slow in comparison to other con-
taminants of concern; a similar result was reported by Weber, et al.42. This is un-
doubtedly due to the macromolecular character of these substances.
Because the type of activated carbon is so important in estimating sorptive capac-
ity of humic substances (Fig. 17), it should also greatly influence the GAC service
time. This was illustrated in the model predictions of Lee etal.39 (Fig. 19a). Pretreat-
ment by chemical coagulation has practical implications as is very well illustrated by
comparing the breakthrough curves predicted by Lee, et al. before (Fig. 19a) and
after coagulation (Fig. 19b). The time scale over which complete breakthrough oc-
curs with pretreatment is an order magnitude longer (80 days) than without pretreat-
ment (ten days).
One practical measure of the removal effectiveness of humic substances by GAC is
the reduction in trihalomethane formation potential (THMFP). TheTH MFP is cal-
culated as the increase in trihalomethane (THM) concentration which occurs upon
reacting chlorine with the water sample for a specified time (usually two to seven
days). Because GAC is expected to adsorb the precursors to THM, a reduction in
83
-------
100
en
c"
o
ID
I10
O
c
o
o
0)
o
10
Leaf Fulvic Acid -
Commercial HumicAcid
-Soil Humic Acid
Soil Fulvic Acid
pH = 7.0
PO4 = 0.001 M
0.01
0.1 1 10
Equilibrium Concentration, mg/l As TOC
100
Figure 15. Adsorption of various types of humic substances37.
100
ra
X,
O>
c
o
I 10
c
5000
MW > 50,000
0.01
0.1
1
10
100
Equilibrium Concentration, mg/l As TOC
Figure 16. Adsorption of molecular weight fractions of soil fulvic acid37.
THMFP should be measured when comparing influent and effluent of the GAC bed.
This should also be qualitatively related to the removal of TOC across the GAC bed
because TOC and THMFP generally reflect the presence of humic substances.
To show that mathematical predictions of breakthrough of humic substances, as
measured by either TOC or THMFP, can in fact be quite useful, EPA pilot plant
data from Jefferson Parish, Louisiana36 on the breakthrough of THMFP was com-
pared with results generated by Lee, et al.39. While the sources of the humic sub-
stances are of course completely different, the similarities in influent TOC, contact
time and carbon type (actually the same) nevertheless justify the comparison. Fig. 20
shows that the TH M FP rises steadily in the pilot plant study to a value of 100 jug/ L
after about 70 days. Lee, et al.39 had correlated removal of TOC with reduction of
TH M FP in the laboratory and concluded that the value of 100 jug/ L TH M FP would
84
-------
0.3
o>
* 0.2
0)
_D
O
0.1
12
18
—i—
24
30
—r—
36
—i—
Pore Radius <70 A
Pore Radius 100-500 A
'ore Radius 100-500 A
O CHA
A PFA
O PFA (>50,000 MW)
V PFA « 1 .OOOMW)
4 6 8 10
Adsorption Constant K, mg/g
12
0.6
0.5
0.4
0.3
0.2
0.1
Figure 17. Adsorption constant of humic substances as a function of pore volume
within a certain range of pore radii of carbon. Nine different carbon
types were tested. CHA refers to commercial humic acid; PFA to peat
fulvic acid; and PFA (250,000 MW) and PFA (<1000 MW) to specific
molecular weight (MW) fractions of the peat fulvic acid38.
1.00
DDW Background
a a
90
120 150 180 210
Time, hours
Figure 18. Effect of inorganic ions present in tap-water on the breakthrough profile
for humic acids in granular carbon adsorption columns. DDW refers
to distilled, de-ionized water40.
85
-------
1.0
o 0.8
c
.0
S 0.6
1
o
O
*-
c
0)
3
0.4
0.2
I I I T
Carbon Type: WV-G
Carbon Type: F-400
- Carbon Type: HD-3000
i i i r
Carbon Type: WV-W
Influent Concentration = 3.56 mg/l TOC
EBCT= 18.85 min
456
Time, days
10
(19a)
o
c
o
c
at
o
c
o
O
_
it
UJ
1.0
0.8
Influent Concentration = 3.56 mg/l TOC
EBCT = 18.85 min Carbon Type: WV-W.
Carbon Type: HD-3000 -
-Treatment Objective, 2.75 mg/l TOC.
Carbon Type: WV-G
i i
Carbon Type: F-400
0.2
Figure 19a and 19b. Model predictions of breakthrough curves for peat fulvic acid
on different brands of activated carbon (a) before, and (b) after
alum coagulation. Extent of TOC removal is shown normalized
to influent TOC38.
be reached when effluent TOC was 2.75 mg/L, or in about 60 days (see Fig. I9b).
This is remarkably close to the time to reach 100 /ug/LTHMFP actually observed at
Jefferson Parish.
An interesting feature of the breakthrough curve for terminal TTHM in Fig. 20 is
the attainment of a plateau value which is significantly less than the influent value.
86
-------
0.30
ob 0.25
E
2 0.20
| 0.15
I 0.10
0.05'
Influent/ °\
'
20 40 60 80 100 120 140 160
Days of Run
180
Figure 20. Performance of terminal TTHM through post filter adsorber (EBCT = 24
min) at Jefferson Parish, LA36
This has also been observed in other studies and explained as being due to biodeg-
radation of humic substances. Weber,40'43 in EP A-sponsored research, showed that
biological activity must be considered for accuracy in predictions of humic sub-
stance breakthrough. M icrobial degradation on adsorbent surfaces will be explored
in more detail later.
Competition for Adsorption Sites
Competition for adsorption sites occurs when two or more contaminants are pres-
ent in the feed to the GAC bed. The consequences of this phenomenon are that (1)
sorptive capacity for each component is less than expected in the absence of com-
petition, and (2) concentrations of some contaminants may actually be higher after
GAC treatment than before. Sorting out of these effects has been a very difficult
problem when analyzing the performance of pilot plants. This is understandable
because raw waters contain so many different components in highly variable concen-
trations. Nevertheless, there was substantial progress in the 1970's in developing a
better concept of how competition occurs in less complex mixtures.
The phenomenon of competitive displacement will be discussed for the case of a
two-component system. The more poorly adsorbed component of the two exhausts
sorption sites more rapidly and thus its breakthrough is expected first. As the active
sorption zone advances down the bed, a region develops in which dynamic re-equil-
ibration is occurring between the two components. Competitive equilibrium leads to
displacement of some molecules of the more poorly adsorbed component which, as
the adsorption zone finally exits the bed, manifests itself as a rise in effluent concen-
tration temporarily above that of the influent.
There were numerous studies of two, and three component competition in GAC
beds throughout the 1970's. An example is given in Figs. 21 a and 21 b for the case of
p-nitrophenol -p-chlorophenol mixture44. It can be seen that as bed length (z), and
thus contact time, increases, displacement of the more poorly adsorbed p-chloro-
phenol causes higher peak concentrations and a broadening of the band in which ef-
fluent exceeds influent concentration. The data show that while increasing contact
time lengthens the service time of the bed, it also accentuates the effect of
competition. The displacement effect may not be as noticeable in mixtures contain-
ing components differing widely in diffusivities. It may also be dampened when more
than two components are competing.
Obviously, it would be counter-productive if the effluent of a GAC bed contained
higher concentrations than the influent, particularly when maximum contaminant
87
-------
(21a)
100
200 300
Time, hours
400
500
2.0r
1.5
6
c
o
o
£ 1.01
te
•
0.5
p-chlorophenol (PCP)
(21b)
100
200 300
Time, hours
400
500
Figure 21 a and 21 b.
Calculated (solid lines) and experimental (data points)
breakthrough curves for PNP and PCP, respectively, in the
PNP/PCP system at different bed depths Mass transfer
controlled by film diffusion and competitive equilibria described
by the IAS model44
levels may be enforced. However, there have been very few pilot plant studies where
the displacement effect could be observed. The most notable cases have involved the
poorly adsorbed TH M 's and other low molecular weight, chlorinated hydrocarbons
such as vinyl chloride25. But even in these instances, a sudden decrease in influent
concentration could have produced the same effluent concentration pattern. Draw-
ing the distinction between the effects of fluctuating influent concentration and com-
petitive adsorption is a common problem in interpreting pilot plant results.
88
-------
Another manifestation of competition is the domination of adsorption sites by
just one component. For example, Suffet, et al.45 reported that an accidental spill of
toluene in the Philadelphia raw water supply resulted in no adsorption of 30 other
contaminants present in much lower concentrations. Following the spill event, these
compounds here again adsorbed; still later, changing feed concentrations of various
components caused another re-equilibration and it was accompanied by desorption
of several components, including toluene.
The overwhelming portion of organic matter is composed of humic substances
which might compete with trace contaminants for'adsorption sites. One interesting
example of competition is that between humic substances and odor causing com-
pounds. The early work of Rosen, et al.46 at the EPA identified specific organic com-
pounds which were responsible for odors, namely methylisoborneol (MIB), a metab-
olite of actinomycetes, and geosmin, a metabolite of actinomycetes and blue green
algae. Herzing, et al.47 showed how these two compounds, when present in trace
amounts, are adsorbed in competition with much higher concentration of humic
substances. Although some competition did occur, the typical displacement effect
was not observed. In fact, once MIB was adsorbed, it was very difficult to desorb.
These observations helped to explain why GAC beds have been so successful in re-
moving taste and odor causing compounds which are well adsorbed and present in
such very low concentrations as to be relatively unaffected by competition.
Competitive adsorption of carbon tetrachloride, which is poorly adsorbed, and
humic substances provides a contrasting example. In EPA studies, Weber et al.43 ex-
amined the breakthrough curve with and without competition. Carbon tetrachloride
was first adsorbed from distilled water until complete breakthrough occurred (Fig.
22). Then, distilled water alone was fed resulting in significant desorption of carbon
tetrachloride. Subsequent to the distilled water feed, a humic acid feed was intro-
duced. This produced the expected rise in carbon tetrachloride concentration as a re-
sult of competitive displacement. In a similar experiment involving removal of the
much more strongly adsorbed component, dieldrin (pesticide), Weber, et al.43 ob-
served little desorption or competitive adsorption in the presence of humic sub-
stances. The point here is that the energy of adsorption associated with the trace con-
taminant controls the extent to which displacement occurs. In the case of strongly
adsorbed MIB and dieldrin, for example, competition may not be important.
It has already been discussed that adsorbability of humic substances depends
upon their source. This characteristic applies to competitive adsorption with trace
contaminants as well. Murin and Snoeyink48 found notable differences in the extent
of competition depending upon the source of humic substance. In the presence of
each of these humic substances, adsorption of the trace contaminant, 2,4,6 trichloro-
phenol was reduced somewhat differently, but in general on the order of 50 percent
due to competitive effects.
The research team at the University of Karlsruhe in Germany, led by Professor
Sontheimer, has been seeking a practical way of evaluating competitive adsorption
between background organic matter, i.e. humic substances, and trace contaminants.
In their approach, they have found that flocculated lignin sulfonic acid simulates the
sorptive behavior of the background organic matter and serves as a convenient
model compound49. Competition with trace contaminants is simulated by addition
of paranitrophenol. Mathematical modeling of equilibrium adsorption gives a prac-
tical method of comparing the effectiveness of different activated carbons under this
test condition.
A reasonably solid foundation for understanding of competitive adsorption
evolved in the 1970's with the aid of mathematical modeling. Research teams in the
United States and Europe, working on this problem from different directions,
seemed to reach some sense of agreement on the principles involved.
Mathematical models of competitive adsorption developed by different groups
share a common framework. The need for a competitive equilibrium model is fore-
most in importance. The dynamic aspect of GAC bed operation further requires a
89
-------
0.30
200 ppb Carbon Tetrachloride
Distilled Water
5 ppm
Humic Acid
E
&0.25-
c
-------
Several investigators have independently applied the IAS model. The best docu-
mentation is provided by, Fritz and Schluender55, Jossens, et al.56, and Singer and
Yen54. Despite the success claimed and the advantages over the Langmuir model,
there have been notable exceptions, too50'55'56'". One problem that seems to persist
is that components differing widely in molecular structure, such as 2,4 dichloro-
phenol and dodecylbenzenesulfonate, do not compete as expected. Crittenden and
Weber50 recognized this difficulty and decided not to adopt the I AS model for use in
dynamic column performance predictions.
Assumptions regarding sorption kinetics are also important. Many of the early
models considered the rate of transport of solutes to be limited by resistances offered
by the external, laminar film surrounding the carbon particles and by internal pore
diffusion. However, in the 1970's, there was evidence presented by several research
teams"'44'50 that surface, rather than pore diffusion was occurring in the sorbed
phase itself. This concept seemed better able to account for the influence of feed con-
centrations. Nevertheless, at least two groups (Crittenden and Weber50 and Fritz, et
al.44) recognized that even this model could not explain the interactions observed
for two components differing widely in diffusivity. In this case, the rate of adsorption
of the faster diffusing component is slowed and that of the slower diffusing compo-
nent accelerated. Only by adjusting diffusion coefficients artificially and somewhat
arbitrarily was it possible to predict breakthrough.
The sorptive affinity of trace contaminants relative to the background humic sub-
stances can be very important in determining the service time of the bed. Wilde58
adopted a dynamic, fixed-bed model of competitive adsorption to show this effect by
computer simulations. In his work, TOC was taken as the indicator of humic sub-
stances and acceptable service time as the time for the effluent to reach one percent of
the influent concentration of the trace contaminant. Fig. 23 shows quite logically
that as the sorptive affinity of the trace contaminant increases relative to that of the
TOC, i.e. as a increases, the service time increases. This is because the trace contami-
nant is able to compete much better with the TOC for sorption sites and is therefore
adsorbed to a greater degree.
One other important competitive effect is that produced by variable component
concentrations in the feed to the adsorbent bed. Weber and co-workers developed
MADAM (Michigan Adsorption Design and Applications Model) to handle this as
well as other problems of practical concern50' 9'60. Another model developed by
Balzli, et al.61 was used specifically to examine the effect of a sudden increase in the
influent concentration of one component on the time to the beginning of break-
through of each component. Increasing the concentration of the less strongly ad-
sorbed component accelerated its breakthrough, but delayed the beginning of break-
through of the more strongly adsorbed component. Practical application, however,
of models to predict the effect of discontinuities in influent concentrations to adsorp-
tion beds is very difficult because of the large number of competing components
which would have to be considered. The recent work of Thacker, Snoeyink and
Crittenden62 shows that at least for two component systems, the displacement effect
can be predicted quite well. Their results indicate, for example, that effluent THM
will remain above the influent concentration for many weeks if the influent THM
concentration is suddenly lowered.
Adsorption on Synthetic Resins
The relatively low sorptive capacities measured for low molecular weight, chlori-
nated compounds such as the THM's, carbon tetrachloride and trichloroethylene
(see Fig. 7) were of considerable concern in the 1970's because these specific sub-
stances may be targeted for removal. The practical consequence of poor adsorbabil-
ity is limited service times of GAC beds. This is well illustrated by the EPA-sponsored
pilot plant studies at Miami, Florida33, which showed that service time for several of
these contaminants was less than 100 days, even at very long contact times (see Fig.
91
-------
800
700
600
-c 500
400
w 300
1)
m
200
100
6 8 10
or Trace/or TOC
12 14
Figure 23. Bed service time vs ratio of trace to TOC background selectivities58
14). In addition, service time on the basis of TOC removal was also severely limited.
TOC is largely a measure of humic substances and must be removed in order to pre-
vent reaction with chlorine to form THM's.
Although activated carbon is an effective adsorbent for a broad spectrum of com-
pounds, it is clear from several pilot plant studies that its performance capabilities
can be unnecessarily underutilized if certain, poorly adsorbed contaminants must be
removed. Alternative adsorbents, therefore, were sought in the 1970's. Synthetic
resins were potentially attractive because design of resins with specific physical and
chemical properties could make them more selective adsorbents than activated
carbon. The possibility therefore existed that these resins could be used to comple-
ment the removal functions of activated carbon.
New resin materials did not fit the classical description of ion exchangers. Some
have porous, polymeric matrices composed of eitherstyrene-divinylbenzene(Rohm
and Haas XAD-2) or an acrylic ester (Rohm and Haas XAD-8). Others developed
more recently are designated as carbonized resins (Rohm and Haas XE 340, XE 347
and XE 348) and have chemical and physical characteristics approaching those of
activated carbon.
Polymeric resins were first used to concentrate and recover organic contaminants
for further chemical identification. Recovery is obtained by first adsorbing the con-
taminants on the resin and then desorbing them into an organic solvent. Weber and
van Vliet30, explained that polymeric resins should perform better than G AC for this
purpose because such resins do not bind the sorbed compound as tightly as does
activated carbon. This feature however, is not necessarily desirable when permanent
removal of the contaminant is the objective nor does it indicate better adsorption
properties
A pilot plant study of XAD-2 and activated carbon adsorbents was conducted in
Philadelphia. Sophisticated GC-mass spectroscopy monitored the removal of spe-
92
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cific contaminants across XAD-2 and GAC adsorbent materials. These results, re-
ported by Suffet, et al.63, did not show the resin to be superior to GAC. Many com-
pounds appeared much earlier in the effluent of resin than the GAC system; in ad-
dition, there was significant desorption of several other compounds.
The adsorption efficiency of carbonized resins for specific contaminants was a
great improvement over that of the polymeric resins. These resins are characterized
by less pore volume than activated carbon and with pore sizes in a much narrower
range than activated carbons; in particular, there are few pores greater than 300 A.
Internal surface area is also much less than found in activated carbon (350 M2/gm to
500 M2/gm vs 650 M2/gm to 1100 M2/gm). The XE 340, XE 347 and XE 348 resins
do differ in their pore size distributions; it is suggested that these differences provide
an opportunity for matching the resin with the treatment objective. Another distin-
guishing feature of resins is the availability of adsorbents with different hydro-
phobicities. XE 340, for example, has a very non-polar, hydrophobic surface which
favors adsorption of such compounds as the THM's.
An EPA pilot plant study64 in Cincinnati showed that XE 340 resin was still re-
moving THM's after 40 weeks (Fig. 24) whereas GAC was only effective for eight
weeks (see Figs. 11 and 12). A more detailed pilot plant evaluation in Miami, Flor-
ida, indicated that in addition to the THM's, dichloroethenes, trichloroethylene,
chlorobenzene and vinyl chloride were all more effectively removed by this resin33.
The amounts adsorbed were about three times greater than that for GAC treatment.
The advantage of resin adsorption was not as clearly shown, however, in other pilot
plant studies at Kansas City, Missouri65 which were sponsored jointly by the EPA
and the American Water Works Association Research Foundation. While XE 340
was a better adsorbent for chloroform than most types of activated carbons evalu-
ated, it was far less effective as an adsorbent for TOC.
Controlled laboratory experiments by Chudyk, et al.66 explained how XE 340
could in some cases adsorb chloroform better than activated carbons. It was rea-
soned that the large volume of small pores favors the adsorption of chloroform while
excluding humic substances. Activated carbons, on the other hand, can adsorb both
because of the much wider range of pore sizes available. This produces competition
which lowers the adsorption capacity with respect to chloroform. In the absence of
competition by humic substances, it was shown in fact, that some activated carbons
adsorb chloroform as well as do the resins.
One concept advanced was use of two different resin beds in series. The first would
remove the TH M precursors, i.e. the humic substances and the second, the low mole-
cular weight, halogenated organic contaminants. European research had indicated
that both strong and weak base ion exchange resins could remove humic substances.
For example, both Kolle67 in Germany and Rook68 in the Netherlands reported suc-
cess with strong base resins in pilot plant and full-scale studies. Later, Rook and
Evans69 switched to a more specific removal measurement, the THMFP test, and
showed that weak base resins could also be used. They indicated the practical ad-
vantage of these resins in terms of chemical regeneration efficiency; service time,
however, was still quite limited.
In studies sponsored jointly by the EPA and the American Water Works Associ-
ation Research Foundation, Boening et al.70 compared various resins for removal of
humic substances. As Fig. 25 indicates, the best adsorbent was a strong base, macro-
porous resin. However, because of the difficulty in regenerating this resin, these re-
searchers suggested use of a weak base resin instead although their results showed
that an activated carbon having a pore-size distribution favoring larger pores would
perform about as well as weak base resins. Pilot plant results as Kansas City, Mis-
souri65 and at Miami, Florida33 generally supported the argument that strong base
resins remove humic substances rather well. It was also apparent that some activated
carbons performed equally well. Though not specifically discussed in the pilot plant
reports, the carbons which performed the best were those having the most pore vol-
ume associated with the largest pores. This was reported independently in more care-
fully controlled, laboratory studies by Boening, et al.70 and by Lee and Snoeyink38.
93
-------
3
6
c
o
o
150--
125--
100--
75 -
50--
25--
TTHM, Column Influent
TTHM, Column Effluent
10
15
—1—
20
25
—l—
30
—i—
35
—i—
40
Time in Operation, weeks
Figure 24. Removal of trihalomethanes by Ambersorb® XE-340 (EBCT = 10 mm)64.
The question of when and how to use synthetic resins is open to debate. Carbon-
aceous resins may provide a solution when a few, specific contaminants must be re-
moved as might be the case in groundwater treatment. When humic substances are
important, as is the case for many surface waters, then GAC seems to perform about
as well as weak base resins. Much of the debate, however, centers about the econom-
ics and efficiency of synthetic resin regeneration.
Regeneration of Adsorbents
For adsorption to be economically feasible, the adsorbent must be regenerated
several times. The initial investment in the adsorbent is very large. For example, the
charge of GAC required to fill the contactors in a plant serving 75,000 people would
be on the order of 500,000 pounds at a cost of about 60 cents per pound. With service
time being from three to six months, it is obvious that the adsorbent cannot be used
on a throw-away basis.
Regeneration consists of (1) transporting the spent adsorbent from the contactor
to the regenerating facility; (2) removing sorbed organics from the sorbent structure
using either thermal reactivation, or in the case of resins, organic solvents, steam, or
brine; and (3) returning the reactivated adsorbent to the contactor. Efficient regener-
ation depends on minimizing the adsorbent losses during transport and regener-
ation, and restoring the virgin adsorbent properties, or nearly so. To be practical,
energy costs must be reasonable and by-products, such as the sludge from slurry
transport, must be easily handled.
Advances were made in regeneration technology in the 1970's, particularly for ap-
plication in water treatment plants. The economic necessity of on-site regeneration
was recognized in Germany7 and Switzerland72. Several plants installed what is re-
ferred to as a fluidized-bed furnace. This new type of furnace has several advantages
94
-------
ioor
s
I
CO
10
0.1
pH5
Strong Base Resin
A pH 9.5
• pH 7.0
O pH 5.5
Weak Base Resin
A pH 8.3
D pH 7.0
T pH 9.5
O pH 5.5
10~3 M Phosphate
Mil
Polymeric Resin
A pH 5.5
D pH 7.0
O pH 8.0
I I I il i l I
I nl
0.01
0.1 1
Equilibrium Concentration, mg/l
10
Figure 25. Commercial humic acid isotherms as a function of pH of weak base resins,
strong base resin and polymeric resin70
over the multiple-hearth furnace, which was so well known in the United States as a
result of advanced waste treatment research, e.g. at Lake Tahoe. In particular, the
fluidized-bed furnace is far less complicated, more compact, provides more rapid
heat and mass transfer and minimizes carbon losses due to abrasion.
European experience during the late 1970's was very encouraging. Results of
equilibrium adsorption tests at the Wuppertal water works of Germany after one,
three and five regeneration cycles are compared in Fig. 26 with those of fresh acti-
vated carbon73. In this plant, the practical parameter used to assess the effectiveness
of regeneration is the amount of ultraviolet (UV) absorbing material (A UV) which
can be removed by the activated carbon when added to the raw water in various dos-
ages. As seen, regeneration restored the carbon to its original effectiveness by this
measure.
A dynamic rather than equilibrium test of effectiveness was adopted at Dusseldorf,
Germany using measurements of the reduction in UV absorbing material across full-
scale GAC beds74. An empirical, semi-logarithmic relationship was developed
between the percent reduction in U V absorbance and the volume of water processed.
95
-------
t
0.020
0.015
0.010
0.005
Legend
1 - new activated carbon
2 - after first reactivation
3 - after third reactivation
4 - after fifth reactivation
10 30
Activated Carbon Dosage, mg
50
Figure 26. Adsorption isotherms after repeated regeneration.73
Fig. 27 is a typical result obtained for fresh and regenerated carbon, showing that
there was little difference in the rate at which adsorption efficiency decreased with
volume processed.
Although European data suggested that regeneration was effective, this issue had
not been addressed in the United States with theexception of one short-lived project
(employing a multiple-hearth furnace) at the Nitro, West Virginia facility. The need
to demonstrate regeneration in the United States was also sharpened by the lack of
information on effectiveness of G AC in removing specific contaminants after several
cycles. In response, three demonstration projects were initiated by the EPA at Man-
chester, New Hampshire, Passaic Valley, New Jersey and Cincinnati, Ohio75.
The fluidized bed is being used at Manchester and Cincinnati; the electric furnace
is used at Passaic Valley. The fluidized bed has already been discussed. The electric
furnace, a relatively new concept, utilizes infra-red energy for regeneration and is de-
scribed as (1) providing an alternative to use of scarce fossil fuels, and (2) enabling
start-up and shutdown without long warm-up and cool-down periods".
Only preliminary results are so far available from these United States projects and
the bulk of the data is from operation of the electric furnace installation at Passaic
Valley. Here, carbon has been regenerated successfully for two cycles76. In this case,
96
-------
100
50
c
o
Q.
O
tn
2
20
Activated
Carbon "F"
Reactivated
Virgin
024 6 8 10 12
Throughput, cm/I of activated carbon
Figure 27. Dynamic testing of regeneration effectiveness in full-scale testing of
GAC bed performance in Dusseldorf, West Germany74
success is defined in terms of the two traditional parameters used to characterize ac-
tivated carbon: the iodine number (indicating pore volume and surface area) and ap-
parent density (a reduction indicating removal of sorbed material), and by such re-
cently adopted parameters as reduction in TOC and total organic halogens (TOX).
Carbon losses were within acceptable limits, i.e. between 4 and 7 percent. Initial re-
ports from the fluidized-bed facility at Manchester also indicate success77. However,
carbon losses were about 11 percent and according to the traditional measures of ad-
sorbent effectiveness, the carbon was restored to near its virgin state. At both Man-
chester and Passaic Valley, important problems were encountered with transport of
carbon to and from the furnace; it is believed that as experience is gained these can be
overcome.
Two other studies in the United States have involved evaluation of the effective-
ness of regeneration, although they did not include on-site facilities. The
EPA/American Water Works Association Research Foundation project at Kansas
City, Missouri examined the ability of regenerated carbon to remove TOC and total
trihalomethanes (TTHM)65. There was little difference in performance of fresh, once
and twice regenerated carbons. Another interesting, although expected observation,
was that simply steaming the GAC bed routinely would be sufficient if the purpose of
regeneration was limited to restoration of TTHM removal capacity; in this case,
steam volatizes the TTHM's.
Similar studies were conducted at the Philadelphia pilot water treatment plant78.
However, here the emphasis was on the effect of the gasification temperature (the
last stage of regeneration in which the carbonaceous residue is oxidized) on the res-
97
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toration of virgin carbon properties. It had been feared that data derived from ear-
lier regeneration experience with carbon in wastewater treatment might not be use-
ful in selection of gasification temperature for regeneration of carbon used in water
treatment. In wastewater applications, the loading of adsorbed organic contami-
nants is far greater and thus the required gasification temperature may be much
higher. These water treatment studies indicated that the sorptive capability of GAC
with respect to chloroform was in fact impaired if the conventional gasification tem-
perature (1750° F) were used; a temperature of 1550° F gave better results. H owever,
if regeneration effectiveness were measured by TOC adsorption rather than by chlo-
roform adsorption, gasification temperature made little difference. It was suggested
that selective destruction of micropore space by high temperature may affect adsorp-
tion of chloroform but not of humic substances, which comprise a significant frac-
tion of TOC.
Experience with regeneration of synthetic resins is even more limited. Steam was
used effectively to regenerate the carbonaceous resin, XE 340, in Kansas City pilot
tests65. However, Chudyk, et al.66 were not able to remove sorbed methylisoborneol,
an odor-producing compound, with either steam or steam plus ethanol. Strong base
resins proposed for removal of humic substances can be regenerated with mixtures
of caustic and sodium chloride. Less caustic is required for weak base resins which
must still be rinsed with hydrochloric acid. As Boening, et al.™ point out, there are
still many questions to be answered regarding regenerant selection and ultimate dis-
posal of the waste brine.
Microbial Activity on Activated Carbon
An intriguing process explored in the 1970's was the combination of biodegrada-
tion and adsorption to achieve more efficient removal of organic contaminants in
water treatment. A thorough review of research findings is provided in a recent Com-
mittee Report of the American Water Works Association79. Microbial growth is
known to occur on sand grains; this was especially true for slow sand filters used
widely to treat water at the turn of the century. However, the extent to which biode-
gradation by attached microbial growths may account for removal of trace organic
contaminants in this process was largely unknown. In contrast to a non-adsorbent
material such as sand, activated carbon affords the opportunity of "temporary stor-
age" for substances which are rather difficult to biodegrade. By providing a longer
time of contact between the microbial population and the substrates, such storage
could promote microbial acclimation and subsequent biodegradation.
An interplay between the adsorption and biodegradation processes may exist.
This could lead to in situ regeneration of sorption sites. Although it has not been
rigorously proven, Sontheimer and his co-workers were first to suggest that in situ
regeneration was occurring in pilot plant studies at the Bremen water works in Ger-
many80. Several research groups followed up with mathematical descriptions of the
interaction between biodegradation and adsorption81"8'. According to most inves-
tigators, biodegradation increases in importance as the adsorption capacity becomes
exhausted and substrate concentration begins to build in the fluid phase adjacent to
the biofilm in the carbon particles.
The only substrate considered available in most models is that external to the par-
ticle. This interaction is depicted schematically in Fig. 28. Biodegradation begins in
the uppermost portion of the bed which, in downflow operation, is first to become
exhausted. As service time increases, the active adsorption zone moves down the
bed. It is followed by the freshly exhausted zone which has yet to build up to maxi-
mum intensity of microbial activity, and then by the active biodegradation zone.
Conceptual mathematical models of this process were an outgrowth of adsorption
models. Typical breakthrough curves, with and without biodegradation, are given in
Fig. 29. Instead of effluent concentration approaching influent concentration as
sorption capacity is depleted, a steady-state removal is attained across the bed due to
98
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'
////,
Time 1
Time 2
Time 3
Figure 28. Depiction of three zones of activity in activated carbon adsorbers when
microbial activity is significant At Time 1, microbial activity has not
developed because the substrate concentration external to the carbon
particles is still low. At Time 2, biodegradation is increased in the upper
region where substrate is available The exhausted zone below will soon
become bio-active, the adsorption zone has been forced lower in the bed as
exhaustion occurs. At Time 3, breakthrough of organic compounds, due to
exhaustion of adsorption capacity, begins. Breakthrough will not be
complete because the biodegradation zone will increase and account for a
final steady-state removal condition.
biodegradation. It should be stressed, however, that such predictions do not assume
1/7 situ regeneration, i.e. that the sorbed substrate becomes available to the biofilm;
instead, bioactivity results only from substrate external to the particle.
Sontheimer's research group continued practical testing of the combined biode-
gradation-adsorption process at the Millheim water works84. This project was re-
ported widely in the United States literature. It marked the beginning of United
States interest in this process for water treatment. An important aspect of the
Mulheim study was the replacement of prechlorination (used to achieve ammonia
removal by breakpoint chlorination) with ozonation prior to GAC adsorption.
Sontheimer stated that this process change was very important to the success of bio-
degradation on the surface of GAC because ozone was able to partially oxidize or-
ganic contaminants such that the end-products were more biodegradable than the
parent compounds. Pre-ozonation extended the service time of GAC beds signifi-
cantly at the Mulheim plant. These results not only suggest a beneficial effect of
ozonation, but also a detrimental effect of prechlorination in that the chlorinated
products seemed to be less biodegradable.
Following the success at Mulheim with pre-ozonation, there were also reports in
the United States literature on other European plants in Switzerland72 and in
France85 where this process has extended GAC service time. The EPA recognized
that such a process development could have a significant impact on the strategy
adopted in the United States for removal of trace contaminants, especially if humic
99
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Weak Adsorption without Biodegradation
/ Weak Adsorption
i with Biodegradation
Strong Adsorption with Biodegradation
Strong Adsorption without Biodegradation
500
1000 1500 2000 2500 3000
Bed Service Time in Arbitrary Units
3500 4000
Figure 29. Predicted effluent concentration as a function of service time with the
GAC bed. The effect of biodegradation on removal of both weakly and
strongly adsorbable organics is shown (After Ying and Weber81 )
substances, the precursors toTHM's, must be removed. Indeed, EPA's initial stud-
ies25 confirmed the European claim that ozone enhanced removals on GAC as shown
in Fig. 30. The removal of THM precursors, as represented by TTHMFP, was def-
initely improved by ozonation, especially as the steady-state removal stage was
reached. However, more careful studies suggested that the non-adsorbent surface of
sand was just as efficient as GAC in promoting biodegradation86. In fact, when ozo-
nation was followed by sand filtration and GAC adsorption, the benefit of ozonation
(in terms of enhanced removal of THM precursors) occurred only through the sand
filtration process. This leaves in doubt the role played by GAC in enhancing biode-
gradation. Definitive, pilot plant studies are now underway at the Philadelphia
water works to answer this question'5.
There is still little data available on the benefits of microbial activity on GAC in
terms of reducing the concentrations of specific organic contaminants. Perhaps the
most valuable insight is given by EPA studies of wastewater reclamation at Water
Factory 21 in Orange County, California. Here, McCarty, et al.87 showed that signif-
icant biodegradation was occurring in terms of the overall organic parameter, TOC.
More important, he compared the removal of trace contaminants by GAC in service
for a very long time (referred to as "old GAC") with that by fresh GAC. Many of
these contaminants were removed about as well by "old GAC" as fresh GAC. This
suggests that when adsorption capacity becomes exhausted, biodegradation can be a
fairly effective removal process.
Influence of Pretreatment
Awareness of the inter-relationship between chemical pretreatment and adsorp-
tion emerged in the 1970's. In United States practice, it is the combination of chem-
ical coagulation and adsorption which will determine the effectiveness of removal of
organic contaminants, while in European practice, it is chemical coagulation, ozon-
100
-------
Values computed from monthly averages of
weekly determinations 2 day, 25°C total
Trihalomethane formation potential
Adsorber
Influent
Effluent - GAC Only
Effluent - GAC+Ozone
45678
Time in Service, months
Figure 30. Influence of ozonation prior to adsorption on tnhalomethane formation
potential removal26.
ation and adsorption. The work of Lee and Snoeyink38, Weber, et al.40 and Randtke
and Jepsen41 in the United States showed very clearly that certain fractions of humic
substances are removed by coagulation thereby improving the adsorbability of the
remaining fractions. They demonstrated also that the residual inorganic coagulant
itself in some way aided in adsorption of the remaining humic substances by activated
carbon. The practical implication of these findings is that pretreatment by
coagulation may be controlled to achieve optimal removal of the residual THM
precursors by adsorption.
Ozonation produces several effects on organic removals which may not always be
advantageous. Although still not well documented scientifically, there is evidence
that ozonation improves the biodegradability of the organic matter found in many
raw water supplies84'88'89. There is also evidence that it aids in the flocculation of
humic substances. On the negative side, ozonation has been shown to reduce the ad-
sorbability of organic matter when measured by TOC or UV absorbance 90'91. This
may seem logical considering that ozonation leads to more polar by-products which
tend to be less adsorbable. The more recent findings from the pilot and full-scale test-
ing at the Mulheim water works suggest that enough ozonation is needed to enhance
biodegradation, but that too much ozone will reduce the overall removal efficiency
of GAC beds because of the detrimental effect on sorptive capacity89. A similar con-
clusion had also been reached by Benedek91 in an earlier study conducted at a water
works in France.
STATE OF THE ART
Existing Adsorption Systems
A 1977 survey conducted by the Organic Contaminants Committee of the
AWWA indicated that only 12 of the 500 plants queried used GAC92. In most of
these cases GAC replaced sand in filters to provide the dual function of filtration and
adsorption. The most notable examples are plants at Mount Clemens, Michigan25,
Davenport, Iowa25, Lawrence, Massachusetts25 and Pittsburgh, Pennsylvania93.
Such GAC beds, however, do not allow sufficient contact time to remove precursors
to THM's or to remove other synthetic trace contaminants. Rather, they are mainly
101
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designed to remove taste and odor causing compounds and in some cases, pesticides.
Moreover, aside from three current EPA demonstration projects, there are no GAC
facilities in the United States which include on-site regeneration. In fact, it has been
standard practice in the United States to operate GAC beds for three or more years
before replacing the media; obviously, such an operating mode severely limits con-
taminant removal capabilities.
The data base on removal of trace contaminants by GAC beds in the United
States should be expanding rapidly as results from full-scale studies at Cincinnati,
Ohio; Manchester, New Hampshire; Jefferson Parish, Louisiana; and Passaic, New
Jersey, become available. In addition, the Ohio River Valley Sanitation Commission
(ORSANCO) is planning four more full-scale tests75.
In Europe, GAC is a common unit operation for removal of trace organic contam-
inants and has been used extensively as is shown in Table 5. While most of these
plants include pre-ozonation, this was not necessarily intended as a pretreatment for
GAC. In most cases, preozonation actually originated out of concern for oxidizing
iron and manganese which were frequently present in groundwater, or in blends of
groundwater and river water pretreated by river bank filtration. In addition, an ap-
parent aversion of European consumers to the taste of chlorine has fostered ozone
treatment to provide disinfection. Only very small dosages of chlorine are applied at
the very end of treatment to provide what is known as "safety chlorination", i.e. pro-
tection from regrowth of bacteria in the distribution system.
Up until about 1970, the traditional role of activated carbon in European practice,
as in the United States, was removal of taste and odor. However, the growing prob-
lem of pollution of the Rhine, Ruhr and other major rivers forced consideration of
adsorption for removal of a wider spectrum of contaminants. Because few treatment
plants used chlorine as the major disinfecting agent, the problem of THM's and
THM precursors was not central to the decision in Europe to include GAC beds. In-
stead, performance criteria were developed which were based on more general pa-
rameters indicative of organic contamination. This will be discussed later.
Process Design
The continuing explosion of information which started in the 1970's makes state-
of-the-art assessment of process design for GAC beds a moving target. The European
experience became very important when the EPA took on the task of preparing a
regulation for control of synthetic organic contaminants. As a result, the late 1970's
became a period of intense information exchange between Europe and the United
States. This began with EPA's translation of proceedings of a special conference on
adsorption techniques94 held at the University of Karlsruhe, Germany in 1975 and
with a review of German experience by McCrearyand Snoeyink95in 1977. Later, the
EPA joined with the NATO Committee on Challenges of Modern Society to spon-
sor two conferences: Oxidation Techniques in Drinking Water Treatment and Prac-
tical Applications of Adsorption Techniques in Drinking Water. Some of these
papers have since been published in the Journal of the American Water Works
Association. There was also a special symposium on the adsorption process spon-
sored by the American Chemical Society96. More recently, the USEPA published a
treatment document as required for the Trihalomethane Regulation64. This docu-
ment is a state-of-the-art review of treatment techniques, including adsorption, for
control of both THM's and their precursors.
Sontheimer summarized the design criteria problem by stating that there is still no
way to postulate rules for controlling GAC bed performance or for selecting the type
of GAC to be used which may hold for all water works". He proposed, however, the
following guidelines: (1) regeneration before the displacement effect produces outlet
concentrations of poorly adsorbed components which exceed inlet concentrations;
(2) removal of at least 50 percent of the UV absorbing material; (3) identification and
monitoring of some specific compounds which are nearly always in the raw water;
102
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Table 5. European Water Treatment Plants Employing GAC75
Locations
Capacity
ML/day
MGD
Preozonation
France
Rouen 38
Morsang 118
Vigneux 38
Viry-Chatillon 76
Federal Republic of Germany
Mulheim 38
Dusseldorf 1 148
Dusseldorf 2 68
Dusseldorf 3 91
Benrath 118
Duisburg 57
Frankfurt
Langenau 156
Wiesbaden
Susel 23
Solmgen 1 5
Siegburg 53
Duren 28
Albstadt 14
Konigswmter 3 8
Fnedsrichshafen 23
Stolberg 23
Vallender 4.5
Stuttgart 66
Switzerland
10
31
10
20
10
39
18
24
31
15
41
6
0.4
14
7.4
37
1 0
6
6
1 2
18
yes
yes
no
no
yes
yes
yes
yes
yes
yes
yes
no
yes
yes
yes
yes
yes
yes
yes
no
no
no
St Gallen
Arbon
Biel
Zurich
Kreuzlmgen
The Netherlands
Rotterdam
46
23
28
194
25
163
12
6
74
51
6 6
43
yes
yes
yes
yes
yes
yes
(4) measurement of the extent of biological activity in the GAC bed; and (5) selection
of carbon types based on tests of performance on the raw water itself and on spiked
samples.
The practical design specifications proposed by Sontheimer as a result of Euro-
pean experience with GAC for different functions are given in Table 6. It is obvious
that removal of synthetic contaminants (see entry in Table entitled "Organic Re-
moval") requires a longer contact time and shorter period between regenerations
than removal of taste and odor compounds. The adaptation of a biologically active
GAC process (see entry in Table entitled "Biological Activated Carbon") according
to Sontheimer lengthens the service time (and thus the period between regeneration)
by a factor of four but does require significantly longer contact times (15 to 25 min-
utes vs 8 to 15 minutes for adsorption only).
103
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Table 6. Design Criteria for GAC Filters in Germany71
Empty Bed Throughout
Filtration Bed Retention Ratio Before
Treatment Velocity Height Time Regeneration*
Method m/hr m mm mVm3
Dechlormation 25-35 2 2-4 1,000,000
Taste & odor
removal 20-30 2-3 8-10 100,000
Organics removal 10-15 2-3 8-15 25,000
Biological
activated carbon 8-12 2-4 15-25 100,000
*m3 of water processed per m3 of activated carbon.
This information exchange clearly shows that treatment objectives, design criteria
and process performance measures are not sufficiently pinpointed in European prac-
tice to satisfy the EPA's need for regulation purposes in the United States. In general,
the concern in Europe is protection from synthetic organic contaminants as deter-
mined by easily measured parameters such as TOC, U V absorbance, or total organic
chlorine (TOC1). In contrast, EPA regulations would probably call for measure-
ment of specific contaminants and regeneration of GAC when certain maximum
limits are reached. This more legalistic approach presents problems in establishing
design criteria which do not actually require pilot plant studies at the site of a pro-
posed treatment facility.
At EPA's request, the National Academy of Sciences' Safe Drinking Water
Committee initiated in 1978 a critical review of the state-of-the-art. With respect to
efficiency of adsorption in controlling toxic organic contaminants, the committee
concluded that GAC is a particularly effective adsorbent fora wide spectrum of con-
taminants and supported its use where certain contaminants are known to pose a
health risk24. There is no evidence to suggest any detrimental effects from the pro-
cess. However, areas cited as needing further evaluation are: control of bacterial con-
tamination of the treated water (resulting from microbial activity of GAC); identi-
fication of any metabolic end-products from microbial activity; documentation of
the effectiveness of regeneration after many cycles; and investigation of possible cat-
alytic reactions on GAC which could produce toxic end products. With regard to
this last point, EPA-sponsored research has shown that when chlorine and humic
substances are introduced into GAC beds, no new chlorinated organic compounds
are produced97. Other possible precursor compounds remain to be investigated, how-
Selection of Activated Carbon
Although there is no method for "designing" an activated carbon to meet a spe-
cific treatment objective, several studies in the 1970's indicated the importance of ad-
sorbent selection. Fig. 31 shows that adsorption profiles can vary within GAC beds
containing different adsorbents. In this case, the most hydrophobic carbon (carbon
L) gave the best removal of the rather polar, bis 2-chloroisopropyl ether molecule.
Other carbons showed definite signs of displacement of this compound.
Another example is the widely varying performance of different activated carbons
tested at Louisville, Kentucky when evaluated on the basis of adsorption of chloro-
form (Fig. 32). Here, exhaustion of sorptive capacity is indicated by the decreasing
slope of these plots as the volume of water treated increases. The better removals
104
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Carbon-L
Figure 31. Adsorption of Bis(2-chloroisopropyl) ether on various activated carbons
at different filter depths95.
were obtained with carbons having the larger fraction of smaller pores; these
probably excluded competition with humic substances. In contrast, if TOC removal
were the design objective, then these carbons would be the least effective (see also
earlier discussion of adsorption of humic substances).
The problem of selecting an activated carbon is therefore quite difficult in treat-
ment situations where prechlorination is practiced because both THM's and unre-
acted THM precursors, i.e. TOC, may need to be removed; one carbon type does
not appear optimum for both. Breakthrough of one or the other classes of com-
pounds will control service time as was amply illustrated by EPA pilot plant studies
at Miami, Florida".
Costs
In anticipation of a regulation requiring G AC treatment at many locations in the
United States, the EPA has developed extensive cost estimating procedures and has
made these available to design engineers in a format which allows for simple updat-
ing and adjustment for specific situations100. Important design options are included
105
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I I 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
Barnebey-Cheney PC-1
NucharWV-W
1 2 34 5 67 8 9 10 11 12 13 14 15 16 17 18
Bed Volumes, x1000
Figure 32. Cumulative adsorption of chloroform by seven brands of granular
activated carbon98
such as on-site vs. off-site regeneration; frequency of regeneration; size of con-
tactors, i.e. EBCT; pretreatment with ozone vs. no pretreatment; and sand replace-
ment vs. separate post filter adsorbers. As an example, the amortized capital costs,
operating and maintenance (O&M) costs and the total costs are compared for sand
replacement and separate adsorbers in Table 7 for plants of several sizes. The table
shows that total cost is quite sensitive to the size of the plant. A 5 MOD filter adsorb-
er facility would cost 25.3 cents per thousand gallons while a 100 MOD facility
would be 12.3 cents per thousand. Post filter adsorbers are more expensive than sand
replacement because of the higher capital costs required for constructing the bigger
contactors. The percentage increase in cost of water treatment by addition of G AC
treatment is anywhere from 43 percent for a small plant (one MOD capacity) to 21
percent for a large plant (100 MGD capacity).
Another important assumption in cost estimation is the regeneration frequency
(Fig. 33). Total costs are seen to rise very sharply as the regeneration frequency in-
creases, i.e. as the time interval between regenerations shortens to less than about
two months. These results are also dependent upon the assumptions used in estimat-
ing carbon loss rate. While available data for more than a few cycles are limited, an
assumption of 6 percent loss for post filter adsorption and 10 percent for sand re-
placement seems reasonable.
Process Control
The basic objective in process control is to determine when to regenerate. Given
the broad choice of treatment objectives discussed, it is clear that monitoring of per-
formance will depend on which organic contaminants have been selected for reduc-
tion. The greater the specification of target trace contaminants, the more expensive
is monitoring and the less likely is real-time control. The success of a monitoring pro-
106
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Table 7. Amortized Capital and O&M Costs for GAC Systems1
Amortized Capital Cost
Design Capacity
ML/day
3.8
19
38
380
570
MGD
1
5
10
100
150
Sand
C/kL
6.4
23
1.6
0.5
04
Replacement
C/1000gal
24.8
9.0
6.2
2.0
1.7
Filter
C/kL
7.6
33
2.8
1.3
1.2
Adsorber
C/1000gal
294
127
98
4.9
4.6
O&M Cost
Sand Replacement
C/kL C/1000gal
51 195
34 13.0
3.0 11.7
23 8.7
2 2 8.4
Filter
C/kL
5.8
33
2.8
1.9
1 8
Adsorber
C/1000gal
22.1
12.8
10.8
7.4
7.1
Sand
C/kL
11.5
5 7
48
28
2.8
Total
Replacement
C/1000gal
443
220
17.9
10.7
10 1
Cost
Filter
C/kL
13.4
66
5.4
3 2
30
Adsorber
C/1000gal
51.5
253
208
12.3
11.7
-------
10
o
o
c
o
2 5
o
ol
"ro
o
I
40
35
30
25
20 <3
c
o
15 1
10 2
o
234567
Reactivation Frequency, months
Figure 33. Total production cost versus reactivation frequency in months for
380-ML/day (100-mgd) post-filter adsorption100
gram also depends on the sampling frequency. This is especially important where ac-
cidental spills may occur.
Suffer recommended that sophisticated GC/MS (gas chromatography/mass
spectroscopy) be used for confirmation purposes only and not for routine assess-
ment of performance. In research this technique has proven very useful in demon-
strating the level of effectiveness of GAC in removing many contaminants"'4^6 .
Non-specific organic analyses are used in European practice, the most common of
these being UV absorbance (which measures aromatic compounds), TOC (which
measures the background compounds such as the humic substances) and TOC1
(which measures the total organic chlorine contained in organic compounds). The
TOC1 test was originally developed at the University of Karlsruhe by Kuhn and
Sontheimer'01 with the aim of measuring compounds containing organic chlorine
which had been adsorbed in full-scale operation of activated carbon beds. Procedures
were then developed and tested in the EPA laboratories by Dressman, et al.102 such
that the organohalide content of water samples themselves could be measured. In-
clusion of all halogenated forms led to adoption of the term total organic halogen
(TOX) to describe the test. Still later, Jekel and Roberts103 modified the TOX test to
distinguish between purgeable (i.e. more volatile) and non-purgeable(i.e. less vola-
tile) organohalides.
Although TOX measurements of the influent and effluent of GAC beds may be
useful as a general guide to performance, the variability in concentrations of indi-
vidual compounds limit interpretation of results. Alternatively, measuring the load-
ing of sorbed TOX allows estimation of the position and shape of the active adsorp-
tion zone and the remaining service time of the bed. A recent example of use of this
technique is presented by Qumn and Snoeyink104 in their EPA-sponsored work at
the Jefferson Parish, Louisiana, full-scale GAC facility (Fig. 34). The buildup in
108
-------
X
o
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
4.2 Months
3.3 Months
2.4 Months
1.5 Months
Top
Middle
Bed Depth
Bottom
Figure 34. Jefferson Parish adsorber TOX profiles104
sorbed TOX along the bed length as service time increases from 1.5 to 4.2 months is
seen very clearly. What is still needed, however, is a method to correlate such data
with effluent quality.
The concern overTHM's in the United States implies not only a need for their re-
moval, but control of their precursors compounds, the humic substances, before the
chlorination step64. Hence, another measure of process performance is reduction of
THMFP. For plants practicing prechlorination as well as post chlorination, both
the THM and THMFP test are needed. The difficulty of the THMFP test is that it
must be carried out over several days in order to provide sufficient time for the chlo-
rine to react with the humic substances. Thus, it does not lend itself to real-time
process control. An alternative parameter which it was hoped could be correlated
with the THMFP test is non-purgeable organic carbon (NPOC)25. This seems logi-
cal because humic substances represent most of the organic carbon in a raw water.
Plant studies at Jefferson Parish, Louisiana36 supported this argument but, un-
fortunately, similar studies at Miami, Florida" did not. The NPOC parameter, how-
ever, still has utility as a general indicator of adsorption efficiency.
Test procedures are also needed to assess the effectiveness of on-site carbon regen-
eration. These must not be limited to the more traditional measures such as density
and iodine number. Rather, they must include some way of evaluating the effective-
ness of the regenerated carbon in adsorbing specific contaminants of concern to a
water works.
Sontheimer, at the University of Karlsruhe, has collaborated with several of the
major German water works over many years to develop new practical test proced-
ures. Central to most of these is the use of UV absorbance and DOC (dissolved or-
ganic carbon) as measures of the general makeup of organic contaminants. The tests
include determination of adsorption rate, adsorption equilibrium and response to
competitive adsorption. In addition, there has been progress on use of thethermo-
109
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gravimetric balance. This device measures the rate of weight loss of the activated
carbon as a function of regeneration temperature. The rate of weight loss can be cor-
related with removal of sorbed organic compounds; when used conjunctively with
GC/MS, it may even be possible to determine which compounds are removed. A
major objective of all these procedures is development of simple ways to monitor
GAC performance.
A Look to the Future
Although not promulgated, the regulation proposed by the EPA in 1978 to con-
trol synthetic organic contaminants by GAC was very important. It precipitated a
scientific debate over the merits of the adsorption process which focused attention
on the gaps in present understanding. The direct consequence has been research that
is extending our knowledge of the principles of adsorption and is demonstrating
through pilot plant and full-scale studies, practical applications of the process.These
efforts are fostering a better understanding of the process and its widespread utility,
and also of its limitations. It would seem that the case for GAC as the best adsorbent
for removal of a broad spectrum of organic contaminants has been made quite
clearly. All that remains is to arrive at an acceptable way of recommending its use in
United States water works without overspecifying process design and performance
criteria.
EPA-funded pilot plant and full-scale demonstrations in the late 1970's certainly
did much to close the gap between adsorption theory and practice. Nevertheless, the
need for more basic research persists. Prediction of adsorber performance is one
area in which research could have very practical consequences. It could in particular
lessen the need for long and expensive pilot plant testing programs. However, the
variability in chemical composition of raw waters from site to site, and from season
to season at any given site, is a great barrier to adaptation of mathematical modeling
principles. The most ideal predictive technique would allow estimation of service
time based solely on knowledge of the mix of organic contaminants present at a
given time. This may be possible as progress is made in prediction of equilibrium ad-
sorption behavior by such procedures as those developed by McGuire and Suffet27,
Manes28, Belfort29, and Weber and van Vliet30, all of which were described earlier.
The rate at which adsorption equilibrium takes place is equally important. The
work of teams led by Weber, Snoeyink, and Sontheimer advanced knowledge in this
area, particularly by recognizing the importance of surface diffusion and by distin-
guishing the slow rate of adsorption of humic substances from the more rapid rate of
trace contaminants. This should allow for better assessment of process dynamics in
GAC beds as these two categories of compounds compete for adsorption sites.
Two relatively unexplored areas of research on the topic of adsorption rate in-
volve the influence of surface topography (i.e. particle shape and roughness) and
macro-micropore diffusion. The recent work of van Vliet, et al.105 shows that higher
external film, mass transfer coefficients are associated with adsorption on activated
carbons than on carbonaceous resins; concomitantly activated carbons have rough-
er surfaces. Their work also suggests that internal surface diffusion cannot explain
the observed slow approach to equilibrium in fixed-bed adsorbers. Instead, they
draw upon past findings of other researchers which indicate internal mass transport
to consist of: (1) rapid migration through macropores and (2) slow migration
through micropores. Evidence of a renewal of interest in this concept is noted in the
recent research of Peel and Benedek106 and Famularo, et al.'07 in which mathematical
models are developed.
Modeling will probably never be a panacea in predicting the breakthrough be-
havior of each and every contaminant to be removed. Nevertheless, it may provide a
means for bracketing the expected performance if a way can be found for subdividing
the contaminants logically into several large classes. In this regard, the development
of new analytical methods may be of considerable help. For example, it is already
110
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possible to separate purgeable, and thus more volatile, from nonpurgeable, and thus
less volatile, organic compounds; these categories can be measured either by organic
carbon or halogen content. The research group at the University of Karlsruhe is
working on such an approach for characterization of the adsorptive behavior of
organic mixtures according to such broad classes of compounds.
The beneficial aspects of microbial degradation on adsorbent surfaces used in wa-
ter treatment needs to be examined more closely. At present, there is limited under-
standing of the interaction between biodegradation and adsorption, particularly as
it relates to removal of specific trace compounds found in water treatment. Just how
sorbed substrates can again become available to the microbial population of a GAC
bed is poorly understood, although there is evidence of "back diffusion" from the in-
ternal surfaces to the external surface108. Assessment of rules governing biodegrad-
ability of synthetic organic contaminants especially at low concentrations is also
needed, as is better understanding of the metabolites produced and their fate in the
GAC bed.
Such research can lead to improved design of GAC beds. For example, if biode-
gradation is an important feature, then the type of pretreatment to be provided needs
to be specified more carefully (perhaps ozone should be used); also, the flowrate
through the bed and the adsorbent particle size may need to be optimized to take ad-
vantage of biological rather.than adsorptive principles. Process control, too, would
reflect concern over minimizing the release of bacteria into the effluent; one method,
for example, would be by more frequent backwashing. EPA pilot plant studies now
underway in Philadelphia, Pennsylvania35 should shed more light on all these areas.
The development of carbonaceous resins in the 1970's showed how advances can be
made in designing new adsorbents and in improving old adsorbents. Past research
has shown that not all contaminants are removed to the same extent by activated
carbon. Therefore, other adsorbents may be important in very specific situations, as
may be the case in treatment of contaminated groundwater, for example. The
contrasting adsorptive behavior of humic substances and low molecular weight,
halogenated compounds shows quite clearly the importance of pore size
distribution. Manipulation of pore size distributions and activation procedures (for
activated carbons) may both lead to development of better adsorbents.
As removal functions to be performed by adsorbents become more specific there is
need to review the minimum standards for judging acceptability of adsorbents. The
old standards'09 adopted by the AWWA in 1974 need to be revised to include better
performance indicators for removal of trace organic compounds of concern in prac-
tical situations. A committee of the AWWA is working on this problem.
Efficient and economical on-site regeneration is also essential for wide acceptance
of the adsorption process. Ongoing pilot plant studies at Cincinnati, Ohio and Man-
chester, New Hampshire using the fluidized bed furnace and at Passaic Valley, New
Jersey using the infra-red furnace should answer the need for practical data. Most
important is the use of activated carbon over the number of cycles corresponding to
that estimated by loss rate per cycle; at a 6 percent loss rate, this would be about 16
cycles.
In looking to the future, it is interesting to reflect on discussions by eminent scien-
tists and engineers110 at the 1958 Conference on "Man Versus Environment." At that
time, the little known area of organic contaminants was recognized as a potential
major concern of the future. Since then, as a result of the introduction of more so-
phisticated analytical techniques, our perception of the problem has sharpened, but,
it must be said a specific control strategy has yet to be adopted for use in the United
States. However, GAC adsorption has emerged as the control strategy with the most
likely chance of success based on its effectiveness and costs. Other adsorbents with
further development, may also prove advantageous in control of special problems,
e.g. those associated with treatment of groundwater. Advances in the understanding
of adsorption phenonema will surely lead to improvement of water treatment prac-
tice. It requires no special insight to foresee that removal of organic contaminants
111
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will generally require use of a separate unit for this purpose. This process, like coag-
ulation, sedimentation, filtration, and disinfection, will be viewed as an essential
component of the conventional water treatment plant.
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from the Aqueous Phase Vol. I and Vol. II, proc. of the 176th ACS Meeting,
Miami, Florida, 1978, Ann Arbor Sci. Pub., Ann Arbor, MI, 1980.
97. McCreary, J.J. and V.L. Snoeyink. "Reaction of Free Chlorine with Humic
Substances Before and After Adsorption on Activated Carbon," Environ.
Sci. and Technol., 15, 193-197, 1981.
98. Mullins, R.L., Jr., J.S. Zogorski, S.A. Hubbs and G.D. Allgerei. "The Ef-
fectiveness of Several Brands of Granular Activated Carbon for the Removal
of Trihalomethanes from Drinking Water," in Activated Carbon Adsorption
of Organics from the Aqueous Phase, Vol. I, I.H. Suffet and M.J. McGuire,
eds. pp. 273-307, Ann Arbor Sci. Pub., Ann Arbor, MI, 1980.
99. Wood, P.R. and P.R. DeMarco. "Treatment of Groundwater with Granular
Activated Carbon," Jour. Amer. Water Works Assoc., 71, 674-682, 1979.
100. Clark, R.M. and P. Dorsey. "The Costs of Compliance: An EPA Estimate for
Organics Control," Jour. Amer. Water Works Assoc., 72, 450-457, 1980.
101. Kuhn, W. and H. Sontheimer. "Several Investigations on Activated Charcoal
for the Determination of Organic Chlorocompounds," Vom Wasser, 15, 65-
79, 1973.
102. Dressman, R.C., B.A. Najar and R. Redzikowski. "The Analysis of Organ-
ohalides (OX) in Water as a Group Parameter," In: Proceedings-Seventh
Water Quality Conference, Philadelphia, PA, December 9-12, 1979, pp. 69-
92, American Water Works Association, Denver, CO, 1980.
103. Jekel, M.R.and P.V. Roberts. "Total Organic Halogen as a Parameter for the
Characterization of Reclaimed Waters: Measurement, Occurrence, Forma-
tion, and Removal," Envir. Sci. and Technol., 14, 970-975, 1980.
104. Quinn, J.E. and V.L. Snoeyink. "Removal of Total Organic Halogen by
Granular Activated Carbon Adsorbers," Jour. Amer. Water Works Assoc.,
72, 483-490, 1980.
105. van Vliet, B.M., W.J. Weber, Jr. and H. Hozumi. "Modeling and Prediction
of Specific Compound Adsorption by Activated Carbon and Synthetic Ad-
sorbents," Water Research, 14, 1719-1728, 1980.
106. Peel, R.G. and A. Benedek. "Dual Rate Kinetic Model of Fixed Bed Ad-
sorber," Jour. Envir. Engrg. Div., Amer. Soc. Civil Eng., 106, EE4, 797-814,
1980.
117
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107. Famularo, J., J.A. Mueller and A.S. Pannu. "Prediction of Carbon
Column Performance from Pure Solute Data," Jour. Water Poll. Control
Fed., 52, 2019-2032, 1980.
108. Li, A.Y.L. and F.A. DiGiano. "The Availability of Sorbed Substrate for
Microbial Degradation on Activated Carbon," Proc. of Water Pollution
Control Federation Research Symposium, Las Vegas, NV, Sept. 29-Oct. 2,
1980.
109. "AWWA Standard for Granular Activated Carbon-B604-74," Jour. Amer.
Water Works Assoc., 66, 672-681, 1974.
110. Proceedings-Conference on Man Versus Environment, H. A. Farber, ed., May
5-6, 1958, U.S. Public Health Service (Grant No. RG-6425), January 1959.
118
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REMOVAL OF ORGANIC SUBSTANCES FROM
WATER BY AIR STRIPPING
Perry L. McCarty, PhD
INTRODUCTION
The value of aeration for improvi. .g the quality of water has long been known, and
this process was among the first to be used for water treatment. As early as 1887,
Charles B. Brush reported in the Journal of the New England Water Works Associ-
ation1 that "The necessity of aeration even in Europe.. .is now being recognized and
its necessity advocated." Use then, and indeed at the present time, has been primarily
for aesthetic purposes, to reduce the level of tastes and odors in water. Initially, this
improvement in water quality was believed to result primarily from the introduction
of oxygen, but later, the benefits were better understood as the result of the removal
of volatile compounds.
In his book published in 1935 on Elimination of Taste and Odor in Water2, John
R. Baylis described many of the principles of aeration and its applications to the
treatment of water supplies. He indicated that aeration was a well established prac-
tice, and was then in use at an estimated 300 water supplies in the United States. He
suggested that use of aeration was not confined to taste and odor removal, but was
valuable for adding oxygen to water, and for removal of iron, manganese, and car-
bon dioxide. He also suggested that many inorganic and organic compounds which
might be responsible for tastes and odors in water could be removed by aeration, and
he provided boiling point and solubility data then available for a large number of
organic compounds, some of which are of lexicological concern today, to aid in
assessing the potential effectiveness of stripping with air in removing them. Thus, the
value of this process for stripping trace organic substances was recognized as early as
1935.
By 1958, a United States Public Health Service survey3 indicated that 2,154 water
treatment plants had aerators. However, the American Water Works Association's
third edition of Water Quality & Treatment, published in 19714, suggested that
aeration was generally not an efficient method for the removal or reduction of
unpleasant tastes and odors because most of the substances causing these traits were
not sufficiently volatile. It was indicated that a number of plants had discontinued
using aeration for taste and odor reduction, and that aeration was then used
primarily for iron and manganese removal. Use of aeration for well waters was also
The Author Dr McCarty received his B S degree in civil engineering at Wayne State University and his
M S. and Doctor of Science degrees in Sanitary Engineering at MIT After working with several consulting
firms in the water and wastewater field, he joined the faculty of the Department of Sanitary Engineering at
MIT In 1962 he left MIT to accept an appointment on the faculty of Stanford University where he serves as
Silas H Palmer Professor of Civil Engineer Dr McCarty is the author of numerous technical papers in
many aspects of water and wastewater treatment He serves on a number of committees of the National
Academy of Sciences - National Research Council and professional organizations. He is active as consultant
for Federal and State agencies and private organizations Dr McCarty is a member of the National Academy
of Engineering
119
-------
questioned because of possible contamination by airborne impurities including
pathogens. Thus, emphasis on the beneficial effects of aeration in more recent times
seems to have declined.
However, in 1977, air-stripping was found to be effective for removing a wide
range of volatile organic materials present in trace concentrations in reclaimed
wastewaters5. It was pointed out that many potentially toxic materials, that were not
effectively removed by activated carbon treatment, or that broke through activated
carbon beds most readily, could be removed efficiently by aeration. This and
subsequent studies6 " indicated that many of the organic contaminants being found
in ground as well as surface waters could be effectively removed by air-stripping.
Thus, the potential value of this relatively inexpensive treatment process for dealing
with specific classes of potentially harmful organic compounds was demonstrated.
For this reason, it is desirable to re-evaluate this old process for application to the
newer problems of today.
PRINCIPLES OF REMOVAL BY AIR-STRIPPING
General
A description of air-stripping principles and their employment in process design
cannot avoid use of equations. The examination presented herein, therefore,
depends more or less heavily on equations which, while familiar for many, may seem
arcane to others. To assist in understanding these principles, some terms that are
widely used in gas exchange phenomena are defined:
Mass Transfer—
Mass transfer refers to the movement of a given mass of a compound from one
point to another within a given medium, or from one medium, such as water, to
another medium, such as air . Transfer of mass between media or between phases is
the objective of air-stripping. In this case the transfer of mass takes place across a
boundary that exists between the two phases, and this boundary can be characterized
by its area. In order to design a stripping process, it is necessary to understand the
factors that affect the rate at which the compound of interest moves across this
boundary, and this is generally expressed as the net movement of contaminant mass
per unit area of boundary per unit of time. This is referred to here as the rate of mass
transfer.
Diffusion—
Diffusion is one of the processes by which a compound moves from one point in a
given medium to another. This is a molecular process and results from the normal
movement of molecules, their random collision with one another, and the tendency
towards an even mixing of the molecules that results. The process of diffusion tends
to move a compound from a point of high concentration within a medium towards
one of lower conentration. This difference in concentration divided by the distance
between the two points is termed the concentration gradient; the rate of mass
transfer between the two points by diffusion is proportional to this concentration
gradient. The proportionality constant is termed the coefficient of diffusion. There is
a relationship between diffusion and mass transfer, and this will be discussed later in
some detail.
Henry's Law—
Henry's law is a statement relating the concentration of a given compound in air to
its concentration in water under conditions of equilibrium. An understanding of
Henry's law is basic to an understanding of air stripping as will be indicated in the
following.
120
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Equilibrium Considerations
Henry's law states the relationship that exists between the amount of a volatile
substance in the gas phase above a liquid and the amount of the substance dissolved
in the liquid at a given temperature and at equilibrium. For an ideal volatile com-
pound which obeys this law, the relationship is given by:
H,= F,e/C,e (1)
where H, is the Henry's law constant, Flc is the partial pressure expressed as a
concentration (mg/m3) of compound j. in the gas phase above the liquid, and C,e is
the concentration in the liquid (mg/m3), under conditions of equilibrium between
the liquid and the gas phases.
If the relative concentrations of the compound in the liquid and the gas phases are
not in equilibrium as indicated by Henry's law, then the gas will tend to move from
one phase to the other in a direction to approach equilibrium. Volatile organic
materials dissolved in water can be removed from water by transfer to a gas such as
air if the concentration in the air is below the equilibrium value with the water phase.
Thus, Henry's law provides the basic equilibrium foundation for determining the
removability of volatile organic materials from water by air-stripping.
As with other equilibrium constants, the Henry's law constant varies with
temperature, and for the case where the enthalpy of dissolution remains relatively
constant, the temperature dependence is given by:
, u AH''
log H, =
where,
AH," = the standard enthalpy of dissolution for gas ± (joule/ mol)
R = universal gas constant (8.314 joules/ deg mol)
T = absolute temperature, K
K, = constant
In general, H, increases by a factor of two to three for each 10°C rise in temperature.
Thus, the temperature effect can be quite significant.
Rates of Mass Transfer
Based upon Henry's law a volatile component can be removed from water by air-
stripping if the bulk concentration C, is greater than the equilibrium concentration
with the bulk concentration F, in the air, that is if OF,/ H,. This condition is shown
pictorially in Fig. 1 . The mass transfer from water to air then occurs in response to a
negative gradient from the bulk water to the water-air interface, across the interface,
and then from the interface into the bulk air. Generally it is considered that the rates
of transfer from the bulk water to the interface, and from the interface to the bulk air
are much slower than the rate of transfer across the interface itself. For this usual
case, the interfacial concentration in the water C,* is in equilibrium with the inter-
facial concentration in the air F,*, thus
C,* = F,*/H, (3)
121
-------
Interface
o
o>
c
Liquid
Phase
Gas
Phase
Figure 1. Transfer of volatile components from liquid phase to gas phase in response
to a concentration gradient.
The rates of mass transfer from the water to the interface and from the interface
into air are given by the following:
N, = K,L(C, - C*)
(4)
N, = K,o (F,* - F,) (5)
where,
N, = The rate of mass transfer (mg/ m2/sec)
K|L = Overall liquid transfer coefficient (m/sec)
K,o = Overall gas transfer coefficient (m/sec)
The rate of transfer from the water to the interface must equal that from the interface
into the air. By combining Equations (3) and (5) with Equation (4), the general
formula for the overall transfer of the volatile component from water to air is found
to be
(6)
for which,
l/H,K,o
(7)
122
-------
From Equation (7) it can be seen that when H, is very large, that is when the volatile
compound is not very soluble in water, the last term in the Equation is negligible and
K, essentially equals the liquid transfer coefficient. The transfer is then said to be
liquid-phase controlled. In the converse situation of very small H,, the overall trans-
fer coefficient essentially equals H,K,G, and the transfer is gas-phase controlled. The
relationships between water concentration, the air partial pressure and the interface
concentration of the volatile component for these different cases are illustrated in
Fig. 2.
It is more generally the nature of the volatile component itself rather than the flow
conditions which determines whether a mass transfer system is gas-phase or liquid-
phase controlled. However, the process design should consider which, if either, phase
is controlling. The values for the individual phase rate coefficients are a function of
the process characteristics as well as temperature. Thus, for example, if the process is
gas-phase controlled, then the process should be designed to achieve a high gas-
transfer coefficient. Also, in comparing the effects of process variables on rates of
transfer of different volatile components, it is important to know whether or not
both are limited by rate of transfer across the same phase. According to Mackay and
Leinonen12 the phase resistances are approximately equal when H, equals 7 X 10~3.
For values well above this, the liquid-phase transfer would control, and f"
-------
o
D>
C
Liquid Phase C Gas Phase
Liquid Phase
Controlled
o
o>
C
Gas Phase
Gas Phase
Controlled
Figure 2. Difference in concentration gradients between liquid and gas phases for
liquid phase and gas phase controlled processes
With gas-phase controlled transfer, the power relationship is intermediate between
the two liquid-phase controlled processes15 as indicated in the following for am-
= (KNH3a)(D,G/DNH3)0
(10)
The above relationships are valuable for extending the current knowledge of sys-
tems for transferring inorganic gases to the removal of volatile organic compounds
through air-stripping.
Equations (8) through (10) suggest that for different situations, the rate of mass
transfer is a function of the diffusion coefficient to some power which varies between
0.5 and 1.0. From the literature there appears to be some differences of opinion on
just what power to use in a particular case, but in any event, the differences in mass
transfer which result are generally not as significant in comparative removals of dif-
ferent compounds as is the effect of their Henry's law constants. This will be seen in a
subsequent section.
Properties of Hazardous Organic Compounds
As the previous discussion shows, an important property of hazardous organic
compounds which governs whether they may be readily removed from water supplies
by air-stripping is their Henry's law constant. Such values have not been evaluated
extensively, and it is not presently known whether the removability by air-stripping
of the many compounds of concern can be described well by the relationship given in
Equation (1) over a sufficient range that extends down to the microgram per liter
concentration. Therefore, additional evaluation is needed. However, an approxi-
mation of the Henry's law constant can be obtained from two properties of organic
compounds which are somewhat more readily available: the compound's vapor
pressure and its solubility in water. The first expresses the equilibrium relationship
between a solid or liquid compound and its gaseous concentration, and the second
expresses that between the compound and its concentration in water. If a compound
is both at equilibrium with the gas phase and the water phase, then the respective
concentrations in the gas and water phases must also be at equilibrium, and thus the
ratio of these respective concentrations should yield the Henry's law constant12,
124
-------
Table 1.
Calculated Henry's Law Constants and Water Diffusion Coefficients at 20°C
for Selected Organic and Inorganic Compounds*
Compound
Vinyl chloride
Pichlorofluoromethane
Oxygen
1,1-Dichloroethylene
1,2-Dichloroethylene
Tr ic hlorof I uoro methane
Methyl bromide
Toxaphene
Carbon dioxide
Carbon tetrachlonde
Tetrachloroethylene
Chloroethane
Beta-BHC
Tnchloroethylene
Hydrogen sulfide
Methyl chloride
1 ,2-Trans-dichloro-
ethytene
Ethyl benzene
Toluene
1 ,1 -Dichloroethane
Benzene
Chlorobenzene
1,1,1-Tnchloroethane
Chloroform
1 ,3-Dichlorobenzene
Methyl chloride
Heptachlor
1 ,4-Dichlorobenzene
Aldrm
1 ,2-Dichloropropane
H,t
mg/m3
mg/m3
270
87
29
7 1
7 1
46
39
26
1 1
1 0
96x10 '
62x10 '
46x10 '
42x10 '
34x10 '
33x10 '
24x10 '
24x10 '
24x10 '
2 1x10 '
1 9x10 1
1 7x10 1
1 5x10 1
1 4x10 '
1 1x10 '
1 0x10 '
96x10 2
87x10 2
87x10 2
83x10 2
°!L
cm2/s
xlO5
1 3
1 3
24
1 2
1 2
_
1 5
-
1 9
1 1
1 0
1 3
081
1 1
1 8
1 5
1 2
093
1 0
1 2
1 1
1 0
1 0
1 2
097
1 4
_
095
_
1 1
Compound
1 ,2-Dichloropropylene
Alpha-BHC
1,2-Dichlorobenzene
Anthracene
1 ,2-Dichloroethane
Hexachloroethane
1 ,1 ,2-Trichloroethane
Bromoform
1,1,2,2-Tetrachloro-
ethane
Naphthalene
Fluorene
Acenaphthene
Phenanthrene
Bts{2-chloroisopropyl)
ether
Acrolem
2-Nitrophenol
Acrylomtnle
Di-n-butyl phthalate
2,4-Dichlorophenol
4,4'-DDT
2-Chlorophenol
Ammonia
Nitrobenzene
Isophorone
Pentachlorophenol
Dimethyl phthalate
Lindane
Phenol
Dieldrin
4,6-Dimtro-o-cresol
H,t
mg/m3
mg/m3
83x10 2
83x10 2
7 1x10 2
58x10 2
46x10 2
46x10 2
32x10 2
26x10 2
1 7x10 2
1 5x10 2
87x10 3
79x10 3
54x10 3
46x10 3
40x10 3
32x10 3
26x10 3
26x10 3
1 7x10 3
1 4x10 3
87«10 "
7 1x10 "
46x10 "
1 7x10 "
87x10 5
1 7x10 5
1 3x10 6
1 1«10 5
7 1xio 6
7 1x10 6
D,L
cmVs
X105
1 1
_
098
_
1 2
097
1 1
1 1
1 0
092
086
082
_
080
1 3
1 1
1 3
058
_
1 0
20
1 0
082
-
078
_
1 1-
*For organic compounds, H, after values from Reference 25 and D|L calculated from Equation 1 3, for
inorganic compounds, H( are from Reference 26 and D^ from Reference 27
fHp expressed for convenience as ratio of mass per unit volume in air to mass per unit volume m water,
for more usual units, multiply by 0 024 to convert to atm-mVmol, or by 1 340 to convert to a mol
fraction ratio
125
-------
H,=F1S/C1S (11)
where
F1S = vapor pressure concentration of compound 1 (mg/ m3 ) at a given
temperature
C1S = solubility of compound _i_(mg/m3) in water at the given temperature
Values of the Henry's law constant for various organic compounds, especially
those on the U.S. Environmental Protection Agency (EPA) priority pollutant list,
and for some inorganic gases of significance, are listed in Table 1.
Table 1 lists coefficients for diffusion of the compounds through water and
through air. The values for water were for the most part calculated from the
Equation by Wilke and Chang16,
D,L = 7.4 X 10-8(T)(XM)05/MVo06 (12)
where
/u = solvent viscosity, centipoise
T = absolute temperature, K
X = association parameter, 2.6 for water
M = solvent molecular weight, 18 for water
V0 = molal volume of solute at normal boiling point, cc/g-mole
Values for V0 were not readily available for the organic compounds of interest. In
order to determine the diffusion coefficients listed in Table 1, V0 was roughly
estimated to equal the quotient of the molecular weight and the density of the pure
liquid chemical at 20°C in g/cm3. Since the latter would normally be smaller at the
boiling point of the compound, the calculated diffusion coefficients are somewhat
larger than they should be. Through use of Equation (13), diffusion coefficients for
other temperatures can be readily calculated.
AIR-STRIPPING PROCESSES
Types
Stripping of volatile organic components from water into air depends upon
bringing the two phases into intimate contact under conditions wherein the forces
for stripping will be most favored. This involves maximizing the interfacial area of
contact between the two. Various processes for doing this are illustrated in Fig. 3. In
diffused air systems, the air stream is broken into small bubbles, providing a large
surface area as they rise through water. This is a common procedure for introducing
oxygen into wastewaters for biological treatment. Mechanical aeration is also used
to produce a similar effect; it results in breaking the liquid into droplets or films
which are thrown through the air. As the water splashes back into the basin, some air
is also entrained as bubbles and additional transfer takes place. Such processes could
be used for removal of volatile organic compounds with mass transfer coefficients
and Henry's law constants similar to that of oxygen.
With less volatile compounds, larger quantities of air than normally used for bio-
logical treatment may be required to obtain the desired efficiencies of removal, and
therefore some type of tower system may be preferred for reasons of economy. In
towers the pressure drop for mixing air and water is much less than in a basin. In a
126
-------
Stripping Basins
Water
if
Air
Air
o
<5,
,O
Air
Water Water
Air
Water
Diffused Aeration
Mechanical Aeration
Air.
Stripping Towers
Air
A
00
o
• Water
* Water
Spray Tower
Air
Air
Water
Air
Air-
Air-
T
" Wai
Water
Packed Tower
Countercurrent Flow
• Water
1 Water
Trayed Tower
Air Air
Water | I Water
I !
&»
Air
^ Water
Packed Tower
Cross Flow
Figure 3. Configurations for different air-stripping processes.
127
-------
spray tower, air and water flow in a countercurrent fashion. The water is broken into
fine droplets by passage through nozzles in order to increase the surface area ex-
posed to the rising air. Power is required to pump the water and force it through the
nozzles. Also, some of the water may be carried out with the rising air. Other kinds of
tower systems tend to be more efficient both in terms of contaminant removal and
energy consumption, although the capital costs may be higher. In a packed tower,
water is broken into slow-moving films which form and reform exposing renewed
surfaces to the rising air. The counter-current flow of air and water is most efficient
for the transfer process. Some towers are built for horizontal or cross-flow of air by
falling droplets of liquid. This type of tower has been used for removing highly
soluble gases such as ammonia from wastewaters, and has good potential for some
of the less volatile organic components. In trayed towers, air is broken into small
bubbles and pass up through a volume of water.
Thus, there are many different mechanical processes which can be used for re-
moving volatile organic materials from water. For highly volatile components which
require relatively low volumes of air to water, stripping in basins may be the best
procedure, while towers tend to be more cost effective if relatively large volumes of
air to water are required. When the choice is not critical on process grounds, other
factors such as aesthetics, noise, land area, compatibility with other processes or ex-
isting facilities, versatility, or other factors may govern the choice.
Considerable experience is available in the water supply and wastewater treat-
ment fields for the introduction of oxygen into water, especially with basin type
systems, and this experience together with knowledge of relative transfer coefficients
and Henry's law constants can be used to predict the efficiency of removing volatile
organic compounds. Towers are widely used in the chemical industry and this experi-
ence can be directly applied to the design of air stripping systems for volatile organic
removal.
Design Factors Affecting Removal Efficiency
In a stripping system, the area of contact between the air and water is generally not
known and cannot easily be measured, thus making design based upon direct use of
Equations (4) or (5) difficult. The overall mass transfer, however, can usually be
measured and this is proportional to (K,a).
For a given process the factors of importance in the efficiency of removal of a given
volatile organic are the Henry's law constant, the relative volumes of air to water,
and the rate of mass transfer. The relationship between these factors is illustrated in
the following for a countercurrent flow packed tower, and then application to other
air-stripping systems is discussed. Also of importance in design is the ultimate
capacity of a system with respect to its ability to pass liquid and gas without flooding.
This aspect is also briefly discussed in the following.
Countercurrent Packed Tower
Consider the countercurrent packed tower illustrated in Fig. 4. Air enters at the
bottom of the tower and liquid at the top. The mass of volatile componentiwhich is
removed from the water must be contained in the air leaving the system. Thus, as-
suming the volume of the air and water streams do not change significantly during
passage through the tower, and that the concentration of volatile component in the
incoming air is zero,
- C,e)= GF,e (13)
where,
128
-------
Water
/I
i
Z
\
H
CD
C
Q.
g.
i i
A
/
}
09
I
Water 1 Air
Q G
C,' C,»
Ci* = F,
Figure 4. Flow scheme and concentration profiles for a countercurrent flow air
stripping tower. Q and G represent water and air flow rate, and C, and F,
represent concentration of volatile component i in the water and air,
respectively. H, is the Henry's law constant.
Q = water flow rate in m3/s
G = air flow rate in rn3/s
C,° = concentration of volatile component i in the influent water, mg/m3
C' = concentration of volatile component i in the effluent water, mg/m3
F,e = concentration of volatile component i in the effluent air stream, mg/ m3
Then, the rate of removal of volatile component at any point within the tower is
given by,
(Q/A)(dC,/dz) = (K,a)(C,-C,*)
(14)
where C,* is the liquid concentration which is in equilibrium with the air phase
concentration and equals F,/ H,, and A is the tower cross-sectional area. The term (C,
- C,*) represents the driving force for removal of the volatile compound and varies
throughout the column as illustrated in Fig. 4. The advantage of countercurrent
operation is that C,* is high when C, is high (at the liquid entrance to the tower) and
C,* is low when C, is low (at the liquid exit from the tower), thus maintaining a more
uniform driving force throughout the tower. Higher air flow rates dilute the concen-
tration F, in the air phase and thus reduce C,*, increasing the driving force and the
removal obtained. In order to determine the overall effect of these forces, Equation
(14) can be integrated:
129
-------
.z _C,e
(15)
C,
I dz - (Q/A(K,a)) I dC,/(C, - F,/H,)
or in a short-hand form which is quite useful,
z = (HTU)(NTU) (16)
where, HTU equals the Q/ A (K,a) portion of the right side of Equation (15) and is
termed the height of a transfer unit, and NTU equals the value under the integral in
Equation (15) and is termed the number of transfer units. An HTU is defined as the
combination of flow and mass-transfer coefficient which gives one transfer unit of
separation.
NTU is a term originated by Chilton and Colburnin 193517 and expresses the total
change in bulk water concentration through the tower divided by the average effec-
tive driving force, C, — C,*. In one transfer unit, the change in concentration just
equals the effective driving force in that unit. An analytical solution for NTU for
volatile components that obey Henry's law over the concentration range of interest is
as follows15
C° - Ce (17)
NTU= _b b_
(C, - C,*)LM
Here, the denominator represents the log mean of the driving force throughout the
column and equals,
(C,° - C,0*) - (C° - C,e*) (18)
(C, - C,*)LM = -
ln((C,° - C,°*)/(C,e - C,e*))
General curves which relate removal efficiency and the height of a transfer unit to
the number of transfer units are given in general references on separation
processes15'18'19. As an example, the importance of a number of variables is illus-
trated in the following for removal of chloroform by decarbonators at Water
Factory 21 in Orange County California6. The two decarbonators are countercur-
rent flow stripping towers designed for removal of carbon dioxide from this
advanced wastewater treatment plant's reverse osmosis effluent, but have also been
found to remove volatile organics as well. Each has a design flow Q of 0.11 m3/ s (2.5
mgd), is 2 m square and contains 2.4 m of polyethylene packing for a total volume
per decarbonator of 9.6 m3. A 5-horsepower (hp) blower on top of the decarbonator
draws upward through the tower an airflow rate of 2.45 m3/sat 50 mm of hydraulic
head, or 22 m3 of air per m3 of water applied. Under these conditions, the measured
removal efficiencies obtained for trihalomethanes are summarized in Table 2.
The values for NTU and HTU for the decarbonators can be determined by first
solving Equation (13) for F,e, using the measured chloroform removal of 83 percent:
F,e = 0.11(10.6 - 0.17(10.6))/2.45 = 0.40 mg/m3
130
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Table 2. Removal of Trihalomethanes by Countercurrent Flow Packed
Tower Decarbonators at Water Factory 21
Influent Percentage Removal
Cone.* 9595
Trihalomethane mg/m3 Mean Conf. Inter.
Chloroform 10.4 79 70-85
Bromodichloromethane 4.1 85 76 - 91
Dibromochloromethane 1.7 71 49-84
'Geometric Mean
From Equation (1) and the value of H, in Table 1,
C,°* = 0.40/0.141 = 2.83 mg/m3
Equation (18) can then be solved to determine the average effective driving force for
chloroform removal,
(10.6-2.83)-(1 -0.83)10.6 3
(C, - C,*)LM - in((10.6 - 2.83)/(l - 0.83)(10.6)) *'w mg/
One transfer unit would thus remove 4.09 mg/ m3 of chloroform from the bulk water.
The number of transfer units in a decarbonator from Equation (17) is,
NTU = (10.6 - 0.17(10.6))/4.09 = 2.15
and the height of a transfer unit from Equation (16) is,
HTU = 2.4/2.15= 1.12 m
The overall mass transfer coefficient can then be determined from HTU = Q/ A(K,a),
thus
(K,a) = 0.11/(1.12)(4) = 0.0246 sec'1
The above values can be used to answer questions about how the efficiency of
chloroform removal would vary with the air to water (G/Q) ratio and with tower
height, and what influence the Henry's law constant has on overall removal effi-
ciency.
The effects of tower height and G/ Q ratio on removal of chloroform are illustrated
in Fig. 5. It was assumed here that Q/A remained constant so that K,a did not
change. The curves illustrate that for chloroform little advantage in removal effi-
131
-------
10
20 30
Air to Water Ratio, G/Q
40
50
Figure 5. Computed effect of air to water (G/Q) ratio and tower height (z) on removal
of chloroform by decarbonator with countercurrent flow. The water flow rate
per unit cross-sectional area of tower is assumed constant.
ciency is obtained with G/Q ratios greater than about 20. At values above this, the
concentration of chloroform in the effluent air stream is sufficiently low so that C,*
approaches zero. The efficiency of removal is then essentially mass transfer limited,
and changes in tower height are required to change removal efficiency. With G/Q
ratios less than about five, however, increased tower height has little effect on re-
moval efficiency. Here the rate of transfer is limited by the low air flow rate which
provides insufficient dilution of chloroform to maintain a high driving force.
The effect of the Henry's law constant on removal efficiency by the Water Factory
21 decarbonators is illustrated in Fig. 6. For this case, the overall mass transfer coef-
ficient (K,a) was assumed to be constant and equal to that for chloroform. Fora
given G/Q ratio, the higher the value of H,, the higher the removal efficiency. Also,
the higher the value of H,, the lower the G/ Q ratio at which the system becomes mass
transfer limited. With the G/Q ratio of 22 used in the decarbonation tower at Water
Factory 21, materials, with an H, value of 0.042 and above should be quite efficiently
removed. From Table 1, this includes a large number of priority pollutants including
chlorinated benzenes, several halogenated one- and two-carbon compounds, and
several aromatic hydrocarbons. Another range of compounds with somewhat lower
H, values would be only partially removed, while several organic compounds includ-
ing phenols and several pesticides would have almost zero removal.
132
-------
100 -
10
20 30
Air to Water Ratio, G/Q
50
Figure 6. Computed effect of air to water ratio and Henry's law constant (H,) on
percentage removal for countercurrent flow stripping tower or decarbonator
at Water Factory 21 The mass transfer coefficient is assumed constant
and similar to that for chloroform.
Flooding
Flooding of a stripping tower results when the downward movement of water is
prevented by the upward movement of air, and must be avoided. Flooding is related
to the ability of the two phases to flow past each other in opposite directions in suf-
ficient quantity and within the confines of the tower. The upward flow of air exerts a
drag force on the falling water. This results in both an increase in pressure drop for
the air and a reduced velocity of downward flow for the water. If the flows are in-
creased, a point will be reached where the pressure drop for the rising air just equals
the gravity head for the falling water which will then be unable to move through the
packing at the desired rate; it will tend to build up in the tower and reduce the air flow
rate. The tower then becomes unstable.
Countercurrent towers are generally designed to operate at no more than about 50
to 80 percent of their flooding limit in order to allow a factor of safety in the opera-
tion. Sherwood, et al.20 developed a flooding correlation for packed towers, and
these correlations have been modified by several researchers including Leva21 and
Peters and Timmerhaus22. These correlations are commonly given in reference
texts15'23 in the form of graphs relating gas and liquid flow rates, their physical prop-
erties, and the characteristics of the packing material.
133
-------
From such correlations, Fig. 7 was prepared to illustrate the relationship between
the G/Q ratio, the water application rate per unit, cross-sectional area of tower
where flooding will occur (Q/ A)t, and the properties of the packing material. The
figure was derived for randomly placed packing. With stacked packing, flooding
occurs at higher flow rates. Design water flow rates should be about one-half of the
(Q/ A)f value obtained from the figure. The figure was also developed for air flow at
one atmosphere of pressure. For operation at altitudes much above sea level, the
actual G/Q ratio would need to be decreased in direct proportion to the decrease in
atmospheric pressure to obtain the proper value for the abscissa of the graph in Fig.
7.
The F values or packing factors are related to the properties of the packing
material, with typical values given in Table 3. The lower the packing factor, the
higher the capacity of the tower as indicated in Fig. 7. Low packing factors are asso-
ciated with low packing surface areas per unit volume of tower and with high void
volume in the packing material. The factors which tend to make F low also tend to
decrease the overall rate of mass transfer so that a compromise between these factors
is necessary in selecting an appropriate packing material.
Fig. 7 shows that with higher G/Q ratios, the liquid loading where flooding occurs
is reduced. Thus, higher air flow not only requires larger power requirements and
larger fans, but also larger tower cross-sectional area.
Cross-Flow Stripping Tower
Cross-flow stripping towers are used extensively for water cooling at chemical
plants and power plants, and also for ammonia stripping at wastewater treatment
plants because they permit a high air flow rate with respect to water flow rate with
relatively small energy usage and without flooding. They also offer promise for re-
moval from water of more soluble trace organic compounds, or those with a rela-
tively low Henry's law constant. The relationship between air and water flow in a
cross-flow stripping tower is illustrated in Fig. 8.
Higher air flow rates are possible with cross-flow towers because the side area of
the tower through which the air flows is relatively large and thus the airflow rate per
unit of area is small. The disadvantage is that the efficiency of contaminant removal
for a given air to water ratio is less than in a counter-current packed tower. As air
flows horizontally through the tower, a volatile contaminant is stripped from the
water which flows downward through the packing. The contaminant concentration
in the air leaving the tower is higher than in that entering the tower, and so the driving
force for removal is less on the leaving side. Therefore, as illustrated in Fig. 8, the
concentration profile for the volatile compound in the water on the two sides of the
tower, and at any point in between, is different.
Thibodeaux, et al.24 have recently developed an analytical solution for volatile
compound removal by cross-flow towers which is similar to the number of transfer
unit approach described in the previous section for counter-current flow towers. The
reader is referred to their article for details. Their approach was taken to analyze the
operation of the large cross-flow stripping towers used at Water Factory 21 for
removal of ammonia from reclaimed wastewater. In this analysis, the efficiency of
ammonia removal measured was used to estimate the overall mass transfer coeffi-
cient for the tower, and with this value in hand, the efficiency that should be obtained
as a function of air to water ratio and Henry's law constant was calculated.
The polypropylene splash-bar packing over which water cascades in each of the
two Water Factory 21 ammonia stripping towers is 55 m long by 4.9 m wide and 7.3
m deep for a total volume per tower of 1970 m3. The packing consists of 1.3 cm
diameter bars spaced 7.6 cm apart in crisscross pattern for alternate rows. Air is
drawn horizontally through the packing by six 125-hp fans, each 5.5 m in diameter.
The fans are operated by 2-speed electric motors which can draw 990 m3/ s through
each tower, or 3000 m3 air per m3 of water at the design water flow rate of 0.66m3/s.
134
-------
0.01
10 20 50 100
Air to Water Ratio, G/Q
200
500 1000
Figure 7. Relationship between the air to water ratio (G/Q) and the water flow
rate per unit cross-sectioned area of tower (Q/A), which produces flooding
in countercurrent flow towers for packing media with various F values.
The pH of the water applied to the stripping towers was generally about 11.2 as re-
quired for efficient ammonia removal. However, for the volatile organic compounds
studied, removal efficiency should be independent of water pH.
The influent and effluent concentrations to the tower for ammonia and a range of
trace organic compounds as determined for two of the three different periods of
Water Factory 21 operation are listed in Table 4. Also listed are the average
percentage removal and 95 percent confidence intervals for the average removals6.
During the second period covering a year and one-half of operation, Water Factory
21 received a trickling filter treated wastewater which had a high ammonia and
volatile organic concentration, and the fans were in full operation. However, during
the third period covering about nine months of operation a nitrified activated sludge
effluent was treated and the fans were not operated as ammonia removal was not
necessary. However, some removal of ammonia occurred and a high percentage
removal of several volatile trace organic compounds was also still obtained. Air
movement then occurred through the tower by natural draft even though the fans
were not in operation.
135
-------
Table 3. Packing Factors F for a Variety of Packing Materials (After Nonhebel18)
Nominal Packing
Type of Packing
Raschig rings
0.8 mm wall
1.6 mm wall
3.0 mm wall
Bert saddles
Intalox saddles
Pall rings
Pall rings
Size (mm)
Material
Ceramic
Metal
Metal
Metal
Ceramic
Ceramic
Plastic
Metal
6
Packing
4590
2300
-
-
2950
1970
-
-
13
Factor-F
2100
985
1115
-
785
655
-
-
16
1250
625
950
-
-
-
320
235
19
855
510
755
-
555
475
-
-
25
525
375
475
-
360
320
170
155
32 38
410 310
-
360 270
-
215
170
105
92
51
215
-
185
-
150
130
82
66
76
120
-
105
125
-
-
-
-
-------
.C
?
'
£.
g>
'»
C, - C,*
Air Entrance
Cross Flow Tower
C,
Air Exit
Cone.
F,e
c,*=TT
Figure 8. Flow scheme and concentration profiles for a cross flow air stripping
tower For air entrance concentration profile, F° assumed equal to zero.
The overall mass transfer coefficient for ammonia removal during the second
period as defined by Thibodeaux, et al.24 was found to be 0.0009 sec"1, and using this
value and their procedure, the efficiency of ammonia removal was found as a
function of the air to water ratio. First, the air flow rate was assumed constant at 990
m3/s for each tower and the water flow rate was assumed to vary. Since H, for
ammonia is low, the transfer is predominantly gas-phase controlled, and with con-
stant gas flow rate the overall mass transfer coefficient should not vary. Next, the
water flow rate was held constant at 0.66 m3/ s and the air flow rate was allowed to
vary. For this case the mass transfer coefficient would be a function of the air flow
rate and was assumed to equal 5.43 (G/990)°64 as cited in the literature24.The results
for both the water variable and air variable cases are shown in Fig. 9. The difference
between the two cases is not great. One might use these results to estimate the air flow
rate during the third period when the fans were not in operation. For 25 percent
ammonia removal the air to water ratio should be 225, or the range based upon the
95 percent confidence interval of removal, the ratio would range from 100 to 350.
These values correspond to gas flow rates per tower of 150 with a confidence interval
range of 66 to 230 m3/s.
The air to water ratios obtainable for the ammonia stripping towers are quite high,
and should be sufficient to remove a wide range of volatile organic compounds. As-
suming the liquid-phase controlled mass-transfer coefficient for ammonia applied,
137
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Table 4. Removal of Volatile Compounds by Cross-Flow Packed Stripping
Tower at Water Factory 21
Compound
Trihalomethanes
Chloroform
Dibromochlorometha ne
Chlorobenzenes
Chlorobenzene
1,2-Dichlorobenzene
1 ,3-Dichlorobenzene
1 ,4-Dichlorobenzene
Tetrachloroethylene
1 ,1 ,1 -Trichloroethane
Fans Operating
Percentage Removal
Cone.* 95% Conf
mg/m3 Mean Interval
1.1 83 70-91
3.0 96 88 - 99
1 .0 97 88 - 99
No Fans Operating
Cone.*
mg/m3
4.1
0.8
0.6
0.1
1.3
2.5
4.7
Percentage
Removal
95% Conf
Mean
79
82
88
83
92
95
91
Interval
64-87
76 - 87
61 -96
60-93
89- 94
88-98
76-96
'Geometric Mean
-------
100
80
60
5:
m
ID
1C
40
20
Variable
Air Flow
I
500 1000 1500
Air to Water Ratio, G/Q
2000
2500
Figure 9. Calculated effect of air to water ratio (G/Q) on efficiency of ammonia
removed by Water Factory 21 cross-flow stripping tower. In one case, the
G/Q was changed by varying the air flow rate, and in the other case by
varying the water flow rate.
percentage removals of volatile materials as a function of air to water ratio and
Henry's law constants were calculated. The results are shown graphically in Fig. 10.
Compounds with Henry's law constants down to almost 1CT3 should be efficiently
removed by the cross-flow stripping towers. Based upon the compilation in Table 1,
this represents a large number of priority pollutants. Unfortunately, adequate data
were not obtained for removals of compounds other than those with relatively high
H, values, and so the true potential of cross-flow towers was not measured. In fact,
most compounds for which adequate data were obtained could be removed well by
the normal counter-current stripping towers. They were all removed efficiently by
the ammonia stripping tower, even when the blowers were not in operation. The
latter indicates that natural-draft towers may have potential for stripping of
relatively volatile compounds and at low energy consumption. The trade-off is
higher capital cost for tower construction.
Clearly, cross-flow stripping towers have good potential for removal of organic
compounds with intermediate water solubility and Henry's law constants. Also, if
designed for natural draft rather than forced draft, they may offer potential for
removal of more volatile compounds at low energy cost. These areas deserve further
study.
139
-------
100
500
1000 1500
Air to Water Ratio, G/Q
2000
2500
Figure 10. Calculated effect of air to water ratio and Henry's law constant on
removal efficiency by Water Factory 21 cross-flow stripping tower
Stripping Basins
In stripping basins for removal of organic contaminants, air is brought into
contact with water either by diffused aeration or mechanical aeration. Such basins
are commonly used for introducing oxygen into biological treatment systems, and
experience developed on mass transport rates for oxygen are directly applicable to
the design of stripping basins.
Fig. 11 illustrates the flow of air, water, and volatile material in diffused aeration
and mechanical aeration stripping basins. The air and water flow rates are easily
monitored in a diffused aeration system, and so control of the air to water ratio is
relatively straightforward. In a mechanical aeration system, however, the air flow is
generally not controlled and varies randomly with such factors as wind speed. This
problem was considered at a mechanical aeration stripping basin designed for the
Palo Alto, California, wastewater reclamation plant operated by the Santa Clara
Valley Water District. Here blowers were installed to draw air across the basin in an
effort to enhance the removal of ammonia from the wastewater. This system is used
as an example in the following to illustrate the variables of importance in a stripping
140
-------
Q C ,°
Water
C,
Surface
Aerator
Q C,
*
Water
Q C ,°
Water
Air
1
Air
F,°
00 °
00 0 °
0 O O
Q C,e
Water
Diffused Air
Aerator
Figure 11. Flow schemes for surface aerator and diffused air-stripping basins.
basin. Diffused aeration systems behave in a similar fashion and so the analysis is
appropriate for their design also.
For this analysis it is assumed that the mechanical aerator results in the complete
mixing of the basin contents so that the concentration of volatile compound C, is the
same throughout the basin. For this case the effluent concentration of the compound
C, would also equal the basin concentration. Also, it is considered that the air being
drawn over the top of the basin contains none of the compound so that F,° equals
zero. The air is also considered to be completely mixed over the top of the basin by
the mechanical aerator so that the concentration there and indeed in the air leaving
the basin is the same and equals F,e. For this case under steady-state conditions of
operation, the difference between the mass flow rate of volatile component flowing
into the basin and flowing out of the basin equals that leaving the basin by stripping,
Q(C,° - C,e) = K,aV(C,e - C,*)
(19)
where V is the basin volume in m3. Since the volatile compound enters the air, then it
must also be true that
GF,e = K.,aV(C,e - C,*)
(20)
The value C,* represents the water concentration of compound i which is in equilibri-
um with the air concentration and from Equation (1) equals F,e/ H,. By combining
this relationship with Equations (19) and (20) and rearranging, the following
equation for the fractional removal of compound i through mechanical aeration
basin stripping results
(21)
GH,
141
-------
Equation (21) was used to evaluate the stripping basin at the Palo Alto reclamation
plant. This basin treats a steady flow of 0.044 m3/ s (1 mgd) and is 16.8 m long, 9.1 m
wide, and 5.8m deep for a total volume V of 887 m3. The basin contains two 50-hp
floating spray mechanical aerators. A 30-hp centrifugal fan draws 19 m3/s of air
under 3.8 cm of static pressure through a series of 0.3 mX0.6 m openings located just
above the basin water surface causing air to flow down and over the basin. Grab
samples were collected of the basin influent and effluent once per week over a period
of about two months and analyzed for removals obtained for several volatile organic
compounds. The results are summarized in Table 5. Removals in general were above
90 percent.
Equation (21) was rearranged and used to calculate the mass transfer coefficient
for the measured chloroform removal, giving a value for K,a of 0.0007 per sec., with a
range based upon the 95 percent confidence interval for removal of 0.0003 to 0.002
per sec. Values within this range were also obtained for the chlorinated benzenes, but
for the chlorinated ethanes and ethylenes, the average values were all near the upper
end of the range. The reason for the higher values for the latter needs further explor-
ation.
It is common in the design of mechanical aeration systems to specify transfer
efficiencies in mass of a compound transferred per unit time per unit of energy input
to the aerator under some standard operating conditions. For example oxygen
transfer ratings are commonly expressed in pounds per hour per unit horsepower.
Through introduction of appropriate conversion factors such values can be con-
verted to give mg transferred per m3 of basin per second (r) as follows:
r = 0.126m/V (22)
where 0.126 converts Ib/hr to g/s, and m is the aerator rating in Ib/hp-hr under
standard conditions. For oxygen the standard conditions are generally 20° C, for
which the saturation concentration for oxygen at one atm is 9.2 g/ m3 and the basin
dissolved oxygen content is zero. The mass transfer coefficient would then be
K,a = 0.126mP/V(9.2-0) (23)
where P is the horsepower rating for the blower. The value for m is not known for the
mechanical aerators at Palo Alto, but typical values are 2 to 4 lb/ hp-hr. This would
give a range of computed K,a values from 0.003 to 0.006/ s. From Equation (8) and
diffusion coefficients in Table 1, the value for chloroform should be about one-half
of that for oxygen. While the data available are not adequate to provide a good test,
the K,a value obtained for chloroform appears in the correct range. Equation (21)
indicates that both the detention time of water in the basin (V/ Q) and the air to water
ratio (G/Q) affect the fractional removal of a volatile component. Using the mass
transfer coefficient determined for chloroform, the effect of these variables was
determined for the Palo Alto stripping basin, and the results are illustrated in Fig.
12. The air flow rate used is obviously much higher than needed to remove
chloroform efficiently. Also, the detention time could be lowered significantly with
little effect on removal, although the present basin is about what is required to
adequately handle the mechanical aerators. The selection of optimum operating
conditions for a desired removal efficiency should consider construction and
operating costs. The biggest factor in the operating costs are for power to operate the
aerators and the blower.
An analysis of a diffused air system is not given here because of space limitations,
but would be similar to that given for the mechanical aeration system. In a similar
way, experience with oxygen transfer by diffused air systems should be most helpful
in design. A limitation is that power requirements for introducing air under a high
142
-------
Table 5. Removal of Organic Compounds by Spray Aeration Stripping
Basins at Palo Alto Wastewater Reclamation Plant
Compound
Total Chlorinated Organtcs
Purgeable
Non-purgeable
Chloroform
Bromodichloromethane
Dibromochloromethane
1 ,1 ,1 -Trichloroethylene
Trichloroethylene
Tetrachloroethylene
1 ,2-Dichlorobenzene
1 ,3-Dichlorobenzene
1 ,4-Dichlorobenzene
1 ,2,4-Tnchlorobenzene
Octylcyanide
Nonylcyanide
Number of
Paired
Samples
Analyzed
5
5
6
6
6
6
6
6
5
5
5
5
5
5
Influent
Cone.
mg/m3
168
151
18
1.9
0.50
106
38
58
8.3
3.1
5.1
14
0.97
0.25
Percentage Removal
Mean
95
5
92
94
82
99
99
99
91
94
92
92
86
70
95% Conf.
Interval
91
-28
84
84
31
97
97
98
68
80
76
79
49
33
-98
- 30
-96
-98
-95
-997
- 99.6
-99.8
-97
-98
-97
-95
-96
-86
hydrostatic pressure is much greater than for a mechanical aeration system, and so
diffused aeration could be quite expensive if a high air to water ratio were required.
They may, however, be more economical for removal of more volatile compounds
where high G/Q ratios are not required.
SUMMARY AND DISCUSSION
Many different methods are available for stripping volatile organic materials from
water, but in general they rely upon the same physical principles. The major char-
acteristic of organic compounds affecting the ease with which they can be removed
from water is Henry's law constant, the higher the value, the more volatile the com-
pound and the more readily it can be removed from water. The rate of mass transfer
of a compound from water is also a function of its diffusivity, but this characteristic
does not vary greatly within the range of organic compounds which are susceptible
to stripping.
The three major factors that affect the efficiency of removal by a given process are
the air to water ratio, the detention time, and the rate of mass transfer. In a stripping
tower the actual detention time of the water is not readily measured, but is a function
of the tower height which is generally the design variable used. For a given system
with a given rate of mass transfer, less volatile compounds require either a greater
detention time in which to accomplish a given efficiency of removal, or a greater air
to water ratio in order to increase the driving force for the transfer. Mass transfer
may be either liquid-phase controlled for compounds with Henry's law constants
well above 7 X 10 3, or gas phase controlled when values are well below this. When
removals are liquid-phase controlled, then mass transfer coefficients determined
143
-------
100 -
100 200 300 400 500
Air to Water Ratio, G/Q
Figure 12. Computed effect of air to water ratio (G/Q) and water detention time
(V/Q) on chloroform removal by the Palo Alto Reclamation plant
mechanical aerator stripping basin.
with oxygen can be used for design after appropriate correction for differences in
diffusivity are made. When removals are gas phase controlled, then transfer
coefficients for other materials such as ammonia can be used. The mass transfer
coefficient for a given system is a function of many factors, but in general the greater
the turbulence of the controlling phase and the greater the exposed air-water surface,
the higher the rate of transfer.
Finally, the most appropriate system for a given situation depends upon
economics plus a variety of other considerations previously mentioned. Perhaps the
major trade-off is between construction cost and power costs. Table 6 presents a
summary of the power requirements and system volume for the two tower strippers
and the mechanical aeration stripper discussed in this chapter. Also included is an
estimate of power requirements for a diffused air system. It is obvious that for high
air to water ratios, stripping towers or a mechanical aeration system with air flow
across the basin are far superior to a diffused air system. Also, in order to achieve
very high air to water ratios without flooding, cross-flow towers have a decided
advantage over counter-current towers.
Experience is relatively limited with the use of stripping towers for removing
hazardous organic chemicals from water supplies. In this chapter an attempt was
made to illustrate that the fundamental principles of strippers have been quite well
developed within the chemical engineering field and are directly applicable to the
water treatment field. One need is for a good economic analysis of the different
144
-------
Table 6. Summary of Operational Characteristics and Power Requirements for Various Stripping Processes
nr-rnti«nn. rhoMrtn,.rti« Horsepower Requirements
i_r|jriaiiuMCTi v^i mi nv,i _ i mi II.IT HydrBUlIC
Volume Q V/Q Kja Head Mech.
Process m3 mVs G/Q min. sec ' Pumping* Aerator Blower
Countercurrent tower
(decarbonator) 9.6 0.11 22 1.45 0025 8.4 0 5
Cross flow tower
(ammonia stripping) 1970 0.33 2700 99 0.0009 61 0 750
Mechanical aeration
(ammonia stripping) 887 0.044 430 336 0.007 0 100 30
Diffused aeration
Horsepower per
mVmin. of flow
Water f AirJ
1 3 0.034
3.1 0.014
38 0.026
1.5§
•Assumed 60% overall efficiency for pumping.
fHydraulic head or mechanical aerator power requirements divided by Q expressed in mVmin.
JBIower horsepower divided by air flow rate in mVmin.
§Calculated using Eq 129, p. 510, Metcalf and Eddy, WastewaterEngineering, McGraw-Hill (1972), for blower
brake horsepower, 70% power efficiency, 30°C ambient temp, and 8 psi discharge pressure.
-------
stripping processes in order to determine more precisely when each is most
appropriate for use. Another concern is that stripping processes result in the transfer
of a contaminant from one media to another, and since the overall objective of
treatment is to reduce human exposure one must be certain that stripping does not
simply substitute one problem for another. In this vein, research is needed to
evaluate methods for cleaning the air of the volatile compounds after removal from
water. Perhaps activated carbon can be used for this purpose. An advantage might
be that the carbon would be spent less rapidly than if used directly with water, and
regeneration costs could be greatly reduced. Additional research is also needed to
evaluate the effectiveness of air-stripping for removal of less volatile compounds.
While two of the stripping processes discussed in this chapter had adequate air to
water ratios to accomplish this, adequate analyses were not available to evaluate
removals for compounds with lower Henry's law constants. Research is also needed
to better evaluate the Henry's law constants and diffusivity for compounds of im-
portance since the values presented here are simply unverified estimates.
In summary, air-stripping has good potential as an economical water treatment
process. It has a long history of usage in the water treatment field for removal of in-
organic gases and some taste and odor producing compounds, and has direct rele-
vance to the organic problems of concern today. It has potential for direct treatment
of surface waters that may contain many different volatile organic chemicals, for
disinfected waters containing high concentrations of trihalomethanes, and for
contaminated groundwaters that in a growing number of cases contain volatile
organic chemicals in relatively high concentration. Today, there appears to be many
new and important applications for this old water treatment process.
REFERENCES
1. Brush, C.B. "Aeration and Filtration of Water," Jour. New England Water
Works Association, 2, 71, 1887.
2. Baylis, J.R. Elimination of Taste and Odor in Water, McGaw-Hill Book Co.,
New York, NY, 1935.
3. U.S. Public Health Service. "Municipal Water Facilities: Inventory as of
January 1, 1958," 1958.
4. Amer. Water Works Assoc. Water Quality and Treatment, McGraw-Hill
Book Co., New York, NY, 197!.
5. McCarty, P.L., M. Reinhard and D.G. Argo. "Organics Removal by Advanced
Wastewater Treatment," Proc., Amer. Water Works Assoc. 97th Annual Con-
ference, Part I, 5-3, 1-26, May 1977.
6. McCarty, P.L., K.H. Sutherland, J. Graydon and M. Reinhard. "Volatile
Organic Contaminants Removal by Air Stripping," Seminar on Controlling
Organics in Drinking Water, Pub. No. 20148, Amer. Water Works Assoc.
1979.
7. Houel,N., F.H. Pearsonand R.E. Selleck." Air Stripping of Chloroform from
Water," Jour. Environmental Engineering Division, ASCE, 105, 111, 1979.
8. Singley, J.E., A.L. Ervin, M.A. Mangone, J.M. Allan and H.H. Land. Trace
Organics Removal by Air Stripping, American Water Works Association,
Denver, May 1980.
9. Matter-Muller, C., W. Gujer, W. Giger and W. Stumm. "Non-Biological
Elimination Mechanisms in a Biological Sewage Treatment Plant," Progress in
Water Technology, to be published.
10. McCarty, P.L., M. Reinhard, J. Graydon, J. Schreiner, K. Sutherland, T.
Everhart and D.G. Argo. Advanced Treatment for Wastewater Reclamation at
Water Factory 21, Tech. Report No. 236, Department of Civil Engineering,
Stanford University, Stanford, CA, January 1980.
11. McCarty, P.L., M. Reinhard, C. Dolce, H. Nguyen and D.G. Argo. Water
Factory 21: Reclaimed Water, Volatile Organics, Virus, and Treatment Per-
146
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formance, EPA-600/2-78-076, 1978.
12. Mackay, D. and P.J. Leinonen. "Rate of Evaporation of Low Solubility Con-
taminants from Water Bodies to Atmosphere," Environ. Science and Tech-
nology, 9, 1178, 1975.
13. Tsivoglou, E.C., R.L. O'Connell, C.M. Walter, P.J. Godsiland G.S. Logsdon.
"Tracer Measurements of Atmospheric Reaeration I. Laboratory Studies,"
Jour. Water Pollution Control Fed., 37, 1343, 1965.
14. Rathbun, R.E., D.W. Stephens, D.J. Shulta and D.Y. Tai. "Laboratory
Studies of Gas Tracers for Reaeration," Jour. Environ. Engineering Division,
Amer. Society of Civil Engineers, 104, 215, 1978.
15. King, C. Judson. Separation Processes, 2nd ed., McGraw-Hill Book Co., New
York, NY, 1980.
16. Wilke and Chang. Jour. American Institute of Chemical Engineers, 1, 264,
1955.
17. Chilton, T.H. and A.P. Colburn. "Distillation and Adsorption in Packed
Columns," Ind. Eng. Chem., 27, 255, 1935.
18. Nonhebel, G., ed. Gas Purification Processes for Air Pollution Control,
Butterworth and Co., London, 1972.
19. Treybal, R.E. Mass Transfer Operations, 3rd ed., McGraw-Hill Book Co.,
New York, NY, 1980.
20. Sherwood, T.K. et. al. "Flooding Velocities in Packed Columns," Ind. Eng.
Chem., 30, 765, 1938.
21. Leva, M. Chem. Eng. Prog. Symp. Ser., 50, 51, 1954.
22. Peters, M.S. and K.D. Timmerhaus. "Plant Design and Economics for En-
gineers," 2nd ed., Chapter 15, McGraw-Hill Book Co., New York, NY, 1968.
23. Kwanten, F.J.G. and J. Huiskamp. "Gas Absorption Towers," Gas Purifica-
tion Processes for Air Pollution Control, ed. G. Nonhebel. Butterworth and
Co., London, 1972.
24. Thibodeaux, L.J., D.R. Daner, A. Kimura, J.D. Millican and R.J. Parikh.
"Mass Transfer Units in Single and Multiple Stage Packed Beds, Cross-Flow
Devices," Ind. Eng. Chem., Process Design Dev., 16, 325, 1977.
25. Environmental Protection Agency, Innovative and Alternative Technology
Assessment Manual (Draft), EPA^30/9-78-009, 1978.
26. Hodgman, C.D., R.C. Weast and S.M. Selby, eds. Handbook of Chemistry
and Physics, 39th ed., Chemical Rubber Pub. Co., Cleveland, OH, 1958.
27. Perry, J.H. Chemical Engineers'Handbook, 4th ed., McGraw-Hill Book Co.,
New York, NY, 1963.
147
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OXIDATION OF ORGANIC SUBSTANCES IN
DRINKING WATER
William H. Glaze
INTRODUCTION
Although oxidation processes occur to some extent in virtually all drinking water
treatment plants, oxidation per se has not been viewed traditionally as an important
unit process. Two classic texts on water treatment by Hopkins and Bean' and by
James2 do not list oxidation as a principal method for drinking water treatment.
Several decades after the original appearance of these texts, two newer monographs
by Gulp and Gulp' and Hammer also do not list oxidation techniques in their
chapter headings. Two reference texts on water treatment by the Nalco Chemical
Company^ and Degremont6 cover oxidation-reduction processes, but the emphasis
in the drinking water sections is on disinfection. No discussion of the use of oxidants
for control of toxic organic substances is given. In a recent two volume review on
water reuse and recycling technology by the firm of Gulp/ Wesner/ Gulp,7 removal of
organic substances is covered briefly with no mention of oxidation as an alternative
treatment method. Ozonation is covered only in a section emphasizing its disin-
fection capability.
This situation most surely will change in the coming decade as more stringent
standards for drinking water quality are prescribed.8 In particular, oxidation
processes offer one of the few means for the removal of potentially toxic organic sub-
stances such as pesticides, industrial solvents, and other synthetic organic chemicals.
These substances generally are not removed by traditional water treatment pro-
cesses, and to the extent that their presence is a source of concern, oxidation
processes may develop into important treatment alternatives
When speaking of oxidation processes in drinking water treatment, most authors
confine their discussion to iron and manganese control and to the destruction of
offensive tastes and odors.1 4 While some citations to these applications will be
listed, this review will discuss mainly the use of oxidants for control of synthetic
organic chemicals. We also shall include in our discussion the oxidation of naturally
occurring organic compounds (such as trihalomethane precursors) which may be
transformed by disinfection processes into by-products of concern to human health.
Three oxidation processes will be emphasized in this chapter: chlorine (in its
various aqueous forms), ozone, and chlorine dioxide. In addition, other oxidants
such as permanganate, hydrogen peroxide and catalytic systems such as ozone with
ultraviolet radiation will be discussed. In most cases, the oxidation process will be
discussed in terms of its basic chemistry, studies which reveal its relative efficacy for
organic compound removal, and the expected costs of applying the process to water
treatment. Evidence concerning by-product formation will also be presented.
The Author Dr Gla/e received his Bachelor of Science degree in chemistry at Southwestern University, his
Master of Science degree at the University of Wisconsin in physical polymer chemistry, and his Ph D at the
same institution in physical chemistry Alter a year of post-doctoral study at Rice University in 1960-61, he
joined the faculty of North Texas State U Diversity where he served as Professor of Chemistry and Director of
the Institute of Applied Sciences In 1980 he moved to the University of Texas at Dallas where he serves as
head of the Graduate Program in Environmental Sciences and Professor of Chemistry Dr Glaze has
published extensively on the subject of environmental sciences and the physical chemistry of water disin-
fection
148
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Basic Discussion of Oxidation Processes
Although one usually thinks of oxidation as a chemical reaction involving oxygen
such as the burning of a fuel, literally speaking, oxidation refers to the loss of
electrons by the substance being oxidized.9 This process need not involve oxygen; for
example, it may happen at an electrode in an electrolysis cell. In water treatment
applications, however, oxidizing agents usually involve an active chemical species
which contains oxygen.
Oxidation always involves the increase in the oxidation state of the oxidized
species; for example, when carbon monoxide is oxidized to carbon dioxide
(Equation 1),
2CO + 02 - 2CO2 (1)
the formal oxidation state of carbon increases from the +2 state to the +4 state.
Simultaneously, and by necessity, the oxidizing agent (oxygen in this case) is
reduced to a lower oxidation state. Water treatment oxidants may involve oxygen as
the reducible species, as in ozone, hydrogen peroxide or oxygen itself. Alternately,
another element may be reduced (i.e., act as the oxidant) as manganese in MnO4 or
chlorine in chlorine dioxide or hypochlorite.
All of these oxidants have sufficient oxidizing power to oxidize organic com-
pounds to carbon dioxide. This power is measured by the standard potential (E°) of
the agent acting as one-half of a redox cell. Values of E° are listed in Table 1 for six
important oxidants.'0 By convention, the reactions are written as reduction
processes.
One may use these thermodynamrc potentials to calculate the overall free energy
change of a redox process in order to establish whether the process should be
spontaneous. For example, reactions (2) through (4) describe the permanganate
oxidation of oxalic acid, a relatively refractory compound.
MnO4~ + 4H+ + 3e~ - MnO2(s) + 2H2O (2)
H2C2O4 - 2CO2 + 2H+ + 2e~ (3)
2MnO4" + 3H2C2O4 + 2H+ — 2MnO2(s) + 4H2O + 6CO2 (4)
E° for the net reaction (4) is calculated from the sum of E° values in Table 1 for the
half reactions. The value is 2.19 volts and the free energy change, —303 kilocalories,
is calculated from -nFE° where n is the total electrons (six) transferred in Equation
(4) and F is the Faraday constant. The high negative free energy change shows that
reaction (4) should occur spontaneously." In actual fact, the process, while
thermodynamically allowed, is too slow for application in a real water treatment
situation. This situation prevails for most organic oxidation processes. Although
oxidation of substrate may occur, the product usually is not carbon dioxide; rather,
a series of partially oxidized by-products are formed. In short, few oxidation
processes even as simple as Equation (4) occur as predicted by thermodynamics.
They must be tested in actual reaction systems to determine their feasibility. In the
case of the oxidants considered here, all are theoretically capable of oxidizing
organic substances to CO2, but in actual practice, conditions are seldom extreme
enough to reach this goal and the oxidation process is usually incomplete.
It is interesting to note that this conclusion does not apply to many biological
systems, where oxidation to carbon dioxide is accomplished routinely.12 Indeed
many life processes consist of the oxidation of carbonaceous substances as a means
149
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Table 1. Potentials of Selected Oxidants at 298°K™
Half-Reaction E° (Volts)
03 + 2H + + 2e~- 02 + H2O 2.07
H202 + 2H+ +2e~-2H2O 1.77
MnO4~+4H+ + 3e~-MnO2(s) + 2H2O 1.70
CIO2 + 4H+ + 5e~- Cl~+ 2H2O 1.51 *
HCIO + H + + 2e~~ Cl~+ H2O 1.49
CIO~+ H2O + 2e~- Cl~+ 20H~ 0.90
2C02 + 2H+ + 2e~- H2C204 -0.49
*Value calculated since couple is unknown.
of deriving energy for the sustenance and reproduction of the species. The biological
degradation of organics in wastewater treatment is merely an exploitation of these
processes for purification purposes. Lately there has been much renewed interest in
the utilization of biological oxidation for the removal of organic impurities from
drinking water. Particular attention is being focused on bacterial oxidations which
may occur on filters, especially granular carbon filters. I3~14 We shall return to that
subject in a later section of this chapter.
Not all organic compounds are biodegradable, of course. Carbon in nature often
occurs in highly reduced states such as coal, peat and petroleum hydrocarbons which
cannot easily be assimilated by biochemical systems. Moreover, it is very common
for organic substances of ecological concern to be in a reduced state. Polynuclear
aromatic hydrocarbons, most pesticides and other synthetic organic chemicals thus
are candidates for oxidation. Because these substances are non-polar, they may
accumulate in lipid tissues.15 As they are oxidized, they become more water soluble
and generally more biodegradable. l6 However, they are not necessarily less toxic in
these intermediately oxidized forms, as the metabolites of benzo[a]pyrene attest.17
In this case, it is the oxygenated metabolites, not the original compound, which have
been identified as active carcinogens.
The implications of all of these considerations are often missed by scientists and
engineers who propose to utilize oxidation processes for removal of toxic substances
in drinking water. Such processes seldom remove the organic matter; they merely
convert it to a set of new by-products of intermediate oxidation states. While the
toxicity due to the original substrate may be removed in the process, other toxic
effects may be substituted due to the formation of by-products. Viewed in this light,
by-product studies become a necessary and important part of R & D programs
which evaluate oxidizing agents. EPA studies directed toward this goal are presently
in progress.
Finally, it should be noted that oxidation processes are often energy intensive and
costly unit processes which may add significantly to the cost of water treatment.18 To
a large extent this explains the lack of common use of these processes in present
treatment plants. Oxidation with permanganate or other chemicals is now used only
as a last resort when other cheaper methods fail, as in taste and odor crises. Nonethe-
less, with the advent of concern over synthetic organic chemicals in drinking water,
and with the increase of interest in water reuse, chemical oxidation processes are
subjects of several recent research and demonstration projects. One may expect this
interest to continue as criteria for water quality grow more stringent.
150
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CHLORINATION AS AN OXIDATION PROCESS
Chlorination has been used in drinking water treatment since before the turn of
the century, primarily as a disinfectant.19 White has reviewed the chemistry of
aqueous chlorine20 and has pointed out it is hypochlorite ion (~OC1) and
hypochlorous acid (HOC1) which under usual conditions are the principle chemical
species (Equation 5). The former predominates at high pH, the latter at low pH with
the transition occurring near pH 7.5.
Cb + H2O H HOC1 + H+ + Cf (5)
H+ + OC1
Both of these species are strong oxidizing agents, having standard electrode
potentials of 1.49 and 0.90 volts, respectively (Table 1). Even when used as a
disinfectant, chlorine acts also as an oxidizing agent of organic material in the
water.21 Indeed most authorities in the field suggest that the majority of the chlorine
demand* in drinking water treatment is due to oxidation processes.22
Chlorine in its aqueous forms has another chemical property which has become of
paramount importance in recent years—the ability to substitute for hydrogen in
certain organic chemical environments.24 It is this property which leads to the
formation of carbon-bound chlorine during drinking water disinfection. This fact
was first appreciated by workers concerned with phenolic tastes and odors who
showed that chlorophenols are produced from chlorine and phenol.25 Reaction (6)
symbolizes this process.
OH OH
HOCI Clxr^\fCI (6>
The product 2,6-dichlorophenol has often been identified as one of the principal
compounds of bad taste produced by chlorination of waters co'ntaining phenol.26
More recently it has been shown that chlorination of waters containing natural
organic materials produces a series of organohalides as complex as the natural
matrix which is their precursor.27"29 Most infamous of these are the trihalomethanes,
CHC13, CHCl2Br, CHClBr2 and CHBr, first discovered by Rook of the Netherlands
early in the past decade.27 These four substances are produced by the chlorination of
natural organics in water by a still unknown process, partly reflecting our ignorance
of the nature of aquatic humus.30 Work on the trihalomethane question in the
United States has centered in Cincinnati, Ohio, at the U.S. Environmental Pro-
tection Agency (EPA) Environmental Monitoring and Support Laboratory
(EMSL) where Bellar, et al. discovered THMs in drinking water almost simul-
taneously with Rook." A detailed discussion of the chemistry of this subject is
beyond the scope of this chapter. We shall note simply that THMs are produced by
the reaction sequence (7) to (8), where the term "precursor" refers to the unidentified
constituents of natural water which lead to THMs upon reaction with chlorine.32
HOCI + Br - HOBr + Cl" (7)
HOX + Precursors — THMs + Other Oxidized and Halogenated Organics (8)
'Chlorine demand is defined as the amount of chlorine dose (in mg/ L) required tojust form a
free hypochlorite residual in a given water source.21
151
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Note that natural bromide in the water is responsible for brominated THMs which
may predominate in highly saline water. Also note that other chlorinated
by-products are produced in this process. Fig. 1 is taken from an EPA-sponsored
study in which several treatment options for organics control are being
investigated.34 The figure shows total trihalomethane (TTHM) and total organic
halogen (TOX) values obtained by laboratory chlonnation of samples taken from a
surface water in the southern USA. These and other recent studies have shown that
the amount of chlorine bound in the form of volatiles other than THMs and non-
volatile organic by-products may exceed THM concentrations by a factor of two to
five.35 The nature of these by-products and their effects on human health are
unknown at this time.
Equation (8) also shows that oxidized organic compounds are produced by the
chlorination process. In one system with a DOC level of 8.4 mg/ L, a dose of chlorine
of 20.0 mg/L leaves a free residual of hypochloriteof0.6mg/L(as Cl) after 72 hours
contact time; thus, the chlorine demand was 19.4 mg/ L (as Cl). The TH M yield was
257 Mg/ L (as Cl), and the yield of other organic halides was 976 Mg/ L (as Cl).36 The
demand for oxidation reactions is calculated as follows:
(20.0 - 0.6 - .257 - 0.976) = 18.2 mg/L
In other words, 91% of the chlorine expended in the reaction acts as an oxidizing
agent and presumably ends up as chloride ion (Cf). This is not an unusual figure and
it symbolizes the oft-forgotten fact that chlorine in water treatment acts mostly as an
oxidant. Chlorinated by-products, particularly the THMs are produced in very low
yields (6% in the case above). This is not to imply, of course, that the yields are
insignificant as the impact of recent legislation attests.8 Indeed, it is the need to
minimize THM levels in finished drinking water that has stimulated much of the
recent research into the use of alternative disinfectants such as ozone, permanganate
and chlorine dioxide. Parallel to this effort, the same chemicals have received re-
newed attention as oxidants for the control of toxic organic compounds including
THMs in water, and it is this work which is emphasized in the following sections.
Returning to the question of the oxidizing power of aqueous chlorine, it has been
known for some time that prechlorination may enhance flocculation/sedimentation
processes.37 As early as 1924 Weston38 discussed the beneficial aspects of prechlori-
nation to improve floe formation. This property, combined with the power of chlorine
to prevent biological growth in intake transmission lines, in settling basins, and on
filter beds, has led to widespread use of the chemical in quantities far above those
required for bacterial disinfection.
It is not known at this time what is the actual mechanism of oxidation of natural
organics by aqueous chlorine. Recently, Christman, et al., in an EPA-sponsored
study39 have identified a series of oxidized, sometimes chlorinated compounds from
the chlorination of aquatic humic acid. (Humic acid and fulvic acids are generic
names for the largest fraction of organic material present in surface waters, soils and
groundwater.30) Glaze and co-workers in another EPA-sponsored project have
shown that chlorination increases the polarity of aquatic organic material so that it
becomes less sorbable on non-polar synthetic resins.40 In work from the group at
Stanford University41 also sponsored by EPA, organic halogen compounds
produced during the advanced treatment of municipal wastewater were shown to be
transported rapidly through the ground after well injection near Palo Alto,
California. The results show these non-volatile compounds are quite polar and not
adsorbed in the aquifer strata.
In principle, the oxidizing power of high concentrations of chlorine is sufficient to
degrade many toxic substances. In fact, the use of large doses of chlorine (2000 mg/ L
to 4000 mg/L) for sludge destabilization is a commercial process.42 Morris has
suggested "that a partial solution to the problem of chlorinated organic compounds
152
-------
1600 -
1400 -
1200
5
g 1000
24
48 72
Time, hours
120
Figure 1. Formation of trihalomethanes and non-volatile organohalides by
chlorination of filtered water from a surface source in southern United
States (20 mg/L dose HOCI, pH 6.5, incubated at 26°C).
in. . . .drinking water is an intensification rather than an abatement of
hypochlorination." However, it is unlikely that chlorine or hypochlorite will be used
for this purpose in drinking water treatment. As shown above, chlorinated
by-products such as THMs inevitably will be produced, and the risk to human
health posed by these substances, however small, will probably deter further
expansion of the use of chlorine as an oxidant. Recent attention has been focused on
alternatives to chlorine for this purpose.
OZONATION«-*V»
Ozone is not used extensively as a water treatment chemical in the United States,
but because of its widespread use in Europe48 the EPA has initiated numerous
internal and extramural research projects to examine potential uses of ozonation.
Many of these laboratory studies and pilot plant projects use ozone as a chemical
oxidant. Four applications of ozonation for this purpose are possible:
I. Oxidation of natural organics as an aid to floe formation
2. Destruction of THM precursors prior to chlorination
3. Destruction of THMs and other toxic organic chemicals
4. Oxidation of natural and synthetic organic materials to enhance their
biodegradability for removal on biological filters.
Ozone is generated on site,50 usually by an electric discharge process in a flowing
air or oxygen stream. Mixtures of I % to 3% ozone/ air and 3% to 5% ozone/ oxygen
are produced, which are then mixed with water in a manner to ensure efficient
transfer into the liquid phase. Fig. 2 is a schematic diagram of a typical ozone
generating and contacting system for water treatment.
153
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Oxygen
or
Air Dryer
Make-up
Oxygen
or
Air
,_J
Ifi
1 '•
*
1
|
1
— ^~~f
\.
Recycle Oxygen or Air
Ozone
Generator
Water
Inlet
~\
1
1
L
1
i
1
Ozone/Water
Contactor
Retention
Basin
Ozonated
Water
Figure 2. Schematic diagram of typical ozone generating and contacting facility for
water treatment (After Rosen50).
-------
As an oxidant, ozone is the most powerful of those used in water treatment, acting
by at least two mechanisms in aqueous solution. At low pH values the ozone
molecule O3 directly attacks organic substances.51'52 Being an electrophilic reagent,
ozone is particularly active towards substances such as phenols which are activated
to electrophilic attack. The ozone molecule also acts as a 1,3-dipole attacking
unsaturated centers such as olefins to form unstable ozonides. These decompose in
water to yield carbonyl compounds as shown in Equation (9).
/
,
I, /Rj R, + R2
— C C R3 ^,C-O-O + O =C
O 0 H ^R3
O
Ozonide
OH
Carboxylic Acid
\
H
Aldehyde
C = O -*
/ -H20
Carbonyl
Oxide
Zwitter
Ion
0-H
Hydroxy
Hydroperoxide
(9)
The intermediate hydroxy hydroperoxide is usually decomposed rapidly in aqueous
media, although in one case (from naphthalene) it is more long lived.53
At high pH values, ozone decomposes by a complex process yielding a variety of
products such as oxygen and hydrogen peroxide. Hoigne'and Bader have shown that
ozone reactions at high pH involve the hydroxyl radical intermediate,54'" a very
reactive, unselective oxidant. Some organic substrates, such as benzene, react much
faster with the hydroxyl radical than with ozone. Others, such as phenol, are more
reactive towards molecular ozone at low pH values. Most significant, the immediate
oxidation by-products will be different in the two cases where different reaction
pathways are taken.56
Ozone as an Aid to Color Removal and Flocculation
This property of ozone has not been exploited significantly in the United States,
although it is well appreciated in Europe and Canada.48'" Only small doses of ozone
are required to enhance floe formation, and in the process significant decrease of
color is obtained. Sontheimer has attributed the effect to the oxidation of humic
material in the water into a floe forming polyelectrolyte.58 The beneficial effects of
preozonation have been documented by Schwartz and co-workers,59'60 by Trussell,
et al.,61 McBride,62 Maier,57 and Tate and Trussell.63 Most significant are the advan-
tages of preozonation in direct filtration systems, where improved flocculation is
accompanied by lower THM values upon post-chlorination. It appears direct
filtration plants with preozonation, followed by post-disinfection with chlorine,
chlorine dioxide or chloroamines offer a viable alternative to conventional treat-
ment for many applications.
155
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Ozone for Destruction of THM Precursors Prior to Chlorination
TH M precursor removal with ozone has not proven to be a satisfactory treatment
method. This is due primarily to the fact that ozonation produces THM precursors
as well as destroys them. This conclusion has been reached as the result of studies by
Rook,64 Riley, et al.,65 Hubbs,66 Glaze and co-workers,67-68 and Love and others.69
Fig. 3 shows the destruction of precursors in a continuous process at a pilot plant
using water from a southern United States reservoir.70 At these low dose rates
THMFP destruction is minimal; however, at higher dose rates more extensive de-
struction of precursors may occur68'69'71 as shown in Fig. 4.
Ozonolysis of Organic Micropollutants—
Ozone is selective in its ability to oxidize organic compounds in water, and in
general the method has not proven to be useful for removal of these substances from
water. There are, in fact, few published reports taken from actual plants or pilot
plants with which to gauge ozone's effectiveness in this application.72
Sontheimer and co-workers73 have reported data on the effect of ozonation of
several organic halides in Rhine River water. Table 2 shows that only the trichloro-
benzenes are removed by ozonation and filtration, whereas all of the halides are
adsorbed by activated carbon. Ozonation has been shown to reduce the levels of low
molecular organic halides such as trichloroethylene from reclaimed wastewater.
However, the effect is largely due to purging in the ozonation process.74 All of this
does not discount the potential for removal of specific organic contaminants in
certain cases where reactivity with ozone is high, as with certain carcinogens. In such
cases, ozonation may prove to be a useful treatment alternative.75
Ozone Combined with Activated Carbon—
This subject is treated in more detail in the chapter by DiGiano. Suffice it to say
here that many authorities feel that ozonation prior to activated carbon filtration
improves the removal of organic constituents in the process.58 While there is by no
means full agreement on the mechanism of this process,76 it is usually contended that
ozonation partially oxidizes natural and anthropogenic compounds, making them
more susceptible to biological metabolism on the filters.77' 8 Current research is
being directed by the EPA and other agencies toward a more thorough
understanding of the role of the preoxidant and of the mechanisms operating on
carbon filters.
Cost of Ozonation in Water Treatment—
Ozonation is a rather expensive process by water treatment standards, requiring
substantial capital investment and operating (energy) costs. Clark and co-workers at
the EPA Municipal Environmental Research Laboratory (MERL) in Cincinnati
have estimated the cost of ozonation facilities.79 Design parameters are shown in
Table 3 along with total costs for several size plants. The firm of Culp/ Wesner/ Gulp
independently calculated capital and O&M costs of ozonation plants in an
EPA-sponsored project.80 If one utilizes these data for a 10 MOD plant with a dose
of 10 mg/ L (a more realistic figure for organics control than 1 mg/ L), the results are
shown in Table 4. The cost estimates in Tables 3 and 4 are presumably out of date
due to substantial increases in energy and other expenses which determine the net
cost of producing ozone. Recent estimates by EPA suggest that the figures in Table 3
should be increased by approximately 20% and 100% for small (1 MOD) and large
(100 MGD) plants respectively. While these costs are high by comparison with con-
ventional unit processes, it should be noted that ozonation properly applied could
save on chemical costs for disinfection and flocculation and possibly on reactivation
costs for GAC filters.
156
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300
200
c
£
o
O.
C
O
E
o
IB
o 100
10
-C
A—A Ozone Contactor Influent
• • Ozone Contactor Effluent (tR 40 min)
-6.3 mg 03/L-»}«-2.0 mg O3/L-4— 2.5 mg O3/L-
8
12
16
20
24
28
Week of Operation
Figure 3. Effect of low doses of ozone on trihalomethane precursors (THMFP) in a pilot plant. Contactor influent
is settled/filtered water at pH 5.5-6.5, TOC 2.7-4.6 mg/L. Contactor turbine type with 85-90% trans-
fer efficiency. THMs measured after dose of 20 mg/L for 72 hours at pH 6.5.
-------
100
o
ID
DC
• 0.44 mg 03/1-min in Oxygen
O 0.28 mg O3/1-min in Oxygen
A 0.14 mg O3/1-min in Oxygen
40 -
20
90
Time, min.
120
150
180
Figure 4. Effect of ozone dose on the destruction of trihalomethane precursors in water from the same source as
Figure 3. THM's measured after dose of 20 mg/L for 72 hours at pH 6.5.
-------
Table 2. Effect of Treatment Methods on Selected Organic Halides in River
Rhine7*
% Remaining in Effluent from
Substance
Chlorobenzene
Dichlorobenzene
(o,m,p)
Tnchlorobenzene
(3 isomers)
Chlorotoluene
Dichlorotoluene
Table 3. Design
River Bank
Filtration
4
38
62
6
38
Parameters and
Ozonation Plus
Sand Filtration
7
42
< 1
9
38
Total Costs for Ozonation79
GAC Filtration
< 1
5
< 1
< 1
< 1
Ozone dose: 1 mg/L
Ozone contact time: 20 mm.
Cost of oxygen: 0046 S/lb
Construction cost index- 256.7
Wholesale price index- 178.1
Direct hourly wage rate 5 19 S/hr
Amortization interest rate: 7 percent
Amortization period: 20 years
Electric power cost- $0.01/kwh
Capacity factor: 70 percent
Total cost (C/1000 Gallon)
air
634
227
1.65
085
0.79
oxygen
7.68
2.53
1.77
0.87
081
Plant size(MGD)
1
5
10
100
150
Summary of Advantages and Disadvantages of Ozone for Water Treatment—
The advantages and disadvantages of ozone for water treatment have been sum-
marized by Sontheimer, from whom Table 5 is taken.71 As a result of the European
experience, United States research efforts on the applications of ozone have in-
creased significantly in the past decade. At the present time these efforts emphasize
ozonation in combination with activated carbon filters but further research using the
full potential of ozone should be forthcoming.
159
-------
Table 4. Estimated Costs for an Ozonation Facility for Removal of Toxic
Organic Compounds
Design parameters
Ozone dose: 10 mg/L
Ozone contact time 20 mm
Cost of oxygen 0 050 S/lb
Plant size 10 MGD
Ozone utilization: 833 Ib/day
Construction cost
Ozone generator.
Ozone contact chamber.
TOTAL
$ 983,690*
50,520f
$ 1,034,210
Amortization periods 20 years
Amortization interest rate: 7%
Amortized capital cost $97,622/year
Operation and maintenance costs $114,700t/year
Oxygen cost- $18,250/year
Total cost $230,572/year
Cost per 1000 gallons of water. 6.3C
•Reference 80, Table 37, 1000 Ib/day. tReference 80, Table 38, 2300O ft3 .
^Reference 80, Table 39, 1000 Ib/day.
Table 5. Advantages and Disadvantages of Ozone in Drinking-Water Treat-
ment73
Oxidation with Ozone
Advantages
Disadvantages
Rapid disinfection and virus mactivation
No commercial chemicals required
Microflocculation,
Formation of degradable
organic substances
Increase of polarity
Transformation of resistant into
biodegradable substances
No formation of harmful substances
Numerous processes and installations
for ozone input
High ozone consumption by organic
substances
High investment and operating costs
After-treatment step and
installation required
Decrease of molecular weight
Increased germ formation in
distribution network
Biological after-treatment and safety
chlorination required
Difficult to control; Mass transfer often
determines ozonation efficiency
160
-------
CHLORINE DIOXIDE
Chlorine dioxide, like ozone, is a powerful disinfectant and oxidant. While not
used as extensively as chlorine, it does find substantial application in portions of the
world including locations in North America.81 Chlorine dioxide is favored particu-
larly in Belgium and other countries of Western Europe for disinfection of ground
waters, for taste and odor control, and for control of iron and manganese.81'83
More recently, chlorine dioxide has been found to compare favorably with chlo-
rine for predisinfection and oxidation of raw surface waters. Masschelein, a leading
authority on the use of chlorine dioxide for water treatment, reports that costs for
pretreatment at the Compagnie Intercommunale Bruxelloise favor chlorine dioxide
over chlorine by a factor of 2.4.82b
Chlorine dioxide is receiving substantial attention by EPA authorities since it was
established C1O2 produces no trihalomethanes.84 This property combined with its
good disinfection potential has promoted chlorine dioxide to a position of promi-
nence among the chemical oxidants.
Chlorine dioxide to be used in water treatment is prepared on site, usually from
sodium chlorite.82a'82b Two methods are used; one described in Equation (10) in-
volves disproportionation of chlorite in acidic media, usually hydrochloric acid:
5NaClO2 + 4HC1 - 4C1O2 + SNaCl + 2H2O (10)
Under appropriate reaction conditions, nearly quantitative yields of C1O2 are
achieved, although chlorite and sometimes chlorate are by-products. Fig. 5 shows a
schematic diagram of a chlorine dioxide generator using this process.
Chlorine dioxide may also be produced by the oxidation of chlorite by chlorine
(Equation 11).
2NaClO2 + C12 - 2C1O2 + 2NaCl (11)
Proper control of the mixing of the two reagents results in excellent yields of C1O2,
which may be fed directly into the water or stored for subsequent dosing (Fig. 6).
Usually a slight excess of chlorine (10%) is recommended; however, this may have
undesirable side effects (see below).
Formation of THMs and Other By-Products
Chlorine dioxide does not react with natural aquatic organic compounds to form
trihalomethanes.83"85 Fig. 7 taken from Stevens, et al.85 from Miltner84 shows no
THM formation from C1O2 with humicacid, a powerful THM precursor with hypo-
chlorite. As expected, mixtures of chlorine dioxide and hypochlorite form THMs,
but this is presumably due to the hypochlorite, not to chlorine dioxide.83 Also shown
for comparative purposes is the yield of THMs from chlorine alone.
The absence of THM by-products does not mean that no chlorinated by-products
are formed when CICh reacts with raw waters. Stevens, et al.85, Rosenblatt,86 and
Hedberg, et al.83 have reviewed the chemistry of chlorine dioxide and all acknowl-
edge that chlorinated derivatives are possible from a variety of chemical precursors.
For example, chlorohydrins are reported from the reaction of chlorine dioxide with
methyl oleate.85 Phenol reportedly forms a series of quinones and chloro-quinones87
(Equation 12).
161
-------
0 0
Cl
(12)
More significant perhaps are the non-chlorinated epoxides reportedly formed
from the reaction of chlorine dioxide with substituted olefins.83 However, thesesub-
stances have not been found in actual drinking waters treated with chlorine dioxide
to this date.
Stevens and co-workers at the EPA MERL in Cincinnati have examined volatile
by-products of Ohio River water treated with chlorine dioxide.85 Low molecular
weight aldehydes were the only by-products identified.
Feed
Water
Treated
Water
Hydrochloric
Acid 1:6
F2 = F, x (6 to 7)
Sodium
Chlorite
Figure 5. Schematic diagram of typical facility for generation of chlorine dioxide
from sodium chlorite and hydrochloric acid (From Masschelem 82b)
162
-------
Transparent
Orifice or Other
Pressure Differential
Device to Insure
Flow Thru System
Che
Feet
Pum
r
Chlorine
Dioxide ^
Generator
(Packed
column)
mical
J
P
Sodium
Chlorite
Solution
i
/
X
J^-at
ir^-
Ru
Ho
— m-frML
\^ J
Sample
Point
i
bber
se i
-*-lx»trt-
t
Min. 5(
C
Solenoid
Valve '
1
50 mg/
antent
Chlori-
nator
J
/
Optional Flow Rate
Acid Indicator
Injection
Point
CI2
Flow
Switch
4-
i MI « r
i
j
Wate
1/60/115
Vac Power
Source
r Line
Transparent
Section
Chlorine
Dioxide
Generator
(Packed
Column)
1/60/115 Vac
Power Source
Back Flow
Preventer
NaCIO2
Solution
Figure 6.
NaOCI
Solution
H2SO4
or HCI
Schematic diagram of typical facility for generation of chlorine dioxide
from chlorine and sodium chlorite (After Sussman and Rauh, In Ref. 47,
p 355).
163
-------
0.6
^ 0.5
o
E
c
0)
o
c
o
O
c
TO
0.3
0.2
F.A. Cl = Free Available Chlorine
pH = 7.4
T = 24°C
1.5 mg/IF.A. Cl
\.3 mg/l CI02 + 1 .5 mg/l F.A. Cl
CI02 Alone
/.
10 20 30 40 50 60
Contact Time, hours
70
80
Figure 7. Trihalomethane formation by CI02 and excess free available chlorine, ERC
pilot plant settled water (From Miltner84)
Chlorine Dioxide as an Oxidant in Water Treatment —
Chlorine dioxide is often used at low doses to control taste and odor compounds
in water works.88 In addition, CICh reportedly oxidizes organic compounds such as
3,4-benzopyrene,82a yielding noncarcinogenic quinones.89 The effect on pesticides is
spotty, however; some compounds such as parathion are destroyed only with diffi-
culty.90 For the elimination of phenols from wastewaters, chlorine dioxide has been
rated between ozone (best) and chlorine (poorest) in effectiveness.91
Hedberg and co-workers have reported interesting experiments on the effects of
chlorine dioxide on haloform yields using raw water from the River Gota Alv in
Sweden.8' Fig. 8 shows that C1O2 at reasonably low doses (<5 mg/L) substantially
lowers the yield of CHCh and CHCbBr obtained by chlorination. The larger effect
on chloroform yield may in some way by related to the oxidation of bromide, al-
though it has been reported CIO: cannot oxidize bromide to bromine to a significant
degree.82"
Perhaps the most important by-product formed when chlorine dioxide is used in
water treatment is chlorite ion. Chlorite is known to cause methemoglobinemia, a
condition in which hemoglobin is oxidized to a metabolically inactive (ferric) state.93
After a careful assessment of the health effects literature, EPA officials have suggest-
ed limitations on the use of chlorine dioxide to keep the sum of residual oxidants
(C1O2, CICh and C1GY) below 0.5 mg/L.94
Cost of Chlorine Dioxide Treatment
Clark, et al.79 have calculated costs for 1 mg/ L doses of chlorine dioxide in plants
ranging from 1 to 150 MGD. Total costs range from 3.3c/ 1000 gallons to 1.2
-------
pg/i
10
u
o
I
10
mg CI02 .
o
o
1 -
mg CI02 /i
Figure 8. Formation of haloformsfrom combinations of chlorine and chlorine dioxide.
A = CHCI3; B=CHCI2Br 1 =chlorineaddedbeforechlormedioxide;2 = chlorme
added after chlorine dioxide (From Hedberg, et al.83).
165
-------
O.Sc/1000 gallons. As noted above, chlorine dioxide economy becomes more
favorable for raw water treatment where its higher oxidative power can be exploited.
OTHER OXIDANTS
Potassium Permanganate
It has been reported that more than 250 North American and more than 30 Euro-
pean water works use potassium permanganate,95 but it is not clear how many of
these installations use permanganate sporadically and to what extent. Usually,
permanganate application is limited to taste and odor control, and in some cases for
lowering of iron, manganese, sulfide and phenol levels. Permanganate has also been
used as a biocide to control algae growths in reservoirs.95 Little work has been done
on the control of organics with permanganate treatment. Singer and co-workers96
studied trihalomethane formation potential (THMFP) reduction using raw water
from Chapel Hill, N.C. and settled water from Durham. Both were non-chlorinated
before treatment with KMnO4. THMFP reductions were observed from 20 to 40
percent but doses of 10 mg/L permanganate were required to give the best results.
The effect was greater at pH 10.3 than at 6.5. Voss and co-workers reported 83% re-
duction in chloroform formation levels using permanganate at an initial pH of
10.3.97 However, the concentration of permanganate was 4.OX 10~4 M (48 mg/L), an
unrealistic dose level, and 3.5 hours contact time was allowed. Workers at the EPA
MERL determined little effect on THMFP in Ohio River water using 5 mg/L doses
of permanganate.92 Fifteen percent removal of TH MFP was observed with a 1.5 hour
permanganate contact time.
Hydrogen Peroxide
Hydrogen peroxide (H2O2) is a strong oxidant, but it has seen little use as a water
treatment chemical. Overath has reviewed the applications of hydrogen peroxide in
wastewater treatment, but cites no applications in drinking water technology except
for disinfection.98 Voss and co-workers97 measured reduction of TH M FP of Kansas
River water, and were unable to detect chloroform (after 10 ppmchlorination) after
treatment with peroxide. However, the dose was 1.1 X 10' M (37 mg/L), an un-
reasonably high level for water treatment purposes, and the contact time was 22
hours. Nonetheless, the results are promising and show that peroxide oxidation, as
stated by Overath "invites application."98
Ferrate
While ferrate ion (FeO4~2) has received sporadic attention for many years, it has
not proven to be a popular disinfectant or oxidant. In acidic media its oxidation po-
tential is high, 2.2 V, marking the reagent as another one of promise. Waite and
Gilbert have reviewed the preparation and properties of ferrate salts,'9 and pointed
out that ferrate treated wastewaters clarify and decolorize "nearly instantaneously."
Ferrate was found to significantly reduce the concentrations of allylbenzene, ben-
zene chlorobenzene, and l-hexene-4-ol. Voss, et al. found no dramatic decrease in
THM precursors with ferrate treatment although simultaneous treatment of humic
acid with ferrate and HOC1 gave lower chloroform yields than HOC1 alone.97
OZONE/UV AND OTHER CATALYTIC OXIDATION PROCESSES
While Overath may be correct in stating that hydrogen peroxide "invites applica-
tion,"98 it is the catalytic oxidation processes which are especially promising. These
166
-------
processes, which involve the generation of highly active chemical intermediates in-
clude the following:
• Ozone with ultraviolet radiation, the most studied and possibly the most prom-
ising catalytic oxidation process
• Hydrogen peroxide with ultraviolet radiation
• Hydrogen peroxide catalyzed ozonations
• Heavy metal catalyzed ozonation and peroxidations.
Ozone with Ultraviolet Radiation
This process has just been the subject of a three-year laboratory scale study by
Glaze and co-workers. lo° The results confirm the power of the Os/ U V method for the
destruction of THM precursors and other refractory organic compounds first sug-
gested by Prengle and co-workers. I01a~d The process is also being studied by a group
at Westgate Research Corporation for application in reclamation of waste waters' 2
and has been applied to hospital wastewater constituents by Lee, et al.103 and Chian
and co-workers. !04a'b As an example of the process, Fig. 9 shows the destruction of
THM precursors with O3/UV and O3 only.100 The effect of 0.31 Watts/liter UV
radiation (X = 254 nm) exceeds that of tripling the ozone dose rate. Prengle, et
aj ioia-d jjave estimated the cost for 90% removal of precursors fromthis system to be
lOc to 14e/1000 gallons. A pilot scale study to further examine the Os/ U V process
for precursor removal is being conducted by SumX Corporation and the University
of Texas at Dallas under EPA sponsorship (CR-808825-01).
Hydrogen Peroxide with Ultraviolet Radiation
Photolysis of hydrogen peroxide also generates active species, presumably hydroxyl
radicals. Berglind and co-workers'05 have described the oxidation of 3,4-benzo-
pyrene, bromodichloromethane, methylisoborneol (a taste and odor compound)
and chloroform. As shown in Table 6 the system is capable of removing these micro-
pollutants rather effectively. Also shown in Table 6 is the effect of U V/ H2O2 on color
reduction in two humus samples. In other work, Ogata and co-workers106 used the
H2O2/UV process for destruction of non-ionic detergents. Later, Dore,etal.107 com-
pared ozonation and H2O2/ UV for phenol oxidations and found the two processes
to give comparable results. Cost data from Berglind, et al.105 are not favorable, but
the process deserves further investigation.
Hydrogen Peroxide Catalyzed Ozonation
Nakayama and co-workers have used hydrogen peroxide as a catalyst for increas-
ing the effectiveness of ozonation.108 Table 7 shows TOC reductions obtained for
runs involving individual compounds initially at 40 mg/ L and H2C>2 at 30 mg/ L. The
results are favorable, although it should be noted that efficiencies should be much
lower using solute concentrations in the ppb range which are more common in drink-
ing water. The process was stated to be approximately one-half to one-third as ex-
pensive as GAC adsorption, but no details of the cost evaluations were given.
Fernandes109 is currently evaluating the O3/H2O2 process for destruction of THM
precursors.
Heavy Metal Catalzyed Ozonation and Peroxidation
Hydrogen peroxide decomposes in the presence of several types of catalysts and
with UV irradiation into highly reactive intermediates. The most studied case is the
Fe+2/H2O2 system,referred to as the Fenton reagent.110'1" Other heavy metals such
167
-------
O 0.14 mg O3/l -min; UV = 0.31 Watts
I
A 0.42 mg03/l -min; no UV
180
Figure 9. Normalized THM formation potential: ozone destruction of THM precursors-
Caddo Lake water.100
Table 6. Removal of Organic Micropollutants With UV Radiation and
Hydrogen Peroxide103
Period
of
Radiation
(min.)
0
30
60
120
240
Percent of Compound
Without H202
B(a)P CHCI3
0 0
-
8
29 58
-
CHCI2Br
0
-
9
25
-
With 0 0035% H2O2 (35 mg/L)
B(a)P
0
-
72
91
98
CHCI3
0
25
32
91
-
CHCI2Br
0
42
56
91
-
MIB Color
0 0
100
100*
100f
•Sample "2", Fig 5, Lee, et al103
fSample "V, Fig 5, Lee, et al.103
168
-------
Table 7. Pilot Scale Oxidation of Model Compounds with Ozone-Hydrogen
Peroxide108
TOC, mg/L
Compounds Raw Treated AO3/ATOC A03/AH202
Methanol
Ethanol
n-Butanol
t-Butanol
s-Butanol
Sodium acetate
Propionic acid
Benzole acid
Acetone
Methyl isobutyl
ketone
Phenol
Ethanol amine
Diethanol amine
Ethylene glycol
monomethyl ether
37.5
38.4
395
390
40.5
43.5
43.0
44.2
41.0
45.3
425
43.0
42.5
41.4
23
8.3
105
10.5
12.5
13.5
7.5
11 1
13.0
20.7
95
45
7.4
40
12.9
11 0
9.5
98
100
9.1
6.9
11 1
9.1
9.9
9.6
11 0
11 6
89
15 2
11.0
9.1
9.1
9.1
9.1
8.3
123
8.3
83
10.5
14.1
135
11 1
Initial concentration of substrates 40 mg/L
Dose rate of H2O2: 30 mg/L
Solution flow rate: 23.9 mL/min.
as titanium (III)"2 catalyze H2O2 decomposition, and the active intermediate gener-
ally is assumed to be a hydroxyl radical (-OH). Hydroxyl radicals may also be gener-
ated by base catalyzed decomposition of ozone." Ferric oxide has been used as a
solid catalyst for increasing the efficiency of COD removal by ozonation."3 While
not thoroughly investigated, these and analogous systems are interesting and de-
serve further research.
FUTURE TRENDS
As noted in the introductory portion to this chapter, the future usefulness of
oxidation processes will probably depend on the water quality criteria which are
established. The promulgation of stricter standards for synthetic organics will most
surely advance the need for advanced treatment processes such as oxidation. For
this reason, and because of the need to evaluate alternatives to chlorine, oxidation
processes should continue to be researched in the coming decade. Listed in Table 8
are some of the more obvious problem areas and exploratory topics which deserve
attention. The list emphasizes identification of by-products and determination of
their health effects; full scale evaluation of oxidation methods, particularly in non-
traditional plant configurations; and basic research to develop novel catalytic
oxidation methods. While certainly incomplete, the list is a formidable challenge to
the scientific and engineering community and to the public who must eventually sup-
port this research effort.
169
-------
Table 8. Research Issues Related to the Use of Oxidants in Drinking Water
Treatment
By-product studies in pilot and full scale application of ozone, chlorine dioxide, and other
oxidizing agents
• emphasis on non-volatile and labile intermediates
• parallel studies of health effects of by-products and undifferentiated fractions
Pilot scale and full scale demonstration of ozone, chlorine dioxide and other available
oxidants
• emphasis on non-traditional configurations such as ozone with direct filtration;
combinations of disinfectants with improved flocculation, etc
• emphasis on standardization of water quality parameters, quality control
procedures, and attention to process details such as ozone efficiency
• critical review of data to compare cost effectiveness on equal terms
Laboratory and pilot scale evaluation of novel oxidation systems
• emphasis on basic research of catalytic oxidation processes
• comparison of by-products and health effects with uncatalyzed oxidation systems
ACKNOWLEDGEMENT
This work was carried out while the author was a member of the faculty at North
Texas State University (NTS U), and represents the state-of-the-science of oxidation
methods as of mid-1980. (However, it was possible during the process of editorial re-
view to include selected papers appearing after this date.) Particular gratitude and
pride is expressed for the work of staff of the NTSU Institute of Applied Sciences,
including Sharon Dumas, Julie Kerestine and Sue Zant, and for the support of the
Director, Dr. Kenneth Dickson. The author is also particularly indebted to editors
of compendia and reviews which were used extensively in this work, especially refer-
ences 7, 19, 43, 44, 46, and 47.
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170
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20. Ibid, pp. 182-227.
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171
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37. White, op cit. pp. 324-325.
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70. Glaze, W. and J. Wallace, unpublished Data from EPA-Sponsored Co-
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Assoc., 6th ed., Austin, TX, pp. 74-132, 1975.
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90. Gomaa, H.J. and S.D. Faust. Adv. Chem. Ser. Amer. Chem. Soc., 111:189-
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92. Symons, J.M. "Utilization of Various Treatment Unit Processes and Treat-
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95. Kotter, K., in Kuhn, op cit. pp. 528-543.
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174
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REDUCTION OF ORGANIC SUBSTANCES IN
MUNICIPAL WASTEWATER BY BIOLOGICAL
PROCESSES
Robert L. Irvine, PhD
PERSPECTIVE AND APPROACH
A primary objective of a municipal wastewater treatment plant is the removal of
organic compounds which have been discharged from residential communities,
commercial establishments and industries. If these organics were allowed, instead,
to flow untreated into a river or lake, the full impact and meaning of the term
pollution would be witnessed. Under the most severe circumstances, oils, greases and
other floatables would cover the surface along with mats of bottom sediment lifted
to the surface by noxious gases produced by microorganisms thriving on the
untreated organics. Such conditions were somewhat commonplace during the first
half of the twentieth century and resulted in the reduction of activities, such as
swimming and fishing, which are associated with natural bodies of water.
Because of efforts at both the local and federal level, steady progress towards
limiting the nature and extent of discharge of organic pollutants was realized during
the fifties and sixties. Intense interest in preserving the nation's waterways, however,
did not blossom at the federal level until the late sixties. The notion that a body of
water should be both fishable and swimmable was not established as a national goal
until October 1972.
The agency responsible for implementing this national goal is the U.S. Environ-
mental Protection Agency (EPA). EPA efforts during the seventies have included
the establishment of national discharge limits, expansion and improvement of
existing treatment facilities, construction of new plants, development of new
treatment technologies and enforcement of regulations. An attempt is made herein
to capture the essence of these efforts by combining the experience gained from
existing treatment facilities with the insights being generated from on-going
research.
Wastewater is comprised of a multitude of inorganic and organic compounds.
This chapter will deal with the organic compounds. Technologies employed for the
removal of organics are selected according to the physical, chemical and biological
properties of the organics present in the wastewater. Simply stated, organics may be
either soluble or insoluble. Some of the solubles have surface active properties which
The Author Robert L Irvine, a native of Boston, received a B S in Chemical Engineering and M S in
Chemical Engineering at Tufts University, and the PhD in Chemical Engineering at Rice University in 1969
He has engaged in consulting and in university-based education and research His research interests are
broad but in recent years have been directed mainly to theoretical and applied aspects of sequencing batch
reactors for the biological treatment of municipal wastewater Since 1974. Dr. Irvine has been a member of
the faculty of the Department of Civil Engineering, University of Notre Dame
175
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make them amenable to adsorption on materials such as activated carbon; others are
volatile; many serve as a food source for microorganisms. Some exhibit all three
properties. In addition to being either adsorbable or consumable, the insolubles can
either settle, float or remain in suspension. This chapter will focus on those soluble
and insoluble organics that are removed in the biological process.
The approach taken herein may be viewed as a severe departure from the classical
descriptions available on the biological removal of organics from municipal
wastewaters. This was done for several reasons. First, the EPA has produced, either
directly or indirectly, an overwhelming collection of reports which cover adequately
the classical approaches. While a summary of these techniques would properly
satisfy the objectives of this chapter, the method employed provides a counter
perspective for those engineers and scientists who are actively engaged in wastewater
treatment. Second, the presentation is also directed at those who are not aware of the
general principles and practices of wastewater treatment. The approach allows a
rather high level technical discussion without demanding an understanding of
specific techniques which are generally practiced in the field today. Third, another
chapter in this monograph deals with the biological reduction of organics in
industrial wastewaters. While the content of that chapter and this must overlap to
some extent, overlap has been limited to the conceptual level with the content of one
reinforcing that of the other. Finally, a decision was made to emphasize biological
removal of organics from a process rather than a hardware point of view. This was
easily accomplished with the method employed.
Eight new terms are introduced in this chapter. These are: INITIAL STATE,
GOAL STATE, OPERATOR, MIX, SPLIT, SEPARATE, BIOLOGICAL
REACT and CHEMICAL REACT. Each is fully capitalized both to avoid conflict
with more common uses for the terms and to assist in clarity of presentation. In
particular the entire organization of the chapter revolves around the use of four of
the terms: INITIAL STATE, GOAL STATE, BIOLOGICAL REACT (sometimes
designated as the OPERATOR: BIOLOGICAL REACT) and SEPARATE. In this
context, the process notion of biological wastewater treatment is emphasized. At no
time is criticism of more conventional presentation (generally used by the author
himself) intended.
DEFINITION OF TERMS
A convenient way to visualize any process flowsheet was described by Powers' for
chemical process synthesis and modified by Irvine2 for use in water and wastewater
treatment system descriptions. Simply stated, any treatment facility can be described
in terms of an INITIAL STATE which is transformed into the GOAL STATE by the
use of the proper sequence of OPERATORS (Fig. 1)*. The INITIAL STATE
describes the concentration and amount of each chemical component in the raw
materials stream that will be transformed in the process. The state (solid, liquid or
gas) of each component is defined along with the temperature and pressure of the
feed stream. The GOAL STATE is a characterization of the final products desired or
achieved, once again in terms of concentration, amount, state, temperature and
pressure. The unit operations and unit processes utilized to carry out the
transformations desired are referred to as OPERATORS and are broadly classified
as MIX, SPLIT, SEPARATE, and REACT (Fig. 2). MIX and SPLIT are simple
terms for blending and separating streams. SEPARATE is primarily used to
designate unit operations involved in phase separations such as in filtration,
•The term OPERATOR is fully capitah/ed when used to designate a system function Lower case is used to
identify any individual (i e , an operator) associated with the running of a treatment facility
176
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INITIAL STATES
OPERATORS GOAL STATES
Discharge
Figure 1. Transformation of initial state into goal state.
-------
sedimentation and centrifugation. REACT may be either biologically or chemically
mediated and defines the unit processes which transform one component into
another.
Applied in this context, treatment of, say, groundwater for human consumption,
industrial wastewater for reuse and municipal wastewater for surface water dis-
charge, are conceptually the same, requiring the selection of the proper sequence of
OPERATORS for the site specific INITIAL STATE and GOAL STATE. This
generic similarity between all forms of treatment also provides the bases for
describing the major differences among process flowsheets.
THE INITIAL STATE
The INITIAL STATE for municipal wastewaters is notably different from that
for most industrial wastewaters. Indeed, depending upon the extent and nature of
the industrial input and the degree of pretreatment practiced by each industry, the
INITIAL STATE for one municipal wastewater can differ appreciably from that of
another. In addition, urban runoff, inflow, infiltration and nature of the collection
system all contribute to site specific variations in the INITIAL STATE. Neverthe-
less, municipal wastewaters are generally quite similar because contributions from
households, restaurants, etc. vary little from site to site and normally constitute a
notable fraction of the total wastewater flow (Table 1). As a result of these contri-
butions, the concentration of total organics, measured as either Total Organic
Carbon (TOC), Biological Oxygen Demand (BOD) or Chemical Oxygen Demand
(COD), is relatively low, with the 5-day BOD ranging between 110 mg/L and 400
mg/L.3
Perhaps the most distinguishing feature of the INITIAL STATE of municipal
wastewater is the presence of suspended solids (SS). Colloidal, supracolloidal and
larger settleable particulate matter constitutes up to 75 percent4 of the COD in
municipal wastewater. Surprisingly, very little is known about the mechanisms
involved in SS degradation, in spite of the fact that approximately 158 million out of
the 220 million people in the United States are served by some form of central treat-
ment facility5 which must adequately handle these solids.
While the organic content of municipal wastewater has been generally character-
ized in terms of non-specific water quality parameters such as 5-day BOD and
volatile suspended solids (VSS), recent efforts by the EPA to determine the treat-
ability and removability of priority pollutants has resulted in the development of
rather sophisticated analytical techniques for the measurement of organics.6 The
analytical procedures being developed are both complex and costly. Attempts to
establish indicator or surrogate parameters such as BOD or TOC have not been
successful.7 As a result, organic priority pollutants have been used to define the
Text for Figure I TRANSFORMATION OF INITIAL STATE INTO GOAI STATE is a phrase that may
be applied to any treatment processes Conceptually, a liquid stream containing water can be treated in such
a manner that the water is extracted from that liquid Because virtually all domestic wastewaters contain well
over 99 percent water, unwanted inorganic and organic compounds are removed instead The
transformation then is one which converts an undesirable liquid stream having limited applications into one
which has a multitude of uses. This notion is conceptually illustrated in Figure 1. In most cases, wastewater
collected from residential, commercial and industrial sources are treated in a Publicly Owned Treatment
Works (POTWs) and discharged to a receiving body of water The extent of treatment required will
determine the selection of OPERATORS needed for the conversion. If a more restrictive GOAL STATE
limits must be met, other OPERATORS must be added In the example shown above, commercial,
industrial and residential discharges from points A thru D are collected for treatment in the POTWs (I) For
purposes of illustration, the OPERATOR selection in (1) is shown to be adequate for both discharge and
limited commercial use but additional treatment (II) is required for selected industrial use and still further
treatment (111) for residential consumption Industry may choose to treat its wastewater for direct discharge
(not shown) to the receiving body of water or discharge without treatment (Point B) to the collection system
or, if required, pretreat the wastewater to acceptable levels (IV) before discharging (Point C) to the collection
system The reader should note that the reuse described above is not generally practiced
178
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Inorganic
Recycle
Raw Materials
(Wastes)
(INITIAL STATES)
(OPERATORS)
Tanks
I. Separate:Settle
II. Chemical React
III. Biological React
IV. Separate:Float
Further
Processing
Treated
Effluent
(GOAL STATES)
Figure 2. The sequence of operators.
-------
Table 1. Typical Initial and Goal State
Initial State Goal State
Total 5-day BOD
Soluble BOD
TOC
COD
SS
VSS
Typical
(mg/L)
200
80
150
500
200
150
Range
(mg/L)
50 to 400
0 to 200
50 to 300
200 to 1000
60 to 400
50 to 300
Typical
(mg/L)
20
None
None
None
20
None
Range
(mg/L)
5 to 30
None
None
None
5 to 30
None
?'i?\r /or Table I TYPICAL 1N11IA1. AND GOAL STATE concentrations for municipal wastewater are
given in Table I for various parameters used to report organics The ranges shown reflect site specific
variations which affect OPERATOR selection Unless there is a significant industrial contribution, the
INI TIAL S f ATE will be defined by concentrations equal to or less than the typical values given Asa result,
the sequence of OPERATORS in most POTWs is as follows bar racks, screens, a grit chamber (this is a
short detention time settler which removes high density particles such as sand and grit), a primary
sedimentation tank, a biological reactor, a secondary sedimentation tank and chlorination If properly
designed and operated, this sequence of OPERATORS will meet or exceed the typical GOAL STATE
shown More stringent GOAL STATE criteria are usually imposed on effluents which enter a water quality
limited segment of a body of water A water quality limited segment is one in which typical effluent
concentrations will impair the uses established for that segment In such cases the OPERATOR sequence
might be modified to include tanks tor chemical reactions, additional sedimentation and filtration It is
interesting to note that there are usually no direct restrictions on soluble organic carbon Only those organics
which can be degraded in a typical biological reactor must be removed The others are considered non-
biodegradable and would require other methods (e g , activated carbon) for removal
INITIAL STATE of municipal wastewaters in limited cases. Nevertheless, the over-
all impact of expanding the INITIAL STATE to include priority pollutants should
dramatically increase the understanding of removal mechanisms for other soluble
and suspended organics and concomitantly improve the design and operability of
treatment facilities.
THE GOAL STATE
Before the proper sequence in OPERATORS which will transform the INITIAL
STATE into the GOAL STATE can be established, the GOAL STATE must be
Te\i for Figure 2 "I HF SEQUENCE OF OPERATORS shown in Figure 2 has been selected to demonstrate
the basic philosophy of treatment and is not necessarily typical of any POTWs The first OPERATOR (I)
allows for the sedimentation of a portion of the settleable solids Such a tank is usually found in POTWs and
is referred to as primary treatment Bar racks and screens for the removal of larger debris often precede this
tank Some treatment facilities are required to remove phosphorus Chemicals (typically either iron or
aluminum salts) are mixed into the effluent from the primary sedimentation tank (I) at point A before
emeimgtank (11) Reactions between the phosphorus and the salts added are allowed to occur in this tank
The reaction mixture containing suspended solids (SS) which were not separated in tank (I), precipitates of
phosphorus formed in tank (11) and soluble matter are mixed with a recycle stream before entering tank (II I)
Microorganisms in this tank convert the organics into carbon dioxide, water and additional cell mass Much
of the insoluble matter is also incorporated into this mixed liquor Finally, the discharge from tank (III)
enters tank (IV) for separation of the solid matter (in this illustration by flotation but usually by
sedimentation) from the treated effluent The biological reactor (II I) and the separator (IV) comprise what is
called secondary treatment In POTWs the treated effluent is chlorinated before discharge Additional
OPERAIORS after the secondary system are called tertiary or advanced waste treatment Solids separated
in tanks (I) and (IV) require further processing As can be seen from this illustrationa basic philosophy of
waste water treatment is either the direct separation of SS or the chemical or biological conversion ol soluble
matter into SS for later separation
180
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defined. While the INITIAL STATE can be determined by sampling and analysis of
existing wastewater flows and projecting population changes, the GOAL STATE is
usually defined by some federal, state or local regulation. For example, a U.S. Court
Consent Decree in 1976 between the Natural Resources Defense Council (NRDC)
and the EPA8 established the need for defining the GOAL STATE (and, therefore,
the INITIAL STATE) in terms of priority pollutants (Table 2). The GOAL STATE
must, of course, include limitations on organics in terms of the non-specific water
quality parameters.
At the present time, the primary parameters defining the concentration of
organics in the GOAL STATE are 5-day BOD and SS. Results presented in a U.S.
Government General Accounting Office (GAO) study conducted in 1977,9'10 how-
ever, indicated that 40% of all publicly owned treatment works (POTWs) failed to
meet the designed GOAL STATE for 5-day BOD removals and 49% failed to meet
SS removals. In all, only 50% of POTWs were in compliance with standards
established by the National Pollution Discharge Elimination System (NPDES).
In a study conducted by the EPA's Office of Research and Development between
June, 1975 and December, I97710'"'12, 287 waste treatment facilities were surveyed
to determine the specific causes of failure to meet the desired GOAL STATE.
Results from the 103 facilities selected for extensive study indicated that the major
causes of poor plant performance were (1) inadequate plant operation and (2) plant
design deficiencies. The poor plant performance resulted directly from factors which
include the following: inadequate operator application of concepts and process
control testing; lack of operator understanding of wastewater treatment; improper
technical guidance; and, design limitations in the areas of infiltration/inflow, sludge
wasting and return, aeration capacity and general process controllability and flex-
ibility. Only 37 of the 103 plants evaluated met the NPDES GOAL STATE consist-
ently. The report noted that an estimated 51 additional plants would be able,
however, to meet NPDES GOAL STATE standards by implementing changes in
operation and/or minor modification of design features. As a result of this study, the
EPA recommended a Composite Correction Program (CCP) for improving the per-
formance of existing facilities. This program was implemented at several sites with
considerable success noted (Table 3). The CCP developed in this study has been
adopted by the USEPA's Office of Enforcement as a part of a national enforcement
strategy for POTWs GOAL STATE compliance.
An obvious conclusion for the CCP is that the OPERATORS (i.e., unit operations
and processes) used at POTWs for biological reduction of organics are fundamental-
ly sound and, if placed in the proper sequence, can transform the INITIAL STATE
into the desired GOAL STATE at least in terms of 5-day BOD and SS. As is
described below, more information is required before the impact of POTWs on the
removal of organic priority pollutants can be assessed.
Organic Priority Pollutants
A preliminary study to obtain data on the occurrence of eight organic priority pol-
lutants in the influent and effluent of two treatment plants was conducted in 1976.n
Results from this rather limited study indicated that the concentrations of the
organic priority pollutants tested were highly variable and that the EPA must
improve the then existing analytical methods for organic priority pollutant measure-
ment. Recently, four major studies were initiated by the EPA14 to expand the data
base for the occurrence and removal of organic priority pollutants in municipal
wastewater treatment plants (Table 4). Two of these studies were conducted by the
EPA Municipal Environmental Research Laboratory (MERL) in Cincinnati. One
was a 25 city survey designed to refine analytical procedures and assess
removability by analyzing data from one 24-hour composite collected from each
influent and effluent. Results from this study indicate that unless the treatment plant
discharge is diluted by a factor of 50 in the mixing zone, the concentration of several
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Table 2. Selected Actions Influencing Goal State Limits
Name Date Selected Key Points
Consent Decree 1976 Established priority pollutant list
Resource Conservation and 1976 Identifies hazardous municipal sludges
Recovery Act (PL 94-580) and solid wastes
Regulates treatment, storage and
disposal of hazardous wastes
Toxic Substances Control Act 1976 Regulates manufacture, processing,
(PL 94-469) distribution, use and disposal of
hazardous chemical substances
Clean Water Act 1977 Amended act passed in 1972 (PL 92-500)
(PL 95-217) Identifies construction grants
program, general effluent limitations,
pretreatment requirements, toxic
pollutant list, innovative and
alternative technologies, and NPDES
permits
Text for Table 2 SELECTED ACTIONS INFLUENCING GOAL STATE LIMITS for the control of
organics are listed in Table 2 In the three laws. Congress identified the terms hazardous wastes, hazardous
chemicals and toxic pollutants The V S. court consent decree defined the term priority pollutant.
Thousands upon thousands of organic chemicals are included on lists which combine these terms There is
also considerable overlap among lists Through the Toxic Substances Control Act, the EPA can prohibit or
limit the manufacture of chemicals which pose an unreasonable risk to the public welfare Highly
concentrated hazardous wastes (e.g , waste liquid organics placed in drums for storage) are controlled by the
EPA through the Resource Conservation and Recovery Act. Both acts help to exclude such compounds
from municipal wastewaters The organic priority pollutants are just 114 of the many organics which could
have been identified for control under the Clean Water Act In this sense the list is somewhat arbitrary For
example, solvents, cleaning agents used in the home, household paints, etc contain a multitude of organic
chemicals which may or may not be included on the priority pollutants list Using the National Pollution
Discharge Elimination System (NPDES) permit program described by the Clean Water Act, the EPA can
enforce national standards for the removal of the selected organics on the priority pollutant list Present
GOAL STATE limits normally include "conventional" pollutants only On-going studies will be used to
determine the need for including specific organic compounds in these limits
of the organic priority pollutants are likely to exceed proposed water quality criteria.
The other MERL study involved pilot plant investigations at the EPA's Test and
Evaluation Facility in Cincinnati to assess the effectiveness of conventional
municipal waste treatment processes in removing priority pollutants. Selected
organic priority pollutants were spiked into the raw wastewater. Typical removals
ranged from 95% to 99%. This does not guarantee, however, that the discharge
would not be either acutely or chronically toxic to aquatic organisms. Futhermore,
the concentration of many compounds could exceed the proposed water quality
criteria. Additional research is presently being conducted at this test and evaluation
facility.
The remaining two EPA studies are being conducted by the Washington Head-
quarters Office of Water Planning and Standards.14'15 One is a 40 city survey similar
to the Municipal Environmental Research Laboratory, Cincinnati, OH (MERL's)
25 city survey. The notable difference is that each plant will be sampled for one week
rather than one day. Preliminary results indicate that five organic priority pollutants
(benzene, chloroform, bis (2-ethylhexyl) phthalate, tetrachloroethylene and
toluene) are in all wastewaters, including those of domestic origin, and five additional
organics (1,1-dichloroethylene, ethyl-benzene, butyl benzyl phthalate, di-n-butyl
phthalate and trichlorylethylene) were consistently detected. More priority
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Table 3. The Improved Performance Resulting from CCP
Effluent
Total 5-day BOD (kg/d)
Facility
A
B
C
D
Before
160
136
35
75
After
51
15
10
33
Effluent
Total SS (kg/d)
Before
154
273
70
111
After
41
9
4
49
Texl for Table j THE IMPROVED PERFORMANCE RESULTING FROM CCP is dramatically
illustrated in Table 3 for four different POTWs Consider, for example, the improvement in the discharge of
SS from facility B On a yearly basis, facility B discharged 96,000 kg (over 100 tons) less solids into the
receiving body of water after the implementation of CCP Similar successes are noted for the other facilities
for both 5-day BOD and SS These results confirm the importance of a basic understanding of the biological
process in design and operation. Many existing treatment facilities are quite capable of meeting present-day
GOAL STATE limits without costly plant expansion programs Of course some systems are extended in
terms of aeration capacity, solids loading to the secondary sedimentation tanks and sludge handling and
require some form of plant expansion
Table 4. The List of Organic Priority Pollutants
Limit of
Detection*
Compounds
Purgeable Organics
by Purge and Trap
Acrolemf 100
Acrylonitnlef 100
Benzene 10
Bromodichloromethane 10
Bromoform 10
Bromomethane 10
Carbon Tetrachlonde 10
Chlorobenzene 10
Chloroethane 10
2-Chloroethylvmyl ether 10
Chloroform 10
Chloromethane 10
Dibromochloromethane 10
1,1-Dichloroethane 10
1,2-Dichloroethane 10
1,1-Dichloroethene 10
trans 1,2-Dichloroethene 10
1,2-Dichloropropane 10
cis 1,3-Dichloropropene 10
trans 1,3-Dichlorpropene 10
Ethylbenzene 10
Methylene chloride 10
1,1,2,2-Tetrachloroethane 10
Tetrachloroethene 10
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1,1,1-Trichloroethane 10
1,1,2-Tnchloroethane 10
Trichloroethene 10
Trichlorofluoromethane 10
Toluene 10
Vinyl chloride 10
Extractable Organics
Acid Extractables
4-Chloro-3-methylphenol 25
2-Chlorophenol 25
2,4-Dichlorophenol 25
2,4-Dimethylphenol 25
2,4-Dmitrophenol 250
2-Methyl-4,6-dinitrophenol 250
2-Nitrophenol 25
4-Nitrophenol 25
Pentachlorophenol 25
Phenol 25
2,4,6-Tnchlorophenol 25
Base-Neutral Extractables
Acenaphthene 10
Acenaphthylene 10
Anthracene 10
Benzo(a)anthracene 10
Benzo(b)fluoranthene 10
Benzo(k)fluoranthene 10
Benzo(a)pyrene 10
Benzo(g,h,i)perylene 25
Benzidine 10
Bis(2-chloroethyl)ether 10
Bis(2-chloroethoxy)methane 10
Bis(2-ethylhexyl)phthalate 10
Bis(2-chloroisopropyl)ether 10
4-Bromophenyl phenyl ether 10
Butyl benzl phthalate 10
2-Chloronapthalene 10
4-Chlorophenyl phenyl ether 10
Chrysene 10
Dibenzo(a,h)anthracene 25
Di-n-butylphthalate 10
1,3-Dichlorobenzene 10
1,4-Dichlorobenzene 10
1,2-Dichlorobenzene 10
3,3-Dichlorobenzidine 10
Diethylphthalate 10
Dimethylphthalate 10
2,4-Dmitrotoluene 10
Dioctylphthalate 10
1,2-Diphenylhydrazine 10
Fluoranthane
Flourene 10
Hexachlorobenzene 10
Hexachlorobutadiene 10
Hexachloroethane 10
Hexachlorocyclopentadiene 10
lndeno(1,2,3-cd)pyrene 25
Isophorone 10
Naphthalene 10
Nitrobenzene 10
N-Nitrosodimethylamme
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N-Nitrosodi-n-propylamine 10
N-Nitrosodiphenylamine 10
Phenanthrene 10
Pyrene 10
2,3,7,8-Tetrachlorodibenzo-p-dioxin 0 003J
1,2,4-Tnchlorobenzene 10
Pesticides and PCB Extractables
Aldrin 10
a-BHC 10
b-BHC 10
d-BHC 10
g-BHC 10
Chlordane (multi-component)
4,4'-DDD 10
4,4'-DDE 10
4,4'-DDT 10
Dieldnn 10
Endosulfan I 10
Endosulfan II 10
Endosulfan Sulfate 10
Endrin 10
Endrin Aldehyde 10
Heptachlor 10
Heptachlor Epoxide 10
Toxaphene (multi-component)
PCB-1016 (multi-component)
PCB-1221 (multi-component)
PCB-1232 (multi-component)
PCB-1242 (multi-component)
PCB-1248 (multi-component)
PCB-1254 (multi-component)
PCB-1260 (multi-component)
*This is a minimum level at which the entire system must give recognizable mass spectra
and acceptable calibration points.
tDetection limits refers to either the GC/MS method or direct aqueous injection
(GC-FID).
^Detection limit for both electron capture and GC/MS detectors.
Text for Table 4 THE LIST OF ORGANIC PRIORITY POLLUTANTS shown in Table 4 is divided into
categories which correspond to analytical procedures used to identify them'. A thorough explanation of the
analytical techniques lor the measurement of organic priority pollutants may be found in that reference. A
conceptual overview of these procedures is presented here. A municipal wastewater contains many organic
compounds The organic priority pollutants must be separated from the other compounds first, then from
each other before being identified and quantified. Separate clean-up procedures for removal of background
orgamcs are usually not necessary for municipal wastewaters. The volatile (purgeable) organics can be
purged from a wastewater sample with either helium or nitrogen gas, trapped and then separated in a gas
chromatograph (GC). The extractable organics are extracted from a wastewater sample with methylene
chloride at pH II for the base and neutral extractables and at pH 2 for the acid extractables, dried and
concentrated before being separated in the GC. A modified procedure for the pesticide and PCB
base/neutral extractables has also been employed. A mass spectrometer (MS) is used to identify and
quantify the organics after separation. Some organics require special clean-up procedures while others need
different extraction and separation techniques (e g., high pressure liquid chromatography). In general, these
procedures are quite time consuming and costly and must be performed in laboratories containing highly
specialized equipment.
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pollutants were found in the influent to larger municipal facilities than smaller ones
having a lower industrial contribution. The final major study is a six city survey
designed to determine the sources of priority pollutants (i.e., domestic, commercial
or industrial). To date, four locations have been sampled. These are Muddy Creek
Drainage Basin, Cincinnati, Ohio; Coldwater Creek Drainage Basin, St. Louis,
Missouri; R.M. Clayton Drainage Basin, Atlanta, Georgia; and the Hartford Water
Pollution Control Plant, Hartford, Connecticut. A collective summary of findings
from these surveys is not yet available.
THE BIOLOGICAL PROCESS
OPERATOR selection for municipal wastewater treatment plants depends upon
more than the parameters dealing with organics. Very often the INITIAL STATE
and the GOAL STATE are defined in terms of additional components such as the
various forms of nitrogen and phosphorus. As a result, OPERATOR selection,
sizing and operation may reflect requirements based on the removal of components
other than organics. Nevertheless, virtually all POTWs not involving land treatment
consists of two OPERATORS: BIOLOGICAL REACT and SEPARATE. In
particular, the biological removal of organics is intimately associated with some
form of phase separation system, typically either sedimentation or flotation.
Filtration is usually not employed unless some rather restrictive GOAL STATE for
SS is mandated. Land treatment, on the other hand, is primarily associated with the
OPERATOR: BIOLOGICAL REACT, in which soil organisms utilize the organics
applied.
The primary function of any municipal wastewater treatment facility is the
removal of soluble and insoluble organics from the raw waste flow. Because of the
many different organic compounds involved (indeed, there are 114 priority
pollutants alone), any combination of unit operations and chemical additions is
either too costly or too difficult to permit ready and consistent removal of the
organics present. As a result, BIOLOGICAL REACT, the conversion of soluble
organics to microorganisms, carbon dioxide, water and other end products, is
employed (Fig. 3). A principal objective of the treatment system is, therefore, the
conversion of soluble organics into microorganisms which in turn constitute an
insoluble mass that can be separated physically from the wastewater flow by some
OPERATOR^EPARATE. Some of the organics, however, are volatile and maybe
stripped from the system and, thus, avoid biological degradation. The volatile
priority pollutants fit, of course, into this category. Insoluble organic compounds
usually become enmeshed or adsorbed in the biological mass and are degraded to a
lesser or greater extent depending upon the treatment system involved.
The microorganisms are typically grown either in suspension or attached to some
surface. The systems may operate either in the presence of oxygen (aerobic) or some
other electron acceptor (anoxic) or in the absence of an external electron acceptor
(anaerobic). The rate of conversion of raw materials to end products depends upon a
multitude of factors including the concentration of the various components involved
in the reaction (e.g., substrate, microorganisms, inorganic nitrogen and phosphorus
and oxygen), temperature, pH and the nature and distribution of the organics
involved. The nature of the end products varies considerably with the environmental
conditions imposed. For example, under anaerobic conditions low molecular weight
acids, alcohols, aldehydes and methane gas are often produced along with a
relatively small quantity of organisms, carbon dioxide and water while larger
quantities of organisms, carbon dioxide and water are produced aerobically.
A complete story cannot be told by visualizing the OPERATOR:BIOLOGICAL
REACT in a municipal wastewater treatment facility as either suspended or
attached or as aerobic, anoxic or anaerobic. In aerobic suspended growth systems,
for example, the microorganisms come together in large floes. The depth of
penetration of oxygen and organics depends, among other factors, upon the
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Raw Waste or
Settled
Effluent
/
Organisms
\ Settle /
Further
Y Processing
^ Further -
Recycle
Processing
(a) Suspended Growth
Support Media
for Biological
Growth
Raw Waste or
Settled
Effluent
(b) Fixed Film
Waste Distribution
Recycle
Further
Processing
Further
Processing
Waste
Distribution
Evapotranspiration
Grass and Vegetation
Runoff
Collection
(c) Overland Flow
Figure 3. The biological utilization of organics
Text for Figure 3 THE BIOLOGICAL UTILIZATION OF ORGANICS can be conducted in a wide
variety of controlled environments Figure 3 shows three such systems In all cases, the reactions which
would otherwise take place at a slow rate in the dilute environment of a receiving body of water are
accelerated in a controlled system by providing increased concentrations of both organics and
microorganisms Asa result, organics can be utilized in a relatively small area as compared to that necessary
in a river or lake without interfering with other uses such as recreation or fishing. In both the suspended
growth and fixed film systems the secondary sedimentation tank is an essential feature Unless the organisms
produced and the SS captured are removed from the wastewater flow, GOAL STATE limits will not be met
In many treatment plants, especially suspended growth facilities, overall treatment plant performance is
controlled by the operabihty of the sedimentation tank. Proper operation of the biological reactor is
necessary for the enhancement of organisms with reasonably good settling characteristics In the case of the
suspended growth system, organisms and wastewater are usually aerated in the reactor with either
mechanical mixers or compressed air Wastewater is distributed over organisms attached to a support
medium in the fixed film system Treated water is recycled to the reactor in order to maintain a relatively
constant hydraulic load on the bio-film and to provide the mixing and oxygen transfer which results from the
free fall of fluid over the bio-film. The overland flow system is one of several land application facilities
available. In this case, there is limited percolation of water thru the soil with the bulk of the reactions taking
place near the soil surface as the wastewater flows down the incline (e g., a hill with a two to four percent
slope).
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concentration of each in the bulk fluid. Depending upon the rate of substrate supply
per unit mass of organisms present (this is often referred to as the mass loading) and
the rate of aeration, the relative depth of substrate versus oxygen penetration in the
floe may switch from hour to hour and from location to location within the entire
reactor system. These temporal and spatial variations occur in attached growth
systems as well. The extent to which these variations are allowed and controlled
determines which reactions are occurring and which are responsible for overall plant
behavior. As a result, two treatment plants receiving essentially the same waste and
employing the same sequence of OPERATORS, may have notably different
performance records.
BIOLOGICAL REACT for organic removal in most municipal wastewater
treatment plants is aerobic with either suspended growth or fixed film (attached)
system employed. Even though the innovative and alternative program
implemented by the EPA promotes land treatment, only a small fraction of the
total number of facilities involves such systems. In particular, of the 158 million
people serviced by 20,666 central collection systems carrying a total of
approximately 22 billion gallons each day, 2,603 systems are without any form of
treatment at all, 2,377 employ physical OPERATORS only, 3,331 are fixed film,
9,909 are suspended growth and 2,446 are classified as miscellaneous.s Virtually all
of the treatment facilities are continuous flow although many land application
systems and one suspended growth system under development are intermittent.
Suspended Growth Systems
There are many variations of the suspended growth systems currently employed
for the removal of organics from municipal wastewaters. The mode of operation can
dictate the net amount of organisms produced, the extent of SS degradation, the
importance of intracellular materials, the settling characteristics of the organisms
and the extent of treatment of priority pollutants. By varying the location of
aeration, raw waste input location and organism concentration in the reactor, each
of the different suspended growth systems can be made to perform, to a greater or
lesser extent, in much the same way. The primary point here is that there is just one
biological process and a multitude of physical variations which can be implemented.
While each new physical variation is often noted as a separate process, the
conversion of organics to cell mass is the primary process involved.
The physical variations of the suspended growth system may be categorized most
simply as: (1) no-recycle or recycle, (2) completely mixed or plug flow, and (3) single
tank or multiple tank. No-recycle systems (e.g., aerated lagoons) containing low
mixed liquor suspended solids (MLSS) concentrations, require relatively large
quantities of land and are found mostly in rural areas. These systems usually put out
high concentrations of SS during warmer periods because of the production and
discharge of non-settleable algae. The tank (actually, probably an earthen ditch)
contents may or may not be completely mixed. Organisms grown may be allowed to
settle either in a quiescent portion of the tank or in a separate quiescent tank. In a
recycle system, the organisms grown are returned to the reactor (i.e., the
OPERATOR: BIOLOGICAL REACT) so that the rate of degradation of organics
can be increased and the volume of the reactor required decreased.
The generic term for a suspended growth system with recycle is Activated Sludge.
The tank may either be completely mixed or channeled such that a general
appearance of plug flow is achieved. Plug flow conditions are also simulated by
operating several tanks in series. Depending upon the location of aeration, the
application points for raw feed, the hydraulic retention time and general physical
appearance, the Activated Sludge system may be referred to as Step Feed, Step
Aeration, Extended Aeration, Conventional, Completely Mixed, Contact
Stabilization, Oxidation Ditch or any one of a variety of other names. Those systems
operated such that the organisms are first exposed to high loadings in either the inlet
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portion of a tank or the first tank of a multiple tank system and then allowed to
"burn-off" (i.e., oxidize) the organics in the remainder of the tank or tanks generally
produce the highest quality effluent.
Process Considerations in Suspended Growth Systems
Unlike the feed streams in the chemical process industry, the INITIAL STATE for
a municipal wastewater varies, virtually uncontrollably, with time. Equalization
systems are sometimes used to regulate hydraulic variations. The impact of
equalization on concentration changes is, however, limited. Design is further
burdened by the fact that the primary components used for performance evaluation
(and, therefore, design) are 5-day BOD and SS.
The 5-day BOD is a rather arbitary measure of waste strength and includes the
oxygen required to convert (1) many (not all) of the soluble organics to cell mass, (2)
a portion of the SS to cell mass and (3) new cell mass into carbon dioxide and water
during respiration (Fig. 4). As a result, removal of 5-day BOD means little as far as
specific component degradation is concerned and overlaps to a certain extent with
SS. Design is clearly hindered by use of this parameter. For example, the 5-day BOD
associated with the SS can conceptually be 100 percent removed in the treatment
process by adsorption on the floe without any biological degradation taking place.
On the other hand, SS which are adsorbed will be degraded at a rate which is notably
less than that for many of the soluble components. To further complicate matters the
SS can be divided into several fractions, each with its own characteristic degradation
rate. The extent of degradation of the SS will depend on the time allowed for the
reactions to take place.
Reactor design involves a systematic accounting of the mass of each major
component entering and leaving the system. On a 24-hour basis, all components
which entered the reactor and were not found in the effluent must have either
accumulated in the reactor or been converted to another form. In the case of soluble
BOD, the individual chemical compounds which are lumped into the non-specific
BOD parameter are used as a food source by the microorganisms and converted into
cell mass, carbon dioxide, water and other end products. A properly sized suspended
growth system will degrade virtually all of the soluble BOD entering. A larger
reactor containing a greater mass of organisms will often not appreciably reduce the
mass of soluble BOD leaving the system but will convert more cell mass to carbon
dioxide and water and degrade more of the incoming SS. As a result, of the two
reactors receiving the same quantity of substrate and containing the same MLSS
concentration, the larger reactor (having, of course, the greater mass of organisms
and, therefore, lower loaded) will produce a smaller quantity of sludge provided the
rate of oxygen supply is adequate.
One of the more difficult tasks in accounting for the mass of each component is the
definition of the rate at which the components are degraded. In particular, the many
physical variations offered as alternatives for the biological process produce
confusion with respect to mixing patterns and render many of these systems virtually
impossible to represent mathematically. More specifically, design formulations for
systems, which are not at least completely mixed in discrete zones, are extremely
difficult to develop especially when the time-varying nature of the system inputs are
considered.
The rate of oxygen supply is one of the most critical factors in biological
treatment. The mass of oxygen supplied each day must be equal to the mass of
ultimate BOD used each day by the organisms less the oxygen equivalent of the
sludge wasted each day. The oxygen equivalent of sludge can range from 1.2 kg to 1.4
kg oxygen per kg sludge. Simply stated, the more organics converted to carbon
dioxide, the less sludge produced and the more oxygen utilized.
The organic portion of the sludge wasted each day is, of course, composed of
unreacted SS, cell mass and cellular debris (insoluble fragments from dead
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f
o
Figure 4. The 5-day BOD is an arbitrary measure.
Text for Figure 4 THE 5-DAY BOD IS AN ARBITRARY MEASUREof organic compounds Thiscan
be readily seen in Figure 4 by considering the plot for cumulative oxygen consumed in a BOD bottle vs. time
for either compound A or B In general, all of the organics are consumed as soon as the plateau in the curve is
reached Further utilization of oxygen after the plateau results from degradation of intracellular materials
(endogenous respiration), consumption of compounds released by dead organisms and respiration by higher
predator forms (e.g , protozoa) As is illustrated above, the total amount of oxygen consumed in five days is
identical for compounds A, B and C even though compound A had been completely removed after the
second day and compound C is not fully consumed Compound D may be considered representative of either
SS or a slowly degraded organic The standard test for BOD measurement does not provide for mixing As a
result, SS which settle during the test period may be more slowly degraded in the bottle than is possible in a
mixed environment In spite of these deficiencies, the 5-day BOD provides an excellent indication of the
general strength of the wastewater and the overall performance of the treatment system Direct use of this
parameter in design, however, can lead to difficulties especially if the relative concentration of soluble and
insoluble organics is not measured separately whenever either the INITIAL STATE or GOAL STATE
differs from that expected for a typical municipal wastewater
organisms). The quantity of VSS wasted each day for each kg of total 5-day BOD
removed (as opposed to degraded) from the raw waste flow ranges from 0.25 to 0.85
and is often referred to as the yield of organisms. Since only a portion of the sludge
wasted each day is organisms, the term cell yield does not correctly describe the
mechanisms involved. The "yield" will be smallest in those systems which
biologically consume the greatest quantity of oxygen (excluding nitrification) for
each unit mass of total BOD removed. Lightly loaded systems with adequate oxygen
supply fit well into this category. The sludge produced from these systems is usually
quite stable, dewatering easily and providing few unreacted organics for methane
production in an anaerobic digester. Sludges from heavily loaded systems are
usually an excellent source of organics for methane production.
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Current Research - Suspended Growth Systems
The discussions which follow describe observations from current research
investigations. While additional studies are necessary to confirm these observations,
results from many existing facilities support the concepts presented.
The adequacy of the oxygen supply system depends upon the variations in the
mass loading over, say, a 24-hour period. Organisms in a suspended growth system
are able to "store" organics during periods of increased load which can then be
utilized during periods of reduced load. The soluble organics are "stored" internally
as intracellular intermediates, nucleic acids, proteins and, perhaps, classical storage
compounds such as glycogen. The SS are "stored" within the general mixed liquor
mass by becoming adsorbed or enmeshed in the floe. This accumulation of organics
occurs whenever the rate of organic supply to the microorganisms exceeds the rate at
which oxygen is supplied for the completion of the necessary oxidative reactions
(expressed, for example, in terms of the carbon dioxide production rate) (Fig. 5). As
may be expected, organic accumulation will eventually result in a deterioration in
effluent quality if allowed to continue until the "storage capacity" of the system is
exceeded. The "storage capacity" can, of course, be exceeded in a matter of minutes
or hours, depending upon the extent of reserve capacity available and the intensity
and duration of an instantaneous mass load.
While organics can accumulate in the reactor whenever the dissolved oxygen
(DO) concentration is above one or two milligrams per liter, the usual situation is to
observe low DOs during times of accumulation and generally higher DOs during
times that the oxygen supply rate is adequate. Adequacy of the oxygen supply rate
depends upon both the existing loading and the quantity of "stored" organics.
Proper system operation would require that the oxygen supply rate be sufficient
during selected periods of a day to allow the complete oxidation of at least the
intracellular organics. The extent of degradation of the "stored" SS is, of course,
critical as well.
There is an intimate relationship between the operation of a biological reactor and
the nature of organisms which develop in that reactor. The organisms must be able
to utilize the organics and possess physical characteristics that allow easy separation
(e.g., by sedimentation). For a "typical" municipal wastewater, the major organisms
which develop in the system depend not only upon the average daily load, aeration
supply rate and resulting DO, but also on the variations in the INITIAL STATE
within a 24-hour period. This has resulted in the rather confused literature with
respect to the presence and control of the poorly settling filamentous organisms. One
study will cite low loading and high DO as the cause of filamentous organisms while
another will cite high loading and low DO (Fig. 6). Actually, the system's ability to
handle adequately "stored" organics (i.e., depending upon the match between hourly
variations in load and oxygen supply rate) and not the average daily loadings and
DO, will contribute significantly to both the presence and absence of filamentous
organisms and, thus, the general performance of the system.
In general, the organisms that are present in a biological reactor are capable of
degrading a wide variety of organic compounds. Whether or not the organism
system does break down a given organic compound, however, depends upon a
number of factors. For example, glucose, an organic compound readily and rapidly
degradable by many of the microorganisms found in POTWs, will only be utilized by
these organisms after a suitable acclimation period has elapsed. In this case, most of
the organisms found in POTWs possess the genetic information for glucose
degradation but do not normally synthesize the enzymatic systems necessary for the
transport of glucose to the interior of the cells because glucose is not usually present
in sufficient concentration in domestic wastewaters. A compound that must be
present for enzyme system development and appears in the wastewater flow only
periodically and/or in extremely low concentrations may not be degraded in the
system. Some organisms manufacture the enzymes for degrading selected organics
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Raw Waste Organics
(Primary Electron Supply)
Electron
Accumulation
in Intracellular
Compounds
Electron Flow
to External
Acceptor and
H20 Production
^='\ Organic By-Products >/"/*/
\ \ (Secondary Electron S jS
\ V Supply) //
Vi ^
Additional
Organic
By-Products
Figure 5. The flow of electrons during biodegradation of organics.
Text for Figure 5 THE FLOW OF ELECTRONS DURING BIOUEGRADA I ION OF ORGANICS
(Figure 5) can be used to illustrate the complex interactions which take place in a biological reactor
Conceptually, electrons (organics) supplied by the untreated wastewaters may either accumulate within a
primary organism system (i e . new cell mass) or be released either to an external electron acceptor (e g.,
oxygen) or in the form of organic by-products Electrons stored in these by-products may then be utilized by
a secondary organism system in the manner described above As a result, the biological population that
develops from the utilization of the same organics each day depends upon the relative importance of each of
these pathways For example, an organism system "fed "at the beginning of a 24 hour period (say, such that
the external source of organics is utilized in less than four hours) will differ dramatically in character from a
population that is supplied the same organics continuously and uniformly over the 24 hour period
Furthermore, a given organism may differ either physiologically or morphologically or both when subjected
to one or the other feeding strategy The intensity, frequency and duration of feeding all contribute to the
selection of organisms and the definition of those reaction pathways that distinguish one organism system as
a whole from another For example, without changing the nature of the food (organics) supplied, some
organism systems may accumulate and utilize storage products on a regular basis while another may
generate and utilize by-products While a Change in food type would clearly complicate the analysis, the
notion that a limited number of major reaction pathways dictates the performance of the system is not
mitigated Additional research must be conducted in order to determine the relative importance of these
pathways in a municipal wastewater treatment plant The mechanisms by which SS are involved must also be
included
independently of the presence of these organics. Even if the proper enzyme system is
always "expressed," the rate of degradation of a given compound may be so slow
that only a limited portion of that compound is utilized. Recall that, because of the
slow degradation rate of SS, a lightly loaded system is generally more effective at
utilizing SS than a heavily loaded system.
The only way to ensure the removal of a given component is to design the reactor
such that the component of interest is reduced to the desired level. Unfortunately,
many of the priority pollutants are at such low concentrations that the reactor
volume required to "guarantee" biodegradation may be so large as to indicate the
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Microscopic View of Activated Sludge
Figure 6. The relative amount of filamentous organisms.
Percent Filamentous Organisms
-------
need for alternate technology. In addition, the composition of the community of
organisms developed to remove the bulk of the conventional components may not
perform as well as demanded in an oversized reactor (e.g., filamentous organism
population may develop under lightly loaded conditions). Clearly, many of the
priority pollutants, including some of those that are biodegradable, will have to be
removed by non-biological methods.
Fixed Film Reactors
While approximately 63 percent of POTWs employing some form of biological
treatment utilize suspended growth reactors, fixed film systems are the second most
common variety of biological treatment at 21 percent.5 Many of the comments
directed at suspended growth systems apply to the fixed film systems. There is just
one biological process and many physical plants to house that process. The
suspended growth system is based on the premise that the microorganisms selected
not only can utilize the organics supplied to the reactor but also can be separated
(usually by sedimentation) from the treated wastewaters. Similarly, the fixed film
system utilizes the organics, both soluble and insoluble, but selects for organisms
which attach to surfaces.
The original fixed film reactors were called Trickling Filters and used rocks for
organism attachment. Later systems have employed synthetic plastic media instead
of rocks. In the Trickling Filters the medium remains stationary and the organics
move past the medium. An alternative form of fixed film system is one in which the
film is attached to a drum rotating through the wastewater flow. This system, the
Rotating Biological Contactor (RBC), has received considerable attention during
the past decade.
Fixed film systems as well as many land treatment applications suffer from a lack
of process control alternatives which can be implemented to meet changes in either
the INITIAL STATE or GOAL STATE. The primary factor responsible for this
limitation is the fixed surface area for organism attachment. In a fixed film reactor,
however, the fraction of the organism population which participates in the treatment
can be expanded by implementing proper control strategies. For example, in a
vertical column (e.g., a Trickling Filter), the relative "work" between the organisms
located at the top of the column and those at the bottom can be varied by changing
the hydraulic flow rate (e.g., by increasing the recirculation ratio). In addition,
strategies which increase the depth of penetration of either oxygen or organics will
improve the reaction potential at a given column elevation.
Text for Figure 6. THE RELATIVE AMOUNT OF FILAMENTOUS ORGANISMS present in a
suspended growth reactor has a dramatic impact on the settling characteristics of the biomass Figure 6
shows that the biological population can be thought of as being composed of discrete floes (the dots) of non-
filamentous organisms and the filaments (the lines) This is clearly an oversimplification since well over 300
species of organisms have been identified in activated sludge with more than twenty of those exhibiting
filamentous growth characteristics. Nevertheless, the suggested microscopic view of Activated Sludge and
the negative impact of an increasing percentage of filamentous organisms on the rate of settling are both
reasonable representations of observed conditions. Suspended growth reactors are often plagued by the
presence of excessive amounts of filaments. Because different filaments have different growth characteristics
and nutrient requirements, a single strategy which mitigates their presence may be difficult to achieve The
literature suggests that difficulties associated with filamentous organisms can be reduced if the solids in the
reactor are turned-over approximately once every three to twelve days (this is referred to either the solids
detention time or the sludge age) Research currently being conducted by the author indicates that the
relative fraction of filamentous organisms to the total cell mass can be controlled by regulating the extent of
storage and by-product formation allowed For example, the organism population established in a system
utilizing organics in the form provided by the feed will differ from an organism population which converts
the organics present in the feed into both new cells and by-products The difference in the two populations is,
of course, that the by-products represent a change in the organics available to the organisms. In a mixed
culture this will result in a population shift Other theories have been proposed Additional research is clearly
necessary.
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In many cases, the organism population of a fixed film reactor can be increased
only by adding support medium. Since the approximate quantity of organics
consumed by a fixed mass of organisms is relatively constant in these systems,
neither additional waste loads nor more restrictive effluent criteria for a given waste
load can be accomodated easily. This, of course, is true of any suspended growth or
fixed film biological reactor. After the apparent limit of treatment potential is
reached, little can be done to extend the useful life of the facility without making a
reasonably significant modification in hardware. By way of contrast, however,
within certain site specific constraints, the organisms present in a suspended growth
system can be increased up to two-fold during the life of the facility. Indeed, such
changes can be made in a matter of days if necessary. This disparity in flexibility
between fixed film and suspended growth systems was highlighted by the EPA's
Office of Research and Development study1 which noted that fixed film facilities
missed GOAL STATE compliance because of deficient design practices and the
suspended growth systems usually missed because of operational problems.
Current Research - Fixed Film Systems
i
Fixed film reactors add a dimension to wastewater treatment that must not be
overlooked. It is a form of biological treatment which has served the nation's
wastewater elimination problems well. Operation is relatively simple. Performance
is generally consistent. Research which expands the knowledge base for fixed film
reactors not only will enhance the capability of existing systems but also will allow
the development of new systems which are necessary for meeting the ever escalating
demands for improved performance.
Apparent limits in flexibility come, in part, from the relatively complex problems
associated with theoretical descriptions of the fixed film system and the resulting
lack of fundamental understanding of system operation. Theoretically sound
mathematical models must include a number of factors such as (1) mechanisms of
attachment of SS on the biofilm and the interference of oxygen transfer resulting
from such attachment; (2) definition of liquid film thickness and the resistance to
mass transfer offered by these films; and (3) definition of the active portions of the
bio-film in terms of aerobic, anoxic and anaerobic reactions and rate of organic
utilization in each of these zones. Results from research efforts in this area will
clearly benefit the general design and operation of both fixed film and suspended
growth reactors. Answers to specific questions such as those involving the impact of
rotational speeds on rotating biological contactors (RBC) performance and the use
of pure oxygen in closed column Trickling Filters can also be addressed.
Other fixed film systems include those in which the organisms are attached to a
medium such as clay, sand or plastic. Attachment may be either on the surface or in
the interior of the medium. The individual organism systems thus created may be
either mixed (i.e., fluidized) or allowed to remain stationary (fixed). Such reactor
configurations (especially the fluidized beds) have features of both fixed film and
suspended growth facilities and are likely to have a dramatic impact on the future of
biological wastewater treatment.
IMPROVING PROCESS PERFORMANCE
The fact that the newer systems are not yet facing their design loads and have not
weathered the full test of time, cannot be overlooked. Nevertheless, two strategies
are apparent from the development made during the seventies. First, there was an
attempt to utilize oxygen more efficiently. The intricate controls instituted in the
oxygen enriched systems, the fine bubble diffusers and the jet aeration systems are
all examples of this attempt. Second, staged reactors, baffled areas creating discrete
zones and channeled flows, allowed contact between either the return sludge or
195
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mixed liquor and elevated concentrations of organics in the raw wastewater.
Virtually all of the systems designed for biological nutrient (e.g., denitrification)
removal possess this feature. In general, these strategies have resulted in reduced
reactor volumes, higher concentrations of organisms and lower energy
consumption.
Improved efficiency of oxygen transfer will, of course, reduce costs. Perhaps more
importantly, the more efficient oxygen transfer systems can respond better to
changes in the oxygen demand due, for example, to increased loadings or elevated
mixed liquor concentrations. This intimate relationship between tank volume and
oxygen supply rate can be readily understood by considering the following example.
A reactor, apparently sized twice as large as necessary but receiving oxygen at
one-tenth the rate required to satisfy the organic demand, will not operate properly.
Increased aerator capacity is necessary for proper operation. Clearly, many older
systems receiving design loads or greater can be made to produce a higher quality
effluent (without adding tankage) simply by installing more efficient oxygen transfer
equipment. On the other hand, systems operated with excessive aeration relative to
the organic load (as in many of the conventional Extended Aeration plants) often
develop high DOs and filamentous organisms.
Improvements associated with staged reactors are more difficult to understand.
Several observations, however, can be made using results from current research
efforts. One is that organisms intermittently subjected to high concentrations of
organics both "store" these organics and have consistently good settling
characteristics. Excessive loadings can cause difficulties. Exposure to very high
concentrations of organics sometimes results in the development of a turbid effluent
caused by the production of dispersed organisms, even though the bulk of the
organism population is associated with sludge with good settling characteristics.
Another observation is that the staged reactor system evidently allows less severe
constraints on the sizing of the aeration device provided that the "stored" organics
are oxidized in the latter portion of the system. In particular, the DO can be
essentially zero in the first portion of the system and exceed 4 mg/ L or 5 mg/ L in the
latter stages without fostering the growth of filamentous organisms. Finally, the
overall kinetics associated with soluble organic utilization (not SS) is increased in
these staged systems. As a result more soluble organics can be utilized in a staged or
baffled system than in a single tank reactor having the same total liquid volume.
Once again the implication with regard to existing older systems is that either the
useful life can be extended or the effluent quality improved by changing the reactor
configuration (e.g., by adding baffles). Such a change may have to be accompanied
by a modification or replacement of the existing aeration system.
Because the bulk of the research efforts and new system applications in the
seventies dealt with suspended growth reactors, examples which demonstrate
improved process performance for fixed film systems are less evident. One strategy,
however, that has been quite successful involves the addition of a short detention
time suspended growth reactor between the Trickling Filter and the final separator.
Other strategies, such as increasing the recycle flow rate, have also been used to
improve the performance of existing facilities.
FUTURE
Efficiency of oxygen transfer and increased concentrations of organics (due to
some form of staging) were the common features among four new treatment systems
described by Barth. These systems, the activated bio-filter," the reactor-clarifier,18
the deep shaft," and the sequencing batch reactor,20 reflect developments during the
seventies for use during the eighties. The activated-bio-filter represents an obvious
means for upgrading fixed film facilities which can no longer meet effluent
standards. The extent of organic utilization in the fixed film portion of the system is,
of course, limited by the quantity of oxygen transferred. As a result, the suspended
196
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growth reactor must be sufficiently large to complete the desired reactions. The
reactor-clarifier is, from a biological process standpoint, quite similar to the
activated bio-filter (Fig. 7). The initial contact of the raw waste is with oxygenated
mixed liquor in a quiescent tank rather than a fixed film medium. The extent of
treatment in the external U-tube used to supply oxygen to the mixed liquor relative
to that" in the quiescent tank used for solids separation depends upon the volumes
and reaction rate of each zone.
The deep shaft employs a vigorously mixed oxygen transfer zone in the region of
contact between the raw waste and the return sludge. This plus the extremely wide
ranges of hydrostatic pressure experienced by the microorganisms distinguishes this
system from the other two. The vigorous mixing in the zone of high loadings
presumably results in a greater removal of organics in this initial contact zone than
the corresponding location in either the reactor-clarifier or the activated bio-filter.
Because of the rapid rate of oxygen utilization, a considerable fraction of the
organics removed must be converted to carbon dioxide and water in the initial
contact zone with the remaining fraction "stored" (e.g., enmeshed unreacted SSand
intracellular organics) for utilization in the remainder of the shaft. Even though the
loading to the deep shaft is up to five times that of conventional activated sludge
plants, the yield of solids is relatively low but still consistent with the observed
oxygen uptake rates. Such low yields suggest both deep penetration of oxygen into
the individual floes and a rather extensive degradation of SS. Such results point to
the need to understand better the mechanisms for SS degradation.
The sequencing batch reactor differs from all of the other systems discussed in that
this "process" operates on a fill and draw schedule while the others are continuous
flow (Fig. 8). Nevertheless, the sequencing batch reactor accomplishes in time what
the continuous flow systems do in space. Perhaps the most notable difference in this
system and those operated on a continuous flow basis is the flexibility offered. In a
continuous flow, staged reactor system, the liquid volume in each tank is fixed and
the "relative work" between any two tanks (or functions) is essentially fixed. In the
sequencing batch reactor, the relative work between two functions can be adjusted as
necessary by simply changing the time allotted to one or both of the functions. Four
additional interesting features of the sequencing batch reactors are: (1) solids
separation takes place under quiescent conditions; (2) sludge is not removed from
the reaction mixture unless it is no longer necessary; (3) if properly sized, selected
desired reactions such as those associated with the removal of specific priority
pollutants can be allowed to go to completion after the tank is filled; and (4)
short-circuiting is all but eliminated.
In order to meet changes in the GOAL STATE, such as those which may be
necessitated by emphasis on organic priority pollutants, sufficient flexibility must be
designed into the system to allow changes in operation. Otherwise, costly and
unnecessary construction of additional facilities might be required. In addition, both
the new and the old systems must be reexamined with the view to simplifying plant
operation so that the personnel available for the needed jobs are not overwhelmed by
sophisticated concepts and machinery. While this may seem counter to current
trends, a concerted effort to accomplish this goal will generally result in improved
plant performance.
On the other hand, the design engineer must seek a more fundamental
understanding of the biological process. For example, if an elevated organic load
followed by a period of oxidation of "stored" organics does, in fact, reduce
filamentous organisms and generally improve system performance, the notion of
maintaining a relatively constant load is not the best control strategy. Likewise, the
benefits of periods of near zero oxygen concentration provided higher DOs can be
achieved when needed should be considered in new system design and operation.
More research is obviously necessary before the biological process can be
understood sufficiently well for an engineer to be able to "guarantee" a design. The
basic research centers presently being developed by the EPA recognize the need for
answering some of the more fundamental, but yet unresolved, questions.
197
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Waste Flight Mechanism
Water " >
i
W^-TT~/~1!^~~^~^ r\
c Clarification Zone
;•;':.; Mixed Liquor \
o ••'/• '.;:.Zone •• \
Reactor
1 1 .J7
~^-
Effluent
V
-*o
i=ii
-r
Recycled Sludge 1
(a) Reactor-Clarifier Waste
Sludge |
Oxygen Gas
V
V
Tl U-Tube
Oxygen
i Transfer
. Unit
T
,t
T
|f
Waste
Water
Bio-Cell I
Lift Station I
Aeration
Flow Control K iTl/
& Splitting K "=^X
Process
Effluent
...,,.
Recycled Sludge
»• Waste Sludge
(b) Activated Bio-filter
Figure 7. The reactor-clarifier and activated bio-filter
Text for Figure 7 THE ACTIVATED BIO-FILTER AND REACTOR CLARIFIER represent variations
of the biological process which possess considerable merit Essential similarities and differences between the
two systems can be seen from the schematic presented in Figure 7 The activated bio-filter is simply a
combination of a Trickling Filter and an Activated Sludge system The reactor-clarifier, on the other hand, is
quite unlike most conventional treatment facilities m operation today The U-tube oxygen transfer unit is
similar to the deep shaft Additional treatment, however, takes place in the reactor which serves as both an
additional reaction zone and a secondary sedimentation tank
As both basic research and experience from full-scale facilities become more fully
bonded, mathematical models which incorporate such principles as metabolism of
intracellular organics and SS utilization will be used in the design and analysis of
biological reactors. At the present time, however, use of such models is quite limited
for several reasons. First, many of the stoichiometric and kinetic coefficients are
198
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Compressor
WaLwater
Head
":'.- .. Tank
-Waste Sludge
Effluent
Flotation Tank
(a) Deep Shaft
Shaft
1.00
1 0.75
E
| 0.50
'S
(D
2 0.25
f- Solids Wasting
V Liquid Level
//// Solids
°< Aeration
-Fill-
-React-
-Settle-
-Draw-
-Idle-
0.25 0.50 0.75
Fraction of Cycle Time
Liquid Volume vs. Time
for One Reactor (Hypothetical)
1.00
(b) Sequencing Batch Reactor
Figure 8. The deep shaft and sequencing batch reactor
199
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difficult to evaluate and must be estimated. Second, many of the concepts are still in
the early stages of development (e.g., in bio-film models) and are not generally
accepted. Third, the older empirical formulations and earlier simpler models have
been used successfully to design reactors which have performed adequately. Fourth,
many individuals have an intuitive grasp of biological treatment which provides
much if not all of that which can be obtained from the newer more sophisticated
models. Finally, some individuals do not have sufficient background to allow easy
understanding of many of the mathematical concepts and, as a result, are unwilling
to use the new approaches.
Each of the above reasons alone is sufficient to limit use of the newer models. The
combined impact is overwhelming. But intuition is not easily transferred from
individual to individual. The newer systems with high organic concentrations in one
zone and reduced concentrations in another force the organisms to carry out
reactions which were not important in many of the older more lightly loaded
reactors. As design loads for the older systems were approached and exceeded,
organism behavior changed and standard operating rules were no longer sufficient.
Under such conditions, the earlier models simply do not contain adequate
information to allow proper analysis. Unfortunately, use of simpler models can lead
to poor design and operational judgment. For example, the more complex model
with estimated coefficients may (correctly) direct the designer to go to the right
(figuratively speaking, of course) while the simpler model not only points to the left
but also indicates how far. A better appreciation of the strengths and weakness of
mathematical modeling must be developed in the eighties.
As a final note, the energy consumption associated with wastewater treatment is
extremely small when compared with consumption on a national level.2' The energy
costs, however, for the operation of a given treatment facility can be quite extensive
and may be responsible for heavy tax burdens to individuals located in small
communities. As a result, methods for the reduction of energy requirements for
existing systems are being investigated. For example, feedback DO controllers have
been used to operate some systems. Because of the virtual complete use of aerobic
biological systems for the treatment of municipal wastewaters, the extent to which
energy utilization can be reduced in the system is somewhat limited. As a result, the
EPA plans to investigate possible benefits associated with anaerobic treatment.
Preliminary results with anaerobic expanded fluidized beds have been quite
promising.'2 The eighties will certainly see developmental work conducted on these
and other low energy anaerobic systems.
Ten for Figure 8 THE DEEP SHAFT AND SEQUENCING BATCH REACTOR are depicted
schematically in Figure 8 Newer versions of the deep shaft are more intricate than the one shown Solids
separation is typically achieved by flotation but sedimentation tanks have also been employed. The deep
shaft is a highly loaded system which often produces a lower than expected yield of solids The high pressures
(the shafts can be as deep as 200 m) and resulting high concentrations of DO (up to 40 mg' L) may both be
responsible for the low yield The pressure changes could mechanically increase the availability of SS to the
organisms The elevated DOs may result in a positive concentration of oxygen throughout the floe Because
the rate of carbon dioxide production is increased for DOs greater than approximately 0 5 mg; L, a high
concentration of oxygen through the floe would result in an increase in the overall rate of carbon dioxide
production and, as a result, reduce solids yield The sequencing batch reactor is basically a fill and draw
suspended growth system. The wastewater is added to a partially full reactor containing activated sludge
during a period identified as FILL Reactions taking place during FILL are controlled by the adjusting of the
mixing and aeration intensity and frequency After the liquid level in the reactor reaches the maximum level,
the wastewater is diverted (say, to another reactor) and the reactor enters a period called REACT During
RE ACT the required reactions are allowed to continue to completion The extent of one reaction to another
is controlled, once again by adjusting both mixing and aeration After the desired time in REACT, all mixing
and aeration provided to the reactor is discontinued so that the organisms (solids) may be separated by
gravity sedimentation from the treated wastewater This period is called SETTLE and is followed
immediately by DRAW, the time during which the treated wastewater is decanted. After DRAW, the settled
organisms remain in the reactor during IDLE waiting for a new supply of wastewater to be added to the
reactor
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ACKNOWLEDGEMENTS
The author is extremely grateful to Mr. John Convery of the EPA Municipal
Environmental Research Laboratory in Cincinnati for the time spent by him and his
staff in providing source material for this chapter. In particular, special thanks
should be given to B. Austin, E. Barth, R. Brenner, R. Bunch, J. Cohen, F. Evans, J.
Kreissel, G. Lubin, E. Opatken, A. Petrasek, and R. Williams.
REFERENCES
1. Powers, G.J. "New Developments in Modeling, Simulation and
Optimization of Chemical Processes. Section 7. Computer-Aided Chemical
Process Synthesis," Notes, Special Summer Program, Massachusetts
Institute of Technology, Cambridge, MA, July 26-August 4, 1976.
2. Irvine, R.L. "ACTIVATED SLUDGE - Stoichiometry, Kinetics and Mass
Balances," American Chemical Society Audio Course, September, 1980.
3. Metcalf & Eddy, Inc. Wastewater Engineering, Treatment, Disposal, Reuse,
McGraw-Hill Book Co., New York, NY, 1979.
4. Hunter, J.V. and H. Heukelekian. "The Composition of Domestic Sewage
Fractions," Jour. Water Poll. Control Fed., 37, 1142, 1965.
5. Barth, E.F. "New Secondary Treatment Processes," Presented at the 54th
Annual Conference, Ohio Water Pollution Control Association, Cincinnati,
OH, June 11-13, 1980.
6. Bishop, D.F. "GC/MS Methodology for Priority Orgamcs in Municipal
Wastewater Treatment," Draft Report, Municipal Environmental Research
Laboratory, Cincinnati, OH, February, 1980.
7. Convery, J.J. "Statement to the Subcommittee on Oversight and Review,"
Committee on Public Works and Transportation, U.S. House of
Representatives, June 24, 1980.
8. Natural Resources Defense Council (NRDC) et al vs. Train 8 ERC 2/20,
DDC 1976.
9. Comptroller General of the United States. "Continuing Need for Operation
and Maintenance of Municipal Waste Treatment Plants," Report to
Congress, Washington, DC, CED-77-46, April, 1977.
10. Smith, J.M., F.L. Evans, III, and J.H. Bender. "Improved Operation and
Maintenance Opportunities at Municipal Treatment Facilities," Proc. 7th
United States/Japan Conference on Sewage Treatment Technology, Tokyo,
Japan, EPA 600/9-80-047, p. 731, December, 1980.
11. Hegg, B.A., F.L. Rakness, and J.R. Schultz. "Evaluation of Operation and
Maintenance Factors Limiting Municipal Wastewater Treatment Plant
Performance," EPA 600/2-79-034, June, 1979.
12. Gray, A.C., Jr., P.E. Paul, and H.D. Roberts. "Evaluation of Operation and
Maintenance Factors Limiting Biological Wastewater Treatment Plant
Performance," EPA 600/2-79-078, U.S. Environmental Protection Agency,
Cincinnati, OH, July, 1979.
13. Municipal Environmental Research Laboratory. "Survey of Two Municipal
Wastewater Treatment Plants for Toxic Substances," Report of the
Wastewater Research Division, Cincinnati, OH, March, 1977.
14. Convery, J.J., J.M. Cohen, and D.F. Bishop. "Occurrence and Removal of
Toxics in Municipal Wastewater Treatment Facilities," Proc. 7th United
States/Japan Conference on Sewage Treatment Technology, EPA 600/9-80-
047, p. 633, December, 1980.
15. Council on Environmental Quality. "Environmental Quality - 1979," 10th
Annual Report, U.S. Government Printing Office, Washington, DC, 1979.
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16. Smith, J.M., J.J. McCarthy, and H.L. Longest, 2d. "Impact of Innovative and
Alternative Technology in the United States in the 1980's," Proc. 7th United
States/Japan Conference on Sewage Treatment Technology, EPA 600/9-80-
047, pp. 515, December, 1980.
17. M & I, Inc. "Evaluation of Activated Bio-Filter Wastewater Treatment
Process at Helena, Montana," Interim Report, U.S. Environmental
Protection Agency, Municipal Environmental Research Laboratory,
Contract No. R-806047010, Cincinnati, OH, May, 1979.
18. Brenner, R.C. Status of Novel Biological Process Development in the United
States, Presented at Interim Technical Seminar between 7th and 8th United
States/Japan Conference on Sewage Treatment Technology, Tokyo, Japan,
May, 1981.
19. Brenner, R.C., and J.J. Convery. "Status of Deep Shaft Wastewater
Treatment Technology in North America," Proc. 7th United States/Japan
Conference on Sewage Treatment Technology, EPA 600/9-80-047, p. 777,
December, 1980.
20. Irvine, R.L., and A.W. Busch. "Sequencing Batch Biological Reactors - An
Overview," Jour. Water Poll. Control Fed., 51, 235, 1979.
21. Wesner, G.M., L.J. Ewing, Jr., T.S. Lineck, and D.J. Hinrichs. "Energy
Conservation in Municipal Wastewater Treatment," U.S. Environmental
Protection Agency, Washington, DC, EPA 430/9-77-011, March, 1978.
22. Heidman, J. "Internal Report," U.S. Environmental Protection Agency,
Municipal Environmental Research Laboratory, Cincinnati, OH, June, 1980.
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REMOVAL OF ORGANIC SUBSTANCES
FROM MUNICIPAL WASTEWATERS BY
PHYSICOCHEMICAL PROCESSES
Walter J. Weber, Jr.
and
Frederick E. Bernardin, Jr.
INTRODUCTION
Physicochemical treatment (PCT) processes are generally categorized as
operations which effect removal and/or destruction of undesirable organic con-
stituents in wastewater by means other than biological degradation or conversion. In
this broad context, PCT embraces a wide range of both traditional and innovative
technologies.
For the present discussion, PCT processes may be defined more pertinently as
technologies which practically and economically supplement or replace conventional
biological treatments to yield equivalent or improved removal of organic substances.
The qualifiers—practically and economically—implicitly exclude a number of
processes which, although technically capable of accomplishing treatment
objectives, are impracticable for one reason or another.
This chapter considers only processes which have been demonstrated on large
scale—or at least proven promising on pilot scale—to be suitable for near-term
application to municipal wastewater treatment. The methods reviewed include
selected oxidation/conversion technologies, phase separation technologies, and
The Authors
Walter J Weber, Jr is Professor of Environmental and Water Resources Engineering and Chairman,
University Program in Water Resources, at the University of Michigan A graduate of Brown (Sc B ),
Rutgers (M S E ) and Harvard (M A and Ph D ) Universities, Dr Weber joined the Michigan faculty in
1963 and advanced to full professor and program chair in 1968 In 1978 the University named him
Distinguished Professor Dr Weber has done extensive work on the research, development, and
implementation of physicochemical processes in water and waste treatment applications He has received a
number of citations and awards for his contributions to the field, and is generally acknowledged as one of the
foremost international experts in adsorption technology Dr Weber is author or co-author of over 200
scientific and technical publications and three books, including the widely-used text and reference Phvsu o-
(hemii'al Processes for Water Quality Control
Frederick E. Bernardin, Jr is President and General Manager of Q E D Corporation, a research and
engineering firm in Ann Arbor, Michigan, specializing in water and wastewater treatment Mr Bernardin
holds degrees in chemistry from Waynesburg College and engineering from the University of Pittsburgh.
Prior to joining Q.E D in 1979, Mr Bernardin held technical and managerial positions with Calgon
Corporation, Thetford Corporation and ICI Americas, Inc. In the period of the late 1960'stothe mid 1970's,
Mr Bernardin was supervisor of the group of engineers and scientists in the vanguard of making municipal
wastewater treatment with activated carbon a viable engineering option During this period, he also worked
closely with EPA's technical staff in developing this pioneering activity Mr Bernardin is a member of
WPCF, AlChE and AWWA and has authored over twenty-five technical articles and presentations
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adsorption/exchange technologies. Particular emphasis is placed on that process
category which has most extensively demonstrated its feasibility for removal of
dissolved organic substances; specifically, adsorption processes utilizing activated
carbon.
Historic Perspectives
Several PCT processes-such as filtration and chemical coagulation—which meet
criteria of practicality and economics, had already enjoyed widespread application
for potable water treatment by the turn of this century. The first well-documented
use of PCT technology for municipal wastewater, conversely, occurred less than 50
years ago. This application involved the chemical coagulation of municipal sewage
to improve removal of suspended solids and biochemical oxygen demand (BOD);
the so-called Guggenheim process.'>: At least part of the motivation for this develop-
ment was rooted in the industrial and population expansion that had occurred
during the preceding three or four decades, and in the resulting increase in pollution
of receiving waters. The Guggenheim process, although technically sound, was not
widely accepted in the recession and post-recession period of the 1930's, in large
measure due to the higher costs associated with chemical treatment and the fact that
traditional biological processes were capable of achieving extant treatment
objectives relative to suspended solids and BOD.
Shortly thereafter the second world war began, and technologic developments in
municipal waste treatment stagnated. In contrast, industrial growth surged in both
magnitude and product sophistication. Hundreds of new synthetic chemicals were
produced, largely in support of the war effort. As the nation and world emerged from
armed conflict in the mid-1940's, the products of the vastly expanded industrial
sector were refocused for use in agricultural and domestic applications. Con-
cornitantly, the prosperity of the late 1940's and early 1950's encouraged major
population expansions in industrialized nations.
During this period, and continuing well into the latter half of the 1950's, principal
activities in the municipal wastewater field related to construction and expansion of
traditional waste treatment facilities. This was aimed at achieving at least "primary"
treatment (sedimentation and disinfection) for areas then discharging raw sewage,
and implementation of "secondary" treatment (sedimentation plus biological oxida-
tion and disinfection) for systems discharging to receiving waters of particular
sensitivity.
In the late 1950's the federal agency responsible for both water supply and waste
treatment—the Division of Water Supply and Pollution Control, U.S. Public Health
Service (USPHS), Department of Health, Education and Welfare (HEW)-
identified two major and convergent national problems; namely, increased water
demand in the face of an essentially fixed fresh water supply and increased pollution
of existing water supplies by inadequately treated waste discharges. Treatment
inadequacies related both to the biologically recalcitrant nature of many of the new
synthetic chemicals and to increases in volumes of sewage to be treated. Concern
began to mount with respect to specific chemicals in wastewater in addition to the
more traditional water quality measures of suspended solids and the oxygen demand
created in receiving waters by residual biologically degradable organic substances.
In response to these problems, the Advanced Waste Treatment Research
(AWTR) Program was initiated by the USPHS in the early 1960's. In July 1961 the
U.S. Congress amended the Federal Water Pollution Control Act to direct the
Secretary of HEW to proceed with plans to. .
"develop and demonstrate practicable means of treating municipal sewage
and other water-borne wastes to remove the maximum possible amounts
of physical, chemical, and biological pollutants in order to restore and
maintain the maximum amount of the Nation's water at a quality suitable
for repeated reuse."3
204
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The AWTR Program, pursuant to the directives of Congress, sponsored investi-
gation of a large number of different PCT processes. Many of these had proven
successful in industrial processing and/or water treatment applications, but had not
previously been applied to municipal waste treatment. This program was continued
through the 1960's under the auspices of the successor water pollution control
agencies, the Federal Water Pollution Control Administration (FWPCA) and the
Federal Water Quality Administration (FWQA).
Conventional wisdom at the outset of the AWTR Program held that PCT
processes would function best—both technically and economically—in "tertiary"
mode; that is, as add-ons to secondary biological systems for advanced waste treat-
ment (AWT), as pictured schematically in Fig. 1 a. The potential of PCT processes in
tertiary treatment had been demonstrated in a number of pilot-plant investiga-
tions,4"14 and on full scale at Lake Tahoe, California,15 and at Windhoek, South
Africa.16 In the late 1960*s and early 1970's, however, several researchers began to
explore the use of such processes as alternatives to biological treatment;17"25 that is,
as independent physicochemical treatment (IPCT) processes, as illustrated in the
schematic diagram of Fig. Ib.
For purposes of coarse comparison, Table 1 provides approximate values for
several major municipal wastewater quality parameters along with values typical of
effluents from conventional, AWT, IPCT, and other treatment schemes.
The Decade of the Seventies
The beginning of the 1970's brought both a reconsolidation of water supply and
pollution control responsibilities within one agency, the U.S. Environmental
Protection Agency (EPA), and a broadened enthusiasm for using both AWT and
IPCT processes to accomplish higher levels of wastewater treatment and water
reclamation. The EPA was instrumental in encouraging and supporting these
developments. Moreover, through development and implementation of the Federal
Water Pollution Control Act Amendments of 1972 (PL92-500), and subsequently
the Toxic Substances Control Act of 1976 (PL94-469), the EPA implicitly redefined
—more positively than had the AWTR Program before it—the principal role of PCT
processes as technologies to address removal of specific undesirable compounds
(e.g., toxic and carcinogenic compounds), not only BOD-producing suspended and
soluble organic matter.
As noted earlier, the concepts and applications of the PCT processes in the tertiary
or AWT mode had been well established and demonstrated on essentially full scale
by the beginning of the 1970's. I PCT was yet emerging as a technology in the sense
that, although firmly established in concept and pilot-plant demonstration, no full
scale plants had been built and operated. The decade was destined to witness a rather
remarkable transfer of I PCT technology from concept to practice. This is evident in
the evolutionary progression of technical literature on the subject.
The IPCT literature of the early 1970's relative to municipal waste treatment is
concerned primarily with bench-scale tests, pilot-plant demonstrations, and
preliminary design developments. The mid-1970 literature presents reports on the
final design details and start-up activities of I PCT plants. By the close of the decade
the literature emphasis had shifted toward evaluation of the performance
characteristics, operational aspects, and economics of operating IPCT processes.
Concurrent with these large-scale engineering applications and evaluations,
notable progress was made in the 1970's toward mechanistic definition of process
variables operative in PCT processes, and toward the translation of these findings
into mathematical models to facilitate computer-aided design and operation. It is
worth noting that, while many text and reference books relating to biological
treatment process concepts and design had been available for several preceding
decades, it was not until 1972 that the first text devoted to PCT technologies
appeared.26
205
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Preliminary
Treatment
Primary
Settling
c
F
Biological
Treatment
nemical Coagula
egimentation (Q
tio
pt
'
n a
on
Filtration
, Makeup
nd „ .
Carbon
at)
Carbon
Adsorption
_** 1
Carbon
Regeneration
Disinfection
Primary Treatment
a. Typical AWT Flow Scheme
Secondary
Treatment
Chemical
Clarification
Carbon
Adsorption
Makeup-
Carbon
Coagulant
b. Typical IPCT Flow Scheme
Figure 1. Applications of activated carbon in municipal wastewater treatment.
-------
Table 1. Approximate Performance Characteristics of Different Municipal Wastewater Treatment Processes
Process Effluent Characteristics
Parameter
BOD, (mg/L)
COD, (mg/L)
SS, (mg/L)
Turbidity,
(Jackson turbidity
units)
P04, mg/L
Color,
(chloroplatmate
color units)
Influent
Waste
300
480
230
250
12
Highly
variable
Primary
150
240
100
150
9
Highly
variable
Primary with
Chemical
Addition
75
130
20
12
2
50
Conventional
Secondary
35
50
25
50
6
50
PAC/
Chemical
15
35
20
10
2
15
PACT*
5
40
15
20
5
20
IPCT
8
30
4
2
1
10
AWT
1
10
1
0.5
05
7
*PACT, powdered activated carbon treatment, is a patented process (E.I du Pont de Nemours and Co
aeration basins of otherwise conventional biological secondary treatment plants
in which powdered carbon is added to the
-------
PCT processes for municipal wastewater have thus progressed rapidly in a
relatively short period. During the 1960's PCT was brought from conceptual design
to successful pilot and small scale demonstration. The decade of the 1970's witnessed
emergence of such technology from pilot stage to design of plants with capacities as
large as 100 million gallons per day (MOD), and start-up and operation of a number
of plants utilizing different designs to implement essentially similar unit processes.
Experience with the operating and performance characteristics of the variety of
designs of the 1970's is serving to bring PCT processes through the complete
evaluation cycle, and to establish a firmer basis for future selection and engineering
design of such processes based on proven relative economic and technical merits.
The pioneering research and applications engineering of the 1970's will in the 1980's
serve to optimize PCT process configurations and ancillary equipment, and to refine
this technology for use in municipal wastewater treatment. The work of the 1970's
will also contribute much valuable information with respect to the use of PCT
processes for related applications in industrial wastewater and potable water treat-
ment.
OVERVIEW OF PHYSICOCHEMICAL TREATMENT PROCESSES
PCT processes may function to modify or degrade chemical structures to form less
undesirable species or to separate these substances from solution for subsequent
disposal or destruction. Several processes that have demonstrated reasonable
suitability for municipal applications are briefly reviewed herein. One major dis-
tinction and advantage such processes share in common over biological treatments
is the degree of operational control which they afford. PCT processes generally can
be designed to accomplish any degree of removal desired, thereby facilitating the
tailoring of cost-effective treatment systems for specific applications.
It was noted at the outset of this chapter that PCT processes function primarily by
non-biologic means. The degree to which the term "non-biologic" accurately
describes the function of a PCT process may vary considerably from one application
to another. With biologically active fluids such as municipal wastewaters, com-
peting, interfering or synergistic biological interactions are virtually assured.
Detailed discussion of the specific ways in which microorganisms can affect the
performance of PCT processes is beyond the scope of this overview. The topic will be
considered briefly in a subsequent section of the chapter as it relates to the most
widely used class of PCT processes—activated carbon adsorption.
Degradation Processes
Degradation reactions entail modification or conversion of substances by chang-
ing their oxidation states. The purpose of such treatment is to convert undesirable
chemical species to species which are neither harmful nor otherwise objectionable.
This is accomplished in some instances by use of chemical agents, and in other
instances by application of heat or some other form of non-chemical energy.
Oxygen," ozone,28'30 permanganate,31 chlorine dioxide32 and chlorine33'34 have
all been examined as reagents for direct chemical degradation processes in a variety
of water and industrial waste treatment contexts, but only on a very limited basis for
municipal waste treatment. Other strong oxidizing agents such as hydrogen
peroxide35 and potassium ferrate36 have received even less attention for a number of
reasons, including considerations such as handling, stability, and cost. Advances
during the late 1970's in the technology of production, handling and applications of
such reagents, particularly hydrogen peroxide are likely to enhance future feasibility
of use."
Several "indirect" degradation processes have shown promise. A number of
studies, for example, have demonstrated the feasibility of induced oxidation by
short-lived radicals generated during treatment of wastes with ultraviolet (UV)
208
-------
irradiation.38 The combined use of ozone and UV irradiation has also proven
effective for oxidation of certain wastewater constituents.39
The direct and indirect degradation processes cited above have had limited
application for municipal wastewater treatment. They are included in this brief
overview largely on the basis of promise demonstrated on laboratory scale or in
applications to industrial water treatment. Thermal degradation processes,
conversely, such as wet -air oxidation40 and incineration41'42 have had comparatively
wide-scale use in municipal applications. Such processes can provide for nearly com-
plete destruction of many organic waste constituents, and have particular advantages
for treatment of high-strength, low-volume waste streams such as municipal sludges.
It must be recognized, however, that extremely high temperatures and relatively
long retention times are required for effective destruction of certain priority pol-
lutants, such as polychlorinated biphenyls (PCBs). Further, careful consideration
must be given to the fact that a number of toxic compounds are readily volatilized in
thermal degradation processes, and may thus create potential air pollution hazards.
Separation Processes
Separation technologies of interest in the context of organics removal from
municipal wastes include:
(1) coagulation/sedimentation;
(2) filtration;
(3) membrane processes; and
(4) adsorption processes.
The first two of the above process categories principally—but not exclusively—
address separation of suspended paniculate organic matter. State-of-the-art tech-
nology for these two categories of processes was conveniently summarized in a
design manual published by the EPA in 1971.43 The membrane and adsorption pro-
cess categories include operations designed—again principally but not exclusively—
for removal of dissolved impurities. The following paragraphs provide a brief
discussion of significant developments that took place during the 1970's relative to
all four categories of separation processes.
Coagulation ISedimentation—
These operations are probably the best understood and most widely employed of
all PCT separation technologies in municipal waste treatment applications. They are
frequently used as pretreatments to reduce the operational difficulties that high con-
centrations of particulate and colloidal material may otherwise pose for subsequent
operations such as filtration, adsorption, and disinfection. Fig. 2 illustrates typical
clarifier units for settling solids after chemicals have been added to promote their
coagulation and aggregation into settleable entities.
Emphasis in the area of coagulation/sedimentation during the decade of the
seventies focused largely on the chemical enhancement of the aggregation-settling
characteristics of sewage solids by addition of inorganic (e.g., ferric chloride,
calcium oxide, aluminum sulfate) and synthetic organic polyelectrolyte coagulants
and coagulant aids. Removals as high as 70% for BOD and 90% for suspended solids
were reported achievable for raw sewage.44"46 Other significant studies by the EPA
compared the relative performance characteristics of various coagulants on a
broader scale.47 In related equipment developments, tube settlers and inclined plate
clanfiers, the concept of which is illustrated in Fig. 3, were investigated48 for their
potential advantages over more traditional sedimentation basin configurations.
Filtration—
There was recognition during this period that filtration processes—used widely for
potable water treatment—could function effectively for removal of non-settleable
209
-------
Effluent
Sludge
Influent
(a) Circular Center-Feed Clarifier with
a Scraper Sludge Removal System
Influent
Effluent
Sludge
(b) Circular Rim-Feed. Center Take-off Clarifier with a
Hydraulic Suction Sludge Removal System
Sludge
(c) Circular Rim-Feed. Rim Take-off Clarifier
Figure 2. Typical clanfier configurations (From Reference 43)
suspended and colloidal organic matter from municipal wastes. Early indications
that filtration could play a major role in upgrading effluent quality spurred activity
in the research, development, design, and evaluation of wastewater filtration
processes.
Filtration of waters and wastes has been carried out in a variety of configurations,
including pre-cpat filtration, low-rate and high-rate deep-bed filtration, and micro-
straining.26'4''40'50 The most common and successful configuration has been of
high-rate filtration through a bed of granular media such as sand, coal, or activated
carbon. Such filtration is usually carried out in open beds which operate under the
gravitational influence of a static head of water over the bed. Pressure filtration in
closed vessels is an alternative design used more frequently for industrial waste
210
-------
Modular Unit of Inclined Settling Tubes.
(Courtesy of Neptune Microfloc, Inc.)
Clarified
Water
Settling
Sludge
Flow of Liquid and Sludge in Steeply Inclined Tubes.
Counter Flow of Water and Sludge in Single Tube.
Figure 3. Tube-settlers for sedimentation (From Reference 43)
treatment than in municipal systems. Pictorial representations of open-bed and
pressure filters are given in Fig. 4. In either system excessive accumulation of solids
in or on the media eventually restricts flow and causes backpressure. At this point
the solids are removed by upward-flow or backwash fluidization of the bed. Back-
washing is commonly augmented by a mechanical or hydraulic surface wash.51
Additionally, it is desirable to inject air at the bottom of the filter to increase physical
contact and scouring of the filtration media, thereby improving release of the
accumulated waste solids during backwash.
211
-------
Operating
Table
Operating
Floor
Pipe Gallery
Floor
Filter Drain
Filter to Waste
Wash Line
Rate of Flow and Loss
-of Head Gages
Wash Troughs
Filter Sand
Graded Gravel
Perforated
Laterals
Cast-Iron/
Manifold
Filter Bed Wash-
/ Water Troughs
Concrete Filter
Pressure Lines to
Hydraulic Valves from
Operating Tables
Influent to
Filters
Effluent to
Clear Well
Drain
Raw
Water
Graded
Gravel
Supporting^
Layers
Concrete
Fill Steel
Legs
Concrete
Piers
Gravity Filter
Graded
Gravel
Supporting
Layers
Transition
Layer
Fine Gravel
Medium Gravel
Coarse Gravel
M-Blocks
Air Laterals
:£TS BackwashSteel Plates Legs
Filtrated
ndBack
wash
rough
Air and
Backwash Water
Transition
Layer
Fine
Gravel
Medium
Gravel
Coarse
Gravel
>*-Air
M-Blocks
Air
Laterals
L Steel
Plates
Pressure Filter-Filter Cycle Schematic Pressure Filter-Backwash Cycle Schematic
Figure 4. Filtration systems (from Environmental Pollution Control Alternatives.
Municipal Wastewater, USEPA Technology Transfer, EPA-625/5-76-012)
Filtration performance is significantly dependent upon the nature of the solids
(i.e , inorganic or organic, gelatinous or discrete), the type of filter and media used,
pretreatment practices (including the use of coagulants as filter aids), and the
frequency and efficiency of backwashmg. Reports of filtration efficiencies of 90% or
more have been published, with 50% to 80% removal being typical. Effective filtra-
212
-------
tion can commonly reduce effluent suspended solids levels to below 10 mg/L, thus
producing water ideally suited for further treatment to remove dissovled organic
materials or, in some cases, suitable for direct discharge or limited reuse.
Membrane Processes—
Requirements for removal of supra-colloidal and dissolved organic and inorganic
impurities in AWT systems spurred interest during the 1970's in membrane
separation processes; actual applications, however, were limited. The two major
membrane processes for removal of organic substances, ultrafiltration (UF) and
reverse osmosis (RO), utilize applied pressure as a driving force for separation of
impurities from water, as depicted in Fig. 5. The membranes employed for UF have
much larger pore sizes than those used for RO, and the pressures involved are
correspondingly lower. Because of the large membrane pore sizes involved, UF is
used primarily to separate fine particulate, colloidal, and macromolecular organic
matter, while RO is employed principally for removal of dissolved substances.
Ultrafiltration, which can produce high quality water with respect to suspended
solids, has been employed in a few cases for direct filtration of biologically treated
fluids to replace sedimentation and improve quality/2'" The largest and most
pertinent application of RO for municipal waste treatment has been as a component
in an AWT system producing water for reuse in the recharging of ground water
supplies.54
Adsorption Processes—
Adsorption emerged in the early years of the I970's as the most generally appli-
cable separation technology for removal of dissolved organic pollutants from
municipal wastewater, and was strongly promoted as such by the EPA. "'S6 Activated
carbon has demonstrated effectiveness for a broad range of organic pollutants and is
the adsorbent of choice for most wastewater applications. The process of adsorption
is depicted conceptually in Fig. 6.
The ability of activated carbon to remove dissolved organic material from
solution has been long known. Before the 1970's, however, full-scale application of
this technology had been limited largely to industrial operations—in which the cost
of the carbon system could readily be justified—and to use for taste and odor control
in potable water treatment. Detailed considerations regarding these applications are
given in companion chapters in this Monograph by O'Brien et al. on industrial
wastes and by DiGiano on municipal water supplies.
Taste and odor compounds are usually present in low concentrations in water
supplies, and often the need to provide removal occurs only periodically. Conse-
quently, the common mode of activated carbon application in water treatment was
one in which an appropriate quantity of carbon was introduced to a given volume of
water, held in contact with the water for a specified period, then separated and dis-
carded. The form of carbon used for such operations was usually powdered, largely
for reasons of lower initial cost, adaptability to intermittent operation, and ease of
removal by existing sedimentation and filtration processes.
This mode of operation was, logically, one of the first attempted for wastewater
treatment, but it did not prove entirely satisfactory. The relatively larger amounts of
organic impurities present in municipal wastes and the need for continuous treat-
ment in this application required more efficient utilization of the activated carbon.
Further, the fact that large amounts of carbon are needed for such operations
requires a scheme of regeneration and reuse of the carbon. These requirements
suggested the desirability of using granular activated carbon (GAC) in continuous
contacting systems.
The potential of adsorption for tertiary treatment of wastewaters was well
demonstrated during the mid-to-late 1960's in several pilot and large-scale PCT
projects. By the beginning of the 1970's, 1PCT had been developed and successfully
demonstrated at the pilot-scale as an alternative to biological treatment for direct
213
-------
Membrane
Pressure
Pump
Permeate
Figure 5. Schematic representation of a membrane process using pressure as a
driving force for separation (from Gregor, H.P and C.D Gregor, "Synthetic
Membrane Technology Scientific American, July 1978).
00- 0 0 _' 0 0 0 -
o o Polluted
ooo Water « ° -
00 000 0 0 ' 0 0
0 OOOOOOO 0
00 0 0 000
ooooooooo
Organics Trapped
in Carbon Granule
Figure 6. Conceptual depiction of the adsorption process (Courtesy of Calgon Corp).
application to raw wastewaters. In this early work the basic IPCT process scheme
consisted of chemical coagulation and sedimentation followed by G AC adsorption
beds. In a number of cases, granular media filtration preceded the GAC.
Data from pilot operations and small-scale plants incorporating GAC operations
indicated consistent organic matter removals of 95% or more despite variations in
waste strength and composition. Toxic substances, which would have adversely
affected biological treatment processes, appeared to have little or no effect on the
IPCT process. The effluents produced by those early pilot plants were reported to be
essentially free of suspended solids and to contain only about 5mg/ L to lOmg/ L of
total organic carbon (TOC) and BOD. IPCT processes utilizing inorganic coagulants
were further reported to achieve generally high degrees (~95%) of phosphate re-
214
-------
moval. This resulted in 1PCT processes—specifically including G AC—being
advanced by the EPA in 1973 as the most sophisticated and complete treatment sys-
tems available.59
In addition to possessing superior adsorptive properties for a broad range of
dissolved organic substances, GAC exhibits favorable filtration capabilities. This
characteristic further broadened the usefulness of GAC in municipal waste treat-
ment. It was used as media for both filtration and adsorption in a single reactor, and
backwashed, cleaned, and handled in a manner similar to that used for more con-
ventional granular filtration media. Its use as a dual-function medium prompted a
number of detailed studies on specific pretreatment requirements for various
operating modes.47'60'61
Granular activated carbon can be transferred hydraulically, and requires no
special handling or storage conditions. The fact that GAC can be regenerated
effectively by thermal means is a key to its successful economic application, and was
a prime factor in opening the field of municipal waste treatment to adsorption
technology in general, and to GAC in particular.
Comparisons of activated carbon and alternative adsorbents-particularly syn-
thetic resins—have shown carbon to be much more suitable for municipal wastewater
treatment due to its applicability to the wide range of organics present.62~66 For this
reason, as well as costs, synthetic resins received little attention during the decade of
the seventies in municipal wastewater treatment. For only a limited number of industrial
waste treatment applications and certain by-product recoveries have carbonized
polymeric adsorbents and macroporous cross-lined polystyrene and polyacrylic
ester adsorbents been effective alternatives to activated carbon.
Biological growth on GAC has been found to be a natural consequence of applying
this high-activity material to municipal waste treatment.1'''2''67'68 Such growth has
been variously reported as deleterious and advantageous. The use of appropriately
designed and operated integrated systems, in which biological growth on the surface
of granular activated carbon is properly controlled, has been shown to provide a
favorable combination of adsorption and biodegradation in single reactors.69"72
The most promising use of powdered carbons for municipal waste treatment has
been in integrated systems in which the carbon is added to the aeration basins of
activated sludge plants and/ or to anaerobic digesters to upgrade the performance of
these systems and enhance removal of toxic materials.73"
Overview Summary
A wide range of different types of PCT processes was evaluated for potential
AWT and 1PCT applications during the 1970's. Of these, adsorption on activated
carbon, frequently preceded by coagulation/sedimentation and/or filtration, was
clearly shown to be the most suitable process for achieving high levels of removal of
organic compounds from municipal wastewaters. The remainder of this chapter
expands upon the concepts of this process, significant design and operational
features, and discusses its specific AWT and IPCT applications.
ADSORPTION AND ACTIVATED CARBON TECHNOLOGY
Principles of Adsorption
The phenomenon of adsorption is of major significance in most physical, chemical
and even biological water and wastewater treatment operations. In his chapter of this
Monograph, DiGiano58 provides a thorough and comprehensive summary and
analysis of significant developments which occurred during the 1960's and 1970's
relative to adsorption principles in aqueous systems. These will not be recounted
here. It is of complementary value, however, to set forth a conspectus of state-of-
215
-------
the-art knowledge with respect to variables which significantly impact the
effectiveness and efficiency of activated carbon adsorption operations in wastewater
treatment.
Fundamentally, adsorption involves the concentration of soluble and quasi-
soluble impurities from solution at an interface or surface. The material concentrated
is termed the adsorbate; the material at whose surface the concentration occurs is
termed the adsorbent. Fig. 7 depicts the distribution of an organic molecule between
solution phase and a solid surface. For each combination of a specific type of organic
substance, solution conditions, and type of surface, a quantitative equilibrium distri-
bution between solution and surface phase is defined; several such distributions are
illustrated in Fig. 7. The character of this distribution markedly affects the feasibility
of adsorption for a particular application. It, in turn, is influenced by a variety of
factors. Prime among these for a specific application is the nature of the adsorbent.
As noted earlier in this chapter, activated carbon has been demonstrated to be the
most effective adsorbent for water and wastewater treatment applications.
Properties of Activated Carbon
The generic term "activated carbon" encompasses a broad range of amorphous
carbon-based materials having high degrees of porosity and extensive associated
surface areas. The large surfaces are formed by selective burning and oxidation of
the raw material during activation, creating a microporous end product. Fig. 8
characterizes the physical form of a granule of activated carbon, illustrating the
extensive intraparticle pore structure. The large surface area of this material derives
from surfaces associated with the near-molecular size pores and capillaries within
the carbon granules.
Commercial carbons typically have total surface areas in the range from 450 m2/ g
to 1500 m2/g. This extensive internal surface area is theoretically available for ad-
sorption of organic compounds from wastewater. The actual surface area available
is, however, dependent on the specific nature of the organic adsorbate, and can be
considerably less than the total. The pore volume of these carbons ranges from 0.5
cc/g to 1.5 cc/g. It is, however, only that fraction of the total pore volume which is
contained in larger pores (>10 A) which determines, to a significant extent, the
effective capacity of carbons for liquid phase applications. The effective capacity
thus depends on the distribution of area or volume with pore size, and the dis-
tribution of molecular sizes to be adsorbed.
The surface properties of activated carbon are additionally important in
determining activity; that is, capacity for adsorption of a specific organic substance.
The chemical properties of the surface depend on the raw material used, the type of
activation process, and the conditions employed in activation. Activated carbon
surfaces can be considered to consist of essentially two different types. Planar,
non-polar surfaces comprise the bulk of the surface for most carbons. Heterogeneous
edges of the carbon planes, which make up the microcrystallites whereon carbon-
oxygen functional groups formed by oxidation in the manufacturing process are
located, constitute the second type of surface. The groups attached to these surfaces
enable activated carbon to undergo halogenation, hydrogenation, oxidation, and to
function as a specific adsorbent and catalyst in a variety of different reactions.
Although it does not directly affect adsorption capacity, the hardness or durability
of individual granule structures is a most important property to be considered in
carbon selection, at least for G AC applications. This property largely determines the
losses which will occur on each adsorption-regeneration cycle as a result of attrition
during handling and burn-off during reactivation. Petroleum, lignite and bituminous
coal-based carbons are typically used in GAC systems, while softer wood and peat-
based materials find more use as powdered Carbons.
Pressure drop or head loss in downflow columns and bed expansion in upflow
columns of GAC are determined in part by particle size distribution. These
216
-------
(a) Graphical Representation of Types of Adsorption Separations
"5
0)
c
o
c
o
o
c
o
o
Il-Linear
Adsorption and
Absorption
Concentration in Solution, C
(b) Typical equations for description of solid/liquid phase distributions
The Langmuir Equation:
<7e =
Q°bC
(1 4- bC)
where Q° represents the ultimate capacity and b the relative
energy (intensity) of adsorption
Two convenient linear forms of the equation are
C 1 C
1 1 / 1 \/1
— = ™H~H~
qe Q° \ba°'\
The Freundlich Equation:
q, =
where KF is a measure of limiting capacity and n the relative
energy (intensity) of adsorption
In linear form:
log q, = log/CF + — log C
Figure 7. Distribution of an organic substance between solid and liquid phases (after
Weber, Reference 26).
217
-------
Cross Section of a Granule of GAC
Cross Section of Pore within Granule
Figure 8. Conceptual drawing of carbon granule (Courtesy of Calgon Corp).
properties, in turn, influence design, installation and capital costs. The smallest size
particle that conditions of efficient operation permit generally should be used, for
this increases adsorption rates, and thus reduces the si/e of the adsorption system
icquired
! he general properties of some commercially available GACsand PACs are given
in Tables 2a and 2b, respectively.
218
-------
Table 2a. Properties of Selected Commercial Granular Carbons
Properties
Raw material
Surface area, (m2/g)
Mesh size, U.S. Std.
Density
Apparent, (g/cc)
Backwashed,
Drained, (Ib/ft3)
Effective size (mm)
Uniformity
coefficient
Ash content, (%)
Iodine no., (mg/g)
Abrasion no. (Ro-Tap)
Moisture, (%)
Mean particle
diameter, (mm)
CALGON
Filtrasorb 300
Bituminous
coal
900-1000
8 x 30
0.50
27
08-0.9
<1.9
<10
>950
>70
<20
1.5-1.7
WESTVACO
WV-L
Bituminous
coal
1000-1050
8x30
0.48
25-27
085-1.0
<1.8
<10
>950
>70
<2.0
1.4-1 7
ICI AMERICAS
Hydrodarco 3000
Lignite coal
550-650
8 x 30
0.43
23
0.8-0.9
<1.9
<18
>550
-
<9.0
1.4-1.6
CARBORUNDUM
GAC-30
Sub-bituminous
coal
900-1000
8x30
0.51
28
0.85
<2.1
<15
>900
>70
<2.0
1.5-1.7
WITCO
517
Petroleum
1050
12x30
052
30
0.89
1.4
<0.5
>1000
>85
<1.0
1.2
-------
Table 2b. Properties of Selected Commercial Powdered Activated Carbons
Properties
Raw material
Surface area,
(mVg)
K> Particle size,
o (% less than 325
r">esh)
pH
Density
Apparent, (g/cc)
Tamped, (g/cc)
ICI AMERICAS
HYDRODARCO C
Lignite coal
550
70 (mm)
10.5
_
070
WESTVACO
NUCHAR SA-15
Wood
1400-1800
65-85
3-6
040
-
CALGON
TYPE RC
Bituminous coal
1100-1300
65-75
-
0.48
-
-------
Adsorption on Activated Carbon
Adsorption of organic compounds from water by activated carbon results from a
variety of binding forces between organic molecules and the carbon surface, all
having their origin in electromagnetic interactions. Two principal types of adsorp-
tion are commonly distinguished; i.e., physical and chemical.
Physical adsorption derives from the action of van der Waals forces, which are
comprised of both London dispersion forces and classical electrostatic forces.
Chemical adsorption results from the reaction of an adsorbate with an adsorbent,
commonly resulting in a transformation of the chemical form of the adsorbate. The
resulting chemisorptive bond is localized at active centers on the adsorbent and is
usually stronger than that derived from the physical van der Waals forces.
Adsorptive interactions between organic molecules and activated carbon also
result from specific but non-transformative interactions between solute molecules
and functional groups on the adsorbent surface. These interactions, designated
"specific adsorptions," may exhibit a broad range of binding energies, from values
commonly associated with physical adsorption to the higher energies usually
involved in chemisorption.
The net dispersion, electrostatic, chemisorptive, and functional-group interactions
broadly constitute a category of adsorptive reactions relating to the affinity of the
carbon surface for specific organic compounds. The extent of adsorption may also
be predicated on various interaction characteristics of these compounds with water
itself, notably those of surface tension and solubility.
Many organic compounds of interest for waste treatment can effectively alter the
surface tension of water. The energy balance of aqueous systems of such compounds
favors their adsorptive concentration at the carbon-water interface. The extent of
adsorption is greatly influenced by the degree of insolubility or hydrophobicity of
the organic compound; that is, its degree of "dislike" for water. Increasing hydro-
phobicity generally increases the extent of adsorption. The effects of solubility can
be interpreted in terms of the necessity of breaking a form of bond between an
organic compound and water before adsorption can occur. The greater the solubility
of the compound, the stronger the bond, and the smaller the extent of adsorption.
A number of parameters specific to a given system will thus contribute to the
adsorption of organic compounds by activated carbon in wastewater treatment ap-
plications. These include—relative to the organic substances—concentration, molec-
ular weight, molecular size, molecular structure, molecular polarity, steric form or
configuration, and the nature of background or competitive organic substances.
Temperature and pH can also markedly influence adsorption performance to the
extent that they can effect changes in the aforementioned parameters.
Rates of adsorption are also highly important. The more rapid the approach to the
particular equilibrium distribution dictated by the above factors, the greater is the
fraction of equilibrium capacity utilized in a given time of contact between the
carbon and the wastewater being treated. There are essentially two consecutive rate-
limiting mass transport steps in the adsorption of organic compounds from
wastewater by activated carbon, as illustrated schematically in Fig. 9. The first of
these is the transport of the compound through a surface film to the exterior of the
carbon (film diffusion). The second is the diffusion of the compound within the
pores of the carbon and/or along pore-wall surfaces (intra-particle diffusion). A
significant amount of research on adsorption technology during the 1970's focused
on development of mechanistic interpretations regarding—and mathematical
models for description of—the time-variable dynamics of adsorption of organic
molecules from aqueous solution by activated carbon. S uch information is essential
for evolution of rigorous and optimum criteria for adsorber—or reactor—design and
operation, and for identification and control of important system variables.
221
-------
Organic Molecule
in Solution
'Bulk Transport Film Transport Intraparticle Transport
Adsorption
Attachment
(Fast)
Figure 9. Significant mass transport steps in adsorption.
Reactor Systems
The manner in which to contact carbon most effectively with a wastewater is of
particular significance for large scale treatment operations. As noted, rates of ad-
sorption on carbon depend significantly on the particle size of the carbon. It is
desirable, therefore, to employ carbon of as small a diameter as conditions of
efficient operation permit. The qualification "efficient operation" is key, for the size
of particle chosen dictates to some extent the type of reactor system. Powdered
carbon, for example, is generally used in either a completely mixed batch (CMB) or
completely mixed flow (CMF) reactor, both of which are pictured schematically in
Fig. 10. Head loss using PAC through a bed or column reactor would be prohibitive.
For granular carbon, on the other hand, suspension in a batch reactor would be
difficult, while a plug-flow (PF) column or fixed-bed operation, the principle of
which is illustrated in Fig. 11, is efficient.
Continuous-flow operations have advantages over batch-type operations because
rates of adsorption depend upon the concentration of organic matter in the
wastewater. Plug-flow, or column reactors have advantages over CMF reactors for
the same reason. In column operations the carbon at the inlet of the bed is
continuously in contact with the concentrated influent waste. The concentration in
the water in contact with a given layer of carbon in a column changes very slowly
under this condition, and high uptake levels of adsorbate by the adsorbent are
achieved. For batch treatment, conversely, the concentration of organic substance in
contact with the entire quantity of carbon continuously decreases as adsorption
proceeds, thereby decreasing the effectiveness of the carbon for removing the
organic material. These considerations, coupled with the difficulty associated with
222
-------
Waste Added as a Batch
O
c>o
Concentration of Waste in
Reactor is Uniform over
Volume but Decreases
Over Time
Effluent Withdrawn as
a Batch after Designated
Treatment Time
Completely Mixed Batch
Reactor (CMB)
Continuous Flow of
of Waste
Concentration of Waste
in Reactor is Uniform
Over Volume and
Constant Over Time for
Steady-State Operations
Continuous Flow of
Treated Effluent
Completely Mixed Flow
Reactor (CMF)
Figure 10. Completely mixed reactors
regeneration of powdered carbon, have made granular carbon the choice for most of
the large scale continuous wastewater treatment operations built during the I970's.
Fig. 12 illustrates several arrangements and reactor configurations typically used
for G AC systems. There are two basic PF modes of conduct for column adsorbers
relating to exhaustion/regeneration of the carbon; namely, fixed beds and pulsed
beds. In the fixed-bed mode an entire GAC bed is removed from service when the
carbon is to be reactivated. In the pulsed bed, only the most exhausted portion of the
223
-------
Continuous
Flow of Waste
Concentration of Waste
Varies over Length
of Reactor and Time
Continuous Flow of
Treated Effluent
Figure 11. Plug-flow reactor
total bed is removed as an increment of new carbon is added simultaneously. For the
relatively low organic content of municipal wastewaters, the increased efficiency of
pulsed bed operation is not worth the additional capital cost for equipment; fixed-
bed systems predominate in this application.
Within the fixed-bed mode of conduct, adsorption beds may be operated in series
or parallel. In the series system, the effluent from the first bed passes to a second,
then, if necessary, to additional beds in series. The advantage gained by this mode of
operation is that the carbon in the lead bed can be exhausted more fully—and, there-
fore, used more cost-effectively—in achieving a desired system effluent concentration
than can the carbon in a single larger bed of size equal to the sum of the sizes of the
smaller beds in series. In a series operation the lead bed is removed for reactivation
when it is exhausted. The next bed in sequence then assumes the lead position and a
fresh bed is added in the final position. The most common series-type system for
municipal waste treatment is comprised of two operating beds with a third on
standby.
224
-------
Untreated
Water
Series
Treated
Water—>-, t
Reactivated
Carbon
Treated
Water
Parallel
Untreated
T
,...ijj
•j^p
l_
t
!M
1
^ Treated
' Water
Untreated \ /
Water —*-C. /
X-*
Spent
Carbon
Fixed-Bed Systems
Pulsed-Bed Adsorber
Figure 12. Typical GAC reactor configurations.
In parallel fixed-bed operations the effuent of all columns is blended prior to
discharge. This provides the potential advantage of ensuring that the average con-
centration of the total effluent flow meets discharge criteria although the effluent
from any one bed or more may exceed these criteria. The adsorption beds are
removed from operation for reactivation in a staggered manner, so that the system is
comprised of beds in varying states of exhaustion. This type of system is very com-
mon for municipal waste treatment.
The adsorption beds of both series and parallel design can be operated in either an
upflow or downflow direction. A downflow mode of operation must be used where
the GAC is relied upon to perform the dual role of adsorption and filtration.
Although lower capital costs can be realized by eliminating pretreatment filters,
more efficient and frequent backwashing of the adsorbers is required. Filtration
rates of 8.2 cm/min to 40.7 cm/min (2 to 10 gallons per minute per square ft.,
gpm/ft2) are employed, and backwash rates of 48.9 cm/ min to 81.5 cm/ min (12 to 20
gpm/ft2) are required to achieve bed expansions of 20% to 50%.
Alternatively, adsorption beds have been designed and operated in the upflow
mode. While prefiltration is normally required, excessive head loss, channeling, and
fouling of the GAC can be avoided by upflow operation. In addition, smaller particle
sizes of GAC can be employed to increase adsorption rate and decrease adsorber
size. In attempts to combine the best of boththese modes of operation, systems have
been designed to operate in an upflow-downflow series mode.76
When wastewater is passed through a stationary bed, for either upflow or down-
flow adsorption operations, non-steady-state conditions prevail in that the carbon
continues to adsorb steadily increasing amounts of organic matter over the entire
period of useful operation. Fig. 13 is a plot of an idealized adsorption pattern or
breakthrough curve for such a non-steady-state adsorber. The "breakpoint" on this
curve is that point at which the effluent from the adsorber no longer meets treatment
objectives, dictating disposal and / or regeneration of the GAC contained in the bed.
Factors which affect the actual shape of the breakthrough curve include the
adsorbate concentration, pH, rate-limiting mechanism for adsorption, nature of the
equilibrium conditions, particle size, depth of the column or bed, and the velocity of
225
-------
Time, or Volume of Water Treated
Figure 13. Idealized breakthrough curve (after Weber, Reference 26)
flow. In general, the time to breakpoint for a specific type of carbon and given
organic substance is decreased by: (1) increased particle size of the carbon; (2)
increased concentration of organic matter in the wastewater; (3) increased pH of the
wastewater; (4) increased flow rate; and (5) decreased bed depth.
Adsorber Modeling and Design
The design of fixed-bed adsorbers involves estimation of the shape of the break-
through curve and the appearance of the breakpoint. A substantial amount of the
time and expense in planning and designing adsorption facilities is involved in
predicting or forecasting the operational dynamics of the process for a given applica-
tion. This generally requires extensive experimental pilot studies. The duration and
costs for such pilot studies frequently can be minimized by mathematic models
which are capable of the required prediction and forecasting given certain basic
information about the system of interest.
Considerable development work relating to predictive models for adsorption
system design was initiated in the late 1960's and continued throughout the
1970's.26'05'66'70'77"92 It is beyond the scope of this chapter to discuss all of these
modeling procedures in detail. To present some perspective, however, it is of value to
consider briefly the Michigan Adsorption Design and Applications Model
(MADAM).65'66'70'83'87'89 This model is based on numeric solution techniques, and
not restricted to over-simplified rate and/or equilibrium expressions to facilitate
mathematic solution. It can accommodate the dynamic aspects of fluid dispersion,
solids mixing, multi-solute interactions, and—importantly, as noted in the next sec-
tion—biological growth on activated carbon surfaces, aspects which must be excluded
because of mathematic complexity from models which are based on analytic
solution techniques.
The MADAM algorithms enable predictive modeling of fixed-bed adsorption
operation; i.e., prediction of concentration breakthrough profiles for various
226
-------
adsorbents, column configurations, hydrodynamic conditions, types of solutes and
operational conditions. An example of actual fixed-bed data and corresponding
predicted breakthrough curves forp-chlorophenol and several different adsorbents
are depicted in Fig. 14. Fig. 15 illustrates the MADAM predicted breakthrough
profile and measured data for adsorption of dodecyl benzene sulfonate from an
aqueous mixture of this organic compound and phenol.
It is clear from these figures that reasonable predictions can be made of the
behavior of fixed-bed adsorption systems with respect to individual organic
substances, both in "pure" solutions and in simple mixtures, through the use of
appropriate mathematical models. Such models have also been used with reasonable
success to predict breakthrough of lumped measures of organic matenal(e.g., BOD,
COD, TOC), and further work is in progress to adapt them to predictions of the
adsorption behavior of individual components in highly complex mixtures.
Bioactive Carbon Systems
Biological activity in activated carbon beds used for wastewater treatment is to be
expected. Municipal wastewaters are bioactive by nature, and the surfaces of
activated carbon provide excellent sites for colonization by microorganisms.6
The adsorptive properties of activated carbon serve to enrich substrate and oxygen
concentrations, the surface provides recesses that are sheltered from fluid shear
forces, and the variety of functional groups on the surface can enhance attachment
of microorganisms. It has been clearly demonstrated that activated carbon provides
a more effective surface for growth of attached organisms than do inert media such
as sand and non-activated coal materials.69"72
Fig. 16 is a scanning electron microscope (SEM) photograph of a carbon granule
taken from a waste treatment application. The prolific growth of microorganisms on
the surfaces of the granule may be readily observed. The SEM photo clearly demon-
strates an ecosystem including bacteria and bacteriovorous protozoans. The micro-
graph also demonstrates that the biofilm is not uniform, but is comprised of a
permeable matrix.
Bioactivity has been variously reported as advantageous and disadvantageous to
the primary adsorptive function of the carbon. The depth and composition of the
biofilm may influence adsorption dynamics if it is extensive enough to block pore
openings or to retard boundary layer transport processes.
There have been speculations that biological activity serves to "regenerate"
activated carbon. Bioregeneration, per se, is not likely, since most bioforms are too
large to enter those pore spaces which contain the majority of the effective adsorption
area. It has been demonstrated that, under proper conditions of design and opera-
tion, biological activity can improve removal of certain types of waste contaminants
and can prolong periods between required replacement or regeneration of the
carbon.68"72'93"95 This, however, likely relates more to reduction of the effective
loading on the carbon—as a result of the biodegradation of a certain percentage of
the organic compounds within interstitial void spaces and at the external and
macropore surfaces of carbon granules-than it does to bioregeneration via
degradation of substances which have already been adsorbed within the internal
meso and micropore structure of the carbon.
Preoxygenation and/ or pretreatment with other oxidants, particularly ozone, can
enhance bioactivity in carbon beds.96"99This enhancement relates in part to oxidative
conversion of organic matter to more readily biodegradable forms, and in part to
enriched levels of oxygen in the bed. It is important to note that preoxidative treat-
ment with ozone is not likely to enhance adsorption reactions, as was suggested in a
few literature reports during the 1970's. Indeed, such pretreatment is more likely to
convert aromatic compounds to oxygen-rich polar aliphatic substances, which are
not as well adsorbed by activated carbon as are the less polar aromatic precursor
compounds.
227
-------
-i W
-------
1.60- •
0.00
0.00
1.20
Throughput
1.50
1.80
2.10
2.40
Figure 15. Dodecyl benzene sulfonate influent and effluent concentration (C2/C20) profiles; phenol - dodecyl benzene sulfonate mixture (After
Crittenden and Weber, Reference 89).
-------
Figure 16. A scanning electron microscope picture of carbon surface with attached
microoroganisms (After Weber, et al.. Reference 94).
As noted earlier, bacteria are too large to significantly penetrate the pore spaces of
activated carbon. Thus, development of a biofilm should present no additional
resistance to internal, or intraparticle, diffusion. Because the biofilm is on the
outside of the carbon particle, substantial resistance to transport of substances
controlled primarily by external, or "film," diffusion should be anticipated. It is
possible to minimize the additional external resistance by providing operating
conditions which ensure that the biofilm is maintained as a thin, active film which is
not allowed to cover the entire external surface of the carbon. Regular and vigorous
air scouring to maintain a thin active biological film has been found essential to
minimize the resistance of the biofilm to transport of adsorbing substances,
minimize adverse effects on the primary function of the activated carbon, and to
control head loss in the adsorber.60'6'- '
In further attempts to utilize observed synergistic interactions of activated carbon
adsorption and biological oxidation, investigators at E.I. DuPont in Deepwater, NJ
performed extensive studies on the effects of adding powdered activated carbon to
activated sludge systems.73"75 In the PACT® * process, which resulted from this
work, virgin or reactivated carbon is added to settled wastewater entering a
conventional activated sludge aeration tank in order to improve the performance of
the biological system. The quantity of carbon added is related to the type of
*PACT is a registered service mark of E I DuPont de Nemours & Co., Inc.
230
-------
contaminant to be removed, the degree of removal to be achieved, and to the sludge
age or sludge retention time (factors related to the extent of recycle of biomass and
carbon solids in the system). The economic application of this technology to organic
removal in municipal wastewater treatment requires a viable means for regeneration
of the PAC. Reactivation of P AC in a conventional multi-hearth furnace presents a
number of difficulties, although it has been successfully accomplished.100"102 The
first full-scale municipal system using PACT® , however, was designed to provide
both excess sludge destruction and PAC regeneration through a wet air oxidation
process;101'104 this system is illustrated in Fig. 17.
Because activated carbon has" strong tendencies to adsorb oxygen from solu-
tion, 105 PAC surfaces can provide both sites of attachment for organisms and sites of
enriched oxygen concentrations in activated sludge systems. In most applications,
the increased biomass and carbon solids associated with PACT® can be accom-
modated in existing equipment, allowing higher loadings and/or improved per-
formance. The ability of activated carbon to adsorb toxic organic and inorganic
materials is an advantage in stabilizing activated sludge operations where spills or
spikes of toxicants are experienced.106 Further, excursions in influent concentrations
of biodegradable organics can be attenuated by PAC, thereby reducing the adverse
effects of shock loadings to the system. Finally, increases in sludge weight and
density resulting from the presence of PAC assist the separation of the mixed liquor
suspended solids from the treated water.
While further research is needed for a full understanding of integrated biological-
PCT processes, it is obvious that such systems have substantial potential for
improving organic removal in municipal waste treatment in the next decade. This is
particularly true for PAC-activated sludge systems because of the economic benefits
to be realized by using existing facilities.
Reactivation and Regeneration
Economic large-scale application of activated carbon, particularly GAC,
generally requires its repeated reuse. This in turn requires periodic restoration of the
adsorptive properties of exhausted carbon, by either reactivation or regeneration.
For this discussion, the term "reactivation" refers to a process in which such
restoration is accomplished using conditions similar to those involved in the
production or activation of the carbon, while "regeneration" connotes the use of
processes other than those employed for initial activation. Several methods are
available to accomplish reactivation and/or regeneration. These include chemical,
solvent extraction, and thermal techniques.
Chemical regeneration makes use of changes in adsorbability which accompany
chemical changes in adsorbed molecules. An illustrative example is phenol which is
adsorbed well at neutral and low pH conditions. At high pH, however, the phenol
molecule is converted to the phenolate ion, which is only weakly adsorbed.
Therefore it is possible to design treatment systems to remove phenol from solution
at neutral or low pH and regenerate the carbon and recover the phenolate under
basic pH conditions.107
For easily volatilized adsorbed compounds, such as benzene, hot gases or steam
can be used to convert the adsorbed material to gaseous form and to strip this form
of the adsorbate from the carbon.
Solvent regeneration makes use of differences in the solubility of certain
compounds in water and in other solvents. An adsorbate with a high affinity for
carbon and/or low solubility in water can be adsorbed effectively from aqueous
phase. Draining the exhausted adsorbent bed of water and contacting it with a
solvent, such as alcohol, in which the adsorbate is highly soluble will result in
desorption of the substance and regeneration of the carbon. A final steaming can be
used to clear the bed of the volatile solvent and prepare it for another adsorption
cycle.
231
-------
Chemical
Addition
(Optional)
p*-
Primary
Clari-
Flocculator i
__,J' ... - _,,,U
Raw Grit T 1
Wastewater Removal — «., —
____^r— 1 ^J
} i
____^__ Primary j 1
1 Thickener 1 1
* Jl
Grit to f
Landfill Vacuum
Filtration
Incineration
V
Ash to Landfill
Carbon
Makeup Liquid
^s Polymer
1 I i>
Air f|-yj 1 ^ Filtration
r~*i ^ | ^_^
*" 1 ^ *" Settling! ^-^
Aeratlon ^, 1 (Optional) ~*,,
Snubbing product
CI2 Contact
< ""-•* EOPump
1 <
1 1 Gravity
1 I Thickener
Storage
Tank
^ Wet Air i'
Regeneration
1
Ash to •* — *
Grit Chamber
Figure 17. PACT® flow diagram (Courtesy of Zimpro, Inc.)
-------
Thermal reactivation, the method most commonly used for GAC, particularly in
municipal waste applications, achieves the greatest degree of restoration of original
adsorption properties. In this process the carbon is removed from the adsorber,
heated under controlled conditions in a furnace and returned to service. The purpose
of reactivation is to restore the original adsorptive capacity of the carbon by removal
and/or destruction of adsorbed organic impurities. This is achieved in the thermal
process by destructive distillation of adsorbed organic material and steam activation
of the char residue. A number of investigations have been carried out on the thermal
r /"• A r^ 108-i 13
reactivation of GAC.
Thermal reactivation generally involves three consecutive steps; namely:
• drying and volatilization;
• charring; and,
• activating.
The three steps are normally carried out in one furnace having zones of increasing
temperature from inlet to outlet. Moisture and volatile adsorbed materials are
removed first. Carbon bed temperatures at this stage do not much exceed 100°C
because of moisture vaporization. Temperature gradually increases to above 600°C
in the subsequent zone, and adsorbed volatile material is driven off by destructive
distillation. Only organic char and ash residues remain in the carbon pores at this
time. The char is activated in the final zone of the furnace at about 900°C with the
addition of steam. Inherently, part of the original carbon is burned along with the
char residue; the extent of this "burn off" relates largely to the degree of reactivation
required to restore the carbon's adsorptive properties.
There are a number of equipment alternatives for use in reactivating carbon.
These include the multiple-hearth furnace, the rotary kiln (either direct or indirect
fired), the fluidized-bed furnace, and the electric-belt furnace. Schematic diagrams
of these several furnace configurations are given in Fig. 18 (a thru d).
The multiple-hearth furnace—the most widely used alternative to date-consists of
a number of fixed stages, or hearths, and a rotating center shaft mounted with rabble
arms which move the carbon through each stage. Heating arrangements are usually
designed to provide a gas temperature gradient from about 200° C at the inlet or top
to 900°C in the lowest hearth.
Fig. 18a illustrates a 6-hearth furnace suitable to reactivate carbon at a municipal
IPCT plant. As for any furnace operation, gaseous and paniculate emissions must
be taken into account; this unit is commonly equipped with an afterburner and a
scrubber. The rabble arms are constructed so that the carbon bed is furrowed and
turned over as it moves across each hearth. This increases the surface of the bed
exposed to the furnace atmosphere, and insures that all of the carbon is effectively
reactivated.
The second most common alternative system for carbon reactivation is the rotary
kiln. In this furnace configuration (Fig. 18b) carbon moves continuously from one
end of the inclined tube to the other under the combined influence of the rotational
movement of the tube and of gravity. Gas flow may be either countercurrent or
concurrent to the carbon flow. A lower input of steam is needed for concurrent flow
operation because water evaporated from the carbon is maintained through the
activating zone; emissions are also reduced. The carbon is exposed to the furnace
atmosphere through gentle tumbling action as the furnace itself turns. The rotary
tube furnace has been used successfully for both activation and reactivation of GAC.
Fluidized bed and electric belt furnaces (Figs. 18c,d) are new to the field of carbon
reactivation and are currently undergoing development and evaluation."2'1"
Reactivation of PAC cannot be accomplished economically in conventional
furnaces without extensive modification of both equipment and operating pro-
cedures.102'"4 More success has been achieved employing wet oxidation processes to
regenerate PAC. In this process, illustrated in Fig. 19, an aqueous slurry of PAC is
contacted with an oxygen-containing gas at temperatures of 100°C to 300°C and
233
-------
Carbon In
Hearth
v. Rabble Arm
Burners
and Steam Inlets
Figure 18a. Schematic diagram of a multiple-hearth regeneration furnace (Courtesy of
ICI Americas, Inc.).
.Girt Gear
Gas Out
Carbon In i
Breeching
p
I
1 J-
Q
iq
?TtrD p
1 — i
in
4J
/
\
Carbon Out
s Burner
Figure 18b. Schematic diagram of a rotary tube regeneration furnace (Courtesy of ICI
Americas, Inc.).
234
-------
Granular
Activated
Carbon Inlet
Gas
Gas Inlet
Reactivated
Granular
Activated
Carbon
Figure 18c. Schematic diagram of a f luidized-bed regeneration furnace (Courtesy of ICI
Americas Inc.).
Spent
Carbon
Inlet
at
Moving —
Belt T~LT
Exhaust
Heating Elements Gas
React Carbon
Outlet
Quench Tank
Figure 1 8d. Schematic diagram of an electric-belt regeneration furnace (Courtesy of ICI
Americas Inc ).
235
-------
Spent
Carbon
Thickener
O
Storage
Tank
Pump
Air
Pressure
Pumps
Air
i r
A
r
>
~~1 *]
r
*»i •>
*-£E) J
PCVs
Heat
Exchangers
Reactor
Steam
Generators
(Start-up)
Wastewater
Process
Air Compressors
Scrubbing Channel
Figure 19. Schematic diagram of a wet air regeneration system (Courtesy of Zimpro, Inc )
To Ash
Handling
-------
pressures of 100 psig to 3000 psig. Actual operating conditions are dependent on the
individual application but excellent recovery of carbon properties has been re-
ported.103'104'115'"6
Carbon loss is an important factor in the operation and operating costs of an
activated carbon system. Losses can occur in a number of ways throughout the
overall adsorption-reactivation system. Carbon can be oxidized and burned if excess
oxygen is allowed to enter the furnace atmosphere. Small particles of carbon can be
entrained in the relativley high velocity gas stream moving through the furnace.
Attrition losses can occur in all stages of intrasystem transfer and handling. A major
cause of carbon losses in many plants is inadvertent overflow and spillage from the
system. Entrainment and carry-over in liquid streams can take place wherever
excessively high localized velocities exist. This may occur during adsorber backwash
or during one of the transport steps in the reactivation system. Finally, apparent
losses in volume may relate to changes in the carbon packing characteristics as the
particles change shape during handling.
One of the most important aspects of carbon reactivation is the percent adsorption
efficiency change incurred during successive reactivations. It is important that the
design of any activated carbon facility employing reactivation or regeneration be
based primarily on the long-term performance characteristics of the carbon rather
than those of the virgin material.
Most operating data from both pilot and full scale installations in which the
carbon has been subjected to multiple adsorption-reactivation cycles show similar
results. After an initial decrease in adsorption characteristics, carbon performance is
generally found to stabilize at a quasi-steady-state level which, with good operational
control, can be maintained over a relatively long period.
As noted earlier in this chapter, resistance of the GAC to attrition during use is
important. One measure of attrition or particle breakdown is given by changes in
mean particle diameter. Data from different applications and installations indicate
that such changes are primarily a function of the handling system design and extent
of adherence to proper handling procedures.
CASE HISTORIES
It is interesting to note that of the 32 municipal PCT plants currently identifiable
in the United States,"7 21 were identified in the 1973 EPA Carbon Design Manual.56
This points up the fact that most of these plants were designed and constructed in
parallel, eliminating the valuable engineering aid of operating history available to
more conventional designs. While many of the plants listed have experienced serious
start-up and/or operating difficulties, it must be pointed out that failures and
problems have been a function of mechanical design and not process performance.
When major failures of seemingly straightforward systems such as sewers still occur
as a result of the well delineated process of sulfide corrosion,'18 it is hardly surprising
that mechanical problems are experienced in complex physicochemical treatment
systems. Indeed, in this light, it is a major accomplishment that advanced municipal
wastewater treatment systems have progressed in the single decade of the 1970's
from conception through pilot testing, design, construction and operation to
demonstrate that high quality water can be produced for discharge or even reuse.
An exhaustive review of all existing PCT plants in the United States-let alone
throughout the world—is beyond the scope and intent of this chapter. A reasonable
perspective can be provided, however, by briefly examining several plants
representing somewhat different design concepts and operational modes. To this
end, five specific PCT facilities in the United States are examined with respect to
design, performance, and current status. Table 3 summarizes the salient features of
the five PCT plants chosen.
237
-------
Table 3. Comparison of Representative PCT Applications
to
w
oo
Site
South Lake Tahoe, CA
Orange County, CA
Niagara Falls, NY
Vallejo, CA
Vernon, CT
Treatment
AWT
AWT/RO
IPCT
IPCT
PACT
Discharge
Point
Indian Creek
Recreational Lake
Groundwater
recharge
Niagara River
Carqumez Strait
Hockanum River
Capacity
(Avg MGD)
7.5
15
48
13
6
Adsorber
Type
GAC, upflow, packed
GAC, downflow, packed
GAC, downflow, packed
GAC, upflow, expanded
PAC, suspended (PACT®)
-------
South Tahoe, CA
The South Tahoe AWT system was constructed in an effort to protect the
exceptional quality of the waters and associated ecosystem of Lake Tahoe in the face
of an expanding population and increased use of the lake and surrounding land. The
Tahoe water reclamation facility, put into operation in 1965, was the first full-scale
municipal plant in the United States to use GAC with on-site reactivation for
organic pollutant removal. The plant remains in operation today, and is unique in
the field for its long operating record and excellent performance. Over the past 15
years the plant has treated approximately 15 billion gallons of wastewater.
A unique feature of this system is that the high quality water produced by the
treatment plant is "exported" from the Tahoe basin and subsequently captured in
Alpine County to form Indian Creek Reservoir, a one billion gallon man-made
recreational lake.
Design, operating parameters, and a summary of performance are given in Table
4. A plant schematic is presented in Fig. 20.
Orange County, CA
The Orange County wastewater treatment plant is an AWT facility designated
Water Factory 21 (WF-21). The name derives from the concept of this facility as a
prototype source of supplemental water during the 21st century. The treatment
processes employed at WF-21 are designed to produce water suitable for
recharge to an underground fresh water aquifer to prevent seawater intrusion. While
this particular application is unique to coastal areas, the quality of the water
provided by this facility has far-reaching implications for renovation of wastewater
for reuse.
The WF-21 facility is designed to treat 15 MGD and, because it discharges the
treated water into an aquifer used for potable water supply, the product must meet
drinking water quality standards. As a result the facility includes reverse osmosis
(RO) as a treatment step subsequent to the conventional AWT treatment steps
Approximately one-third of the AWT effluent is treated with RO prior to reblending
for injection.
The GAC treatment facility at WF-21 went on-stream in January 1976, with
groundwater injection beginning in October 1976. The addition of RO treatment
was made in March 1978 and the system has operated successfully since that time.
Table 5 summarizes design, operating parameters, and the performance of WF-
21. Fig. 21 presents a plant schematic.
Niagara Falls, NY
With a capacity of 48 MGD, the 1PCT treatment facility at Niagara Falls repre-
sents the largest such system yet designed. The 1PCT system was chosen because of
the concomitant requirements of secondary quality effluent and treatment of signifi-
cant quantities of industrial wastes. System design was completed in 1972, and
therefore relied heavily on pilot data accumulated at Niagara Falls and other sites
The facility was partially completed in 1977 and began to treat wastewater in April of
that year. The full 1PCT system, including the GAC adsorbers, went on-stream in
January 1978, but has operated on only an infrequent basis due to mechanical
problems.
The design of Niagara Falls plant includes direct application of coagulated, settled
wastewater to the GAC adsorbers. This concept utilizes the GAC as a medium for
both filtration and adsorption, and represents an economic advantage by
eliminating separate filters. Operation of this facility will generate performance and
cost data necessary for evaluation of one of the most innovative and promising
treatment methods developed in the recent history of municipal waste treatment
239
-------
Table 4. Design, Operating Parameters, and Performance Summary
(South Tahoe, CA, Type AWT)
PLANT DESIGN
75
DESIGN FLOW(MGD)
CONTACTOR TYPE dosed, steel
MODE OF OPERATION upflow- packed bed' parallel
CONTACT TIME (EBCT) 17 mm
HYDRAULIC LOADING (gpm/ft2) 65
CARBON DEPTH (ft) 145
CARBON TYPE 8x30, GAC
REACTIVATION multlple hearth (6>
PLANT PERFORMANCE
BOD
COD
1-3 mg/L
10 mg/L
9q < 1 mg/L
TURBIDITY
PO,
COLOR
MBAS
02 JTU
0 8 mg/L
5-6 units
0 1 mg/L
A schematic diagram of the Niagara Falls plant is given in Fig. 22. Table 6 is a
summary of salient design, operation, and performance characteristics.
Vallejo, CA
The Vallejo facility is an IPCT system which employs filtration ahead of upflow,
expanded-bed GAC adsorbers. This design incorporates several new concepts
developed during the 1970's specifically for municipal wastewater treatment applica-
tions of GAC The facility was designed early in the decade. Construction proceeded
through the mid- 1970's and plant start-up occurred during 1978. While limited data
are available and system modifications have taken place, it appears that the facility
can meet the original design criteria.
Design and operating parameters as well as a summary of performance are given
in Table 7. A plant schematic is shown in Fig. 23.
Vernon, CT
Vernon, CT is the first municipality to operate a powdered activated carbon sys-
tem which includes carbon regeneration. The Vernon facility utilizes the PACT ®
concept of combined PAC and activated sludge treatment with wet oxidation for
simultaneous destruction of excess sludge and regeneration of the PAC. With an
early 1979 start-up and no other full-scale operating data available, this facility, like
the IPCT systems at Niagara Falls and Vallejo, characterizes the innovation and
development encouraged and supported by the EPA during the 1970's. The attempt
240
-------
Preliminary
Treatment
Primary
Settling
Biological
Treatment
Chemical
Coagulation and
Sedimentation
Ammonia
Stripping
Recarbonation
and Settling
i
To Indis
Res<
in Creek
srvoir
Chlorination
Carbon
Adsorption
t 1
Carbon
Regeneration
Mixed Media
Filtration
Figure 20. Tertiary treatment schematic South Lake Tahoe, CA (from Reference 56)
-------
Table 5. Design, Operating Parameters, and Performance Summary
(Orange County, CA, Type AWT/RO)
PLANT DESIGN
DESIGN FLOW(MGD) 1S
CONTACTOR TYPE closed, steel
MODE OF OPERATION downflow, parallel
CONTACT TIME (EBCT) 30 min
HYDRAULIC LOADING (gpm/ft2) 23
CARBON DEPTH (ft) 28
CARBON TYPE 8x30 GAC
REACTIVATION multiple hearth (6)
PLANT PERFORMANCE GAC RO
BOD : -
15 mg/L 1 5 mg/L
<1 mg/L <1 mg/L
THRRiniTY
<1JTU
1 3 mg/L <1 mg/L
COLOR
MBAS
to combine the ability of activated carbon to adsorb recalcitrant organic materials
with the proven economic advantages of activated sludge treatment may well result
in a blend of the best of both processes.
The potentials for high quality performance and economic benefits of
regenerating the powdered carbon combine to identify the Vernon facility as one to
monitor closely, for this may represent a trend for future municipal wastewater
treatment.
The design, operating parameters, and performance of the Vernon plant are
presented in Table 8. Fig. 24 provides a schematic diagram of this facility.
THE FUTURE OF PCT IN MUNICIPAL WASTE TREATMENT
Research and development needs relative to the application of PCT processes—
specifically GAC and PAC processes—in municipal waste treatment are much akin
to those in water and industrial waste treatment applications. DiGiano58 and
O'Brien, et al.57 have provided complementary discussions of these needs, and little
more need be said here in this regard. A larger question, however, is what the future
holds for PCT applications in municipal waste treatment.
The economic climate of the United States, indeed the world, during the closing
years of the 1970's forced a re-evaluation of the concept that clean water is worth any
price. Rigorous redeterminations of cost-effectiveness and cost-benefit relationships
have notably slowed the progress of PCT in the municipal waste treatment field.
Future considerations of PCT processes for removal of conventional pollutants will
242
-------
Primary
Settling
Biological
Treatment
Chemical
Coagulation and
Sedimentation
Ammonia
Stripping
Recarbonation
and Settling
Blending
and Storage
Breakpoint
Chlorination
Membrane
Filtration
(RO)
Carbon
Adsorption
Mixed Media
Filtration
Carbon
Regeneration
To Injection
Wells
Figure 21. Tertiary treatment schematic Orange County, CA (from Reference 56).
-------
48 MGD
Raw — *•
Sewage
Bar
Screen
Chemical
Clarification
Carbon
Adsorption
i .
i
t
Carbon
Regeneration
Chlorination
\
To Niaj
Rive
t
at a
r
Figure 22. Physical chemical treatment schematic Niagara Falls, NY (from Reference 56).
-------
Table 6. Design, Operating Parameters, and Performance Summary
(Niagara Falls, NY, Type IPCT)
PLANT DESIGN
DESIGN FLOW (MGD) 1?
CONTACTOR TYPE open' concrete
MODE OF OPERATION "owntlow. Parallel
CONTACT TIME (EBCT) 40 mln'
HYDRAULIC LOADING (gpm/ft2) 1'7
CARBON DEPTH (ft) a75
CARBON TYPE 8x3° GAC
REACTIVATION mult|Ple hearth <6'
PLANT PERFORMANCE
BOD
r-rirt 75% removal
„„ 90% removal
TURBIDITY
P04
COLOR
MB AS
Table 7. Design, Operating Parameters, and Performance Summary
(Vallejo, CA, Type IPCT)
PLANT DESIGN
DESIGN FLOW (MGD) 13
CONTACTOR TYPE open, concrete
MODE OF OPERATION "Pflow, expanded bed, parallel
CONTACT TIME (EBCT) 25mm
HYDRAULIC LOADING (gpm/ft2)
CARBON DEPTH (ft) 16
CARBON TYPE 12x40GAC
REACTIVATION mult""e hearth (6)
PLANT PERFORMANCE
BOD ___ 25_mg/L_
COD .
45
SS _ 7_mg/L_
TURBIDITY _
P04 _
COLOR _
MBAS _ ___ _
245
-------
13 MGD
Sewage
Grit
Removal
Chemical
Clarification
Recarbonation
Chlorination
Dual Media
Filtration
To
San Francisco Bay
Carbon
Adsorption
Carbon
Regeneration
Figure 23. Physical chemical treatment schematic Vallejo, CA (after Reference 56).
-------
Preliminary
Treatment
Primary
Settling
Makeup _
Carbon
PACT
^^ppT i
Wet Air
Regeneration
Chemical
Coagulation and
Sedimentation
-
Filtration
Disinfection
To
->• Hockanum
River
Figure 24. PACT® treatment schematic Vernon, CT
-------
Table 8. Design, Operating Parameters, and Performance Summary
(Vernon, CT, Type PACT®)
PLANT DESIGN
DESIGN FLOW (MGD) b
CONTACTOR TYPE suspended - aeration
MODE OF OPERATION PACT
CONTACT TIME (EBCT) I
HYDRAULIC LOADING (gpm/ft2)
CARBON DEPTH (ft)
CARBON TYPE PAC
REACTIVATION Wet Oxidation
PLANT PERFORMANCE
<20 mg/L
BOD
COD :
eg <10 mg/L
TURBIDITY 1
P04 :_
COLOR ^5 APHA units
MB AS ~
depend on the specific needs of critical geographic regions, and on performance and
economic data yet to be obtained from currently operating systems.
Conversely, concern with respect to the control of toxic and hazardous waste
materials is expanding markedly, and it is in this area that PCT processes, and
particularly activated carbon, are most generally applicable. Legislation has taken
specific issue with discharge of toxic and hazardous substances from publicly-owned
treatment works (POTWs). Moreover, it has given the municipal governments
responsible for operation of POTWs the authority to identify and control all
discharges of such substances to tributary sewer systems. Based on these
developments, the future of PCT processes in municipal waste control—either as
treatment operations under the direct control of municipalities or as pretreatments
required of dischargers to the collecting sewers of POTWs-seems predestined.
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Chemicals Plant Wastewater with the DuPont PACT Process," Proc. AlChE
National Meeting, Dallas, TX, Feb. 1972.
74. Hutton, D.G. "Combined Powdered-Activated Carbon—Biological
Treatment," in Carbon Adsorption Handbook by P.N. Cheremisinoff and F.
Ellerbusch eds. Ann Arbor Science, Ann Arbor, MI, pp. 389-447, 1978.
75. U.S. Patents 3,904,518 and 4,069,148 granted to DuPont. Sept. 9, 1975
and Jan. 17, 1978.
76. Strudgeon, G.E. and B. A. Carnes. "Upflow-Downflow Carbon Adsorption,"
Presented at the Chem-Engineering Conference, Los Angeles, CA, November
19, 1975.
77. Keinath, T.M. and W.J. Weber, Jr. "A Predictive Model for the Design of
Fluid-Bed Adsorbers," Jour. Water Poll. Control Fed., 40, 5, pp. 741, 1968.
78. Erskine, D.B. and W.G. Schuliger. "Graphical Method to Determine the
Performance of Activated Carbon Processes for Liquids," Amer. fnst. Chem.
Engr. Symp. Ser. 68, pp. 185, 1972.
79. DiGiano, F.A. and W.J. Weber, Jr. "Sorption Kinetics in Infinite-Bath
Experiments," Jour. Water Poll. Control Fed., 45, pp. 713, 1973.
80. DiGiano, F.A. and W.J. Weber, Jr. "Sorption Kinetics in Finite-Bath
Systems," Jour. Envir. Engrg. Div., Amer. Soc. Civil Engrs., SA6, pp. 1021,
1973.
81. Brauch, V and E.U. Schlunder. "The Scale-up of Activated Carbon Columns
for Water Purification, Based on Results from Batch Tests—II," Chem. Eng.
Sci., 30, pp. 539, 1975.
82. Hashimoto, K., K. Miura and S. Nagata. "Intraparticle Diffusivities in
Liquid Phase Adsorption with Nonlinear Isotherms,"/. Chem. Eng. Japan,
8, pp. 367, 1975.
83. Weber, W.J , Jr. and J.C. Crittenden. "A Numeric Method for Design of
Adsorption Systems," Jour. Water Poll. Control Fed., 47, pp. 924, 1975.
84. Westermark, M. "Kinetics of Activated Carbon Adsorption," Jour. Water
Poll. Control Fed.. 47, pp. 704, 1975.
85. Neretnieks, I. "Adsorption in Finite Bath and Countercurrent Flow with
Systems Having a Non-Linear Isotherm," Chem. Engrg. Sci. (G.B.), 30, pp.
107, 1976.
86. Mathews, A.P. and W.J. Weber, Jr. "Effects of External Mass Transfer and
Intraparticle Diffusion on Adsorption Rates in Slurry Reactors," AIChE
S\m. Series, 73, pp. 91, 1977.
87. Crittenden, J.C. and W.J. Weber, Jr. "Predictive Model for Design of Fixed-
Bed Adsorbers: Parameter Estimation and Model Development," Jour.
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Environmental Engineering Div. Amer. Soc. Civil Engrs., Vol. 104, No. EE2.,
pp. 185, 1978a.
88. Crittenden, J. C. and W.J. Weber, Jr. "Predictive Model for Design of Fixed-
Bed Adsorbers: Single-Component Model Verification," Jour.
Environmental Engineering Div. Amer. Soc. Civil Engrs., Vol. 104, No. EE3.,
pp. 433, 1978b.
89. Crittenden, J.C. and W.J. Weber, Jr. "A Model for Design of Multi-
component Adsorption Systems," Jour. Environmental Engineering Div.,
Amer. Soc. Civil Engrs., Vol. 104, No. EE6., 1978c.
90. Liapis, A.L. and D.W.T. Rippin. "The Simulation of Binary Adsorption in
Activated Carbon Columns using Estimates of Diffusional Resistances
within Carbon Particles Derived from Batch Experiments," Chem. Eng. 5V/.,
33, pp. 593, 1978.
91. Liu, K.T. and W.J. Weber, Jr. "Determination of Mass Transport
Parameters for Fixed-Bed Adsorbers," Chem. Engrg. Comm., 6, pp. 49, 1980.
92. Famularo, J., J.A. Mueller and A.S. Pannu. "Prediction of Carbon Column
Performance from Pure-Solute Data.'Vowr. Water Poll. Control Fed., 52,1,
pp. 2019, 1980.
93. Magsood, R. and A. Benedek. "Low-Temperature Organic Removal and
Denitrification in Activated Carbon Columns," Jour. Water Poll, Control
Fed., 49. pp. 2107, 1977.
94. Weber, W.J., Jr., M. Pirbazari and G. Melson. "Biological Growth on
Activated Carbon: An Investigation by Scanning Electron Microscopy,"
Environ. Set. and Techno/. 12, pp. 817, 1978.
95. Latoszek, A. and A. Benedek. "Some Aspects of the Microbiology of
Activated Carbon Columns Treating Domestic Wastewater," Environ. Sci.
and Technol, 13, pp. 1285, 1979.
96. Guirguis, W., T. Cooper, J. Harris and A. Unger. "Improved Performance of
Activated Carbon by Preozonation," Jour. Water Poll. Control Fed., 50, 2,
pp. 308, 1978.
97. Stopka, K. "Ozone-Activated Carbon Can Remove Organics," Water and
Sewer. Works, 125, 5, pp. 88, 1978.
98. Rosen, H.M. "Ozonation-Its Time Has Come," Water and Waste Engrg., 15,
9, pp. 106, 1978.
99. Weber, W. J., Jr. "Discussion of I mproved Performance of Activated Carbon
by Pre-Ozonation," Jour. Water Poll. Control Fed., 50, 12, pp. 2781, 1978.
100. Burant, W. and T.J. Vollstadt. "Full-Scale Wastewater Treatment with
Powdered Activated Carbon," Water and Sewer. Works, 120, 11, pp. 42,
1973.
101. Gulp, G.L. and A.J. Schuckrow. "What Lies Ahead for PAC?," Water and
Wastes Engrg., 14, 2, pp. 67, 1977.
102. Heath, H.W. "The PACT® Process to Treat 40 MGD of Industrial Waste,"
Presented at WWEM A Industrial Pollution Conference, Houston, TX, June
4-6, 1980.
103. Meidl, J.A. "Full Scale Wet Oxidation of PACT Spent Carbon," Water and
Sewage Works, 125, 12, pp. 32, 1978.
104. Berndt, C.L. "Innovative Technology: Wet Air Regeneration," Presented at
the 53rd Annual Conference of the Water Pollution Control Federation,
September 28-October 3, 1980.
105. Prober, R., J.J. Pycha and W.E. Kidon. "Interaction of Activated Carbon
with Dissolved Oxygen," Amer. Inst. Chem. Eng. Jour., 21, pp. 1200, 1975.
106. Sundstrom, D.W., H.E. Klei, T. Tsui and S. Nayar. "Response of Biological
Reactors to the Addition of Powdered Activated Carbon," Water Research,
13, pp. 1225, 1979.
107. Himmelstein, K.J., R.D. Fox and T.H. Winter. "In-Place Regeneration of
Activated Carbon." Chem. Engrg. Prog., 69, 11, pp. 65, 1973.
253
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108. Juhola, A.J. "Optimization of the Regeneration Procedure for Granular
Activated Carbon," U.S. Natl. Tech. Inform. Serv., Springfield, VA, PB
Rept. 208205, 1970.
109. Krieger, E. "Thermal Regeneration of Activated Carbons," Amer. Chem.
Soc., Div. Petrol. Chem. Preprints, 14, 1, pp. 13, 1970.
110. Loven, A. W. "Perspectives on Carbon Regeneration," Chem. Engrg. Progr.,
69, 11, pp. 56, 1973.
111. Zanitsch, R.H.and R.T. Lynch. "Selecting Thermal Regeneration System for
Activated Carbon," Chem. Engrg., 85, 1, pp. 95, 1978.
112. Johnson, H.R. and M.L. Massey. "Thermal Regeneration of Granular
Carbon Using a Fluidized Bed," Presented at the 7th Industrial Pollution
Conference of WWEMA, Philadelphia, PA, June 5-7, 1979.
113. Inhoffer, W.R. "Sorptive Properties of Granular Activated Carbon and
Infrared Regeneration at Little Falls, New Jersey," Presented at the Annual
Conference of the American Water Works Association, Atlantic City, NJ,
June 27, 1978.
114. Smith, S.G. "The Thermal Transport Process," Chem. Engrg. Progr., 71, 5,
pp. 87, 1975.
115. Knopp, P.V. and W.B. Gitchel. "Wastewater Treatment with Powdered
Activated Carbon Regenerated by Wet Air Oxidation," Proc. 25th Industrial
Waste Conf., Purdue Univ., Lafayette, IN, 137, pp. 687, 1970.
116. Gitchel, W.B., J.A. Meidl and W. Burant, Jr. "Powdered Activated Carbon
Regeneration by Wet Air Oxidation," AIChE Symposium Series No. 151,
Vol. 71, 1975.
117. Carnes, B.A. and D. Burstein. "Alternative GAC Adsorber Design,"
Presented at WWEMA Industrial Pollution Conference, Houston, TX, June
4-6, 1980.
118. Kienow, K..K. "The San Diego-West Loma 114-inch Interceptor Failure,"
Water and Sew. Works, 127, 5, pp. 44, 1980.
254
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THE IMPACT OF ORGANIC SUBSTANCES ON
MUNICIPAL WASTEWATER REUSE
J. Carrell Morris, PhD and John F. Donovan
INTRODUCTION
Rationale for Reuse
With few exceptions, the development of municipal wastewater-reuse facilities
and current research in this field have focused on reuse for nonpotable purposes.
Nonpotable reuse can be defined as the substitution of "reclaimed water" for waters
of potable quality that are now being used for nonpotable purposes. The U.S.
Environmental Protection Agency (EPA) encourages reuse by defining it as an
"innovative/alternative technology" that may be eligible for additional funding
under the Clean Water Act of 1977. The benefits in this type of water reuse are
several1:
• The demand on limited, high-quality sources of water is relieved, so that
such sources can serve larger populations or so that the useful life of these
sources can be extended.
• The cost and energy requirements of providing reclaimed water may be less
in some instances than the cost of developing additional high-quality
freshwater sources, even when these are availabe.
• Discharge of pollutants to watercourses is reduced.
• Other priorities—such as the preservation of open spaces, of recreational
areas or of agricultural lands—may be realized, because a supply of water
necessary for each development may be made available that otherwise
could not be dedicated to these uses.
• The risks to the population from the life-long ingestion of the harmful
organic contaminants in polluted waters are virtually eliminated, since
high-quality waters are reserved for potable use.
The Authors Dr J Carrell Morris received his Bachelor of Science degree in chemistry at Rutgers
University and his Ph D in physical chemistry at Princeton On receiving his doctorate in 1938 hejomed the
faculty in Sanitary Engineering at Harvard University where he is now Professor of Sanitary Chemistry.
Since 1964 Dr Morris has also been associated with the International Institute for Hydraulics and
Environmental Engineering at Delf University, Holland His research interests extend to the physical-
chemical aspects of water purification and wastewater treatment, and particularly to drinking water
disinfection. Dr Morns serves on numerous committees of the National Academy of Sciences - National
Research Council and professional organizations and serves as consultant to many Federal and State
agencies and private organizations He has published extensively on the physical chemistry of waters and
wastewaters and on the management of water quality
Mr John F Donovan is a project engineer in the Environmental Engineering Division of Camp, Dresser
and McKee, Inc . which he joined on receiving his Bachelor of Science and Master of Science degrees in civil
engineering at Northeastern University. He has participated in several major projects involving design
evaluation of wastewater collection and treatment facilities, studies of sludge management and effluent
disposal, and research on innovative methods of sludge stabilization and water reuse He served as overall
project manager for "Guidelines for Water Reuse" which was published as an EPA manual Mr Donovan is
the author of several technical papers on water reuse
255
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The last point is particularly important in the context of this monograph:
wastewater reuse is one way to avoid the use of polluted sources for potable supplies.
Two other broad options are available for producing potable waters from polluted
sources: (1) eliminating the pollutants at their source—source control—or (2)
monitoring and removing the pollutants by treatment. Both of these approaches can
be difficult where sources drain urban and industrial areas.
The difficulty in source control is exacerbated by the continual development of
new chemicals. Some 1,000 new chemicals are introduced by industry annually. The
U.S. General Accounting Office (GAO), reviewing the status of the control of
chemical exposures in industry, has concluded that. . ."It will take more than a
century to establish needed standards for substances already identified as
hazardous. . .The problem is compounded because new substances, which may
warrant standards, are being introduced faster than standards are being established
on existing substances" If this is the case for chemicals in the industrial setting, the
situation is far more critical in the environment.
Monitoring and removing organic substances by treatment is also difficult.
Treatment may be accomplished at a wastewater-treatment facility discharging to a
waterway that serves as a source of potable supply, or at a water-treatment facility
drawing from a polluted source, or both. Routine monitoring and treatment for
removal for a wide variety of organic chemicals is not currently practiced because the
technology is complex and projected costs appear too high for the thousands of
relatively small municipal and private authorities responsible for wastewater
disposal and water supply.
Impact of Organic Substances on Reuse
Organic substances present in reclaimed water have not been considered an
important factor in most reuse planning. Where reuse is projected for nonpotable
purposes only, the presence of potentially harmful organic chemicals in the
reclaimed water has not been found generally, to be of great consequence.
Such is not the case, of course, where reuse for potable purposes is contemplated.
In such instances, the full range of concerns explored in the water supply section of
this monograph must be considered. Reclaimed water generated from domestic
wastewaters and treated to sufficient levels may be of better quality than water taken
from sources subject to heavy contamination from synthetic organic substances.
Such reuse applications as urban landscape and agricultural irrigation, industrial
processing (other than food processing), cooling, toilet flushing, most commercial
applications (e.g., car washing), and recreation generally are not affected by the
presence of organic material of the types and concentrations normally encountered
in municipal secondary effluents. Only where irrigation is used on crops that are to
be used for animal feed, or directly for human feed need there be a concern for
human health. Up to now, most attention in such situations has been given to
concentrations of heavy metals, but as the number of reuse projects continues to
grow, more attention needs to be given to the possible accumulation of organic
substances in crops and the significance of these substances both to the growth of the
crops themselves and to the health of animals and humans who use these crops for
food.
The following sections deal with types and examples of municipal reuse, the need
for control of organic substances in waters intended for reuse, quality requirements
for organic substances, methods of control of organic substances, an assessment of
present problems, and future outlook.
TYPES AND EXAMPLES OF MUNICIPAL REUSE
Reuse can be classified broadly as direct and indirect. Direct reuse is the planned
and engineered use of treated wastewater for some beneficial purpose such as
256
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irrigation, recreation, industry, or groundwater recharge. Indirect reuse of
wastewater occurs when wastewater already used for domestic or industrial
purposes is discharged into fresh surface or ground waters and withdrawn
downstream in diluted form. An example of indirect reuse is the Upper Occoquan
AWT Plant in Virginia. There, highly treated effluent is discharged to a stream
tributary to the Occoquan Reservoir, a potable water source for a large portion of
Fairfax County, Virginia. This text deals solely with direct, engineered "pipe-to-
pipe" use of municipal wastewater.
Fig. 1 and Table 1 illustrate the number and variety of nonpotable reuse appli-
cations already used in pilot- or full-scale installations throughout the United States.
Potable reuse has not been pursued as strongly as nonpotable reuse, because of
health concerns; however, potable reuse is being evaluated on a pilot-scale basis in
Denver, Colorado. The figures used in Table 1 are taken from two nationwide
surveys ''4 completed in 1971 and 1979, respectively.
The 1979 survey reported reuse projects underway in about 25 states.
The California Department of Health Services in 1977-78 found reuse underway
at 363 locations in that state.5 As in the nationwide surveys, the great majority of
projects—88 percent—involved types of agricultural or landscape irrigation. Most of
the individual irrigation users reportedly utilize only relatively small volumes of re-
claimed water. Another similarity among the California and nationwide surveys is
the finding that relatively large volumes of reclaimed water a re being used in industry
and for groundwater-recharge purposes.
The State of California, where the majority of all U.S. reuse is taking place, offers
several good examples of various types of reuse projects. At the Irvine Ranch Water
District, a coagulated, filtered, and disinfected secondary effluent is fed through a
50-mile reclaimed-water distribution network for use in landscape and agricultural
irrigation. Eventually, the district will be reclaiming and distributing up to 15 mgd
for nonpotable uses, including expanded agricultural irrigation and possible
industrial use.
Drawing wastewater directly from an interceptor has worked successfully since
1962 at Whittier Narrows in a reuse program administered by the Los Angeles
County Sanitation District (LACSD). Wastewater diverted from a major regional
interceptor is given conventional activated-sludge secondary treatment, filtration,
and chlorination at the Whittier Narrows Water Reclamation Plant. The reclaimed
water (along with reclaimed water from the San Jose and Pomona plants also
administered by LACSD) is then applied to spreading areas for recharge of two large
groundwater basins a few miles east of Los Angeles. The ope ration has several major
attributes:
• The reclamation plant receives mostly domestic wastes. It is upstream of
major industrial discharges that might make the plant's effluent unsuitable
for recharge purposes.
• Sludge from the reclamation plant is diverted back to the sewer for
eventual treatment downstream, serving to reduce system complexity and
cost at the upstream reclamation plant, and also to improve site landscap-
ing possibilities, which enhances public acceptance.
• There are savings in capacity—and total costs—for conveyance and treat-
ment facilities downstream.
• Because the plant draws a constant flow from the interceptor, its treatment
performance is improved.
• In the event of a treatment-plant upset, the unsuitable effluent can be
diverted back to the sewer for treatment downstream.
This system proved so successful that LACSD has since constructed four additional
reclamation facilities (Pomona, Los Coyotes, Long Beach and San Jose Creek).
A major EPA-funded wastewater reclamation facility is the Orange County,
California, Water District's Water Factory 21. This 15-mgd facility is designed to
treat unchlorinated secondary effluent from a nearby municipal wastewater treat-
257
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NJ
00
(4) Number of systems in the subregion Reference: 4
Figure 1. Distribution of existing water reuse projects in the United States
-------
Table 1. Reuse Projects in the United States
Number of Projects
Type of Reuse
Agricultural/landscape irrigation
Industry
Groundwater recharge
Fish propagation, recreation and other
Totals
1971
400
15
10
5
430
1979
470
29
11
26
536
ment facility. The unit processes include lime clarification with sludge recalcining,
air stripping, recarbonation, prechlorination, mixed-media filtration, granular
activated carbon adsorption with carbon regeneration, final chlorination and
reverse osmosis demineralization. The effluent is blended with potable water and
injected into the groundwater to protect potable groundwater aquifers from salt-
water intrusion.
The State of California Department of Health Services has set restrictions on
reuse programs that involve recharge of groundwater aquifers.6 Recent research
results indicate that viruses and organic contaminants show measurable rates of
migration in aquifer recharge systems. An amendment to the agency's regulations
calls for a strict, case-by-case review of projects based on factors such as "level of
treatment provided; effluent quality and quantity; spreading area operations; soil
characteristics; hydrogeology; residence time; and distance to withdrawal."
In Contra Costa County, California, the Central Contra Costa Sanitary District is
responsible for much of the wastewater treatment and disposal function in the
county's industrial areas. Most of the major oil and chemical companies in the area
purchase water from the Contra Costa Water District. In 1972, the Sanitary and
Water Districts signed a contract providing for sale of the Sanitary District's effluent
to the Water District for further treatment and resale to several industrial users. The
Sanitary District provides secondary treatment and filtration of 15 to 30 mgd, with
delivery to a clearwell operated by the Water District. The Water District operates
an ion-exchange softening facility and a storage and distribution system to supply
industries with 15 mgd for cooling-tower usage. The project is designed to meet an
ultimate 30-mgd demand for various industrial uses, but mostly for cooling. In the
development of the Contra Costa County project, consideration was given to the
prospect that various organic substances might contribute to potential cooling-
tower corrosion by increasing biological growths. A test program showed that
biological growth could be controlled by periodic use of chlorine and other
nonoxidative biocides.
Reuse projects are also in operation in other countries: The Netherlands, Israel,
South West Africa and Singapore. At Dordrecht, The Netherlands, pilot testing of
potable reuse is underway to evaluate the use of reclaimed water as an alternative to
the highly contaminated Rhine and Meuse Rivers as sources of supply.7 Schematics
of the physical-chemical lime process train and the reverse-osmosis process train are
shown in Fig. 2. The lime-process employed was found not to be effective in
removing organic compounds. No removal of volatile halogenated organic
substances or non-polar organic chlorine was noted. However, many organic
compounds in the activated-sludge facility effluent were actually lower than mean
concentrations in the drinking water of Dordrecht (see Fig. 3).
Reverse-osmosis test results at Dordrecht are shown in Table 2. The groups of
halogenated aliphatics, benzene and Ci and C2 alkyl benzenes, naphthalene,
259
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Lime
Sec. Effluent
Zeoliet
jRecarbonation |Dry Filtration Filtration
Activated Carbon
| Filtration
Reference: 7
a. Physical-Chemical System
Brine
FeCI3
NaOCI.
HCI
-"L
Upflow Downflow Reverse
Filter Filter Osmosis
Activated Marble-
Carbon Filter
NaOCI
Reference 7
b. Reverse-Osmosis System
Figure 2. Schematic diagram of pilot testing systems for potable reuse, Dordrecht,
The Netherlands
alkalated naphthalenes, sulfonamides and alkylphenols were not removed effectively
by this process. However, effective removal was attained for alkanes, alkylated
benzenes with long side chains, mdane and alkylated indanes, organic phosphates,
chlorophosphates, phthalates and cyclic hydrocarbons. With subsequent activated-
carbon filtration, substantial additional removal of nearly all organic substances
was achieved.
The largest and most advanced wastewater reclamation scheme in Israel is the
Dan Region Project located near Tel Aviv.8 Planning for this facility began in 1965.
260
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25 20
*• Retente tijd
Figure 3. Chromatogram of Rhine River water and secondary effluent - Dordrecht, The
Netherlands
Wastewater undergoes three major treatment processes prior to groundwater
recharge by means of spreading basins:
• Biological treatment in oxidation ponds
• High-lime treatment, and
• Detention in polishing ponds.
The recharged water will be extracted from the ground by means of recovery wells
and conveyed to the southern part of the country in a separate transport main
system. This high-quality water will be available for unrestricted agricultural,
industrial and municipal nondomestic uses. An earlier objective, production of a
potable product, was dropped in 1975, due primarily to public-health considerations.
The well-known Windhoek reclamation plant in South West Africa is still the only
full-scale facility in the world at which wastewater is reclaimed for direct potable
reuse. Intermittently since 1968, the 1-mgd facility has provided 10 to 20 percent of
the potable-water demand for the City of Windhoek. A Water Research
Commission has carried out studies on health effects of using reclaimed water.9 In
one study, 18 known toxic compounds were inoculated into the feed water of a pilot
261
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Table 2. Removal of Organic Micropollutants in the Reverse-Osmosis System
Compounds Secondary Feed Effluent Effluent
Effluent R O. R.O. Marble Filter
Halogenated aliphatic
hydrocarbons 21 8.5 11 1.1
Benzene +C1+C2 alkyl-
benzenes
Sulphonamides
Alkylphenols
Naphthalene + alkyl-
naphthalenes
Hydronaphthalenes
Alkanes
C3-C8 alkylbenzenes
Indane + alkylmdanes
Phosphates + chloro-
phosphates
Phthalates
Cyclic hydrocarbons
0.3
3.3
0.69
0.2
0.12
0.2
5.7
0.75
0.5
10.2
1 13
0.3
3.3
0.69
0.1
0.19
0.9
3.36
0.75
05
1.7
1 13
0.25
3.1
0.31
0.05
0.06
005
0.76
0.05
0.06
023
0.11
0.1
-
0.36
0.01
-
0.05
0.13
-
0.13
0.2
reclamation plant. The plant reduced the toxicants by more than 99.9 percent, to
values from 0 mg/L to 6 mg/L in the final water. In another study, only two poly-
nuclear aromatic hydrocarbons were detected in the plant effluent, while ten were
detected in the influent. The two in the effluent (pyrene and fluoranthene)are not of
toxicological importance.
In Singapore, reuse of wastewater effluents has been practiced since 1915. Since
the mid-1960s, a coagulated, filtered, and disinfected reclaimed water has been used
by industry, for irrigation and through a separate distribution network for toilet
flushing in high-rise residential apartments.
NEED FOR CONTROL OF ORGANIC SUBSTANCES IN WATERS
INTENDED FOR REUSE
Wastewater effluents from standard secondary treatment plants, even from those
using intensive or extended biological oxidation, contain a wide variety of organic
materials, amounting overall to 20 to 100 mg per liter. Although much of this
organic material is proteinaceous, fatty or carbohydrate-like, derived from micro-
bial metabolism, a significant fraction consists of synthetic organic chemicals
resistant to aerobic biochemical attack. Such resistance is indicated by the high
ratios of chemical to biological oxygen demand found for secondary effluents, often
as great as 5 to 1.
Although the proportion of synthetic organic materials may increase with the
fraction of industrial wastewater admitted to the municipal sewerage system, the
absence of industrial discharges does not mean that synthetic organic structures will
be absent. Such substances as polycyclic aromatic hydrocarbons (PAH) pesticides
and polychlorinated biphenyls are nearly ubiquitous. In addition, household and
small business use of organic cleaning solvents, deodorants like the dichloroben-
262
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zenes, cresols and terpenoid materials contributes additional synthetic organic
structures to purely domestic-type sewages. Standard treatment is ineffective for
removal of many of these substances, so that they remain as components of the
treated effluents.
When the effluents are discharged to riverine waters, the organic materials present
are subjected to further processes of dilution, biochemical or chemical reactions,
volatilization, and adsorption. Nonetheless, many of the added compounds
continue to exist in the receiving rivers in currently measurable concentrations over
long distances and for extended periods of time. Intensive investigations of large
receiving rivers like the Ohio, the Mississippi, and the Rhine have shown that they
each contain 500 or more trace organic compounds of which 50 to 100 are of specific
concern with regard to toxic or carcinogenic properties.
The situation is more critical when the effluents are to be employed for some sort
of direct reuse, such as groundwater recharge and/or potable reuse. Because the
factor of dilution is missing or greatly reduced, the opportunity for long-term
environmental changes is also diminished. For example, Garrison and Miele pointed
out a few years ago: "the constituents of major concern in groundwater-recharge
systems are trace organics of unknown health effects"10; others have expressed
similar concerns with regard to these contaminants in waters intended for similar
types of reuse. These health concerns are not as critical, however, in water intended
for nonpotable reuse.
Unfortunately, there are few data relating to the magnitude of the problems posed
by these trace organic substances. Recently, Roberts, McCarty and coworkers"
acquired imformation on some California wastewaters, including both standard
secondary effluents and those treated by advanced methods. Smith, Englande and
coworkers12 reported trace organic analyses of the effluents of a number of advanced
waste treatment installations around the country. These studies show that many
identifiable organic compounds of toxicological interest are present in normal
sewage effluents and that several of these persist through current advanced waste
treatment processes, usually in much reduced concentrations.
McCarty, Reinhard and Argo, in their study of the Orange County Advanced
Waste Treatment Plant, Water Factory 21, adopted as tentative maximum con-
taminant levels (MCLs) for trace organic substances the concentrations of 1 micro-
gram per liter for nonchlorinated substances and 0.5 microgram per liter for
chlorinated ones." The procedure is a crude one, but the attempt is worthwhile and
the selected values appear reasonable in the absence of specific toxicological and
carcinogenic data. Based on their values, the types of substances presenting the
greatest problems and the greatest need for controls are the volatile chlorinated
hydrocarbons, dichloro-benzenes and phthalate esters.
Researchers caution against overreaction to the presence of trace organic sub-
stances in reclaimed wastewater. As Lennette and Spath14 state, "In struggling with
the complexities of establishing goals or standards for treatment of recycled
wastewater in order to provide reasonable assurance against health risks, one should
be wary of insisting that nothing less is acceptable than the ultimate degree of purity
which modern technology can achieve. This technology is immensely costly and
should be reserved for those situations in which there is a genuine and demonstrable
need. Because modern technology permits measurement of such minute traces, it
does not necessarily follow that public health will be better protected or the public
interest better served by embracing such sweeping goals as the complete removal of
all viruses of human origin from any waters that man may contact or equivalent
ideals such as 'zero-levels' and 'no-threshold levels' of toxic or carcinogenic
substances."
In some instances, the presence of organic matter in general, as a medium for
undesirable biological growths, for the clogging of soils or other finely apertured
materials, or for sensory problems, may call for regulation of parameters relating
broadly to the organic content, such as BOD, COD, TOC, CCE, color, odor, M B AS
and phenols. Necessary limitations on these more traditional engineering parameters
263
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can now be met in almost all instances by advanced treatment procedures developed
during the past decade. Thus, they do not present as great a need for future research
and acquisition of data as the trace organic substances do.
QUALITY REQUIREMENTS FOR ORGANIC SUBSTANCES IN
REUSE OF MUNICIPAL WASTEWATERS
General Aspects
With the exception of general parameters relating to organic content such as
BOD, COD or TOC, the organic substances that might affect the nonpotable reuse
of municipal wastewater had received little attention prior to 1975 and are hardly
mentioned in any formalized tabulations of criteria for water reuse. This is partly
because suitable techniques for measuring most individual organic substances were
not generally available before 1975. Perhaps a more important factor, however, is
that the concentrations of organic contaminants, other than those natural materials
associated with usual household activities, are generally so small—a few micrograms
per liter or less—that deleterious effects in nonpotable reuse have not been
considered significant.
Accordingly, it is necessary to discuss organic substances of potential concern,
based upon what has been learned about the types of compounds present in waste-
waters and treated effluents during the past decade, rather than to describe standards
and control measures for established hazards. Until the middle of the decade, the
only specific organic contaminants to attract much attention in connection with
reuse were phenolic compounds and synthetic detergents, the former because of
tastes and odors, the latter because of foam-producing properties. Beginning about
1974, however, with the development of gas-chromatographic/mass-spectrometnc
techniques for the isolation and identification of specific organic compounds from
effluents and polluted waters, interest in other classes of organic substances began
expanding rapidly. Among the types of compounds receiving particular attention are
those, like chlorinated hydrocarbons or nitrosoamines, that appear to present
significant health hazards and those, like some chlorinated aromatic compounds or
phthalate esters, that exhibit strong resistance to chemical and biological degrada-
tion in the environment. Until now, the concern has reached the stage of quantitative
expression as criteria or standards only in connection with the reuse of treated waste-
water for groundwater recharge. For most other types of reuse, as Russell Gulp15
pointed out in a paper at the Groundwater Recharge Symposium of September,
1979 in Pomona, California, "The trace organics problem is no greater in reclaimed
wastewater than it is in natural water supply sources. All indications are that
resolution of the trace organics problem in reclaimed water will be simultaneous
with resolution of the same problem in drinking water supplies, or sooner."
Dr. Takashi Asano of the California Office of Water Recycling has identified five
major categories for municipal water reuse in a descending order of anticipated
volume of reuse. These are: land application (irrigation for crops, landscape plant-
ings or natural vegetation); impoundment for recreational facilities; industrial
cooling or process water; groundwater recharge; direct consumptive use. Generally
speaking, the same order of ranking applies with regard to quality requirements,
those for direct potable use being the most stringent, of course. With some excep-
tions, criteria for specific organic pollutants will rank similarly.
Agricultural and Landscape Use
Historically, quality requirements on reclaimed water for use agriculturally have
not been too demanding. Attention has been focused chiefly on saline properties and
on biological health hazards. Adequately disinfected standard secondary effluent
264
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has been satisfactory for irrigation in many cases. Even primary municipal effluent
has on occasion been accepted as suitable for use on forage and grain crops.
A recent Office of Water Research and Technology (OWRT) review of water
quality criteria for reuse lists only one specific organic requirement in connection
with general agricultural irrigation.4 Phenols should be restricted to a maximum
concentration of 50 mg/L, quite a high limit. However, when truck crops are
involved, a maximum level of 20 mg/L for 5-day BOD is also recommended.
Stipulations with regard to reuse of effluent for landscape irrigation are only
slightly greater. The OWRT report found a consensus criterion that oil and grease
should be absent for this use—on aesthetic grounds—in addition to the previous
phenol and BOD criteria.
Standards for reuse of wastewater effluent in watering of livestock and wildlife are
almost equally meager. Phenol, at a maximum of 1000 mg/ L, and floating oil, not
supposed to be present, are the only specifically listed criteria. However, it is also
recommended that pesticide concentrations be less than the corresponding MCLs
for drinking water.
For the most part, lack of concern for the trace organic substances that may be
present in effluents otherwise suitable for agricultural reuse is a result of the fact that,
until now, no damages to crops or livestock have occurred because of the presence of
usual concentrations of such compounds. But portions of the water applied for
irrigation eventually reach either the groundwater table or surface receiving waters.
The organic substances present in the interim may have been oxidized, in which case
they are of minor concern, or they may pass through the soil with the remaining
water to pollute the groundwater or the receiving stream or they may be absorbed
and concentrated in the soil with eventual effects as yet undetermined.
With regard to the current situation, Jones and Lee16 have made probably the
most direct and penetrating remarks in their paper for the conference on health
effects of land application of wastewater held in San Antonio in December 1977,
stating, "It is presently impossible to assess the risk associated with land disposal of
wastewater... containing hazardous chemicals. There is no justification for assuming
that land disposal is environmentally less damaging than conventional water
disposal practices. Water disposal systems tend to be dispersive and diluting,
whereas terrestrial systems concentrate the chemicals in the soil and enhance the
opportunity for uptake by biological systems. Terrestrial systems also pose greater
risk of groundwater contamination."
The fates of organic chemicals during soil passage of water thus are matters of
important concern that need much research and investigation during the coming
decade.
Recreational Uses
The primary criterion for recreation water is, of course, hygienic safety or freedom
from pathogenic microorganisms. Stipulations relating to organic materials have
generally been indirect, expressed in terms of sensory or physiological effects, rather
than as specific organic constituents. Thus, virtual freedom from color, odor, oil and
scum have been specified by the EPA and other agencies, but no standards for
individual organic compounds or even classes of them have been recommended in
connection with general recreation. However, two types of organic substances have
regulatory limits when fishing or fisheries are involved. Polychlorinated biphenyls
(PCBs), which are representative of toxic substances that may bioaccumulate in fish,
to be transferred to man, have been limited to 10 6 mg/ L, and phenols, which are
examples of substances that may cause tainting of fish flesh, have a recommended
limit of 10"4 mg/L. The latter standard, if established, may cause difficulties,
because Englande and Reimers have found that currently operating advanced waste
treatment systems are unable generally to reduce phenols to less than 10~3 mg/ L."
265
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Other organic substances, such as aliphatic hydrocarbons in the kerosene range,
aromatic hydrocarbons and chlorinated compounds like paradichlorobenzene and
chloroanihnes will undoubtedly require future consideration with regard to these
possibilities.
Industrial Uses
Industrial uses of water are so diverse and the quality requirements are so varied
that a single set of criteria is impossible. For use as cooling water, either in power
generation or in other industries, the major stated criterion is that there be no
floating oil. Otherwise, normal clarified secondary effluent appears suitable. For use
as boiler feed, the requirements increase as the operating pressure increases. To feed
very high-pressure boilers, 100 atmospheres or greater, carbon-tetrachloride ex-
tractable matter should be virtually zero, and detergents should be less than 0.1
milligrams per liter. Otherwise, for this use and also for reuse as process water in
most chemical, petroleum, paper, and primary metals industries, the organic com-
position of clarified secondary municipal effluents seems generally satisfactory as
received.
The industries for which specific organic substances are important are those
concerned with food, beverages and kindred products. Overall, water of potable
quality is required, so that the MCLs relating to organic materials in drinking water
apply. In some instances, standards for sensory properties, such as color or organo-
leptic quality may be even more rigorous. Presumably, however, any water meeting
drinking water MCLs would be acceptable for further in-plant treatment.
Groundwater Recharge and Potable Reuse
Reclaimed wastewater intended for groundwater recharge to potable aquifers
clearly must meet more restrictive and extensive criteria than those for other types of
reuse. As a minimum, compliance with MCLs for drinking water is required.
However, some parameters, especially those relating to a number of specific organic
substances, need to be limited even more strictly. The particular types of organic
substances that need stricter limitation are those that are not degraded in the
groundwater, those that may build up in the water during recycle and those that may
interact reversibly with the soil during passage into the groundwater.
When recharge is performed by direct injection, then the full organic content of
the recharged water impacts on the groundwater aquifer. Moreover, opportunities
for subsequent degradation of many organic compounds under the nearly sterile
conditions of most aquifers are comparatively limited. Thus, relatively volatile
substances like tetrachloroethylene and methyl-chloroform, which would be lost
from surface waters by volatilization over a period of time, appear to persist almost
indefinitely in confined, underground water. Studies by McCarty and others have
shown that these small chlorinated compounds pass quite readily through advanced
waste treatment processes." For example, effluent from the advanced waste
treatment facility in Orange County—Water Factory 21-contained more than 0.5
micrograms per liter each of trichloroethylene, tetrachloroethylene and methyl
chloroform about one-third of the time, and these concentrations were substantially
maintained even through reverse osmosis.
When recharge occurs by percolation or infiltration, there is opportunity for
reaction of organic matter in the upper soil layers and for removal of organic
substances by interaction with soil particles, thus providing for some degree of
purification. In the latter instance, however, the retained material, if truly persistent,
may later break through into the aquifer, as Roberts and McCarty have shown for
substances like chlorobenzene, dichlorobenzenes, benzonitrile and styrene at Palo
266
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For more than 30 years, the Dutch have been recharging treated Rhine river water
by infiltrating it into the groundwater aquifers of their coastal dunes. Within recent
years, as highly sensitive analytical techniques became available, they have found
that numerous persistent organic pollutants, such as dichlorobenzenes, chloro-
anilines and phthalate esters have penetrated into their groundwaters. They are
currently conducting exhaustive studies on the organic materials that may have
interacted with and been accumulated by the filtering soils over more than 30 years,
to determine both the degree of pollution of the soil and its effects on the soil's
characteristics and utility. Results of this study should have a major effect on our
knowledge of requisite organic criteria for groundwater recharge.
Recent analyses of concentrates from six advanced waste treatment plants have
shown that each contains between 100 and 500 identifiable organic compounds.
Each effluent probably also contains a larger number of other unidentified
compounds. Discovering which of these need limitation and developing the
necessary criteria for all these compounds with regard to different types of reuse is a
formidable task for the coming decade. It is also, however, one that must be
accomplished, at least for those compounds with high toxicity and great persistence,
if potable water reuse opportunities are to be realized with security.
METHODS OF CONTROL OF ORGANIC SUBSTANCES
Two broad alternatives are available to reduce or eliminate organic substances in
municipal wastewater intended for reuse. The first is source control of the
substances, and the second is treatment for removal. This section is intended as an
overview of potential control methods. Detailed reports on organic substance
removal by biological processes and physical/chemical processes were discussed
earlier in this monograph.
Source Control
Industrial source control is the reduction or elimination of the discharge of
pollutants to municipal collection systems, a very effective way of improving the
quality of reclaimed water, especially for municipal facilities not designed to remove
specific organic substances. By in-plant recycling of flows that would otherwise be
discharged, the net amount of water that needs to be treated to meet these strict
standards is reduced.
Another major incentive toward source control is the recovery of materials with
value. The realization by industry that "waste" by-products represent a potential
source of revenue to offset production costs has led to the decrease of industrial
discharges. The cost of water itself has compelled industry to conserve and recycle
water, which reduces pollutant discharges to municipal systems. This trend is
expected to continue, as water costs escalate, energy costs increase, and stricter
water-quality standards necessitate costlier treatment of wastewater.
Treatment
Not many municipal wastewater-treatment facilities are designed to remove
synthetic organic substances. This section provides a brief overview of the capability
of selected treatment processes to reduce the concentrations of such organic
substances. These processes have been used at several advanced wastewater
treatment facilities where water reclamation is practiced.
Lime Treatment—
Lime is used for removing phosphate from wastewater. IfthepH is raised to about
11.5, the effluents produced exhibit nearly total phosphorus removal and BOD and
267
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COD reductions in the range of 65 to 80 percent." Most of the BOD and COD
reduction is due to coagulation and adsorption of the suspended and colloidal
organic matter by the calcium carbonate and magnesium hydroxide precipitates.
Soluble BOD and COD in the form of organic acids also are removed by
precipitation of their calcium salts.
Studies at Water Factory 21, Orange County, California give a comprehensive
overview of organic removal achieved by lime and other treatment processes.20
Table 3 shows trace organic removals during two periods representing different
influent conditions. Some removal of pesticides, phthalates and PCBs was recorded
during lime treatment. Removals were somewhat higher when trickling-filter
effluent was used as a feed because of higher initial solids concentrations and higher
proportions of industrial wastes, which resulted in higher concentrations of the
organic substances themselves. Many other trace organic substances were not
reduced during lime treatment, especially when better quality wastewater was fed to
the facility. The lime treatment effluent may have been contaminated, which would
account for the concentration increases. Firm conclusions cannot be drawn for these
substances because of the large statistical range for the 95-percent confidence
interval, but lime treatment, in general, can be characterized as relatively ineffective
for removal of trace organic substances.
Activated Carbon Adsorption—
Residual dissolved organic compounds from secondary-treatment plant effluents
can be effectively removed by either granular or powdered activated carbon. Treated
COD values of less than 10 mg/L are possible when the COD of the secondary
effluent is in the range of 50-150 mg/ L. Carbon adsorption is more effective for large
nonpolar organic molecules than for smaller or more polar compounds.
If aerobic conditions are maintained in a granular carbon bed or in a fluidized
suspension of powdered carbon, the biological degradation of adsorbed organics,
unless they are totally refractory, can be accomplished with resultant in situ partial
regeneration of the carbon. Apparently, the presence of activated carbon in an
aerobic biomass, such as activated sludge, can enhance the biodegradation of
organics that are difficult or slow to biodegrade. The organics that are completely
refractory remain adsorbed on the carbon and are removed with the carbon, which
can be heat-regenerated.21-"
If the wastewater effluent is contacted with ozone ahead of a granular carbon bed,
many of the complex nonbiodegradable organic compounds are broken down to
simpler compounds which are adsorbed and biodegraded in the carbon bed, and the
bed will be maintained in an aerobic condition as a result of the pre-ozonation.
Removals of organic materials by granular activated carbon at Water Factory 21
are shown in Table 4. Chlorinated benzenes, phthalates, aromatic hydrocarbons and
brominated trihalomethanes were removed as effectively as COD. Breakthrough of
COD, TOC and trihalomethanes occurred at regular intervals, indicating a need for
carbon regeneration. According to McCarty et al.20, when granular activated carbon
is being used to maintain low concentrations of trihalomethanes, COD, or TOC,
regeneration must be done more frequently than when only more hydrophobic
materials (e.g., pesticides and PCBs) are being controlled.
Air Stripping—
At Water Factory 21, air stripping has been highly effective for removal of many
trace organic contaminants.24 Table 5 indicates that the chlorinated benzenes and
the halogenated methanes, ethanes, and ethylenes are effectively removed.
Volatile compound removals were also recorded in decarbonators that follow
reverse osmosis units. Originally, these removals were attributed to the reverse
osmosis unit itself. Seventy percent or greater removals of bromodichloromethane,
chloroform, dibromochloromethane, and tribromomethane have been achieved.
268
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Table 3. Removal of Trace Organic Substances by Lime Treatment at Water Factory 21>
to
CTv
With Trickling
Contaminant
Di-n-butyl phthalate
1 ,2,4-Tnchlorobenzene
Bis-(2-diethylhexyl)-phthalate
Lindane
1 ,3-Dichlorobenzene
Dimethylphthalate
Diisobutylphthalate
1 ,4-Dichlorobenzene
PCBs as Aroclor 1 242
1 ,2-Dichlorobenzene
2-Methy (naphthalene
Bromodichloromethane
Chlorobenzene
Dibromochloromethane
Trichloroethylene
Heptaldehyde
Naphthalene
Chloroform
1 ,1 ,1 -Trichloroethane
Tetrachloroethylene
p-Xylene
Ethylbenzene
Styrene
Tribromomethane
1 -Methylnaphthalene
m-Xylene
Inf.
Cone.
/wg/L
0.46
068
2.1
2.4
0.09
2.5
0.9
0.57
1.6
4.7
0.6
1.4
Second
Eff.
Cone.
«!/L
0.22
0 12
1.02
1.2
0.21
3.0
021
0.21
1.09
0.94
0.16
0.23
Filter Influent
Period
% Removal
(95% Cl)
52 (-96 to 88)
82 (15 to 96)
51 (-7 to 78)
50 (-71 to 85)
-133 (-1340 to 62)
-20 (-174 to 47)
77 (-19 to 95)
63 (-22 to 89)
32 (-26 to 63)
80 (-30 to 97)
73 (10 to 92)
83 (62 to 93)
Inf.
Cone
A*O/L
0.79
0.11
11
0.14
0.16
48
4.7
1.85
0.47
0.64
0.01
0.53
0.14
0.69
0.74
0.10
0033
3.2
3.3
1.67
0.015
0.043
0.048
0.40
0.008
0035
With Activated Sludge
Eff.
Cone.
«3/L
0.23
0.035
3.8
<0.05
0.10
3.1
3.2
1.29
0.37
0.56
0.009
0.56
0.15
0.79
0.86
0.12
0.041
4.1
4.7
2.5
0.023
0.067
0.076
0.67
0.019
0086
Influent
% Removal
(95% Cl)
71 (-80 to 95)
68 (2 to 90)
65 (32 to 82)
>64
38 (10 to 57)
35 (-5 to 60)
32 (12 to 48)
30 (16 to 42)
21 (-15 to 46)
1 2 (-54 to 50)
10 (-253 to 77)
-6 (-33 to 1 6)
-7 (-80 to 36)
-14 (-42 to 8)
-16 (-270to 63)
-20 (-132 to 38)
-24 (-548 to 76)
-30 (-79 to 5)
-45 (-1 98 to 30 )
-50 (-141 to 7)
-53 (-206 to 23)
-56 (-202 to 20)
-58 (-294 to 36)
-68 (-143 to -15)
-138 (-510 to 8)
-146 (-329 to 41)
-------
Table 4. Removal of Organic Materials by Granular Activated Carbon at Water Factory 21
Contaminant
COD
TOC
1 ,4-Dichlorobenzene
1 ,2-Dichlorobenzene
Dnsobutylphthalate
Tribromomethane
Dimethylphthalate
Chlorobenzene/o-xylene
Bromodichloromethane
Dibromochloromethane
m-Xylene
Naphthalene
Di-n-butylphthalate
Carbon tetrachloride
Ethyl benzene
Bis-(2-ethylhexyl)-phthalate
Tetrachloroethylene
Methylene chloride
1 ,1 ,1 -Trichloroethane
2-Methy (naphthalene
Chloroform
Inf
Cone
42
14
0.02
0.17
0.09
4.8
1.4
0.06
0.04
1.5
8.2
With Trickling
"Eff.
Cone
16.6
7.0
0.02
0.03
0.05
1.3
0.23
0.03
001
1.6
6.7
Filter Influent
% Removal
(95% Cl)
60 (58 to 63)
51 (48 to 54)
1 7 (-750 to 90)
82 (0 to 97)
46 (-5 to 72)
72 (46 to 86)
84 (58 to 94)
45 (3 to 69)
72 (-260 to 98)
-7 (-98 to 43)
21 (-70 to 63)
Inf.
Cone
24
0.07
0.02
2.0
0.41
1.3
0.11
1.8
0.65
0.05
0.05
0.59
0.07
0.02
3.4
0.16
0.16
0.07
5.
With Activated
Eff
Cone.
12.3
0.001
0.002
0.27
008
0.47
0.04
0.81
0.31
0.023
0.023
0.33
0.0
0.019
3.1
3.1
0018
0.02
7.5
Sludge Influent
% Removal
(95% Cl)
59 (46 to 52)
98(43 to 100)
91 (-150to100)
87 (61 to 95)
81 (40 to 94)
64 (24 to 83)
63 (30 to 80)
54 (5 to 77)
52 (2 to 77)
50 (7 to 73)
50 (-8 to 76)
44 (-90 to 84)
20 (-110 to 70)
17 (-77 to 61)
9 (-69to 51)
-6 (-190to 61)
(-430 to 76)
-18(-150 to 45)
-41 (-146 to 19)
'Geometric mean cone, in /jg/L except TOC and COD which are in mg/L.
-------
Table 5. Removal of Trace Organic Substances by Air Stripping at Water Factory 21
Contaminant
Tetrachloroethylene
1 ,4-Dichlorobenzene
1,1,1 -Trichloroethane
1 ,2-Dichlorobenzene
Tribromomethane
Heptaldehyde
1 ,3-Dichlorobenzene
Bromodichloromethane
Dibromochloromethane
Chloroform
Tnchloroethylene
Styrene
1 -Methylnaphthalene
1 ,2,4-Tnchlorobenzene
Ethylbenzene
Diisobutylphthalate
Chlorobenzene/o-Xylene
m-Xylene
Naphthalene
Dimethylphthalate
PCB as Aroclor 1 242
2- Methylnaphthalene
Heptylcyanide
p-Xylene
Inf
Cone
Aig/L
1.0
0.94
1.2
0.12
021
1.1
0.21
0.22
0.23
30
0.21
With Trickling
Eff.
Cone.
U9/L
0.03
009
0 18
0.02
008
0.18
0.013
011
0 10
0.11
0.18
Filter Influent
% Removal
(95% Cl)
97 (88 to 99)
90 (0 to 99)
85 (-10 to 98)
83 (-46 to 98)
62 (-320 to 97)
83 (70 to 91)
94 (-73 to 100)
50 (-440 to 95)
57 (-110 to 91)
96 (89 to 99)
14 (-257 to 79)
Inf
Cone.
A85 —
>83 —
83 (60 to 93)
>82 —
82 (76 to 87)
79 (64 to 87)
>77 —
51 (-180to 92)
42 (-170 to 88)
40 (-290 to 91)
39 (-58 to 76)
28 (-1 to 49)
27 (-46 to 64)
1 9 (-84 to 64)
10 (-840 to 91)
10 (-79 to 54)
3
0 (-740 to 88)
-2 (-1 60 to 60)
-35 (-200 to 39)
-------
Reverse Osmosis—
Filtered secondary-treatment effluents having COD values of about 30 mg/ L can
have this parameter reduced to less than 5 mg/L by reverse osmosis.25 At Water
Factory 21, COD decreased from 14.6 to 1.3 mg/L (91 percent reduction) during
reverse-osmosis (RO) treatment Because membrane fouling by organic matter is a
problem, a program of membrane cleaning is required when soluble and colloidal
organic material is removed in a RO system. Reverse osmosis is not effective in the
removal of many low-molecular-weight trace organic contaminants, as shown in
Table 6, which is based on a pilot unit at Water Factory 21.
Chlorinatwn—
Very few soluble organic substances are removed completely by chlorination.
Although chlorine-substituted products are not known to be formed from more than
3% to 5% of the applied chlorine, the toxicities and biological inertness of many
chlorinated compounds may render chlorinated wastewaters potentially more
harmful from a chemical point of view than non-chlorinated ones.26'27'2 Thus,
although chlorination is used regularly to reduce levels of pathogenic organisms in
wastewater effluent, it tends to exacerbate rather than alleviate the problem of
organic compounds in waters intended for reuse.
Ozonalion—
Ozone can oxidize many organic structures and thus accomplish partial reduction
in ultimate COD and BOD. It is used principally to reduce the concentration of
organic matter following secondary treatment and filtration, but it can be used in
conjunction with carbon columns for removal of partially oxidized organic
substances. Ozone breaks down many complex organic compounds to simpler
structures which can then be readily biodegraded. Actually, most of the reactions of
ozone with organic pollutants in wastewater are of this type. The specific reaction
chemistry between organic substances in water and ozone is quite complex and not
completely defined. Apparently, some intermediate ozonides are formed that may
have certain toxic properties, but they appear to be quite unstable.26 Many types of
organic compounds, such as aliphatic hydrocarbons, primary alcohols, carboxylic
acids and chlorinated hydrocarbons are resistant to ozone treatment and so cannot
be removed by normal ozonation.
Ozone may also be used in combination with ultraviolet light. A U.S. Army study
indicated that UV light catalyzed the oxidation of refractory organic materials by
ozone.29
PRESENT PROBLEMS AND FUTURE OUTLOOK
An Assessment of Present Problems
During the 1970s, construction of municipal wastewater-reuse projects increased
tremendously. Two problems, however, have constrained the potential utilization of
reclaimed water These interrelated problems are: a lack of monitoring and testing
techniques and the lack of standards for various types of reuse. Both of these
problems are connected with the presence of organic substances in wastewater.
Monitoring and recording of plant-process performance and sustained high
reliability are imperative conditions for most reuse applications. Some reclamation
facilities have redundant process trains or the ability to recirculate or discharge
water that does not meet use criteria. Reuse would be greatly enhanced if there were
fast, inexpensive testing procedures by which organic substances and pathogens
could be identified in the product water. Health agencies in many states have taken
very conservative positions on certain types of reuse applications because of the
possibility that periodic upsets in treatment would result in the utilization of
272
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Table 6. Removal of Contaminants by Pilot Reverse Osmosis Systems at
Water Factory 21
Contaminant
Chloroform
Bromodichloromethane
Dibromochloromethane
Tnbromomethane
1 ,1 ,1 -Trichloroethane
Trichloroethylene
Tetrachloroethylene
Chlorobenzene
1 ,2-Dichlorobenzene
1 ,3-Dichlorobenzene
1 ,4-Dichlorobenzene
1 ,2,4-Tnchlorobenzene
Heptaldehyde
Heptylcyanide
Ethylbenzene
m-Xylene
p-Xylene
Naphthalene
1 -Methylnaphthalene
2- Methy (naphthalene
Styrene
Dimethylphthalate
Diethylphthalate
Di-n-butylphthalate
Dusobutylphthalate
Lmdane
Inf.
Cone
/fl/L
5.5
0.87
8.82
0.63
4.9
1 5
1 5
0.14
0.46
012
1 2
0.04
0 18
004
005
0.08
0.02
005
0.02
002
002
1 3
0.45
0 69
29
0.08
Eff.
Cone.
Afl/L
5 5
0.95
091
0.65
6.3
1 7
1 8
007
038
0065
0.99
0.11
0.05
002
0.029
0045
0022
0.067
0.02
002
0.01
1 0
0.52
0.73
2.5
006
% Removal
(95% Cl)
0(-51 to 34)
-9(-72 to 31)
-1K-41 to 13)
-3(-61 to 34)
-28(-151 to 35)
-12(-118 to 42)
-21(-79 to 18)
49(-18 to 78)
1 7(-72 to 60)
46(10 to 67)
17(-9to37)
-150(-541 to 3)
71(39 to 86)
49(-90 to 86)
44(-66 to 81)
45(-67 to 82)
8(-236 to 75)
-31 (-865 to 82)
5(-191 to 69)
-19(-264 to 61)
44(-373 to 93)
19(-47 to 56)
-16(-168 to 50)
-6(-194 to 62)
14(-22 to 39)
27(0 to 47)
reclaimed water of unacceptable quality. Present monitoring methods for several
important parameters take too long to make in-plant process changes before
discharge occurs.
An example of the wide variation of reuse standards is found in comparing
standards for irrigation of golf courses set by the California Department of Health
Services with those of the State of Arizona. California's Title 22 criteria call for an
oxidized, disinfected effluent meeting a bacterial concentration of 22 coliforms per
100 ml. A similar project in Arizona would require only that the effluent contain a
monthly average of no more than 5,000 coliforms per 100 ml.
For nearly all nonpotable reuse activities, the principal health concern has been
the presence of pathogenic organisms and the degree of control needed to reduce
risks. For some types of nonpotable uses, the question of organic substances is also
very important and requires further study.
EPA has recognized the constraints imposed on reuse by the lack of sound
standards. A national conference in July 1980 examined protocol for the
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development of both potable and nonpotable reuse standards.30 Some of the
conclusions reached were:
• A single set of water quality standards should apply at the tap, regardless of
the source of the raw water.
• Additional information is needed for raw water coming from highly
polluted sources in order to treat it properly.
• The most difficult and as yet unanswered question is associated with health
effects. New parameters are needed to deal with organic compounds in
drinking water.
• A broader spectrum of indicator organisms and disinfectants may be
required to deal with pathogens and their toxins.
• Scientists are unable to predict the transport and transformation of metals,
organics, and biological substances in groundwater
Overview of Current Research Efforts
Current research efforts in reuse of municipal wastewater in the U nited States are
a continuation of studies made in the late 1960s and early 1970s. Foreign research
has been concentrated primarily in Israel and South Africa. Research efforts can be
broadly classified as process and nonprocess.
In the 1970s, major EPA-sponsored process research projects included the
experimental plants at Blue Plains, Washington, D.C., Orange County, California
Water Factory 21 and the Los Angeles County Sanitation District's Facility in
Pomona. Other federal agencies involved in reuse are the Department of the
Interior, Office of Water Research and Technology, the Department of Agriculture,
and the Army Corps of Engineers. Many of the reuse projects shown in Table 1 are
furnishing valuable information useful to other municipalities considering reuse.
A major potable-reuse research effort is being sponsored by EPA, OWRTand the
Denver, Colorado Water Board. A 1-mgd advanced wastewater treatment facility
will produce reclaimed water from secondary effluent for possible potable reuse in
the City of Denver. Denver is the only city in the U S. pursuing a major direct
potable-reuse program. However, several other cities have demonstrated or are
planning groundwater recharge with reclaimed water. In some cases, the
groundwater basins are used for potable purposes.
Many of the process-related research projects of the last ten years have resulted
from stringent EPA effluent-discharge regulations. The effluents produced at these
facilities have been of too high a quality simply to dispose of. At many major reuse
facilities, the incentive for reuse was to either avoid high treatment requirements for
discharge or to better utilize a good quality product water.
Nonprocess research has been carried out by many agencies including EPA's
Municipal Environmental Research Laboratory (MERL) and Health Effects
Research Laboratory (HERL). A major MERL effort is a "Guidelines for Water
Reuse" manual that addresses a broad range of the technical, economic, and
institutional issues faced by water/wastewater managers planning municipal
wastewater reuse systems.1 Major health studies have focused on the obvious
hazards associated with bacterial and viral contaminants, as well as on the long-term
effects of small quantities of organic compounds.
OWRT has sponsored several studies dealing with the social, legal and
institutional aspects of reuse. Public acceptance surveys have been conducted in
many areas of the country. Several studies point out that public officials have lagged
behind the public in their assessment of public opinion on reuse.31
Compilation and dissemination of information on reuse activities is carried out by
several agencies including the American Water Works Association (AWWA)
Research Foundation. The Foundation sponsored a water reuse conference in 1979
that attracted over 160 speakers. A three-volume set of conference proceedings is a
274
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significant source of useful information. The research foundation also publishes a
monthly "Municipal Wastewater Reuse News." A major reuse research symposium
held in 1975 was sponsored by EPA, AWWA, and Water Pollution Control
Federation (WPCF). Entitled "Research Needs for Potable Reuse of Municipal
Wastewater" the workshop reached these principal conclusions:
1. The identified needs pertain to indirect Wastewater reuse as well as possible
direct potable reuse.
2. Identification of contaminants in present drinking waters and the
development of optimum control processes is urgent.
3. Development of rapid, sensitive, and accurate testing procedures for all types
of contaminants and establishment of on-line continuous monitoring where
required.
4. Toxicological studies using in vitro screening and testing are of high priority.
5. Long-term health effects of reuse involving retrospective and prospective
epidemiological surveys of populations exposed to indirect Wastewater reuse
should be undertaken as an international collaborative activity.
6. Improved exchange of information relative to reuse is needed here and
abroad. One potential coordinating body in existence is the World Health
Organization International Reference Centre for Community Water Supply
located at The Hague.
Future of Reuse
Municipal reuse planning of the last decade was based on projected long-term
water shortages. Reuse projects can free up for potable use the limited high-quality
sources of water in communities, while assuring sufficient quantities of water of
adequate quality for industrial, agricultural, and other uses. In the future, municipal
water-supply planning will involve the tailoring of different qualities of water to
users' needs.
A study completed in 1979 for OWRT, projected that reuse would undergo a
seven-fold increase by the year 2000, growing from about 680 millions of gallons per
day to 4,750 millions of gallons per day. Reuse would be centered largely in
agricultural irrigation and industrial cooling.
This expected increase in reuse will require closer scrutiny of the impact of organic
materials in reclaimed water. Since potable reuse will only make up a very'small
portion of the total demand, research on organic substances will not receive a large
portion of the research spending. Nevertheless, for those municipalities investigating
potable reuse, health effects of long-term ingestion of small amounts of organic
substances need to be studied carefully. Other types of reuse projects involving high
water-quality standards—such as irrigation of specific food crops—will also
necessitate an examination of organic substances in reclaimed water.
As reuse projects become more common, planning will become more
sophisticated. Because reuse usually involves marketing of reclaimed water, its
success in each case depends on establishing a working relationship between the
water purveyor and potential reclaimed-water user. The future of reuse will be
closely intertwined with the rethinking of the traditional, separate roles of water and
wastewater management and greater attention to institutional matters.
REFERENCES
1. Camp Dresser & McK.ee Inc. "Guidelines for Water Reuse," U.S.
Environmental Protection Agency, Municipal Environmental Research
Laboratory, Cincinnati, OH, EPA 600/8-80-036, August 1980.
2. U.S. General Accounting Office. "Delays in Setting Workplace Standards for
Cancer Causing and Other Dangerous Substances,"Report No. HRD-77-71,
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Washington, D.C., May 10, 1977.
3. Schmidt, C.J. and E.V. Clements, III. "Demonstrated Technology and
Research Needs for Reuse of Municipal Wastewater," EPA 670/2-75-038,
U.S. Environmental Protection Agency, Cincinnati, OH, 1975.
4. Gulp, Wesner, Gulp and M.V. Hughes, Jr. "Water Reuse and Recycling,
Volume 1, Evaluation of Needs and Potential," OWRT/RU-79/1, Office of
Water Research and Technology, U.S. Department of the Interior, 174 pp.,
April 1979.
5. Ling, C.S. "Water Reclamation Facilities Survey Report," Sanitary
Engineering Section, State of California Department of Health Services,
1978.
6. "Wastewater Reclamation Criteria," California Administrative Code, Title
22, Division 4, California Department of Health Services, Sanitary
Engineering Section, Berkeley, CA, 1978.
7. Hrubec, J., J.C. Schippers and B.C.J. Zoeteman. "Studies on Water Reuse in
The Netherlands," In: Proceedings of the Water Reuse Symposium, Volume
2, Amer. Water Works Assn. Research Foundation, Denver, CO, pp. 785-
807, 1979.
8. Idelovitch, E., T. Roth, M. Michail, A. Cohen and R. Friedman. "Dan
Region Project in Israel: From Laboratory Experiments to Full-Scale
Wastewater Reuse," In: Proceedings of the Water Reuse Symposium,
Volume 2, Amer. Water Works Assn. Research Foundation, Denver, CO, pp.
808-829, 1979.
9. Slander, G. "Micro-Organic Compounds in the Water Environment and
Their Impact on the Quality of Potable Water Supplies," Water SA, Volume
6, No. 1, pp. 1-14, January 1980.
10. Garrison, W.E. and R.P. Miele. "Current Trends in Water Reclamation
Technology," Jour, of the Amer. Water Works Assn. 69 (7), pp. 364-369,
1977.
11. Roberts, P.V., P.L. McCarty, M. Reinhard and J. Schreiner. "Organic
Contaminant Behavior during Groundwater Recharge," Journal of the
Water Pollution Control Federation (in press).
12. Smith, J.K., A.J. Englande, N.M. McKownand S.C. Lynch. "Characterization
of Reusable Municipal Wastewater Effluents and Concentration of Organic
Constituents," U.S. Environmental Protection Agency, Municipal
Environmental Research Laboratory, Cincinnati, OH, EPA 600/2-78-016,
February 1978.
13. McCarty, P.L., M. Reinhard and D.C. Argo. "Organic Removal by
Advanced Wastewater Treatment," Proceedings Annual Meeting Amer.
Water Works Assn. Part I, pp. 1-26, 5-3, May 1977.
14. Lennette, E.H. and D.P. Spath. "Health Considerations Associtated with Land
Treatment of Wastewater," Proceedings: International Symposium on Land
Treatment of Wastewater, Volume 1, Published by U.S. Army Cold Regions
Research and Engineering Laboratory, Hanover, NH, August 20-25, 1978.
15. Gulp, R.L. "Selecting Treatment Processes to Meet Water Reuse
Requirements," Municipal Wastewater Reuse News No. 26, pp. 8-12,
November 1979.
16. Jones, R.A. and G.F. Lee. "Chemical Agents of Potential Health Significance
for Land Disposal of Municipal Wastewater Effluents and Sludges,"
Proceedings: Conference on Risk Assessment and Health Effect of Land
Application of Municipal Wastewater and Sludges, B.P. Sagik and C.A.
Sorber eds. Center for Appl. Res. and Techn., University of Texas, San
Antonio, San Antonio, TX, pp. 27-60, December 1977.
17. Englande, A.J., Jr. and R.S. Reimers, III. "Water Reuse-Persistence of
Chemical Pollutants," Proceedings of Water Reuse Symposium, Volume 2,
Amer. Water Works Assn. Research Foundation, Denver, CO, pp. 1368-
1389, 1979.
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18. McCarty, P.L., D.G. Argo and M Reinhard. "Reliability of Advanced
Wastewater Treatment," In: Proceedings of Water Reuse Symposium,
Volume 2, Amer. Water Works Assn. Research Foundation, Denver, CO, pp.
1249-1268, 1979.
19. Brouzes, R.J.P. "The Use of Lime in the Treatment of Municipal
Wastewaters," Ontario Ministry of the Environment, Research Report No.
21, Toronto, Canada, 1972.
20. McCarty, P.L., M. Reinhard, J. Garydon, J. Schreiner, K. Sutherland, T.
Everhart and D. Argo. "Advanced Treatment for Wastewater Reclamation at
Water Factory 21," Technical Report No. 236, Department of Civil
Engineering, Stanford University, Palo Alto, CA, January 1980.
21. DeWalle, F.L. and E.S.K. Chian. "Biological Regeneration of Powdered
Activated Carbon Added to Activated Sludge Units," Water Research,
Volume II, No. 5, p. 439, 1977.
22. Snyder, A.J. and T.A. Alspaugh. "Catalyzed Bio-oxidation and Tertiary
Treatment of Integrated Textile Wastewaters," EPA Report No. 660/2-74-
039, June 1974.
23. Miller, G.W. and R.G. Rice. "European Water Treatment Practices: The
Promise of Biological Activated Carbon," Civil Engineering, p. 81, February
1978.
24. McCarty, P.L. "Organics in Water - An Engineering Challenge," Jour, of the
Environmental Engineering Division ASCE, Volume 106, No. EE1, pp. 1-17,
February 1980.
25. U.S. Department of Commerce. "Reverse Osmosis Renovation of Secondary
Effluent," NTIS PB-293-761, April 1979.
26. New York State Department of Environmental Conservation. "Residual
Chlorine in Water Effluents Resulting from Disinfection," Technical Paper
No. 38, Albany, NY, March 1975.
27. Morris, J.C. "Formation of Halogenated Organics by Chlorination of Water
Supplies," EPA 600/1-75-002, Office of Research and Development, U.S.
Environmental Protection Agency, Washington, D.C., 54 pp., 1978.
28. Barnhart, E.L. and G.R. Campbell. "The Effect of Chlorination on Selected
Organic Chemicals," NTIS Publication No. PB2II 160, 105 pp., 1972.
29. "Oxidation of Refractory Organic Materials by Ozone and UV Light,"
Contract DAA K02-74-C-0239. Report of U.S. Army Mobility Equipment
R&D Center, Ft. Belvoir, VA, 1974.
30. Johnson, C.C. "Water Reuse: An Unfinished Agenda," Environmental
Science and Technology, Vol. 14, No. 11, pp. 1304-1306, November 1980.
31. Johnson, B.B. "Waste Water Reuse and Water Quality Planning in New
England: Attitudes and Adoption," Water Resources Research, Volume 15,
No. 6, pp. 1332-1334, December 1979.
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MANAGEMENT OF RESIDUALS SEPARATED FROM
MUNICIPAL WASTEWATER
Richard I. Dick
INTRODUCTION
A common feature of nearly all processes used for treatment of municipal
wastewaters is that they are intended to remove contaminants in the form of
suspended solids. If a constituent that is to be removed does not exist as particulate
matter, then chemical, physical, or biological means are used to make the
constituent a part of a particle that can be removed by solids-liquid separation
procedures. As a result of this approach, residues from municipal wastewater
treatment consist of suspensions of particles in water. The residue is called "sludge,"
and its management is a major challenge and expense in environmental quality
control.
Contaminants that are associated with settleable solids in the raw wastewater can
be removed in primary sedimentation processes without the need for prior
treatment. In the past, primary sedimentation often was the only form of wastewater
treatment provided for municipal wastewaters. At present, primary treatment
ordinarily is provided prior to more extensive secondary wastewater treatment.
When wastewater constituents that are to be removed do not arrive at the
treatment plant in association with suspended solids, then treatment processes are
used to produce suspended solids containing the contaminant. These solids are then
removed by solids-liquid separation processes (commonly sedimentation) to effect
removal of the contaminant. Chemical precipitation of phosphorus offers an
illustration of a chemical process for converting a soluble material (orthophosphate)
into an insoluble form (typically phosphorus in association with iron, aluminum, or
calcium) so as to achieve removal.
Biological processes for removing organic material from wastewater are, in effect,
techniques for biological precipitation of organic material. While some organic
material is oxidized to carbon dioxide and lost as a gas, the microorganisms in
wastewater treatment processes oxidize only enough organic material to obtain the
energy required to synthesize new microorganisms. These organisms are,
effectively, the precipitated organic material removed from the wastewater. Because
of the usual flocculent nature of the microorganisms, they agglomerate into particles
amenable to separation by sedimentation or filtration.
There are a few exceptions to the usual rule that constituents removed from
municipal wastewaters are removed in the form of suspended solids to yield sludges.
One exception is organic constituents with sufficiently low vapor pressure to be lost
The Author. Richard 1. Dick is the J oseph P Ripley Professor of Engineering at Cornell University in Ithaca,
New York. His teaching and research primarily concern water and wastewater treatment. Previously he was
Professor of Civil Engineering at the University of Delaware and the University of Illinois, and. before
entering academia, he was in consulting engineering practice and with the U S. Public Health Service. He
received a Bachelor's degree 'rom Iowa State University, a Masters degree from the University of Iowa, and a
Ph.D from the University of Illinois.
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to the atmosphere as a gas during the process of treatment. In the case of adsorption
using granular media such as activated carbon, removal occurs by association with a
solid phase, but not in the form of a sludge. Disinfection is another example of a
municipal wastewater treatment process that does not produce a particulate residue.
Other examples are processes (like ion exchange and reverse osmosis) that produce a
concentrated brine—these processes, however, are not in common use in
municipal wastewater treatment.
General Characteristics of Municipal Wastewater Treatment Plant Sludges
All wastewater constituents removed in the course of conventional municipal
wastewater treatment, excepting those lost by oxidation and volatilization, are
contained in wastewater treatment plant sludges. Inasmuch as the ultimate removal
mechanism for these materials is solids-liquid separation, the contaminants exist
primarily in the solid phase. The substantial liquid phase of sludges is derived from
the wastewater, and, hence, sludges also contain materials not preferentially
removed in wastewater treatment. The chemical properties of municipal wastewater
sludges can best be characterized as being heterogeneous and variable. Chemical
characteristics of sludges are highly dependent upon industrial contributions to
municipal wastewater treatment systems. Not all adverse chemical properties of
sludges can be attributed to industries, however1.
Conventionally, municipal wastewater treatment plants have not been designed
specifically to remove toxic organic compounds; they have been designed primarily
to remove suspended solids contained in raw wastewater and organic compounds
that are biologically degradable. Occasionally, especially in recent years, designs
have also been developed to include removal of the nutrients phosphorus and
nitrogen. Additionally, disinfection processes for inactivation of microorganisms
and viruses are common.
While municipal wastewater treatment plants are not currently designed
specifically to remove toxic organic chemicals contained in municipal wastewater,
appreciable removal occurs2. This incidental removal of toxic organic compounds is
caused by mechanisms such as volatilization, adsorption, absorption, and chemical
or biological oxidation (see chapter in the Monograph by Irvine). Aside from those
organic compounds lost by volatilization and oxidation, the organic materials
removed from wastewater would be expected to be contained in the solid phase of
the sludge. It is important to recognize, however, that even if no preferential
association of toxic organic compounds with a solid phase of sludge occurred, toxic
organics in wastewaters would still be included in sludges. This is because the
compounds would be in the liquid phase of the sludge which typically represents
about 95 percent of the volume of sludges generated in primary treatment and about
99 percent of the volume of sludges produced by secondary wastewater treatment.
Sludges from biological wastewater treatment consist principally of excess
organisms synthesized in wastewater treatment. Additionally, a high degree of
removal of the many pathogenic organisms and viruses contained in municipal
wastewaters occurs during wastewater treatment, and, hence, such organisms and
viruses are found in sludges. The organisms in sludge that may be pathogenic to
humans include many species of bacteria, protozoa, and helminths (worms) together
with human viruses.
The physical properties of municipal wastewater treatment plant sludges are
complicated by the fact that the solid phase of sludges is comprised of assemblages of
flocculent particles. Many of the particles in sludges are hydrophilic, and significant
amounts of water are oriented to the surface of the particles. As particle
agglomeration occurs, additional water is entrained. Because of the high water
content of the agglomerated particles, their specific gravity may be barely greater
than one. This leads to poor settleabihty and accounts for the dilute suspended solids
content of typical municipal wastewater treatment plant sludges. A further
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complication of the physical structure of sludges is that when a pressure gradient is
applied in an attempt to remove the excessive moisture in sludges so as to reduce
sludge volume, the loose flocculent structure tends to deform creating a highly
impermeable matrix through which movement of moisture is impeded.
Nature and Magnitude of Municipal Sludge Management
Traditionally, the focus of water pollution control activities has been on the
processes used to treat wastewater. Yet, roughly half of the overall cost of wastewater
management is for management of the residues (sludges) produced by wastewater
treatment. Furthermore, the complexity and cost of responsible residue management
has kept pace with the cost and complexity of processes needed to achieve higher
degrees of water pollution control. When primary wastewater treatment plants are
converted to secondary plants, for example, the mass of residual sludge is nearly
doubled (on a dry solids basis), the volume of wet sludge is more than doubled
(because of the high moisture content of secondary sludges), and the cost of sludge
management is escalated significantly (because of the typical adverse physical
properties of secondary sludge). Similarly, the need for phosphorus removal at some
wastewater treatment plants (to control eutrophication of receiving waters) has
severe implications in sludge management.
Secondary wastewater treatment produces in the order of 0.1 kg of residual solids
(on a dry basis) per capita per day. Regrettably, the sludge does not consist of dry
solids, but, rather, as has been noted, it is a very dilute aqueous suspension of solids.
At a typical average moisture content of about 3 percent by weight (for a mixture of
primary and secondary sludge), the total mass of residual material per person
exceeds 3 kg/day. The relative quantities of treated effluent and sludge and the
comparative costs of wastewater treatment and sludge management are illustrated
further in Fig. 1.
Estimates of total municipal sludge production analyzed by a committee of the
National Academy of Sciences' indicated that current (1982) annual production is
about seven million tons (about 6.3 million metric tons) on a dry solids basis.
Estimated municipal production in 1990 is nearly ten million tons (nine million
metric tons). This represents a twofold increase since 1972.
The opportunities for ultimate discharge of the residues remaining after treatment
of municipal wastewaters are extremely limited. They can be reclaimed or disposed
to air, land, or water-no other alternatives exist. A wide variety of processes
have been developed to treat sludges so as to prepare them for one of these possible
means for reuse or disposal. In later sections of this chapter, processes used in
municipal sludge treatment are described and reclamation and disposal possibilities
are considered in more detail. Because of the focus of this book on organic
compounds (particularly priority pollutants) in water and wastewater, special
attention is given to the fate of organic compounds in processes for municipal
sludge treatment and in the environment. In addition, available information on the
effects of organic compounds in sludge on the performance of sludge treatment
processes and on the environment is reviewed.
CHEMICAL COMPOSITION OF MUNICIPAL SLUDGE
The composition of sludges produced by municipal wastewater treatment is as
complex and diverse as the activities served by the wastewater collection system.
Even in the absence of industrial wastewater discharges, municipal sludges are highly
heterogeneous and difficult to characterize Municipal sludge characteristics
described herein are for "typical" municipalities with some commercial and
industrial activities. lf must be emphasized that the chemical composition of any
particular municipal wastewater sludge could vary widely from the composition
described here.
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Effluent
Sludge
a. Daily Production
Effluent
X X
Sludge
^
b. Potential Annual Accumulation
Effluent
Sludge
c. Cost
Figure 1. Relative quantities of treated effluent and sludge and the comparative costs
of wastewater treatment and sludge management.
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Organic Chemical Constituents of Sludges
Human wastes are a prominent source of wastewater treated at municipal
treatment plants, and the sludge produced by conventional treatment processes
contains large amounts of microbial mass. Accordingly, typical municipal sludges
contain an abundance of common and comparatively inoffensive organic
compounds. Indeed, approximately 70 percent of the solids in typical municipal
sludges are comprised of organic matter.
Some of the major categories of organic compounds in primary and secondary
sludges as reported by Rudolfs and Gehm4 are shown in Table 1. The high protein
content of activated sludge is noteworthy. Furthermore, the essential amino acids are
well representedd in the protein5. In addition to the naturally occurring organic
compounds found in abundance in sludges, trace quantities of growth factors and
vitamins also are present6—indeed, vitamin B,, has been extracted from activated
sludge7.
The naturally occurring organic compounds that constitute the predominant part
of the solid phase of municipal sludges are readily biodegradable, and, hence, could
cause oxygen depletion and odors when sludges are returned to the environment. As
described in sections that follow, biological stabilization is a common method of
sludge treatment to control possible adverse environmental impact from such
organic compounds in sludge. The product of biological stabilization still contains
large amounts of natural organic materials (often about 50 percent by weight of dry
solids), but they degrade far more slowly than the organic compounds in untreated
sludges.
In recent years it has become clear that, in addition to the wide assortment of
naturally occurring organic compounds, typical municipal sludges also contain a
number of synthetic organic chemicals of possible health significance. Limited
available data suggest that many such compounds are ubiquitous in the environment
and should be expected to be present in any municipal sludge, even though there is no
reason to associate the compound with the municipality and, indeed, even if
production of the compound has ceased8.
Limited available data on polychlorinated biphenyl (PCB) concentrations in
municipal sludges serve to illustrate the ubiquity of recalcitrant organic compounds
in municipal sludges. While exceptionally high PCB concentrations have been found
in some municipal sludges (for example, 352 mg/kg of dry solids at Bay City,
Michigan9 and 1,700 mg/kg at Bloomington, Indiana10), some PCB's seem to be
present in all municipal sludges. In their survey of 16 American cities, for example,
Furr, et al." found PCB's even in sludge from a wastewater treatment plant which
received almost entirely domestic wastes. Similarly, Epstein and Chancy reported
that a Michigan Water Resources Commission study showed PCB's in sludge from
each of ten Michigan wastewater treatment plants surveyed (excluding Bay City,
concentrations ranged from 0.1 to 32 mg/ kg). Lawrence and Tosine8, who surveyed
PCB concentrations at wastewater treatment plants in Ontario, noted that effluents
were relatively free of PCB's and that high PCB concentrations in sludges "clearly
indicate the ultimate fate of PCB's in treatment plants."
Data on the presence of other organic priority pollutants in municipal wastewater
sludges are even more limited than PCB data. However, as might be expected, the
data suggest that many organic priority pollutants are typical constituents of
ordinary municipal sludges. Furr, et al.11, for example, found dieldrin to be a
common constituent in sludges from 16 municipal wastewater treatment plants
surveyed, and Jones and Lee12 tabulated reported data on 16 different specific
pesticides indicating that they are common constituents of domestic sludges.
* Lower case superscripts designate notes listed at the end of this chapter
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Table 1. Major Conventional Organic Constituents in Municipal Sludge*
Grease
Primary
Constituent
and fat
Cellulose and hemicellulose
Lignin
Protein
Primary
Sludge
7-35
7
5.8
22-28
Activated
Sludge
3-17
7
**
32-41
*ln percent by weight of dry solids
"Included with cellulose and hemicellulose
DeWalle, et al1 monitored the fate of 36 organic priority pollutants in three
municipal wastewater treatment plants. On the average, about 50 percent removal
occurred in primary treatment and an additional 60 percent removal of remaining
organic material occurred in secondary treatment. Removal was attributed primarily
to association of the trace organics with solids, and, hence, they would be
incorporated in sludges. In related studies, DeWalle and Chian" identified 67
organic priority pollutants at three different treatment plants and detected most of
the compounds in sludge. Even some of the volatile organic compounds were
concentrated in sludge due to adsorption on solids.
Current ability to predict the amounts of organic priority pollutants to be found
in sludge from specific communities is limited. DeWalle, et al1 anticipated that
industrial wastewater pretreatment would be effective in controlling chlorinated
solvents, chlorinated and alkyl benzenes, and phenols, but not phthalates.
Ramanathan, et al14 concluded that it currently is not feasible to predict sludge
characteristics as they relate to toxic organic substances.
Inorganic Chemical Constituents of Sludges
While emphasis in this monograph is on organic chemicals, it is appropriate to
briefly consider major inorganic constituents also. Inorganic constituents influence
sludge treatment, utilization, and disposal practices, and, hence, affect the fate
and behavior of organic constituents in sludges.
Municipal sludges contain appreciable concentrations of the nutrients nitrogen
and phosphorus1". Biological sludges are rich in nutrients, but even mixed primary
and biological municipal sludges may have nitrogen and phosphorus contents in the
order of 3 to 5 percent by weight of dry solids. As compared to commercial inorganic
fertilizers, municipal sludges ordinarily contain inadequate amounts of potassium in
relation to nitrogen and phosphorus. Appreciable solubilization of nitrogen and
phosphorus occurs during anaerobic digestion and, depending upon the amount of
liquid removed after digestion, substantial amounts of the nitrogen and phosphorus
in sludges may be recycled back to the wastewater treatment processes.
Heavy metals are other inorganic constituents of sludges and they are of concern
because of possible entry into the food chain. While industries can be major
contributors of heavy metals and source control and pretreatment can significantly
reduce their concentration in municipal sludges, heavy metals are found even in
purely domestic sludges15.
Median heavy metal concentrations in sludges from 57 wastewater treatment
plants in Michigan16 were: 12 mg/kg cadmium, 380 mg/kg chromium, 700 mg/kg
copper, 52 mg/ kg nickel, 480 mg/ kg lead, and 2,200 mg/ kg zinc (all on a dry solids
basis). Values far from these median concentrations were observed at specific cities,
however.
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SLUDGE TREATMENT PROCESSES
Almost inevitably, sludges produced by municipal wastewater treatment are
treated prior to being reclaimed or disposed of. Appropriately, the type of
treatment provided varies widely depending upon local circumstances. The purpose
of treatment processes is either to remove liquid from sludge or to alter sludge
quality'7. Sludge volume reduction is often associated with both types of treatment.
Some sludge treatment processes in the two treatment categories are described in the
sections that follow.
Processes for Removing Water from Sludge
The moisture content of municipal sludges has a significant effect on the cost and
performance of most processes for sludge treatment, utilization, and disposal. The
performance of an anaerobic digester (see following section), for example, depends
on the length of time organic solids are retained. Thus, if the volume of sludge to be
anaerobically digested is reduced by 50 percent by removal of water, the volume of
the digester can be reduced by the same fraction, and capital costs can be significantly
reduced. Similarly, the cost of facilities for storing and transporting sludge is highly
dependent upon the amount of water with which the sludge solids are associated.
Economical performance of sludge combustion facilities is, in the current energy
market, highly dependent upon extensive removal of water from sludge prior to
incineration.
The extent of sludge volume reduction that can be achieved by processes for
moisture removal is dramatic. For example, if the moisture content of a sludge is
reduced from 99 percent (a common value for waste activated sludge) to 97 percent,
sludge volume is reduced to essentially one-third of the initial value even though
much water still remains in the sludge. As described in the following sections,
removal of water is accomplished by thickening, dewatering, drying, and
dehydrating processes, and the ease with which sludge releases water is improved by
conditioning processes
Thickening—
Thickening processes are those which accomplish significant removal of water
while yielding a product that still flows as a liquid'. Thickeners usually produce
significant reductions in sludge volume at comparatively low cost, and, thus, they are
typical components of municipal sludge management schemes.
Thickening processes are of little consequence with regard to organic compounds
in sludges. Organic compounds are predominately in the solid phase of municipal
sludges, and their concentration is merely increased as a result of thickening. If long
retention times are used (most likely, in gravity thickeners), then some solubilization
of biodegradable organic compounds might occur during thickening, in which case
some organic matter would be recycled back to the wastewater treatment plant with
the liquid removed by thickening.
Thickening of municipal sludges is accomplished by use of gravity, centrifugal, or
flotation thickeners. Gravity thickeners are simply sedimentation basins which
afford the opportunity for suspended solids with a specific gravity greater than that
of water to be concentrated at the bottom and clarified liquid to be removed from the
top. Combined primary and secondary sludges from a municipal wastewater
treatment plant at a concentration of, say, 3 percent solids by weight might thicken to
a suspended solids concentration of about 6 percent solids by weight.
Although centrifuges'1 appear to be quite different than gravity thickeners, they,
in fact, accomplish thickening by the same mechanisms that are involved in gravity
thickening. Centrifugal force replaces the force of gravity as the cause of solids
separation. Centrifuges are, thus, compact, intensive gravity thickeners. A centrifuge
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operated in the thickening mode at a municipal wastewater treatment plant might be
expected to produce a thickened sludge concentration in the general order of 5 to 14
percent solids by weight.
Flotation thickeners also perform like gravity thickeners except that gravitational
forces cause particles to move upward instead of downward because of the reduction
in their specific gravity to a value less than that of the suspending medium—water.
The specific gravity of the flocculent particles in municipal sludges normally is only
slightly greater than that of water because of the large amount of water that they
contain. By associating a gas (logically, air) with the light, flocculent, solids, their
specific gravity can be readily reduced to the extent that flotation occurs. In most
municipal sludge treatment applications, flotation is achieved by aerating water
under pressure, reducing the pressure, and blending the supersaturated water with
sludge to achieve reduction of the specific gravity of the solids. Flotation thickening
has most commonly been applied to waste activated sludge. Typically, activated
sludge enters a flotation thickener at around 1 percent solids by weight and is floated
to a concentration of about 4 to 6 percent solids by weight (which is a much higher
concentration than can be achieved by gravity thickening).
Dewatering—
Dewatering processes are used to achieve a higher degree of removal of water from
sludges than is possible by thickening. The distinguishing characteristic of
dewatering processes is that the product has a moisture content sufficiently low as to
require handling by solids transporting techniques (for example, a conveyor belt) as
opposed to the liquid transporting facilities (for example, pumping and piping) used
to feed the sludge to the dewatering process. It should be recognized, however, that
the water content of sludges fulfilling the definition of a de watered sludge is still quite
high. Municipal sludges containing 80 percent moisture typically display
properties of a solid.
Dewatering of sludges is necessary prior to landfilling to reduce volume and
achieve bearing capacity. It is also essential prior to combustion to limit fuel costs.
Dewatering also may be justified to reduce transportation costs'.
Like thickening, dewatering ordinarily does not affect the organic constituents of
sludges other than by increasing their concentration because of removal of water.
An exception is when sludge has previously been conditioned by heat treatment (see
following section) in which case significant amounts of suspended organic material
are solubilized and recycled back to the wastewater treatment plant.
Techniques used for dewatering municipal sludges vary from comparatively
simple sand drying beds (on which liquid sludge is placed for gravity drainage and
subsequent air drying) to complex mechanical dewatering devices such as vacuum
filters, centrifuges'1, belt filter presses, and pressure filters.
Typically, sludge dewatered by mechanical techniques has a suspended solids
content in the range from slightly less than 20 percent to somewhat more than 30
percent. Because of the large pressure differentials which can be used in pressure
filtration, however, dewatered sludge suspended solids concentrations in the order of
50 percent often can be achieved (albeit at some sacrifice because of extensive
conditioning requirements—see next section). Depending on the advantage which is
taken of possibilities for air drying, sand bed dewatering facilities can be operated to
yield dewatered sludge with a moisture content transcending the entire range of
moisture contents achievable by mechanical sludge dewatering processes.
Conditioning—
The purpose of sludge conditioning processes is to alter the physical properties of
sludges so as to improve dewatering characteristics. None of the mechanical
dewatering processes mentioned in the previous section would ordinarily perform in
an economically satisfactory matter without prior conditioning of sludge.
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Mechanisms involved in sludge conditioning processes are poorly understood.
Based on theoretical concepts of flow through porous media, they must involve
factors such as increased porosity and sludge particle size, and result in increased
permeability and reduced compressibility of sludge. The precise mechanisms by
which these alterations in the physical properties of sludges are achieved remain to be
clarified, however, and assessment of conditioning requirements and effects must
currently be carried out empirically.
Inorganic chemicals have traditionally been the most common means for chemical
conditioning of sludges. In the United States, lime and ferric chloride have been the
chemicals most often used. Additionally, organic conditioning agents (synthetic
anionic, cationic, or nonionic organic polyelectrolytes) are common. Other means of
sludge conditioning include thermal treatment, slow freezing and thawing, and use of
admixtures such as fly ash.
Of the various conditioning techniques, heat treatment is of most concern with
respect to organic compounds in sludge. This conditioning process involves
exposure of sludge to "pressure cooker" conditions (approximately 200° C at about
200 psig for about 30 min). The improvement in sludge physical properties that
occurs as a result of thermal conditioning is gained at the expense of appreciable
solubilization of organic compounds. Garrison, et al18, for example, indicated that
40 percent of the organic material contained in sludge as suspended solids was
solubilized by heat treatment. Typically, heat treatment liquors contain 10,000 to
20,000 mg/L of chemical oxygen demand19. Effective heat treatment, thus, creates a
substantial amount of organic material that requires treatment. Return of the heat
treatment liquor to the wastewater treatment plant from which the sludge originated
significantly increases oxygen requirements and sludge production. And further
iterations result in more sludge production. Additionally, concern has been expressed
about the effects of the nonbiodegradable organic constituents produced by heat
treatment20.
Drying and Dehydration—
Drying occurs following drainage on sand bed dewatering facilities and prior to
burning in combustion facilities. Additionally, heat drying has been used as a
separate process to achieve a high degree of moisture removal from solids. Such
treatment is needed when it is necessary to give sludge a "shelf life" to allow
marketing as an organic fertilizer. Separate heat drying is an energy intensive
process and it is not common at municipal wastewater treatment plants'.
In recent years, means for reducing drying costs by use of solvent extraction21 or
oil immersion22 dehydration have been evaluated. Recently, an extensive study in the
Los Angeles-Orange County Metropolitan Area23 identified oil immersion
dehydration as a potential process for future use in the Los Angeles metropolitan
area.
Processes for Altering Sludge Quality
Some alteration of sludge properties may be necessary to make sludge suitable for
ultimate disposal or reclamation. Processes such as sludge stabilization and
combustion are comparatively common, established means for altering sludge
properties. Other processes such as selective removal or destruction of toxic
materials in sludges or inactivation of organisms and viruses are not currently
common but are undergoing research and development.
Stabilization—
The purpose of sludge stabilization processes is to avoid nuisances that may
accompany decomposition of organic material. Additionally, most stabilization
processes accomplish significant reduction in the number of pathogenic organisms
and viruses contained in the sludge.
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The most common types of sludge stabilization processes involve biological
degradation of organic materials under controlled aerobic or anaerobic conditions.
Chemical stabilization processes have also been used. Chemical stabilization is
achieved by creating conditions under which decomposition cannot occur. That is,
chemical stabilization controls odor nuisances associated with decomposition of
organic materials by preventing decomposition from taking place. In lime
stabilization, the most common chemical sludge stabilization technique,
decomposition of organic materials, is prevented by creating a pH high enough to
slow the microbial breakdown of organics34. It should be emphasized that such high
pH conditions should not be expected to prevail indefinitely in the environment;
ultimately, the pH of lime stabilized sludge will fall and decomposition of organic
materials will occur. Sludge chlorination is another chemical means of stabilization.
In this case, the possibility for creating chlorinated organic compounds (and for
solubilizing heavy metals and organic compounds25) may exist.
Biological stabilization of sludges has most frequently been accomplished using
anaerobic processes. Anaerobic stabilization of sludges may simplistically be
considered to involve two stages of microbial conversion of organic material. In the
first stage, solubilization of complex organic solids occurs with production of
carbon dioxide and simple organic compounds with comparatively low molecular
weight. Then, obligately, anaerobic organisms convert the end product of the first
stage into the final products, carbon dioxide and methane.
It might be noted that, whereas production of fuel from waste organic materials has
become a popular topic in recent years, it has been common practice at municipal
wastewater treatment plants throughout most of the century. Typically, anaerobic
digester gas (which is about two-thirds methane) is used to maintain the temperature
of anaerobic digesters at about 95° F, to fuel internal combustion engines, and/ or to
generate electricity. In the past, flaring of digester gas in excess of that needed for
digester heat was common—especially at small municipal wastewater treatment
facilities. Because of increasing energy costs, it is to be anticipated that flaring will
become less common as the anaerobic digester gas is used to provide an increasing
fraction of the total energy required for wastewater treatment26.
Anaerobic stabilization commonly achieves about 50 percent reduction in the
conventional organic solids contained in sludge. The product is a comparatively
stable material which, while continuing to undergo biological change in the
environment, does so at a rate that does not produce aesthetically objectionable
conditions.
A wide variety of organic compounds can be biologically degraded under
anaerobic conditions. To illustrate, Hovious, et al27 found that all
chromatographically identifiable organic compounds in petrochemical wastewater
were destroyed under anaerobic conditions in a lagoon. Similarly, Healy and Young28
reported that catechol and phenol were degraded anaerobically (following ring
cleavage, products were stoichiometrically fermented to carbon dioxide and
methane). Some organic compounds (notably, some of synthetic origin) are not
degraded anaerobically, however, (or are degraded at very low rates). Iwata and
Gunther29 reported that DDT was metabolized in anaerobic digestion, but PCB
remained intact. Numerous investigators, including Klein'0, have reported that NTA
(nitrilotriacetic acid, an organic detergent builder) is not effectively degraded under
anaerobic conditions8. Another synthetic organic detergent builder, CMSO
(carboxymethyloxysuccinate), has been reported1' to be anaerobically
biodegradable following a period of acclimation.
Synthetic organic compounds in sludge dispatched to anaerobic digesters may
also be of concern because of their toxicity to the organisms responsible for
anaerobic stabilization. In this case, the digested product would contain not
only the offending organic compound but, also, higher concentrations of ordinarily
degradable organic compounds. Benson and Hunter32, for example, found 0.4 mg/ L
of trichloroethane to be inhibitory to anaerobic digestion, and gas production halted
temporarily at 1 mg/ L. Comparable concentrations for trichlorotrifluoroethane
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were 4.8 and 20 mg/ L. English investigators" have found anionic organic detergents
in municipal sludges to inhibit anaerobic sludge stabilization.
Aerobic digestion is similar to anaerobic digestion in that it involves biological
stabilization of degradable organic constituents suspended in liquid sludge. A major
difference between the performance of the two processes is the higher
oxidation-reduction potential of aerobically digested sludge (this is reflected in the
presence of reduced compounds such as ammonium ions and hydrogen sulfide in
anaerobically digested sludge as opposed to their oxidized counterparts, nitrate and
sulfate ions, in aerobically stabilized sludges). Operationally, a major difference
between the two processes is that anaerobic digestion produces energy, while aerobic
stabilization (because of the need for supplying air or oxygen) consumes energy. One
effect of the "energy crisis" has been to reduce the size of a wastewater treatment
facility at which aerobic stabilization may prove advantageous because of
operational convenience.
A third technique for biological stabilization of sludge is composting, which
involves aerobic stabilization of dewatered sludge (or liquid sludge mixed with
sufficient previously composted sludge or other agents to render it, effectively
dewatered). A significant feature of sludge composting is its ability to achieve high
temperatures (55° to 60° C for 5 days34) so as to inactivate microorganisms, viruses,
and helminths contained in sludge11. Higgins, et al35 reported that compost
stabilization processes produce a higher degree of destruction of organic
compounds than do competitive stabilization processes.
Composting of sludge is carried out by one of three basic means:
1. The windrow technique,
2. The static pile technique, and
3. Use of mechanical composting equipment.
These three composting processes differ only in the way that oxygen is supplied to the
aerobic organisms responsible for sludge stabilization. In windrow composting,
oxygen is supplied by periodic mechanical mixing of a long pile (a windrow). Static
pile composting involves use of air compressors to pull air through elongated piles of
dewatered sludge into perforated air headers located at the pile bottom (or to force
air in the opposite direction) Mechanical composting involves use of a variety of
different commercial equipment36—much of it developed in Europe—for assuring
that oxygen is made available to composting sludge masses. A significant feature of
the static pile composting method is that raw (previously unstabilized) sludges can be
composted. Odors produced by turning of windrows preclude practical application
of that technique to unstabilized sludge. The accommodation of unstabilized sludge
in the pile technique of composting is achieved by discharge of gases removed from
the compost pile through stacks of previously composted sludge where odor
reduction occurs. An additional benefit of the pile composting method, if the pile is
properly insulated with a blanket of previously composted sludge, is that all portions
of the composting pile may reach the higher temperatures necessary for inactivation
of organisms.
Combustion—
The combustion is the sludge treatment process that effects the greatest change in
organic compounds--it destroys them. The principal end products of properly
controlled combustion are carbon dioxide, water vapor, and inorganic ash. The high
moisture content of sludge (even after dewatering) and the high organic content of
sludge solids, results in appreciable volume reduction; however, ash and exhaust
scrubber water still remain.
The potential air pollutants in municipal sludge incinerator exhaust include
particulates, sulfur and nitrogen oxides, carbon monoxide, heavy metals, heat, and
organic compounds. On the basis of limited available data', the pollutants of
greatest potential concern in the discharge from properly designed and operated
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incinerators equipped with scrubbers would seem to be lead, cadmium, and organic
compounds.
Thermal decomposition is, of course, a common means for destroying
concentrated hazardous organic wastes; however, temperatures prescribed for such
operations are higher than those usually achieved in municipal sludge incinerators.
Nevertheless, the limited information available suggests that appreciable removal of
organic priority pollutants occurs at the lower temperatures prevailing in municipal
sludge incinerators. For example, an EPA Sewage Sludge Incineration Task Force"
monitored five different pesticides and PCB's in operating municipal fluidized bed
and multiple hearth furnaces and reported that the pesticides and PCB's could not be
detected in ash or scrubber water. The TaskForce inferred that the compounds were
destroyed in the furnace and were absent in the furnace exhaust. In other studies38,
the fate of PCB's and pesticides added to sludges was monitored; 99 percent of the
pesticides and 94 percent of the PCB's were destroyed.
In the United States, sludge combustion using an excess of oxygen (from air) in
multiple hearth or fluidized bed furnaces is most common. Possible advantages of
starved air combustion (pyrolysis) have been investigated. Although pyrolysis of
municipal sludges has been demonstrated at full scale39, the process is not in
continuous full scale use. Another combustion variation is wet air oxidation, in
which high pressure is used to allow combustion of organic compounds while sludge
remains in liquid form'. The largest municipal installation of its kind, in Chicago, has
been taken out of service-presumably because of maintenance problems.
The two most common types of municipal sludge combustion equipment
(multiple hearth furnaces and fluidized bed furnaces) affect organic compounds
differently: gases leaving a fluidized bed furnace are hotter than those leaving a
multiple hearth furnace. Because of the countercurrent flow of sludge and
combustion gases in multiple hearth furnaces, the sludge remains "cool" until its
appreciable water content is evaporated. The evaporation cools the exhaust gas,
increasing the possibility that organic compounds are volatilized and removed from
the furnace unburned. Such organic compounds could be odor-causing substances,
photochemical reactants, or toxic materials. Afterburners can be used to control
organic emissions from multiple hearth furnaces, but at an appreciable cost for fuel.
Zang40 has argued that the operation of municipal multiple hearth sludge
incinerators can be controlled to eliminate the need for afterburners; and 95 percent
destruction of PCB's has been achieved in a multiple hearth furnace with no
afterburning41.
Facilities for sludge combustion are intimately associated with the conditioning
and dewatering processes considered in a previous section. A major cost for energy in
sludge combustion results from evaporation of the large amounts of water associated
with sludge solids. This cost can be reduced by effective dewatering prior to
combustion; however, achieving a high degree of dewatering is also expensive.
In recent years, production of energy through combustion of organic compounds
in municipal sludge has frequently been suggested; however, sludge combustion
facilities usually consume energy1. Future advancements in sludge combustion
might include improved combustion and heat recovery technology and improved
capability for conditioning and dewatering sludges.
Inactivation of Organisms and Viruses—
Inactivation of microorganisms, helminths, and viruses occurs incidentally in
some processes for sludge treatment. Sludge incineration is one obvious example.
Inactivation of organisms and viruses also occurs in thermal conditioning and
certain stabilization and chemical conditioning processes. Biological stabilization
processes operated at mesophilic temperatures can achieve appreciable reduction in
the number of pathogenic organisms and viruses, but not their elimination. A high
degree of destruction of organisms and viruses may be achieved by use of
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thermophilic biological stabilization processes (composting or thermophilic aerobic
or anaerobic digestion).
The use of special processes to inactivate organisms and viruses in sludge prior to
sludge application on land has been suggested in recent years. Whereas anaerobic
digestion (the process most commonly used prior to land application) causes a
dramatic reduction in the population of pathogenic helminths, microorganisms, and
viruses, they may still be present in substantial numbers in sludge being applied to
land42. Epidemiological evidence justifying the need for destruction of organisms in
anaerobically digested sludge prior to land application has not been developed,
however1.
The inactivation processes of principal interest are heat pasteurization, and
radiation using isotopes or high energy electrons. All three of these processes have
been demonstrated at comparatively large scale"1 but none is in common use.
Use of high-energy electrons for inactivation of organisms in sludges may be of
interest from the standpoint of control of organic materials in sludges because of the
possible concurrent alteration of organic molecules resistant to biological
degradation. Similarly, the possibility of using gamma radiation to degrade
persistent organic compounds in waste has been suggested (for example, see
Cappadona, et al.43 and Vollner, et al.44). Trump, et al.45 has demonstrated that
PCB's in water are destroyed using high energy electron dosages far less than those
which would be used to inactivate organisms. However, because such compounds
become associated with organic material such as lipids, their inactivation in
municipal sludges may not be so readily achieved.
Detoxification Processes—
Detoxification processes remove potentially offensive constituents from sludges.
Because detoxification technology is emerging, it is difficult to cite examples of
current applications in municipal sludge management practice. The extent of future
development and implementation of detoxification processes will depend upon:
(1) the degree to which source control and pretreatment serve to modify the quality
of sludges from municipal wastewater treatment, and (2) the stringency of sludge
quality standards necessary to protect environmental quality and human health.
Removal of heavy metals and destruction of toxic organic compounds are the
best illustrations of detoxification processes applicable to municipal sludges.
Attempts at heavy metal removal ordinarily take the form of acidification (to
solubilize heavy metals and permit their extraction by removal of moisture from
sludge). The extraction process can be reasonably effective46, but it leaves behind a
low pH wastewater containing an assortment of heavy metals.
Work on destruction of toxic organic compounds in municipal wastewater
residues has been limited principally to the preliminary work on high energy
electron radiation and radioisotope radiation described in the previous section (and,
of course, sludge combustion is used).
In view of the relatively dilute concentrations of toxic metals and organic
compounds in municipal sludges, detoxification will be expensive (certainly costs
per unit of toxic material removed will be high). It is to be hoped that effective
source control and pretreatment practices will minimize the necessity for
implementing such processes.
ULTIMATE RECLAMATION OR DISPOSAL OF MUNICIPAL
SLUDGES
After treatment, municipal wastewater sludges must either be recycled into
productive use or disposed of as waste materials. The options available for ultimate
reuse or disposal are extremely limited. Reclamation opportunities are scarce, and
disposal is possible only to air, water, and land. Opportunities for environmentally
acceptable disposal of sludge constituents to air are limited (combustion and
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biological stabilization result in some disposal to air, but either of these processes
leaves residues requiring reuse or disposal). Practicable options, thus, are limited to
sludge reclamation or sludge disposal to water or land. These possibilities are
considered in this section.
Reclamation of Sludge Constituents
Municipal sludges are constituted from a remarkably heterogeneous collection of
substances—each contained in comparatively dilute concentration and
frustratingly difficult to separate from the others. To date, this inherent characteristic
has prevented widespread use of reclamation schemes. The only common sludge
reuse scheme which can be cited—application of sludge to agricultural
land—makes use of sludge in heterogeneous form. This practice is considered in a
later section on land application. Some additional, but largely unrealized,
possibilities for reclamation are reviewed here.
The potential attractiveness of reclamation of sludge constituents is illustrated by
Hurwitz's6 estimate that the amino acids in municipal sludge are worth $12,000 to
$25,000 per ton of dry solids (in 1957 dollars). However, the appreciable reward for
amino acid extraction (as well as for extraction of other constituents such as vitamins
and precious metals) awaits development of practicable technology.
Extraction of protein from waste-activated sludge has been considered , but not
carried out at full scale. Use of activated sludge as a component in the diet of a wide
variety of animals has been practiced by certain industries48 which can control sludge
quality, but use of waste-activated sludge as an animal food supplement is not
common in municipal waste management practice.
Development of construction materials, production of activated carbon49,
extraction of metals46, and recovery of waste water treatment chemicals50 have been
considered and, in some instances, carried out. However, as a practical matter, none
of these reclamation practices are in common use.
Ultimate Disposal to Water
Disposal of sludge to fresh water is not a viable option. However, the vastness of
oceans has led to their use in assimilation of wastewater treatment residues. Ocean
discharge of sludge, as currently practiced, is strictly a disposal technique—ocean
disposal systems are not being designed to make productive use of sludge
constituents in mariculture projects.
It was generally considered that ocean dumping of sludges in the United States
was barred after December 31, 1981" by a 1977 amendment51 to the Marine
Protection, Research and Sanctuaries Act. Recently, however, a Federal District
Court judge ruled that statutory conditions in the Marine Protection, Research and
Sanctuaries Act must be considered in determining whether ocean dumping of
sewage sludge unreasonably degrades the marine environment52. Regardless of the
ultimate status of ocean discharge of sludge, the legislation had a major impact on
coastal cities because of the economic attractiveness of ocean dumping, and the
difficulty in identifying environmentally suitable land-based sludge disposal
alternatives that would be accepted by the public. Indeed, much of the municipal
sludge management activity during the 1970's represented efforts by coastal cities to
identify suitable land-based alternatives (and to influence federal ocean dumping
policy). The reader is referred to reports of activities at Philadelphia51'54, New York
City"'56, and Los Angeles21'57 for a perspective on the complexity of management of
residues from large metropolitan areas when a major change in disposal practices is
required".
Some of the potentially adverse effects of ocean discharge of municipal sludge
include toxicity and food chain impacts from heavy metals, unwanted coastal water
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enrichment from nutrients, disease transmission from pathogenic organisms and
viruses, oxygen depletion from biodegradable organics, and toxicity and food chain
impacts from recalcitrant organic compounds. In accordance with the focus of this
Monograph, the last two possible effects are considered briefly here.
Control of oxygen depletion m ocean waters due to the presence of biodegradable
organic materials in municipal sludges is not a substantial technical challenge1".
Traditionally, municipal sludges have been biologically stabilized prior to ocean
discharge so that their oxygen-demanding capability is diminished. Dispersal serves
as an additional means to avoid high local demand for oxygen.
As with other techniques for sludge utilization and disposal, organic priority
pollutants in sludge may present a more formidable challenge. Control of such
organic compounds at their source would represent the preferred solution. The
degree of structuring of marine food webs was studied by Young and Mearns58 so as
to anticipate the extent of food chain biomagnification. Heavy metal
biomagnification did not occur because of absence of adequate structure in the
food chain, but the sum of DDT and a PCB did increase with trophic
level—presumably because of the long biological half-life of these constituents.
Similar results have been reported by Bascom59.
In addition to long-term food chain effects, organic priority pollutants also may
produce effects in marine environments of more immediate nature. Fin erosion in
bottom-dwelling fish has been reported in areas of waste discharge off both the
Atlantic and Pacific coasts of the United States as well as elsewhere in the world. It
has been hypothesized6"'61 that exposure to PCB's in bottom sediments may be a
factor in development of fin rotq. Another possibility for short-term effects arises
from the influence of toxic organic substances on aquatic food chains. O'Connor,
et al62, for example, suggested that PCB's reduced the size of marine phytoplankton
and, thus, might affect the amount of harvestable fish.
Ultimate Disposal to the Land
As a practical matter, the most residues produced by wastewater treatment
currently must be placed on land. This is necessitated by the absence of suitable
technology for recycling sludge constituents, and by controls on the disposal of
wastewater treatment residues elsewhere in the environment
Strategies in placing sludge on land involve either concentration or dispersal of
residues. If the concentration strategy is used, the result is some form of lagoon or
landfill. The dispersal strategy involves use of liquid, dewatered, or dried sludge in
agriculture or silviculture, application to park land, use in rehabilitation of spoils or
low quality land, and similar practices in which sludge is widely distributed on land.
Even though most attention is given to application of sludge on agricultural land,
more municipal wastewater treatment residues currently end up in lagoons or
landfills. Farrell61 estimated that 40 percent of municipal sludge was being placed in
landfills in 1972 and that the fraction would remain the same in 1985. Beneficial
utilization on land was estimated to account for 20 percent of municipal sludge in
1972 and 25 percent in 1985. (Corresponding 1972 and 1985 estimates for
incineration were 25 percent and 35 percent while ocean discharge was estimated at
15 percent and 0 percent, respectively.)
Landfills for municipal wastewater treatment sludge vary widely. The nature of
preceding wastewater and sludge treatment, the extent of inclusion of other waste
materials, climatic conditions, and soil and hydrogeological characteristics, all can
vary widely from one sludge landfill to another. Common characteristics of
municipal sludge landfills are that: (1) decomposition of biodegradable organic
compounds occurs (ordinarily under anaerobic conditions with the production of
methane gas which may be collected more commonly in the future), (2) depending on
local conditions, production of a leachate containing high concentrations of organic
compounds may occur, and (3) they fill in time. Readers are referred to the U.S. EPA
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Process Design Manual on Municipal Sludge Landfills64 for details on variations in
municipal sludge landfill practices.
Recent interest has focused on municipal solid-waste landfills and industrial
hazardous-waste landfills. Little attention has been given to purely municipal sludge
landfills (which generally differ only in degree from the other two types). Design and
operational aspects such as gas generation and capture, leachate production and
treatment, and avoidance of groundwater contamination and similar. Readers are
referred to Chapter 14 of this Monograph for additional information.
The past decade produced dramatic changes in practices concerning application of
municipal wastewater sludges to agricultural land. Use of sludge on agricultural
land was commonplace at both the beginning and end of the 1970's, but the
philosophy guiding application practices changed. At the beginning of the decade,
the focus was on sludge disposal with a tacit goal of maximizing the areal rate of
sludge application. By the end of the decade, the focus had shifted to environmental
quality control and health protection with a goal of restricting sludge application
rates to levels necessary to control water, land, and crop quality'.
The beneficial effects of application of controlled amounts of "clean" municipal
wastewater sludge to agricultural land are generally not debated. However,
differences in views are pronounced regarding the potential detrimental effects of
municipal sludge application of land. The principal potential detrimental effects are:
1. Disease transmission by pathogenic organisms.
• 2. Plant toxicity due to phosphorus.
3. Infant methemoglobinemia (particularly via ground water) or surface water
enrichment due to nitrogen compounds.
4. Phytotoxicity due to salts.
5. Phytotoxicity or food chain effects of heavy metals.
6. Toxicity to soil organisms and food chain effects of persistent organic materials.
In this chapter, these potential detrimental effects are considered only in the context
of organic compounds in sludge. For more comprehensive discussion, readers are
referred to documents such as the EPA Technical Bulletin65, a bulletin prepared by
committees of the North Central and Western State Agricultural Experiment
Stations66, and the appraisal of heavy metal hazards by the Council for Agricultural
Science and Technology67.
The high concentrations of "ordinary" organic compounds contained in municipal
wastewater sludges (even after biological stabilization processes) significantly affect
soil properties and the fate of other sludge constituents in the soil. Organic
compounds in soil can play major roles in preventing plant uptake and migration of
heavy metals because of factors such as low solubility of metal complex species,
adsorption of the metal complexes on soil, and reduced coulombic repulsion68. Some
organic compounds, on the other hand, can serve to mobilize heavy metals because
of formation of soluble complexes. The synthetic organic detergent builder, NTA, is
an example68'69. Furthermore, biodegradation of organic compounds that inhibit
heavy metal adsorption might be slowed when they are associated with heavy
metals70.
"Ordinary" organic compounds from sludge applied to soil might also influence
the fate of synthetic organic chemicals in the same sludge. Terry, et al.71 have
reported that the rapid decomposition of sludge organics following sludge
application to land exerts a "priming"effect which hastens the rate of degradation of
other organics in the soil. Indeed, Ward, et al.72 and others have considered the
possibility of detoxifying herbicide wastes by applying them to soil enriched with
sewage sludge8. Cometabolism of synthetic organic compounds has been discussed
in detail by Alexander .
In addition to adsorption and microbial degradation, the fate of synthetic organic
substances added to soil in sludge may be affected by photodecomposition, chemical
reactions, volatilization, plant uptake, leachate and runoff74. The relative
significance of these various mechanisms depends on the nature of the synthetic
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organic compounds, soil type, soil organic content, soil moisture, temperature,
nutrient availability, and other factors. The combined influence of the various
mechanisms is reflected in data such as those presented by Nash and Harris75 on the
persistence of nine different chlorinated hydrocarbon insecticides in soils. After 16
years, they found that 7 to 49 percent of the original amounts was still present.
Data on the persistence of synthetic organic compounds other than insecticides,
pesticides, and herbicides are limited. Polybrominated biphenyl (PBB—the
contaminant in Michigan animal feed76 has been found to have low biodegradability,
low volatility, low solubility, low plant uptake, and therefore, is a "rather permanent
component of contaminated soils"77. PCB's have been found to resist leaching by
water, and their volatility is reduced by adsorption to soil and soil organics78. Slightly
chlorinated PCB's are more readily biodegraded than highly chlorinated PCB's, and
nearly all PCB's apparently taken up by carrots were associated with the peel29. In an
incident where high concentrations of PCB's were contained in municipal sewage
sludge applied to soil79, PCB's were detected in cow's milk (at 5 ppm) and in
vegetables. The detergent builder, NTA, was nearly completely degraded in aerobic
soil but not at all in anaerobic soil80, while another builder, CMOS
(carboxymethyloxysuccinate), was degraded in aerobic soils following acclimation81.
Very little information is available about the possible long-term health risks
associated with those synthetic organic compounds that do persist in soils. Majeti
and Clark82 recently called for more research on crop uptake of organic chemicals
and on pertinent chronic human health effects.
FUTURE DEVELOPMENTS IN MUNICIPAL SLUDGE
MANAGEMENT
Traditionally, sludge management has received a disproportionately small share
of the attention of those concerned with the development of technology for waste
management. There is, therefore, a corresponding need for research and
development to improve current capabilities to effectively manage wastewater
residues, to reduce the costs of residue management, and to control the
environmental effects of residues. Particular attention is given here to desirable
developments that concern organic compounds in sludges. First, however, a few of
the major needs associated with other aspects of sludge management are reviewed.
Desirable Overall Municipal Sludge Management Developments
Present municipal sludge management practices evolved primarily on an empirical
basis. Basic understanding of sludge management has lagged behind the actual
development of sludge management techniques. Elaboration of the fundamental
factors that control the physical, chemical, and biological properties of sludges, the
fundamental mechanisms that determine the performance of sludge management
processes, and the fundamental concepts that establish the environmental effects of
sludge management practices remains, for the most part, to be completed.
Improved understanding of the basic chemical, physical, and biological properties
of sludge is essential for future improvements in sludge management capabilities.
Such understanding could lead to source control and pretreatment practices and to
wastewater treatment systems that would yield sludges with properties most suitable
for economical and environmentally acceptable treatment, utilization, and disposal
procedures.
Major reduction in the cost of municipal sludge management might be achieved
by improved understanding of the physical properties of sludges, the factors that
influence these properties, and the ways by which they can be altered. The need for
these improvements may be appreciated by considering that, at present, some
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municipal sludges from biological wastewater treatment cannot be concentrated by
gravity sedimentation to a suspended solids concentration as high as one percent by
weight. Dewatering of such sludges is exceedingly difficult and expensive. Yet,
removal of moisture is essential prior to combustion, landfilling, and composting
and can significantly reduce the cost of other processes such as digestion,
transportation, and storage.
An improved understanding of the chemical properties of sludges is needed to
permit effective control of chemical constituents of concern and to provide a basis
for controlled alteration of the physical properties of sludges. Similarly, improved
understanding of the biological characteristics of sludges could provide a basis for
controlling and enhancing chemical and physical properties. For example,
development of capability to optimally regulate bioflocculation could significantly
reduce current sludge management costs, and improved ability to control the
microbial uptake, utilization, or release of wastewater and sludge constituents could
extend current sludge management capabilities.
There is need to move from the current empirical approaches for designing and
operating sludge treatment processes to approaches that derive from basic
understanding of the fundamental mechanisms involved. This requires further
knowledge of the influence of the physical, chemical, and biological properties of
sludges on the performance of each of the various sludge treatment processes. The
efforts to regulate sludge properties described in previous paragraphs should be
linked to advancements in treatment process design to achieve economical and
effective processing. An important by-product of such research could be
fundamental insights that lead to sludge management processes that are unimagined
currently.
On the basis of resource, energy, and environmental considerations, it seems
inevitable that present emphasis on sludge "disposal" must, in the future, be replaced
by emphasis on sludge "reclamation." Ideally, the advances in basic understanding of
sludge properties and treatment processes considered in previous paragraphs would,
in time, lead to an ability to economically sort sludges into their basic constituents for
recycle into the productive cycles. Practically, even something short of this goal
would be a vast improvement over current practices.
Capabilities for returning unreclaimed sludge constituents to the environment
must be improved. This requires a much more rigorous understanding of the fate and
effects of sludge constituents in soils and in oceans. These advances, too, must be tied
to work on basic properties of sludges and basic performance of treatment processes
in order to achieve overall integrated waste management schemes that are effective
and economical.
Sludge management inevitably involves risks. Abilities to monitor and assess risks
of alternative approaches need to be improved. This should come, in part, from the
advances in basic understanding advocated earlier.
Sludge management also inevitably involves people. As in the past,
implementation of future developments in municipal sludge management will be
influenced by perceptions of the public. Improved ability to control, predict, and
monitor the performance of sludge management schemes should aid in achieving
public acceptance, but it would seem that more attention also is due in the future to
the social aspects of residue management.
Desirable Future Developments Concerning Organic Compounds in Sludge
A major step in controlling the environmental impact of organic compounds in
municipal wastewater treatment residues is to control the quality of the wastewater
from which the residues originate. Source control and pretreatment of industrial
discharges is a necessary, but, probably, inadequate means of sludge quality control.
Modification of product formulations and, even, lifestyles may, in the long-term, be
warranted.
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Improved understanding is needed of the mechanisms that control incorporation
of organic compounds into the various residues produced at municipal wastewater
treatment facilities. Such knowledge could lead to ability to control the
concentration of organic compounds in various residues. Perhaps, for example,
most synthetic organic compounds could be caused to accumulate in a single
residue—possibly skimmings. Alternatively, improved understanding of basic
removal mechanisms might lead to development of new wastewater treatment
practices to effect selective removal of organic compounds of concern.
Improved understanding is needed of the mechanisms of biodegradation of
synthetic organic compounds in wastewater treatment processes, in sludge
management processes, and in the environment. Process modifications needed to
maximize degradation could then be implemented, and economical means for
altering organic compounds to achieve biodegradation could be sought.
The fate of synthetic organic compounds in each sludge management process
needs to be determined and related to fundamental causative mechanisms. Because
the fate of organics in many sludge treatment processes is established by their
distribution between the liquid and solid phases of sludge, more information is
needed on the nature of the association of specific organic molecules with the solid
phase of sludge and the influence of various processing techniques on that
association. Loss of organics to the gaseous phase (perhaps during vacuum filtration)
also must be explored.
Based on fundamental knowledge of the fate of organic compounds in sludge
treatment processes, opportunities for removing or destroying them can be sought.
Given the ubiquitous distribution of some recalcitrant organics and their tendency to
concentrate in wastewater sludges, effective sludge treatment might represent an
opportunity to extract some organic compounds from the environment.
The possible role of organic compounds in influencing the removal or behavior of
other sludge constituents must be determined. Mobilization of metals through
formation of soluble complexes and interference with biological degradation of
ordinary organic compounds are examples.
The fate and effects of organic compounds in sludges in reclamation and disposal
schemes must be more clearly established. Mechanisms that control the destruction
and transport of organic compounds in soils and oceans must be understood so that
discharge to the environment can be accomplished in a fashion that minimizes risk.
Similarly, biological uptake of organic compounds from sludge and food chain
implications in terrestrial and marine environments must be understood more
completely so as to minimize their adverse environmental and health effects.
NOTES
Some have noted that sulfur-containing amino acids are deficient—a problem
common to single cell protein production also.
Some emphasize the low nutrient content of municipal sludges. They use
commercial inorganic fertilizers as their basis for comparison. In making
beneficial use of sludge nutrients in agriculture, sufficient sludge can be applied
to achieve the desired amount of nitrogen or phosphorus. Limitations on areal
rates of application of nutrients, in fact, ordinarily limit the amount of sludge
that can be applied to land.
Even at very dilute concentrations81, sludges display complex flow behavior.
Because of the existence of a solid phase, sludges behave as pseudoplastic or
plastic materials84 Additionally, sludges usually are thixotropic*5—that is,
their flow properties change with time. These characteristics of sludges lead to
complexities in design of pumping and piping installations as well as in effective
design and operation of other facilities whose performance depends on the
rheological properties of sludges.
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d. Note that centrifuges can be used as either thickening or dewatering devices. In
the former case, they produce a product displaying fluid-like properties, while in
the latter, the product must be moved by solids-handling techniques.
e. Note, however, that dewatering of sludge is expensive. Significant
transportation distances ordinarily are required to justify dewatering solely on
the basis of transportation cost reduction. Illustrative calculations by the
author86, for example, indicate that dewatering is not economically justified in
municipal sludge management schemes involving less than 20 mile transport to
agricultural land disposal sites regardless of the size of the wastewater treatment
facility.
f. Indeed, the Metropolitan Sanitary District of Greater Chicago recently has
announced its intention of closing down heat drying facilities.
g. In contrast, Moore and Barth87 did obtain NTA degradation under anaerobic
conditions. Biodegradation occurred only when the digester received aerobic
biological sludge that was acclimated to NTA.
h. It should be noted, however, that maintenance of thermophilic temperatures
without energy addition also is possible in anaerobic and aerobic digestion
processes. In anaerobic digestion, thermophilic temperatures are achieved by
burning methane gas produced in the course of digestion. The process is rare in
the United States (although it has been demonstrated at full scale88) but
comparatively common in the Soviet Union89. Thermophilic conditions in
aerobic digestion processes can be achieved by use of pure oxygen90 to reduce
the loss of heat associated with the large volumes of nitrogen involved when air is
used as the oxygen source or, as demonstrated recently by Jewell and Kabrick91,
by use of a self-aspirating mechanical aerator that avoids excessive heat loss.
i. Information on municipal sludge incineration emissions has been summarized
by Farrell and Salotto", Shen92 and Helfand93.
j. Wet air oxidation of sludge involves the same type of equipment as is used in
some thermal conditioning processes. A continuum of temperature and pressure
conditions exists with higher degrees of oxidation of organic compounds
occurring as temperature and pressure are increased. Like thermal conditioning,
wet air oxidation produces a liquid phase with high concentrations of dissolved
organic materials94.
k. Note that accounting practices for determining energy balances in sludge
combustion processes are not highly developed. Reports of self-sustaining
sludge combustion often exclude related primary or secondary energy
consumption. An example of a related, but usually excluded, energy
requirement is the increased energy needed to supply oxygen in biological
treatment processes that receive recycled liquid containing high concentrations
of organic compounds from thermal conditioning processes used to achieve
highly dewatered sludge prior to combustion. Another illustration is the
coincineration of sludge and municipal refuse. Self-sustaining combustion is
readily achieved by coincineration, but inclusion of sludge reduces net power
production from refuse combustion or decreases the amount of available RDF
(refuse derived fuel).
1. As noted by Love, et al.95 epidemiologic data on enteric diseases are
notoriously poor, however. For reviews of the potential significance of sludge
in disease transmission, see Hays96 and Love, et al.95.
m. Developments related to sludge pasteurization were reviewed by Stern97. Use of
high energy electron radiation at a 380 m3/ day facility at the Boston Deer Island
Wastewater Treatment Plant was described by Trump, et al.98. Experiences at
the Abwasserverband Ampergruppe in Geiselbullach in the Federal Republic of
Germany with a 30 m3/ day facility for sludge radiation using 60Co were reviewed
by Lessel, et al.99. Brandon100 summarized related work at Sandia Laboratories
as a part of research on beneficial use of radioisotopes from nuclear reactor
wastes.
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Ocean dumping was, effectively, previously banned by the criteria for issuing
ocean dumping permits developed under provisions of 1972 Marine Protection,
Research, and Sanctuaries Act and the Federal Water Pollution Control Act.
Thus, planning to develop land-based alternatives to ocean dumping was
underway before the 1977 legislation was inacted.
The banning of ocean discharge of municipal wastewater sludge has been—and
continues to be—a controversial environmental issue. Thus, the General
Accounting Office101 noted that proposed alternatives to ocean disposal may be
more environmentally harmful, two committees of the National Academy of
Sciences3-102 advised against protection of one segment of the environment
without consideration of impacts on other segments, and, on the basis of studies
of the effect of sludge management, the Commission on the Coastal Water
Research Project103 considered that the public opinion on the ocean "has been
mislead by the press, television, and 'documentary' motion pictures."
Bascom104, quoting Isaacs, noted that a single marine species, anchovies, off the
southern California coast produces ten times as much fecal matter daily as does
the population of Los Angeles.
Indeed, Sherwood61 has suggested the use of fin erosion in bottom-dwelling fish
as an environmental indicator.
The 1970's also brought new terminology, with Sands receiving sludge according
to the former philosophy now being termed "dedicated" lands.
On the other hand, soil organics may reduce the biodegradability of adsorbed
persistent organic compounds105.
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REDUCTION OF ORGANICS BY BIOLOGICAL
TREATMENT
Raymond C. Loehr, PhD
INTRODUCTION
General
Biological waste treatment processes utilize microorganisms to remove organic
substances from wastewaters and thereby minimize pollution of the nation's water
resource and help meet the nation's water quality objectives.
The majority of biological processes used to produce a well treated effluent are
aerobic in nature. Examples include activated sludge units, trickling filters, aerated
lagoons, oxidation ditches, and rotating biological contacting units. This chapter
describes only the major aerobic processes, the factors affecting removal of organics,
and typical process performance.
Anaerobic biological treatment processes are not discussed. While such
processes can be a component of a waste treatment system, they rarely are used as the
major biological process to produce an effluent suitable for direct discharge to
surface waters.
The intent of this chapter is to identify the type and quantity of organics that can be
removed from untreated industrial as well as municipal wastewater by typical
aerobic biological treatment processes. The chapter is not intended to be a detailed
guide to design and operation of biological wastewater treatment systems, but
rather, to provide the general scientist a basis for improved understanding of the
processes most commonly used in treating wastewaters.
Treatment Systems
Biological treatment processes normally are one component of a total treatment
system (Fig. 1). Preliminary and primary treatment processes remove the large and
settleable solids while biological treatment processes remove soluble compounds
and solids not removed in the primary processes. A secondary clarification unit
generally is part of the biological treatment process. The secondary clarification unit
removes the microorganisms discharged from the biological processes before the
treated effluent is discharged either directly to receiving waters or to additional
treatment processes. With many biological treatment processes, part of the microbial
The Author- Dr Raymond C Loehr received his Bachelor of Science and Master of Science degrees in civil
and sanitary engineering at Case Institute of Technology, and his Ph.D. in sanitary engineering at the
University of Wisconsin. His research has been directed mainly to problems of pollution control in
industry with emphasis on the special problems in agriculture He has authored many technical papers and
books on agricultural wastes and wastewater treatment, has served on numerous committees of the National
Academy of Sciences — National Research Council and professional organizations, and serves as a
consultant to Federal and State agencies and private organizations He is at present Professor of Agricultural
Engineering and Environmental Engineering, Cornell University
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Untreated
Wastewater
Preliminary
Processes
Primary
Treatment
Processes
Biological
Treatment
Processes
Sludge Handling,
Treatment and
Disposal Processes
Secondary
Clarification
Processes
•*•
Additional
Processes
As Needed
Treated
Effluent
1
1
1
1
|
Figure 1. Typical components of a wastewater treatment system.
-------
solids removed in the secondary clarification processes are returned to the biological
unit to enhance its performance.
Depending upon the degree of treatment needed, additional processes such as
chlorination to reduce the bacterial concentration, filters to reduce the suspended
solids concentration, activated carbon to adsorb soluble organics, and chemical
precipitation to remove phosphorus may be needed after biological treatment. The
primary treatment processes, the biological treatment processes, and some of the
additional processes produce solids, known as sludge, that must be handled, treated
and processed prior to disposal.
Treatment systems are designed and operated to achieve certain goals: either
specific effluent limitations or water quality criteria (Fig. 2). The effluent
limitations represent those limits felt to be practically and economically achievable
by available treatment technology. Water quality criteria are established to
safeguard uses of the receiving water, such as public water supply, recreation, or
irrigation. These goals are established by federal and state agencies.
Legislation
The objectives of wastewater treatment that are to be achieved by municipalities
and industries have been established by federal legislation. The Federal Water
Pollution Control Act Amendments of 1972 required industrial dischargers to meet
effluent limitations that can be achieved by the application of the best practicable
control technology currently available (BPT) and the best available technology
economically achievable (BAT). Pretreatment standards were to be developed for
industrial wastes discharged to publicly owned treatment works (POTW). The Act
also imposed an obligation on the Environmental Protection Agency (EPA) to
promulgate regulations restricting the discharge of toxic chemicals.
EPA was unable to meet all of the deadlines set by the Act and was sued by
environmental groups. The settlement agreement of the lawsuit required EPA to
develop a program for promulgating BAT effluent limitations guidelines,
pretreatment standards, and new source performance standards for 65 "priority"
pollutants and classes of pollutants. The 65 classes were expanded to a list of 129
specific pollutants. These can be grouped into ten categories (Table 1).
The Clean Water Act of 1977 (CWA) incorporated the basic elements of the
settlement agreement for toxic pollutant control. As a result, pollutants were
grouped into three broad categories:
• conventional — biochemical oxygen demand (BOD), total suspended solids
(TSS), fecal coliform bacteria, pH, and oil and grease (O&G)
• toxic — the priority pollutants and others that may be so identified by EPA
• nonconventional — all others which are not specifically listed as either toxic or
conventional; examples include color, chlorides, total dissolved solids, and
turbidity.
The effluent limitations guidelines and standards that EPA identifies for each
industry are technology-based. Treatment processes that are capable of controlling
the pollutants from an industry are identified and the long-term performance,
operational limitations and reliability of each process are assessed. The costs
associated with such control methods also are evaluated. After consideration of these
and other factors, specific control and treatment technologies are identified as
applicable to an industry and effluent limitations and standards are specified for each
industrial subcategory. The regulations do not require the installation of a particular
technology. It is the responsibility of each industry to use or develop technology
that can economically achieve the effluent limitations.
Where technically and economically possible, effluent limitations can reflect the
complete control of wastewaters and thereby achieve the ultimate goal of
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Untreated
Industrial or
Municipal
Wastewater
Waste
Treatment
System
Treatment Goals
-Effluent Limitations
-Water Quality
Criteria
Treated J j
;,., ..^. .
>V 1
7^
(Deceiving
Stream
or Lake
Figure 2. A waste treatment system is designed and operated to achieve specific goals
"no-discharge" of pollutants from an industry. This can be accomplished by recycle
and reuse, by "dry" waste cleanup procedures, or by controlled land treatment of the
wastewaters.
Thus, there is an emphasis on the control of conventional, nonconventional, and
toxic organic pollutants in industrial wastewaters. Efforts are underway to quantify
the removal of all types of organics, especially the organic priority pollutants, in
various treatment processes and systems.
ORGANIC POLLUTANTS
General
There are many organic compounds in wastewater and many methods are used to
identify and quantify them, and to remove them. Organic compounds are composed
of carbon in combination with one or more elements, generally hydrogen and
oxygen. Other elements commonly found in naturally occurring organic compounds
are nitrogen, phosphorus, sulfur, and iron. The principal natural organic
compounds found in wastewater are proteins, carbohydrates and fats and oils (also
known as oils and greases). Synthetic organic compounds containing halogens,
metals and many other elements also can be found in wastewaters. Generally,
nonspecific methods are used to measure these organic compounds in wastewaters
(Table 2).
Biodegradable Organics
The most widely used parameter to measure organic matter in wastewater is the
five-day biochemical oxygen demand (BOD5). BOD is the amount of oxygen
required by bacteria while decomposing organic matter under aerobic conditions.
The BOD test is a bioassay which measures the oxygen consumed by living
organisms as they metabolize the organic matter in a waste. The test is conducted
under conditions as similar as possible to those that may occur in nature.
Theoretically, infinite time is required for complete oxidation of the organic matter.
Normally, a large fraction of the oxidation takes place in five days. Standard
conditions of 20° C incubation and 5 days are used to assure consistent and
reproducible results.
The BOD test is the only available test that provides a reasonable measure of the
impact that untreated and treated wastewaters will have on the dissolved oxygen
resources of surface waters. The microorganisms used in the test are the link between
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Table 1. Categories of the Priority Pollutants1
Pollutant
Remarks
Pesticides
Polychlorinated biphenyls
(PCBs)
Metals and other inorganics
Halogenated aliphatics
Ethers
Phthalate esters
Monocyclic aromatics
(excluding phenols,
cresols, and phthalates)
Phenols
Polycyclic aromatic
hydrocarbons
Nitrosoamines
Generally chlorinated hydrocarbons — concentrated
through the food chain
Used in electrical capacitors and transformers,
paints, plastics, insecticides, other industrial
products — fat soluble, subject to biomagnification
Antimony, arsenic, beryllium, cadmium, copper,
lead, mercury, nickel, selenium, silver, thallium,
zinc, asbestos, cyanide — not biodegradable,
persistent, some subject to biomagnification
Used in fire extinguishers, refrigerants, propellents,
pesticides, solvents, and in dry cleaning — can cause
damage to central nervous system and liver
Used mainly as solvents for polymer plastics —
carcinogens
Used chiefly in production of polyvinyl chloride and
thermoplastics as plasticizers — can be teratogenic
and mutagenic in low concentrations
Used in the manufacture of other chemicals,
explosives, dyes and pigments, and in solvents,
fungicides, and herbicides — central nervous
system depressant, can damage liver and kidneys
Chiefly used as chemical intermediates in the
production of synthetic polymers, dyestuffs,
pigments, pesticides and herbicides — toxicity
increases with degree of chlonnation, can impart
objectionable odors and taste to drinking water
Used as dyestuffs, chemical intermediates,
pesticides, herbicides, motor fuels, and oils —
carcinogenic in animals, indirectly linked to cancer
in humans
Used in the production of organic chemicals and
rubber — tests with laboratory animals indicate
nitrosoamines are some of the most potent
carcinogens
the laboratory BOD test and the real environment. BOD data are used routinely to
measure the pollutional strength of wastewaters, to design biological treatment
processes, and to measure the performance of treatment facilities.
Chemical oxygen demand (COD) is a chemical oxidation test used to estimate the
total oxygen demand of a wastewater. The test is based on the fact that with few
exceptions, organic compounds can be oxidized by strong oxidizing agents under
acid conditions with the assistance of inorganic catalysts. The COD test measures
biodegradable organic compounds, organics resistant to biological oxidation, and
those that may be toxic to the organisms used in the BOD test.
The total organic carbon (TOC) and total oxygen demand (TOD) tests also
measure organic matter in wastewater. The TOC is measured by oxidizing organic
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Table 2. Methods for the Identification of Organics in Wastewater
Method
Remarks
BOD
Biochemical oxygen demand
COD
Chemical oxygen demand
TOC
Total organic carbon
TOD
Total oxygen demand
GC, HPLC, MS
Gas chromatography,
High performance liquid
chromatogaphy, Mass
spectroscopy
— measures biodegradable compounds under
conditions related to those that occur in nature
— measures the oxygen required by bacteria as they
metabolize the organics
— a general, nonspecific parameter
— estimates the oxygen demand using strong
oxidizing agents, acid conditions, and certain
inorganic catalysts
— measures biodegradable and many
nonbiodegradable organic compounds
— a general, nonspecific parameter
—measures organic carbon using high
temperatures and a catalyst
— measures the oxygen required for the combustion
of organics at high temperatures and with a
catalyst
— identifies specific organic compounds including
those that are nonbiodegradable and that have
been designated as priority pollutants
carbon to carbon dioxide in a high-temperature furnace in the presence of a catalyst.
The TOD test measures the amount of oxygen required for the combustion of
impurities in an aqueous sample at a high temperature using a platinum catalyst. The
oxygen demand of carbon, hydrogen, nitrogen, and sulfur in a wastewater sample
can be measured by the TOD test.
BOD, COD, and TOD utilize an oxygen approach while TOC uses a carbon
approach to the measurement of organics in wastewaters. There are no fundamental
correlations among the measurements. However, when wastes have reasonably
uniform characteristics, there can be a fairly constant relationship between BOD and
the other measurements. Once such a relationship is established,
COD, TOC, or TOD can be used in place of BOD for routine monitoring.
An important difference in these measurements is that the BOD test is an attempt
to indicate the impact of biodegradable organic matter on the aquatic ecosystem.
COD, TOC, and TOD measure both biodegradable and nonbiodegradable organics.
Nonbiodegradable and Toxic Organics
Gross values of organic substances such as BOD, COD and TOC are of little value
where there is a need to identify specific organic compounds in a wastewater. No
general method, comparable to the BOD test for biodegradable organics, has been
developed to measure nonbiodegradable or toxic organic compounds in
wastewaters. Narrow spectrum or single-chemical methods of analysis have been
developed to measure priority pollutants and other specific organic pollutants. In
addition, broad spectrum measurement methods are being developed to quantify a
range of specific chemicals or classes of chemicals found in industrial wastewaters
and treated effluents.
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Guidelines establishing test procedures for 113 organic toxic pollutants are
available8. For each pollutant, two acceptable methods are proposed: (a) either gas
chromatography (GC) with selected detectors or high performance liquid
chromatography (HPLC) depending on the particular pollutant, and (b) GC coupled
with MS.
Analytical methods for nonbiodegradable and toxic organic compounds continue
to undergo refinement and evaluation. Sophisticated and expensive analytical
equipment is necessary for the GC, MS, and HPLC methods. In spite of these
limitations, both single-chemical and broad spectrum analytical methods will see
increasing use.
Historical Data Base
Standardized analytical methods have been used for BOD since the 1940's, COD
since the 1950's and TOC since the 1960's. Thus, information on the concentration
of organic substances as measured by these parameters has been available for
decades. At many industrial plants, untreated and treated waste waters are analyzed
for one or more of these parameters at least weekly, and in some cases, every day. As
a result, a significant historical data base is available on the BOD and COD
characteristics of untreated industrial wastewaters and on the reduction that occurs
in biological and other treatment processes. This data base has proven valuable in
developing technology-based effluent limitations and in identifying effluent
characteristics from various treatment processes.
In contrast, interest in nonbiodegradable and toxic organic pollutants has been so
recent that there is only a small historical data base on the occurrence of these
organics in industrial wastewaters and their reduction in biological and other
treatment processes. Although there has been long-term interest in the measurement
of some organic compounds such as phenol and certain pesticides, only since about
1978 have efforts been initiated to identify organics such as the priority pollutants in
industrial wastewaters. EPA now has a strong effort to acquire information on the
occurrence of priority pollutants in wastewaters and their removal by treatment
processes.
Relative Concentrations of Organic Pollutants
Concentrations of conventional and nonconventional pollutants in industrial
wastewaters usually are measured in terms of milligrams per liter (mg/ L), whereas
the concentrations of priority pollutants are measured in terms of micrograms per
liter (/u/L)- In the effluent of many municipal and industrial wastewater treatment
systems, a number of the priority pollutants are either not detected or are found
only at about the levels of detectability for the analytical methods that were used.
BIOLOGICAL TREATMENT
General
Biological treatment, the microbial degradation of organics, is a process that
occurs throughout nature. Uncontrolled biological treatment occurs in streams,
soils, and swamps while controlled biological treatment processes occur in
man-made treatment tanks and systems. The controlled processes are designed to
accomplish a required amount of organic matter removal from wastewaters.
Because microorganisms are the key to the success of biological treatment
processes, an understanding of the processes must be based upon microbiological
fundamentals, and on the microbial transformations that occur in the biological
treatment units. If this understanding can be achieved, rational predictions of
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performance become possible, and the capabilities of a process can be better
utilized. Without a sound understanding of the fundamentals, the processes can be
treated only as "black boxes" in which the performance is subject to parameters
seemingly out of man's control. In such cases, successful design and operation can be
based only on prior performance of similar processes. Such performance may be
difficult to transfer to different wastes and environmental conditions.
The objectives of biological treatment are to stabilize, coagulate and remove the
soluble, colloidal, and suspended organics in wastewater. Until the past few decades,
the concepts relating to the biological oxidation of organics were developed either
from the treatment of domestic wastewater, or from the oxidation of simple chemical
substrates such as glucose. Industrial wastes can have different characteristics, and
direct application of such design and performance data to the full-scale treatment of
industrial wastes is not always possible. Fortunately, many industrial wastewaters
have now been treated using biological treatment processes and successful design
and operational criteria have evolved for the biological treatment of industrial
wastes.
The removal of organics in biological treatment processes is related to the
characteristics of the wastes. Suspended solids are incorporated in the microbial floe
and are removed by flocculation and settling. Colloidal matter can be adsorbed on
the microbial surfaces. The biodegradable organics are metabolized by the
microorganisms. High levels of organic removal are related to the type, composition,
and the concentration of the organics present.
This section summarizes some of the key factors that influence the performance of
biological treatment processes and describes common biological treatment systems.
Important Factors
The performance of biological treatment processes is influenced by those factors
that affect the microorganisms in the processes. These include temperature,
solids-retention time, nutrients, oxygen, pH, and inhibition and toxicity.
To reproduce and function, microorganisms must have a source of energy, carbon,
nutrients such as nitrogen and phosphorus, and trace elements for cell synthesis.
Carbon dioxide and organic carbon are the sources of carbon used by
microorganisms. If an organism obtains its carbon from organic carbon, it is
heterotrophic; if it uses carbon dioxide it is autotrophic. The overwhelming majority
of microorganisms in biological waste systems are heterotrophic.
Temperature—
Variations in temperature affect biological processes by influencing the metabolic
activities of the microorganisms, the transfer of oxygen and other gases, and the
settling characteristics of biological solids. According to the temperature range in
which they function best, microorganisms may be classified as psychrophilic,
mesophilic, and thermophilic. The optimum temperature ranges for these
microorganisms are 12° to 18°C, 25° to 40° C, and 55° to 65° C, respectively. Almost
all biological waste treatment processes operate in the mesophilic range. With the
exception of certain anaerobic treatment systems, temperature is not controlled in
biological treatment processes.
The effect of temperature on the reaction rate of a biological treatment process is
commonly expressed as:
where T, is the temperature of reaction rate kTr T2 is a different temperature at
which the rate kT2 is desired, and 6 is the temperature coefficient. 6 values are
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generally in the range of 1.04 to 1.10. If 9 = 1.072, a reaction rate is doubled for an
increase in temperature of 10° C. This has led to the generalization that biological
reaction rates are doubled for each 10° C rise in temperature. This estimation is useful
for temperatures around 20° C and is less applicable at extreme temperatures.
In a biological treatment process, the reduction of the temperature by about IO°C
will require about twice as many active organisms in the process to achieve
equivalent process efficiencies. This can be accomplished by increasing the mixed
liquor suspended solids (MLSS) concentration thereby increasing the microbial
solids retention (SRT) of the unit. Systems with solids recycle permit the operator
to compensate for lower temperatures by increasing the microbial solids in the unit.
Systems without solids recycle do not have this flexibility.
Solids Retention Time—
Fundamentally,
weight of microorganisms in the system
weight of microorganisms leaving the system per unit time
SRT should be determined using the quantity of active organisms in the system.
However, measuring the active microbial mass in a biological treatment unit is
difficult and other parameters are used. Assuming that complete mixing and
therefore uniform distribution of the active microorganisms and other solids occurs,
SRT can be determined in terms of parameters such as volatile suspended solids,
total suspended solids, or total solids. Thus SRT can be defined by
where Xa = average mixed liquor volatile or total suspended solids, mass per reactor
volume (such as grams per liter), V = unit volume (such as liters), and AX = solids
loss or wastage, mass per day (such as grams per day).
The actual SRT of a biological treatment system must be greater than the
minimum time it takes for the organisms to reproduce in the system. If this does not
occur, the microorganisms will be removed from the system in the effluent at a faster
rate than they can multiply, and failure of the biological system will result. The
minimum or critical SRT (SRTC) is that SRT at which a stable microbial cell
population can be maintained. The SRTC for biological processes ranges from 2 to 6
days for anaerobic systems and from 0.1 to 0.4 days for aerobic systems. The range of
SRTC values is a result of the impact that other process variables, such as
temperature and inhibition, have on microbial growth. Temperature and SRTC are
interrelated, with SRTC increasing as the temperature decreases.
Preliminary treatability studies are important with industrial wastes to establish
design parameters and the relationship of SRT and other control parameters to
desired effluent quality and process performance.
Nutrients—
To achieve satisfactory biological treatment, the wastewaters being treated must
contain sufficient basic nutrients such as carbon, nitrogen, phosphorus, and trace
minerals to sustain the desired rates of microbial activity. Certain industrial wastes
may not be nutritionally balanced.
The common approach of avoiding nitrogen or phosphorus limitations in aerobic
systems is to add these nutrients to obtain a BOD:N:P ratio of 100:5:1. This assures
313
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that there is no deficiency of these nutrients and will result in measurable amounts of
nitrogen and phosphorus in the effluent. Studies with nutrient deficient wastes
have established that 3-4 kg N per 100 kg BOD5 removed and 0.5-0.7 kg P per 100 kg
BOD removed will avoid nutrient deficient conditions. This results in a BOD:N:P
ratio of about 100:3:0.6 and assures minimum nitrogen and phosphorus in the
effluent.
Oxygen—
Aerobic biological treatment processes require adequate oxygen input to meet the
needs of the microorganisms. The total oxygen requirements are related to the
oxygen needed for: (a) biological oxidation of the carbonaceous organics (O^), (b)
biological oxidation of the organic and ammonia nitrogen (Ojsj), (c) endogenous
respiration of the biomass (Og), and (d) any immediate oxygen demand. Not all of
these oxygen demands occur in every biological treatment process. The actual
oxygen demand is related to the characteristics of the wastewater and the desired
effluent quality. As a minimum, oxygen is needed to meet the carbonaceous and
endogenous requirements.
pH-
The pH in the biological treatment process will affect the growth of the
microorganisms. Organisms in the treatment systems normally will not function at
pH levels above 9.5 or below 4.0. The optimum pH range for most treatment systems
is about 6 to 8. The most critical pH is that of the mixed liquor in contact with the
organisms and not that of the entering wastewater. The entering wastewater will be
diluted by the aeration tank contents and partially neutralized by the buffer capacity
of the mixed liquor. When there is inadequate buffering capacity in the unit, acidic
and caustic industrial wastewaters must be neutralized before being treated in a
biological unit.
Inhibition and Toxicity—
Compounds that inhibit or retard the metabolism of the microorganisms can
affect the performance of biological treatment processes. Certain cations, such as
lead, copper, and arsenic, and certain anions, such as cyanides and fluoride, can be
toxic to microorganisms. Some organic compounds in industrial wastewaters also
can be toxic. Treatability studies can identify whether industrial wastes are
inhibitory to biological systems and the relative degree of inhibition.
Biological Treatment Processes
There are many types of biological treatment systems that can be used for
wastewater treatment. These can be defined by the presence or absence of oxygen,
i.e., aerobic or anaerobic, by their photosynthetic ability, or by the mobility of the
organisms, i.e., suspended or attached growth. The major biological treatment
processes are identified in Table 3 and examples described below.
Activated Sludge—
Activated sludge processes are widely used to treat municipal and industrial
wastes since they are versatile, flexible, and can be used to produce an effluent of
desired quality by varying process parameters. The process was so-named because it
produces an active mass of microorganisms capable of aerobically stabilizing a
waste. Many versions of the basic process exist but all are fundamentally similar.
The term activated sludge is applied to both the process and to the biological solids
in the treatment unit. The mixed liquor suspended solids or activated sludge contains
a variety of heterotrophic microorganisms such as bacteria, protozoa, fungi, and
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Table 3. Common Biological Treatment Processes
Aerobic Suspended Growth
Activated sludge Activated sludge
Trickling filters Aerated lagoons
Aerated lagoons Oxidation ditch
Oxidation ditch Mixed digesters
High rate algal ponds
Rotating biological Attached Growth
contactors Trickling filters
Facultative Packed bed reactors
_ . . . . Anaerobic filter
Oxidat.on ponds Rotatjng blological
Anaerobic contactors
Anaerobic lagoons Photosynthetic
Anaerobic f.lters Oxidation ponds
Digesters
Anaerobic contact process
High rate algal ponds
larger microorganisms. The predominance of a particular microbial species depends
upon the waste that is treated and the way in which the process is operated.
The microorganisms will aggregate into flocculant masses which can be kept in
suspension while the activated sludge is mixed but which will settle when the mixing
is stopped. These properties are important because they permit the microbial solids
to mix with the influent wastewater yet separate readily in a subsequent separation
unit. A well-treated effluent containing few suspended solids will result.
The system shown in Fig. 3 illustrates the components of an activated sludge
system. This system incorporates recycle of the settled microbial solids to the
aeration basin to maintain a high microbial population and a long SRT to treat the
settled wastewater. Excess waste activated sludge and settled solids from the primary
clarification units are sent to an anaerobic or aerobic digester for stabilization prior
to final disposal. The aeration unit is maintained aerobic by diffused or mechanical
aeration which also serves to mix the contents of the unit.
Aerated Lagoons—
Aerated lagoons are similar to an activated sludge unit in that oxygen is supplied
continuously by mechanical or diffused aeration. An aerated lagoon is essentially the
same as a long SRT activated sludge system except that a shallow (8 to 10 ft) earthen
basin serves as the reactor, primary clarification may not be used before the
wastewaters enter the aerated lagoon, a separate final clarification unit may not be
used and solids recycle does not occur. When final clarification is not used, the
effluent end of the aerated lagoon is not well mixed and many of the mixed liquor
solids settle out in the lagoon.
The aerated lagoon contains fewer microbial solids than an activated sludge unit
and mixing may not be sufficient to keep all of the solids in suspension. A schematic
of an aerated lagoon is shown in Fig. 4.
The lagoon temperature depends upon the mixing that takes place and the rate at
which heat is lost. Because of the large surface area and the long liquid detention
time, the lagoon temperature can be close to the ambient air temperature.
Aerated lagoons require more land area but less care and maintenance than
activated sludge systems and are commonly used by many industries.
315
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Influent
Settled
Activated
Sludge
To Ultimate
Disposal
Figure 3. Components of an activated sludge system.
Mechanical Surface
Aeration
Earthen
Dikes
Influent
Wastewater
f
ifip
Effluent
£b()i ) ) M ) /3
Figure 4. Schematic top view of an aerated lagoon.
Oxidation Ponds —
Oxidation ponds are the simplest aerobic biological treatment process used to
treat wastewaters. Fig. 5 portrays the processes that take place in an oxidation
pond. The pond effluent contains bacteria, algae, and soluble organic and inorganic
compounds.
These ponds are relatively shallow, diked structures with a large surface area.
No mechanical aeration is used, and the aerobic conditions are a result of
photosynthetic algae and wind action. Where land is relatively flat, inexpensive, and
available, oxidation ponds can be more economical than other types of aerobic
biological treatment. They generally are used in rural areas with adequate sunlight,
wind action, and available land. They have been used for the treatment of
316
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Solar
Energy
Effluent
Algae
OxygelT Synthesis
8
Figure 5. Schematic cross-section of an oxidation pond
petroleum refining and petrochemical wastewaters as well as wastewaters from
food processing, textile, pulp and paper and other industries.
An activated sludge unit, an aerated lagoon and an oxidation pond used to treat
the same wastes will differ greatly in size, as well as in the control that can be exerted
over the units. The environment within an oxidation pond cannot be controlled by
the operator or design engineer. Only physical factors such as size or loading rate can
be controlled, and reliance is placed upon natural microbial reactions. In aerated
lagoons, mechanical aeration is provided. With an activated sludge system, the
degree of aeration and mixing, solids recycle and the mixed liquor solids
concentration all can be controlled closely.
These relative levels of control are manifest in the different liquid detention times
that occur in each unit. Table 4 indicates the period of time that a volume of liquid
(liquid detention time) will stay in the respective units.
The term "aerobic" does not completely describe the microbial reactions that take
place in an oxidation pond. While ample dissolved oxygen may exist in the upper
portion of the pond, there may be little or no dissolved oxygen in the lower depths.
Oxidation ponds are designed and loaded so that the anaerobic conditions in the
lower portions have little impact on the quality of effluent from the ponds.
Both bacteria and algae are key microorganisms in an oxidation pond. Soluble
organics in the pond are metabolized by the bacteria and end products of that
metabolism, such as carbon dioxide, ammonium and nitrate ions, and phosphate
ions, become available for growth of the algae. As the autotrophic algae generate
new cells using solar energy, oxygen is produced which can be used by the
hetrotrophic bacteria. Surface reaeration due to wind action rarely can meet the
oxygen needed for the oxidation of the organic matter in the pond, and the oxygen
produced by the algae is essential to maintain aerobic conditions.
Trickling Filters—
Trickling filters are an aerobic attached growth biological process. They are not
filters but are aerobic oxidation units which absorb and oxidize the organic matter in
the wastewater which passes over the microorganisms on the permeable media.
Trickling filters consist of a bed of media to which microorganisms attach and over
which the wastewater passes. The media generally used are stone or rock of large size
(2 to 4 inches, 50 to 100 mm) or plastic or wood of various configurations.
The waste to be treated is distributed over the top of the media by a movable,
generally a rotary, distributor. Trickling filters are designed to treat liquid wastes
with few solids. Readily settleable solids are removed in a primary treatment unit
before the wastewaters are treated in a trickling filter.
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Table 4. Range of Liquid Detention Times in Different Biological Treatment
Units
Detention Time
Unit (days)
Activated sludge 0 2-1
Aerated lagoon 5-10
Oxidation pond 20-60+
The organic matter in the wastewater is removed by the population of
microorganisms that are attached to the media. As the microorganisms grow, the
thickness of the microbial layer increases, and microbial solids periodically wash off,
permitting a new microbial layer to grow. The solids in the trickling filter effluent are
removed in a secondary sedimentation unit. A flow diagram of a trickling filter
system is shown in Fig. 6.
The trickling filter media are supported on an underdrain system which collects
the treated wastewater and microbial solids from the filter for transport to the
secondary sedimentation unit. The underdrain assists in keeping the filter aerobic by
facilitating air movement through the filter.
In contrast to an activated sludge system, there is little opportunity to increase the
microbial solids in a trickling filter above that which would be there normally. The
return of settled microbial solids from the secondary clarifier, as in an activated
sludge system, can clog the openings in the filter media and impair the filter
performance.
With proper design and concern about inhibitory compounds and consistent
loading, trickling filters can successfully treat many industrial wastes.
PACT Process-
Biological treatment does not remove nonbiodegradable and toxic compounds
unless such compounds become part of the microbial floe or are removed in the
clarification systems. The PACT process is a patented process that incorporates
powdered activated carbon in the biological treatment unit (Fig. 7). The activated
carbon adsorbs many organics not readily biologically degradable and that
otherwise may not be removed. The process also aids the removal of BOD, COD,
and color. Carbon dosages have ranged from 15 mg/L to 200 mg/L of influent
wastewater.
The process has been used successfully with several industrial wastes including
those from the manufacture of complex organic chemicals and from oil refining. In
one full-scale study2, greater than 82% removal of the priority pollutants was
achieved with the PACT process. With a carbon dosage of 100 mg/L, 99.6% removal
of benzene and 84% removal of 2,4-dichlorophenol was achieved.
Sedimentation—
In all treatment systems, clarification units are an important component
(Figs. 1, 3, 6, 7). The majority of these units are gravity sedimentation units.
Separate units are needed to remove: (a) readily settleable organic solids that would
add an additional load to the subsequent biological units or would impair the
performance of units such as a trickling filter, and (b) the settleable microbial solids
synthesized in the biological units. To achieve the desired effluent quality, a
biological treatment system must include properly designed and operated secondary
solids sedimentation units.
318
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Trickling Filter
with a Rotary
Distributor
Influent
Settled
Microbial
Solids
To Ultimate
Disposal
Figure 6. Components of a trickling filter system.
Primary
Clarification
Effluent
Figure 7. Schematic of the PACT process in which activated carbon is added to an
activated sludge unit.
Effluent Quality—
Well-designed and well-operated biological treatment facilities are capable of a
high degree of organic removal and low concentrations of organics in the treatment
plant effluent. Because processes such as aerated lagoons and oxidation ponds
normally do not have secondary clarification units, the effluent from such processe;
will have higher concentrations of solids and associated BOD than will processes
such as activated sludge systems and trickling filters. Table 5 indicates the typp o)
BOD removals and effluent quality that result with the biological treatment system;
discussed previously.
319
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TableS. Typical Range of BOD Removals and Effluent BOD and
Suspended Solids Concentrations for Biological Treatment
Systems*
Activated sludge
Trickling filter
Aerated lagoon
Oxidation pond
BOD
Removal
(%)
85-95+
85-95
80-95
75-95
Effluent Concentrations (mg/L)
BOD
5-25
5-25
30-80
30-80
Suspended
Solids
10-30
10-30
60-200**
60-150*'
* Actual removals and effluent concentrations are related to system design and
operation and influent wastewater characteristics.
**These solids consist of microorganisms, inert material and algae; the concentration
depends on the amount of settling that takes place prior to discharge.
Variability of raw waste flow and characteristics, and changes in process
temperatures can affect the effluent quality from industrial waste treatment systems.
The impact of these factors is more pronounced with systems that have short solids
and hydraulic retention times and low concentrations of microorganisms. Increased
BOD loads can affect performance of the biological units by creating oxygen
demands in excess of the aeration system oxygen transfer capabilities. Adverse
effects from toxicants, salts, nutrient shortages, and temperature changes can be
heightened at marginal dissolved oxygen levels in the biological units.
There are, however, approaches that can minimize effluent variability. Table 6
indicates several possibilities. Many of these are necessary to achieve consistent and
satisfactory performance by industrial waste treatment plants. Especially effective
are: (a) monitoring of the influent so as to adjust plant operation when there are
large variations in pH, flow, toxic compounds, and organic load, (b) equalization
basins or tanks, (c) proper operation of the biological process, and (d) flexibility to
utilize solids recycle.
Pollutant Removal Mechanisms
Removal of pollutants in a treatment system occurs as a result of physical,
chemical, and biological processes that occur within the system. Although aerobic
biological treatment systems are designed to remove soluble organics by microbial
degradation, there actually are several removal mechanisms that take place:
microbial degradation, air-stripping and volatilization, and adsorption on the
sludge (Fig. 8). Other possible mechanisms include hydrolysis and photolysis, but
these play a minor role in biological treatment processes.
Air-stripping and adsorption can be important removal mechanisms for organic
compounds that may not be readily biodegradable but may have high volatility or
high adsorption characteristics for the microbial sludge respectively. While
air-stripping and adsorption can be important mechanisms to remove organic
chemicals from industrial wastewater and to meet effluent limitations, they merely
transfer the pollutants to another media, i.e., the atmosphere and the sludge. In
solving one problem, care must be taken to avoid exacerbating or creating other
problems. Thus it is important to comprehend how pollutants are removed in
biological treatment systems.
320
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Table 6. Approaches to Minimize Effluent Variability
• Monitoring of process
discharges
• Holding and blending waste
streams before discharge
• Separate treatment of problem
process wastewaters
• Automated monitoring of
influent quality
• Equalization basins
• Control of biomass concentrations
and recycle
• Ability to control adverse
pH levels
• Adequate oxygen supply
Air Stripping
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_ — Adsorption
-> on Microbial
Solids
Air for Mixing
and Oxygen
Requirements
Figure 8. Mechanisms for pollutant removal in biological treatment processes.
Aeration—
Aeration in biological treatment units provides air-liquid contact and an
opportunity for the transfer of volatile organic compounds from the liquid phase to
the gas phase. Stripping of organic compounds by aeration may result wherever
significant surface turbulence occurs such as caused by mechanical aeration,
spraying, or bubble diffusion. Many organic compounds discharged in wastewaters
from the organic chemicals industry are air-strippable3. The removal of an
organic by aeration is governed by two major factors: (a) the tendency of a
compound to establish an equilibrium between the gas and liquid phases, and (b) the
intensity and duration of the aeration.
Adsorption on Sludge—
Adsorption of organics on sludge provides an additional removal mechanism in
biological treatment systems. Primary sludge, mixed liquor suspended solids, and
secondary sludges have surface areas which favor sorption of organics. Organics
which are insoluble or only slightly soluble can be sorbed to these surfaces and can be
removed in a biological treatment process even though the amount of their
biodegradation may be small. As an example, pesticides have been found to
accumulate in grease scum layers. The factors affecting sorption include solubility,
pH, concentration, temperature, molecular weight and available surface area.
321
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Microbial Degradation—
The factors affecting the microbial degradation of organics in biological treatment
systems have been discussed earlier. These include solids retention time, nutrients,
oxygen, pH, inhibition, toxicityand acclimation. Several control approaches can be
used to optimize microbial degradation. Those commonly in use center on:
(a) maintaining proper solids retention time to achieve the desired degradation,
(b) acclimatizing the microorganisms to the organic compounds, and
(c) maintaining proper nutrient and oxygen balances.
MUNICIPAL WASTE TREATMENT
General
Publicly owned treatment works (POTWs) treat the combined domestic,
commercial and industrial wastes of a community. As a result, conventional,
non-conventional and priority pollutants can be present in the POTW
influent. POTWs are designed to remove conventional pollutants.
Generally, they accomplish 85% or better removals of BOD and suspended solids.
Although POTWs are not designed to remove toxic pollutants, in many cases high
removals do occur. Those priority pollutants that can be (a) volatilized or degraded
in the biological treatment units, (b) sorbed to the biological solids, and (c) deposited
in the primary or secondary sedimentation units may be removed substantially.
Since about 1977, considerable effort has been directed to determining how
effectively POTW biological processes remove priority pollutants. These efforts
include: (a) an extensive EPA sampling and analysis program to evaluate the
occurrence and fate of priority pollutants in 40 POTWs and (b) research activities
geared to provide additional removal and occurrence information. This section
summarizes the results that have been obtained.
The 40 City Study
From 1978 to 1980, EPA conducted a study of the occurrence and fate of priority
pollutants in 40 POTWs. The first phase was a two-plant pilot study designed to set
operating parameters for the remainder of the study. In 1980, EPA published an
interim report4 which summarized data from the two POTWs evaluated in the
pilot study plus eighteen additional POTWs. At the time that this section was
written, only the interim report was available. It is understood that the complete
study which will include results from all 40 POTWs is to be published later in 1982.
All of the POTWs had common secondary treatment processes with activated
sludge and trickling filters being the most common. While few of the POTWs had
effluents that always met secondary treatment requirements (30 mg/ L BOD and
30 mg/ L total suspended solids), the POTWs included in the study were selected
only- if their operation was considered to be reasonably good. Therefore, the results
of the study can be considered as coming from well-operated plants.
At each plant, a minimum of six days of 24 hour sampling of influents, effluents
and sludges was completed. Each sample was analyzed for conventional and priority
pollutants as well as selected non-conventional pollutants.
Combining data from all liquid and sludge samples, 103 priority pollutants,
including metals, were detected at least once. The organic priority pollutants not
detected were acrolein, benzidine, hexachloroethane, hexachlorocyclopentadiene,
nitrobenzene, several ethers, all nitrosoamines, endosulfans and metabolites, endrin
and metabolites, all but two PCB mixtures, and toxaphene.
Table 7 identifies the organic priority pollutants that were found in at least 25% of
the influent samples and Table 8 identifies the organic priority pollutants that were
322
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Table?. Occurrence* of Organic Priority Pollutants in POTW Influent
Samples4
Chemical
Toluene
Tetrachloroethylene
Chloroform
Methylene chloride
Trichloroethylene
Bis (2-ethylhexyl) phthalate
1 ,1 ,1 -Trichloroethane
Ethylbenzene
Phenol
Benzene
Di-n-butyl phthalate
Diethyl phthalate
Butyl benzyl phthalate
1 ,2-Trans-dichloroethylene
Naphthalene
1,1-Dichloroethane
1,1 -Diehloroethylene
1 ,2-Dichlorobenzene
Pentachlorophenol
Anthracene
Phenanthrene
*Found in at least 25% of the
TableS. Occurrence* of
Samples4
Chemical
Bis (2-ethylhexyl) phthalate
Methylene chloride
Chloroform
Tetrachloroethylene
Di-n-butyl phthalate
1 ,1 ,1 -Trichloroethane
Trichloroethylene
Toluene
Gamma — BHC
Phenol
Percent of
Times Detected
98
97
96
95
95
94
91
86
83
68
63
62
59
58
55
40
35
30
27
27
27
influent samples.
Organic Priority Pollutants in
Percent of
Times Detected
91
88
88
82
59
57
54
48
36
27
Range of
Concentrations
Detected (fjg/L)
2 - 500
2- 1,100
1 - 430
1-11 ,000
1 - 860
2 - 390
1 - 1,600
1 - 448
1 - 380
1 - 1,560
1 - 105
1 - 33
2- 140
1 - 97
1 - 150
1 - 24
1 - 243
2- 440
2 - 94
1 - 93
1 - 93
POTW Effluent
Range of
Concentrations
Detected (fjg/L)
2 - 370
1 - 1 ,570
1 - 61
0 - 320
1 - 92
1 - 370
1 - 97
0 - 1,100
0.01 - 0.84
1 - 89
"Found in at least 25% of the effluent samples.
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found in at least 25% of the effluent samples. Two facts are obvious from the
information in these tables. First, the concentrations of organic priority pollutants in
wastewater influents and effluents are at least three orders of magnitude less than the
concentrations of conventional pollutants in similar wastewaters. For example, the
concentration of BOD and suspended solids in untreated municipal wastewater is in
the range of 150-300 mg/ L. In the effluent of secondary treatment plants, both BOD
and suspended solids concentrations are in the 20-40 mg/ L range. The tables show
the influent and effluent concentrations of priority pollutants are generally in the
0.001 to 1 mg/L (1 to 1000 Mg/L) range.
Second, the organic priority pollutants were found less frequently in the effluent.
The decrease in frequency is expected since a number of mechanisms can remove
such pollutants in POTWs.
Not all of the organic priority pollutants were degraded microbially or were
volatilized. Many were sorbed to solids and settled in the primary and secondary
clarifiers to become part of the sludge. Table 9 identifies the organic priority
pollutants that were found in at least 25% of the samples of untreated sludges. Many
of the organic compounds were concentrated in the sludges and were found here with
greater frequency than in the plant influent or effluent.
Because many of the organic priority pollutants were detected in the influent and
effluent wastewater at concentrations only barely above detection limits, percent
removals of only a few of the organic compounds could be determined. Table 10
summarizes data for the organic compounds for which there were 10 or more usable
data points to determine percent removals. For purposes of comparison, percent
removals of organics as measured by common parameters are also included in
Table 10.
On the average the POTWs achieved high BOD and suspended solids removals (91
and 92% respectively). The removals of the organic priority pollutants were variable,
as high as 94% for toluene and as low as 55% for methylene chloride. It must be noted
that the POTWs were designed and operated to remove conventional pollutants
(BOD and suspended solids) rather than the priority pollutants.
Other Studies
Information is available on the removal of PCB compounds in POTWs5. A
survey of PCBs in the wastewaters of 33 municipalities and of their fate during
conventional secondary treatment showed the following: PCB concentrations in
untreated wastewaters ranged from less than 0.1 to 1.8 jug/L; primary treatment
removed, on the average, 50% of the PCB load, and secondary systems averaged 66%
removal; PCB concentrations in digested sludges ranged from 0.6 to 76.6 ppm
(dry weight); PCB removal during wastewater treatment occurred largely by
accumulation in the primary and/or waste activated sludge.
The behavior and fate of twenty-two potentially toxic organic compounds6 was
evaluated in a small scale pilot plant that had treatment facilities comparable to
those at a POTW secondary treatment plant. The 22 compounds consisted of four
pesticides, three phenols, six phthalates and nine polynuclear aromatic
hydrocarbons (P AHs). The compounds were added to typical municipal wastewater
to obtain a concentration of about 50 Mg/ L of each compound. Removals of the
compounds were typically in the 95-98 percent range and the activated sludge
effluent concentrations generally were less than 10 Mg/L with many concentrations
less than 3 pgl L. Many of the compounds were concentrated in the sludges.
Summary
The data indicate that; (a) well operated POTWs remove variable but significant
removals of organic priority pollutants, (b) POTW effluents contain some organic
priority pollutants in low concentrations (generally less than 100 Mg/L), and
(c) many of the priority pollutants can be found in POTW sludges.
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TableS. Occurrence* of Organic Priority Pollutants in Untreated POTW
Sludges4
Chemical
Toluene
Bis (2-ethylhexyl) phthalate
Ethylbenzene
Benzene
Methylene chloride
1 ,2-Trans-dichloroethylene
Anthracene
Phenanthrene
Phenol
Trichloroethylene
Pyrene
Di-n-butyl phthalate
Butyl benzyl phthalate
1,1-Dichloroethane
Tetrachloroethylene
Fluoranthene
Naphthalene
1,2-Benzanthracene
Chrysene
Percent of
Times Detected
95
95
73
69
63
62
53
53
52
50
46
45
42
42
37
34
33
26
25
Range of
Concentrations
Detected
(/ug/L wet weight)
1 -
20-
1 -
1 -
1 -
1 -
6-
6-
19-
1 -
6 -
14-
1 -
1 -
1 -
8-
11 -
9-
9-
42,300
35,000
4,200
694
3,310
96,000
3,200
3,200
17,000
4,690
1,700
1,600
45,000
2,880
2,800
1,200
5,200
1,500
1,500
*Found in at least 25% of the sludge samples; some of the sludges were mechanically
thickened sludges.
INDUSTRIAL WASTE TREATMENT
General
Biological treatment is used to treat many industrial wastewaters and achieve the
effluent quality necessary to meet BPT limitations. Process performance commonly
is measured in terms of the concentration of BOD, COD, suspended solids and other
pollutant parameters in the effluent or of percent reduction of these parameters. As
a result, most of the existing information on the removal of organics from industrial
wastewater relates to the removal of conventional pollutants. In recent years,
however, many studies have been directed to the removal of priority and
non-conventional organic pollutants in industrial treatment systems. The following
examples describe the performance that has been achieved.
Chemical Industry
A study of wastewater treatment facilities at five chemical industry plants was
designed to evaluate the effectiveness of biological treatment in removing toxic
pollutants in wastewaters that result from manufacture of organic chemicals, and
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Table 10. Removals of Organic Compounds Achieved by POTW Secondary
Treatment4
Compound Median Removals* (%)
BOD 91
COD 83
TOC 71
Total suspended solids 92
1,1,1-Tnchloroethylene 88
Ethylbenzene 89
Methylene chloride 55
Bis (2-ethylhexyl) phthalate 60
Tetrachloroethylene 86
Toluene 94
Trichloroethylene 90
"Values obtained using data where the influent concentration was greater than 10/^g/L.
This concentration was accepted as the reasonable detection level for the analytical
methods that were used.
plastic and synthetic industries.7 All of the plants used activated sludge treatment
systems which included neutralization and equalization where needed, one or more
aeration basins, associated primary and secondary clarifiers, and biological solids
recycle systems.
All influent and effluent samples were analyzed for the organic priority pollutants,
except pesticides and polychlorinated biphenyls, by gas chromatography (GC) or
gas chromatography/mass spectroscopy (GC/MS) procedures. Not all organic
priority pollutants were analyzed at each plant. Certain conventional and
non-conventional pollutants also were measured. Data were collected over a four to
six week period at each plant.
Table 11 presents data on the conventional and non-conventional pollutants
collected at each plant during the period of evaluation. The information represents
mean values of the collected data.
The performance of the plants, with the exception of Plant E, was good with high
removals of BOD, COD, and TOC. The low performance of Plant E was attributed
to the low concentration of pollutants in the influent. The BOD and TOC effluent
concentrations at Plant E were comparable to those at the other plants with the
BOD concentrations at each plant being less than 20 mg/ L.
To preserve the confidentiality of organic priority pollutant data, the influent and
effluent data from all five plants were pooled. Tables 12, 13, and 14 present the
treatment plant performance data for three classes of organic priority pollutants.
The data were found to be distributed in a log normal pattern and the geometric
mean was used to determine the percent removal and as a measure of the "average"
values. Only data based on at least five detections in the influent are presented in these
tables.
With only a few exceptions, high removals of the organic priority pollutants
occurred in the biological treatment systems. In addition, the effluent concentrations
for most of the noted compounds were close to or below the detection limit of
10 Mg/ L accepted for the analytical methodologies that were used. The data also
showed that generally there were fewer observations of a specific compound in the
treated effluent than there were in the influent.
326
-------
Table 11. Wastewater Treatment System Performance at Five Organic Chemical Industry Plants — Conventional and
Non-Conventional Parameters7
Plant
A
B
C
D
E
Influent*
(mg/L)
580
450
840
1280
48
BOD
Effluent*
(mg/L)
18
14
3
19
14
COD
Removal Influent* Effluent*
(%) (mg/L) (mg/L)
97 1180 153
97 750 95
99 1 740 1 23
99
•7 i
Removal Influent*
(%) (mg/L)
87 378
87
93 485
650
54
TOC
Effluent*
(mg/L)
39
35
60
23
Removal
(%)
90
93
91
57
•Mean values.
-------
Table 12. Wastewater Treatment System Performance at Five Organic
Chemical Industry Plants — Volatile Organic Priority Pollutants7
Compound
Acrylomtnle
Benzene
Bromomethane
Bromodichloromethane
Chlorobenzene
Chloroethane
Chloroform
Dibromochloromethane
1,1-dichloroethane
1,2-dichloroethane
1,1-dichloroethene
t-1 ,2-dichloroethene
1 ,2-dichloropropane
1 ,3-dichloropropene
Ethylbenzene
Methylene Chloride
Tetrachloroethene
1 , 1 ,1 -trichloroethane
1 ,1 ,2-tnchloroethane
Trichloroethene
Toluene
Influent*
(A
-------
Table 13. Wastewater Treatment System Performance at Five Organic
Chemical Industry Plants — Base/Neutral Organic Priority
Pollutants7
Compound
Acenaphthene
Anthracene/Phenanthrene
Benzo(a)anthracene/Chrysene
Benzo(a)pyrene
Bis (2-ethylhexyl) phthalate
Di-n-butyl phthalate
1 ,2-dichlorobenzene
Dimethyl phthalate
Dioctyl phthalate
Fluoranthene
Fluorene
Naphthalene
Nitrobenzene
Pyrene
Influent*
(/wg/L)
84
62
20
13
24
86
331
46
28
17
56
802
3,000
17
Effluent*
(Aig/L)
##
6
8
t
28
6
28
**
**
6
t
6
91
6
Removal
(%)
90
60
0
93
92
65
99
97
65
*Geometric mean.
"All detections less than
tLess than five detections.
Table 14. Wastewater Treatment System Performance at Five Organic
Chemical Industry Plants — Acid Organic Priority Pollutants7
Compound
2-chlorophenol
2,4-dichlorophenol
2,4-dimethylphenol
2,4-dinitrophenol
2-nitrophenol
Pentachlorophenol
Phenol
2,4,6-trichlorophenol
Influent*
(Aig/U
53
347
270
673
40
216
171
100
Effluent*
-------
Table 15. Organic Priority Pollutants* in the Influent and Effluent Wastewaters of an Organic and Inorganic Chemicals
Manufacturer8
U)
o
Influent (ug/L)
Chemical
Benzene
Bis (2-ethylhexyl) phthalate
Bromoform
Butyl benzyl phthalate
Carbon tetrachlonde
Chlorobenzene
Chlorodibromomethane
Chloroform
Di-n-butyl phthalate
1 ,2-Dichtorobenzene
1 ,4-Dichlorobenzene
1,2-Dichloroethane
Diethyl phthalate
Dimethyl phthalate
2,4-Dinitrotoluene
Ethylbenzene
Fluoranthene
Methylene chloride
Methyl chloride
Naphthalene
4-Nitrophenol
2-Nitrophenol
Phenanthrene
Pyrene
Phenol
Tetrachloroethylene
1,1,1 -Trichloroethane
Trichloroethylene
Toluene
Range
1 -
1 -
6-
1 -
1 -
1 -
4-
1 -
1 -
1 -
07-
2-
02-
1 -
03-
1 -
10-
1.2-
15-
16-
1.7 -
04-
1 -
1 -
1 -
3 -
3 -
949
500
10
410
1,130
500
338
7,600
150
310
30
t
31
130
t
300
500
10,000
100
17,800
29,200
8,650
30
6
116,000
200
20
100
1,000
Geometric Mean
147
142
8 1
128
196
376
208
21 4
53
146
52
t
•49
48
t
179
1 4
439
31 0
942
290
258
106
24
50
15 1
62
120
289
Effluent
Range
2-
1 -
3-
1 -
1 -
15-
2-
1 -
06-
10-
t
10-
1 -
08-
1 9-
8-
t
2-
t
t
4-
t
t
t
8-
1 -
1 -
t
1
33
1,400
10
180
20
100
15
500
8
1,470
25
10
92
30
300
10,000
374
130
120
40
600
(ug/L)
Geometric Mean
62
85
6 5
43
63
42 2
6 3
103
2 1
76 1
t
136
26
9 6
80
275
t
300
t
t
208
t
t
t
189
80
59
t
207
'Pooled data from many facilities, compounds detected at least three times
fLess than three detections
-------
Leather Tanning and Finishing
Leather tanning and finishing wastewaters contain soluble and paniculate
organics, dirt, and inorganics such as chlorides, sulfides, and metals. Biological
treatment processes are commonly used to treat these wastewaters. These processes
include activated sludge, oxidation ponds, aerated lagoons, rotating biological
contactors, and trickling filters9. One or more of the following unit processes is
part of the total treatment system in addition to a biological treatment process: grit
removal, equalization, coagulation and primary sedimentation, and screening. The
total treatment system can achieve high degrees of pollutant removal.
During its development of effluent guidelines and standards for this industry, EPA
sampled a number of treatment facilities and analyzed their performance for
conventional, nonconventional, and priority pollutants9. Table 16 illustrates the
results that were obtained and indicates that, in general, treatment facilities that
accomplish high removals of the conventional organics (BOD, COD, oiland grease)
also achieve high removals of the priority pollutants.
Volatile organics measured in the influent were removed in the treatment facilities,
probably due to air-stripping. A variety of metals were present and significant
removals were obtained probably in the solids separation process. The actual fate of
the pollutants was not determined.
Specific Organic Compounds
In addition to the evaluations noted above, there have been studies on the removal
of specific organic compounds produced by industry.
In one such study, the biodegradation and toxicity of 12 organic compounds was
evaluated using respirometric measurements and acclimated activated sludge tests10.
Many of these compounds are considered potential carcinogens or toxic chemicals.
The summary in .Table 17 notes that many are biodegradable although, in certain
cases, acclimation of the microorganisms was necessary before high removals
occurred.
Summary
The available information indicates that biological treatment processes can
achieve high removals of both general organic pollutant parameters, such as
measured by the BOD test, and specific organic compounds. In general, when high
removals of BOD, COD and other general pollutants occur in biological treatment
systems, high removals of the priority pollutants also occur. There are, however,
organics that are inhibitory to biological treatment systems. More information is
needed on such compounds and the concentration at which they are inhibitory.
It is clear that microbial degradation is not the only mechanism of removal of
organic compounds in biological treatment systems. Because of the aeration that
takes place in such systems, air-stripping and volatilization as well as adsorption on
suspended solids play a major role in the removal of organics.
FUTURE DEVELOPMENT AND NEEDS
General
Biological treatment is one of a variety of technologies that can treat industrial
wastewaters and aid in reducing degradation of the environment. Most of these
technologies are "end-of-pipe" or curative technologies. In addition, there are
preventive technologies which reduce the quantity of pollutants requiring treatment
331
-------
Table 16. Performance of Industrial Waste Treatment Systems Treating Leather Tanning and Finishing Wastewaters19*
Pollutant
Conventional and
nonconventional
pollutants (mg/L)
BOD
COD
TSS
TKN
oil and grease
sulfide
Priority pollutants (/i/g/L)
benzene
chloroform
toluene
1,2 dichlorobenzene
1,4 dichlorobenzene
phenol
2,4,6 trichlorophenol
pentachlorophenol
bis (2-ethyhexyl) phthalate
naphthalene
phenanthrene/ anthracene
ethylbenzene
chromium
copper
cyanides
lead
nickel
zinc
mf
1240
2560
1100
250
170
50
ND
41
tr
215
99
480
10500
9500
ND
49
56
ND
31000
57
20
100
5
230
Biological Treatment Process
Separate Activated Sludge or Extended Aeration Systems
System A System B System C
rem rem
eff (%) mf eff (%) mf eff
920
1780
560
190
90
30
ND
tr
tr
69
21
435
4300
3100
ND
15
tr
ND
20000
37
40
30
34
140
26
31
49
26
47
40
-
98
-
68
79
9
59
67
-
69
98
-
35
35
-
70
-
39
2000
4030
2250
290
550
16
tr
tr
>100
ND
ND
5500
ND
ND
32
ND
29
MOO
1 70000
220
50
3100
75
2100
300
890
130
160
17
6
tr
ND
tr
tr
tr
1400
12
12
6
23
1 4
tr
1700
8
40
60
30
170
86
87
94
43
97
63
_
_
99
_
_
75
_
_
83
_
52
99
99
96
20
98
60
92
1530
5950
6370
750
250
19
tr
ND
tr
49
19
845
1700
2900
ND
19
76
43
6400
200
100
100
60
460
49
550
340
280
35
17
tr
ND
tr
ND
ND
ND
38
200
26
ND
ND
tr
170
25
400
50
30
59
rem
(%)
97
91
96
63
86
9
_
_
_
100
100
100
98
93
100
100
98
97
88
50
50
97
Aerated Lagoon
rem
mf eff (%)
1870
5530
2900
500
720
200
tr
ND
>100
255
54
4400
880
ND
51
•24
ND
88
1 60000
50
60
1100
60
500
20
220
155
105
17
04
tr
ND
tr
ND
ND
ND
ND
ND
2
ND
ND
ND
1100
5
150
80
30
49
49
96
95
79
98
99
_
99
100
100
100
100
96
100
100
99
90
93
50
90
'Average results from a three-day sampling program at the noted plants
tr - compound detected only in trace amounts, generally less than 10 ug/L
ND- compound not detected
-------
Table 17. Biodegradability of Specific Organic Compounds
10
Compound
Result
Methylethyl ketone (MEK)
Dimethyl amine (DMA)
Dimethyl formamide (DMF)
p-nitrophenol (PNP)
o-chlorophenol (OCR)
Trichlorophenol (TCP)
-easily biodegradable, does not inhibit microbial
activity up to 800 mg/L
-easily biodegradable at concentrations not
surpassing 300 mg/L
-easily biodegradable, microorganisms require
acclimation before high removals occur
- biodegradable at concentrations up to 50 mg/L;
at higher concentrations PNP is toxic; activated
sludge removes PNP with high efficiency;
microorganisms require acclimation
- biodegradable only at low concentrations, up to
10 mg/L; at concentrations up to 200 mg/L,
OCP is not biodegraded but does not significantly
inhibit biological processes; microbial
acclimation needed
- biodegradable at concentrations up to 35 mg/L
but needs acclimated microorganisms; 25 mg/L
in activated sludge inhibits the process
2,2'dichlorodiethyl ether (DCE) —biologically inert, no degradation even at 0.7
mg/L; a volatile compound that may be stripped
during biological treatment
Fluorescent whitening agents
(FWA) (used in household
detergents) (five were
evaluated)
-did not cause any oxygen demand but were
partially decomposed in the respirometric tests,
no effect on the treatability of the activated
sludge units until the concentration was greater
than 80 mg/L
by curative methods. Preventive technologies involve recycle and reuse of
wastewaters before discharge as well as product substitution and source control to
minimize unwanted pollutants in industrial wastewaters. These technologies can
have a significant impact on biological treatment by reducing the toxic compounds
in industrial wastewaters and the volumes of wastewaters requiring treatment.
Important innovations will occur with both preventive and curative technologies.
However, while preventive technologies offer great potential for industrial pollution
control, industrial wastewaters containing conventional, nonconventional, and
toxic pollutants will continue to be produced and require treatment.
Developments
Current biological treatment processes will continue to be used to treat
industrial wastes since they have been shown to provide excellent removal of
conventional pollutants and frequently high removal of nonconventional and toxic
pollutants.
Biological treatment systems have been used to remove organics that have an
effect on oxygen resources of the receiving body of water. As a result, waste strength
and treatment plant performance have been measured in terms of general parameters
such as BOD, COD or TOC. With the increasing interest in toxic pollutants, greater
attention is being paid to the removal of specific organic compounds such as the
333
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priority pollutants. This interest, plus the continuing need for treatment processes
that are less energy-intensive, more reliable, and which have lower operating costs
will result in improved and more innovative biological treatment processes being
used in the future.
Land application of primary or secondary treated wastewaters incorporates
biological treatment and offers another approach for the management of all or a part
of the discharges from industry. Proper use of this treatment technology offers
opportunities for nutrient recycling through crops grown on the site and for recharge
and renovation of the applied wastewaters. Land application incorporates treatment
with resource recovery. The movement and fate of toxic materials applied to the soil
need to be more fully understood; however, preventative technologies such as
pretreatment of industrial wastes and source control should minimize this problem
in the future. Where land is available, greater use will be made of land application of
wastes since it is less energy-intensive, less costly, and makes use of the biological,
physical, and chemical treatment capabilities of the soil.
Process control of biological treatment offers considerable opportunities for
development in the future. This control will result in increased reliability and more
effective and consistent treatment. Better control of dissolved oxygen, nutrient
addition, solids wasting, and equalization will occur. The design engineer and plant
operator will have a better array of processes and methods to treat industrial wastes
and achieve effluent or water quality goals.
In summary, the potential for improved and innovative preventative and curative
technologies is significant. Biological treatment technologies which are more
energy-efficient, are easier to control, are more reliable, and include resource
conservation will see the greatest development and use.
Needs
The improved processes and systems will result only if significant gaps in
knowledge are filled. The information that needs to be acquired if biological
treatment processes are to be used with maximum effectiveness includes:
Mechanisms of Removal—
Insufficient information exists on the mechanisms of removal of organic
pollutants, particularly potentially toxic compounds, in biological treatment
systems. Although abundant information is available on the removal of conventional
pollutants in a process or a system, there is only a limited understanding of whether
the pollutants are removed by stripping, sorption, or degradation. Treatment plant
performance data that permit input and output mass balances of pollutants in all parts
of the treatment system must be obtained if we are to know how and where specific
conventional and priority pollutants are removed. With such fundamental
knowledge available, better treatment processes can be developed and improved
system performance can be attained.
Full-scale Systems—
Much of the available information on the removal of specific organics has
resulted from small-scale and relatively short-term studies. Longer term studies on
large full-scale operating systems are needed to provide a better understanding of the
removals of conventional, nonconventional and toxic pollutants that occur in the
real world and the influence of seasonal, operational, and design factors on such
removals.
Low Pollutant Concentrations—
Biological treatment processes and systems can achieve high removals of
conventional and toxic pollutants. However, low concentrations of these pollutants
334
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remain in the effluent. Effluent concentrations of conventional pollutants generally
are less than 20 mg/L and toxic pollutants generally are less than 100 Mg/L. Both
fundamental and applied studies are needed to determine how lower concentrations
of these pollutants can be achieved in effluents from biological treatment processes.
Information thus derived must then be applied to improve design and operation
guidelines. The current approach to remove the remaining contaminants in
biological treatment process effluents is to add additional treatment processes such
as multimedia filtration and activated carbon after biological treatment processes.
Such add-on processes increase the capital and operational costs of a treatment
system and increase treatment plant operating difficulties. Improved biological
treatment processes that achieve lower effluent pollutant concentrations may be
more cost-effective.
Measurement—
The lack of practical monitoring and analytical techniques for toxic compounds
is a critical deficiency for both municipal and industrial waste management. This
deficiency impedes the specification and development of better treatment processes
and the clearer definition of health related issues. Less costly analytical methods
need to be developed to reduce monitoring and compliance costs and to provide
greater information on the amount and type of toxics entering and discharged from
industrial and municipal wastewater treatment plants. Such methods should be
suitable for analysis of wastewater, sludge, soil, and the atmosphere.
Improved Processes—
Continuing emphasis on the development of improved biological treatment
processes is warranted. Rather than focusing on high technology processes, research
on processes that are more cost-effective, less energy intensive, and more efficient in
removing potentially toxic compounds should be emphasized. These improved
processes should be evaluated at a large enough scale to obtain reliable information
on operating and maintenance costs.
REFERENCES
1. Council on Environmental Quality. "Environmental Quality — The Ninth
Annual Report of the Council on Environmental Quality," U.S. Government
Printing Office, December 1978.
2. Hutton, D.G. and S. Temple. "Priority Pollutant Removal: Comparison of
Du Pont PACT Process and Activated Sludge," paper presented at the 52nd
Annual Water Pollution Control Federation Conference, Houston, TX, 1979.
3. Thibodeaux, L.J. and J.D. Millican. "Quantity and Relative Desorption
Rates of Air-Strippable Organics in Industrial Waste water," Env. Science and
Technology 11: pp. 879-883, 1977.
4. Feiler, H. "Fate of Priority Pollutants in Publicly Owned Treatment
Works — Interim Report," Effluent Guidelines Division, Environmental
Protection Agency, DPA-440/1-80-301, Washington, DC, 1980.
5. Shannon, E.E., F.S. Ludwig, and I. Valdmanis. "Polychlorinated Biphenyls
(PCBs) in Municipal Wastewaters: An Assessment of the Problem in the
Canadian Lower Great Lakes," Environmental Protection Service,
Environment Canada, Ottawa, Research Report 49, 1978.
6. Petrasek, A.C., I.J. Kugelman, B.M. Austern, T.A. Pressley, L.A. Winslow,
and R.H. Wise. "Fate of Toxic Organic Compounds in Wastewater
Treatment Plants," Technology Development Support Branch, Wastewater
Research Division, Municipal Environmental Research Laboratory,
Environmental Protection Agency, Cincinnati, OH, 1981.
7. Engineering Science, Inc. "CMA/EPA Five-Plant Study," Report prepared
for the Chemical Manufacturers Association, April 1982.
335
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8. Wood, K.N. "Analysis of DuPont Plant Site Wastewaters for Priority
Pollutants," Engineering Department, E.F. DuPont de Nemours and
Company Inc., Wilmington, DE, January 1980.
9. "Development Document for Effluent Limitations Guidelines and Standards
for the Leather Tanning and Finishing Point Source Category — Proposed,"
Effluent Guidelines Division, Office of Water and Waste Management,
EPA 440/1-70/016, U.S. Environmental Protection Agency, Washington,
DC, July 1979.
10. Dojlido, J.R. "Investigations of Biodegradability and Toxicity of Organic
Compounds," Municipal Environmental Research Laboratory, Office of
Research and Development, EPA-600/2-79-165, Environmental Protection
Agency, Cincinnati, OH, December 1979.
336
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REMOVAL OF ORGANIC COMPOUNDS FROM
INDUSTRIAL WASTEWATERS USING
GRANULAR CARBON COLUMN PROCESSES
Robert P. O'Brien, Joseph L. Rizzo,
and Wayne G. Schuliger
HISTORICAL REVIEW
During the last decade, one unit process gained wide acceptance for the removal of
biorefractive and toxic dissolved organic compounds. The process is adsorption,
using granular activated carbon. Specifically, it is the unit process of choice for the
control of many of the hazardous and toxic organics among the priority pollutants,
and particularly for the removal of the chlorinated hydrocarbon pesticides.
Several innovations in the adsorption area took place during the early seventies.
In May of 1970, a commercial application of the clarification/ adsorption process for
wastewaters was able to reduce suspended solids and oil and grease to 50 mg/ L and
10 mg/ L, respectively. This reduction is accomplished in conventional clarifiers with
polymer addition followed by an adsorption system which removes the remaining
suspended solids and dissolved organic contaminants. Prior to the announcement of
this process, many studies clearly indicated that granular carbon was capable of
serving the dual role of removing suspended matter and adsorbing the dissolved
organics.
Dr. Walter J. Weber of the University of Michigan was carrying on some very
interesting work with a unique adsorption system design1 called the "Upflow
Expanded Bed Design." In this type of system, because the granular carbon bed is
expanded approximately 10%, suspended solids in the influent tend to pass through
the carbon bed. Naturally, if the removal of suspended solids is an objective, another
process has to be employed downstream of the carbon. Two of the largest industrial
wastewater treatment plants in the United States employ the expanded bed design:
American Cyanamid, in Boundbrook, New Jersey since 1978; and MobayChemical
Company in New Martinsville, West Virginia since 1974.
In mid-1973, two companies signed contracts with Calgon Corporation for a
unique adsorption service. One of these companies (Crompton Knowles)
The A uthors: Robert P. O'Brien has been with Calgon Corporation since 1973 During that time he has been
responsible for the design of adsorption and reactivation systems for Calgon's customers. He is presently the
manager of Applications Engineering
Joseph L. Rizzo is the Director of Commercial Development for Calgon Corporation, Pittsburgh,
Pennsylvania, 15230. He was formerly a Marketing Director for Adsorption Systems for Calgon
Corporation He is a member of the Water Pollution Control Federation
Wayne G Schuliger has been wilh Calgon Corporation since 1966 During that time he has been responsible
for the design of adsorption and reactivation systems for Calgon's customers He is presently the manager of
the Carbon Applications Group.
337
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manufactures organic dyes, and the other (Hardwick Chemicals) produces organic
chemicals. A key feature of this new adsorption system is that although the
adsorption system is located at the user's plant site, the spent carbon is returned to a
central reactivation center. The major benefits to industrial clients are (1) they do not
have to make a major capital investment for an adsorption system, since they lease
the equipment and carbon, and (2) they are relieved of any future liabilities
associated with one category of waste substances. The adsorbed organics since then
are completely destroyed. Furthermore, all the economies of scale can be realized
with centralized reactivation facilities. Over one hundred industrial plants are now
enjoying the benefits associated with the adsorption service.
In the mid-seventies, the U.S. Environmental Protection Agency (EPA) proposed
effluent standards for toxic chemicals in accordance with Section 307 (a) (2) of the
Federal Water Pollution Control Act. Studies by Calgon Corporation had clearly
indicated that the pesticides endrin, aldrin, dieldrin, toxaphene, DDT, DDD, and
DDE could be removed down to nondetectable levels.2 Several adsorption systems
including two at Rocky Mountain Arsenal in Denver, Colorado and Redstone
Arsenal in Huntsville, Alabama are now in operation removing these pesticides. The
majority of the 129 pollutants identified as priority pollutants by EPA are amenable
to carbon adsorption. The use of adsorption as a key industrial wastewater
treatment process of the future is evident.
TYPES OF ORGANICS REMOVED IN EXISTING SYSTEMS
During the 1970's, the use of granular activated carbon for dissolved organics
removal in industrial wastewater evolved into an effective unit process. More than
200 adsorption systems of various sizes currently operating in the United States are
removing a broad spectrum of undesirable dissolved organic compounds from
wastewaters.
Review of Carbon Feasibility
Numerous studies conducted over the past ten years have shown the feasibility of
using carbon adsorption in the treatment of organic chemical wastes. Some of the
classes of organic compounds shown to be amenable to carbon adsorption are listed
in Table 1.
These compounds are very effectively removed from water by adsorption, even in
the presence of other organic compounds. In general, adsorption is favored when the
contaminants: (1) have high molecular weight, (2) have limited solubility, (3) are
nonpolar, and (4) contain functional groups.
Laboratory results have also demonstrated granular carbon's ability to reduce
toxic compounds such as pesticides, to very low levels. Listed in Table 2 are
equilibrium (isotherm) results for a number of toxic compounds. Work performed
in the 1960's also showed that carbon has a great affinity for parathion and butoxy
ethanol ester of 2, 4, 5 - T.
Table 3 shows some results of adsorption isotherm studies conducted over a
number of years on wastewaters collected from all over the United States. The
wastes are classified using Standard Industrial Classification Numbers (SIC
numbers) and Table 3 reflects the major industries involved. This major survey
evaluated 326 wastewaters of which 204 or 63 percent achieved greater than 90
percent removal of Total Organic Carbon (TOC) using carbon. Over 76 percent of
all samples tested showed greater than 85 percent removal of TOC.
Review of Operating Systems
Because it is capable of adsorbing a broad spectrum of organic compounds,
carbon is used in a wide variety of industries. Among the major industries using
338
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Table 1. Classes of Organic Compounds Amenable To Adsorption On
Activated Carbon3
Aromatic Solvents
Benzene, toluene, xylene
Polynuclear Aromatics
Naphthalene, biphenyls
Chlorinated Aromatics
Chlorobenzene, PCB's, aldrin, endnn, toxaphene, DDT
Phenolics
Phenol, cresol, resorcinol
High Molecular Weight Aliphatic Amines and Aromatic Amines
Aniline, toluene diamine
Surfactants
Alkyl benzene sulfonates
Soluble Organic Dyes
Methylene blue, textile dyes
Fuels
Gasoline, kerosene, oil
Chlorinated Solvents
Carbon tetrachloride, perchloroethylene
Aliphatic and Aromatic Acids
Tar acids, benzoic acids
Table 2. Equilibrium Removal Capability of Activated Carbon for Selected
Toxic Compounds*4
Concentration of Compound, mg/L
Compound
Aldrin
Dieldrin
Endrin
DDT
ODD
DDE
Toxaphene
Arochlor 1 242
(PCB)
Arochlor 1254
(PCB)
pH Before Carbon
7.0
7.0
7.0
7.0
7.0
70
7.0
7.0
7.0
48
19
62
41
56
38
155
45
49
After Carbon
1 0
0.08
0.07
0.15
0.14
1.0
1.0
0.05
0.5
% Removal
99+
99+
99+
99+
99+
99+
99+
99+
99+
Capacity*
30.0
15.0
100.0
11.0
130.0
9.4
42.0
25.0
7.2
'Determined by standard isotherm testing procedures.
**Mg/L of the compound adsorbed per g of carbon.
339
-------
Table 3. Summary of Isotherm Studies of Wastewaters from Major Industries5
Number of Samples/TOC Reduction
SI.C.
Category
2000's
2200's
2600's
2800's
2900's
3300's
Subtotal
% of subtotal
All other samples
Total study
% of study
Industry
Food
Textile
Paper
Chemical
Petroleum
Metal
>99%
12
35
7
103
15
9
181
63%
23
204
63%
85-90%
4
9
0
23
1
1
38
13%
5
44
14%
<85%
3
9
3
51
2
3
71
24%
8
76
23%
Total
19
53
10
177
18
13
290
100%
34
324
1 00%
granular activated carbon are those producing: plastics, textiles, dyes, agricultural
chemicals, munitions, general organic chemicals, steel and pesticides. Others using
carbon are refineries, chemical waste processors, ore processing and truck washing
facilities.
Table 4 presents a tabulation of important design and operating parameters for
carbon adsorption systems.6 Adsorption systems are currently treating wastewaters
with flows from as little as 10,000 GPD to as high as 20,000,000 GPD.
Superficial contact times vary from a low of 23 minutes to a high of 800 minutes.
Carbon exhaustion rates vary, due to flow and concentration of contaminants, from
a low of 55 Ibs/day to a high of 120,000 Ibs/day.
Treatment objectives also vary widely. In some cases they are expressed as
pollutants measured in terms of such gross parameters as TOC, COD and color; in
other cases, the objectives are expressed as specific chemical structures. Required
effluent standards range from the mg/ L range for the gross parameters to the low
/ig/L ranges for the specific compounds. The concentration of organics in
wastewaters also varies widely, ranging from a wastewater containing as much as
15,000 mg/ L TOC, in thecase of one herbicide manufacturer, to as lowas 10 jug/ L of
chlorinated organic compounds, in the case of leachate from one industrial landfill.
The wide range in influent and effluent contaminant levels reflects the flexibility of
a carbon system and demonstrates that carbon is often used to supplement other
forms of treatment, e.g., biological. A 1982 review of industrial systems utilizing the
Calgon Adsorption Service showed that they could be classified as follows:
Carbon adsorption only 43 percent
Carbon adsorption preceding biological treatment 24 percent
Carbon adsorption combined with biological treatment 7 percent
Carbon adsorption only, with reuse 10 percent
Carbon adsorption with other treatment/disposal process 16 percent
Carbon adsorptions systems preceding biological treatment are generally used to
remove compounds which are toxic to the micro-organisms. In cases where the
biological treatment is utilized ahead of the adsorption system, the carbon is added
because biological treatment alone cannot meet the treatment objective.
340
-------
Table 4. Review of Operating Adsorption System Parameters
Client Plant
Landfill
Leachate
Landfill
Leachate
Specialty
Chemicals
Specialty
Organics
Herbicide
Manufacturing
Resin
Manufacturing
Specialty
Chemicals
Engine Repair
Landfill Leachate
Flow
(GPD* 103)
24
10
200
40
144
36
500
200
30
Pretreatment
N
E
C
f
P
C
F
P
E
P
E
E
P
B
S
F
L
F
P
System
* Design
2 Stage
1 Tram
2 Stage
1 Train
2 Stage
1 Train
2 Stage
1 Train
1 Stage
1 Train
2 Stage
2 Tram
2 Stage
1 Train
2 Stage
1 Tram
Superficial
Contact Time
(Minutes)
628
40
200
48
193
32
78
40
Types of
Organics In
Wastewaters
Hexachloroethane
Hexachlorobutadiene
Hexachlorobenzene
—
—
Phenol
—
Phenol
Alkanes, Phenol
Dioctyl azelate
Chloroform
Carbon Tetrachlonde
Trichloroethylene
Treatment
Objective
Hexachlormated
Organics
TOC Removal
TOC Removal
TOC Removal
Phenol Removal
Phenol Removal
TOC Removal
Phenol Removal
Phenol Removal
Chlorinated
Hydrocarbon
Removal
Influent
(mg/L)
10^/g/L
30
4000
1650
250
275
400
20
30
50
Effluent
(mg/L)
non-
detect
4
750
20
<1
<01
<20
5
<01
0.001
Carbon
Exhaustion
Rate
(Ibs/day)
223
55
438
2250
274
5300
1440
700
Perchloroethylene
-------
Coke
Manufacturer
Resin
Manufacturer
Specialty
Manufacturers
Pesticide
Manufacturing
Pesticide
Manufacturing
Plastic
Manufacturing
Specialty
Chemicals
Chemical
Washing &
Waste Processing
Pesticide
Manufacturing
Specialty
Resins
Herbicide
Manufacturing
255
200
220
144
86
350
10
200
250
50
43
C
F
E
E
E
O
f
E
F
B
F
E
E
F
E
C
O
E
F
E
2 Stage
2 Tram
2 Stage
2 Tram
2 Stage
1 Tram
2 Stage
1 Tram
2 Stage
1 Tram
2 Stage
1 Train
2 Stage
1 Tram
2 Stage
1 Tram
2 Stage
1 Train
2 Stage
1 Tram
2 Stage
1 Tram
135 Phenol
Methyl ethyl ketone
78 Alkyl phenol
64 Methylene Chloride
Chloroform
Toluene
Benzene
Carbon Tetrachloride
55 Methylene Chloride
90 —
23 Phenolics
800 MBAS, Phenol,
0-Cresol, Ethers
40 Phenols
Xylene, Alky!
Benzene
Benzaldehyde,
32 Atrazme
160 Phenol, Naphthalene,
Diethyl phthalate,
Methylene Chloride
191 —
Phenol Removal
Phenol Removal
COD Removal
Pesticide Removal
Pesticide Removal
Phenol Removal
TOC Removal
Phenol Removal
Atrazme
SOC
Phenol Removal
TOC Removal
150 1
1 50 <0 1
200 20
125 01
1-2 0 1
05 <0 1
3600 60
80 <0 1
35 <01
45 <0 002
3500 16
40 0.5
600 60
6250
6000
3300
700
1200
1600
700
525
2700
4900
1370
Table 4. (Continued).
-------
Microbiocide
Manufacturing
Specialty
Chemicals
Specialty
Chemicals
Synthetic Textile
Manufacturing
Waste Processing
Ore Processing
Dye
Manufacturing
Pesticide
Manufacturing
Specialty
Chemicals
Herbicide
Manufacturing
Synthetic Fibers
Manufacturing
50
43
500
115
CO
80
150
P
170
150
23
120
E
E
F
F
N
S
F
P
E
P
E
P
E
P
P
P
E
2 Stage
1 Train
2 Stage
1 Train
1 Stage
2 Tram
1 Stage
1 Tram
2 Stage
1 Tram
2 Stage
1 Tram
2 Stage
1 Tram
2 Stage
1 Tram
2 Stage
1 Tram
2 Stage
1 Tram
2 Stage
1 Tram
138
350
32
55
175
100
55
40
46
400
75
Dimethylamme
Toluene diamine
Benzene
Toluene
Phenol
Naphthalene
_
Various Hydrocarbons
—
Organic Dyes
—
Benzene
Aniline 180 mg/L
Dmitrophenol
480 mg/L
Nitrobenzene
1300 mg/L
Phenylenediamme
Methylester
Dimethylamme
Chloroform
TOC Removal 1 500 2
COD Removal 480 50
Naphthalene 10 <0 1
Removal
Color Removal — —
TOC 52 <20
Surfactant
Removal 225 <75
Color Removal
(APHA) 1 300 <1 50
TOC Removal 600 20
Benzene Removal 280 <2
TOC Removal 15,000 2000
Solvent Removal 120 <0.01
2100
330
550
275
2300
329
4000
8700
4500
19,000
3300
Table 4. (Continued).
-------
Organic 20
Intermediates
Herbicide 300
Manufacturing
Specialty 35
Chemicals
Naval Stores 1080
Organic 1950
Chemicals
Organic 504
Chemicals
Organic 20,000
Chemicals
General 1 5
Chemicals
JE
F
S
F
E
P
O
C
B
E
B
E
P
L
E
C
A
F
E
2 Stage
1 Tram
2 Stage
2 Train
2 Stage
1 Tram
Parallel
Moving
Beds
Parallel
Moving
Beds
Parallel
Moving
Beds
Parallel
Moving
Beds
1 Train
3 Stage
400 Chloroform
Phenol
55 Xylene
Triethylamme
Nitrophenol
Chloromtrobenzene
199 2-MIN Dimethyl
Sulfate
— —
— Toluene diamine
— Phenol
Urea
30 Dyes, Pigments,
Various Chemicals
540 Xylene
TOC Removal 6400 1600
TOC Removal 1300 <600
Phenol Removal <0 01
Odor Removal — —
TOC — —
TOC — —
TOC — —
TOC — —
TOC 6400 400
COD 28,500 1300
10,000
8400
219
22,000
11,000
30,000
1 20,000
6500
'Abbreviations as follows N=none, E=equalization, C=clanfication, F=filtration, P=pH adjustment (control), B=biological, S=setthng, L=sedimentation,
O=oil and grease removal, A=aeration
-------
PRETREATMENT REQUIREMENTS FOR ADSORPTION
SYSTEMS
The pretreatment process or processes required will be dictated by the adsorption
system design employed and the wastewater constituents. This section discusses the
types of pretreatment processes utilized with the various adsorption system designs.
Conditions Requiring Pretreatment
Industrial wastewaters may contain many types of contaminants which interfere
with, and reduce the effectiveness of, the adsorption process, indicating that
pretreatment should be considered. Suspended solids, insoluble or free oils and
grease, and chemically unstable wastewaters could be particularly troublesome.
The objective of the suspended solids removal process is to prevent a premature
excessive pressure drop through the carbon bed. The maximum desirable
concentration of suspended solids that a backwashable adsorption system can treat
economically is approximately 50 mg/ L to 65 mg/ L at a hydraulic loading of 4
gpm/ft2. At this suspended solids loading, one backwash using approximately 2.5%
of the total process water will be required for each 24-hour operating period.
Obviously, higher concentrations of suspended solids or higher hydraulic loadings
will require more frequent backwashing. Normally, the backwash water is returned
to the head of the plant. If the clarifier is operating at capacity, a backwash water
collection tank may be employed to permit control of the rate at which the
suspended-solids-laden backwash water is returned to the system. It should be noted
that the suspended solids in the backwash water settle more rapidly than do the
solids in the wastewater entering the treatment plant initially. Where necessary,
conventional clarifiers, with and without polymer addition, have been utilized to
reduce the suspended solids concentration. If the suspended solids float, air flotation
units have been successfully used to achieve reduction. High-rate pressure filters
using sand or mixed media also have been successfully employed.
Insoluble oil and grease in the influent to a carbon adsorption system can coat the
granular activated carbon and cause loss of adsorption capacity. Skimmers in the
clarifiers and air flotation units have been successfully used to reduce the levels of
these contaminants to the maximum desirable concentration of approximately 10
mg/ L. Concentrations of insoluble grease and oil as high as 50 mg/ L have been
successfully treated in adsorption systems. In these systems, the oils and grease are
removed in the upper portion of the carbon bed which, as a consequence, loses its
adsorptive capacity and must be sacrificed.
High concentrations of calcium carbonate or calcium sulfate in wastewater coat
the granular activated carbon and cause a loss of adsorptive capacity. Such carbon
may have to be discarded because it cannot be reactivated. The objective of
pretreatment in such a case is to increase the wastewater stability. This can be
accomplished by simple pH adjustment or the addition of a scale-inhibitor such as
sodium hexametaphosphate.
Four types of adsorption systems have been employed in industry for wastewater
treatment, each imposing different requirements on pretreatment processes. The
system using fixed downflow beds in parallel operation is flexible, since backwash
capability may be designed into the system. This capability may include air scour or
surface washers to break up the solids which collect on the top of the carbon bed.
Experience indicates an adsorption system having backwash capability can handle
the aforementioned 50 mg/ L to 65 mg/ L suspended solids at a hydraulic loading of 4
gpm/ft2 on a continuous basis when being backwashed once every 24 hours.
Backwashing capability may also be designed into the adsorption system using
downflow beds in series; the parameters used for the fixed-bed parallel system are
applicable here, too. However, if backwash is not included, experience indicates the
system influent should contain no greater than 10 mg/L of suspended solids. The
345
-------
first bed in the series will collect the solids and must be designed to be taken off
stream for reactivation prior to being limited by a high pressure drop.
The adsorption system consisting of upflow-packed fixed beds in series has
virtually no capacity for handling suspended solids. If a premature high pressure
drop occurs, the entire carbon bed must be removed. In this case pretreatment to
reduce the solids to approximately 5 mg/L to 10 mg/L is essential.
The adsorption system employing upflow expanded beds permits the suspended
solids to pass through the granular activated carbon. This occurs because the
granular carbon bed is expanded approximately 10 percent. If suspended solids
removal is an objective, it must be accomplished by a final solids removal process.
KEY DESIGN PARAMETERS
The design of an efficient and economical adsorption system for treating
wastewater should be approached in the same manner as the design of any unit
process in a chemical plant. Careful planning and testing are essential in order to
generate the firm data on which design decisions must be based. The sequence of
steps needed to provide the data required to design a carbon adsorption system has
been detailed by Hansen and Froelich7 and Rizzo and Shepherd.8
Wastewater Survey
The first and most important step in solving an industrial wastewater treatment
problem is to conduct a thorough wastewater survey, which includes analysis of
flows and of the quantity and types of pollutants at key points in the wastewater
system. The three basic objectives of a wastewater survey are: (1) thorough
characterization of the constituents present, (2) definition of conditions representing
problems in use of activated carbon treatment, or favoring treatment processes other
than adsorption and (3) assistance in the wastewater process design. The wastewater
survey may improve the quality of the feasibility study design, or it may actually aid
in determining the most effective point at which to apply adsorption and thus assure
a more economical use of the carbon.
Analytical tests outlined by Hansen and Froelich7 permit a reliable general
characterization of a wastewater when considering treatment by activated carbon.
This should include determination of total and soluble organic carbon, and total and
soluble chemical oxygen demand which indicate the nature and quantity of organics
present in the wastewater. The presence of inorganic components including calcium,
sodium, chlorides, phosphates and sulfides suggests potential problems in the
reactivation process such as air pollution, slagging or corrosion. Measurement of
pH, suspended solids, immiscible oils and grease, and chemical stability are needed
in determining whether pretreatment or post-treatment to activated carbon is
necessary. More specific analyses should be conducted when toxic and refractory
organics are present. In this case, sophisticated testing is required, since chemicals of
interest may be present in very low concentrations. A reliable characterization of
such compounds can usually be obtained by gas chromatograph/mass
spectrophotometry.
Evaluating Adsorption Feasibility
The first step in assessing the feasibility of granular activated carbon treatment for
a specific application is development of a liquid-phase adsorption isotherm in the
laboratory. Data for isotherms are obtained by treating fixed volumes of the
contaminated wastewater with a series of known weights of carbon. The carbon-
wastewater mixture is agitated at a constant temperature for one to three hours.
After the carbon and the wastewater reach adsorptive equilibrium, the carbon is
346
-------
removed and the residual contamination in the wastewater is measured by an
appropriate analytical method. Specific information on how to construct isotherms
and interpret the data from single component isotherms is provided in a number of
references, including the article by Hansen and Froelich.
The isotherm plots of mixed-solute waste waters commonly found in industry are
frequently nonlinear and more complex to interpret. Such plots may exhibit a series
of straight lines with dissimilar slopes, each representing one of the components in
the mixture. In such cases, each of the lines can be treated as a separate isotherm and
a theoretical carbon demand can be calculated by summing the weights required for
removing each contaminant. An example of such an isotherm is presented in Fig. 1.
In general, isotherm studies can provide the following information:
1. the attainable effluent after granular carbon treatment
2. the equilibrium adsorption capacity of the carbon (maximum weight pickup
of organics)
3. the optimum pH for adsorption
4. the effect of concentration of the contaminants on the adsorption capacity
5. the effect of competitive adsorption on capacity
This information is invaluable for making a quick determination of the feasibility
and economic viability of granular carbon treatment. However, additional
information from pilot column studies is needed before the detailed design of a
carbon treatment system can be optimized.
Dynamic Column Testing
Assuming isotherm tests prove that granular carbon adsorption is capable of
producing an effluent of suitable quality and that the theoretical carbon usage is
economically acceptable, one may then proceed to determine the full-scale
adsorption system design parameters. Of primary importance are the
determinations of the optimum operating capacity and contact time which are
needed to establish the column dimensions and the number of units needed for
continuous treatment. Optimum contact time depends upon the rate at which the
contaminant is adsorbed by the carbon and can be determined by dynamic pilot
column testing.
The test is usually conducted with a series of columns containing carbon. The
superficial contact time usually is in the range of 60 to 150 minutes but can be
greater. A typical pilot-column arrangement is shown in Fig. 2.
Wastewater is pumped through the column system, and effluent samples are
collected at appropriate intervals from each of the columns. The amount of impurity
remaining in the samples is plotted as a function of volume throughput for each
column. The result is a series of curves that vary in shape, depending on the dynamics
of the system.
The point at which the impurity in a column effluent exceeds the treatment
objective is called the breakpoint. That part of the curve between the initial leakage
and the point where the column effluent concentration is the same as the influent is
called the breakthrough curve (Fig. 3).
During adsorption, the carbon in the inlet section of a column becomes saturated
with impurities, whereas the carbon in the outlet section remains relatively virgin.
Between these two extremes lies the adsorption zone, where the removal of impurity
is actually taking place. As the carbon becomes saturated, the adsorption zone
moves through the bed. The time required for appearance of the breakpoint in the
effluent, as well as the slopes of the breakthrough curves, provides an index of the
relative depth of the adsorption zone.
From the breakthrough curve, the volume of acceptable effluent collected for each
contact time can be read directly. From isotherm curves, the minimum carbon
exhaustion rate (pounds of carbon used per unit volume of wastewater treated) can
also be calculated. With these data, one can plot a curve of contact time vs.
347
-------
x^
M,
M
C V
c,
5
o>
O
O
100
90
80
70
60
50
40
30
20
10
1 I I I T
100
200 300 400 600 800 1000
TOC Concentration, mg/l
Figure 1. Isotherm plot of wastewater containing multi-components.
The carbon usage rate is determined by using the following equation:
Co- C,
X
C, -C2
X
L Mr, "~ Me,
—
LBS/1000 gal. =
From Figure 1 obtain the following values:
Co = 700 mg/L ^-~ = 70 mg/g
C, = 575 mg/L ^ = 49 mg/g
C2 = 220 mg/L — = 36 mg/g
Substituting these values into the equation gives:
X 8.337
LBS/1000 gal. =
700 - 575
70
575-200
Theoretical Carbon Usage = [l5.14] X 8.337 = 126 Ibs/1000 gal.
*Example assumes 100% removal of TOC
348
-------
To Drain
Bleedoff Valve H
I
Sample In . — -
"Ur
Pump
Backwash Inlet
i
1
— te
T f
F
Carbon
•'••'•'•'••'••'•'i
T.
ample Out f
-T-t ^r.7
f?
Carbon
-••••••••••••••••
Sample O
r
Hj
ut
Sample
Valve
•t H*-
T Ss
19
Carbon
.v.w.v.v
r
imple Out
i Backwash
^
4
i
i-th-
Carbon
4-in height
Shot Gravel
/ Filter mesh
S f'*' Strainer
T \ Output
Sample Out f _. ,
K w ' Flow-control
Flow-control Valve
Figure 2. Pilot system flow diagram for evaluating the feasibility of carbon adsorption.
-------
100
10 20 30 40 50 60
Volume throughput, gpm
70
Figure 3. Typical breakthrough curves from pilot carbon adsorbers.
exhaustion rate (Fig. 4).9 Such a curve reveals at a glance whether there is anything
to be gained from long contact times. At the point where the curve becomes
asymptotic to the minimum carbon usage line, there is no advantage in increasing
adsorber volume to extend contact time.
Depending upon the type of full-scale adsorber system contemplated, further pilot
test work may be necessary. Hansen and Froelich provided procedures for
evaluating the operation of fixed beds in series.7
Reactivation Testing
Economics favor the use of granular carbon because it can usually be reactivated
and reused. It is, therefore, important to confirm that the spent carbon generated in
the column test can be successfully reactivated.
Tests need to be conducted to determine the quality of reactivated carbon which
can be obtained and the degree of difficulty in reactivating the carbon. The difficulty
of reactivation is an important parameter in sizing a furnace.
Tests also need to be conducted to determine the possible emissions generated
during reactivation and to determine the corrositivity of the spent carbon. These
parameters are necessary in order to design air pollution control devices for the
reactivation system and to select the proper materials of construction.
DESIGN OPTIONS
After it has been decided that adsorption on activated carbon is the applicable
technology, the following questions must be answered: (1) Should powdered or
granular activated carbon be used? (2) Should/can the carbon be regenerated on-site?
If not, should it be regenerated off-site? (3) What type of adsorption system should
be used?
350
-------
(0
OB
30
£ 20
ID
QC
C
o
-------
On-Site Vs. Off-Site Regeneration
If carbon is to be regenerated several factors determine whether regeneration
should be done on-site or off-site: (1) availability of capital, (2) availability of space,
(3) availability of people, (4) capital investment for types of processing operations
required on-site, (5) local air and water pollution guidelines, and (6) possible
recovery and reuse of organics.
Adsorption System Type
When the system designer has chosen the form of carbon and the type of ion
carbon regeneration, he still must select the type of adsorption system which will
provide a quality effluent most reliably and economically. This discussion is limited
to those systems using granular carbon since these generally offer greater
advantages.
Currently, there are two basic types of adsorbers which can be designed to operate
under pressure or at atmospheric pressure. These are commonly referred to as
moving or pulse-bed and fixed-bed. Each can be operated as either packed or upflow
expanded beds.
In the pulse-bed adsorber, the liquid enters the bottom cone and leaves by way of
the top cone. Periodically, when the maximum effluent criterion in terms of, say,
TOC is reached, the flow of liquid is stopped. Spent carbon is withdrawn (pulsed)
from the bottom and virgin or reactivated carbon enters the top.
In a fixed-bed adsorber, the liquid passes through the carbon until the carbon is
spent or the maximum effluent criteria is reached. At that point, the entire volume of
carbon is removed and replaced by virgin or reactivated carbon.
The choice of system depends on many factors. One which has not been widely
appreciated is the variation in the effluent quality in the two types of adsorbers. The
concentration of organics in the effluent from the system will have the greatest
fluctuation in a single fixed-bed system. A typical profile of effluent concentration
variation is depicted in Fig. 5. In this case the carbon in the adsorber must be
changed each time the effluent reaches the maximum specified level of impurity.
Thus, the average quality of the effluent is significantly better than the specification
value. There would be an obvious advantage here in operating several columns in
parallel and blending the effluents.
Since a pulse-bed system is generally operated so that less than 10% of the carbon
is pulsed at any one time, the variation in the effluent is much less than that of the
fixed-bed system. A profile is shown in Fig. 6.
It will be noted that the average effluent concentration is maintained close to the
maximum specification value. This type of operation results in a lower carbon
dosage. The magnitude of this variation can be changed by varying the size of and/ or
frequency of the pulse.
In a well designed continuous countercurrent system the effluent quality would be
constant as shown in Fig. 7. As of now there are none of these in operation.
Some of the advantages and disadvantages of each system are given below:
Fixed-Bed System (Non-Expanded)—
Advantages
• Has lowest building profile
• Requires least operator attention
• Has less carbon fines in the effluent
• Lends itself to on-site regeneration of carbon
• Can be inspected internally as often as the adsorber is emptied
• Can treat water containing up to 50 mg/ L suspended solids
• Can be backwashed and air scoured
• Requires lower inlet water pressure to pass through the system
352
-------
c
g
c a
-------
shown in Fig. 6.
• Has lowest carbon dosage in applications having a long adsorption mass
transfer zone.
Disadvantages
• Cannot treat liquids containing >5 mg/ L of suspended solids
• Adsorber is not emptied in normal operation; therefore, inspection and/or
repairs can only be done by taking an adsorber off-line and emptying the
entire contents of carbon into temporary containers.
• Effluent usually contains carbon fines after each pulsing operation
Dp-Flow Expanded Bed
Both fixed-bed and pulse-bed systems can be operated in the up-flow expanded
mode. This mode of operation permits the suspended solids in the influent to pass
through the carbon bed. To assure that this will occur, the designer must provide a
pilot column utilizing the actual wastewaters operated at the expected flow
conditions. In addition, the type of liquid distribution system proposed for use in the
fixed-bed system should be tested to insure that plugging will not occur. Utilizing
backwashing to expand the bed and remove solids defeats the purpose of this type of
adsorber since the carbon particles will be redistributed, thus destroying the mass
transfer zone. Also, carbon losses are higher in this type of system.
System Configuration
Each of the two types of adsorbers can be arranged in various ways. Those shown
in Fig. 8 are: (1) single adsorber, (2) two or more adsorbers in series, (3) two or more
adsorbers in parallel, and (4) four or more adsorbers in series - parallel.
Selection of a particular system is generally based on the following factors:
Fixed Bed—
Single Adsorber
• Mass transfer zone is short, i.e., saturation of the carbon occurs shortly after
breakthrough.
• Carbon dosage is low and the cost of replacing or reactivating the carbon is a
minor operating expense.
• The investment required fora multiple column system cannot be justified by the
resultant lower carbon dosage.
Two or More Beds in Series
• The performance is such that saturation of the carbon occurs a longtime after
breakthrough, i.e., the mass transfer zone is long.
• Influent concentration fluctuates widely.
• It is economically attractive to have more stages in order to more completely
exhaust the carbon.
Two or More Beds in Parallel
• The flow rate is high and the size of the adsorbers in a single pass would be too
large to be economical or feasible.
• It is necessary to operate with a minimum pressure drop.
• Average effluent quality is desired to be essentially equal to specification value.
This is achieved through blending of the effluents from the adsorbers.
• Space limitations prevent the use of large diameter or extremely high vessels.
• The adsorption process must be continuous.
Two or More Beds in Parallel - Series
• Flow rate is high.
• It is desirable to have a lower carbon dosage than is possible with parallel only
operation.
-------
Single
Untreated <
Water
Treated
Water
Series
Untreated
Water
Treated
Water
Parallel
Water
I
\
Treated
Water
Series-Parallel
Untreated
Water
*- Treated
Water
Figure 8. Fixed bed system configurations.
Pulse Bed—
• The mass transfer zone is extremely long and there is an incentive for obtaining
the minimum carbon dosage.
• Suspended solids are <5 mg/L.
• Minimum capital investment is desired.
•There is a desire to minimize wide fluctuations in the effluent.
355
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Selection of Type of Fixed Bed—
If fixed beds are selected, a choice must be made between two modes of operation,
i.e., pressure or atmospheric.
The advantages of the pressure vessel are:
• Carbon removal can be fairly easy.
• Allowable pressure drop due to suspended solids is limited only by design
pressure of adsorber and frequency of backwashmg.
• Installation is above grade.
• Operation in series is easily accomplished.
•Selection of type of material is not limited.
Atmospheric open-type adsorbers are used under the following conditions:
• Available pressure is determined by gravity only.
• There are many beds in parallel.
• Pressure build-up due to solids will be less than 5 psi per 24 hours.
• Flows are high.
•Concrete or lined concrete is a suitable material of construction.
REGENERATION
If the carbon must be regenerated to assure an economical carbon process, the
following regeneration techniques may be considered: steam regeneration, chemical
regeneration and thermal regeneration. Usually steam regeneration is used when the
adsorbed organic compound(s) has a boiling point less than 150°F and/ or when it
has a reuse value which makes this technique economical; however, few applications
are known. Likewise, chemical regeneration is a limited technique and is
infrequently used, although caustic has been used to regenerate carbon which has
been used to treat wastewater containing phenol. In the latter case the sodium
phenolate which is formed is reused in the process. The most widely used
regeneration technique is thermal reactivation, for which two basic process modes
generally are used: direct fired and indirect fired.10
Types of Equipment for Thermal Reactivation
The following types of equipment currently are being employed for thermal
reactivation of carbon which has been used to treat industrial wastewater(presented
in order of on-stream carbon reactivation capacity and number of operating units):
• Multiple Hearth Furnace
« Vertical Tube-Type Furnace
« Direct Fired Rotary Kiln
• Fluidized Bed
• Indirect Fired Rotary Kiln
• Infrared Furnace (Indirect Heating)
The multiple hearth furnace is the most widely used for thermal reactivation; its
advantages and disadvantages and those of rotary kilns have been widely reported."
The fluidized-bed furnace and infrared furnace have only recently been applied to
reactivating granular carbon used in treating industrial wastewater. Experience to
be obtained in the next few years will determine whether or not these types of
equipment will have a significant role in future carbon reactivation, particularly
when the adsorbate on the carbon causes corrosion and formation of slag, and is
difficult to remove in the reactivation process.
Environmental Considerations
Thermal reactivation of carbon which has adsorbed impurities from industrial
wastewaters conceivably may present health hazards. These may be adequately
356
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controlled with properly designed equipment. Wet or dry scrubbers are used to
remove components such as SO:, HF and HC1 from the exhaust gases. The
scrubbers are also effective in removing carbon fines and ash components.
Organic compounds which are volatilized in the furnace can be oxidized to CO2
and H2O in a direct fired afterburner which operates at temperatures of 1400-2000° F
and contact times of one to two seconds.
Energy Considerations
An important factor in selecting the type of equipment is its energy requirement
and that of an ancillary afterburner and/ or scrubber. Since the capital and operating
costs of these units are related to the volume of gases being treated, the selection of
direct or indirect heating systems has a major impact on costs. The advantages are all
in favor of indirect systems. The volume of gases from an indirect system is only 25%
of that from a direct fired system, and the fuel consumption in the afterburner is
reduced 75%. Because the volume of gases is less, the size of the induced draft fan,
ducting, scrubber and stack are smaller with subsequent savings in capital and
operating costs.
Additional savings are obtained because of the elimination of steam addition. In
the indirect units, carbon and evolved gases can flow concurrently. Thus, instead of
having to add steam to serve as the oxidizing gas in the high temperature zone of the
furnace, the water which is evaporated from the pores of the carbon in the inlet
portion of the furnace serves this function.
CAPITAL AND OPERATING COST ESTIMATES
Since a carbon system consists of adsorption and, in some cases, carbon
reactivation equipment, the costs are presented separately for the two systems. Also,
since a small adsorption system can have a large reactivation system or a large
system can have a small reactivation system, combining the costs is not meaningful.
Adsorption System Costs
The capital costs shown in Fig. 9, have been estimated for fixed-bed adsorbers
with and without backwash and air scouring.
The design basis is as follows:
Flow, MOD 0.5 - 3.0
Surface Loading, gpm/ft2 3
Superficial Contact Time, hr. 1
No. of Adsorbers in Series 2
Backwash Rate, gpm/ft2 12-15
The cost includes site preparation, foundations, building, feed and backwash
pumps, air compressor, electrical, automatic controls, engineering, overhead and
profit. It is assumed that all utilities and off-site facilities are available at the battery
limits of the system.
The operating costs associated with the adsorption process are minor. The only
utility which is required is power. Based on4c/KWH, the dollar costs per day are 16,
32 and 64 for flows of 0.5, 1.0, and 2.0 MOD, respectively. Operator attention for
this type of system is minimal, i.e., periodic checks two or three times per shift is
normally all that is required.
357
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3000
S 2500
2000
1500
a
c 1000
§ 500
0.5 1.0 1.5 2.0 2.5
Wastewater Flow, mgd
3.0
Figure 9. Installed costs for fixed bed adsorption systems.
Reactivation System Costs
The total installed costs shown in Fig. 10, are based on using multiple hearth
furnaces or rotary kilns. The basis for developing the installed costs is that used for
the adsorption process. The design basis is as follows:
• Furnace size based on 80 lb/Day/ft2 of hearth area
• Afterburner and water scrubber is included
• Spent and Reactivation Storage Tanks, 40-80,000# of carbon capacity
• Operating costs for reactivating spent granular carbon are shown in Table 5. The
basis for the costs is as follows:
• One operator per shift at $16/hr, plus 25% for supervision
• Fuel for furnace and afterburner - 8,000 BTU/lb carbon at $4/ million BTU
• Steam rate of 0.6 Ib/lb carbon at $5/1,000 Ib
• Power at 4
-------
(0
o
o
3000
2000
1000
i
I
J_
10 20 30
Reactivation Rate, 1000 Ib/d
40
Figure 10. Installed costs for reactivation systems.
Table 5. Operating Costs ($1,000/yr) For Reactivating Spent Granular
Carbon
Reactivation Rate, Ib/Day
5,000
10,000
30,000
60,000
Fuel
Power
Steam
Makeup carbon
Maintenance
Labor and supervision
General plant overhead
Total operating cost, $1,000/yr
Operating cost, C/lb carbon
55
15
5
90
90
175
50
480
288
110
30
10
180
100
175
65
670
20.1
320
75
30
540
150
175
130
1,420
14.5
640
105
60
1,080
200
175
225
2,485
125
mg/L TOC). Their calculation showed that 48.2 x 106 BTU's per day would be
required for the activated sludge system and 39.4 x 106 BTU's per day for the
reactivated carbon system.
In a similar study the United States Army Corps of Engineers found that an
independent physical-chemical treatment system would consume less energy and
resources than either an advanced wastewater treatment system (AWT) that
included biological plus polishing, or land irrigation.'3 The energy consumed for
each of these systems is shown in Table 6.
359
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Table 6. Comparative Consumption of Energy for Treatment Systems of
Similar Capacity Studied by the U.S. Corps of Engineers
Treatment Type
Energy Consumed — Mega Watts
Independent physical-chemical
Advanced waste treatment (biological
plus polishing)
Land irrigation (on private land)
Land irrigation (on public land)
350-420
400-530
1100
1142
A more detailed look at the individual elements of cost associated with the
reactivation of carbon is indicated since this process accounts for a major portion of
the energy required by the adsorption system.
MONITORING TECHNIQUES
Monitoring of a carbon adsorption system refers to measurement of the organic
contaminants in the influent and effluent; it does not mean a measurement of the
carbon itself. Monitoring of this system is required for the following reasons: (1)
insure that the treatment objective is consistently being met; (2) determine when the
carbon needs to be reactivated; and (3) provide a baseline of information which is
necessary in investigating problems that may develop in the adsorption system.
The frequency and type of monitoring which is required are site specific and are
dependent on the frequency of carbon reactivation and the specific treatment
objective. A system in which the carbon needs to be changed every day will obviously
require more frequent monitoring than one in which the carbon needs to be changed
only every six months. Regardless of the actual number of analyses to be performed,
the objective of a good monitoring program remains the same - to provide useful
information at a reasonable cost. The program should consider several ways by
which that end might be served. If the treatment objectives are COD or BODs
removal tests should be conducted to see if a relationship exists between these
parameters and TOC. The use of TOC is encouraged since it is a way of obtaining
information at low cost. Assuming TOC is used as a surrogate parameter, COD or
BODs could be run on a spot check basis or as necessary to confirm the relationship
and to meet discharge permit requirements. If the treatment objective requires the
total removal of a number of priority pollutants, the tests should be conducted to see
which of the compounds will first leak or break through the carbon columns.
Monitoring for the least adsorbable priority pollutant will reduce costs, while
guaranteeing that the remaining priority pollutants are being controlled.
In a well-designed adsorption system, the carbon will be completely spent before it
is replaced with virgin or reactivated carbon. The time to replace it is usually when
the concentration of the organics in the effluent is the same as in the influent. This
can occur in pulse beds or fixed beds in series systems.
When the carbon is reactivated on-site the conditions for reactivation are
controlled so that the reactivated carbon is as effective as virgin carbon. The quality
of the reactivated carbon can be determined by conducting comparative adsorption
tests and standard carbon quality tests as iodine and carbon tetrachloride numbers
and apparent density. The latter can usually be corrected with one or more of the
other tests for a given application. It may then be used by plant operators to control
the reactivation process.
360
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FUTURE DEVELOPMENT NEEDS
The use of adsorption in treating industrial wastewaters is important today, and it
will grow more so in the future. Full use, however, is retarded by a number of
problems that have been experienced in the design and application of the process.
Each of the following problems, in particular, represents a significant research need.
Simpler Analytical Tools & Tests Required
Existing dynamic experimental tests to establish key design parameters for an
adsorption system are very timeconsumrngand expensive. Fasterand lessexpensive
techniques than these utilizing carbon column studies are needed.
More Efficient Reactivation Technology Needed
Techniques and procedures must be improved in order to minimize energy utilized
in reactivating carbon and to reduce carbon burning and attrition losses during
reactivation.
Improved Materials of Construction for Adsorption Systems Needed
Since a carbon-water slurry is quite corrosive as are many industrial wastewaters,
the improvement of anti-corrosive vessel linings and the selection of the correct
materials of construction for other system components is extremely important.
Although some work has been done in this area, more needs to be done.
Improved Materials of Construction for Reactivation Systems Needed
The complete destruction of halogenated organic compounds in a thermal
reactivation furnace can cause problems particularly for such equipment as furnace
rabble arms and teeth, flights in the rotary kilns, belts and tubes in the infrared
furnace and vertical tube-type furnaces. It is obvious the improvement of materials
of construction is a key factor in minimizing these problems. It has also been
observed that inorganics may attack the refractory lining of reactivation equipment.
More work must be done in this area to assure availability and proper selection of
the correct type of refractories, and/ or to remove the refractory attacking inorganics
from the spent carbon prior to reactivation.
Improvement Needed In Adsorption System Underdrains
An underdrain is the so-called heart of an adsorber. It must provide good
distribution, be of proper materials of construction, and be sturdy enough to handle
high backwash water rates. More work is needed in this area to assure proper design
of underdrains.
More Effective Methods of Recovery Needed
Activated carbon serves as a mechanism for concentrating organic materials in
wastewaters. Treatment costs, in a number of applications, could be significantly
reduced if a practical technology for recovering valuable organic chemicals from
spent carbon was available. Low boiling chemicals have been successfully recovered
using steam distillation. However, recovery of high boiling compounds has been
retarded by unsuitable materials of construction and high energy requirements.
361
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REFERENCES
1. Weber, W.J., C.B. Hopkins and R. Bloom. "Physical-ChemicalTreatment of
Wastewater," Journal of WPCF, Volume 42, No. I, p. 83, 1970.
2. Hager, D.J. and J.L. Rizzo. "Removal of Toxic Organics from Wastewater
by Adsorption with Granular Activated Carbon," presented at
Environmental Protection Agency Technology Transfer Seminar, Atlanta,
GA, April 19, 1974.
3. Rizzo, J.L. "A Comparison of Unit Processes and Dissolved Organic
Removal," presented at the Adsorption Conference sponsored by the Calgon
Corporation, Pittsburgh, PA, November 17-18, 1975.
4. Froehch, E.M. "Determining Feasibility for Adsorption of Toxic and
Refractory Organics," presented at the Adsorption Conference sponsored by
the Calgon Corporation, Pittsburgh, PA, November 17-19, 1975.
5. Hager, D.G. "Wastewater Treatment via Activated Carbon," Chemical
Engineering Progress, Vol. 72, No. 10, October, 1976.
6. Brunotts, V.A. "A Review of Operating Calgon Adsorption Service
Systems," paper presented at the Adsorption Conference sponsored by the
Calgon Corporation, Pittsburgh, PA, November 17-19, 1975.
7. Hansen, K..H., Jr. and E.M. Froelich. "The Use of Activated Carbon
Adsorption in Water Pollution Control," Chemical Times & Trends, July,
1978.
8. Shepherd, A.R. and J.L. Rizzo. "Treating Industrial Wastewater with
Activated Carbon," Chemical Engineering, Vol. 84, No. 1, p. 95, January 3,
1977.
9. Erskine D.B. and W.G. Schuliger. "Activated Carbon Processes for Liquids,"
Chemical Engineering Progress, Vol. 67, No. 11, p. 41-44, November, 1971.
10. Juhola, A.J. and F. Tepper. "Regeneration of Spent Granular Activated
Carbon," Robert A. Taft Water Research Center, Report No. TWRC-7,
February, 1969.
11. Zamtsch, R.H. and R.T. Lynch. "Selecting a Thermal Regeneration System
for Activated Carbon," Chemical Engineering, Vol. 85, No. 1, p. 95, January
2, 1978.
12. Bernardin, F.E. and J.C. Petura. "Energy Considerations in Adsorption as a
Wastewater Renovation Technique," presented at Second National
Conference on Water Reuse, Chicago, 1L. Published by AlChE, New York,
NY, pp. 147-154, 1975.
13. Detroit District U.S. Army Corps of Engineers. "Southeast Michigan
Wastewater Management Survey," Final Report, Scope Study Report, pp.
108-110, May, 1974.
362
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SEPARATION OF ORGANIC SUBSTANCES IN
INDUSTRIAL WASTEWATERS BY
MEMBRANE PROCESSES
Ronald F. Probstein, PhD, Calvin Calmon, PhD,
and
R. Edwin Hicks, PhD
INTRODUCTION
Membrane Processes
A membrane is a selective barrier that permits some entities to pass through it
while preventing the passage of others. In particular, membranes have the ability
to prevent the passage of dissolved molecules while allowing the solvent, usually
water, to pass through under the influence of a driving force.
It is the ability to reject particles of molecular dimensions that principally
distinguishes a membrane from an ordinary filter. Another distinguishing feature is
the fact that the driving force for a filter is always the pressure difference across the
filter, which forces the water through while the suspended particles are retained. In
the case of membranes there are a number of driving forces in addition to pressure
including gradients in concentration, electrical potential and temperature.
The three principal membrane separation processes are reverse osmosis (RO),
ultrafiltration (UF), and electrodialysis (ED). Of the three, ultrafiltration bears the
closest resemblance to ordinary filtration, in that the process is pressure-driven and
the particle sizes held back by the membrane are determined by the membrane pore
The Author: Dr Ronald F Probstein is Professor of Mechanical Engineering at M.I.T. and Chairman of
Water Purification Associates.
Following receipt of the Ph.D degree from Princeton U niversity in 1952, his principal work centered in the
field of fluid mechanics About 15 years ago he became concerned with the application of his studies to
desalination and water purification and since then he has earned international recognition for his work in
these fields. He has been honored for his contributions by election to the National Academy of Engineering
and American Academy of Arts and Sciences and by an award from the American Society of Mechanical
Engineers
In 1974 he formed Water Purification Associates to deal with problems of water purification, reuse and
pollution control. His activities at W P A. since that time have centered on water needs and treatments for
synthetic fuel conversion processes.
For more than ten years now he has headed up the water purification activities of the M.I.T. Fluid
Mechanics Laboratory and is presently responsible for M I.T 's teaching program in water purification, and
synthetic fuel production. Among his studies while at M.I T. have been his use of fluid mechanics to redesign
reverse osmosis, ultrafiltration and electrodialysis equipment, resulting in much improved efficiencies
Dr. Probstein holds a number of patents in water purification and desalination including methods for
continuous ion exchange, freeze desalination, continuous flocculation of waste water, and lamella
sedimentation. In addition to his texts, he has authored more than a hundred technical papers which have
appeared in various scientific and engineering journals He is a member of many engineering societies,
including A S M.E., A I Ch.E and W.P C F , is a registered professional engineer, and is listed in Who's
Who in America.
363
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sizes. However, the pore sizes of a UF membrane are typically some 1000 times
smaller than in an ordinary filter. Although reverse osmosis is also a pressure driven
process, the controlling factor in particle-rejection is not size directly but generally
relates to the diffusion properties of the molecules or ions in the membrane. In
electrodialysis, under the action of a D.C. electric field, only dissolved ions are
removed or concentrated from the bulk solution based on the selective character of
the membrane with respect to particle charge. Since the particles must be charged,
the utility of this method for organics separation is limited and shall not be
considered further.
One other membrane process applicable to organics separation is
liquid-membrane permeation. This process employs a solvent in which an aqueous
solution of caustic soda is emulsified. The emulsion acts as a one-way membrane that
is permeable only to the organic by virtue of the organic reacting with the caustic
soda and forming a compound which stays in place because it is not soluble in the
solvent. This procedure will be discussed separately from reverse osmosis and
ultrafiltration because of the limited number of organics which can be separated by
this method and also because the process is still under development.
In ultrafiltration the selective membrane accomplishes separations in cases where
the solute particles to be removed are at least an order of magnitude larger than the
solvent molecule, of which the water molecule is of principal interest in this review.
On the other hand, in reverse osmosis the membrane can separate out solute
molecules which are of the same order or only slightly larger than the water molecule.
Reverse osmosis is, however, a relatively high-pressure process with typical applied
pressures of 3 to 7 megapascals (MPa) (400 to 1000 psi), compared with
ultrafiltration where typical pressures range from0.07 to0.7 MPa (10 to 100 psi). The
membranes in both processes are fabricated from synthetic polymers with the RO
membranes continuous gels and the UF membranes relatively porous materials.
The capabilities of reverse osmosis and ultrafiltration in terms of particle size
rejection are compared in Table 1. RO membranes will reject almost completely most
species with molecular masses greater than 150. Readily available commercial UF
membranes will have high rejections for molecular masses greater than 10,000 and
will usually have almost complete rejections for molecular masses greater than
35,000.
Mechanism and Operation of Reverse Osmosis
Reverse osmosibM is a pressure-driven membrane process that is used to separate
relatively pure water from solutions containing salts, dissolved organic molecules,
and colloids. Under the action of a hydrostatic pressure applied across the membrane
separating the feed from the relatively pure water, the water passes through the
membrane and the dissolved materials remain behind.
The Author Dr. Calvin Calmon is a physical chemist with 43 years of industrial experience He has been in
the field of water and wastewater treatment since he received his Ph D degree from Yale Universityin 1938
Until his retirement in 1973, he worked almost continuously with different divisions of Sybron Corporation,
holding important positions with the research and chemical divisions On his retirement he was Senior Vice
President and Research Consultant to the Chemical Group of the Sybron Corporation Since then he has
been a principal of Water Purification Associates and a private consultant
Most of his industrial work has been in the application of ion exchangers, adsorbents, polymers and
membranes to water treatment and pollution control He has written about 80 technical publications and
holds 19 patents in these areas He is internationally known for his work in the synthesis, development,
production and use of ion exchangers His most recent activities involve the development of treatment
methods for the removal of potentially toxic pollutants from industrial wastewaters
Dr Calmon has held many important editorial and advisory posts and has frequently been honored for his
many contributions to environmental chemistry, including the Distinguished Service Award from the
American Chemical Society in 1972 and a Certificate of Appreciation for Contributions to Environmental
Chemistry from the Environmental Protection Agency in 1976
364
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Table 1. Useful Particle Size Ranges of Reverse Osmosis and Ultraf iltration
and Reference Particle Sizes
Membrane
Process
Reverse osmosis
Reverse osmosis
Ultrafiltration
Ultrafilt ration
Particle Size
Separated
(nm)*
X3.3-0.5
-
>3-5
-
Max. Particle
Size for Which
Membrane Used
(nm)*
-
60-100
-
3000-5000
Reference
Particle
Water
molecule
Smallest
bacteria
10 x water
molecule
Typical
bacteria
Size
(nm)"
0.3
200
3
2000
*nanometer: 10"9 meters
Fig. 1 shows the simplest arrangement of a reverse osmosis system consisting of a
pump to pressurize the system, usually to pressures of from 3 to 7 MPa, a membrane
element, a pressure container to house the membrane element, and controls to
regulate the flow and pressure. Normally, a reverse osmosis system is employed
with some pretreatment or postreatment which may range from a simple filter to
protect the high-pressure pump from particulate matter, to ion exchange
demineralization to upgrade the water.
Most reverse osmosis systems in use today employ semipermeable "asymmetric
membranes" made from cellulose acetate or polyamide, or "composite membranes"
made with a dense thin polymer coating on a polysulfone support film. The
asymmetric cellulose acetate membrane is a high-water-content gel structure
consisting of a thin rejecting "skin" on the order of 0.1 to 0.5 micrometers thick
integral with a much thicker porous substrate 50 to 100 micrometers thick. It is the
skin which offers the main hydraulic resistance to the flow. The porous substructure
gives the membrane strength but offers almost no hydraulic resistance. The dense
rejecting skin of the composite membranes can be up to 10 times thinner than the
skin of the cellulose acetate membranes.
The rejection of the dissolved materials is not complete; the incompleteness
depends not only on the size of the rejected species, but also on the chemistry of the
membrane and the rejected species. With the use of the correct membrane the percent
The Author: Dr. Richard Edwin Hicks received an M.S and a Ph.D. degree, both in chemical engineering,
from the University of the Witwatersrand, Johannesburg, South Africa During this time he was employed
by the South African Council for Scientific and Industrial Research. His advanced studies were in the
field of heat and mass transport phenomena, in particular the optimization of systems containing turbulence
promoters as used in reverse osmosis and electrodialysis units for brackish water desalination. Also included
was an in-depth study of packed bed transport related to water purification, gasification and liquefaction. He
is a principal of Water Purification Associates with 18 years of practice.
Concurrently with his academic studies he was engaged in design and troubleshooting work at the world's
first commercial scale electrodialysis plant at the Free State Geduld gold mine in South Africa
Since joining W.P A. in 1977, Dr. Hicks has made detailed studies of water problems in coal conversion
and oil shale plants in the U S. A and has visited several of this country's synthetic fuel pilot plants. He has
designed many water treatment plants and, in particular, has been concerned with recycle and reuse of
contaminated effluent water
365
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Low Pressure
, H20
RO Membrane- Gel Structure
with Rejecting "Skin"
Monitor
Optional
Filter
Wastewater
Treated Water
(Permeate)
Pressure
Regulating
Valve
Concentrated Water
(Concentrate)
Figure 1. Outline of simple reverse osmosis arrangement.1
rejections of inorganic salts will be in the high nineties and there is, as already noted,
almost complete rejection of most species with molecular masses greater than 150.
The rejection of low-molecular-mass nonelectrolytes, such as small organic
molecules, is generally low with the asymmetric cellulose acetate and polyamide
membranes. However, with the use of newer composite membranes and by proper
pH adjustment, moderate-to-good rejections can be obtained with many
intermediate and even low-molecular-mass organics. Because the percent rejections
tend to be constant over a wide range of concentration, the concentration in the
water passing through the membrane will be proportional to the concentration
retained. The higher the fraction of feed which passes the membrane, that is, the
higher the "recovery" of water, the higher will be the concentration in the product
water. The retained water is often termed the "concentrate" and the product water
the "permeate."
If an ideal semipermeable membrane separates an aqueous organic or inorganic
solution from pure water, the tendency to equalize concentrations would result in the
flow of the pure water through the membrane to the solution. The pressure needed to
stop the flow is called the osmotic pressure. If the pressure on the solution is
increased beyond the osmotic pressure, then the flow would be reversed and the
fresh water would pass from the solution through the membrane, whence the name
"reverse osmosis." In actual reverse osmosis systems the applied pressure must be
sufficient to overcome the osmotic pressure of the solution and to provide the driving
force for adequate flow rates.
Osmotic pressure is a property of the solution and does not in any way depend on
the properties of the membrane. For dilute solutions the osmotic pressure is
proportional to the mass concentration of the dissolved material divided by its
molecular mass, so that at the same mass concentration sugar has a much lower
osmotic pressure than salt. For dilute solutions the osmotic pressure is given by the
modified van't Hoff equation
TT= ic(R/M)T
(1)
366
-------
where
TT — osmotic pressure (Pa)
c = mass concentration (mg/ L)
M = molecular mass
R = universal gas constant (8.314 J/mol-K)
T = absolute temperature (K)
i = number of ions formed if the solute dissociates (e.g., i = 2 for NaCl).
In Table 2 are shown typical osmotic pressures of aqueous solutions at the standard
temperature of 298 K. (77° F).
The amount of water (solvent) that will pass through a membrane is proportional
to the excess of the hydrostatic pressure, p, over the osmotic pressure, TT, and for
commercially useful flow rates p should be large compared to TT. The volume of water
flux crossing a unit area of membrane per unit time may be written approximately as
J = A(Ap - ATT) (2)
where Ap is the hydrostatic pressure difference across the membrane and ATT the
osmotic pressure difference corresponding to the solute concentrations immediately
adjacent to the membrane surface on both sides. The coefficient A is the membrane
water permeability coefficient. It is inversely proportional to the thickness of the
solute rejecting portion of the membrane, a quantity which is generally not known
precisely. The value of A is determined empirically using Eq. (2).
Although the solvent flux is inversely proportional to the "active" thickness of the
membrane, the solute rejection is independent of this thickness. The driving force for
the solute flux is mainly the difference in solute concentration across the membrane
between the feed and product. Increasing the applied pressure increases the water
flux proportionally more than the solute flux, with the result that the quality as well
as the quantity of product increases with increasing pressure.
As already observed, the rejection characteristics of the membrane are generally
better the smaller the membrane pore size as compared to the solute molecule, while
the solvent flux increases with decreasing membrane thickness. Although Reid and
Breton4, during the period from 1957 to 1959, found that cellulose acetate was
capable of rejecting electrolytes from aqueous solutions, they concluded that such
membranes could not be made thin enough practically to achieve reasonable water
fluxes. Loeb and Sourirajan, at about this same period (1958-1961), produced a
major breakthrough in membrane fabrication by casting "asymmetric" cellulose
acetate membranes which had a very dense, thin rejecting skin on a relatively porous
substructure. The thin skin had good rejection characteristics and allowed for
practical flux rates because of its relatively low hydraulic resistance, which is
proportional to thickness. On the other hand, the integral porous substructure
offered little resistance to the flow but provided support for the thin skin.
All the membranes have limits in their tolerance to pH, temperature and
chemicals such as chlorine and other strong oxidizing agents. Cellulose acetate
membranes have greater susceptibility to biological attack and have limited pH
range, while polyamide membranes have a wide pH range but have no tolerance for
chlorine or other oxidizing agents. All reverse osmosis membranes are also very
susceptible to blockage by deposition of solids, a phenomenon termed "fouling." The
concentration must never exceed the solubility of the least soluble material in
solution, and suspended solids must be removed from the feed stream.
367
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Table 2. Typical Osmotic Pressure Data at Standard Temperature
Dissolved Species
NaCI (M = 58.5)
Urea (M = 60)
Sugar (M = 180)
Concentration
(mg/L)
50,000
10,000
5,000
50,000
10,000
5,000
50,000
1 0,000
5,000
Osmotic Pressure
|kPa)»
4,550
833
416
2,100
420
210
375
75
38
*kilopascals
Commercial membranes are generally limited to applied pressures of less than
7 MPa because of compaction, which leads to long-term decline in water flux.
Osmotic pressure is one limitation already mentioned. In order to leave an adequate
driving force, the maximum concentration reached, which is the concentration of the
effluent concentrate, usually should not exceed 3 MPa osmotic pressure. In this
regard, it should be noted that passage of water through the membrane tends to carry
the solute to the membrane surface and the concentration at the membrane surface
tends to be higher than in the bulk of the liquid. This phenomenon is called
"concentration polarization" and is reduced by increasing the flow speed and/or by
promoting turbulent flow. The extent to which this can be done depends on the
module type. As sold today membranes are fabricated as sheets, as hollow fibers, and
as hollow tubes. Depending on the membrane geometry they are packaged in four
main module types: spiral wound, hollow fiber, tubular, and plate and frame. Details
of the module types along with details of the limitations of commercial membranes
will be discussed in the section, "Membranes and Module Developments."
Mechanism and Operation of Ultrafiltration
Ultrafiltration''6 is also a pressure driven-membrane separation process.
Pressures usually from 70 to 700 kPa are applied and water passes through the
membrane. Material which does not pass through the membrane includes paniculate
matter, colloids, suspensions and large dissolved molecules of molecular mass
usually greater than 2,000 and more generally greater than 10,000. Rejection is
usually close to complete.
From a theoretical point of view, ultrafiltration is like ordinary filtration except
that very small particles are held back by the membrane. The size of the rejected
particles depends on the pore size of the membrane; a choice of membranes is
available. The most common UF membranes are of the Loeb-Sourirajan asymmetric
type and, like RO membranes, these have been cast from a variety of polymers
including cellulose acetate and polyamides. Today's practical ultrafiltration flux
rates are usually within the range 600 to 3,000 liters per square meter per day (L/m2-d).
This range is about 1 / 200th of the usual practical filtration rates.
In general the flux through the membrane is linearly proportional to the applied
pressure difference across the membrane since in ultrafiltration osmotic pressure
effects are usually negligible (see Eq. 2). The accumulation of solutes or particulates
368
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at the membrane surface is termed concentration polarization, as in reverse osmosis,
and it can give rise to an apparent fouling of the membrane. The effect of dissolved
solutes is better understood than the effects of accumulated particulates and
undissolved species.
At sufficiently high solute concentrations at the membrane surface, gelation of the
macromolecular solute can take place. This gel formation is similar to cake
formation in ordinary filtration. When gelation occurs the permeate flux is
controlled by the transport rate through the gel layer, which is lower than through
the membrane, and it approaches a limiting value independent of the membrane
permeability and applied pressure. This effect can be reduced by increasing the
velocity of the solution past the membrane in order to "mix up"or"sweepaway"the
accumulated layer and thereby reduce the solute concentration at the surface. This
effect is illustrated in Fig. 2. The module types used in ultrafiltration are similar to
those for reverse osmosis.
MEMBRANE AND MODULE DEVELOPMENT
The development of the asymmetric Loeb-Sourirajan membrane was an
outgrowth of work sponsored by the then Office of Saline Water of the U.S.
Department of the Interior. The goal of the work was the desalination of saline
waters, and until recently most of the applications of reverse osmosis have been in
this direction. Although not normally considered a wastewater application, reverse
osmosis has been extensively used for processing foods, such as concentrating fruit
juices and sucrose solutions. However, pollution control has often been a byproduct,
as in its application to whey processing. With the more recent development of
ultrathin composite membranes, reverse osmosis is becoming an important process
with exciting potential for the separation of low molecular mass organics. It is
beginning to be applied extensively for water reuse, organics recovery, and pollution
control. Ultrafiltration, on the other hand, has long been employed for the
separation of high molecular mass organic materials and for pollution control
applications in the treatment of organically contaminated wastewaters.
Parallel with the advances in membrane fabrication there have been a number of
developments in the assembly of membrane modules. Most of the module
development efforts have had as a goal the reduction of the polarization and fouling
effects which limited flux, without at the same time incurring energy penalties in the
form of excessive pressure drops through the system.
Membrane Development
The principal types of reverse osmosis membranes that have been fabricated are
the Loeb-Sourirajan flat sheet and tubular membranes, hollow fiber membranes and
dynamic membranes. All the membranes except the composites ha've also been
developed for UF applications. A review of the state of development of synthetic
membranes is given in Reference 8.
It is useful to have some picture of how reverse osmosis membranes reject
molecules since it provides some insight into the membrane developments. As
already noted, in rejecting molecules, reverse osmosis membranes do so not only by
mechanical filter action but also by other mechanisms. Two among these are the
solution-diffusion mechanism and the charge-exclusion mechanism. In the
solution-diffusion mechanism the movement of each species can be described as a
solution of that species in the membrane which diffuses through the polymer matrix
under the action of pressure and concentration differences across the membrane. It
is easy to visualize that a high-water-content gelatinous membrane will only be
capable of sorbing a low salt concentration relative to the external concentration,
while the difference in water concentration across the membrane will be small. The
369
-------
0.04
100 200 300
Applied Pressure, kPa
400
Figure 2. Effect of solution flow rate past membrane on ultrafiltration flux 6
charge-exclusion mechanism is applicable to ionized solutions, whereby the
membrane in contact with the ionized solute acquires a charge of opposite sign, as do
cellulosic materials, and thereby acts to slow down the movement of charged
particles through it.
Loeb-Sourirajan Membranes—
These membranes were originally "cast" from a solution of cellulose acetate
dissolved in an acetone solvent. The viscous solution is spread on a glass plate and the
solvent allowed to evaporate. This forms the thin skin at the solution-air interface
with the underlying solution still fluid below. The membrane is then "quenched" by
plunging it into a cold-water bath. The water diffuses into the interior of the film,
serving to precipitate the remaining polymer with consequent gelation. In this
quenching the porous backing is formed. The membrane is then peeled off the glass.
The solute rejection properties of these membranes were not good and Loeb and
Sourirajan found that by "annealing" the membrane in hot water for a suitable
370
-------
period the strain was removed, the polymer shrank, and its porosity decreased,
giving the membranes better rejection properties although lower permeabilities.
Cellulose acetate membranes are today routinely manufactured for commercial
use. They have water permeabilities in the range 600 to 1,200 L/ m2-d at 5.5 M Pa with
salt rejections from 95 to 99 percent. They do, however, have poor chemical
resistance and are hydrolyzed below pH 4 and above pH 8. The membranes are also
hydrolyzed above 38° C. The membranes are resistant to chlorine and can be
operated continuously with up to 2 mg/L, and for 15 minutes with up to 5 mg/L
chlorine. Asymmetric membranes have been prepared from different polymers
including cellulose triacetate, aromatic polyamides that are generically related to
nylon, and other polyamides. Although some of these membranes exhibit high salt
rejections they generally have lower water permeabilities than the cellulose acetate
membranes. The polyamide membranes which are in wide use are, however, much
more resistant to biological attack and can be operated over a pH range from 3 to 11.
Their temperature limitation is about the same as for the cellulose acetate
membranes but unlike these membranes, they are not very resistant to chlorine,
their limits being 0.1 mg/L at pH <8 and 0.25 mg/L at pH ^ 8.
Asymmetric ultrafiltration membranes are prepared in the same way as the reverse
osmosis membranes, except that they are much more "open" membranes and have
correspondingly higher permeabilities. These membranes also have been cast from
a wide range of polymers with the aim of seeking better solvent resistance, pH
resistance, and thermal stability. Antifouling ultrafiltration membranes have been
prepared from sulfonate polymers9. These polymers incorporate the negatively
charged sulfonate group, derived from sulfuric acid. This charged group tends to
prevent adherence to the membrane of colloidal foulants in the water, most of which
are negatively charged. Table 3 shows some properties of asymmetric membranes
produced by the Amicon Corp. and used for organics separation.
Hollow Fiber Membranes—
One asymmetric membrane geometry which has been fabricated that enables the
more effective use of polymers with lower permeability is the hollow fine fiber. These
fibers are about the thickness of a human hair. Because of their small diameter they
permit the packaging of large surface areas per unit volume of container. DuPont has
pioneered the hollow polyamide fiber membrane. The fibers, which are actually
thick-walled pressure vessels, have an outside diameter of about 80 micrometers and
an inside diameter of about 40 micrometers. The feed flows outside the fiber
countercurrent to the product water, which flows inside as shown in Fig. 3. The
DuPont fiber has a product flux of about 75 L/m2-d at 2.75 MPa, which is about an
order of magnitude less than for the cellulose acetate membranes. However, the
surface area packing densities are on the order of 40,000 m2/ m3. Dow Chemical Co.
has fabricated a cellulose triacetate hollow fiber membrane with a flux rate of 60
L/m2-d at 4 MPa. These membranes generally have greater resistance to biological
attack and higher salt rejections than the more commonly used cellulose "diacetate,"
although the water permeability is lower.
As may be expected, because the spacings between the hollow fibers are small they
are subject to clogging, and extensive pretreatment of even relatively clear waters
must be carried out.
Hollow fiber membranes are also produced for ultrafiltration applications,
although generally the internal diameters of the fibers are from 10 to 30 times larger
than the DuPont fibers.
Composite Membranes—
The composite membranes mentioned earlier are a more recent development of
particular importance in organics separation since they allow for greater ease in
tailoring the membrane properties. These membranes are prepared by casting a dense,
homogeneous ultra-thin polymer film onto a relatively porous substrate. Different
371
-------
Table 3. Amicon Corp. Ultrafiltration Membranes
Membrane
UM 05
UM 2
YM 5
YM 10
YM 30
Mol Mass
Cutoff
500
1,000
5,000
10,000
30,000
Water Flux
@ 380 kPa
(L/m2 d)
1 50-300
450-850
1,150-1,700
1,700-2,600
4,800-29,000
polymers can be used for the two layers, each having the best property for its
particular function. These thin-film composite membranes are now fabricated with
rejecting skins a factor of ten times thinner than the skin of the cellulose acetate
membranes. Not only does the thin skin permit much higher water fluxes, but
polymers can be used which have higher rejection capabilities and which are more
chemically and thermally stable. The successful flat sheet composite membranes
developed originally by North Star Research and Development and now made by
FilmTec Corp. are composed of either a sulfonated polyfuran or PEI
(polyethylenimine) film on a polysulfone support film10. These membranes show
very high salt rejections at high flux rates. They do, however, suffer from low
resistance to chlorine. Recently, FilmTec has developed a new membrane of
proprietary composition (FT-30) which shares some of the properties of the original
North Star membranes but which shows appreciable resistance to degradation bv
chlorine1"'". On simulated sea water, salt rejections of 99.5% are reported with flow
rates of 1,200 L/m2-d for pressures of 7 MPa at 25° C. Thin film composite
membranes with similar properties are also made by other manufacturers'2-13.
Another advantage of the composite membranes which may be mentioned is their
improved resistance to long-term compaction. The organics separation capabilities
of these promising membranes will be discussed in the next section, "Organics
Separation by Reverse Osmosis."
Dynamic Membranes—
Because irreversible fouling so often limits the membrane life, a great deal of work
has been done on dynamically formed membranes. The membrane is made by
depositing, from a slurry, inorganic-ion-exchange materials such as zirconium oxide
or organic polyelectrolytes such as polyacrylic acid. The deposit is formed inside
porous tubes. When the dynamically formed membrane is fouled it may be pumped
out of the tubes and replaced in situ without moving the tubes. In general, the
rejections of these membranes are somewhat lower than the rejections of
"permanent" membranes. However, extremely high fluxes are reported with values
of 4,000 to 8,000 L/m--d at 6.5 MPa. The reproducibility of these membranes does
present some problems. The membranes are also useful for ultrafiltration
applications.
Module Development
The membranes described in the previous section are normally packaged for both
reverse osmosis and ultrafiltration applications in one of four configurations: spiral
wound, hollow fiber, tubular, or plate and frame. The two most commonly used
configurations are the spiral wound and hollow fiber.
372
-------
Feed
Concentrate
Figure 3. Hollow fiber flow configuration.
-------
The concept of the packaging of the spiral wound module is shown in Fig. 4. The
configuration consists of two sheets of membrane sealed on three edges to form a
flexible envelope, or leaf. The open end of the envelope is attached to a perforated
central tube. A mesh-like spacer is placed adjacent to the membrane envelope and the
layers are wrapped around the central tube like a window shade. The function of the
spacer is to separate the membrane layers so that the feedwater can flow axially
between the leaves under pressure and also to promote turbulence in the feed
stream. Permeate flows through the membrane into the support fabric and is
conducted spirally to the perforated central tube where it is collected and removed.
The spiral wound module has the advantages of a high packing density, low
manufacturing cost, and relative ease of cleaning by both chemical and hydraulic
means. Its main disadvantage is that it cannot handle feeds containing suspended
solids without extensive pretreatment because the feedwater flow passages are thin
and subject to clogging.
The hollow fiber module is shown in Fig. 5 in the configuration developed by
DuPont. The fibers are packed together in a cylindrical bundle about a central
feedwater distributor tube. The fibers are bent in a half-loop with the free ends
potted in epoxy resin at one end with the bores of the fibers exposed. The bundle is
placed in a pressure vessel. The pressurized feed is introduced through the
distributor tube and flows outside of the fibers (see Fig. 3) with the permeate passing
through the fiber walls into the bores of the fibers. The permeate flows up through
the bores to the open fiber ends at the epoxy head. The concentrate is removed from
the opposite end of the unit.
As already noted, the packing density of this configuration is very high, with the
result that hollow fiber systems are compact. In addition, the capital cost is relatively
low. The main problem with this module is that it is very susceptible to clogging
because of the small spacing between the fibers. It is also very difficult to clean. For
these reasons, hollow fiber units usually require expensive pretreatment for removal
of materials which might plug them.
The tubular configuration is illustrated in Fig. 6. The membrane is either cast on
the inner surface of a porous tube or placed within the tube, which is typically 1 or 2
cm in diameter. A module is normally formed by connecting a number of tubes in
series or in parallel. The feedwater circulates through the tubes under pressure and
the permeate drips off into a collection system for removal. These systems present a
relatively small membrane area per unit volume of feed and therefore have high
capital costs. However, they do have the advantage of being able to handle feeds
containing suspended solids, so long as they do not foul the membranes, and they can
be cleaned either mechanically or hydraulically with relative ease.
The last module type is the plate and frame configuration in which membranes are
attached to both sides of a rigid plate (Fig. 7). The plates are constructed from a
number of materials, including solid plastic with grooved channels on the surface,
porous fiberglass materials, or reinforced porous paper. The plate and frame units
are put in a pressurized vessel through which the wastewater is fed under pressure.
The permeate passes through the membranes, which are in contact with the high
pressure feed, and is collected from the plates which are at low pressure. The design is
complex and is expensive for large scale use but is useful where sanitary or clean
conditions must be maintained.
ORGANICS SEPARATION BY REVERSE OSMOSIS
As discussed previously, most commercially available UF membranes have high
rejections for large macromolecular organic species, regardless of the type of species.
On the other hand, until recently, the removal of smaller organic molecules by
membranes was not widely practiced. Reverse osmosis membranes now
commercially available reject the smaller organic molecules with the extent of the
rejection for a given membrane dependent principally on the size and type of species.
374
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Module Core and
Permeate Collector Tube
Feed Flow
through Open Mesh
Outside Membrane
Envelope
Sealed on Edges
with Support Fabric
Between
Mesh
Membrane
Support Fabric
Figure 4. Packaging of spiral wound module '
Large amounts of data on organics rejection by reverse osmosis membrane can be
found in the literature. However, there is to date no general methodology for
predicting the rejection characteristics of a given compound as a function of such
variables as membrane type, solution pH, concentration, temperature, mixture
properties, etc. The bulk of data available for organics rejection is from laboratory
experiments with single solute aqueous solutions and selected membranes.
Generally, the solutes are tested at concentrations maintained low enough that
osmotic pressure effects are not important. On the other hand, most field
applications of reverse osmosis for organics removal involve complex mixtures of
organic compounds at varying concentrations which can make interpretation
difficult. Moreover, the availability of such data is limited.
The ability of membranes to handle organic-containing wastes can be
conveniently discussed in terms of the several classes of organic compounds. The
molecular size of compounds within each class varies greatly and, as discussed, this
has an important bearing on rejection. Several other properties specific to each
class strongly determine membrane performance14. These include the extent to
which the molecules are ionizable and the charge on the ion, the solubility of the
compound in water and in organic solvents, and to some extent the shape of the
molecule. Rejections are further dependent on the type of membrane, whether these
be asymmetric cellulose acetate or aromatic polyamide, composite, or dynamic
membranes. It may be possible to select an optimum type of membrane for each
class of compounds. Operating conditions of temperature and pressure have only a
secondary effect on rejection, although these variables largely determine the flux for
a given membrane type.
Available data are principally for hydrocarbons and oxygenated compounds, in
particular acids and alcohols. From a pollution control point of view other organics,
especially nitrogen and sulfur containing species, are important. While some
information is available for amines, the nitrogen-containing organic bases related to
375
-------
Open Ends
of Fibers
"O" Ring Seal Concentrate Outlet
Snap Ring
Porous
Back-up Disc Snap Ring
End Plate
Porous Feed
Distributor Tube
Epoxy Tube Sheet \ End Plate
"O" Ring Seal
Figure 5. DuPont hollow fiber module.1
-------
Porous Tube
Membrane
v_ x N Concentrate
Pressurized ,
Feed
I ; 1
Permeate
Figure 6. Tubular membrane element.
ammonia, there are unfortunately limited data published for other species important
in industrial wastewater control. The Environmental Protection Agency has
published a list of 129 "priority pollutants" which are of particular concern in
effluent control. Considerable attention has recently been directed towards these
compounds although their rejection by membranes has not been extensively
reported.
Hydrocarbons
Data for rejection of hydrocarbons by cellulose acetate membranes15 indicate that
the rejection increases with decreasing solubility in water. This is in accord with the
solution-diffusion rejection mechanism discussed previously, wherein the solute and
solvent dissolve in the membrane material and then diffuse through it. The relative
rate of permeation of the solute and solvent through the membrane determines the
composition of the permeate. Low membrane solubility of the solute therefore leads
to high rejection. Membranes are basically a high-water-content gelatinous
substance and the solubility of a species in the membrane is related to its solubility in
water. Consequently, all other factors being equal, the rejection of a class of
compounds can be expected to increase with decreasing water solubility as was found
for the hydrocarbons.
For a given solubility and membrane, the rejection will of course relate to
molecular size and also to its shape. For hydrocarbons of equal solubility, rejection
decreases in the order
aromatics > cyclic compounds > chain compounds
Benzene, which forms the basis of the aromatic compounds, has a relatively high
solubility and a moderate rejection of about 75%. Chain hydrocarbons of the same
solubility have lower rejections in the order of 40%. Compounds with solubilities of
less than about 100 mg/L generally have rejections in excess of 90%.
Oxygenated Compounds
These compounds include classes of organics whose degree of ionization in
solution can be high, and those which are only slightly ionized in solution. The acids,
and the phenols, for example, are ionized at high pH. Other oxygenated compounds
including the alcohols, ketones, aldehydes, esters and ethers are only slightly
ionizable at any pH. Some of these materials, in particular the aldehydes and ketones,
are polar, that is, they have a differential charge across the molecule, and so behave
like ions to some limited extent.
The degree of ionization plays a very important part in determining the rejection
characteristics of a species, as is clearly shown in Figs. 8 and 9. In general a dramatic
377
-------
Center Bolt
End Flange
Flow
Spacer Plate
Membrane Support Disc
—^ Permeate
Neck Ring
Membrane
Filter Paper
Membrane Support Disc
Filter Paper
Membrane
Feed
r
Zl
1
1
1
j
13
ZJ
Zl
1]
I]
Zl
Zl
1
1
c
c
(Z
c
c
c
c
1
1
t
•1
[I
c
c
Permeate
Concentrate
Figure 7. Plate and frame membrane configuration showing cross section of
membrane unit and flow in module '
378
-------
100
80
60
c
o
0>
oc 40
20
Formic Acid
Benzoic Acid
Membrane
NS-100 Composite (Ref. 9)
Cellulose Acetate (Ref. 10)
23456
pH
Figure 8. Rejection of carboxylic acids as a function of pH.
increase in rejection occurs in the pH range over which the species changes from
mainly dissolved molecules to mainly ions. The predominant rejection mechanism is
clearly that of charge exclusion. In this respect it is interesting that while the
carboxylic acids are essentially ionized above pH 7, phenols are only ionized at pH's
about 9 to 10. As cellulose acetate membranes cannot be used at these high pH's, it
was not until the arrival of composite membranes that treatment of phenolic
containing wastes by membrane processes became feasible. The composite
membranes have a further beneficial effect in addition to their application at low and
high pH. Because the active film is considerably thinner than in asymmetric type
membranes, they can be significantly more dense or "tighter" for the same flux.
Consequently, for the same degree of ionization, composite membranes will
generally perform better. This is in principal a size effect.
The availability of these tighter composite membranes has had a tremendous
impact on treating nonionizable compounds such as the alcohols, where molecular
size appears to predominantly determine rejections. For example, rejections of
alcohols by composite membranes range from about 40% for methanol, the smallest
alcohol with a molecular mass of 32, to greater than 90% for alcohols greater than
propanol, molecular mass 60. Equivalent rejections for cellulose acetate
membranes are 1% to 20%19.
379
-------
100
Figure 9. Rejection of phenols by cellulose acetate membranes as a function of pH.1
Aldehydes, ketones, esters and ethers also exist as unionized molecules in aqueous
solutions. Apart from molecular size, the rejection of these substances can be
correlated with their polarity, which for the organic substances discussed decreases
in the following order:
acids > alcohols > aldehydes > ketones > esters > ethers
Rejections of ethers are consequently determined essentially by the shape and size of
the dissolved molecule15.
Nitrogen Containing Compounds
The amines are weak bases and dissociate into ions at low pH. As with acids and
phenols, the rejection of amines is largely determined by the degree of dissociation as
is shown in Fig. 10. Secondary and tertiary amines have higher rejections than
primary amines.
Pesticides and Priority Pollutants
Although not belonging to specific classes of organic compounds, pesticides and
the priority pollutants are of importance in industrial wastewater control.
Both cellulose acetate and composite membranes showed greater than 98%
rejection for chlorinated and organophosphorous pesticides20. However it was
concluded that a significant fraction of the removal is due to adsorption on the
membrane. Long-term data are necessary on the effect of exhaustion of adsorptive
capacity on removal of pesticides, and procedures for extraction of adsorbed
pesticides must be developed.
380
-------
100 -
ae
c
o
I
80
§ 60
40
20
- \
Methylamine
\
• Aniline
\
Membrane
— NS-100 Composite (Ref. 9)
— Cellulose Acetate (Ref. 11)
\
10
12
14
pH
Figure 10. Rejection of two amines as a function of pH.
Data available for removal of organic priority pollutants by reverse osmosis are
summarized in Table 4. Removal of trace organic pollutants can be enhanced by
proper selection of the membrane and operating conditions, pH adjustment if the
compounds are ionizable, and complexation of the compounds with high molecular
mass substances, such as humic acids in natural waters.
INDUSTRIAL APPLICATIONS
Textile Industry
Both ultrafiltration and reverse osmosis have been successfully applied, on pilot
scale, to the treatment of textile effluents for the reduction of waste water volume and
the recovery of water, process chemicals and heat24'26.
The ultrafiltration of effluents from the scouring of raw wool results in a clear
permeate, which can be recycled or reused, and a concentrate containing the wool
grease, which can be subsequently recovered and refined into lanolin. Similarly,
ultrafiltration of effluents from the desizing of fabric permits separation, recovery
and recycle of water and sizing agent. In both applications, the flux through the
ultrafiltration membrane decreases with time due to concentration polarization and
possible gel formation and blinding of the membrane. Since ultrafiltration removes
381
-------
Table 4. Rejections of Some Organic Priority Pollutants by Reverse
Osmosis
Rejection
Membrane (%) Reference
Hydrocarbons
Benzene
Toluene
Ethylbenzene
Cellulose acetate
Cellulose acetate
Cellulose acetate
75
72
78
21
21
21
Chlorinated Hydrocarbons
Trichloroethylene
Tetrachloroethylene
Esters
Dimethyl phthalate
Phenols
Pesticides
Aromatic polyamide*
Poly (ether/amide) composite
Poly (ether/amide) composite
—
—
>74
>93
95
pH dependent
>98
22
23
23
See Fig. 9
See Text
"DuPont B-9 hollow fiber.
organic compounds by a sieving mechanism, recycle of the permeate to the process
will result in a buildup of low molecular mass compounds dissolved in the process
water. Discharge of a portion of the permeate is necessary if the buildup is
detrimental to the process.
Reverse osmosis has been used for recovery of up to 90% of the water from
effluents of textile finishing plants. The quality of the permeate is satisfactory for
reuse in scouring, bleaching, dyeing and finishing processes. Specific pretreatments
over and above cartridge filtration may be required, depending upon the membranes
used. Cellulose acetate membranes, for example, require pH control to minimize
hydrolysis; hollow fiber membranes are susceptible to plugging and may require
extensive prefiltration; and poly (ether/amide) membranes require the reduction of
residual oxidizing agent such as peroxide in effluent from bleaching processes.
Tests with cellulose acetate and dynamically formed zirconium oxide-polyacrylate
membranes showed a sharp decline in flux at the beginning of the test followed by
equilibration to a constant value, along with an increase in rejection, suggesting the
formation of a secondary membrane from constituents in the feed. For both UFand
RO, development of membrane cleaning procedures and optimization of
pretreatment, equipment design and operating parameters may improve the
long-term flux of membranes in textile applications. Development of membranes
which can tolerate high temperatures, such as the dynamically formed membranes,
would permit recycle of hot permeate and significant energy savings.
Food Industry
Ultrafiltration and reverse osmosis are used in the food industry for concentration
of process effluents and for separation and recovery of suspended and dissolved
solids. Because membrane processes are nonthermal, do not involve phase changes
and, in general, do not require chemical addition other than for pH adjustment, they
are suitable for recovery of heat-sensitive organics such as proteins. In most food
processing applications, UF and RO not only concentrate or recover valuable food
products but also reduce the contamination in the effluents to be discharged.
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Major problems in the operation of UFand RO in food processing are membrane
fouling and membrane cleaning and sanitation. Concentration polarization and
formation of a protein gel on the membrane are the usual reasons for decline in flux.
Flushing and circulation of cleaning solutions containing enzymes, detergents
and/or complexing agents is the most common procedure for restoring the
membrane flux.
In the dairy industry, ultrafiltration is used to recover proteins from cheese whey
or skim milk27-28. Since the membranes are selected for low rejection of lactose and
salts and essentially complete rejection of proteins, the composition of the
concentrate is controlled as a function of the extent of water recovery. The
concentrate can therefore be used as the raw material for a variety of products. Data
from ultrafiltration of cheese whey show that gel formation occurs with associated
limiting flux (see Fig. 2) when the protein concentration at the membrane surface
reaches approximately 20 percent by mass. Optimum operating conditions for the
ultrafiltration of whey and milk are determined not only by the tradeoff between flux
and energy requirements, but also by the properties of the recovered product. Flux,
for example, increases with temperature; proteins, however, are denatured at
55-60° C so that UF is usually carried out at 45-50° C.
Reverse osmosis can be used for preconcentration of cheese whey prior to thermal
evaporation with a resulting savings in overall energy requirements27. Reverse
osmosis is used to concentrate lactose and salts in the permeate from ultrafiltration
of cheese whey or skim milk. The nutritional value of the UF permeate is recovered
and the RO permeate is recycled or reused. Since the UF permeate is protein free, the
flux decline during concentration by RO is primarily due to an increase in osmotic
pressure and not to membrane fouling.
The combination of U F for protein recovery followed by RO for recovery of lower
molecular mass organics and salts has been tested for other foods including soy
extracts and citrus fruit wash water2'''0. Ultrafiltration has also been used to recover
proteins from potato starch process water and fish processing wastewater, and
reverse osmosis to concentrate fruit juices and food processing wastewater31"33.
Oily Wastes
Membranes have proved particularly satisfactory for removing emulsified oils
from water34. The usual system design is semibatch as shown in Fig. 11. Clean water
leaves through the membrane. Oil is concentrated until it separates into a floating
layer where it is taken off for burning or recovery. A settled sludge is often also
removed. The advantages of this system are that concentrations in the oil layer are
very high and the membrane is not exposed to large amounts of free oil which can
foul the membranes.
Various sources of oily waste have been treated; for example, the emulsified oils
used as coolants and lubricants in machining operations. These oils become
degraded and must be disposed. Ultrafiltration can concentrate machinery oils and
similar oily wastes from about 3-5% to about 40-50% so that the concentrate can be
incinerated without fuel consumption. Fluxes are generally between 800 and 1200
L/m2.d with permeate concentrations less than 250 mg/L. The permeate can be
further cleaned using RO35.
Pulp and Paper Industry
Membranes have been tested on many wastewaters in pulp and paper plants.
Although commercial development is still required, applications involving the
recovery of water for recycling show good promise. The initial work has indicated
that the best applications are for dilute wastes including pulp and screen wastewaters
and bleach plant effluents. Fouling, particularly by fibrous solids, is a problem.
383
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Oily Waste Water
| Oil Layer
Mixed Layer
Permeate
Clean Water
Sludge Drain
Figure 11. Semi-batch system for treatment of oily wastes.
Dynamically formed membranes seem particularly applicable for color removal.
Ultrafiltration has been used for byproduct recovery. The Institute of Paper
Chemistry has tested many RO membranes, mostly cellulose acetate, on various pulp
and paper mill wastewaters36. The wastewaters contained 0.3-2% suspended solids
and had a biological oxygen demand (BOD) range of 2,000 to 35,000 mg/ L. Rux was
limited by fouling and generally ranged between 200 and 400 L/m2-d.
The recovery of commercially valuable chemicals from pulping waste is regularly
practiced in the paper industry. Ultrafiltration has been used to recover
lignosulfonates which are used as binders in plywood, particle board and other
products37.
Oil Shale Industry
Oil shale retorting produces water, partly by chemical reactions within the retort,
and partly by the release of combined and free moisture in the shale. The amount of
water produced ranges from about one quarter to one-and-a-half times the volume
of oil produced depending on the retorting procedure and site conditions. This
represents a significant quantity of wastewater for a 50,000 barrel-a-day oil shale
plant and it is essential that a means of reusing or disposing of this wastestream in an
environmentally acceptable fashion be available if an oil shale industry is to develop
in this country.
Process water that condenses within the retort and is known as retort water has
proven to be particularly difficult to treat. This water contains both inorganic and
organic contaminants, is black in color and has an obnoxious odor. While the
inorganic constituents can be controlled by processes such as steam stripping,
chemical precipitation and ion exchange, these treatments do not remove the
organics. Nor do the conventional processes for organic control such as solvent
extraction, carbon or resin adsorption, and biological treatment have adequate
effect. At most a combination of some of these processes results in a 70% removal of
the organics which reduces the organic carbon from about 4,000 mg/L to 1,200
mg/ L. This is still too high for either disposal or reuse within the plant.
In a search for a feasible retort water treatment scheme, it was recommended that
reverse osmosis be evaluated on the basis that it had potential for controlling both
inorganics and organics38. Several membranes including cellulose acetate and
modern composite membranes were tested in the laboratory on actual retort water
samples. Screening tests demonstrated that while good inorganics rejection could be
obtained under most operating conditions, organics rejections were dependent on
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pH. At elevated pH's of 10 to 11, using composite membranes, in excess of 95%
organics rejection was obtained in a batch cell. The high pH required is a reflection of
the acid and phenol content of the retort water.
These exciting results were confirmed in a flow system in which even better
rejections were obtained (see Table 5). The flux decline shown with increased
recovery in Table 5 is attributable entirely to the increased osmotic pressure. Fluxes
were within expectations and suggest commercial application is viable. The
economics are particularly attractive in that the expensive inorganic control steps
such as ion exchange can be eliminated. Flux deterioration by fouling was not
detected in the laboratory, nor in subsequent field tests. However, long term test data
are required before the effects of fouling can be fully evaluated.
LIQUID MEMBRANES
Liquid membranes were developed by Li39 and are formed by dispersing an
emulsion of two immiscible phases, for example, an aqueous solution of caustic
soda and a hydrocarbon solvent-surfactant solution, in a third phase, for example,
an aqueous solution containing phenol. Small emulsion globules, approximately 1 to
5 mm in diameter, are formed upon dispersion and distribute themselves in the third
phase (see Fig. 12). Many small "encapsulated" aqueous reagent droplets, having
diameters in the range of 1 to 10 /im, are contained in the larger emulsion globules.
The liquid membrane is the emulsion between the encapsulated droplets and the
third phase and usually contains additives, surfactants and a base material which
is a solvent for all of the other ingredients.
The wastewater solution contacts the emulsion in a mixer. The small emulsion
globules are stable and do not disintegrate as long as the system is agitated.
Agitation maintains good dispersion of the emulsion globules in the wastewater.
Organic acids or bases, which are weakly dissociated, permeate across the liquid oil
membrane into the very small encapsulated droplets (see Fig. 13). The droplets
contain reagents which enhance the permeation rate and neutralize the weak acids or
bases. Neutralization converts the acids or bases into salts which are highly
dissociated. The insoluble organic solvent or liquid membrane acts as a barrier to the
ionic components preventing them from permeating back into the wastewater
solution. The ionized salts remain encapsulated inside the emulsion globules. After a
certain degree of removal, mixing is stopped and the emulsion globules coalesce and
form an emulsion layer which can be easily separated from the continuous
wastewater phase. The highly concentrated ionized salt solution can then be
separated from the oil emulsion.
Liquid membranes have been used primarily for the extraction of inorganic
substances from aqueous solutions and for hydrocarbon separations41. A limited
amount of data is available on the removal of organics from wastewater40'42.
Phenols, acetic acid and other organic acids have been removed using a caustic
reagent liquid membrane and amines have been removed with a sulfuric acid liquid
membrane. For example, wastewater containing 200 mg/ L of phenol is treated in a
two-stage contactor. The phenol concentration can be reduced to 15 to 25 mg/ L in
the effluent of the first stage and to 0.25 to 2.5 mg/ L in the effluent of the second
stage, assuming that only 50 percent of the caustic is spent. The phenol concentration
in the caustic can exist in equilibrium with the wastewater even if the phenol
concentration in the caustic encapsulated droplet is 10,000 times that in the
wastewater.
If a wastewater solution contains other weak acids, they will compete with phenol
in permeating through the liquid membrane and reacting with the caustic reagent in
the encapsulated droplet. However, if strong mineral acids are present, they will tend
not to diffuse across the oil membrane because of their high degree of dissociation
and low solubility. Chlorides and sulfates will therefore be excluded from the
encapsulated droplet. Thus, the liquid membrane method can be used to remove
385
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TableS. Reverse Osmosis Treatment of Oil Shale Retort Water
Parameter
Feed (pH 10)
Rejection, '
Initial
80% Recovery
Total dissolved solids, mg/L
Total organic carbon, mg/L
Flux (5.5 MPa), L/m2-d
13,500
4,400
99.3
99.5
560
98.7
99 6
20
Wastewater
Drops of Emulsion
Oil Surfactant Membrane
Aqueous Reagent
Droplets (10~3-10-2mm)
Figure 12. Water treatment by liquid membrane emulsions.34
weak acids even in the presence of large amounts of chloride or sulfate. However, in
view of the limited data, further development work and testing are required before
this system can be considered to be commercially viable.
COSTS
The overall costs of a UF or RO system include the costs of pretreatment and
concentrate disposal in addition to the costs for the membrane system itself. Costs
for pretreatment depend on both the quality and quantity of the waste stream, and
will not be considered here in detail. Values of some 4
-------
Liquid Membrane
Surfactant
Solvent
Surfactants
Additives
Phenol Reaction Inside
Droplets
Phenol-I- NaOH->-
Sodium Phenolate
(Non Permeable)
Aqueous Feed Outside
(Continuous Phase)
Figure 13. The mechanism of removal of phenol by liquid membranes.34
The most reliable means of estimating costs for the membrane system is from
current data for similar plants. Experience in the use of membranes for treatment of
organically contaminated wastes is limited and we will not attempt to give a detailed
cost breakdown. Rather, major equipment and operating costs will be indicated, and
engineering factors used to arrive at estimates for overall costs.
Capital Costs
Major capital items include the membranes, the pressure vessels in which the
membranes are housed, and the pumps. Reverse osmosis membrane replacement
costs range between S40-85/ m2, with ultrafiltration costs 20 to 50 percent higher. The
ultrafiltration pricing appears to be based on higher flux. It is important to
emphasize that original membrane cost when computed as part of the manufacturer's
total equipment sale price may be only 30 to 40 percent of the replacement cost. More
detailed pricing information is not available as such information is treated as
proprietary by the manufacturers.
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Total equipment and construction costs curves for RO systems43 can be
represented by:
Capital S = 605 (Q m1/d)086 [Q < 106 m'/d]
These costs include cartridge filtration pretreatment only, and are based on costs for
municipal drinking water systems. The equation gives a cost of $2.9 x I06 for a
19,000 m3/ d (5 mgd) plant; reported bids for a similar sized municipal RO system44
ranged from $2.5 x 106 to $3.8 x 106. The estimated costs for RO treatment of a
composite textile finishing plant waste, based on data in Reference 26, is $2 x I06
without membranes, for this flow rate.
Ultrafiltration costs for small systems can be estimated from the following
equation based on a tannery unhairing bath wastewater45:
Capital $ = 27,000 (Q m3/d)07 [Q < I02 m3/d]
Operating Costs
Operating costs are very dependent on membrane life. We recommend that a
conservative estimate of, say, annual membrane replacement be made until the
extended lifetimes of two and three years, sometimes claimed by manufacturers for
municipal systems, are realized in practice with industrial wastewaters. Membrane
replacement is generally the major operating expense. The minimum energy required
is that needed to pump the feed to the operating pressure. This amounts to about
2 kWh/m3 for a 5.5 MPa pressure. Total energy required, including that for
recirculation for high recoveries, is probably not more than twice this minimum
value.
Total operating costs reported in Reference 43 for municipal systems range from
$0.1 to $0.2 per m3 for plants of up to 106 m3/d capacity and are in accord with
published experience44. These costs include maintenance and building energy in
addition to the process energy and membrane replacement costs. A membrane
replacement life of three years was assumed in these estimates; total operating costs
for industrial systems are therefore considered to be some two to three times higher.
Total direct operating costs for UF based on the tannery study45 are estimated to
be about $1.6/m3. However, these costs are for a small plant of 11 m3/d and more
than half the operating costs are for labor. Additional economies of scale should be
realized for larger UF plants. In this regard we note that economies of scale are not
as large as for other chemical processing equipment due to the modular construction
of both UFand RO plant. Scaling capital cost with the 0.8 power of the throughput is
probably safe.
FUTURE DEVELOPMENTS
Both reverse osmosis and ultrafiltration have been shown to have great potential
for the treatment of wastewaters containing dissolved organics. A limitation of
currently available reverse osmosis membranes is that they still do not have high
rejections for small size, unionized organic molecules. A significant advance in
improved rejection characteristics has been made with composite membranes.
Further development of these and other membranes that can be more closely
"tailored" to reject specific organics is needed46. This may be particularly useful
388
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with respect to the priority pollutants. Another limitation of current membranes is
that operation is generally restricted to below 40 to 50° C. However, many industrial
processes and waste streams require treatment at much higher temperatures. Both
membranes and membrane systems require development for continuous operation
at temperatures of 65° C and higher.
An important practical limitation of both ultrafiltration and reverse osmosis is
that the effective treatment of many waste streams is hampered by the fouling of the
membrane surfaces by many of the dissolved species. As a result there is a need for
the development of membranes which are resistant to fouling, which once fouled
can be easily cleaned, and which if fouled are not irreversibly damaged. Moreover,
there is also a need for improved module design which through control of the flow
can inhibit fouling.
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3. Porter, M.C. "Membrane Filtration," in Handbook of Separation
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4. Dresner, L. and J.S. Johnson, Jr. "Hyperfiltration (Reverse Osmosis)," in
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7. Probstein, R.F., W.F. Leung and Y. Alliance. "Determination of Diffusivity
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9. Gregor, H.P. and C.D. Gregor. "Synthetic-Membrane Technology,"
Scientific American, 238, pp. 112-127, July 1978.
10. Cadotte, J.E. et al. "A New Thin-Film Composite Seawater Reverse Osmosis
Membrane," Desalination, 32, pp. 25-31, 1980.
11. Cadotte, J.E. and R.J. Petersen. "Thin-Film Composite Reverse-Osmosis
Membranes: Origin, Development, and Recent Advances," in Synthetic
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12, Riley, R.L. et al. "Recent Developments in Thin-Film Composite Reverse
Osmosis Membrane Systems," Desalination, 36, pp. 207-233, 1981.
13. Nielsen, W.K. and S. Kristensen. "The Application of Thin-Film Composite
Membranes in Plate-and-Frame Systems for Water Purification,"
Desalination, 38, pp. 383-396, 1981.
14. Jonsson, G. "Overview of Theories for Water and Solute Transport in
UF/RO Membranes," Desalination, 35, pp. 21-38, 1980.
15. Sourirajan, S. and T. Matsuura. "Physicochemical Criteria for Reverse
Osmosis Separations," in Reverse Osmosis and Synthetic Membranes,
S. Sourirajan, ed. Chapter 2, National Research Council Canada, Ottawa,
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16. Fang, H. H.P. and E.S.K. Chian. "Removal of Alcohols, Amines and Aliphatic
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pp. 1347-1358, 1975.
17. Matsuura, T. and S. Sourirajan. "Physicochemical Criteria for Reverse
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Aqueous Solutions Using Porous Cellulose Acetate Membranes," J. Appl.
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18. Matsuura, T. and S. Sourirajan. "Reverse Osmosis Separation of Phenols in
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19. Rozelle, L.T., J.E. Cadotte, K..E. Cobian and C.U. Kopp, Jr.
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20. Chian, E.S.K., W.N. Bruce and H.H.P. Fang. "Removal of Pesticides by
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22. Light, W.G. "Removal of Chemical Carcinogens from Water/ Wastewater by
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23. Riley, R.L., R.L. Fox, C.R. Lyons, C.G. Milstead, M.W. Seroy and
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24. Groves, G.R., C.A. Buckley and R.H. Turnbull. "Closed Looped Recycle
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25. Brandon, C.A. and J.J. Porter. "Hyperfiltration for Renovation of Textile
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PB279 451, Research Triangle Park, NC, March 1978.
27. Delaney, R.A.M. and J.K. Donnelly. "Application of Reverse Osmosis in the
Dairy Industry," in Reverse Osmosis and Synthetic Membranes,
S. Sourirajan, ed. Chapter 20, National Research Council Canada, Ottawa,
1977.
28. deBoer, R. and J. Hiddink. "Membrane Processes in the Dairy Industry,"
Desalination, 35, pp. 169-192, 1980.
29. Lawhon, J.T., D. Mulsow, C.M. Cater and K..F. Mahil. "Production of
Protein Isolates and Concentrates from Oilseed Flour Extracts Using
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pp. 389-394, 1977.
30. Matz, R., E. Zisnerand G. Herscovici. "Pressure Driven Membrane Processes
in the Treatment of Industrial Effluents," Desalination, 24, pp. 113-128,1978.
31. von Bockelmann, I., P. Dejmek, G. Eriksson and B. Hallstrom. "Potential
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Canada, Ottawa, 1977.
32. Porter, M.C. "Ultrafiltration of Colloidal Suspensions," in Recent Advances
in Separation Techniques, AIChE Symp. Series, 68, (120), pp. 21-30,
AIChE, New York, NY, 1972.
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34. Wahl, J.R. et al. "Ultrafiltration for Today's Oily Wastewaters: A Survey of
Current Ultrafiltration Systems,"in Proc. 34th Purdue University Industrial
Waste Conference, Ann Arbor Science, MI, 1980.
35. Sonksen, M.K., P.M. Sittig, and E.F. Maziarz. "Treatment of Oily Wastes by
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University Industrial Waste Conference, pp. 696-705, Ann Arbor Science,
MI, 1979.
36. Bansal, I.K. and A.J. Wiley. "Application of Reverse Osmosis in the Pulp and
Paper Industry," in Reverse Osmosis and Synthetic Membranes,
S. Sourirajan, ed. Chapter 22, National Research Council Canada,
Ottawa, 1977.
37. Eriksson, P. "Ultrafiltration for Recovery of Lignosulfonates from Spent
Sulfite Liquor," in Water-1979, AIChE Symp. Series, 76(197), pp. 316-370,
AIChE, New York, NY, 1980.
38. Hicks, R.E., R.F. Probstein and I. Wei. "Water Management in Oil Shale
Production, Proc. Industrial Wastes Symposia, 53rd Annual WPCF
Conference, Las Vegas, NV, September, 1980.
39. Li, N.N. "Separation of Hydrocarbons by Liquid Membrane Permeation,"
Ind. Eng. Chem. Process Des. Develop., 10, pp. 215-221, 1971.
40. Cahn, R.P. and N.N. Li. "Separation of Phenol from Wastewater by the
Liquid Membrane Technique," A?/?. Sci., 9, pp. 505-519, 1974.
41. Frankenfeld, J.W. and N.N. Li. "Liquid Membrane Systems," in Ion
Exchange for Pollution Control, C. Calmon and H. Gold, eds. Vol. II, Chapter
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42. Cahn, R.P. and N.N. Li. "Separation of Organic Compounds by Liquid
Membrane Processes,"/ Mem. Sci., 1, pp. 129-142, 1976.
43. Gumerman, R.C., R.L. Gulp and S.P. Hansen. "Estimating Water Treatment
Costs, Vol. 2," Report No. EPA 600/2-79-162B, 1979, and "Estimating Costs
for Water Treatment as a Function of Size and Treatment Plant Efficiency,"
Report No. EPA 600/2-78-182, U.S. Environmental Protection Agency,
Cincinnati, OH, 1978.
44. Argo, D.G. "Water Factory 21. Design and Operating Data, Orange County
Water District, CA," NTIS No. PB 300-602, U.S. Office of Water Research
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Cost Estimating," Section IV.3.7, Report No. 600/8-80-042d, Office of
Research & Development, U.S. Government Printing Office, Washington,
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39-58, 1980.
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THE IMPACT OF ORGANIC SUBSTANCES ON
WASTEWATER RECYCLING IN INDUSTRY
Robert H. Culver, PhD
and
John F. Donovan
INTRODUCTION
The Concept of Recycling
Recycling is the application of an industry's wastewater to meet all or a part of its
water requirements. A distinction is commonly made between recycling and reuse.
Recycling in this text is defined as internal reuse: an industry uses a portion of its
wastewater again for the same or some other in-plant function. Reuse, on the other
hand, involves an industry's use of a wastewater effluent or "reclaimed water" from a
municipal or another industrial wastewater treatment facility. Reuse is discussed in
another chapter of this monograph.
The concept of recycle in industry is not new. In the United States alone, there are
thousands of closed-cycle cooling systems. Noncontact process steam is frequently
recycled through a condensate return system for boiler water make-up. Water
recycle has been used for a long time in air conditioning and power generation for
cooling water. However, while recycle of water in heat-transfer systems has been
common for years, attempts to recycle process water have been, in general, relatively
recent. Even so, several industries have been engaged in process-water recycling for
over 50 years
The Need for Recycling
Recycling in industry is carried out to reduce operating costs. The cost equation
for water use and reuse has changed drastically in recent years. The cost of water is
no longer just the cost of pumping it out of the ground or the nearest clean surface
water source, plus the cost of cheap energy for heating orcooling it if needed. It now
The Author Robert H Culver received his B S in Chemical Engineering and M S in Civil Engineering at
the University of Florida, and his Ph D in Sanitary Chemistry at Harvard University In 1953, after teaching
at North Carolina State College and Harvard University, he joined the staff of Camp Dresser and McKee
Incorporated, and has worked on a wide variety of sanitary engineering problems Dr Culver is Vice
President of Camp Dresser and McKee, Incorporated
John F Donovan is a project engineer in the Environmental Engineering Division of Camp
Dresser and McKee, Incorporated which hejomed after receiving his B S and M S in Civil Engineering at
Northeastern University He has participated in several major projects involving design evaluation of
wastewater collection and treatment facilities, studies of sludge management and effluent disposal, and
research in innovative methods of sludge stabilization and water reuse He served as overall project manager
for the "Guidelines for Water Reuse" which was published as an E PA manual. M r Donovan is the author of
several technical papers on water reuse
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includes the cost of expensive energy, the cost of treatment and disposal and the cost
of bad public relations if excessive amounts are wasted.
Federal, state and local regulation of industrial discharges has added a significant
cost element to the overall cost of water use. Increased water quality requirements by
many industrial processes have increased the cost of process water preparation.
Increased energy costs have made hot or cold water more valuable. Public awareness
of the limitations of the water resource has sensitized management to the value of
water conservation and waste reduction.
Recycling therefore is now perceived, in many situations, to be a cost-effective
means for supplementing a plant's water requirements. Recycling can often reduce
the cost of (a) pumping fresh water (sometimes from great depths), (b) heating or
cooling water, (c) treating and disposing of wastewater, and (d) pretreating
wastewater before discharge to a publicly owned treatment works or paying
penalties for excessive pollution loads. In some cases recycling is essential in meeting
the plant's water needs. An added benefit may be recovery of valuable materials
previously lost.
Freshwater Use by Industry
The Nation's total withdrawal of water from natural sources in 1975 was about
360 bgd (billions of gallons per day).1 Industry used about 40 percent of this amount.
Table 1 lists the estimated amounts used by the manufacturing industries. The
remainder was used by the electric power industry, principally for cooling water.
Status of Wastewater Recycling by Industry
A 1979 study for the Office of Water Research and Technology (OWRT), U.S.
Department of the Interior, predicted significant reduction in freshwater
withdrawals by the year 2000 for the primary metals, chemical and paper industries
due to wastewater recycling.2 The recycle ratio for these industries, which currently
averages about two cycles per unit of water, is expected to increase by that year to
about 17 cycles per unit of water.
Industrial wastewater recycling is practiced in most industries in varying degrees.
In many, organic substances are absent or are present in concentrations so low that
they do not pose processing problems. In other industries, organic substances are
used in the manufacturing process and therefore occur in the wastewater, sometimes
rendering it unfit for recycle without treatment. The latter industries include
petroleum refining, textile manufacturing, coal conversion, polymer and plastics
production, pesticide manufacturing and organic chemicals production. However, it
is only in recent years that serious widespread efforts have been made by industry to
recycle water. While reliable data are not presently available to indicate the degree to
which recycling has actually progressed, industries in which recycling is widely
practiced are described below.
Petroleum Refining Industry—
The petroleum refining industry involves a complex combination of operations
yielding many intermediate and finished products.3 Typical production processes
include crude desalting, crude oil fractionation, thermal or catalytic cracking,
solvent refining, grease manufacturing, asphalt production, and product finishing.
Among the more common recycling strategies are:
• Recovering wastes from catalytic cracking units and recycling as
washwater to the crude oil desalting operation.
• Using stripper bottoms for make-up to crude desalters.
• Using blowdown from high pressure boilers as feed to low pressure boilers.
• Using steam condensate as boiler feedwater and for make-up in cooling
towers.
-------
Table 1. Use of Freshwater by Industry, 1975.
Water Use
Industry
Primary metals
Chemicals and allied products
Paper and allied products
Petroleum and coal products
Food and kindred products
Transportation equipment
Textile mill products
All other
Billions of Gallons
per day
17.6
13.5
8.4
25
2.5
1 3
0.6
47
Percent of
Total
34
26
17
5
5
3
1
9
51 1
100
These techniques reduce "end-of-pipe" wastewater treatment, which usually
involves neutralization or equalization prior to biological treatment. Some plants
recycle this treated wastewater as cooling water or scrubber water.
Textile Industry—
The textile industry is perhaps the most diversified of industries in terms of
processes, technologies, and materials employed. One might say that each plant is
unique. The materials processed range from cotton, wool, linen and silk, to a
seemingly endless list of synthetic manmade fibers. The chemicals employed number
in the thousands, ranging from the most complex organic dyes to the simplest
inorganic salts, such as magnesium sulfate. The chemicals include dyes, soaps,
disinfectants, fungicides, permanent press and waterproofing products, insecticides,
acids, alkalies, carriers, fixers and many others.
The value of dyes and other processing chemicals, as well as the cost of water, has
for many years provided an incentive for textile engineers to develop systems for
recycling and reusing spent solutions. Dialysis for the recovery of caustic soda in the
mercerizing process has long been practiced. However, since the treatment of
wastewater to control environmental pollution has become mandatory, recycling
both water and chemicals is even more economically attractive than before.
One of the largest uses of water, in textile manufacturing, is the washing of the
material. Washing is involved in nearly every process, starting with preparation of
the raw fibre and ending with washing after dyeing or other finishing of the cloth.
Reduction in the amount of water used for washing and rinsing can result in
substantial savings. One of the most successful methods by which water can be saved
in rinsing is by countercurrent washing. Fig. 1 illustrates the principal of countercurrent
rinsing.
Pulp and Paper Industry—
Completely closed-cycle bleached kraft pulp mills have been designed and built.4
The Great Lakes Paper Company built such a mill in 1976. In this system, the wood
room operates entirely on recycled water with the solids being removed by
sedimentation, filtration or centrifugation. Screen rejects and other solids are
recycled or burned. Unbleached pulp is washed entirely by continuous diffusion
using recycled chlorinated filtrate from the bleach plant. Black liquor is evaporated
and burned with chemical recovery. White liquor is evaporated in two steps
permitting separation and recovery of sodium chloride. The salt is electrolyzed to
394
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Cloth
Travel
To Drier
Fresh
Water
Dye Bath or
Washer
1st Rinse
2nd Rinse
3rd Rinse
To Waste
Treatment
Reference: Camp Dresser & McKee Inc.
Figure 1. Countercurrent rinse system commonly used in the textile industry
produce chlorine and caustic for reuse in the bleach plant. Evaporator condensate
from both black liquor and white liquor is used to replace fresh water sources.
Recovered sodium sulfate and soda ash are directly reused in the mill.
Organic Chemical Production—
The organic chemical industry is so diversified that even a general description is
beyond the scope of this chapter. However, the example of recyling given below
illustrates how one plant profited from recycling. In this case a major chemical
company manufactured an insecticide for the control of cattle grubs and internal
parasites in cattle and sheep. The process generated a high percentage of by-products
in the form of tars and a wastewater stream containing dissolved organics and salt.5
The two tar streams generated up to 650 Ibs of waste per 1,000 Ibs of product. The
wastewater stream amounted to about 18,000 gal per 1,000 Ibs of product
manufactured.
A pilot plant was constructed to investigate methods of treating the wastewater
and recycling valuable compounds. The first step in the recycling process involved
blending the tar and waste stream with sodium hydroxide. The second step—the key
to the recycling—was high-temperature caustic hydrolysis. This was followed by
neutralization and coalescence to achieve separation of the organic phase from the
brine. The organic phase consisted of 95 percent pure 4-t-butyle 2-chlorophenol,
which was distilled and recycled as a raw material in the pesticide manufacturing
process.
The brine was subjected to activated carbon treatment to remove chlorophenol
prior to biological wastewater treatment. Regeneration of the activated carbon
allowed recovery and recycling of the chlorophenol. By-product recovery and
utilization increased yield of the process to approximately 90 percent versus about
40 percent before recycle. Overall waste treatment costs were also minimized by
reducing the load on the biological treatment units and by reducing the amount of
tar previously incinerated.
Photographic Processing—
One of the few photofinishing companies practicing complete recycle of process
wastewater is the PC A International Co., Matthews, North Carolina. In 1977, the
company initiated a system, illustrated in Fig. 2, that recycles 90 percent of all
washwater and 75 percent of all process chemicals.6 Process washwater is pumped
through reverse osmosis units, which recover 90 percent of the washwater for direct
recycling to film and print processing operations. The reverse osmosis concentrate is
395
-------
Photographic Chemical
Processing
t *
Reverse
Osmosis of
Washwater
Recovery
~*"
t
Waste |
Ammonia
Reference: 5
t
Sludge to
Silver Refinery
Figure 2. Example of photographic processing recycling system
combined with chemical recovery wastes and treated in a three-stage evaporation
system. The water from the evaporators is condensed, collected and passed through
an ion exchange unit. The clean water is then recycled.
Two by-products are the sludge from the evaporation unit and the ammonia from
the ion exchange unit. Silver is recovered from the sludge and re-refined. In 1979, the
company estimated the value of this silver at approximately $700,000. The ammonia
extracted is used to fertilize lawns on the company's 57-acre site.
These brief summaries indicate the diversity of industries which presently employ
recycling of wastewater. Others will be described in more detail.
GENERAL CONSIDERATIONS
Approach to Plan Development
Recycling is a highly special activity unique to each plant and therefore, the
approach to recycling can be described only in general terms. Recycling can be as
simple as returning to the process effluent process water from a particular unit
operation, or it can be as complex as providing multistage treatment for all of the
wastewater from a large industry with recycle of the treated water for all plant uses
(Pulp and Paper Industry). However, regardless of the simplicity or complexity of a
recycling scheme, certain information is required before any scheme can be
implemented. Each plant must be studied to determine the quality and quantity of
the water required for each plant process. Water quality requirements should be
developed to define the minimum acceptable water quality for each unit process. The
effects of each use on quality should be understood. A water use matrix may then be
drawn up showing water quantity and water quality needs, and the quantity and
quality characteristics of the wastewater discharges available. A hierarchical
ordering of uses can be assembled and appropriate treatment processes devised to
meet the minimum requirements.
Defining the minimum acceptable water quality for a process is not always easy,
particularly where it is proposed to substitute recycled water for water of the highest
available quality. Plant managers are understandably reluctant to make changes
that may adversely affect the quality of their products. This concern is often justified
because of the tendency for pollutants to build up in the recycled stream. Once
minimum acceptable water quality standards have been established for each water-
using unit process, several strategies are available for designing an appropriate
system to maximize recycling and minimize the use of fresh make-up water.
The water quality required by each process might be defined in relatively simple
qualitative terms, as shown in Table 2. The wide differences in water quality for
396
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Table 2. Water Quality Limitations on Water Use
Class Quality Acceptable Uses
I Highest quality Final product wash or rinse, pump
seal flush, etc
II Some dissolved solids. Make-up washing solutions, first
minor amounts of rinse, cooling water, etc.
suspended solids, low
color
III Some dissolved solids, Area wash, equipment wash-up
suspended solids and
color
different industries and for different uses within an industry make it impractical to
recommend specific numerical limitations for designated water uses except on a
case-by-case basis in each plant. This is generally true even within various categories
of a single industry. Each plant will have to establish the water quality characteristics
required for the individual water using processes within the plant.
Once the minimum water quality requirements for each process within the plant
have been established and classified, it then becomes necessary to determine the
characteristics of the process waste streams. A comparison of these characteristics
with the minimum water quality requirements may show that the wastewater from
one process can well serve as the source water for another process within the plant
with little or no treatment. For example, a tannery might use the effluent of the last
hide-rinsing bath to make up the dehairing solution for the next batch of hides, or a
textile factory might use the last rinse of the cloth in a washing operation to make up
the next batch of soapy washwater. An example from the food processing industry is
the use of the final washwater for vegetables going to canning or freezing as the first
wash for fresh vegetables arriving from the field. Such successive use without
intermediate treatment is termed cascading use of water, in which the wastewater
from the unit process is used in another process that can tolerate water of a lower
quality.
It is probable that cascading use of water has limited application in most ind ustrial
situations, although it should always be investigated to reduce the volume to be
treated. In almost every recycling situation some form of treatment will be required
before the water can be recycled. The treatment may be as simple as cooling the
water in a cooling tower before recycle or as complex as a multi-stage biological-
chemical treatment system. Even a system as simple as cooling the water in a cooling
tower probably will require chemical treatment to prevent biological fouling and
control corrosion.
The conventional unit processes available for treatment of water containing
objectionable suspended and colloidal solids for recycle consist of screening,
chemical coagulation, sedimentation, filtration and ultra-filtration. Dissolved
matter can be removed by such processes as chemical precipitation, ion exchange,
reverse osmosis, carbon adsorption, or synthetic resin adsorption, or distillation.
For many wastewaters biological oxidation may be used to remove organic material.
In some instances chemical oxidation may be advantageous. Typical treatment
systems usually consist of a combination of several of these processes.
Recycle of the wastewater from a particular process is often objectionable because
of a single characteristic, i.e. suspended solids. In such cases, it may prove feasible
and economical to treat the water to remove the objectionable property—for
example, to filter out the objectionable solids—and recycle the water directly to the
397
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original process. A good example occurs in poultry processing, where cold water
used for chilling processed birds may be upgraded by filtration and recycled directly,
thereby saving the cost of cooling fresh water. In the textile industry, dilute dye baths
or starch solutions have been concentrated by ultra-filtration, with recovery of both
the dye or starch and the water. Direct recycle of a process waste stream separated
from the general plant wastewater provides the opportunity for conserving energy,
product and water. Until recently, the savings inherent in such techniques generally
have been overlooked. So far, most treatment for recycling has been concerned with
the inorganic materials in the water. Organics have been of little concern in
industrial water recycling except in special cases.
Impact of Organic Substances on Recycle
Until recently industrial wastewaters contaminated with organic matter have been
considered unfit for recycle and following treatment, have been discarded to the
environment, usually to surface waters, there to undergo further degradation as a
result of microbial metabolic activity. Biological degradation of organic matter is by
aerobic oxidation as long as dissolved oxygen is available in the receiving waters,
and thereafter continues as an anaerobic process. The impairments to aquatic life
associated with low concentrations of dissolved oxygen are well known as are the
gross nuisance conditions that may result from the absence of dissolved oxygen.
These need not be detailed here. Now of course industries must remove harmful
materials, organic and inorganic, from their wastewaters to comply with the
requirements of regulatory agencies. To treat or not to treat is no longer a choice.
Since the cost of wastewater treatment is a fixed factor of operation the decision to
reuse the treated water is easier to make.
Based on their particular impacts, organic materials in wastewaters may be
classified as follows:
• Suspended solids which may contaminate the product, foul equipment or
clog pipes or nozzles. Examples are non-soluble fats, feathers, vegetable
parts, non-soluble tars, fruit pits, coffee grounds, fish, animal or fowl
parts, sawdust and gelatinous chemicals. This list is of course not complete.
• Suspended liquid substances which may contaminate the product, react
with process chemicals to reduce effectiveness, or otherwise interfere with
processes. Examples are oils, emulsified or free, insoluble organicssuchas
carbon tetrachloride, benzene, dyes or emulsions.
• Dissolved organic substances which may contaminate the product, and
react with process chemicals to produce damaging side reactions.
• Examples are: sugars, alcohols, some amines, pesticides, color bodies and
similar materials.
Suspended material, both solid and liquid, can be removed by
sedimentation/flotation and filtration. Chemical additions may be required to
break emulsions or coagulate colloidal or finely divided particles to accelerate their
removal by sedimentation/flotation and filtration. Common coagulant chemicals
are alum (aluminum sulfate), ferric chloride, lime (calcium oxide or hydroxide) and
synthetic polymers. pH adjustment is frequently necessary to assist coagulation.
Lime, caustic soda or sulfuric acid are commonly used pH adjusting chemicals. Soda
ash and sodium bicarbonate may be used in special situations where the carbonate
ion provides a pH buffering action. Chemical coagulation is often effective for color
removal. Flotation of materials may often be enhanced by the introduction of the
dissolved air under pressure. As the pressure is released fine bubbles attach
themselves to suspended substances causing them to float to the surface to form a
scum which may be removed by skimming. Solids which settle to the bottom are
removed as a semi-liquid sludge. Suspended solids not removed by
sedimentation/ flotation may be removed by filtration. Some suspended liquids such
as heavy oils may also be removed in filters.
398
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A number of processes are available for removing dissolved organic substances
including: adsorption on activated carbon or polymeric adsorbents, distillation,
reverse osmosis, ion exchange, electro-dialysis and chemical precipitation. The use
of activated carbon may involve either passing the wastewater through a bed of
granular activated carbon or adding powdered activated carbon to the wastewater in
a mixing tank and removing it with the adsorbed materials by coagulation,
sedimentation and filtering. Detailed descriptions of the adsorption process are
presented in other chapters. In any event pilot studies will normally be needed in
each situation where carbon adsorption is being considered.
The types and concentrations of organic substances in industrial wastewaters are
as varied as the industries themselves. Their effects on the recycling of the water are
usually very specific to the particular system under consideration. For example,
small concentrations of colorless organic substances may not affect adversely the
recycling of water for dyeing in a textile mill, but even the smallest concentration of
colored organic material in a process water will preclude its use for dyeing, especially
where pastel shades are used. A similar restriction applies to the manufacture of
white paper; however, a slight color is of little importance in the manufacture of
brown draft liner board.
The presence of organic compounds in water is often troublesome in the food
processing industry where, for example, they can affect product taste in beverage
production, and provide a food source for growth of bacteria in poultry processing.
Harmful or noxious organic by-products may result from the excessive use of
oxidizing agents used as disinfectants.
Problems in recycling wastewater containing organic pollutants are similarly
diverse in industries in addition to the food industries. However, such problems are
not widespread. In most instances, the organic content as such is of little importance.
For example, there are no limitations on the soluble organic concentrations in water
used for washing vegetables before canning, although the presence of pesticides or
other toxic compounds may be of concern, or for washing and cooling the finished
cans. Water used in many industries, including those involved in the manufacture of
organic chemicals, production of primary metals, etc., generally requires no
treatment at present for reduction of dissolved organic matter. This situation may
change, of course, if wastewaters from other types of industries are considered for
recycling.
Specific Plant Experience
The following case studies illustrate the use of the processes described above to
treat wastewater for recycle.
Chemical R&D Facility—
FMC Corporation's Chemical Research and Development Center at Princeton,
NJ meets a "zero discharge" of contaminated wastewater and treats the wastewater
to produce a high quality water suitable for recycle.7 As in any research type
laboratory and pilot plant facility, the quality of the wastewater fluctuates widely.
Fig. 3 is a flow diagram of the treatment plant which treats about 14,000 liters per
day. The pilot plant wastewater is treated by a rotating biological contactor (RBC),
clarification, filtration, and carbon adsorption. The effluent from the latter stage
thenjoins the laboratory wastewater for further treatment. The combined wastesare
clarified by coagulation with a synthetic cationic polyelectrolyte, and alum, followed
by sedimentation. Final preparation for treatment by reverse osmosis is carried out
by a precoat vacuum filter using diatomaceous earth. The pH is then adjusted to
between 4.5 and 5.5 to prevent calcium carbonate fouling of the reverse osmosis
membranes. Reverse osmosis units recover 81 to 88 percent of the feed water.
399
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PILOT PLANT _
WASTEWATER
RBC
MIXED
MEDIA
FILTER
LABORATORY
WASTEWATER
WASTE
SOLVENTS""
-ci2
BRINE TO
OFF-SITE DISPOSAL
DISCHARGE TO
R&D FACILITIES
Figure 3. Flow diagram of advanced waste treatment plant at FMC Corporation's
chemical research and development facility.
-------
The quality of the influent water varies widely. The organic content, measured as
TOC (Total Organic Carbon), varies from about 200 mg/ L to 1,500 mg/ L. Selected
effluent characteristics are shown in Table 3.
The construction costs (1978) for the plant are shown in Table 4. The total cost for
treating and recycling the water was 0.247 dollars per gallon including depreciation
(straight line for 10 years), labor, power and materials. The single most expensive
material cost was reverse osmosis membranes at 0.0136 dollars per gallon. The cost
for a larger scale plant of the same type is estimated at 0.085 dollars per gallon.
Poultry Processing—
The USDA regulations require that the chilling tanks which are part of poultry
processing have an overflow rate of at least one-half gallon of water per bird chilled,
with the makeup being provided by water of potable quality. At Foster Farms,
Livingston, CA, 125,000 gallons of water per day are used in the chiller system. The
chiller water is maintained at 34° F.
A recycling system consisting of a pump, a diatomaceous earth filter, two
automatic control valves and a heat exchanger for cooling the water to the proper
temperature was investigated. A rough schematic diagram of the system is shown in
Fig. 4. The results of the treatment as measured by total bacterial count and
suspended solids are shown in Table 5. The table shows that the water quality of the
chill tank was superior with recycled influent. The bacterial count was only slighter
lower but the suspended solids were only 23 percent of those in the conventional
chiller.
Furthermore, an analysis of the overall costs indicated that a savings of between
$67.84 to $128.34 per day over the existing system in which chiller overflow was
permitted to discharge to waste.
Recycling in a Dairy—
Data were collected in Cornell's Animal Science Teaching and Research Center9
where approximately 350 cows are milked twice each day in a double-ten or rotary
milking parlor with appropriate C1P (clean-in-place) machinery. Twelve
combinations of three water temperatures with hard and soft water cleaning wash
cycles were used to study water and detergent conservation techniques. Special
equipment was designed and used to save detergents and washwater for reuse in
additional cleaning cycles.
The milking center's double-ten herringbone parlor with a separate identical C1P
system for each half made it possible to use one side as a control and the other for
experimentation.
Washing and sanitizing a CIP system following milking consists of the following
steps in sequence: rinsing, washing with detergent solution, rinsing, sanitizing with a
disinfectant solution, and final rinsing. This sequence is called the post-milk cycle.
Prior to milking this washing cycle is repeated as a pre-milk cycle. The recycling
scheme described below permits the reuse of the detergent solution several times. An
additional modification is designed to save water by using the sanitizing solution
from the pre-milk cycle (which is essentially uncontammated since it followed
washing and rinsing) to be used for the first rinse in the post-milk cycle.
The basic concept of detergent recycle is illustrated in Fig. 5. During the wash
portion of the post-milk cycle, the stored detergent is released into the CIP sink
(valve B open). This detergent is circulated through the CIP lines several times by
returning and passing it through the detergent storage (valve A open to detergent
storage). Thus the detergent does not leave the system during this portion of the
cycle. Toward the end of the wash portion of the cycle, the valve at the bottom of the
storage tank (valve B) is closed and the detergent is captured. During other portions
of the pre- and post-milk cycles, the valve at the top of the storage (valve A) diverts
all returning water to waste.
401
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Table 3. Selected Effluent Characteristics at FMC Corporation's Chemical
Research and Development Facility7
Characteristic Concentration
TOC 2 0 mg/L
Specific conductance 250Aimhos/cm
Total dissolved solids 248 mg/L
Turbidity 36 NTU
Foaming agents 0.06 mg/L
Color 5 units
Table 4. Construction Cost for Zero Discharge Treatment Plant at FMC
Corporation's Chemical Research and Development Facility7
Item Cost, dollars (1978)
Steel frame building (4,500 ft2) 550,000
Recycle treatment equipment 223,000
4 Storage tanks for processed water 212,000
Incinerator, evaporator, fuel oil
storage tank, collection and feed tanks 624,000
Miscellaneous, cooling tower treatment
equipment, chemical feed system, etc 211,000
$1,820,000
One modification of the equipment, from the flow shown in Fig. 5, would allow
reuse of the sanitizing solution from the pre-milk cycle for the first rinse of the post-
milk cycle. The basic concept for detergent recycle with sanitizer reuse is illustrated
in Fig. 6. In this modification, during the pre-milk cycle detergent wash step, valves
A and B would be open as described above. After the detergent wash was completed
and the detergent captured in the detergent storage, valve C would be opened to
waste and the rinse water sent to the sewer. Then during the sanitizing step valve C
would be turned to divert the sanitizing solution to the Cl P sink and from there to be
recycled to the C1P lines. The sanitizing solution would then be collected in the C1P
sink and valve C adjusted to discharge the subsequent rinse to waste. On the next
post-milking cycle the sanitizing solution in the CIP sink would be pumped to the
CIP lines to provide the first rinse.
The results of these procedures were presented in a series of bacteriological
samples from the two parallel milking lines, one operated in the normal manner, the
other with recycled sanitizing solution as described. The results of these tests are
shown in Table 6. No significant difference between the two modes of operation is
apparent.
It was concluded that -
1. Recycling will save about 60% of the energy and at least 80% of the detergent.
2. Water usage in cleaning and sanitizing may be reduced by about 50%.
3. Savings of about S830/year (1976 dollars) can be made by recycling in a 100-
animal dairy.
402
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Cold Water Ice (30 gp
Recirculation
Line
NOTE A turbidity meter to detect pressure of DE and control valves, B for safety purposes, that is,
prevent DE from entering chiller
Figure 4. Rough schematic program of proposed diatomaceous earth filter system to
treat recycled poultry chilling wastewater.
Table 5. Suspended Solids and Total Bacterial Count Comparison of
Conventional System and System Using Recycled Wastewater
to Chill Poultry
System
Conventional chill tank
DE filter effluent
Chill tank with recycled
influent
Total Bacterial Count
(no /mL)
1.5
7.9
1.1
x 106
x 103
x 106
Total Suspended Solids
(mg/L)
809
20
186
ELEMENTS OF A STRATEGY TO INCREASE RECYCLING
The first need to increase recycling in industry is to make managers more aware of
the advantages of wastewater recycling. This awareness can be increased by
publication of the results of successful recycling projects and sponsoring of
demonstration projects by official agencies.
It is unlikely that any startling new technology will be developed in the near future
applicable to industrial recycling. This will continue to depend upon the selection of
403
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C.I.P. LINES
Water
i
Detergent
Storage
C.I.P. I SINK
I
WASTE
Figure 5. Initial milking parlor recycle system.
C.I. P. LINES
>,
Water
I
Detergent
Storage
C.I.P.I SINK
I
WASTE
Figure 6. Modified milking parlor recycle system.
404
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an appropriate combination of unit wastewater treatment processes to achieve the
desired water quality. Existing unit processes will require modifications to meet
individual needs. For example, materials of construction may have to vary from
those traditionally used to prevent corrosion in some cases or to protect recycled
water quality in other cases. Another example would be the development for a
particular application of a microporous membrane having the exact porosity needed
to minimize the passage of undesirable organic materials while at the same time
maximizing the water flux. The use of polymers which would reduce evaporator
tube fouling when distilling water containing organics is needed.
Instrumentation is needed to monitor automatically trace quantities of specific
organics in the recycle stream to prevent product contamination. If operators could
be assured that the recycled water met quality standards at all times they would be
more receptive to its use. Detection methods are available such as TOC (Total
Organic Carbon) analyzers but they have to be modified and developed for on-line
use.
Two of the case studies in this article, the dairy waste cycle and the poultry cooling
recycle, illustrate the need to demonstrate that recycled systems can yield an
improvement in the performance of the industrial process, i.e. lower bacteria counts.
However, more studies are needed to develop an understanding of system reliability
and consequences of system failure.
Very little is known of the impact of organics in recycled wastes, principally
because experience in recycling wastes containing organics is scanty. The Versar10
evaluation of the published literature in the reuse/recycle area uncovered only one
published work on the removal of organics from reused process water. Hence studies
are needed by industry to determine the impact on product quality of organics in
process waters. It may be found that in many cases the impact is negligible and
recycled water quality may require less treatment than previously thought. Much
is known about the removal of organics from wastewater but little is known about
the need to do so.
Future application of adsorption techniques will inevitably increase as new
strategies for recycling of industrial wastewater, recovery of valuable constituents
and reduction in discharge of organic substances are developed.
Future applications of recycling will involve analysis of manufacturing processes
and specification of recycling strategy for the individual process or plant. The
objective will be to develop a comprehensive plan to reduce discharge of pollutants.
Existing wastewater treatment technology will be used only to the extent that it fits
into the plan.
Table 7 shows the results of one analysis of industrial recycling." For this
situation, a generalized representation, recycling was slightly less costly than the
current practice. A more detailed analysis would also take the account accelerated-
depreciation accounting, tax incentives, or other devices available to industry that
favor selection of recycling. One research need involves determination of ways
economic factors influence the decision to implement a recycling program.
An example of a potentially new technology is the use of conventional biological
systems using genetically modified bacteria capable of degrading so-called
"nonbiodegradable" wastes containing organic substances. Battelle Columbus
Laboratories recently announced research efforts using bacteria to decompose toxic
chemicals and pollutants such as DDT.12
The unit separation processes discussed in the previous section will all gain more
widespread use in recycling strategies. However, much research is needed to
optimize these processes for specific wastes. Concurrently, efforts must be made to
reduce the amount of energy needed to achieve the required levels of treatment. This
may involve modifications such as the use of both active and passive solar energy or
the use of low-pressure waste steam to maintain optimum temperatures in biological
treatment units to accelerate the rate of bio-oxidation of organics. Industry can use
heat in this manner more easily than can municipalities.
405
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Table 6. Comparison of Total Bacterial Counts of Dairy Equipment Cleaned
with Recycled and with Conventional Sanitary Solutions.
Arithmetic Mean of Bacterial Counts
(no./ml) of 68 Samples
Sampling Sites
Random teat cup
2nd random teat cup
Receiver jar float
Pipeline joint
Final rinse water
Conventional
Side
2,500
990
210,000
55
1 9,000
Recycled
System
6,400
2,800
1 90,000
620
24,000
Table 7. Cost Comparison of System Using Various Degrees of Recycled
Wastewater.*
Metals removal
Wastewater treatment
In-plant modifications
for recycle
Water charges
Sewer charges
1 00% to Sewer
0% to Recycle
$ 80
70
0
490
2,000
40% to Sewer
60% Recycle
$ 80
1,810
120
240
700
10% to Sewer
90% Recycle
$ 80
1,910
150
50
140
TOTAL $ 2,640 $ 2,950 $ 2,330
"Costs in $1,000's over a 10-yr period. Derived from Reference 11.
The extent to which energy can be conserved will significantly influence the
economics of industrial recycling. Industry uses over two-thirds of its water supply
for cooling needs. Methods of conserving energy used in cooling or of minimizing
cooling requirements will attract increased research efforts. One promising
alternative is simply to cool water only to the level actually required through the use
of sophisticated instrumentation. Presently, cooling is often in excess of
requirements. Cooling water additives are available to minimize organic fouling, or
corrosion promoted by deposits of organic substances in the water.
In summary, research needs for recycling are not limited to technology-related
problems. Economic, energy, institutional, and other factors will need closer
scrutiny in order to increase the number of recycling facilities and their magnitude.
REFERENCES
1. U.S. Water Resources Council. The Nation's Water Resources, Vol. 2, Tables
III 18-25, pp. 46-54, Second National Water Assessment, Washington, D.C.,
Dec. 1978.
2. Williams, R.B. and G.M. Wesner. "Water Reuse and Recycling - An
Assessment of the Potential," Consulting Engineer, Vol. 53, No. 3, pp. 103-
406
-------
117, September 1979.
3. Hackman, E.E. Toxic Organic Chemicals - Destruction and Waste
Treatment, Noyes Data Corporation, Park Ridge, New Jersey, 1978.
4. Rapson, W.H. "The Closed-Cycle Bleached Kraft Pump Mill," Chemical
Engineering Progress, 72(6), p. 68, 1976.
5. Kennedy, T.L., R.E. Bailey, R.I. Keller and R.A. Gaska. "Process Changes in
an Organic Chemicals Production Plant to Improve Yields and Decrease By-
Products," Industrial Process Design for Pollution Control, Vol. 4, AIChE
Environmental Division, Charleston, \VV, Oct. 27-29, 1971.
6. Miller, S.S. "A Zero Discharge Waste water Treatment System,"
Environmental Science and Technology, 12(9), p. 1004, 1978.
7. Gurvitch, M.M. Description of An Advanced Treatment Plant To Recycle
Water at a Chemical Research and Development Facility, Proc. 34th
Industrial Wastes Conference, Purdue University, Lafayette, Indiana, p. 184,
1979.
8. Rogers, C.J. Recycling of Water in Poultry Processing Plants, EPA 600/2-
78-039, March 19, 1978.
9. Zall, R.R., D.R. Price, D.P. Brown, A.T. Sobel and S.A. Weeks. "Reuse
Value of Cleaning Fluids in Milking Centers With Varied Wash Conditions,"
Water - 1976: 1 Physical, Chemical Wastewater Treatment, AIChE
Symposium Series, 166 Vol. 73, 1977.
10. Rissmann, E.F., E.F. Abrams and R.J. Turner. "An Evaluation of the
Published Literature in the Water Reuse/Recycle Area," Industrial
Environmental Research Laboratory 68-03-2604, U.S. Environmental
Protection Agency, 1980.
11. Mace, G.R. "Using Water Once More with Treatment," presented at Water
Pollution Control Federation Annual Conference, October 1978.
12. "Clean Water Report," Business Publishers Inc., Silver Spring, MD, p. 1949,
July 15, 1980.
407
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MANAGEMENT OF ORGANIC RESIDUALS
SEPARATED FROM INDUSTRIAL WASTES
C. Lue-Hing, T.B.S. Prakasam and D. Zenz
INTRODUCTION
A significant development in the strengthening of the control of water pollution in
the United States was made in 1970 by consolidating various pollution control
agencies spread throughout the Federal Executive Branch into one organization viz.
the United States Environmental Protection Agency. The passage of the Federal
Water Pollution Control Act in 1972, the Safe Drinking Water Act in 1974, the
Toxic Substances Control Act (TSCA) in 1975, the Resource Conservation and
Recovery Act (RCRA), in 1976, and the Clean Water Act in 1977 reflected the
nation's determination to control pollution While the Federal Water Pollution
Control Act and its amendments seek to remove pollutants from wastewater, the
RCRA, and to a minor extent TSCA, provide measures for the regulation and the
ultimate disposal of solid wastes and the residuals separated from wastewater
streams. In this chapter, some salient aspects of RCRA and a state-of-art of the
treatment and disposal of organic residuals, particularly those which are hazardous
and are separated from industrial wastes are discussed.
Resource Conservation and Recovery Act
The two basic objectives of RCRA are (1) protection of public health and
environment, and (2) conservation of natural resources. To comply with RCRA
regulations, the generators of hazardous wastes are required to keep records, insure
The Authors Dr Lue-Hing received his Bachelor of Science degree in civil engineering at Marquette
University, his Master of Science Degree in sanitary engineering at Case Western Reserve, and his Ph D in
environmental and sanitary engineering at Washington University in St Louis After serving on the faculty
in Sanitary Engineering at Washington University, he joined the consulting firm Ryckman, Edgerly and
Tomlmson, Associates in which he was appointed Vice President He left this position to accept appointment
as Director of Research and Development for the Metropolitan Sanitary District of Greater Chicago His
research interests cover all phases of waste management and wastewater treatment, including industrial as
well as municipal wastewaters Dr Lue-Hing is the author of many technical papers on waste and wastewater
treatment and water quality management He serves on numerous committees of the National Academy of
Science - National Research Council and Federal and State agencies
Dr Zen? received his Bachelor of Science degree in civil engineering and his Master of Science degree and
Ph.D in environmental engineering at the Illinois Institute of Technology Since receiving his Ph D he has
been affiliated with Metropolitan Sanitary District of Greater Chicago, and since 1972 has served as
Coordinator of Research for this organization His fields of special interest include wastewater treatment,
biology and soils science with focus on land reclamation
Dr Prakasam received his Bachelor of Science in Chemistry at Andhra University, his Master of Science in
Biochemistry at Nagpur University, India, and his Doctor of Philosophy in Environmental Sciences at
Rutgers University Alter serving on the faculties of Food Science and Agricultural Engineering at Cornell
University, Dr Prakasam joined the staff of the Metropolitan Sanitary District of Greater Chicago as
Project Manager His special interests are directed to advanced waste treatment and biological waste
treatment processes He serves on various committees of the Water Pollution Control Federation and the
National Academy of Sciences
408
-------
proper packaging and labeling of wastes shipped off-site, and prepare a transport
manifest document for each shipment describing content, amount, and destination.
Although generators of hazardous wastes do not need permits, the RCRA
regulations place considerable responsibility on them. They must determine whether
their waste is hazardous and comply with RCRA regulations covering every step in
waste storage, treatment and ultimate disposal including collecting information on
the quantities of the hazardous waste generated.
Methods for Determining Quantities of Waste Residuals
A reliable estimate of the quantities of waste residuals generated in varoius
industrial categories would of course be extremely valuable in planning,
implementing, and regulating the disposal of such residuals. Unfortunately, there
has been a lack of information in this area for a long time. Only recently, with the
enactment of RCRA, has a concerted effort been initiated at the national, state, and
local levels to obtain reliable estimates. However, the Illinois Environmental
Protection Agency and the Metropolitan Sanitary District of Greater Chicago
(MSDGC) began implementing their manifest systems in 1979; a summary of data
taken from the MSDGC sludge manifest report forms for the year 1979 is given in
Table 1.
The data indicate that less than 1 % of the industries violated the requirement for
reporting their waste generation. The fats, oils and grease (FOG) wastes representing
about 21 percent of the waste generated are included as organic residuals. Most of
the industrial sludges generated within the MSDGC jurisdiction are disposed of in
three landfills in the Chicago area.
State and National Hazardous Waste Quantities
In a recently published study by the Illinois Institute of Natural Resources', an
attempt was made to compare the quantity of hazardous waste residuals produced in
Illinois with that generated nationwide as derived from various estimates. In arriving
at these estimates, the Institute made two basic assumptions: (1) Illinois contributes
7% of the waste generated in USA, and (2) 10% of the total industrial waste produced
is hazardous. No estimate of the organic fraction of the waste is given in the
Institute's summary. Assuming that 60% of the total solids contained in hazardous
waste streams is organic in composition, Table 2 presents an estimate of the total
quantity of hazardous wastes and its organic component generated annually by
industry.
The USEPA has estimated that the wastewater sludges generated by 14 major
industries (petroleum refining, waste oil re-refining, organic chemicals, pesticides,
explosives, inorganic chemicals, paint and allied products, Pharmaceuticals,
primary metals smelting and refining, textile dyeing and finishing, leather tanning,
electronic components, rubber and plastics, electroplating) would increase from
22.7 x 106tons/yrin 1977 to 42.8 x 106 tons/yr in 1978. Assuming as before, that 7
percent of this would represent the quantity produced by industries in Illinois, the
total amount of wastewater sludges in Illinois would be 1.94 x 106 tons in 1977 and 3
x 106 tons in 1978.
WASTES FROM PRIMARY INDUSTRIES
Among the major industrial categories that generate waste residuals containing
hazardous organics are those related to textiles, paint formulating, petroleum
refining, and organic chemicals manufacturing (including pesticides and
explosives).
409
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Table 1. Data Extracted from the Metropolitan Sanitary District of Greater
Chicago's Sludge Manifest Reporting System for February -
December, 1979
(a) Number of manifest forms
returned by industry 14,145
(b) Number of violations* 117
(c) % Violations < 1
Type of Waste Total Pounds
Metal/cyanide bearing wastes 161,800,000
Acid/alkali wastes 71,600,000
Fats, oils and grease wastes (FOG) 109,500,000
"Other" wastes 169,400,000
Total 512,300,000
"Violation means failure to return a manifest form
Textile Industry
There are seven categories of plants in this industrial sector: (1) wool scouring, (2)
wool fabric dyeing and finishing, (3) Greige goods (which employ dry operations like
knitting and weaving and hence generate no hazardous wastes), (4) woven fabric
dyeing and finishing, (5) knit fabric dyeing and finishing, (6) carpet dyeing and
finishing, and (7) yarn and stock dyeing and finishing.2
About 69% of the plants are engaged in woven or knit dyeing and finishing
operations. This industrial sector collectively produces 5.3 millions metric tons of
product/year.
Textile waste streams contain hazardous toxic organics. Basic dyes, some acid
dyes, and some dispersed dyes have been shown to be toxic to fish and hence are
potentially hazardous when discharged in wastes. Some of the dyes may degrade in
an anaerobic environment such as in landfills, and may produce a leachate of
carcinogenic metabolites. Other organics that are most likely to be hazardous are
dye carriers, solvents, and chemical finishes that find their way into textile waste
streams. Dye carriers include biphenyl, butyl benzoate, methyl salicilate,
trichlorobenzene, perchloroethylene, and other chlorinated aromatics. Some of
these dye carriers are known to be non-biodegradable in activated sludge treatment
systems. Biphenyl, toluene, and naphthalene are known to be toxic to activated
sludge biota. Chlorinated organics which are hazardous compounds are reported to
be present in various textile mill residuals.
The estimated quantity of waste residuals generated for the entire textile industry
along with the quantities of potentially hazardous dye and chemical container
wastes for the years 1974, 1977 and 1983 are given in Table 3. The estimated
quantities of potentially hazardous wastewater treatment sludges are given in Table
4.2
The suspended solids concentration of sludges originating in the different types of
textile mills is highly variable. Hence, the toxic organics associated with these solids
may also be highly variable. A concentration as low as 0.008% suspended solids was
reported for wool fabric dyeing and finishing sludges, whereas about 9.8 percent
suspended solids was reported for wool scouring waste sludges.
Although a complete analysis of the organics and their concentration in sludges is
not available, a range of 0.11 to 64.7 mg of chlorinated organics/kg of sludges
generated from different types of textile mill sludges was observed.2
410
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Table 2. Estimates of Total Solid and Hazardous Industrial Waste Generated and Organics Contained in it in USA and the State of
Illinois (106 tons/yr)
Source*
Michael Rapps
A.D Little Inc.
Booze-Allen Applied
Research Inc.
Stephen James
USEPA 1977 Estimates
Robert S. Glaubinger
Total
300-500
379
283
506
413
300-450
USA
Hazardous
30-50
37.9
28.3
50.6
41.3
18-27
Organics
18-30
22.7
17
30.4
24.8
18-27
Total
21-35
26.5
19.8
35.4
28.9
21-31.5
Illinois
Hazardous
21-3.5
2.65
1.98
3.54
2.89
2.1-3.15
Organics
1.26-2.1
1.59
1.2
2 12
1.73
1 26-1.89
Complete citation of the source is listed in Reference 1.
Assumptions made in the computation:
(1) 10% of the total waste is hazardous.
(2) Illinois generates 7% of the nation's waste.
(3) 60% of the hazardous waste is organic in nature.
-------
Table 3. Estimated Quantities of Waste Residuals Generated in the Textile
Industry (dry kkg/yr)2
Type 1974 1977 1983
(1) Total waste 310,000 336,000 534,000
(2) a. Potentially hazardous
dye and chemical
container wastes 10,000 10,400 12,300
b Hazardous constituents 123 130 154
Potentially hazardous wastes and sludges from the textile industry originate from
process operations and from wastewater treatment plants. All segments of the textile
industry except the Griege goods category produce wastewater sludges that contain
toxic organics. Wastewater from the textile industry plants is processed by the
industry itself on-site or is discharged to the sewer for treatment by a municipality.
Wastewater treatment sludges arise primarily from dyeing and finishing mills.
Sludges generated in activated sludge plants and lagoons concentrate heavy metals
and potentially hazardous organics, such as chlorinated organics and non-
biodegradable dyestuffs.
While process wastes can be reduced or eliminated through good housekeeping
and waste segregation, this is not the case for wastewater treatment sludges. It is also
not possible to predict the degree of hazard associated with wastewater treatment
sludges, although reasonably good estimates of process wastes can be made based on
production schedules, quantities of raw materials used, and inventories of materials
expended in the process.
Sludges are for the most part retained or stored in wastewater treatment systems.
Sludge generated in aerobic treatment systems such as lagoons and activated sludge
units is permitted to build up in the system to a maximum level at which time other
holding basins become necessary In some textile plants, the sludge is disposed on-
site without taking advantage of its fertilizer value, whereas in others it is used in
irrigation to grow crops.
An estimate of the number of textile plants and the level of technology they
employ for disposing of waste treatment plant sludges is given in Table 5
While a few textile mills incinerate their wastes, it is not a common practice in the
industry because disposal of ash is a problem and air pollution may result.
In addition to the typical process wastes and wastewater treatment sludges
generated by the textile industry, other atypical wastes are also generated. These are
listed in Table 6 with the expected quantities of wastes generated and the technology
for their disposal. As can be seen, these wastes are primarily solvents. Unlike the
toxicity of wastewater treatment sludges, the hazard associated with these atypical
wastes lies in their flammability These wastes are adequately disposed of by
recovery and incineration. Atypical waste such as lint contaminated with dye is
disposed of by landfill. Indiscriminate use of landfills does pose a hazard, but this
can be alleviated by disposing of the waste in an approved and secured landfill or by
incineration.2
Plastic Materials and Synthetics Industry
This industrial category (Standard Industrial Classification Code 282), includes
plants that produce (I) plastic materials and resins, (2) synthetic rubber, (3)
cellulosic man-made fibers, and (4) organic fibers that are non-cellulosic. These
industries in 1972 produced about $16 billion worth of goods.1
412
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Table 4. Total Quantity of Potentially Hazardous Wastewater Treatment Sludges, dry kkg/yr Generated in the Textile Industry2
1974
Type
Total
Total chlorinated organics
Dye stuff
Total hazardous constituents
Retained*
Sludges
29.9
756.6 x 1CT6
1117
x 1 0~^
1321
x 10-3
Wasted
Sludges
38,400
230 x 10~3
645
900
1977
Retained*
Sludges
29
790 x 10-6
1150 x 10'3
1350x 10"3
Wasted
Sludges
39,100
240 x 10~3
680
940
1983
All Sludges
167x 103
2500 x 10~3
7,700
9,200
*Sludge retained or accumulated in the waste treatment system
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Table 5. Number of Plants Employing Current and Best Technology for the Disposal of Waste Treatment Plant Sludges in the
Textile Industry*2
1.
2.
3.
4
5.
6.
Category
Wool scouring
Wool fabric dyeing and finishing
Woven fabric dyeing and finishing
Knit fabric dyeing and finishing
Carpet dyeing and finishing
Yarn and stock dyeing and
finishing
Retention
of Sludge
8 (50)
39 (35)
208 (32)
125 (17)
43 (30)
65 (19)
Current Technology
Land Disposal
of Wasted Sludge
8 (100)
0 (0)
86 (41)
0 (0)
0 (0)
0 (0)
Line-Pond
Retention
of Sludges
5 (67)
0 (0)
0 (0)
15 (12)
0 (0)
0 (0)
Best Technology
Approved
Landfill of
Wasted Sludge
0 (0)
0 (0)
0 (0)
15 (12)
0 (0)
0 (0)
•Values in parenthesis are percentages and are based on the total number of plants visited.
-------
Table 6. Atypical Waste Residuals Generated in the Textile Industry2
Category
Waste
Waste
Quantity
kg/kkg
of Product
Disposal Technology
1. Wool fabric dyeing
and finishing
2. Woven fabric dyeing
and finishing
3. Woven fabric dyeing
and finishing
4. Knit fabric dyeing and
finishing
5. Knit fabric dyeing and
finishing
6. Carpet dyeing and
finishing
7. Yarn and stock dyeing
and finishing
Still bottoms from
recovery of dry cleaning
chlorinated solvents
Hydrocarbon solvent
and sludges
Finishing sludges
containing adhesives,
silicones, and solvents
Acetone recovery still
bottoms
Perchloroethylene still
bottoms
Lint with wet dye
Solvent resin slurries
50
333
63
10
12
0.09
Present-sealed in drums and sent
to land fills or dumps
Future-reclamation/or incineration
Present-landfillmp
Future-reclaiming/incineration
Present-landfilling
Future-reclaiming/incineration
Present-incineration, environ-
mentally adequate
Present-reclaimed by contractor,
environmentally adequate
Present-landfilling along with trash
Future-includes washing, approved
landfilling or incineration
Present-sealed in drums and stored
on site
Future-reclamation, approved land-
filling or incineration
-------
The two major processes used in this industry are polymerization of simple
molecules to forma polymer, and spinning of the product polymer to form a fiber. In
the production of polymers several chemicals are used. In addition to the monomers,
compounds characterized as modifiers, plasticizers, chemical additives, dyes and
pigments, and processing chemicals are used to obtain polymers of desired
properties.
The following types of wastes are generated from the polymerization process: (1)
off-grade products, resultingfrom upsets in production, mechanical failures, etc., (2)
still bottoms resulting from monomer or solvent recovery and consisting of short
chain polymerized material and other degradable compounds of solvent and
product, (3) spent adsorbents and filters used to remove water for conditioning the
product, (4) spent catalysts which in some instances are discharged into the
wastewater treatment plant and thus become a part of the sludge, (5) waste oils, a
part of which may find their way into the sewer, the remainder being collected in
drums, and (6) wastewater treatment plant sludges.'
The synthetics manufacturing plants usually provide wastewater treatment. The
sludges generated in polymerization process alone are minor in quantity. These
treatment plants may include a pretreatment step involving pH adjustment and
addition of coagulants prior to clarification. Secondary treatment may be provided
in which compounds such as alcohols discharged from the polymerization process
may be oxidized to form additional sludge.
The waste generation factors for the non-olefinic and olefinic polymer industry
were estimated for various products and are given in Table 7.'
In addition to the wastes generated in the manufacturing of the textile products,
others are generated in the spinning operations; for example, in the spinning of
acrylics and rayon, considerable amounts of wastewater sludges are generated in
addition to other wastes such as off-grade products, still bottoms, and oils. The
wastewater sludge associated with the spinning of acrylics and rayon amounted to
20% of the product. In the rayon industry, difficulties are experienced in meeting
prescribed effluent guidelines and as a result several rayon plants have been closed.
In addition, the wastewater sludges have been characterized as hazardous. The
quantity of potentially hazardous waste generated in this industry was estimated at
5660 kkg/yr on a wet basis. However, most of the hazardous constituents in sludges
are metals, primarily zinc. Wastes from phenolic resin production are considered
hazardous because of the presence of high concentrations of phenol. The
concentration of organic material is highly variable, ranging from 5 to 15%, and
includes both phenol and off-grade products. Similarly, wastes from amino-resin
manufacture are considered hazardous because of the presence of formaldehyde and
methanol.
The still bottoms generated in the production of ABS-SAN, polystyrene,
polypropylene, polybutadiene, and neoprine are also considered potentially
hazardous because of the presence of highly flammable solvents and may contain
polycyclic aromatics. However, the aqueous still bottoms which are generally
discharged to the wastewater treatment system do not pose any particular problem.
The total estimated quantity of potentially hazardous waste for the years 1974,
1977, and 1983 for the plastics and synthetics industry was reported to be 740,000;
896,000; and 1,156,000 kkg (wet), respectively.3
The largest contribution to the potentially hazardous wastes that are destined for
land disposal is reported to be from plants producing polystyrene, phenolics and
amino-resins and from the acrylic spinning process. The largest single contribution,
however, is from the phenolics industry with an estimated 366,000 kkg of an aqueous
solution of phenol and formaldehyde in the year 1974.
The disposal of wastes generated in the plastics and synthetics industry is presently
carried by controlled incineration, landfilling, lagooning, and storage.
Two distinct types of hazardous waste streams are generated in the phenolics
segment as indicated earlier. These are the low viscosity pumpable stream of phenol
and/or formaldehyde waste stream and the semi-solid waste stream containing the
416
-------
Table 7. Waste Generation Factors as Percent of Total Production in Various Polymer Production Categories3
Category
Non-olefinic polymers
Acrylic and modacrylics
Nylon 6 (polyamide)
Nylon 6,6 (polyamide)
Polyester resin
Phenolic resin
Amino formaldehyde
Coumarone-indene
Alkyd resin
Silicone products
Olefinic polymers
Styrene butadiene
Rubber polyethylene
Low density
High density
Vinyl chloride
Vinyl acetate
ABS-SAN*
Polypropylene
Off-grade
Products (%)
0.2
0.02
0.3
3.
0.8
1.0
NA
0.1
5.4
2.5
1.0
0.3
2
1
2
0.5
Still Bottoms
and
Waste Oils (%)
0.7
NA
NA
0.01
38
1.2
NA
NA
5.8
1 15
1.0
0.05
NA
0.1
0.5
1.0
Other (%)
—
—
Wastewater sludge and spent
filter cake. 0.3
Terephthalic acid. 0.1
Wastewater sludge. 6.0
Filter cake. 0.24
Spent clay. 3.6
(40% organic residue)
Sweepings 0.01
Spent adsorbent & scrap. 3.3
Wastewater sludge: 1 3
—
Spent adsorbent & catalyst <0.1
Wastewater sludge 1.1
NA
Wastewater sludge 1 .0
Wastewater sludge 0.2
(dry)
Estimated Quantity
of Potentially
Hazardous Waste
1974
kkg/yr (wet)
278,000
368,000
20,735
5,328
32
1 8,440
10,260
2,200
4,940
ABS-SAN: Acrylonitrile butadiene styrene, SAN: Styrene acrylonitrile
-------
oligomers and phenol. The liquid stream is incinerated on-site by supplementing it
with waste solvent stored on-site from other industries. The semi-solid waste stream
is lagooned, but this is not a satisfactory method of disposal. Other wastes that are
incinerated either on-site or off-site by a contractor consist of amino-resins,
polyester catalyst waste stream and still bottoms. The incineration of still bottoms
results in practically no ash. This is environmentally safe and also economical
because of the high energy value of the still bottoms.
Paint Formulating Industry
The major products of the paint formulating industry (SIC 2851) are (1) trade
sales paints, which are primarily exterior and interior paints for buildings and
structures, and (2) industrial sales paints for application to products such as cars,
aircraft, furniture, etc. In addition, varnishes and lacquers are also manufactured by
this industry. A survey showed that there are 1374 plants in the USA manufacturing
paint; the paint sales were estimated at $1871 x 106 in 1974. Another segment of this
industry manufactures allied products such as putty, caulking compounds, sealants,
stripping compounds, and paint thinners.4
The rinsing and washing operations of mixing tanks and filling equipment
contribute the bulk of wastewater from this industry. Other sources such as floors,
spill cleaning operations, laboratory sinks, air pollution equipment using water
rinses, raw material tanks, and boiler and cooling water blow-down also contribute
wastewater. Generally, sanitary waste and non-contact cooling water are segregated
and discharged to the municipal sewers. Practices promoting a reduction in the
amount of water used in the manufacturing process and reuse of the wastewater are
widely used to reduce the amount of wastewater discharged.
According to an EPA report4, the paint industry generates annually an estimated
5.7 million liters (1.5 million gallons) of process wastewater. About half of the
wastewater (2.8 million liters) is discharged whereas the other half is reused in the
plants, evaporated or disposed of in sealed drums.
Of the 22 plants surveyed in the above report, 17 treated their wastewaters by
means of a batch physico-chemical treatment system involving coagulation, mixing,
and settling. Three other plants used a continuous flow physico-chemical treatment
system, whereas the other two used neutralization followed by gravity settling for
treating their wastewaters.
Biological treatment has been reported to be applicable for latex-bearing wastes.
Pretreatment is required to remove the heavy solids prior to subjecting the
wastewater to biological waste treatment. Although activated sludge and trickling
filters can be used for processing latex wastewaters, aerated lagoons predominate in
the industry.
The characteristics of the sludge separated from the wastewater with particular
reference to the organics contained are presented in Table 8.
A study by EPA's solid waste management branch in 1976 estimated that the paint
manufacturing industry generated 436,000 metric tons of waste in 1974 (wet basis).
Of this, 105,000 tons were included in the category of potentially hazardous
materials. This study has also found that most of these wastes are disposed of in off-
site landfills.4
It is also estimated that about 420,000 liters/day of sludge would be produced if
the entire volume of 2.8 million liters of waste that is currently discharged by the
industry is processed by physico-chemical treatment. This quantity of sludge would
amount to 15% of the waste volume. Based on the average concentration of organic
constituents listed in Table 8, the toxic organic load would be about 72 kg/day (159
lb/ day); the major contribution to this load being methylene chloride (63%), toluene
(23%), and ethyl benzene (7%). If m-plant controls for wastewater volume reduction,
such as wastewater recycle and improved tank cleaning and rinsing procedures are
employed, the quantities of sludge produced will be reduced. Thus, it is estimated
418
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Table 8. Concentration of Toxic Organic Pollutants in Sludge from Paint
Formulating Industry4 _
Average Number of
Concentration Samples Having
Organic Compound fJQ/L about 10//g/L
Benzene 414 4/8
Carbon tetrachloride <10 —
Chlorobenzene 176 2/9
1,2-Dichloroethane 17 1/9
1,1,1-Trichloroethane 866 4/9
1,1,2,2 Tetrachloroethane 13 1/9
2,4,6 Trichlorophenol <10 —
Chloroform 920 2/9
2,4 Dimethyl phenol <10 —
Ethyl benzene 14,277 8/9
Methylene chloride 120,201 8/9
Napthalene 366 3/9
2,4 Dinitrophenol 18 1/9
Pentachlorophenol 346 4/9
Phenols 325 3/9
Total phenols 552 30/37
Di (2-ethylhexyl) phthalate 455 6/9
Butyl benzyl phthalate 10,410 4/9
Di-N-butyl phthalate 3,622 4/9
Diethyl phthalate 370 3/9
Dimethyl phthalate <10 —
Anthracene <210 1/9
Pyrene <10 —
Tetrachloroethylene 2142 4/9
Toluene 44,740 8/9
Trichloroethylene <39 2/9
Aldnn <10 —
/3-endosulfan <10 —
(4/8 means four samples out of a total of eight samples analyzed, etc.)
that an 80% reduction in wastewater discharged would result in a sludge quantity of
3% of the wastewater volume discharged. For the entire volume of wastewater
generated in the industry, the total hazardous waste produced was estimated
between 1 50,000 to 300,000 metric tons/ yr. Also, the toxic organic pollutant loading
would amount to 140 kg/day. Although procedures are employed to reduce
wastewater flows, the total organic pollutant load will not significantly affect the
quantity of toxic pollutants to be discharged. Nevertheless, the costs for contract
hauling would be lowered because of the reduction in the volume of the wastes.
An assessment study conducted by WAPORA, Inc., for EPA on the industrial
waste disposal practices in the paint and allied products industry reported that
landfilling of the waste residuals was by far the single largest practice employed in
this industry. The quantities of various hazardous waste residuals generated in this
industry and their mode of disposal are summarized in Table 9.4
Petroleum Refining Industry
A report prepared by Jacobs Engineering Company, Pasadena, California, in
1974 for the EPA, states that there are 247 refineries (SIC 3 1 1 1 ) in the United States.5
419
-------
Table 9. Quantities of Hazardous Waste Residuals and Mode of Disposal in Paints and Coatings Industry (SIC 285)4
Waste Type
Raw material
Packaging wastes6
Wastewater sludge
Spills and spoiled
batches
Waste organic
Cleaning solvent
Dust from air pollution
control equipment
Total Quantity of
Hazardous Waste
Tonnes/Year, 1977
(Wet-basis)
2000
2300
11800
94800
1800
Disposal Methods-
(Number of Plants, 1972
Basis: 1544 Plants)
Incineration Landfill8 Level I
55 1 540 Off-site
landfill
— 1120 Hauled from
off-site land
disposal
— 1540 Off-site
landfill
25 1000 Off-site
landfill in
drums
— 1000 Off-site
landfill, in
drums
Technology
Level II
Same as Level I
Same as Level 1
Same as Level 1
Off-site solvent
reprocessing,
incineration of
still bottoms.
ash disposal in
landfill
Reuse of waste
as a pigment ex-
tender in low-
grade paint
Level III
Segregation of waste in
bags and incineration,
ash disposal in secured
landfill
Sedimentation,
Dewatering, followed
by disposal in a secured
landfill.
Incineration followed
by ash disposal in a
secured landfill.
Same as Level II
(1) Same as Level II
(2) Secured landfill
disposal.
a "Landfill" may include open dumps, sanitary landfills, secured landfills, etc.
b Plant Total for disposal methods adds to more than the total number of plants since some plants use two or more disposal methods
c Level I means prevalent technology, Level II best available technology, Level III technology necessary to meet strict health and environmental
standards.
-------
However, the Development Document for Proposed Effluent Limitation Guidelines
for the Petroleum Refining Point Source Category, identified 285 refineries in the
USA and its territorial possessions.6 The Jacobs Engineering Company report gives
the following distribution for the refineries producing different products: (1)
residual oil, 227; (2) distilled fuel oil, 226; (3) motor gasoline, 212; (4) unfinished oil,
172; (5) kerosene, including range oil, 154; (6) coke, 148; (7) wax products, 60; and
(8) lubricating grease, 18.
Refineries can be classified into 4 categories depending on the refining processes
they incorporate. They are given in Table 10, along with the types of residuals they
generate.
Several factors do influence the composition and quantity of specific residual
waste streams. Among the more important of these factors are the following: (1) the
composition of raw crude used, (2) raw water employed, (3) degree of pretreatment
of water, (4) process used for refining, (5) metallurgy of various unit processes
employed, (6) plant house keeping, (7) air pollution control and water pollution
control technologies employed, and (8) refining plant capacity.
The characteristics of sludge generated in the wastewater treatment processes are
influenced by composition of the wastewater processed, type of biological treatment
and primary treatment employed, and sludge conditioning and concentration
techniques used.
Numerous organics were reported to be discharged in petroleum refinery
wastes.5'6 Several hazardous organics were identified in petroleum refinery wastes
by examination of the "oil" fraction. Polynuclear aromatics, e.g., benz-a-pyrene,
and phenols are integral parts of residuals separated from various refinery waste
streams. The 16 types of residuals generated as indicated in Table 10, have highly
variable solids concentrations within the range of 1-100%. Wastewater treatment
sludges for example were reported to have a solids concentration of 5 to 44%. The
concentrations of oil, phenols, and benz-a-pyrene were also highly variable in these
sludges, averaging 2800 mg/kg, 4.5 mg/kg, and 0.003 mg/kg, respectively, in waste
biological sludge. In addition to these hazardous constituents, several heavy metals
are also found in these sludge streams.
In an inter-laboratory comparison of the concentrations of the above constituents
made by Jacobs Engineering and the refineries, a wide variation in the observed
concentrations was noted. This suggests a need for scrutiny and standardization of
the analytical methodology involved.
The spent lime sludge stream contributes the highest amount of sludge and
amounts to about 50% or higher of the total amount of sludge produced in any of the
types of refineries. In contrast, the wastewater treatment sludges amount to about 2%
of the total residual waste solids produced in a refinery.
The quantities of some of the hazardous substances in wastewater treatment
sludges and in all sludges generated in the petroleum refinery industry are given in
Table 11. For comparison, the expected quantities of potentially hazardous wastes
as a result of water pollution regulations for the years 1974, 1977 and ] 983 are given
in this table. As can be seen from these data, the hazardous constitutent of these
sludges is essentially oil.
A reduction in the total quantity of hazardous constituents is expected in 1983
because of increased oil recovery, reduced use of lead and metals such as chromium
and zinc in cooling towers. Since additional refineries are expected to use secondary
biological waste treatment, the quantity of wastewater treatment sludges is likely to
increase.
Reported studies on the fate of organics, particularly those of hazardous organics
in biological waste treatment systems are relatively sparse. However, when
petroleum refinery wastewaters are discharged into a publicly owned treatment
works (POTW), they do exert an influence on the characteristics of the secondary
effluent and sludge generated. Data reported from one study, in which the fate of
organics discharged into a POTW from 5 refineries was investigated, showed the
421
-------
Table 10. Types of Refineries and the Wastes They Generate6
Refinery
Type
Type of Processes Included
Wastes Generated
III
IV
Crude Vacuum distillation;
LP recovery; Hydrotreating;
Reforming; Alkylation; Iso-
merization; Visbreaking.
All processes included in
Type I in addition to fluid
catcracking and hydroflowing.
All processes included in
Type II in addition to fluid or
delayed coking.
All processes included in
Type III in addition to lube oil
and petrochemical operations.
1) slop oil emulsion solids,
2) once-through cooling sludge,
3) Spent lime from boiler feed
water treatment,
4) exchange bundle cleaning
sludge,
5) API separator sludge,
6) dissolved air flotation float
(DAF),
7) kerosene filter clays,
8) crude tank bottoms,
9) leaded tank bottoms,
10) non-leaded tank bottoms,
11) neutralized HF alkylation
sludges,
12) storm water silt, and
13) waste bio sludge.
All of above and 14) FCC catalyst
fines
All in II above and 15) coke fines.
All in III above and 16) lube oil filter
clays.
Table 11. Production of Potentially Hazardous Constituents in Wastewater
Treatment Sludges and all Waste Residuals from Petroleum
Refining Industry in 1974, 1977, 1983 (tons, dry wt.)5
Parameter
Phenols
Benz-a-pyrene
Oil
Total Weight
% H2O
Total Weight of
Hazardous Components
Wastewater
Treatment
Sludges
1974
0.4
0.0002
228
10,800
87
36.7*
All Sludges
1974
541
0003
109,000
625,000
110,000
1977
6.34
0.113
110,000
715,000
1 1 7,000
1983
7.28
0.153
1 1 2,000
811,000
114,000
*0il is not included
422
-------
profiles of concentrations of various organics in the refinery effluent, POTW
influent, secondary effluent, and sludge.5 These data are presented in Table 12. It can
be seen that many of the compounds listed did survive the treatment process and
appeared in the secondary effluent. Also, the oil and grease fraction was
concentrated in the sludge and was not completely degraded in the digestion process.
Results of profiling of another POTW into which wastewater from only one refinery
was discharged indicated that most of the organics monitored were removed and the
effluent contained at best traces of these compounds. However, oil and grease, ethyl
benzene, toluene, napthalene, butyl benzyl phthalate, phenol, and diethylphthalate
were detected in significant concentrations in primary sludge. Oil and grease and
phenol were also found in significant amounts in secondary sludge.
The prevalent methodology for the disposal of petroleum refining sludges consists
of (1) landfilling, (2) lagooning, (3) incineration, and (4) land spreading. Estimates of
the on-site and off-site disposal of petroleum refinery sludges usingthese procedures
for the years 1973 and 1983 are given in Table 13.
The figures given in Table 13 for 1983 which are projected by Jacobs Engineering,
Pasadena, California, show a very significant decrease in off-site landfilling and
lagooning practices with a simultaneous increase in the on-site disposal practices.
This trend is a reflection of the emerging environmental regulations. Disposal of
petroleum wastes in municipal landfills is being gradually eliminated. The current
trend in the industry is to achieve self sufficiency in their disposal practices.
In land disposal of refinery sludge, the application rate varies from 1" to 2" in
northwestern USA to as much as 3" to 4" in the warmer southwestern USA. The
degradable fraction of the oil is metabolized by the soil microorganisms. Depending
on the application rate, agronomic practices, and environmental conditions, the
degradation may take about 1 to 6 months.5
The Arthur D. Little Company reported results of a land farming study involving
the application of a refinery sludge contained in holding ponds.7 This sludge
contained 13% solids, 25% oil and 62% water, with an extractable organic matter
concentration of 1100 mg/L (composed as follows: non-volatile aliphatic
hydrocarbons, 41%; volatile aliphatic hydrocarbons, 39%; non-volatile aromatic
hydrocarbons, 10%; volatile aromatic hydrocarbons, 3%; and non-volatile polar
organics, 7%). Approximately 68,000 gallons of this sludge was applied on 1.7 acres
and allowed to dry over a period of several weeks. Then it was rototilled to a depth of
5-6". Samples taken at a depth of 0-6" and 6-12" after such a treatment still contained
extractable organics at a concentration of 3,000 mg/kg and 520 mg/kg in
comparison to the control area concentration of 140 mg/kg and 89 mg/kg. Thus,
complete degradation was not achieved. It was also observed that there was a shift
towards polar organics, and a substantial quantity of aliphatic and aromatic
hydrocarbons had not undergone degradation. Non-volatile aliphatics appeared to
have a higher rate of degradation than non-volatile aromatics. Also, the volatile
aliphatic and aromatic hydrocarbons appeared to have been preferentially lost from
the application site. The cost in disposal at this site was estimated to be 37e/gallon of
waste treated which includes both capital and operating expenses.
Lagooning of sludge as a disposal method is more of a convenience rather than
one of an environmentally accepted practice. This practice is being looked down
upon by several states and is permitted only as a temporary measure.
The disposal of sludge containing organic lead poses a hazard particularly because
of health considerations. The disposal of organic lead sludges involves placing the
settled sludge into a diked area and allowing it to weather and degrade.
Alternatively, it is transported to a site in the refinery and rototilled into the soil.
Incineration is used in some refineries to dispose of dissolved air flotation sludge,
slop oil emulsion solids, wastewater treatment sludges, and API separator sludge.
But, due to the increase in energy costs, need to dispose of ash in an environmentally
safe way, and pressure to recover more oil from sludges, incineration will become
more costly and unfeasible in the future.
423
-------
Table 12. Concentration of Organics in Influent, Effluent, and Sludges from a POTW Receiving Refinery Waste water*5
Parameter
Benzene
1,1,1-Trichloroethane
Chloroform
Ethyl Benzene
Toluene
2,4-Dimethylphenol
Phenol
Naphthalene
Anthracene
Phenanthrene
Oil & Grease mg/L
Effluents
From
Refineries
T-1200
ND-15
ND-21
ND- 18,000
ND-48,000
ND-9300
ND- 14,000
ND-425
ND-81
ND-81
3.1-774
Primary
Effluent
27-71
95-252
13-111
41-51
72-197
ND-750
210-840
23-35
ND-T
ND-T
82-113
Final
Effluent
38-62
97-364
12-19
48-53
ND-80
180-740
160-660
ND-T
ND-T
29.3-347
Primary
Sludge
40
ND-50
ND-13
70-150
260
ND
ND-470
70-305
ND-T
ND-T
3,100-6,580
Digested
Sludge
17
ND
ND
55-75
75
ND
1 300- 1 900
125-565
ND-T
ND-T
2,420-2,640
Centrate
ND
ND
ND
15
65
ND
4600-7300
340-480
ND-T
ND-T
1,660-1,680
Filter
Cake
A*9/kg
6
ND
ND
15-25
8
ND
ND
ND-90
ND-T
ND-T
NA
+ 5 Refineries discharged into this POTW in California. All values are pig/L, unless otherwise indicated.
ND Not Detected, T- Trace
-------
Table 13. Percent Estimates of Refinery Sludge Disposal Methodologies
for 1973 and 1983*
1973
Disposal
Procedure
Landfilhng
Lagooning
Incineration
Landspreading
Total
Onsite
(%)
16.8
18.3
0.8
8.4
44.3
Offsite
(%)
34.3
21.4
0
0
557
1983
Onsite
(%)
24
12
3
34
73
Offsite
(%)
20
7
0
0
27
Concentration techniques usually used for increasing the solids concentration of
municipal sludges may also be used for some petroleum refinery sludges. For
example, sludge streams such as crude tank bottoms, and API separator bottom
sludges can be concentrated by dewatering with polyelectrolytes and by alternate
freeze-thaw cycles where climate permits. The concentrated sludges may then be
disposed of on land.
Only in special cases are both liquid and solid wastes generated in petroleum
refining industry processed by chemical fixation and disposed on land. Chemically
inert precipitates are produced, for example, by the addition of chemical coagulants.
In one patented process, solidification of about 20 million gallons of primarily
lagooned API separator bottoms and crude tank bottoms was achieved by addition
of substances such as cement. Such chemically fixed sludges had not produced a
significant leachate problem.8 The process has been reported to stabilize materials
with as much as 38% solids; leaching of various ions is controlled by the amounts of
silicates added. However, methods such as solidification and encapsulation of
wastes in plastic or concrete are not widely used in the petroleum refining industry.
Composting of waste activated sludge and petroleum refinery sludges has been
reported. However, it is not practiced on a large scale and its full scale application in
this industry has yet to be proven.9
The prevalent technology for the disposal of sludges in this industry is mostly by
landfilling. Leaded gasoline sludges are pumped into lagoons and allowed to dry.
When drying is complete, the lagoon is covered. Landfilling of sludges as practiced
currently is inadequate for the disposal of sludges from an environmental stand-
point. However, the disposal of waste biological sludge is done in secured landfills
and hence is considered to be adequate. Specification of an accepted level of
technology for safeguarding health and environmental protection calls for
landfilling in a secured area which is hydrogeologically safe.
Organic Chemicals Industry
About 87 million metric tons of products are estimated to be produced annually
by industries manufacturing gum and wood chemicals, cyclic crudes and
intermediates, industrial organic chemicals, pesticides and pesticide formulations,
and explosives.10 Waste residuals from these industries include solvents, acids and
bases, sludges (heavy metal and paint), still and tank bottoms, oils, organic and
inorganic toxic compounds, and waste treatment sludges.
The quantities of wastes produced from selected organic chemical and pesticide
manufacturing plants along with the hazardous organic constituents contained in
their wastes are presented in Table 14. The different levels of technology reflecting
425
-------
Table 14. Quantities of Hazardous Components and Methods of Disposal of Residuals from Organic Chemicals Industry10
Product
Perchloroethylene
Nitrobenzene
Chloromethane
Solvents
Selected
Waste Stream
Liquid heavy ends
from purification
column
Liquid still
heavy ends
Solid tails
from solvent
recovery system
Hazardous and
Other Components
Hexachlorobutadiene
Hexachlorobenzene
Chioroethane
Chlorobutadiene
Tars and Residues
Other compounds
(Unknown)
Nitrobenzene
Heavyends
Hexachlorobenzene
Crude Hexachloro-
butadiene and other
chlorinated paraffins
Quantity of
Hazardous
Components,
metric tons/yr
8050
700
350
350
700
350
50
300
small
Total Waste
Quantity
metric tons/yr
10,500
50
300
Level I
Deep well
injection
On-site
Landfill
Contractor
Landfill
Technology
Level II Level III
Controlled Same as
incineration Level II
,,
,-
Epichlorohydrin Still bottoms 1,2.3-Tnchloropropane 2783
Tetrachloropropyl- 557
Ethers
Dichloropropanol 425
Epichlorohydrin 80
Chlorinated aliphatics 1 30
and alcohols
Water and unidentified 100
products
Toluene Centrifuge resi- Lower polyurethane 523
Dusocyanate due sludge polymers and tars
Waste isocyanates 16
FeCI3 37
Unknown 16
Vinyl Chloride Still bottoms Halogenated aliphatic 3780
3,975
On-site
Storage
592
3,800
On-site
Landfill
Contractor
Controlled
incineration
Same as
Level II
-------
Mononer
Acrylonitnle
Maleic Anhydride
Lead Alkyls
Pesticides
Aldnn
Atrazine
Parathion
Malathion
Still bottoms
Sludge & Residue
from distillation
column
Lead Sludge from
settling basins
Area & Equipment
Washdown
Scrubber spent
alkali solution
Sulfur sludge
from chlormator
Filter cake solid
(semi-solid)
Hydrocarbons
Tars
Inorganics
Higher nitnles &
Polymers
Maleic anhydride,
Tars, Fumaric acid.
and chromogenic
compounds
Organic lead compounds,
& other organics
Water & other
inorganic salts
Aldnn
Water & Dissolved Salts
Cyanunc acid
insoluble
Residue
Water
NaCI & NaOH
Sulfur Si Organo
phosphorus compound
Dimethyl Dithio-
phosphoric acid
Toluene and in-
soluble reactor
reactor products
Filter lands
5
15
160
333
6000
24,000
29
288,971
120
3,240
202,140
19,100
2,300
70
756
1,000
incineration
160 Uncontrolled
incineration
333 Landfill
30,000 Incineration
289,000 Lined
Pond
224,600 Deepwell
Disposal
2,300 Uncontrolled
incineration
1,826 NaOH
Treatment
followed by
approved site
Secured
Landfill
Controlled
incineration
Same as
Level I
Same as
Level I
Secured
Landfill
Same as
Level I
Secured
Landfill
Same as
Level II
Same as
Level I
Chemical
Detoxifi-
cation
by ozona-
tion and
deepwell
disposal
Same as
Level II
Same as
Level I
a Level I means prevalent technology. Level
standards.
best available technology. Level III technology necessary to meet strict health and environmental
-------
the current state-of-art and treatment that is acceptable and required for the disposal
of residuals to maintain an adequate health standard (Levels II and 111) are also
given in this table.
Data reported for selected organic manufacturing plants indicated that only
24,325 metric tons of sludge originated from biological treatment processes and the
remainder of about 420,000 metric tons of waste residual solids originated from
other unit processes. Thus, it appears that only a small portion of the total sludge
quantity in this industrial category is produced in wastewater treatment processes.
As it can be seen from Table 14, many of the waste residuals are recalcitrant and
are contained in non-aqueous streams. Thus, their treatment and disposal are quite
different from treatment and disposal of residuals separated from industrial
wastewaters which are biodegradable. The major disposal technique for the disposal
of many of these wastes is incineration (about 70% on a dry solids basis). However,
disposal in approved landfill sites is also an accepted method of disposal for some
wastes (about 15% on a dry solids basis). For the organic chemicals category, it
should be noted that on a wet weight basis, disposal by deep well represents the most
commonly employed method (about 64%) as presented in Table 15.
Explosives Industries
The explosives industry manufactures products such as (1) initiating agents and
primers, (2) propellents of different kinds, (3) high explosives, (4) pyrotechnical
incendiaries, (5) not control agents, and (6) chemical warfare agents. The wastes
containing hazardous organic compounds of importance include waste explosives,
inerts contaminated with explosives, spent activated carbon resulting from
processing aqueous wastes, red water from TNT production, organic solvents from
propellent manufacture, and wastewaters containing RDX and HMX (high
explosives). A characterization of these wastes and their hazardous constituents
contained in each of these types of wastes is given in Table 16.
Waterborne wastes are also discharged from the explosives industry. These wastes
may be processed by biological waste treatment and the sludge generated can be
lagooned and disposed of on land.
The ultimate disposal of explosive industry waste residuals is carried out by (1)
open burning on-site by a contractor, (2) detoxification followed by landf\lling, (3)
deep well disposal, and (4) storage in lagoons. Spray irrigation of ammunition
wastes has been practiced in South Africa.11 Open burning of explosive wastes is
considered to be environmentally objectionable because of resulting air pollution
problems it causes. Controlled incineration which has been developed recently
appears to be promising. In a study in which six different incineration designs were
evaluated the fluidized bed type of incinerator was judged to be the most promising
system from the standpoint of low emission rates and operational cost.12
Food Industry
In addition to the organic residuals generated by the major industries listed above,
significant quantities of organic sludges are produced by such Indus ries as food
processing and allied industries which generate sludges that can be considered as
non-hazardous.
The amount of sludge to be disposed of ultimately depends on factors such as the
character of wastewater, type of wastewater treatment process used, and sludge
treatment applied.
Information on the total quantities of sludge produced from industrial wastes
considered to be non-hazardous is meager, although estimates of waste generation
factors for industries producing food, meat products, canned foods, and beverages
have been reported." These are usually expressed as the pounds of waste
produced/1,000 pounds of product divided by the value added/1,000 pounds of
428
-------
Table 15. Hazardous Waste Disposal Practices in the Organic Chemicals Industry"
Industrial
Category
Organic
Chemicals
(SCI 2861, 2865,
2869 except 28694
le , pesticides )
Pesticides
SIC 28694
2879
Explosives
SIC 2892
Private
Industry
Government owned
and contractor
operated segment
Type of
Waste Residuals
All hazardous waste
residuals from these
categories
Waste Pesticides,
Pesticide contami-
nated items, waste-
water, solvents, floor
sweeping, etc
Blasting agents &
Fixed high explosive
waste
Explosive wastes &
explosive con-
taminated inert
wastes
Other hazardous
wastes include spent
a 4 carbon from
aqueous waste
processing, red water
from TNT, puri-
fication, organic sol-
vents from propellent
manufacture, waste-
waters containing
dissolved and
suspended RDX/HMR
Quanity/yr Disposal Methods
-10 3 million On-site landfilhng
tons/yr Off-site landfilhng
wet basis, 1977 Controlled on-stte
incineration
Uncontrolled on-site
incineration
On-site deep well
Biological treatment
lagoon
Recovery
-625,000 On-site landfilhng
wet basis, Off-site incineration in
1 977 privately owned incinerators
Storage in drums or open
piles
Recovery
Undetermined landfill sites
Unknown
-5500 wet tons Open-burning on-site
Detoxification, followed by
landfilhng, deep well
disposal, spray irrigation
and lag oon ing
19,600 dry tons Open burning
240 dry tons Landfill
Openburning
Sold
Very little disposal in
municipal plants
Wet Wt
Basis (%)
147
t 1
6 8
154
635
55
26
28
16
13
8
12
23
Dry Wt
Basis {%)
148
21 7
480
20
02
83
832
6
943
583
37 5
4
-------
Table 16. Characterization of Explosive Propellent and Warfare Material
Wastes
Class Hazardous Constituents in Wastes
Initiating Agents and Mercury fulminate. Lead azide, Lead
Primers styphnate, tetrazene, BPEHN
Propellents, Nitrocellulose Smokeless gunpowder, nitrocellulose,
based gelatinized nitrocellulose, rocket
propellent (double base), ballistite,
pyrocellulose, composition D-Z,
nitroglycerin.
Propellents, Rocket propellent TP-H-1011,
composite/other Rocket propellent TP-H-1016,
Rocket propellent ANB 3066
Ammonium perchlorate
High Explosives Ammonium picrate, glycoldinitrate, picric
acid, PETN, TNT, dynamite, compositions,
A, B, B2, C, RDX, HMX, Plastic
explosives.
Riot Control Agents Chlorcielophenone tear gas (N)
Tear Gas irritant (CS), BBC(Bromebenzyl
cyanide) CN-DM (Burning mixture of CN
DM) CNS (Chloracetophenone &
chlorpicrin), DA (diphonyl chloroarsine),
DM (Adamsite, diphenslammo chloroar-
sine)
Chemical Warfare Agents GB, VX Lewisite, Sulfur mustard
(H, HD) Nitrogen mustards
Pyrotechnics and Napalm, Thickened gasoline w/napalm,
Incendiaries PTI (incendiary mixture), SGF2 smoke
mixture
product. For food manufacturing (SIC Code 20), meat products (SIC 201), canned
foods (SIC 203), and beverages (SIC 208), the waste factors were 1.42,0.89,4.2, and
0.2 pounds per dollar, respectively. Since these factors are based on the value of the
1972 dollar, appropriate adjustments have to be made to compensate for the effect of
inflation. It is not clear, however, what fraction of the above waste generation
factors represents wastewater treatment sludge.
Primary treatment of meat packing wastes was reported to yield about 8 tons of
sludge per million gallons of waste processed at one plant.14 This large sludge yield
was due primarily to the high solids content of the raw waste. Secondary treatment
of the waste by trickling filter produced about 1.6 tons of sludge per million gallons.
Generally for biodegradable industrial wastes, the quantity of sludge generated in
secondary treatment depends on the degree of stabilization or oxidation achieved.
For example, dairy wastes when completely oxidized are known to produce little or
no sludge requiring disposal.
The nature and mode of disposal of sludges containing non-hazardous organics,
derived from meat packing plants, canning plants and grain mills are presented in
Table 23. Treatment and disposal similar to those cited in this table can also be used
for the sludges from dairy, fermentation, sugar, and starch industries.
430
-------
Waste Residuals from Other Industrial Categories
In addition to those already discussed the following industrial waste categories are
known to discharge hazardous organic compounds: (1) industrial inorganic
chemicals, (2) metals mining, (3) Pharmaceuticals, (4) petroleum rerefining, (5)
leather tanning, (6) metal smelting and refining, (7) electroplating and metal
finishing, (8) special machinery manufacturing, (9) electronic components
manufacturing, and (10) storage and primary batteries.1S Table 17 summarizes the
quantities of hazardous waste generated in some of these categories which generate
residuals that are primarily organic in nature and the disposal technology in use.
Industries such as battery manufacture, leather tanning and finishing, meal smelting
and refining, etc., produce potentially hazardous wastes. The degree of their hazard
is primarily due to the heavy metals contained in them. Leather tanning and
electronic component industries generally depend on treatment technology
conventionally used for municipal wastewater to process their wastewaters. The
primary and secondary sludges produced in these industries are considered
potentially hazardous because of their heavy metal and fluoride content.
As the level of treatment is raised and as pretreatment of wastewater is provided in
some of these industrial categories, the production of biological sludge will increase.
The ultimate waste disposal technology for most of these wastes is land disposal
either on-site or off-site. Incineration is also used for the disposal of some industrial
waste residual streams as is indicated in Table 17.
WASTES FROM SERVICE INDUSTRIES
The nature of waste residuals generated in a service industry depends of course on
the type of service provided. The agricultural service industry for example provides
services involving use of pesticides. The hazardous residuals are associated with
unused waste pesticides, pesticide containers, and dilute pesticide solutions
generated from rinsing operations.16
Pesticide Application Industry
In a study dealing with the disposal of waste pesticides, it was reported that on an
average, 93 ml of residue was left unused per container with a range of 45-221
ml/can.16
The common methods for disposal of dilute pesticide solutions resulting from
rinsing cans and applicator tanks include discharge to ground surface with
subsequent uncontrolled runoff, collection and application to small land areas near
the mixing site, contractor removal of residues settled in a pond, discharge to a pond
with subsequent seepage into the ground, and crop application.
With the promulgation of RCRA regulations, disposal practices will be upgraded.
Triple-rinsed containers will have to be disposed of in a licensed landfill, an
approved practice. Controlled incineration will be needed for unwanted pesticides.
Dilute solutions of pesticides may be applied to crop land, but this may not always be
feasible. An alternative technology employing a system of mounds involves
collecting the pesticide waste solution in a lined earth impoundment and permitting
it to evaporate and degrade. The concentrated pesticide with its associated soil can
then be disposed of.
The cost of disposing unrinsed containers in a secured landfill was estimated to be
$55/ metric ton according to an EPA funded study. Incineration costs of unwanted
pesticides was estimated to be $80/ metric ton. The operating costs of a high quality
soil mound system for the disposal of 450 gal/ day of dilute pesticide rinse water at an
average of 225 days/yr was estimated to be 0.095c/gal. The approximate total
capital cost for this soil system was estimated at $12,850.n
431
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Table 17. Type and Quantity of Hazardous Waste Residuals and Waste Disposal Practices in Industrial Inorganic Chemical,
Pharmaceutical, and Petroleum Rerefining Industries
Industrial Category
Industrial Inorganic
Chemical, SIC 281
Pharmaceuticals
Type of
Hazardous
Waste Residuals
Chlorinated hydrocarbons
Mixed solvents
Non-Halogenated solvents
Halogenated solvents
Organic chemical residue
Quantity
yr 1977
1 ,200 (wet
tons)
1 5,400 (")
26,900 (")
3,900 (")
1 5,000 (")
Hazardous Waste
Quantity Produced
as Percent of Total
-006
-21
~37
5
-21
Disposal
Methods
Secured landfill incineration
Incineration
Incineration
Incineration
Incineration 79.5%
Aqueous mixed solvents
2,800 (")
-4
Landfill 12.5%
Sent to biological wastewater treatment
plant, -8%
Biological wastewater treatment
-4.6%, incineration 95.4%
Aqueous alcohol
Antiviral vaccines
toxoids serum
Returned good and conta-
minated or decomposed
active ingredients
Petroleum Rerefining Oil in acid sludge
Oil in caustic and
other sludges
Oil in spent clay
700 (")
580 (")
600 (")
13,000 (dry tons)
5,600 (")
4,000 (")
<1
<1
-1
17.5
7.5
5.4
Incineration
Crushed, diluted and
wastewater
Treatment -20%
Landfill -70%
Incineration -10%
Landfill
Recycled or reused
Landfill
sent to biological
-------
Drum Reconditioning Industry
In the drum reconditioning industry, firms accept empty drums from various
sources for restoration. This industry may also arrange for disposal of any residual
oils and chemicals contained in the drums.
The waste streams from drum cleaning operations include: (1) pre-dump wastes, if
any, (2) caustic sludge, and (3) incinerator sludge/ash. These wastes vary widely in
their characteristics not only from plant to plant, but also from hour to hour. The
caustic bath may contain in addition to several inorganic compounds, organic
compounds such as toluene, xylene and naptha which are used as paint solvents.
Oil drums can be reconditioned with little difficulty. Waste oil is readily
incinerated and pre-dump wastes are sold for reclaiming operations. Some plants
discharge their wastes to sewers. A status of the current waste disposal practices and
those specified by RCRA has been reported in a summary form16, and is presented in
Table 18. The cost for off-site disposal of wastes originating from drum
reconditioning plants has been reported to be $55/metric ton.16
Gasoline Service Stations and Related Industry
It has been estimated that about 171,000 gasoline service stations, auto repair
shops and other related industries were in operation in 1978.
The main waste streams from these commercial firms are waste oil from crank
cases and hydraulic oil from automatic transmissions. In addition to containing high
concentrations of lead, zinc, and barium, the waste oil is also reported to contain
organic compounds such as PCBs, 2, 3, 7, 8-tetrachloro dibenzodiorin, 2, 4, 5-
tnchlorophenol, and polycychc aromatic hydrocarbons.
The estimated quantity of waste crank case oil and hydraulic oil generated by the
above categories of industries in 1978 is summarized in Table 19.
Waste oil from this industry is either stored on-site in underground tanks
(capacity 500 gallons) or stored above ground in 55 gallon drums. It is generally
collected by a waste oil collector who then disposes of it by using it for any of the
following purposes: (1) fuel, (2) road oil (including dust control), (3) re-refining, (4)
asphalt extender, and other construction uses, (5) weed control, and (6) input to
miscellaneous manufacturing processes. It was reported that about 470 million liters
of waste oil was re-refined in 1977.16
RESIDUAL MANAGEMENT TECHNOLOGY
The treatment of industrial waste residuals, such as the biological waste treatment
sludges and non-aqueous sludge streams may be accomplished by various unit
processes. They range from those applied usually in municipal sludge treatment
(e.g., chemical conditioning, thickening, dewatering, sludge stabilization by aerobic
and anaerobic digestion, etc.) to chemical fixation, detoxification, encapsulation,
and incineration, which are applied selectively to some categories of industrial
sludges. Process sludge streams if not hazardous may be transported by pipe, road or
by barge for land reclamation. Also, they may be detoxified, chemically fixed, or
incinerated and the residue may be disposed of on land in a safe manner. Some of the
important unit operations are briefly described below.
Sludge Conditioning
The conditioning of biological waste treatment sludges and some industrial
sludges such as petroleum refinery sludges is usually accomplished by addition of
chemicals such as ferric chloride, lime, and a combination of ferric chloride and lime.
Organic polymers have also been used with success. The dosing rate for these
433
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Table 18. Summary of Wastes and Disposal Practices in Drum
Reconditioning16
Waste Stream
Pre-dump
Caustic Sludge
Incinerator
ash/sludge
Current Disposal
Landfilled or incin-
erated
To municipal sewer,
municipal or private
landfill
Municipal or private
landfill
RCRA Specified Disposal
Secure landfill or
controlled incineration
Secure landfill
Secure landfill
Table 19. Annual Waste Oil Quantities Produced in the Automotive Service
Industry
Type Total Waste Oil (Million Liters)
Gasoline Service Stations 695 9
Automobile Repair Shops 206.1
New Car and Truck Dealers 272.2
Automobile Fleets 88.7
Total 1,262.9
conditioning agents varies quite widely depending on the nature of the sludge, the
dewatering process and the chemical used. Since several conditioning agents are
available on the market, it is advantageous to test and screen a number of them in the
laboratory in choosing one for a given sludge stream. Criteria for the selection of a
conditioning agent are usually based on (I) solids captured upon dewatering a sludge
after its conditioning, (2) clarity and amount of supernatant, (3) floe strength of the
dewatered sludges, and (4) the dosage rate and the cost of the conditioning agent.
There are several unusual methods described in the literature for conditioning
residuals separated from wastewaters. These include (1) alternate freezing and
thawing, (2) heat treatment, (3) solvent extraction, (4) electrical treatment, (5)
ultrasonic treatment, and (6) bacterial treatment. Application of these techniques is
not extensive, however.
Phase separation by the use of solvent extraction has been reported for petroleum
refining sludges.18 Sludges derived from industries such as pulp and paper
manufacture, steel production, wool scouring, slaughter house operations, (paunch
manure and blood), wastes from shrimp packing, and some mixed industrial sludges
are also amenable to such a treatment.
Sludge Concentration
Sludges are concentrated prior to their stabilization. One such process is
thickening in which liquid is removed to increase the solids concentration of the
sludge to about 10%. The degree of thickening depends on the amount of water
removed. Thickening accomplishes the following: (1) reduce the sludge volume to be
handled, (2) reduce the cost of chemical conditioning, a process which is generally
practiced prior to sludge dewatering, (3) improve digester operation and reduce the
434
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cost of digester operation because of less space required due to increased solids
loading per unit digester capacity and less supernatant production, (4) reduce the
amount of water to be handled prior to digestion and dewatering operations and thus
produce a lower volume of sludge to be handled, and (5) equalize to a significant
extent the quality and quantity of sludge.
Thickening is accomplished by (I) gravity, (2) dissolved air flotation, and (3)
chemical conditioning. These unit processes are used in several industries as in the
case of a municipal treatment plant to increase the solids concentration of dilute
sludge suspensions.
Another process is dewatering in which water is removed from a sludge
suspension so as to concentrate it to a suspended solids concentration of greater than
10%, preferably to over 15%. Dewatering usually connects sludge into a non-fluid
form and renders it amenable to ultimate disposal by landfilling, heat drying
followed by utilization and/or disposal, lagooning, composting followed by land
application, and incineration.
There are several ways to accomplish dewatering. These include unit processes
such as: (1) vacuum filtration, (2) centrifugation, (3) filter pressing, (4) screening,
and (5) sand bed dewatering. In filtration processes such as vacuum filtration,
screening and sand bed dewatering, a dilute suspension of solids moves through a
porous medium such that suspended particles are retained by the porous medium
and a relatively clear liquid is produced. The centrifugation process involves the
separation of solids from the liquid by means of sedimentation under a centrifugal
force. This process is generally competitive technically and economically with other
dewatering devices such as vacuum filters and filter presses, and is more capable of
dewatering sticky and gelatinous sludges.
In comparison to other processes, filter presses yield a sludge cake with the highest
percentage of solids. These units are being used increasingly for dewatering
wastewater treatment sludges.
Table 20 summarizes common thickening and dewatering equipment with typical
operating conditions and applications to the processing of sludges.20
Industrial sludges which are biodegradable may be processed initially by chemical
conditioning and thickening, and then stabilized by processes such as aerobic
digestion, anaerobic digestion, composting, etc. The aerobically or anaerobically
digested sludges still have a very high water content and need to be dewatered for
ultimate disposal. Techniques such as sand bed dewatering, centrifugation, vacuum
filtration, or storage in lagoons may be employed to obtain a product with a high
solids content. The centrifuged and vacuum filtered sludges may be heat-dried, or
composted, to obtain a product with a relatively low moisture content which can be
disposed or utilized on land, or, it may be incinerated. The ash produced in this
operation is disposed of on land in a safe manner.
Incineration
Incineration may be applied for the treatment and disposal of sludges and involves
drying and combustion of waste residuals. It has been applied to many types of
sludges from industries such as organic chemicals, pesticides, explosives, paint,
rubber, plastics, textiles, petroleum refineries, etc.
The factors that govern the process include the fuel value of the residuals, rate of
air supply, time for combustion, temperature of the incinerator, and turbulence.
Any water contained in the sludges needs to be evaporated before combustion of the
waste residuals can be initiated. Heat is generated in the incineration process. The
difference between the heat generated and the heat lost via radiation, stack gases,
and ash, is conserved and used for heating the sludges and the process air. Additional
heat required to burn the sludge is supplied by supplemental fuel. The end products
of incineration are usually CCh, water vapor, SCh, and inert ash. Traces of other
stack emissions such as halogens and oxides of nitrogen are also usually detected.
435
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Table 20. Sludges Thickening and Dewatering Equipment20
Equipment
Type
Stirred Gravity
Air Flotation
Centrifuges
Disc
Scroll
Basket
Filters
Vacuum Drum
Vacuum Belt
Pressure
Belt
Dual Cell Gravity
Feed Rate
(gpm)
10-3,000
10-10,000
25-300
5-300
10-60
0-250
1-50
10-250
5-200
10-500
Solids Solids
Concen- Capture
tration % (% solids)
— 95-98
— 92-98
02-1 90-95
05-15 60-95
1-10 90-95
8-10 50-99
5-10 75-90
2-5 95-99
4-6 95-99
0 5-5 90-98
Output
Cake
Cone.
Range
(% solids)
5-10
5-10
5-10
20-45
5-25
20-40
15-25
20-50
20-35
10-20
Applications
Primary
Primary and secondary,
metallic, chemical
Waste-activated, lime,
flocculated alum
Waste-activated
Raw or digested
primary or secondary,
alum, lime
Waste-activated, lime,
alum
Digested primary or
secondary
Industrial
Alum, waste-activated,
hydroxide
Secondary biological
Primary and secondary
Secondary
Industrial
Primary, secondary,
industrial trickling
filter
Industrial
Hydroxide
Chemically precipitated
Clarifier overflows
Municipal, industrial
chemical
Petrochemical,
industrial
Metal hydroxide
-------
Incineration of waste residuals may be conducted on a batch or a continuous basis
depending on the amount of material to be incinerated.21 Various types of
incinerators are available for the disposal of industrial residuals containing
hazardous organics including the popular multiple hearth and the promising
fluidized bed incinerator.22'23 The advantages and disadvantages of a number of
incinerator types are presented in Table 21.
A detailed study of the applicability of thermal destruction processes such as
incineration, pyrolysis, and wet air oxidation, was performed jointly by the TRW
Defense and Space Systems Group and by the staff of the Arthur D. Little Company
for the USEPA. This study focused on the treatment of various organic industrial
wastes, including an API separator sludge, a phenolic sludge, and a polyvinyl
chloride waste sludge.21 These wastes were treated by pyrolysis, fluidized bed
incineration, and rotary-kiln incineration, respectively. The characteristics of these
wastes, operating conditions of the unit processes, and results of performance are
given in Table 22.
The API waste sludge was processed through a pyrolysis-incineration system
consisting of a rotary-hearth pyrolyzer coupled to a rich fume incinerator for the
combustion of the pyrolyzer effluent gases. The results of this study indicated that a
70% conversion efficiency of the waste to a more useful form was realized for
utilization in the incineration system. In light of the test results, the TRW and A.D.
Little Company study recommended that pyrolysis should only be considered in
conjunction with a heat recovery system particularly with wastes having a high BTU
value. The estimated capital and operating costs for processing 22,800 metric tons per
year was estimated respectively at $6.07 million or $ 140 / metric ton (mt) of which the
auxiliary fuel costs were $37.50/mt.
The incineration of phenolic waste sludges produced from the scrubbing
operation of gasoline with alkali to remove H2Sand phenol was found to be feasible.
The results of this investigation, indicated that a high efficiency of waste destruction
(>99.9%) was achieved (Table 22). Constituents originally present in the wastes
could not be detected in the combustion gases, scrubber water, or sand used in the
incinerator.
A rotary kiln incinerator was used to process a PVC waste sludge which had high
solids content. This sludge is potentially hazardous because of the vinyl chloride
monomer contained in it. The operating conditions of the tests are given in Table 22.
The results of these tests indicated that very high efficiencies were achieved in
destruction of waste and organics. No vinyl chloride monomer was found in the
combustion gases. The capital and operating costs for processing 6,700 metric tons of
PVC waste/year were estimated at $7.8 million or $582/mt.
The successful development and implementation of a fluidized bed incineration
system by the Battelle Laboratories for the disposal of wastes resulting from wood
pulp production led them to study its applicability for the disposal of waste residuals
from paint, plastics, rubber and textile industries.22
Based on a detailed investigation of the conditions of operation of the pilot scale
incinerator, including analysis of data on influent wastes, effluent gases and
scrubber water, the study made the following conclusions:
(1) Fluidized bed incineration was technically feasible for all the wastes studied.
In incinerating solvent recovery sludges from the paint industry, supplemental fuel
was not necessary because of their high heat content. No noxious materials were
found in the exhaust gases, gas scrubber effluent, and residue. An operating cost of
l.lc/lb waste (1972 basis) was estimated for an incinerator serving a typical plant.
(2) Incineration of PVC and styrene wastes resulted in no noxious or toxic
materials, with the exception of HC1 in the gas scrubber effluent. Although these
wastes are currently disposed of in landfills, incineration may provide an economic
alternative.
(3) No noxious or toxic compounds were generated in the incineration of the
rubber waste. The operating cost was estimated at 0.4c/ Ib in an installation sized for
a typical plant.
437
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Table 21. Advantages and Disadvantages of Various Types of Incinerators1
Type
Uses and Advantages
Disadvantages and Limitations
Solid Stationary
Hearth
1. Low capital
2. Potential of tight air
control with an airlock
feeder.
3. Used for incineration of
solids.
4. Can be designed to
include liquid incinera-
tion.
1. No turbulence, mixing or
aeration.
2. Slow burning rates.
3. Batch operation.
4. Manual ash removal.
5. Does not lend itself to good air
pollution control
Solid Hearth
(Rotary Hearth
or Rabble Arms)
1. Continuous ash
discharge.
2. Capable of incinerating
waste solids
independently or liquids
and solids in combina-
tion.
3. Widest practical turn
down ratio. (Maximum
to minimum operating
range).
4 Incinerating materials
will not fall through
hearth.
5. Adaptable for use with a
gas scrubbing system.
1 Rabble arms or plows are
susceptible to damage.
2. Limited turbulence and air
contact.
3. Partly combusted materials
may flow out ash discharge.
4. Solid wastes fed at intervals.
An air lock system should be
used to improve combustion
characteristics and control.
5. Arched, self-supported
multiple hearths made of
refractory material are
vulnerable to abrupt
temperature variations with
resultant downtime and cost
increase.
Rotary Kiln 1. Will incinerate a wide
variety of liquid and solid
wastes.
2. Capable of receiving
liquids and solids
independently or in
combination.
3. Not hampered by ma-
terials passing through a
melt phase.
4. Feed capability for
drums and bulk con-
tainers.
5. Wide flexibility in feed
mechanism design.
6. Provides high turbulence
and air exposure of solid
wastes
7. Long inventory time for
slow burning refuse.
8. Continuous ash dis-
charge.
9. No moving parts within
the kiln.
10. Adaptable for use with a
wet gas scrubbing
system.
1. High capital cost installation
for low feed rates
2. Cannot utilize suspended
brick in kiln.
3. Operating care necessary to
prevent refractory damage.
4. Airborne particles may be
carried out of kiln before
complete combustion
5. Spherical or cylindrical items
may roll through kiln before
complete combustion.
6. Kiln incinerators frequently
require excess air intake to
operate due to air leakage into
the kiln via the kiln end
seals and feed chute, which
lowers fuel efficiency.
7. Drying or ignition grates,
if used prior to rotary kiln, can
cause problems with plastics
melt plugging grates and
grate mechanisms.
438
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Fluid Bed
1. Capable of incinerating a
moderate range of liquid
and solid wastes.
2. Rapid heat transfer from
gas to solid.
3. High combustion rate.
High turbulence and air
exposure.
4 Low excess air re-
quirements.
5. Large heat sink to
smooth out fluctuations
in feed rate or fuel value.
1. Requires fluid bed preparation
and maintenance.
2 Feed selection must avoid bed
damage.
3. May require special operating
procedures to avoid bed
damage.
4. Incineration temperatures
limited to a maximum of
about 815°C(1500°F).
Stationary Liquid
Waste Burner
1 Capable of incinerating a
wide range of liquid
wastes
2. May use suspended
brick
3 No continuous ash
removal system required
other than air pollution
controls
Must be able to atomize tars
or liquids through a burner
nozzle except for certain
limited applications.
Heat content of liquids must
maintain adequate
temperatures or a sup-
plemental fuel must be
provided.
Must provide for complete
combustion and prevent
flame impingement on
refractory.
(4) Incineration did not appear promising for viscose rayon waste because of the
presence of low melting point inorganic waste materials. Other wastes generated in
textile mills are too dilute and may not be amenable to incineration.
Wastes from the manufacture of explosives are also amenable to incineration.
This was the conclusion reached by the Manufacturing Technology Directorate
Picatinny Arsenal, Dover, New Jersey who evaluated six incinerator designs and
concluded that the fluidized bed incinerator was superior to others.12
Evaporation
Evaporation may be used for concentrating waste residuals. This process may be
particularly attractive in areas where adequate solar radiation is present and rainfall
is low.
Evaporation ponds are appropriate where land is inexpensive and climatological
conditions permit rapid rates of evaporation. The size of the evaporation basins is
dependent on the amount of waste to be processed and the rate of evaporation.
Concrete tanks, large earthen tanks constructed in reasonably impervious areas, or
ponds constructed with impervious lining materials to prevent seepage are generally
used as evaporation basins. Where large ponds are used appropriate measures
should be taken to divert runoff, protect wildlife, and prevent odor problems.
In order to enhance the rate of evaporation, surface aerators may be provided. In
addition to evaporation, other solids concentration processes may occur in ponds
including sedimentation of particulates and precipitation. Chemical and
biochemical degradation of some compounds may also proceed. Sludge deposited in
these ponds over a period of time should be dredged period ically and disposed of in a
secured landfill. Evaporation also takes place in lagoons where sludges are stored
thereby increasing their solids content.
439
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Table 22. Destruction of Industrial Sludges by Thermal Processes
Wastes & Characteristics
Unit Process & Operating
Conditions
Performance
API Separator Bottoms
a Water 70%
b Organic Materials 13%
(~42 7%, aliphatics rest
aromaticsl
c Ash ~ 10%
d BTU 2500/lb
Phenol Waste Sludge
a Water 86%
b Solids 14%
c Ash 5 5%
d <1% Phenol & Cresols
e BTU < 1050/lb
PVC Waste Sludge _
a Water 72%
b Solids 28%
c Ashr 2%
d VC monomer Z 200ppm
e BTU 400/lb
Pyrolysis/lncmeration
a Feed rate 4 7 to 25 3 kg/hr
b inert gas flow 42.5 mVhr
c Temp 760°C
d Residence Time 125rnm
e Temp Incinerator 830 C
Fluidized Bed Incineration
a Feed Rate 34-501/mm
b Residence Time 12-14 sees
c Bed Temp 740-757°C
d Free Board Temp 813-899°C
e Waste/Auxiliary fuel, 1/1 2 3-3 0
Rotary Kiln Incineration
a Feed Rate 0.845 mt/hr
b Residence Time 3 sees
c Primary combustion zone
temperature 870°C
d Secondary combustion zone
temperature 980-1090°C
70% of feed organic conversion to a
useful form Ash=20% of total feed
85% of the ash is inorganic
Particulate Stack emission
23-88 mg/m3
a Destruction Efficiency
i Total orgamcs 99 93 - 99 95%
ii Waste Constituents >99 999%
b Particulate stack emission 1280-
1430 mg/m3
c Total orgamcs in scrubber water-
Not detected
a Destruction efficiency
i Total orgamcs >99 995%
n Total Waste 99 8-99 88%
b Particulate stack emission 70 9-
71 4 m3
c Total orgamcs in scrubber water
0-5 6 mg/L
-------
Certain waterborne wastes like those from organic chemicals and pesticide
industries are too dilute to be effectively disposed of on land or by incineration. A
review of pollution control technology in the pesticide formulation industry
indicated that solar evaporation of the wastes is practiced extensively. Evaporation
ponds without any discharge are also identified as the best practicable technology
for the disposal of these wastes.10
Other liquid wastes that are amenable to evaporation as a disposal method are
wastes from the inorganic chemicals and explosives industries, and certain types of
radioactive wastes. Wastewater from scrubbers associated with an incinerator may
be concentrated by evaporation, and the residue disposed of on land.
Multiple effect evaporative techniques similar to those used for distillation of
sea water have been applied for de watering sewage sludges. This technique, known as
the Carver-Greenfield process,24 facilitates the evaporation of water contained in
the sludge by using oil as a fluidizing medium. Following evaporation, the oil is
separated from the sludge by a centrifuge and is recycled to the incoming sludge. A
very dry product is obtained in this process. This process has been applied with some
modifications to process residuals from pharmaceutical and pet food industries."
With these wastes, after dry solids are obtained by the evaporative technique, a
pyrolized unit decomposed the organic residue to hydrocarbons which were then
used as a supplemental fuel to the boiler burner. The ash obtained in this process can
either be used in road paving asphalt or disposed of on land.
Other Methods
Various other methods may be used for the processing of residuals. An Arthur D.
Little study conducted for the EPA surveyed 47 different processes for the treatment
of solid, semi-solid and liquid industrial wastes.26 It identified a number of methods
for the treatment of organics contained in industrial residuals including distillation,
calcination, and hydrolysis. (These are usually employed in special cases for the
stabilization and disposal of industrial waste sludges.)
Distillation may be used for the recovery of organic solvents and is basically used
in solvent recovery plants. It may also be used for processing wastes from paint
industry, and lube oil recovery. However, this method is not suitable for thick slurries
and sludges.
Calcination is a process that may be operated at atmospheric pressure in which
theoretically no interaction between material being processed and a gaseous phase
(e.g., air as in incineration) takes place. At high temperatures, usually 650-1100°C,
vaporization of water takes place, organics are destroyed and a solid mass results
due to the fusion of the inorganic material. Additives like silicates and phosphates
may be used to decrease the leachability of the solid residue. This process may be
utilized as a one-step process for treating complex wastes to concentrate, destroy
and detoxify organic residues. A disadvantage of this process is that the costs will be
very high, if the wastes have a large amount of water. About 20 X 106 BTU/ton of dry
solids would be required for the calcination of a sludge having 90% moisture. Hence,
supplemental fuel will be needed. The greater the organic content and the lower the
moisture content of a sludge, the lower is its fuel requirement. Gaseous and
particulate emissions produced in this process cannot usually be discharged to the
ambient air. Therefore, air pollution control equipment is generally required.
Different types of calciners are available including (1) open hearth, (2) rotary kiln,
and (3) fluidized bed units. The choice of equipment depends largely on the type of
waste to be handled. For example, the fluidized bed calciner can be used for liquids,
slurries, sludges and solids, whereas the multiple hearth or rotary kiln types are
better suited for solids and sludges. A fluid bed calciner at an oil refinery in India has
been reported to process a mixture of sludges from various units viz, dissolved air
flotation unit, API separator, tank cleaning, wastewater treatment, etc.
441
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Although not widely used for processing industrial sludges, hydrolysis reactions
may be carried on with suitable feed streams such as waste slurries and sludges at
elevated temperatures usually with the addition of acids, alkalies, or enzymes. In the
petroleum refinery industry, the sludge from acid treatment of light oils may be
hydrolyzed to recover sulfuric acid. A tarry product containing some organics
formed in this process may be disposed of by incineration. Hydrolysis has also been
shown to be a useful technique for production of sugar from refuse, and the
conversion of organic sludge to animal feed.
The above methods can be applied to different hazardous and non-hazardous
industrial waste sludges. The appropriateness of a given method for a given sludge
depends on its characteristics, amenability to a specific technology, and economics.
Table 23 shows the technology that may be considered for the treatment of some
non-hazardous and hazardous industrial sludges. The term "land-based technology"
used in this table encompasses any ultimate disposal technique by which sludge is
applied to land either for land reclamation, growing crops, orfor the sole purpose of
using land as the final locus for sludge deposition.
Disposal - Current Practices
Land based technology and incineration (controlled and uncontrolled) are the
methods most widely employed for the ultimate disposal of industrial waste
residuals.
Industrial waste residuals may be disposed of either on-site where they are
generated or off-site. On-site and off-site treatment/disposal methods include
storage, material recovery, landfilling/dumping, lagooning, landspreading, and
incineration.
Landfilling operations may range from indiscriminate dumping to a controlled
management of disposal of wastes in secured landfills, that is landfills which ensure
adequate environmental and health protection. Landspreading involves the
transportation of a sludge to a site where it is applied. The sludge is usually hauled by
a truck and is applied uniformly on the land. The actual application rate is usually
determined by experience and varies from sludge to sludge. It depends on the nature
of the waste, the assimilation capacity of the soil, climatological conditions, and the
amount of land available. The sludge moisture is allowed to evaporate and then the
residuals are rototilled into the land. Where the soil has the ability to detoxify or
render innocuous the hazardous organic compounds contained in the sludge,
landspreading of sludge may be considered as a treatment and disposal technique.
Disposal of sludges in lagoons, ponds, sumps, and open pits is also a mode of
ultimate disposal but one that is not as environmentally satisfactory as disposal in
secured landfills. Indiscriminate disposal of sludges in lagoons, open pits and ponds
may pose environmental and health hazards. However, when lagoons are properly
constructed and operated, the environmental damage is reduced. Their
environmental acceptability depends primarily on the method of construction,
materials used in construction, site-specific considerations such as hydrogeologic
properties of the land involved, and the type of waste disposed.
Irrespective of whether the residuals are disposed on-site or off-site by a given
method, an important technological consideration is to ensure that the land
receiving the wastes is suited for application of the hazardous wastes without causing
any environmental degradation. Considerations such as availability of land,
machinery to apply the sludges, man power, availability of an incinerator and other
site-specific factors dictate whether the sludge may be disposed of on-site or should
be disposed off-site. Where land is available and incineration is not required to
dispose of the sludge, obviously the choice would be of an on-site land-based
technology. Even though land is available, if incineration is necessary to dispose of
an industrial sludge and one is not available on-site, the sludge has to be hauled to an
off-site facility. Data on the quantity of waste residuals disposed by various
industries either on-site or off-site are summarized in Table 24.
442
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Table 23. Residuals Generated from Non-Hazardous and Hazardous Industrial Wastes and Their Treatment and Disposal
Industrial Category
Grain Mills
Hazardous Wastes
Paper & Allied
Products
Paint Formulation
Type of Sludge
or Semi-Solid Waste
General Treatment
Sludge produced in municipal
treatment plants by
discharging canning wastes
Liquid organic residuals
Steep water
Fiber Solids
Sludge produced in municipal
treatment plants by
discharging grain mill
wastes.
Sludges
Latex Paint Wastes
ensilage production)
Incineration
Municipal treatment technology
By product recovery
Lagooning
Municipal treatment
Conditioning, Dewatering
Landfarming
Physico-chemical procedures.
Sedimentation, Municipal
Treatment Technology
applied to sludges from latex
paint wastes
Disposal Methods
Non-Hazardous Wastes
Meat Packing
Canning
Waste Treatment
Fruit and Vegetable Waste
Anaerobic and Digestion
Lagooning
By product recovery (alcohol and
Land-based Technology
(Irrigation)
Animal Feed
Land-based technology
Spray irrigation
Land-based technology
Land-based technology
Land-based technology
Incineration
Land-based technology
-------
Leather Tanning
Petroleum Refinery
Medical and Other
Health Services
Organic Chemicals
Pesticides and
pesticides
formulation
Explosive
Sem i-solid Wastes Containing
hair, fleshing, trimmings,
dirt, and manure
Sludge produced in municipal
treatment plants due to
discharge of leather tanning
wastes
Different types of aqueous and
non-aqueous sludges
Not sludge, but semi-solid
and solid wastes comprised
of isotopes, bacteriological
specimens, drugs, disposable
medical supplies, general
rubbish
Process waste streams, and
off-grade products
Various process streams
containers with residual
pesticides
Red water, explosive
contaminated inert wastes
Waste explosives
Explosive contaminated
inert wastes
Screening and Sedimentation
Municipal Treatment Technology
Incineration
Conditioning thickening and
Digestion
Land Farming
Stabilization, Solidification
Dilution of some streams and
disposal to sewage treatment
plant
Containment in lagoons and
evaporation
Evaporation of red water and
incineration of concentrates
Red water acidification and
steam stripping
Incineration
Composting of waste explosives
Incineration
Land-based technology
Land-based technology
Land-based technology
Incineration
Encapsulation of isotopes,
autoclavmg of pathological
specimens, and ultimate
disposal by land-based
technology
Land-based technology
Deepwell disposal of liquid
wastes, incineration
Incineration, storage in
sealed drums and burial in
land in a secure manner
Secure landfill, controlled
incineration
Spray irrigation
Incineration
-------
Table 24. Quantities of Residuals Disposed of by Different Methods in On-Siteand Off-Site Facilities by Various Industries (Wet
Tons/yr, 1977 basis)
Industrial Category
Plastic & Synthetics
Pharmaceuticals
Paints & Coatings
Waste
Liquid Phenolics
Phenolic sludges
Amino resins
Still bottoms
Wastewater Sludges
Organic chemical
residue
Mixed solvents
Non Halogenated
solvents
Halogenated solvents
Aqueous Mixed
solvents
Aqueous alcohol
Antiviral vaccines
Other Biologicals
Returned goods and
contaminated or
decomposed materi-
als
Wastewater sludge
Method of
Disposal
Drum storage
Drummed or
Lagooned
Incineration
Incineration
Mostly unknown
Incineration
Landfill
Incineration
Incineration
Incineration
Incineration
Incineration
Incineration
—
Incineration
Incineration
Landfill
On-site
1 61 ,000
44,000
—
27,100
—
6,120
—
6,240
10,740
870
970
280
115
—
60
5
70
Off-site
—
—
20,700
27,100
—
5,800
1,800
9,160
1 6,200
3,000
1,700
400
120
—
—
50
1,470
Unknown or Other
—
—
284,000 (some land disposal)
1 ,530 (disposal to biological)
treatment plant after dilu-
tion)
90 (disposal to Biological
Waste Treatment Plants)
115 (autoclaved on-site and
ash disposed off-site)
270 (autoclaved on-site and
ash disposed off-site)
120 (slurned with water and
disposed in biological
Wastewater treatment plant)
-------
Waste organic
cleaning solvent
Spills and spoiled
batches
Organic Chemicals Organic chemical
waste
Pesticides All types of
pesticide wastes
£
Ov
Explosives
Private industry Fixed High Explosive
waste
Government owned and Explosive wastes
contractor operated
explosives Explosive conta-
minated inert
wastes
Other (red water,
RDX/HMX waste)
Incineration
Landfill
Landfill
Landfill
Incineration
Biological
Treatment/
Lagoon
Deepwell
Landfill
Incineration
Storage in
drums or piles
Unspecified
Open Burning
Chemical de-
toxification
followed by
land disposal
Open Burning
Open Burning
Landfill
Open Burning
5 20
50 950
70 1,470
483,000 1 1 3,000
-2,250,000 51,000
565,000 —
6,540,000
175,000 75,000
— 100,000 (>95% in off-site)
81,000
— — 144,000 (usually detoxified
and disposed either on-site
or off-site)
>387 —
— — 26
4,800
1 3,700
1,000
90
-------
Petroleum Refining
Rubber Products
Leather Tanning
(water, organic
solvents, etc.)
All wastes
Oily Wastes
Air pollution
equipment dust
Trimmings, shavings,
Landfill
Landfill
Lagoon
Landspreading
Incineration
Landfill
Landspreading
Landfill/dump
Landfill
140
355,000
284,000
334,000
40,000
—
70
1,950
4,800
535,000
289,000
4,000
1,000
—
39,200
97,000
and Finishing
Electronic
Components
finished and un-
finished leather trim,
wastewater
screenings residues
from buffing and
finishing, waste-
water treatment
sludge
Other sludges
Waste Halogenated
solvents & still
bottoms
Waste Nonhalo-
genated solvents &
still bottoms
Wastewater Treat-
ment Sludges
Lubricating and
Hydraulic Oils
Paint Wastes
1,900
40,600
Dumps
Certified facility
Lagoons pits,
ponds, etc.
Landfill
Landfill
Landfill
Landfill
Landfill
Incineration
5,300
200
1,000
7,600
9,700
13,500
2,200
14,900
43,200
2,400
210
6
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Arthur D. Little Company reported that on a national level the principal methods
of hazardous waste disposal are landfilling and lagooning and stated that they will
continue to be the principal methods of disposal in the future (Table 25).27 This study
also described the on- and off-site disposal practices in the various industrial
categories. The extent of on-site disposal practices in these industries is given in
Table 26. As can be seen from this table, on-site disposal of industrial residuals is
quite prevalent in many industries. However, this mode of disposal has limited
opportunities in densely populated areas.
Another national survey directed to the impoundments of wastes in the industrial
sector, reported that there were about 110,000 industrial waste lagoons in the USA.'
About 78% of impoundments served wastes from oil and gas, food products,
textiles, chemicals, refining and mining industries, with another 12% servicing coal
mining and paper making industries.
An excellent survey reported by Allegheny County, Pa., with respect to its
industrial waste management, described landfilling of all types of sludges, reclaiming
of materials, and incineration of tank bottoms and other organics as the primary
modes of disposal.28
A Massachusetts survey based on the permits issued for the disposal of industrial
residuals, reported that 85% and 24% of the waste solvents and oils, respectively,
produced by the industries were reclaimed, whereas the remainder were incinerated
for the most part.29 This survey also indicated that while almost all the aqueous
waste liquids produced (99.1%) were discharged to the local municipal wastewater
treatment plants, other solids and sludges generated by the industries were disposed
of by landfilling (56%) and incineration (41%).
In an elaborate survey conducted in 1978 via questionnaires submitted to State
agencies, it was revealed that there were 14,392 landfills in the USA.'
A report by the Illinois Institute of National Resources indicated that landspreading of
sludges was practiced by 71 industrial plants. Most of the sludges were assumed to be
originating from food processing and similar industries. These sludges are
considered to be organic and are biodegradable in nature and are hence appropriate
for land treatment and disposal. The report also stated that the practice of
landspreading sludges in Illinois appeared to be consistent with that of a 1977
USEPA estimate, which cited that 41% of the vegetable processing plants and 37%
of the fruit processing plants used landspreading of sludge.' From the foregoing, it
may be stated that land disposal of industrial sludges is by far the major prevalent
practice
Leachate and Ash Disposal—
Two important considerations in the ultimate disposal of industrial organic
sludges are the management of leachate produced in land disposal sites and the
disposal of ash resulting from incinerator operations.
A leachate may be formed from the sludges applied on land disposal sites under
conditions of excessive precipitation. The quantity and characteristics of the
leachate depend on the nature of the waste, processing of the waste, ability of the
waste to leach, water holding capacity of the soil, initial moisture content of the
waste and soil, permeability of soil, density of the waste, and the time over which the
decay of the waste has occurred either in storage prior to disposal or in the soil itself.
Depending on the hydrogeologic characteristics of the disposal field, the leachate
may contaminate the groundwater table. In order to minimize the risk of
groundwater contamination, the leachate should be managed properly. This can be
achieved by installation of impervious liners in a land disposal site to intercept the
movement of the leachate, and by collection and treatment of leachate.
Liners should be compatible with the waste disposed. Asphalt liners for example
are not desirable when sludges containing solvents are disposed of because they
react. The results of an EPA study, in which the compatibility of various liners with
different types of industrial wastewaters and sludges was investigated, are presented
448
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Table 25. Present and Projected Hazardous Waste Management Methods
Percent Disposal
Method Present (1977) Projected
Landfill 61.6
Secure landfill - 68.2
Lagooning 29.7 21.5
Incineration 5.2 8.2
Landspreading 0.9 1.3
Deep well injection 30 3.0
Ocean <0.1
Table 26. On-Site Disposal of Hazardous Wastes in Various Industries
Industry % of Waste Managed On-Site
Electronic Components 13
Leather Tanning and Finishing 10
Petroleum Refining 44
Textiles 49
Pesticides 87
Plastics 80
in a recently published book on impoundments of industrial and hazardous wastes.20
A list of various liner materials is also given in this book along with their properties.
Polymeric membranes which are made of rubber and plastic membranes are gaining
more acceptance because of their low permeability to water and other fluids. The
procedure for the installation of a membrane liner is illustrated in a recently
published report.30
Other lining materials for ponds include concrete, clay, and asphalt. Preliminary
evaluations reveal the following ranking for durability of lining materials: (1)
concrete, (2) plastic, (3) rubber, and (4) asphalt.20 However, lined facilities are
generally considered to be unacceptable in Illinois as a substitute for
hydrogeologically safe landfills.31
The leachate formed is permitted to flow by gravity into collection sumps from
which it can be removed and treated. The leachate may be processed by recycling,
chemical fixation, encapsulation, evaporation, biological treatment, and advanced
wastewater treatment. Physical-chemical processes were reported to be ineffective in
removing the organics from leachate and biological pretreatment for biodegradable
organics was required. Leachate from solid waste disposal sites may be treated in a
municipal activated sludge treatment plant. However, a high strength leachate from
such facilities could not be processed effectively in a test unit at a flow rate of >4% of
the sewage flow rate.32 Thus, the composition of leachate and availability of public
treatment facilities are important considerations in selecting a leachate treatment
method when the leachate is amenable to biological treatment.
It is important of course that all practicable steps be taken to minimize leachate
production. In a study designed to evaluate methods to accomplish this objective the
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following alternatives were examined: (1) revegetation, (2) regrading to increase
surface runoff, (3) surface sealing with an impermeable material, (4) construction of
a slurry trench cut-off wall to prevent the movement of groundwater table, (5)
installation of diversion ditches for the surface water away from the landfill, (6)
lowering the groundwater table by developing a drainage system, and (7) counter-
pumping to lower the groundwater table. In this study the first three methods were
found to be most effective in minimizing leachate generation in a Connecticut
landfill."
Groundwater monitoring should be an integral part of any leachate management
program. For soil and groundwater monitoring, samples from lysimeters and test
wells located at different depths may be obtained and analyzed for their quality.
Measurement of resistivity of soil core samples may also be employed to monitor the
changes in ionic concentration of the soil samples. Baseline data on groundwater
quality prior to the start-up of a new land disposal site should be collected so that
deterioration resulting from landfill operations may be detected. Downstream
monitoring of the groundwater quality is very important and if contamination takes
place protective measures may be taken immediately.
The management of leachate involves long term planning and care, and should
include the maintenance of records pertaining to waste inventories, analysis of
monitoring data collected on a routine basis, continued analysis of groundwater
even if the landfill site is closed, and clearly laid out contingency plans for removing
the waste from the site, if needed, or to repair the landfill site or liners as and when
required.
Disposal of ash produced by incineration of organic residuals comprises a facet of
residual management. Ash is inorganic in nature and its content of various organic
residuals depends on their nature. Some residuals like oily waste may contain as low
as 1% of ash by weight whereas some sludges from papermills may contain as much
as 50% or more. The ash content of different types of coal may range from 3 to 32%.
High quantities of fly ash, and bottom ash or boiler slag are produced in coal
burning electric power utilities. Particulate emissions scrubbed from incinerator
stacks also resemble ash except that they are generally present as a sludge. The
disposal of ash may constitute a serious problem if large quantities are encountered
in incineration operations as in the case of service industries such as power
generation facilities.
Ash is generally disposed of by wet ponding or dry landfilling. In wet disposal of
the ash it is usually made into a slurry and pumped into a pond. After settling, the
supernatant of the pond is returned to the plant and reused. Ponds are designed so
that the wet ash slurry may be pumped to an elevated slope when the ash is separated
from the liquid which is collected at the lower end of the pond. Ashcanbedewatered
by as much as 60-90% in well-designed and carefully operated ponds.
Landfilling of ash is usually carried out on-site in open dumps or by spreading. In
order to avoid surface and groundwater contamination by the leachate, ash must be
disposed of in a secured landfill or in a surface impoundment equipped with liners,
leachate management protocols, monitoring wells, fencing, etc.
Ash may also be disposed by chemical stabilization, although this practice is not
extensively practiced. This method has reportedly been used for flue gas
desulfurization sludge, a wet-scrubbed particulate emission from power generation
plants. The leachate of properly chemically stabilized ash should conform to RCR A
regulations.
Evolving Technology
Organic compounds that are known to be toxic but which occur in low
concentrations in industrial waste sludges may be treated by unit processes such as
aerobic digestion, anaerobic digestion, composting, solidification, encapsulation,
chemical detoxification, thermal destruction or by special landfill methods.
450
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Aerobic and anaerobic digestion may be applied to sludges which are non-
hazardous as is the case with sludges from food industries. This technology is fairly
well established. Although composting is practiced for the stabilization of solid
waste and municipal sludge, its application to the processing of industrial sludges is
of recent origin. It has been reported that oils, greases, certain tars and wastes from
the explosives industry have been composted with a certain degree of success, but
application of composting for the stabilization of organics in different types of
industrial sludges is not yet widely established.
Because of an inadequate technology for processing certain types of hazardous
wastes and sludges, they are simply stored in containers. Examples of such wastes
include various pesticides, hexachlorobenzene, PCB's and any number of unknown
chemical wastes. In order to eliminate problems, such as leakage from containers,
new processes have emerged including stabilization/solidification and
encapsulation. This technology is of course not completely novel, as it has been in
use for some time for the containment and disposal of radioactive waste. The
methods employed are designed to produce a material that neither degrades nor
produces a leachate.
Stabilization/solidification processes may be characterized as: (I) cement based,
(2) lime based, (3) thermoplastic (using bitumen, paraffin, and polyethylene), (4)
organic polymer, and (5) encapsulation.34 Although these techniques are mostly
applied to hazardous wastes containing primarily inorganics, it has been claimed
that some proprietary processes have been used for sludges containing organics. A
brief description of these techniques is given below.
In cement based solidification, the sludge is mixed with cement and other
additives such as fly ash or incinerator ash to form a rock like material. Other
proprietary materials are also used as additives. Materials such as clay and
vermiculite are good adsorbents and can be incorporated into the cement. Sodium
silicate has been found to be a good binding agent for contaminants in sludge, when
it is added to the sludge-cement mixture.
There are several advantages to this technology. These are: (1) the process is
amenable to variations in sludge characteristics and components, (2) the proportion
of cement can be manipulated to control the strength and permeability of the
product, and (3) the necessary equipment and materials to implement this
technology are readily available. However, there are also some disadvantages: (1)
the possibility exists that leaching of acid wastes will occur particularly from low-
strength cement-waste mixtures, (2) the process is vulnerable to impurities in the
waste which may affect the solidification process; hence, expensive pretreatment of
the sludge may be required, (3) the bulk and weight of the finished product are
increased.
The two most common additives in the method using lime for solidification are fly
ash and cement kiln dust, which are also waste materials. Several proprietary
additives are also available for solidifying the sludges. This method is similar to the
cement-based in most respects and has similar advantages and disadvantages.
In the thermoplastic method, the waste is dried and mixed with materials such as
bitumen, paraffin, and polyethylene at temperatures usually higher than 100°C. The
mixture solidifies upon cooling. Often the mixture is placed in a steel drum or in a
thermoplastic coating prior to disposal. Wastes containing solvents or chemicals
that solubilize the additives or produce deterioration are not suitable for this
process. The advantages of this process are: (1) leaching rates of pollutants are
generally lower because the thermoplastic materials are resistant to most aqueous
solutions, and (2) wastes adhere well to the thermoplastic materials. The
disadvantages are: (1) waste must be in a dried form, (2) thermoplastic materials are
flammable, (3) skilled labor and special equipment are required and hence, the
process is expensive, (4) the method cannot be used for sludges containing solvents
of thermoplastics, and (5) volatilization of low boiling temperature compounds may
take place and hence special precaution is required.
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In the organic polymer solidification method, urea-formaldehyde resin is usually
added and thoroughly mixed with the waste. A catalyst is subsequently added to
complete the polymerization process. A spongy type of material is formed which
entraps the paniculate matter and separates the liquid component of the waste. This
spongy material may be dried prior to disposal. However, it is often loaded in
containers and buried.
The above method can be applied to either dry or liquid wastes and its use usually
requires only small amounts of additives. The resulting waste-polymer mixture has a
lower bulk density in comparison to the cement-fixed sludges. However, there are
some disadvantages: (1) the catalysts added are highly acidic and will leach out any
metals contained in the waste, and (2) the waste particles are bound in a loose resin
matrix, and some of the organic polymers are biodegradable.
Encapsulation is usually referred to as a process in which the waste particles are
coated with a binding material. The most commonly used process for sludges
involves the use of a polybutadiene binder. This is followed by the application of a
thin polyethylene jacket by mechanical means around the polybutadiene
impregnated sludge to form an encapsulated block, which is disposed of. This
process provides an impervious and chemically inert barrier between the sludge
constituents and the environment, and hence even very soluble constitutents can be
contained. The process requires skilled labor and specialized equipment. Also, heat
is required to form a thermoplastic. Some type of jackets are flammable. The
materials used are often expensive and the waste sludge has to be dried before it is
encapsulated.
Solidification and encapsulation processes have been used successfully for many
industrial waste residuals containing potentially hazardous organics. These
processes are evaluated primarily on the basis of teachability and strength of the
solidified or encapsulated material. A number of proprietary processes have been
reported to stabilize sludges from industries such as paint, tannery, pharmaceutical,
PCB, chemical, electronic component manufacturing, and oil refinery wastes. These
sludges are known to contain potentially hazardous organic compounds.34 These
proprietary processes all claim a low rate of leachability of the solidified wastes.
teachability of inorganics is reported to be low from these wastes when they are
properly stabilized/solidified. No information on the leachability of organics from
the solidified wastes is available. Based on the low leachability of inorganics, a low
degree of leachability of organics may also be expected because of the impervious
nature and resistance of the materials used for solidification and encapsulation.
Techniques such as ultrafiltration, solvent extraction, carbon adsorption, resin
adsorption, steam stripping and air stripping may be applied to industrial wastes for
the removal of organics contained in the aqueous fraction, and for the reduction of
the quantity of these organics adsorbed onto the solid fraction. The removal of such
organics by pretreatment may also permit the biological stabilization of the
industrial wastes. Although these techniques are developed, they have not been used
commonly in industrial sludge treatment.26
Certain organic liquid wastes may be detoxified and rendered non-hazardous by
processes such as chlorinolysis and microwave bombardment. Chlorinolysis can be
termed as a manufacturing process instead of a waste treatment process because
carbon tetrachlonde is produced from waste chlorinated hydrocarbons. The waste
chlorinated hydrocarbons are made to react with excess chlorine at a temperature of
about 500°C and a pressure of about 200 atmospheres.26
The detoxification of hazardous organic wastes may be accomplished by
microwave disharge treatment. An assessment of this technology which may be
applied to small quantities of highly toxic materials which cannot be handled by any
other means is being made by the Lockheed Company. Organophosphorus
pesticides, chemical warfare agents, organonitrogen compounds wastes that are
potential carcinogens and teratogens, and halogenated hydrocarbons and related
pollutants are reported to be amenable to treatment by this method.1%'6 This
452
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technology is not in wide use and little information is available on its application to
industrial organic sludges. However, data reported from laboratory experiments
utilizing oxygen plasmas and pure substrates have shown that the treatment of
hazardous organics by microwave discharge was successful.1'''36 Table 27 shows the
degradation products reported for specific wastes tested.
Thermal destruction processes such as incineration are well established. The
fluidized bed type of incineration process offers great promise for the destruction of
hazardous wastes and has been investigated with several types of wastes. Another
thermal process known as calcination is also well established for the treatment of
industrial wastes. This process can handle complex wastes containing both
inorganic and organic constituents. The organics are destroyed at about 1000°C,
and a sintered solid mass is formed as an end product. This process is likely to emerge
as a practicable process particularly for the processing of tars, sludges and other
residuals whose disposal poses a difficult problem.
Other methods for the treatment of hazardous organic residuals have been
described such as molten salt combustion for the degradation of PCBs, U V radiation
for the degradation of PCBs and kepone, and degradation of complex organic
structures by ozone and chemicals such as chloroiodides."
Performance of Current Systems - Overview
The general operating and performance characteristics of various thickening and
dewatering equipment have been summarized in Table 20.
Very little is known on the effect of conditioning on the organics contained in the
sludges or how the contained organics affect the performance of a conditioning
technique. However, it can be assumed that techniques involving heat are likely to
volatilize some sludge organics. Dewatering techniques which remove progressively
more quantities of water would also remove greater quantities of water-soluble
organics.
Biodegradable sludges may be thickened and digested either aerobically or
anaerobically in order to stabilize them prior to their disposal on land. The degree of
stabilization of a biodegradable sludge by these methods can be measured by the
extent of destruction in its volatile solids content. Depending on the loading rate,
temperature and detention time employed in the process, about 25 to 50% of the
initial volatile solids content may be destroyed. Non-hazardous industrial wastes
such as those from food and vegetable processing industries are stabilized generally
by digestion. Where industries discharge their wastes to municipal systems, the
sludge solids resulting from the industrial waste are digested along with those
contributed by the sanitary wastes. The performance of such systems generally
parallels those processing municipal wastewater primarily.
In aerobic stabilization processes, biodegradable organic compounds are
oxidized to CO2 or utilized in the synthesis of new cells. In the anaerobic digestion
process, the end products are methane and COz. In addition, volatile fatty acids and
unused substrate may also be present if digestion is not completed. Toxic industrial
organics may inhibit the digestion process. Laboratory studies have shown that
acrylonitrile, at 5 mg/ L, carbon tetrachlonde, at 10 mg/ L, and 1,2- dichloroethane,
at <1 mg/L, were toxic for the anaerobic digestion process.38 Several aromatic
compounds have also been reported to be resistant to aerobic digestion. It has been
found that the presence of two or more hydroxyl groups on the benzene ring
increased the resistance of the compound to biological oxidation. Progressive
addition of nitro groups on an aromatic ring increased recalcitrance of the aromatic
compounds to microbial degradation. The presence of chloro, methoxy, or phenoxy
groups on the ring also increased resistance toward microbial degradation.
However, there are microorganisms that can degrade partially some of the normally
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Table 27. Degradation Products Formed by Microwave Detoxification of
Selected Wastes ^_^
Pesticide/Wastes Conversion Reaction Products
Malathion >99.98 SO2, CO2, CO, H2O,
"Cythion" ULV HPO3 or H3PO4.
PCB
Aroclor, 1242 >99.9 CO2, CO, H2O(CI2O,
COCI2)
Methyl Bromide 99 C02, CO, H20, Br2,
Br02.
Phenylmercuric acetate 99.9 Hg, CO2, CO, H20.
"Tyrosan" PMA - 30
recalcitrant compounds. A list of these organisms and the substrates they oxidize is
summarized in a recently published report.39
As indicated earlier, composting of oily wastes and explosive wastes which
contain hazardous organic substances was at least partially effective. It has been
reported that land treatment and disposal has been used for non-hazardous organic
wastes and some industrial wastes containing hazardous organics such as pulp and
paper mill sludges, iron and steelmill sludges, and pesticide manufacturing wastes.40
The performance of a landfill disposal system for hazardous wastes and sludges
depends primarily on the criteria used in their design and on climatological and
characteristics of the soil, nature of the waste applied, and the operational practice
employed.
The performance of incinerator systems may be judged on the basis of the
following: (1) ability to process a given type of waste, e.g., plastic, wood, or other
high moisture residuals, (2) ability to comply with the local air pollution ordinances,
(3) capital and operating costs, (4) disposal capacity, i.e., pounds of waste
processed/ ft2 of incinerator bed area/ time, (5) ease of removal of ash, and (6) ease of
operation and maintenance. The performance characteristics of several incineration
systems for treating various types of industrial waste sludges have already been
presented.
Incineration converts the carbonaceous fraction of the sludges primarily to CO2.
If the oxidation is not complete, some CO may be found. Other gaseous emissions of
incineration may include oxides of nitrogen, SO2, and halogens depending on the
original characteristics of the sludge. If nickel is found in sludges, the stack emissions
may include nickel carbonyl, which is toxic.
Information on performance of other non-conventional processes cited earlier for
the treatment of industrial sludges is meager. Methods such as distillation can be
applied economically for reclaiming waste oils, but it cannot be applied for the
treatment of sludges or slurries. Evaporation may be effective for both aqueous
wastes and sludge suspensions. Dilute pesticide wastes, TNT wastes, paper mill
wastes, molasses wastes, and distillery wastes may be effectively evaporated in
ponds. Multiple stage evaporation techniques using solvent extraction of sludges
may be effectively used for the production of a very dry sludge as was demonstrated
with pet food and pharmaceutical waste sludges.
The performance of various stabilization/solidification and encapsulation
techniques appears to be quite satisfactory for processing PCB and pharmaceutical
residuals in terms of the formation of a product which will produce no leachate."
The performance of land disposal systems may be optimized by assuring that
applications of industrial waste residuals are compatible with characteristics of the
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landfill site. In order to achieve this objective, both landfill sites and wastes are
classified by the State of California and guidelines are prescribed for matching
residuals with the landfill sites.
In this classification of landfill sites, a Class I or Secured Landfill facility cannot
discharge to usable waters and hence can receive all types of residuals. An aquifer
must be separated from the disposal bed by an impervious geological formation and
the site must be protected against flooding and washout. A limited Class I site, in
contrast to the Class I sites which receive all types of wastes, places restrictions on the
types and amount of waste it receives due to the possibility of flooding.
A Class II-I Landfill type of facility can be located without severe restrictions
regarding aquifers. If natural geological conditions do not protect against the
movement of contaminants of applied sludges, such protection may be achieved by
containment of wastes by use of artificial barriers. Non-water soluble and non-
decomposable solids, and wastes containing chemically or biologically decomposing
materials that will not impair the quality of ground water are accepted at these sites.
Some toxic wastes may also be accepted.
A Class II-2 Landfill is a disposal site where a vertical and lateral continuity or
movement of the disposed residuals is permitted with usable groundwater.
Nevertheless, some protection is usually ensured due to naturally occurring
hydrogeologic features of the site.
Class III Landfill sites are disposal areas that provide no protection for
groundwater quality. Wastes when disposed in such sites may reach the
groundwater. Only wastes which are not soluble in water and inert solids may be
disposed of in such sites.
As can be seen from the above classification, secured landfills are preferred for the
disposal of sludges containing hazardous organics, to ensure adequate
environmental protection. Performance of surface impoundments where liquid
wastes or slurries are contained may be improved by constructing them either in
hydrogeologically safe areas, or by installing impervious liners prior to introduction
of wastes into them. In addition, a free board of 2 ft or greater should also be
provided to avoid flooding of the neighboring terrain.
Economics
The economics of industrial residual treatment and disposal is a complex subject.
The costs are a function of several factors including, (1) nature of the waste, (2)
considerations such as on-site vs. off-site disposal, (3) transportation distance, (4)
disposal site location and site preparation costs, (5) pretreatment or modification of
waste to render it suitable for disposal, e.g., encapsulation, chemical fixation,
detoxification, etc., (6) supplemental fuel for incineration, (7) cost recovered from
reclaimable materials, and (8) monitoring costs, etc.
A recent report from the Illinois Institute of Natural Resources summarized the
cost estimates published by USEPA in 1978 for various methods of disposal: (a)
secured chemical landfill, $30-55/ton, (1.5-2.75e/lb), (b) incinerator, $75-265/ton,
(3.75-13.25c/ Ib), (c) landspreading, S2-25/ ton (0.1-1.25c/Ib), (d) chemical fixation,
$10-30/ton (0.5-1.5c/lb). For purposes of comparison, this report presented the
costs published by the Manufacturing Chemists Association (MCA) in 1979 for off-
site disposal excluding pretreatment and transportation costs: (a) incineration, 1-
8e/lb, (b) chemical landfill, 0.2-4c/lb, (c) deep-well, 1.8-3c/lb, and (d) sanitary
landfill, 0.8-1.5c/lb. The MCA reported that transportation costs vary between 0.6-
3.2e/lb within a radius of 600 miles from the waste generation site.1
Some of the above costs compare favorably with those published for Allegheny
County in Pennsylvania where costs were based on actual monitoring of the
expenses incurred. These are given in Table 28.2S
The following benefits are anticipated when the regulatory program of RCRA
goes into effect: (1) reduction of groundwater pollution caused by leaching of toxic
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Table 28. Net Unit Costs of Disposal of Industrial and Municipal Residuals
Allegheny County, Pa.28
Type Unit Cost
Oil Reclamation 2.7 C/lb.
Solvent Recovery 67 C/gal.
Industrial Organic Sludge 0.2 C/lb
(disposed to Wastewater
Treatment Plant)
Municipal Organic Sludge 0.2 C/lb.
Scientific Landfill 2 6 C/lb.
Sanitary Landfill 016 C/lb.
Incinerator 0.08 C/lb.
pollutants from improperly designed and operated landfills and impoundments, (2)
reduction in poisoning and injury due to direct contact with toxic pollutants
contained in wastes and sludges, (3) reduction in pollution of surface waters from
hazardous wastes stored or disposed of in field and river banks, (4) reduction in illicit
dumping of hazardous wastes in soil and water environment, (5) abatement of air
pollution from poorly operated incinerators, (6) reduction in mishaps at hazardous
waste disposal sites due to improved training of personnel and management of
disposal sites, (7) prevention of accidents and mishaps in communities located near
disposal sites as a result of quick implementation of contingency plans, and (8)
reduction in future adverse impacts due to decontamination or other secured
measures taken at the time of closure of a disposal site.41
Although the expected improvements are not quantifiable, it is expected that they
will be substantial. In addition, costs that have hitherto been borne mainly by the
public for environmental clean-up necessitated by improper, surreptitious disposal
of industrial residuals, will in the future be borne by the responsible party.
The implementation of RCRA regulations hopefully will force industries to
comply with such regulations for hazardous waste management, thereby minimizing
the possibility of repetition of incidents similar to the Love Canal episode.
A detailed account of the impact of RCRA on a national level is beyond the scope
of this paper. However, based on certain assumptions, the USEPA predicts that the
implementation of RCRA will have a broad impact on industry while deriving the
potential benefits listed above.
In 1980, RCRA will affect about 67,000 hazardous waste generators producing
approximately 41 million metric tons of hazardous waste annually. A major portion
of this waste is generated by the chemical and allied industries. In a study covering
29,000 hazardous waste generators which produced 13.7 million metric tons of
hazardous waste in 1978, the USEPA predicted that they will incur an incremental
cost of $510 million/year for hazardous waste treatment and disposal. This cost was
stated to be less than 0.2% of the receipts on marketed goods. According to the EPA,
six major industrial categories will be significantly affected in terms of potential
closure, loss in jobs, and possible price increases. These are (1) textiles, (2) leather
tanning, (3) electroplating, (4) inorganic chemicals, (5) organic chemicals, and (6)
non-ferrous smelting and refining. Of these, textile mills and organic chemical
industries are known to generate sludges containing toxic organics. Some plant
closures are also possible in segments of the explosives, petroleum re-refining,
Pharmaceuticals, and plastics industries as a result of the difficulty that may be
experienced in complying to the RCRA regulations. All of these industries generate
sludges containing toxic organics.
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At the state and local level, the impact of the implementation of RCRA will also
be felt eventually. An immediate effect will be the implementation of a manifest
system for identifying and accounting for the hazardous waste generated in various
industries including the method of disposal. If the manifest system is strictly
enforced it should reduce illegal dumping of hazardous wastes and sludges on open
sites and in local sewers. However, if it is poorly managed, it could result in more
illegal dumping than at present. The RCRA will require careful monitoring of
existing hazardous disposal sites. This will possibly decrease the number of available
hazardous disposal sites and will increase the disposal costs. Unfortunately, many
small municipalities neither have the financial resources nor the administrative and
technical manpower to implement their manifest system properly and consequently
will continue to encounter illegal dumping of wastes and sludges into their sewerage
systems.
As a guard against illegal dumping and to comply with the NPDES permit
restrictions at a local level, the Metropolitan Sanitary District of Greater Chicago
has implemented its own manifest system, and an evaluation of the performance of
this program over a year revealed that <1% of the industries, (117 industries),
violated the requirements for reporting the amount of waste generated on their
premises. It is hoped that even these industries like all others will comply with the
MSDGC manifest requirements in the future.
REFERENCES
1. Patterson Associates, Inc. "Hazardous Waste Management in Illinois."
Document No. 79/32 of the Environmental Management Division,
Illinois Institute of National Resources, October, 1979.
2. USEPA. "Assessment of Industrial Hazardous Waste Practices, Textiles
Industry." SW-I25C (PB-258 953), June, 1976.
3. USEPA. "Assessment of Industrial Hazardous Waste Practices: Rubber and
Plastics Industry." SW-163C (PB 282-072), 1978.
4. USEPA. "Assessment of Industrial Hazardous Waste Practices. Paint and
Applied Products Industry, Contract Solvent Operations, and Factory
Application of Coatings." SW 119C, September, 1975.
5. USEPA. "Assessment of Hazardous Waste Practices in the Petroleum
Refining Industry." SW-129C, 1976.
6. USEPA. "Development Document for Effluent Limitations Guidelines and
Standards for the Petroleum Refining Point Sources Category."
440/1-79/014-b, 1979.
7. Berkowitz, J.B., et al. "Field Verification of Land Cultivation/Refuse
Farming—Disposal of Hazardous Waste," Proceedings of the 6th Annual
Research Symposium, U.S.EPA, 600/9-80-010, pp. 260-273, 1980.
8. Conner, J.R. "Ultimate Disposal of Liquid Residues by Chemical Fixation-
Proceedings of Management and Disposal of Residues from the Treatment
of Industrial Wastewater," Information Transfer, Inc., Washington, D.C.,
pp. 197-208, 1975.
9. Deever, W.R. and R.C. White. "Composting Petroleum Refinery Sludges--
Paper Presented at Governors Conference on Economics, Waste and the
Environment," 1978.
10. USEPA. "Assessment of Industrial Hazardous Waste Practices, Organic
Chemicals, Pesticides, and Explosives Industries." SW 1I8C, 1976.
11. Laver, N.A. "Disposal of Nitrogenous Liquid Effluent from Modderfontein
Dynamite Factory—Proceedings of the 21st Purdue Industrial Waste
Conference," pp. 902-925, 1966.
12. Carroll, J. W., et al. "Assessment of Hazardous Air Pollutants from Disposal
of Munitions in a Prototype Fluidized Bed Incinerator." J. Am. Ind. Hyg.
Assoc. 40:2, pp. 147-158, 1979.
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13. USEPA. "A Study of Hazardous Waste Materials, Hazardous Effects and
Disposal Methods," Vols. I-III. 670/2-72-014, 015, 016, 1973.
14. Johnson, A.S. Meat. "Industrial Wastewater Control." Ed. C.F. Gurnham.
Acad. Press, N.Y., 1965.
15. VanNoordwyk, H.,etal. "Quantification of Municipal Disposal Methods for
Industrially Generated Hazardous Wastes." EPA—600/2-79-135, 1979.
16. USEPA. "Economic Impact Analyses of Hazardous Waste Management
Regulations on Selected Generating Industries."SW 182C, December, 1979.
17. SCS Engineers. "Disposal of Dilute Pesticide Solutions—A State of the Art
Report." Draft Report for EPA. October, 1978. Cited in Ref. 4.
18. Krishnan, K. and H. Halvin. "Alternatives for Hazardous Waste Management
in the Petroleum Refining Industry—Proceedings of the 3rd Annual
Conference on Treatment and Disposal of Industrial Wastes and Residues."
pp. 236-243, 1978.
19. Olson, R.L., et al. "Sludge Dewatering with Solven Extraction—Proceedings
of the National Conference on Management and Disposal of Residues from
the Treatment of Industrial Wastewaters." pp. 175-181, 1975.
20. Cherimisinoff, N., et al. "Industrial and Hazardous Wastes Impoundment."
Ann Arbor Sciences, Inc., Ann Arbor, Mich., 1979.
21. USEPA. "Destroying Chemical Wastes in Commercial Scale Incinerators-
Phase II." USEPA SW155C, 1978.
22. Te wksbury, T. L., et al. "Fluidized Bed Incineration of Selected Carbonaceous
Industrial Wastes," Wat. Poll. Cont. Res. Ser. 12120 FYF 03/72, 1972.
23. Rubei, F.N. "Incineration of Solid Wastes." Poll. Tech. Rev. No. 13. Noyes
Data Corp., Park Ridge, N.J., 1974.
24. USEPA. "Sludge Treatment and Disposal." Technology Transfer
Manual, 1979.
25. Sadana, A. "Multiple Effect Evaporation and Pyrolization of Industrial
Wastewater Residues and Energy Recovery—Proceedings of Management
and Disposal of Resiues from the Treatment of Industrial Waste water, "pp.
191-196, 1975.
26. USEPA. "Physical, Chemical, and Biological Treatment Techniques for
Industrial Wastes." Vols. I-II. SW-148C, 1977.
27. A.D. Little Inc. "Draft Economic Impact Analysis for Subtitle C, Resources
Conservation and Recovery Act of 1976 (RCR A)." USEPA, January, 1979 (as
cited in Ref. 1).
28. Berman, D.R., E.J. Consentino, Martin and J. David, Jr. "An Industrial
Residue Management System for Allegheny County, Pa.—Proceedings of the
9th Mid-Atlantic industrial Waste Conference." pp. 39-58, 1977.
29. Fennelly, P.P., et al. "The Generation and Disposal of Hazardous Wastes in
Massachusetts—Proceedings of the 1977 Conference on Treatment and
Disposal of Industrial Wastewaters and Residues." Houston, Texas, April 26-
28, 1972.
30. Schultz, D.W. and M.P. Miklas, Jr. "Assessment of Liner Installation
Procedures: Disposal of Hazardous Wastes—Proceedings of the Sixth
Annual Research Symposium on Hazardous Waste Disposal." EPA 600/9-
80-010. pp. 135-159, 1980.
31. Miller, M. "Special Waste Disposal - Illinois Style." Toxic and Hazardous
Waste Disposal, Vol. Ill, Ed. R.B. Pojasek, Ann Arbor Sci. Pub. Inc., Ann
Arbor, Mich. pp. 67-79, 1980.
32. Chian, S.K. and F.B. Dewalle. Sanitary Landfill Leachates and Their
Treatment. J. Env. Engg. Divn. ASCE 102 EE2, pp. 411-431, April, 1976.
33. Emrich, H.G., W.W. Beck, Jr. and A.L. Tolman. "Top-sealing to Minimize
Leachate Generation: Disposal of Hazardous Waste—Proceedings of the 6th
Annual Symposium." EPA 600/9-80-010, pp. 274-283, 1980.
34. Pojasek, R.B. Toxic and Hazardous Waste Disposal. Vol. I-IV.
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Ann Arbor Science Inc., Ann Arbor, Mich., 1980.
35. Bailin, L.J. and B.L. Hertzler. "Development of Microwave Plasma
Detoxification Process for Hazardous Wastes-Phase I."EPA-600/2-77-030,
pp. 65, 1977.
36. Bailin, L.J. "Microwave Plasma Detoxification Process for Hazardous
Wastes-Phase II: Systems Evaluation." EPA-600/ 2-78-080, pp. 21, 1978.
37. Edwards, B.H.and J.N. Paullin. "Emerging Technologies for the Destruction
of Hazardous Waste Molten Salt Combustion: Treatment of Hazardous
Wastes—Proceedings of the 6th Annual Research Symposium." EPA 600/9-
80-011, pp. 7785, 1980.
38. Barth, E.F. and R.L. Bunch. "Biodegradation and Treatability of Specific
Pollutants." EPA 600/9-79-034, 1979.
39. SCS Engineers. "Selected Biodegradation Techniques for Treatment and/or
Ultimate Disposal of Organic Materials." EPA 600/2-79-006, 1979.
40. Sittig, M. "Land Disposal of Hazardous Wastes and Sludges." Noyes Data
Corp., Park Ridge, N.J., 1978.
41. USEPA. "Hazardous Waste and Consolidated Permit Regulations." Federal
Register, May 19, 1980.
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