v>EPA
United States
Environmental Protection
Agencv
Robert S. Kerr Environmental Research
Laboratory
Ada OK 74820
EPA-600 2-78-200
September 1978
Research and Development
Treatment of Petroleum
Refinery, Petrochemical
and Combined
Industrial-Municipal
Wastewaters With
Activated Carbon
Literature Review
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the ENVIRONMENTAL PROTECTION TECH-
NOLOGY series. This series describes research performed to develop and dem-
onstrate instrumentation, equipment, and methodology to repair or prevent en-
vironmental degradation from point and non-point sources of pollution. This work
provides the new or improved technology required for the control and treatment
of pollution sources to meet environmental quality standards.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/2-78-200
September 1978
TREATMENT OF PETROLEUM REFINERY, PETROCHEMICAL
AND COMBINED INDUSTRIAL-MUNICIPAL
WASTEWATERS WITH ACTIVATED CARBON
Literature Review
by
John E. Matthews
Source Management Branch
Robert S. Kerr Environmental Research Laboratory
Ada, Oklahoma 74820
ROBERT S. KERR ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
ADA, OKLAHOMA 74820
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DISCLAIMER
This report has been reviewed by the Robert S. Kerr Environmental
Research Laboratory, U.S. Environmental Protection Agency, and approved
for publication. Approval does not signify that the contents necessarily
reflect the views and policies of the U.S. Environmental Protection
Agency, nor does mention of trade names or commercial products constitute
endorsement or recommendation for use.
ii
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FOREWORD
The Environmental Protection Agency was established to coordinate
administration of the major Federal programs designed to protect the
quality of our environment.
An important part of the agency's effort involves the search for
information about environmental problems, management techniques and new
technologies through which optimum use of the nation's land and water
resources can be assured and the threat pollution poses to the welfare
of the American people can be minimized.
EPA's Office of Research and Development conducts this search
through a nationwide network of research facilities.
As one of these facilities, the Robert S. Kerr Environmental
Research Laboratory is responsible for the management of programs to:
(a) investigate the nature, transport, fate and management of pollutants
in groundwater; (b) develop and demonstrate methods for treating waste-
waters with soil and other natural systems; (c) develop and demonstrate
pollution control technologies for irrigation return flows, (d) develop
and demonstrate pollution control technologies for animal production
wastes; (e) develop and demonstrate technologies to prevent, control
or abate pollution from the petroleum refining and petrochemical in-
dustries, and (f) develop and demonstrate technologies to manage pol-
lution resulting from combinations of industrial wastewaters or indus-
trial/municipal wastewaters.
This report contributes to the knowledge essential if the EPA
is to meet the requirements of environmental laws that it establish
and enforce pollution control standards which are reasonable, cost
effective and provide adequate protection for the American people.
William C. Galegar, Director
Robert S. Kerr Environmental
Research Laboratory
iii
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ABSTRACT
The Environmental Protection Agency, Effluent Guidelines Division, has
prominently mentioned activated carbon adsorption as an attractive treatment
concept for satisfying wastewater treatment objectives established by P. L.
92-500, which calls for "best available treatment economically achievable"
(BATEA) by July 1, 1983. The recent interest in removal of toxic compounds
generated by the settlement agreement between EPA and the Natural Resources
Defense Council has also led to consideration of activated carbon adsorption
as a part of an industrial wastewater treatment scheme.
A review of the literature on activated carbon adsorption as a treatment
concept for petroleum refinery, petrochemical plant, and combined industrial-
municipal wastewaters is presented in this report. The principal time period
reviewed was 1963-1976. A total of 241 references are cited. These references
cover the various aspects of carbon adsorption and its application in the
treatment of industrial and municipal wastewaters. An additional 65 references
are listed in the Bibliography. These include literature from foreign sources,
literature not located during the original search, and literature published
after the original search was completed.
There is ample evidence in the literature reviewed to suggest that
activated carbon adsorption, using either granular or powdered carbon, should
be considered when evaluating treatment alternatives for industrial waste-
waters. Successful applications of this mode of treatment have been claimed
at numerous municipal, industrial, and combined municipal-industrial instal-
lations.
It must always be remembered, however, that there is no single wastewater
treatment^system which can be applied in all cases. There is enough variations
in the adsorption behavior of organic compounds so that adsorption may not
always provide a suitable removal process. Each industrial operation has its
own effluent characteristics and requirements. Availability of land, complexity
of operation, cost of treatment facilities, and variations in wastes all
combine to make each wastewater treatment system unique. The optimum treatment
system for any given operation can be designed only after careful study of
the entire problem and preliminary evaluation of several alternate designs.
iv
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CONTENTS
Foreword ill
Abstract , iv
I. Introduction 1
II. Application of Carbon Adsorption In Wastewater Treatment . 5
Combined Industrial-Municipal Wastes 5
Industrial Wastewater Treatment 10
Petroleum Refining 13
Organic Chemical 20
III. Design Considerations 27
Basic Principles and Concepts of the Carbon Adsorption
Process 27
Adsorption Models 39
Properties of Activated Carbon 45
Granular Carbon Systems 47
Powdered Carbon Systems . 58
IV. Discussion 64
References 66
Bibliography 86
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SECTION I
INTRODUCTION
Petroleum refineries and petrochemical plants are faced with the problem
of disposing of huge volumes of wastewaters from a wide variety of sources
(1,2). Characteristics of these wastewaters vary considerably. Both quality
and quantity may fluctuate significantly within a plant. Differences may be
even greater between different facilities.
As the petroleum industry strives to comply with provisions of the 1972
Federal Water Pollution Control Act (P.L. 92-500), an intensified interest in
use of the adsorption process to treat plant effluents or selected wastewater
streams within the plant complex has developed (2,3).
In recent years it has become apparent that conventional biological
treatment may not be the optimum solution to all waste treatment problems.
Although conventional biological processes are designed to remove organic
material from wastewater, neither the trickling filter nor the activated
sludge process effectively removes the last portion of this organic material
(4). This material, soluble or colloidal, resistent to biological degradation,
is often termed "refractory." Industrial wastewaters may contain either
refractory substances or materials deleterious to the performance of biologi-
cal systems (5). Furthermore, operating difficulties, sludge handling prob-
lems, and large land requirements are intrinsic to biological processes.
These factors have led to consideration of alternative treatment processes.
Of all advanced waste treatment processes, activated carbon adsorption is
getting the most attention by industry and appears to be applicable in refinery
and, to some extent, petrochemical operations (6). It is not expected that
any one process will be universally applicable. The varied nature of the
problem, the character of the wastes to be treated, the geographical and
logistics considerations, the extent of reuse required, etc., make this prob-
ability unlikely.
The use of activated carbon for removing dissolved organics from drinking
water and wastewater has long been known to be feasible. The increasing need
for highly polished effluents, necessary to accommodate stringent requirements
for both surface water quality and water reuse, has stimulated great interest
in carbon treatment systems. Adsorption makes it possible to remove compounds
that are not readily degradable by biological methods and gives excellent
removal of taste, color, and odor (7). Cooper and Hager (8) stated that
activated carbon is effective in removing refractory compounds. Furthermore,
carbon is effective in adsorbing organics below the concentration where bio-
logical treatment systems are efficient. Hager and Reilly (9) presented data
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showing that the adsorption process can produce a considerably higher removal
of organic contaminants than could be expected from the activated sludge
process alone. Kwok (10) observed that activated carbon exhibits a strong
adsorptive affinity and an appreciable adsorptive capacity for a wide variety
of organic compounds.
Adsorption on activated carbon, both granular and powdered, has been
investigated extensively in recent years because of the ability of carbon to
adsorb organic materials from wastewater (11). Shell et al. (12) reported
that soluble organic materials can generally be removed from wastewaters by
activated carbon adsorption; however, certain organic materials do not adsorb
on activated carbon, or do so slowly.
Coughlin (13) states that activated carbon is one of the most promising
solid adsorbents for removing organic compounds owing to its commercial
availability, high adsorption capacity, and affinity for a broad spectrum of
chemical compounds. Hager and Reilly (9) report that a wide variety of
industrial wastewaters are presently being treated by granular activated
carbon.
Swindell-Dressier Company in their manual on carbon adsorption prepared
for the Environmental Protection Agency Technology Transfer (14) report that
the use of activated carbon for removal of dissolved organics from water and
wastewater has long since been demonstrated to be feasible. In fact, it is
one of the most efficient organic removal processes available. Both the great
capability for organic removal and the overall flexibility of the carbon
adsorption process have encouraged its application in a variety of situations.
The process readily lends itself to integration into large, more comprehensive
waste treatment systems.
Loven (15) reported that the carbon process, usually in conjunction with
other processes, functions to remove gross oxygen-demanding and refractory
organics, color, and specific pollutants such as phenolic compounds and
chlorinated hydrocarbons. Loven feels that as the trend continues toward
increased limitation on discharges of hazardous substances, the need for
carbon treatment grows.
Physicochemical treatment has proved to be a viable method for achieving
improved effluent and receiving water quality. The nucleus of most physico-
chemical treatment plants is an activated carbon system (16). The authors
reported that in 1975 more than 20 municipalities in the U.S. were designing,
constructing, or operating physicochemical facilities for wastewater treat-
ment; the number of industrial facilities was an order of magnitude higher.
According to Weber and Crittenden (16), this number is expected to increase
rapidly in the next decade.
Sigsworth (17) has reported the use of active carbon in a number of
industrial applications for reclaiming solutions which otherwise might consti-
tute wastes. Burleson et al. (18) reported that activated carbon treatment of
some industrial effluents will remove up to 85% of the refractory organic mate-
rial expressed as total organic carbon (TOG). The concentration of organic
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substances measured as COD has been reduced to 5-15 mg/1 depending on previous
treatment, carbon dosage, and contact time in the adsorber (7). A broad-range
study of a number of industrial wastewaters has shown activated carbon adsorp-
tion to be applicable as a viable treatment alternative (19, 20).
Granular activated carbon treatment of water and wastewater in large
volume systems began in the U.S., England, and Germany in the early 1960's
(19). Hager purposed that the development of granular activated carbon capa-
ble of reactivation and reuse made adsorption an economic alternative for
removal of dissolved organic contamination from wastewater. He also stated
that a comprehensive list of all installations since the early 1960's is not
readily available, but the technology is being applied worldwide in a number
of European countries, Japan, and Africa.
Gulp and Gulp (21) stated that the use of granular activated carbon for
the adsorption of organic materials from wastewater has become firmly estab-
lished as a practical, reliable, and economical unit process for water pollution
control. Although no unit operation represents a universal cure for dissolved
organic removal, the adsorption process utilizing granular activated carbon as
a domestic tertiary system has been reported as leading all others in acceptance
(22, 23) Cover and Wood (24 reported that tertiary wastewater treatment with
granular activated carbon has been found to be an efficient and reliable process
for organics removal. The authors stated that granular carbon has proved itself
capable of removing such organic chemical compounds as phenol, polyols, herbi-
cides, pesticides, detergents, trinitrotoluene, dyes, and a host of pollutants
measured as BOD2, COD, TOG, color, and odor. Joyce et al. (25) have shown
that the organic contents of secondary sewage effluent can be reduced signifi-
cantly by granular activated carbon treatment in a column process under a wide
range of conditions. Davies and Kaplan (26) reported that granular activated
carbon systems can remove about 70% of the organics from biologically treated
effluents.
Davies and Kaplan also noted that powdered carbon systems have
faster adsorption rates than granular systems but that utilization had been
limited because of difficulties in handling and regeneration. Powdered
activated carbon was first reported as an aid to sewage treatment for overloaded
activated sludge plants in 1935 (27). Carbon was observed to improve sludge
compaction and filtration. Berg et al. (28) reported that adsorption on
powdered activated carbon has been proven to be a feasible method for removing
the bulk of dissolved organic materials from a municipal secondary effluent;
however, the cost of carbon is such that it must be regenerated and reused.
Operating under all the typical quantity and quality variations encoun-
tered in a full-scale sewage treatment situation, Burant and Vollstedt (29)
demonstrated that powdered activated carbon used in an activated sludge type
process is capable of producing an unusually high-quality effluent. Exten-
sive, full-scale field tests involving a variety of industrial and municipal
wastewaters have demonstrated that powdered activated carbon improves organics
removal, solids settling, color removal, and foam reduction when added to
activated sludge treatment processes (3, 30, 31).
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Effluent Guidelines development documents published by the Environmental
Protection Agency prominently mention carbon adsorption as an attractive
treatment concept for satisfying wastewater treatment objectives established
by P.L. 92-500, which calls for "best available treatment economically achiev-
able" (BAT) by July 1, 1983.
No one process, however, can be expected to be the ultimate solution for
all wastewater treatment problems. Carbon adsorption is no exception. It is
only one of several alternative treatment processes which should be considered
for use in a given situation. An optimum treatment system can be selected
only after careful evaluation of these alternatives.
The purpose of this report is to present a review of the available
literature on activated carbon adsorption as it pertains to the treatment of
petroleum refinery, petrochemical plant, and combined industrial-municipal
wastewaters. A review of the literature pertaining to adsorption kinetics
and process design criteria is also presented, since these aspects are crucial
for successful application of the adsorption process for treatment of any
wastewater.
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SECTION II
APPLICATION OF CARBON ADSORPTION
IN WASTEWATER TREATMENT
The technical literature includes numerous case histories regarding the
viability of adsorption for removal of many potential contaminants from waste-
waters. Full-scale granular activated carbon installations are currently
removing toxic or biological refractory contaminants from wastewaters, and
exhausted carbon is being reactivated for reuse (32).
Cover and Pieroni (33) presented a literature review of tertiary waste-
water treatment giving special attention to activated carbon adsorption.
Hager and Fulker (34) discussed the use of granular carbon in wastewater
treatment. Examples of industrial and municipal application are given. Ford
(35) prepared an excellent treatise presenting pertinent and current informa-
tion on activated carbon treatment of municipal and industrial wastewaters.
This comprehensive paper contains 33 references, 24 figures, and 30 tables.
Basic concepts are included as well as case histories with which the author is
familiar.
The effectiveness of tertiary treatment for domestic and some industrial
wastewaters has been demonstrated in many bench and pilot-plant studies and
amply confirmed by plant-scale operations (36). However, tertiary treatment
is not without limitations. Operation of the secondary stage can be adversely
affected by: (a) fluctuations in temperature and pH; (b) surges in flow of
incoming wastewater; (c) sudden inflow of toxic substances. These limitations
led to investigations of direct applications of advanced waste treatment
processes to raw wastewater usually after primary separation of visible sol-
ids. The author noted that cumulative data from bench and pilot-plant re-
search have been impressive.
COMBINED INDUSTRIAL-MUNICIPAL WASTES
Municipal wastewater treatment using activated carbon adsorption as both
a tertiary and direct physical-chemical application was summarized and dis-
cussed by Swindel-Dressler Company for the EPA Technology Transfer (14). Much
of the information presented is based on evaluation and operation of pilot,
demonstration, and full-scale plants.
Cohen (5) presented a comparison of carbon adsorption and activated
sludge treatment processes for removal of organic material from a primary
effluent. Results of the 6-week study conducted at Lebanon, Ohio, showed that
carbon adsorption was more efficient in the removal of organics than activated
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sludge. Cohen and Kugelman (37) described a system of physical-chemical
treatment using carbon adsorption, surveyed the performance of some pilot
plants, and gave cost estimates for various sized plants. Joyce et al. (25)
reported on a study to determine the economic practicality of using
granular activated carbon adsorption to treat secondary effluents. Not
only was the organic content significantly reduced, but the feasibility
of reactivation and reuse of granular carbon in wastewater treatment was
demonstrated.
Direct application of adsorption by granular activated carbon to a
primary effluent was examined and reported on by Weber et al. (38) and Hopkins
et al. (39). This concept was derived partly from observations by Joyce et
al. (25) and Parkhurst et al. (40) regarding the apparent difficulty of
removing final traces of organic materials from secondary effluents by treatment
with activated carbon as well as the relative economics of a two-stage versus
a three-stage system. In a 7200-gallon-per-day pilot-scale facility operated
for one year, these workers observed overall organic removals from the com-
bined industrial-municipal wastewater of 95-97%. These levels of removal
were maintained constantly despite variations in organic loadings and the
presence at times of toxic chemicals which would affect normal biological
processes adversely. The work described by Weber et al. (38) and Hopkins et
al. (39) represents a significant contribution to the literature on granular
carbon treatment systems. A substantial amount of well-documented data was
presented. The authors most importantly point out the limitations of such
systems—in particular, the difficulty in controlling hydrogen sulfide forma-
tion in the columns and the presence of non or slowly sorbable organic frac-
tions in the process effluent.
Hatheway (41) summarized results of studies undertaken to determine
methods of removing total organic carbon (TOC) from municipal and industrial
wastewaters with the discussion focusing on the physical-chemical process of
activated carbon adsorption. Granular activated carbon adsorption, when used
in conjunction with chemical precipitation and filtration, was found to
remove 95% or greater of TOC, BOD, COD, Total Phosphates, and Suspended
Solids and 78% of Total Nitrogen. Examples of plant variables, data, and
costs are also given.
The concept of applying direct physical-chemical processes utilizing
activated carbon adsorption for treatment of municipal wastewaters was pursued
by Weber (42). Operating results from several different pilot installations
were summarized.
Fuchs (43) reported that the first major full-scale physical-chemical
plant for treatment of municipal wastewaters had gone on stream and that many
more were nearing completion. The status of four of the plants was reviewed,
including the on-line plant in Rocky River, Ohio, which treats almost totally
domestic sewage. Construction of the Niagara Falls plant was reported to be
completed in late 1976. Influent to the plant will be approximately 75%
industrial wastewater high in phenol and chlorine. The toxic nature of this
waste eliminated consideration of biological treatment. A unique feature on
the final design for a 15.3-mgd plant in West Fitchburg, Massachusetts, is
separate primary clarifiers for industrial (two paper mills) and municipal
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wastewaters. Effluents from the separate clarifiers will be combined for
feed to 12 carbon adsorbers.
Parker and Callahan (44) reported on the proposed construction of two
advanced wastewater treatment plants at Fitchburg, Mass. The East Fitchburg
plant is a 12.4-mgd two-stage activated sludge facility with phosphate removal
and nitrification to treat domestic wastewaters, The West Fitchburg plant
is a 15.3-mgd activated carbon plant that will treat primarily industrial
(paper mill) wastewaters. Pilot studies showed that the activated carbon
process achieved better and more consistent removal of BOD, COD, and color
under all industrial loading conditions. Callahan and Pincince (45) reported
that the two plants had been constructed and were in operation. They examined
the performance based on available operating information.
Flynn and Thompson (46) reported on extensive pilot-plant studies at
Niagara Falls which demonstrated that physical-chemical treatment could be
used successfully for providing the equivalent of secondary treatment on
combined industrial-municipal wastewaters with variable characteristics.
Industrial surveys, interviews, and pilot-plant work were supplemented by
close liason with the industries. This industrial-municipal cooperation
benefited both parties, since technical problems of collection and treatment
were mutually solved and the costs were shared.
Joyce and Sukenik (47) reported that activated carbon in packed-bed
column contactors removed much of the organic matter in secondary effluents
from municipal sewage treatment plants. They noted, however, that for the
process to be economically attractive, carbon must be reactivated and reused.
Sulick (48) discussed various methods of tertiary treatment of sanitary
wastewaters with particular emphasis on activated carbon adsorption. He
proposed adsorption with granular activated carbon as the technological tool
applicable for removal of nondegradable organics from these wastewaters.
Furthermore, adsorption could remove either dissolved or suspended organics
and suspended or settleable inorganic solids.
In an additional paper, Sulick (49) reviewed the operation of three
municipal tertiary wastewater treatment plants. With a granular carbon ex-
haustion rate of approximately 250 pounds per million gallons of wastewater
treated, the Lake Tahoe plant has consistently produced an effluent with 10
mg/liter of COD and 1 mg/liter of BOD-. Water produced by the plant in
Orange County, California, meets drinking water requirements and is injected
into the Orange County ground-water.basin. The plant at St. Charles, Missouri,
has two interesting features which make it unique. First, the granular
carbon process is not preceded by filtration; the. secondary effluent is
applied directly to the carbon. The second unique design feature is that
chlorination takes place prior to carbon filtration for ammonia-nitrogen
removal.
The effectiveness of granular activated carbon for continuous treatment
of an unfiltered activated sludge effluent was demonstrated in Pomona, Cali-
fornia, (50). A 0.3-mgd, four-stage, fixed-bed, granular activated carbon
pilot plant was operated for a 4-year period. High-quality product water,
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characterized by a COD of 10 mg/liter was produced on a routine basis.
Regeneration of the carbon was shown to be a feasible process; and successful
backwashing of the first-stage carbon, which serves as a filter and an ad-
sorber, made pretreatment of the effluent unnecessary.
Zanitisch and Morand (51) reported on results of a study at a 120-mgd
activated sludge treatment plant receiving combined industrial-municipal
wastewater containing organic dyes. Applying the plant effluent to granular
activated carbon columns produced a colorless effluent with an average BOD<.
and suspended solids of 3 mg/liter each in the 61-day trial.
Barnes et al. (52) discussed in detail the operation and performance of
a pilot-scale waste treatment facility at Owasso, Michigan, which demonstrated
the feasibility of using chlorination followed by dechlorination with granular
activated carbon for removal of ammonia-nitrogen from a domestic wastewater
source. The pilot facility removed an average 85% of the ammonia-nitrogen
applied to the system. Barnes stated that complete removals could be obtained
if desired.
Bishop et al. (53) reported on the physical-chemical treatment of raw
wastewater in a 50,000 to 100,000-gpd pilot plant consisting of two-stage
lime precipitation with intermediate recarbonation, filtration, pH control,
ion exchange or breakpoint chlorination for nitrogen removal and carbon
adsorption. The complete system with breakpoint chlorination removed approxi-
mately 98% of phosphates, 94% of COD, and 86% of total nitrogen. Addition of
10 mg/liter of chlorine to influent filter controlled biological growth and
produced filter runs of greater than 50 hours. Chlorination oxidized ammonia
to nitrogen gas leaving a residual ammonia-nitrogen concentration of less
than 0.4 mg/liter. Without chlorine addition, heavy biological activity in
the columns resulted in hydrogen sulfide production with 2-3 mg/liter in the
effluent.
A carbon adsorption plant to recover p-cresol from a wastewater process
effluent stream by adsorption on granular activated carbon followed by chem-
ical regeneration was piloted, designed, and constructed to meet air pollution
standards (54). Additionally, it was noted that the unit had been in satis-
factory operation for more than a year, not only reducing emissions to ac-
ceptable levels but also returning a valuable product to the process, elimi-
nating odor problems, and reducing the discharge of pollutants in wastewaters
being discharged to the Metropolitan Sanitary District of Greater Chicago.
Design and installation of the system was based on laboratory and pilot-plant
work cited plus an economic evaluation indicating an after-tax return which
would pay back the installation cost in less than two years.
The Chipman Division of Rodia, Inc., has succeeded in stripping dangerous
levels of toxic phenolic compounds from wastewaters flowing from its herbicide
plant by installing an adsorption-filtration process using activated carbon
(53). The system was installed to pretreat wastewaters prior to introduction
into the municipal sewage treatment facility. The carbon system reduced the
phenolics concentration below the required limit of 1.0 mg/liter. Cost and
performance of the system outmatched several alternatives including a conven-
tional biological system, alterations to the then-existing chlorination system,
8
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substitution of bromine for chlorine, ion exchange, and oxidation using
ozone, peroxide, and permanganate. Total expenditures were $300,000 for a
plant that treats 150,000 gallons of wastewater per day. The 1971 operating
costs were estimated to be 35.6 cents per 1,000 gallons of treated water.
Shuckrow et al. (56) developed and pilot-tested a powdered activated
carbon treatment system for raw and combined sewage. Shuckrow and his co-
workers (57) successfully demonstrated this physical-chemical process utiliz-
ing powdered activated carbon on a 100,000-gpd scale at Albany, New York.
This project established the technical and economic feasibility of this proc-
ess for sewage treatment while removals in excess of 90% COD, 94% BOD,., and
99% suspended solids were consistently achieved. The workers noted that the
carbon dosage could be adjusted to effect the degree of organics removal
required. Additionally, a residual, nonsorbable fraction ranging from 10-20
mg/liter BOD and 20-50 mg/liter COD existed at times which could not be
removed at carbon dosages as high as 1,000 mg/liter.
Burns and Shell (58) discussed treatment of a municipal wastewater by
chemical coagulation-precipitation, powdered activated carbon adsorption, and
granular media filtration. The Powdered Activated Carbon-Physical Chemical
Treatment (PAC-PCT) system as depicted in a nominal 100-gpm pilot plant
produced a treated effluent similar to that expected for biological treatment
followed by tertiary treatment for phosphate removal. Solids contact units
were used for chemical treatment and carbon contacting. Spent carbon was
gravity thickened, vacuum filter dewatered, and thermally regenerated in a
fluidized-bed furnace. The process produced a highly clarified effluent with
COD values of 8-36 mg/liter for carbon dosages of 350-75 mg/liter.
Additional pilot-plant work with the above process was reported by Burns
et al. (59) and Wallace and Burns (60). During the 16-month study period, a
high-quality effluent was consistently produced. Soluble organic materials
were found to be removed by a combination of chemical coagulation, anaerobic
biological activity, and adsorption on powdered carbon. Using alum or ferric
chloride pretreatment and two-stage countercurrent carbon contacting (100
mg/liter), the PAC-PCT process consistently produced an effluent of 5 mg/liter
COD and suspended solids and 0.3 mg/liter phosphate. Powdered carbon regenera-
tion using a fluidized-bed furnace resulted in fixed carbon recoveries on the
average in excess of 90%. Thermally regenerated carbon did not exhibit full
recovery of adsorption capacity; but when reused in the PAC-PCT process, no
significant loss of treatment effectiveness was found.
Burant and Vollstadt (29) reported on a full-scale demonstration of
sewage treatment with powdered activated carbon and carbon regeneration with
wet air oxidation at Rothschild, Wisconsin. This study successfully demon-
strated that powdered activated carbon could be used in the conventional
activated sludge process to provide highly efficient wastewater treatment and
that spent carbon could be effectively regenerated for reuse by a partial wet
air oxidation process. The authors reported that powdered carbon acts as a
treating and weighting agent compacting a large number of organisms into a
small volume. Data generated reflected improved removals of BOD,- and sus-
pended solids. It was observed that an unusually high-quality product water
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could be produced at a cost only slightly higher than that of conventional
activated sludge treatment.
Powdered activated carbon was added to the aerator of the activated
sludge process of the Norfolk, Nebraska, water pollution control plant to
help solve operational problems (61). The plant had a 2.1-mgd average flow
containing 35-50% meat packing wastes. Despite the fact that flows were less
than 60% of design capacity, the plant effluent was characterized by high and
variable solids content. After the powdered carbon concentration reached 200
mg/liter, average effluent solids concentration decreased 67%, sludge volume
index decreased 33%, secondary sludge solids decreased 28%, and sludge bulking
was essentially eliminated. Furthermore, despite a 10% higher organic load,
effluent BOD,, concentrations were maintained at about 4 mg/liter. Other
noticeable improvements were reduction in effluent color, plant odors, and
aerator foam.
Adams (62, 63) discussed laboratory and full-scale tests using powdered
carbon in upgrading the performance of anaerobic digesters. In both laboratory,
reactors lignite carbon increased methane production fivefold. At a 2-mgd
activated sludge plant, a carbon dosage of 100 pounds per day significantly
reduced sludge volume by providing sites for anaerobic reactions to occur,
thus breaking down more volatile solids. Adams proposed that long-term use
of powdered carbon could reduce sludge handling costs by 60-70%. He also
noted that powdered carbon would adsorb toxic materials such as pesticides or
heavy metals that inhibit the anaerobic system.
Adding powdered activated carbon to anaerobic digesters at the Norris-
town, Pennsylvania, sewage treatment plant markedly improved operation of
that plant (64). In addition to adsorbing organics and reducing odors,
powdered carbon reduced solids settling and increased gas production.
INDUSTRIAL WASTEWATER TREATMENT
Activated carbon adsorption systems, widely used in the chemical process
industries for several decades, are now assuming an important role in cleaning
up plant effluents (36, 65). Sigworth (17) reviewed the uses of active
carbon in treatment of trade wastewaters, particularly for the reclamation of
valuable products. Myrick et al. (66) discussed the theory of adsorption,
with particular emphasis on the use of active carbon as an adsorbent for
polluting materials in trade wastewaters. Cohen (5) briefly reviewed the use
of physical-chemical processes to remove polycyclic hydrocarbons (carcinogens),
metals, pesticides, and viruses from wastewaters.
Adsorption studies have indicated that most of the EPA proposed dissolved
toxic organic chemicals may be removed from wastewaters by activated carbon
(32). Furthermore, other similar chemical contaminants, aromatic, nonpolar,
high-molecular-weight compounds, such as OSHA-defined carcinogens and other
chemicals under examination by EPA for inclusion on the toxic chemicals list,
are also predicted to be adsorbable from wastewater by activated carbon. Hager
(20) reported that data from adsorption isotherms on water samples contain-
ing aldrin, dieldrin, endrin, toxaphene, DDD, DDT, DDE, and PCB (toxic
10
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chemicals defined by EPA) revealed that the toxic chemicals in all samples were
reduced by more than 99%.
Rizzo (67, 68) discussed case histories of the successful use of acti-
vated carbon adsorption in treating industrial wastewaters. He proposed that
these case histories depict the versatility of granular carbons in respect to
removal of many different kinds of organic compounds.
Activated carbon adsorption has been demonstrated as an effective method
for improving the effluent quality of textile dying wastes (69). Porter (70)
conducted a pilot-plant study on a textile waste stream using chemical coagu-
lation carbon adsorption as the treatment system. Results indicated that
color and organic contaminants could be removed by both. Carbon adsorption
was also found to be suitable for regenerating the raw wastewaters for reuse
without chemical clarification. MacCrum and VanStone (23) discussed the
successful use of granular activated carbon in the treatment of wastewaters
from two textile mij.ls. In both instances treated wastewaters are reused in
normal plant operations.
»
Phipps (71) described the wastewater reclamation system at the Hollytex
Carpet Mills plant in Southampton, Pennsylvania. The key element in the
reclamation system is an adsorber containing 50,000 pounds of granular acti-
vated carbon. The adsorber reclaims 80% of the wastewater discharged from
the plant's carpet dying operation. The remaining 20% is discharged inter-
mittently to the municipal sewer system.
Full-scale field tests have shown that powdered activated carbon added
to activated sludge systems can resolve unique operating problems and upgrade
the quality of biological treatment (72). Several case histories are presented
which show that powdered carbon addition to activated sludge plants has
improved removals of organic pollutants, aided solids settling, improved
aeration efficiency by reducing foam, ,adsorbed color bodies, and protected
biological systems from shock or toxic loadings. The carbon has also been
found to level effluent quality at plants subject to periodic organic or
hydraulic overloads.
Georgia-Pacific developed a temporary treatment system for investigation
of the feasibility of using granular activated carbon to decontaminate im-
pounded resin plant wastewaters before discharge to a natural stream (73).
Operation of the temporary system, as well as bench-scale laboratory and
field trials upon which the system was developed, is discussed.
Lang et al. (74) conducted a 4-year pilot-plant program to investigate
the technical and economic feasibility of treating unbleached Kraft pulp and
paper mill effluent for reuse. It was concluded that water of reusable
quality could be provided by several combinations of treatment utilizing
activated carbon. Reusable water quality was defined in this study as 100 color
units and 100 mg/liters TOG. A microlime-carbon process that uses low dosages
of lime and clarification followed by carbon adsorption in down-flow granular
carbon beds was the most economical treatment scheme indicated by estimated
capital and operating costs. A novel continuous countercurrent process
(FACET) for carbon adsorption using stirred tanks of carbon slurries (75, 76)
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was shown to have considerable promise for reducing capital and operating
costs, and its development is being continued.
Kraft mill evaporator condensates have a relatively small volume with a
high polluting load. Treatment of these wastes by activated carbon before
they are mixed with other mill effluents was investigated by Hansen and
Burgess (77). In batch and continuous-flow tests using two types of carbon,
it was found that activated carbon could remove 75% of the organic material
from the Kraft mill condensates; this could not be greatly improved by vary-
ing load and contact periods. Extended contact periods in batch tests reduced
toxicity to the mussel, Mytilus edulis, by factors up to 17; but in column
tests, with shorter contact periods, toxicity was reduced only by factors of
2-5.
Kroop (78) concluded from laboratory tests that granular activated
carbon adsorption with thermal regeneration was the best treatment process to
use for treating large volumes of phenolic wastewaters generated by aircraft
paint stripping. Carbon adsorption would provide better reduction of phenolics
and COD and would be less expensive to construct and operate than other
methods. The phenolic concentration was reduced to less than 10 mg/liter in
the first 5 minutes of contact time, after whichvremoval proceeded at a
slower rate. A contact time of 75-100 minutes was required to reduce phenolics
to 3.1 mg/liter or less and a residual COD of less than 200 mg/liter. Kroop
also noted that the total chromium concentration was substantially reduced by
"plating out" on the activated carbon.
Based on information from actual field studies conducted at a steel
producing plant, VanStone (79) reported that the phenol content of coke
plant wastewaters could be reduced to less than 0.1 mg/liter by a treatment
system consisting of clarification, adsorption with granular activated carbon,
and catalytic oxidation. Additionally, it was noted that the process could
handle toxic materials such as phenolics and cyanides and would not be affec-
ted by fluctuations in the concentration of dissolved organics. .The economics
were reported to be favorable in most cases.
El-Dib et al. (80) investigated the efficiency of granular carbon columns
in removals of two carbamate pesticides, Sevin and Baygon, from polluted
waters. Adsorption was found to conform with Freundlich and Langmuir iso-
therms. The workers concluded that adsorption of total carbamates was depen-
dent on their chemical structure and branching of side chains.
Results of various stages of a demonstration pilot-plant study applying
the activated carbon process to treat actual rinse waters from a hard chrome
plating operation indicated that activated carbon adsorption for chromium
removal may have practical application in many small plating plants (81). A
demonstration pilot-plant study was conducted by Batelle-Columbus employing
actual rinse waters from a zinc cyanide plating operation (82). The pilot
system operated at 99% efficiency of cyanide removal. Cost estimates for the
process were given.
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Hager (19, 20) reported on a broad-range adsorption study on industrial
wastewater purification. A series of adsorption isotherms tests was run on
327 samples of wastewater representing 104 manufacturing operations to deter-
mine the effectiveness in reducing TOG concentrations. Selected removals of
color and phenol were also evaluated. Results of the tests were tabulated by
Standard Industrial Classification (SIC) for convenient reference. Hager
noted that results revealed only the degree of purification that might be
achieved and the approximate amount of carbon required to reach the treatment
objective. Examination of the survey data from the six major water-consuming
industries shows subtle differences in the degree of purification achievable
by carbon adsorption.
Subsequent to tests conducted on the initial 220 samples (19), several
plants elected to conduct pilot studies to determine system design data.
Following these pilot studies, adsorption systems for treatment of a wide
variety of contaminants were installed at 15 plants. Hager (20) presented
design parameters covering prominent features of the 15 systems. He noted that
in every case some form of pretreatment has to be used prior to carbon
adsorption. Flows for the 15 systems range from 6,000 to more than 1 million
gpd with carbon contact times ranging from 8 minutes to approximately 23 hours.
In some instances carbon adsorption has been installed to remove toxic chemicals
prior to biological treatment. These chemicals, toxic in nature, would have
a tendency to retard biological activity of the system. In two cases, adsorption
has been added after a biological system because the biological system could
not meet stated objectives. In both cases, the TOC has been reduced to 1.0
mg/liter or less. All systems rely on off-site reactivation where both rotary
kilns and multiple-hearth furnaces are employed. Hager observed that treatment
objectives have been met at all 15 installations.
PETROLEUM REFINING
Numerous comparative pilot granular studies and several full-scale in-
plant powdered carbon evaluations have been conducted by ICI United States,
Inc., to determine the effectiveness of carbon in solving refinery wastewater
treatment problems (3). Results of these studies have shown that refinery
wastewaters can be successfully treated with granular carbon in columns or
powdered carbon added to activated sludge systems.
Ford and Buerklin (83) reported on extensive pilot-plant evaluations
using activated carbon as both a total treatment process and as an effluent
polishing unit for treating petroleum refinery and petrochemical wastewaters.
The authors stated that, generally speaking, the total carbon system is not
satisfactory because of "excessive" leakage of organics. Limitations of using
carbon adsorption as a total treatment process are underscored. Synergism of
biological-carbon treatment was demonstrated. Practical realities and eco-
nomics of system operation were also discussed.
Carbon treatment has been proposed by Hale and Myers (84) as a relatively
efficient method for removing organics from refinery wastewaters. Data are
presented illustrating that an activated sludge treatment system reduced
saturates and aromatic organic materials to low levels in the final clarifier
effluent but was relatively ineffective in removing some of the polar type
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compounds. Carbon treatment removed polar type compounds as well as saturates
and aromatics. The authors note that none of the wastewater treatment systems
now in use at petroleum refineries will remove all of the organics from the
water.
Hale et al. (85) used adsorption isotherms to indicate the effectiveness
of activated carbon for treatment of specific petroleum refinery wastewaters
under controlled conditions. Eleven commercially available virgin carbons
were evaluated. Adsorption varied with the carbon being evaluated and the
wastewater sample. Since refinery wastewaters are composed of a rather com-
plex mixture of materials, some of which are resistent to biological degrada-
tion, these workers also conducted pilot-scale studies to determine if carbon
adsorption might be employed to further reduce the organic material in refinery
effluents. Two complete pilot plants were installed and operated simultane-
ously, one treating API separator effluent and the other treating final
clarifier effluent.
In addition to Hale et al., Hale and Myers (84), Short and Myers (86),
and Short et al. (87) have reported and discussed results of these pilot-scale
studies. Data presented and discussed by these workers are based on a specific
activated carbon for one refinery. Comparable results might not be achieved
using a different carbon or the same carbon at another refinery. Dual-media
filtration pretreatment was used for oil and suspended solids removals to
prevent premature plugging of the columns. Both biological treatment and
carbon treatment were found to produce a significant reduction in all organic
parameters; e.g., BOD, COD, TOC, phenols, and oil. Biological treatment
provided better BOD reduction than activated carbon treatment. COD reductions
were equivalent for the two systems, while TOC removals were greater for
carbon treatment. Best results of reduction of all parameters were obtained
with biological treatment followed by carbon adsorption. Concentrations of
the metals, chromium, copper, iron, and zinc were significantly reduced by
carbon treatment. Sulfide buildups, apparently a byproduct of anaerobic
activity, were observed to be a problem in the carbon columns.
Short et al. (87) noted that although both biological and activated
carbon treatment systems exhibited a high capacity for removal of phenols,
biological systems appear to become easily "upset" with changes in phenol
concentrations. Furthermore, activated carbon systems can provide excellent
treatment capability for phenol removal if the hydrogen ion concentration of
the waste stream is controlled. It is particularly important to avoid caustic
conditions in the carbon columns. Other workers have reported that activated
carbon treatment of phenolic wastewaters seems to be competitive with biological
treatment (51, 88).
Results of laboratory tests conducted on primary and secondary waste-
waters from 10 refineries selected by the API committee on chemical wastes
indicated that activated carbon could be an economically attractive means for
improving the quality of refinery effluents (89, 90, 91). Paulson reported
that granular carbon treatment could produce an effluent containing less than
20 mg/liter BOD and less than 10 units of Recognition Odor Number (RON) from
either primary or secondary effluents. Those organic compounds contributing
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to odor were found to be preferentially adsorbed relative to the total mixture
of organic contaminants. Pretreatment would be required for the majority of
primary effluents and some secondary effluents to effect suspended solids
reduction below 60 mg/liter and oil below 20 mg/liter. Paulson concluded that
granular activated carbon treatment could be used in lieu of biological treat-
ment or following biological treatment to improve the quality of refinery
effluent wastes. Capital cost estimates and direct operating costs based on
BOD removal are presented for 1-, 5-, and 10-mgd plants. Essentially, capital
costs estimates will vary directly with the total wastewater volume to be
treated.
Burleson et al. (18) presented isotherm data for two refinery wastes
using four different carbons and one petrochemical waste using two different
carbons. The data indicated that up to 85% of the refractory organic materials
expressed as TOG could be removed from secondary effluents of refineries and
petrochemical plants by activated carbon treatment.
Carbon adsorption capacities in terms of pounds of COD removed per pound
of carbon have been found to range from less than 0.1 to 0.55 in the petroleum
refining industry and from 0.2 to 0.4 in the petrochemical industry (35).
These are lower than reported carbon capacities for municipal wastewaters,
emphasizing the inaccuracies which can occur by extrapolating results from the
treatment of one wastewater and using them as the basis for predicting another.
Examination of the data presented by Hager (19, 20) reveals that 15 of 18
samples in the petroleum SIC category showed greater than 90% TOC reduction.
In the chemical SIC category, 103 of 177 samples showed greater than 90% TOC
reduction, but 51 samples showed less than 85% reduction.
Stensel et al. (92) described results of a pilot-plant study at the
Atlantic Richfield (ARCO) refinery in Carson, California, to determine the
most feasible method of treating combined storm-water runoff and refinery
process water to achieve imposed COD limitations. It was concluded that a
biological system alone could not produce the low effluent COD concentration
required. Activated carbon treatment, either after biological treatment or
alone, would produce the desired effluent quality.
Mehta (93) presented a chronological description of the evaluation of the
wastewater treatment system installed at the ARCO refinery in Carson, Cali-
fornia. Actual plant cost data are shown. Mehta reports that carbon adsorp-
tion is an effective means of treating refinery effluents to achieve high
purity levels relative to color, odor, and COD. He also noted, however, that
economics would vary greatly with each refinery, depending on specific require-
ments and feed quality.
Prosche et al. (94) described and discussed the design and operation of
the carbon adsorption process for treatment of intermittent wastewater from
the ARCO refinery in Carson, California. The process proved easy to maintain,
easy and quick to start up, simple to shut down and leave in the standby
stage, and very reliable in its performance. For the unique intermittent type
of operation required at this refinery, the authors believe that the carbon
adsorption process proved successful; however, they do not believe that this
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conclusion would necessarily be the same for continuous operation requirements
or some other unique situation. Problems with varying feed COD and algal
growth were experienced. It was noted that these are real problems that exist
when treating refinery wastewaters.
A thorough and complete discussion of the ARCO project is presented in
the final EPA report prepared by Loop (1). This report describes the carbon
treatment system and its first two years of operation. During the project
period, the plant processed 172 million gallons of water, removed 408,000
pounds of COD, and regenerated 1,644,000 pounds of carbon. Carbon was ex-
hausted at the rate of 9.5 pounds per 1,000 gallons of water processed. At an
average feed COD concentration of 250 mg/liter and an average effluent concen-
tration of 50 mg/liter, the carbon was loaded to an average of 0.26 pound per
pound of carbon. Following solution of initial start-up problems, the system
was operated at a cost of 40 cents per 1,000 gallons of water treated, or 18
cents per pound of COD removed. The plant demonstrated excellent reliability
and the ability to start up or shut down without delay or difficulty. This
gives the process a distinct advantage over biological systems for use in
handling intermittent runoff. Operation of this system has now been discon-
tinued because of the change of treatment requirements imposed by the Los
Angeles Regional Water Quality Control Board.
Although the ARCO project demonstrated that activated carbon can be used
on a commercial scale to reduce COD concentrations of some petroleum refinery
wastewaters, Loop recommended several areas which need further investigation:
1. Further determination of quantities and types of COD materials that
do and do not adsorb an activated carbon.
2. Feasibility of pretreatment to reduce the load on carbon.
3. Optimum number of stages in an adsorption process with controlled
feed concentrations.
Peoples et al. (88) reported on a pilot investigation to determine the
most suitable method of treating process wastewater from an oil refinery. The
workers felt that results clearly demonstrated the applicability of activated
carbon treatment, even with widely varying effluent concentrations. In addi-
tion, Krishman et al. (95) reported that this pilot investigation demonstrated
that high-rate, deep-bed sand filtration could remove significant amounts of
oil and suspended solids from the API separator effluent and produce an
effluent suitable for direct application to activated carbon. Activated
carbon could reduce the remaining organics to a very low level; thus, a
combination sand filter-activated carbon treatment system could provide a high
degree of treatment for refinery wastewaters. The system also would require
low land area and costs when compared to the conventional activated sludge
system.
McCrodden (96) reported on operation of a 2.2-mgd filtration-carbon
adsorption wastewater treatment plant placed on line at the 105,000 BPD Marcus
Hook refinery of BP Oil Corporation in March 1973. Data from bench-scale single-
media filtration tests, adsorption isotherm tests, and operation of a sand
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filtration pilot plant followed in line by a dynamic carbon column test were
used as the basis for process design. Final design consisted of three parallel
dual-media filters for oil and suspended solids removal, an intermediate basin
to control flow surges and equalize influent loads, three parallel activated
carbon adsorbers, and a regeneration system with a multiple-hearth furnace for
thermally reactivating spent carbon. The plant demonstrated varying perform-
ance over the first 12-month period as a function of individual operating
characteristics. During the first year of operation, the plant demonstrated
76, 89, 42, 38, and 39% reduction for suspended solids, oil, first-stage
ultimate oxygen demand, COD, and phenol, respectively. An increase in sulfide
concentration was indicative of anaerobic digestion in the carbon columns.
Carbon losses were 10% per regeneration cycle.
McCrodden (96) and DeJohn and Adams (3) reported that after 18 months of
plant operation, the adsorptive performance of regenerated virgin carbon had
fallen off by about 37% for phenol removal and 82% for COD removal. In an
effort to maintain purification levels obtained with virgin carbon, the carbon
regeneration rate was increased from the design figure of 125 to 250 pounds per
hour; the carbon dosage was increased from the design figure of 0.86 to 2.7
pounds of carbon per 1,000 gallons throughput. The regeneration rate was 5,000
pounds per day, reflecting a carbon dosage of 2.3 pounds per 1,000 gallons
throughput and representing an approximate 14% increase in carbon makeup alone.
The major reason for decreased performance of the system and increased operating
costs was that the system design and operating costs were estimated from data
developed in studies using virgin bituminous coal carbon. This particular
wastewater stream contains predominately small molecules. Since micropores are
lost as carbon is reactivated, the carbon exhaustion rate was much faster than
anticipated. Therefore, carbon had to be regenerated more often, causing an
increase in the carbon usage rate which simultaneously increased operating
costs. Subsequently, the company has reviewed operational data and process
performance and decided to install a biological oxidation secondary treatment
system (97). The new system is planned to be placed in operation in 1981.
DeJohn (98) and DeJohn and Adams (3) reported on the comparative ad-
sorption performance of lignite and bituminous coal carbons using oil refinery
wastewaters. Studies were conducted on the total refinery effluent at two
East Coast refineries and at a selected sulfonate waste stream at a West Coast
refinery. Although effluent COD concentrations from columns containing both
types of carbon were equal, carbon dosage (pounds carbon per 1,000 gallons
wastewater) was greater for bituminous coal; conversely, the loading on carbon
(pounds COD remover per pound carbon) was greater for lignite coal.
Results of these studies indicate that when treating a total refinery
effluent or an effluent from a selected refinery wastewater stream where the
pollutants are relatively large molecules, breakthrough is due to transitional
pores becoming exhausted. Lignite carbon, having a higher transitional pore
surface area and pore volume, should be expected to perform better. For
treatment of a selected refinery wastewater stream or total refinery effluent
which contains predominantly small molecules, virgin bituminous carbon should
perform better because of its higher surface area in the small pore range.
However, after a number of regenerations, properties of the two carbons would
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tend to converge, and both should perform equally in removing small molecules.
On a long service basis, however, economics may still favor use of lignite
carbon.
Huang and Hardie (99) conducted a research program to test the applica-
bility of using physical-chemical processes employing.activated carbon ad-
sorption for treatment of refinery wastewaters. The study was conducted on
wastewaters from the American Oil Company, Wood River, Illinois, refinery.
Effluents produced from both fixed-bed and expanded-bed adsorbers were con-
sistently of high water quality. Combined use of chemical clarification-
coagulation and carbon adsorption was able to reclaim the water successfully
since the effluent had a TOC of less than 3 mg/liter. Since physical-chemical
treatment of wastewater does not encounter any of the complexities often
associated with biological processes, use of this approach for future water
reclamation seems promising. The potential value of physical-chemical treat-
ment employing activated carbon adsorption must be considered greatest when
applied to nondegradable and/or nutrient deficient wastewaters for which
biological treatment often cannot be successfully or economically used. It
must be understood, however, that all complex wastewaters contain some organic
materials which cannot be adsorbed effectively.
Huang et al. (100) summarized results from a research study designed to
investigate effectiveness of carbon adsorption for treatment of three selected
industrial wastewaters: a refinery waste, a high-strength acid waste, and a
pharmaceutical waste. The extent of organic removal not only varied from one
industrial waste to another but also from one carbon to another for the same
kind of waste.
In the treatment of the refinery waste, the carbon bed was very effective
in adsorbing organic pollutants. Neither ammonia nor organic nitrogen was
removed by the carbon adsorber. Huang and his coworkers noted that, although
a very high percentage of organic removal can be achieved by carbon treatment,
there are always certain amounts of residual organic materials which resist
carbon adsorption. This was evidenced by the fact that some variable amounts
of COD or TOC were invariably present in the produced effluent. The nature
of these leaked organics was not examined.
Refinery and petrochemical wastes can be treated biologically by the
activated sludge process; however, conventional systems often experience
many effluent quality and operating problems: (1) Oil that is not removed
in the API separators can pass through the aerator and clarifier and will be
measured as TOC or COD. (2) Oil can also entrap solids, prevent them from
settling, and lead to high effluent suspended solids. (3) Surface active
agents often cause foaming in the aerator and on the receiving stream. (4)
Shock toxic loads can kill the active biomass. (5) Oil characteristics of
waste sludges can make them difficult to dewater and handle. DeJohn and
Adams (3) noted that addition of powdered activated carbon to activated sludge
systems has proved to be a satisfactory method for solving such problems. In
addition to improved operation of the activated sludge system, the use of
powdered activated carbon also may result in real operating cost savings.
It was reported that four refinery and/or chemical plants have evaluated the
use of powdered activated carbons in full-scale activated sludge systems. In
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all systems, addition of powdered carbon improved organic removals, aided
solids settling and sludge handling, provided protection from toxic or shock
loadings, and monitored nearly colorless effluents with a more consistent
effluent quality.
In an effort to expand the performance of the existing biological treat-
ment facilities, full-scale trials utilizing powdered activated carbon were
conducted at the Sun Oil Company, Corpus Christi, Texas, refinery (101). The
main objective of adding the powdered carbon was to reduce the effluent sus-
pended solids loading for compliance with 1977 NPDES and State permit condi-
tions. Although powdered carbon addition could not guarantee compliance with
1977 suspended solids criteria, improvements in performance of the existing
system were significant for BOD and COD removals as well as suspended solids.
Treatment costs ranged from 1.7-4.3 cents per 1,000 gallons, depending on
influent flow and quality. Compiled data showed reduction in final effluent
loadings of up to 56% for suspended solids, 76% for BOD, and 36% for COD.
Other improvements noted were more uniform effluent quality, a clearer efflu-
ent, elimination of foam in the aeration system, more consistent sludge wast-
ing at two-thirds the volume, and reduced chances of biological upsets. The
system was maintained at the carbon operating level by batch addition of about
100 pounds per day of carbon. The carbon used in these trials had a bulk
density of 44 pounds per cubic foot, which is an important factor when improved
settleability is the primary objective of carbon addition to the system.
Stenstrom and Grieves (102) and Grieves et al. (103) reported on the
evaluation of an alternate process to granular activated carbon treatment of
refinery activated sludge 'effluent. The proposed process would be used by
refineries to meet their BAT effluent quality goals in 1983. The new treat-
ment alternative involves using powdered activated carbon to enhance the perform-
ance of the activated sludge section of the BPT treatment sequence. The degree
of enhancement was found to be considerably affected by the physical character-
istics—in particular, surface area—of the activated carbon used. In addi-
tion to commercially available carbons, an experimental carbon with an excep-
tionally high surface area was tested. The experimental carbon was found to
be several times more effective than the best commercial grade—that is, an
equivalent effluent quality can be attained at a lower carbon addition rate
using the new carbon. The advantage of the high surface area carbon was no re
more pronounced at high sludge ages or mixed-liquor suspended solids (MLSS)
concentrations.
These workers concluded that the powdered activated carbon-activated
sludge process is a promising technique for meeting BAT effluent requirements
for oil refineries. Data collected during this study generally met or ex-
ceeded the target effluent quality. The process was reported to offer
significant cost incentives over add-on granular activated carbon columns
while being much easier to operate. It was also found that the use of high
sludge age is a viable method of maintaining process performance while lower-
ing carbon addition requirements. The increased sludge age, resulting in
higher MLSS concentrations, reduces carbon make-up requirements and size of
regeneration facilities. Process technology covered in these papers formed the
basis for a number of U.S. and foreign patent applications.
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ORGANIC CHEMICAL
To investigate the possibility of using activated carbon technology on
effluents from biological treatment plants treating organic chemical waste-
waters, a series of carbon isotherm tests was run at standard conditions using
a contact time of 30 minutes (104). Results of these tests are presented.
Inspection of the specific data shows that carbon adsorption has varying
degrees of amenability with regard to cost effective wastewater treatment;
however, the data do indicate that specific wastewaters are readily treatable
using activated carbon. Results of a plant survey program of six activated
carbon plants treating raw wastewaters are also presented. The most interest-
ing fact about the data generated is that, while domestic wastewater treatment
experience indicates that efficient treatment is provided with contact times
between 10 and 50 minutes, the design contact times at the six plants surveyed
varied between 22 and 660 minutes (calculated on an empty column basis).
These higher contact times are required because of the much higher raw waste
loads generated by industry.
The technical literature describes numerous laboratory studies regarding
the viability of adsorption for the removal of organic compounds from waste-
water. A list of organics amenable to carbon adsorption has been presented by
Rizzo (22). Rizzo noted that carbon adsorption is favored by higher molecular
weight, nonpolarity, and limited solubility of the contaminants.
Union Carbide has studied the application of activated carbon adsorption
as tertiary treatment to follow biological (secondary) treatment of waste-
waters from their integrated, multiproduct petrochemical facilities for several
years. Several of these studies, along with studies on the amenability to
adsorption of specific petrochemicals, have been reported in the open litera-
ture (105, 106, 107, 108).
These studies have revealed that, in contrast to experience with domestic
secondary effluents, activated carbon treatment of petrochemical wastewaters
may not produce an effluent essentially free of organics. Even assuming high-
quality biological effluents (BPT) and a long contact time in the adsorbers,
activated carbon removes only 50-70% of the COD remaining in effluents from
typical petrochemical plants. As the quality of the activated sludge effluent
improved, the percent of additional organic removal in a tertiary adsorber
increased. Increasing contact time increased the percent of organic removal
and the adsorber effluent quality. In all cases, however, there appeared to
be a limiting percentage organic removal above which no additional removal
could be achieved, even at truly massive carbon dosages.
Giusti et al. (105) screened 93 compounds from 10 functional groups com-
monly found in petrochemical waste streams. Single dosage tests were used to
evaluate their relative amenability to activated carbon adsorption. In addi-
tion to the single dosage tests, isotherm studies with four commercially
available carbons were conducted on representative compounds from five of the
functional groups. Carbon loadings for each compound studied were determined
on the basis of grams of sorbate removed per gram of carbon. A discussion of
results for each of the functional groups is presented.
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Some conclusions drawn by Giusti and his co-workers were:
1. As molecular weight increases and polarity, solubility, and branching
decrease, the degree at which pure components are adsorbed by activated carbon
increases somewhat predictably.
2. Of the classes of compounds studied, aromatics exhibited the greatest
amenability to activated carbon adsorption because of their relatively low
solubilities in aqueous solution and bonding to the aromatic surface of the
activated carbon.
3. Functionality was seen to have a substantial effect which was inter-
related with solubility and polarity. For the straight chain compounds,
relative amenabilities to carbon adsorption for compounds of less than four
carbons were: undissociated organic ac.ids>aldehydes>esters>ketones>
alcohols>glycols. For compounds above four carbons, alcohols moved ahead of
esters.
Data were presented by Lawson (106) on the relative amenability to adsorp-
tion of 23 typical petrochemical wastewater constituents. Under the test
conditions (5 grams/liter powdered carbon dosage), benzene and nitrobenzene
were the most amenable compounds to adsorption (95-96%) while ethylene
glycol and monethanolamine were the least amenable (7%).
The problem in applying adsorption to petrochemical wastewaters lies in
the limited amenability of many common low molecular weight oxygenated petro-
chemicals to adsorption on activated carbon (107). These difficult-to-adsorb
compounds are produced in high volume in the industry. They often persist in
effluents from biological treatment processes because of their high initial
concentration and the seemingly inherent problems in efficiently treating
petrochemical wastes in biological systems. Activated carbon adsorption can
be an excellent upgrading technique for dilute wastewaters contaminated with
large, nonpolar organics. It is not a panacea, however. Upgrading multi-
component petrochemical effluents to reuse quality likely would require a
multifaceted attack:
1. Reduction of organic pollutants discharged at the production
unit source through process changes.
2. Application of best practical biological treatment techniques
to the combined wastewaters.
3. Application of activated carbon adsorption to waste streams
containing organics which have a high adsorptive capacity or where small
wastewater volume can help offset a limited adsorptive capacity.
4. Application of yet-undeveloped techniques for final polishing to a
low residual organic level.
5. Application of techniques such as ion exchange or reverse osmosis to
remove dissolved inorganic salts.
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Giusti et al. (105) reported that, in a typical flow scheme for a facility
treating petrochemical wastewaters, carbon treatment could be applied at two
positions. A tertiary treatment stage for removing refractory organics follow-
ing conventional secondary biological treatment is usually visualized. How-
ever, because of the high concentration of impurities found in various individual
waste streams contributing to the discharge, some of the streams might be
treated more effectively at their sources. This latter approach could be
beneficial, particularly in handling process unit wastes that produce shock
loads to a treatment plant, produce materials that inhibit biological activ-
ity, or produce economically recoverable materials.
Lawson and Hovious (108) presented an excellent summation of the studies
conducted by Union Carbide. Some noteworthy conclusions were:
1. In pure component studies, specific organic chemicals were shown to
differ widely in their amenability to adsorption, depending on molecular
weight, structure, polarity, and solubility.
2. The relative ease of adsorption of different functional groups can
vary strongly with pH, depending on the chemical nature of the adsorbates. An
optimum pH cannot be predicted for a multicomponent wastewater of unknown or
varying composition.
3. While pure component data could be used to predict binary adsorption
capacity in isotherms fairly closely, a four-component mixture isotherm showed
only about 60 percent of the adsorptive capacity predicted.
4. While isotherm capacities were somewhat extrapolatible to continuous
column behavior in pure component adsorption and simple mixture studies, such
extrapolations have not proved to be possible with complex mixtures, particu-
larly biotreated effluents.
5. The key parameters of interest in real wastewater treatment situa-
tions, percentage organic removal achievable, and water volume treated per
pound of carbon before breakthrough cannot be predicted from isotherm tests.
6. The physical differences between an equilibrium adsorption situation
in a powdered carbon isotherm and the dynamic, multicomponent interactions in
a continuous granular carbon bed are too great to permit prediction of column
performance from isotherms or pure component data.
Examples of refractory materials which are difficult or impossible to re-
move by conventional biological processes but are removable by adsorption
techniques include benzene sulfonate and heterocyclic organic compounds (109).
Phenols, nitriles, and substituted organics can also be adsorbed from waste-
waters when they are present in low concentrations.
Geiblcr (110) reported that benzene hexachloride and other chlorinated
aromatics had been successfully removed from pesticide manufacturing plant
effluents by using carbon.
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Treatment of a high-strength industrial chemical wastewater which had a
COD of approximately 3,000 mg/1 and a pH of 3.0 was attempted by activated
carbon adsorption to evaluate feasibility of yielding effluents of reusable
qualities (111). Experimental methods included both batch and column studies.
The COD of the test solutions was reduced to approximately 300 mg/1, which
indicated that activated carbon adsorption was very efficient in removing COD
from the chemical waste. Initial pH adjustments to 7.0 did not improve COD
removal efficiency. Removal of COD from the test solutions was generally
accompanied by a corresponding degree of color removal from the wastewater.
Hardwicke Chemical Company, Elgin, South Carolina, manufacturer of a
variety of specialty organic chemicals, conducted an evaluation of different
wastewater treatment processes that might reduce their average BOD,, of 3,000
mg/liter to less than 62 pounds per day to meet discharge requirements imposed
by the South Carolina Pollution Control Authority (112). Laboratory and
pilot-plant studies demonstrated both the technical and economical feasibility
of removal of organics by activated carbon adsorption. Following the favor-
able pilot studies, an adsorption system was installed consisting of two
upflow carbon adsorbers, each containing 20,000 pounds of granular carbon.
The author reports that the system has consistently met the 62-pound BOD-
objective since going on stream. The entire system is owned and maintained by
a supplier who also performs analyses of weekly samples and provides monthly
reports. Hardwicke pays a monthly service charge for the entire system.
Data presented by Schimmel and Griffen (113) indicate that the adsorption
process is capable of significantly upgrading the quality of liquid industrial
wastes. Results of an investigation of alternate treatment systems for a
complex industrial wastewater containing phenols is discussed. The waste
effluent resulting from operation of the Rechhold Chemicals, Inc. (RCI), plant
at Tuscaloosa, Alabama, may be considered as typical of that from multiproduct
chemical plants. The aqueous waste results from both batch and continuous
operations contain both organic and inorganic wastes and vary in both
composition and concentration. The report describes selection of carbon
adsorption as the most dependable process for secondary treatment of this
waste and details the evaluation of a full-scale installation treating some
500,000 gpd of wastewater.
A biological oxidation process applied to the RCI wastes resulted in a
significant reduction in BOD5 loading, substantially complete removal of
phenols, and an adequate reduction in COD loading. However, lack of reliabil-
ity ascribed to the biological process led to development of an activated
carbon process that, when placed in commercial operation, resulted in average
removals of 90, 75, and 99% of the COD, BOD5, and phenol loads from the RCI
process wastewaters. The authors concluded that, although biological oxida-
tion should not be overlooked for treating industrial wastewaters, its useful-
ness is limited with respect to bacterial poisons such as phenol and by
ambient temperature changes that result in variable biological activity.
Also, biological oxidation is more easily adapted to wastewaters from contin-
uous type, as opposed to batchwise, chemical processes. The carbon adsorption
process should be considered if the wastewater to be treated is variable in
composition or concentration. The process will handle many refractory organic
compounds that contribute to the COD load but are not susceptible to biologi-
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cal oxidation. Adsorption should not be considered if a substantial portion
of the pollutional load consists of low-molecular-weight organics. Organic
compounds having a molecular weight below about 50 are poorly adsorbed, if at
all, by activated carbon.
Neville Chemical Company, Neville Island, Pa., manufacturer of a variety
of organic chemicals and synthetic resins, had to upgrade wastewater treatment
to meet Ohio River discharge requirements. After considering economic and
technical aspects of various alternatives, a treatment system was selected
consisting of API separators followed by flow equalization, pH adjustment,
multimedia filtration, and granular carbon adsorption (114). The waste treat-
ment system was placed in operation on March 1, 1974. During the first year
of operation, the effluent from the system contained less than 0.1 mg/liter of
phenol, less than 250 mg/liter of BOD,., and was free of oil and suspended
solids. The complete system, including installation, operation, and mainte-
nance, is contracted from a supplier for a monthly service fee.
Brunotts et al. (115, 116) described techniques used to develop a physical-
chemical treatment system capable of treating a complex chemical plant waste-
water. Operating data compiled from operation of the full-scale treatment
system at the Stepan Chemical Company plant located in Fieldsboro, New Jersey,
are compared to data predicted by laboratory and pilot-plant studies. After
10 months of operation, the plant achieved the goals that were set when the
plant was designed. This illustrates that, given the proper design criteria
and a clearly defined set of objectives, a physical-chemical system employing
granular carbon adsorption can be constructed to meet the desired goals.
Wiseman and Bawden (117) described an industrial wastewater treatment
plant using activated carbon adsorption to process wastewater from a synthe-
sized terpene plant that uses isoprene as the raw material. Overall perfor-
mance of the total system is indicated by a final discharge representing over
90% removal of BOD, TOC, and COD.
Dawson et al. (118) conducted a study designed to determine the effective-
ness of activated carbon for removing fishery chemicals from water and to
evaluate some physical and chemical influences on the adsorptive capacity.
The study demonstrated that activated carbon was effective in removing fish
toxicants and anesthetics from water solutions. All seven chemicals tested
were adsorbed by carbon to some extent, with their absorptive capacities
ranging from 0.1-64.0 milligrams per gram of carbon. While each chemical
probably has a different rate of adsorption and some may not be adsorbed at all,
factors such as the concentration of other organic constituents in the water and
variations in activated carbons also affect adsorption. For these reasons,
the investigator should evaluate the efficiency of adsorption of a particular
system in situ to accurately establish the adsorptive capacities.
Zeitoun and Mcllhenny (119) studied several alternative processes for the
treatment of the saline wastewater resulting from the production of polyhydric
organic compounds by the chlorohydrin process. Carbon adsorption was found
not to be feasible because of the low capacity of the activated carbons for
the glycols. The capacity is further reduced by the competitive adsorption of
the chlorinated organics present in the wastewaters.
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Zeitoun et al. (120) concluded that carbon adsorption of relatively high
concentrations of organics from petrochemical wastewaters is technically
achievable but economically feasible only if the organics can be recovered and
recycled to the production facility. In addition, it was concluded that low
concentrations of organics that are highly adsorbable on activated carbon
cannot be removed from petrochemical wastewaters at a reasonable cost because
of the cost of regenerating the carbon thermally. These workers recommended
that in-place regeneration of activated carbon be investigated for each par-
ticular wastewater in order to reduce the cost of carbon adsorption treatment.
A 100-gpm pilot plant was constructed and operated for 1 year to demon-
strate the feasibility to remove and recover phenol and acetic acid from an
18% sodium chloride brine by adsorption on fixed beds of activated carbon
(121). Regeneration of the carbon was accomplished by desorption with dilute
sodium hydroxide. The phenol desorbed was recycled to the phenol manufacturing
plant, while the acetate regenerant was processed to underground disposal
wells. More than 23 million gallons of brine were purified. Fourteen cycles
of phenol adsorption and regeneration and 105 cycles of acetic acid adsorption
and regeneration were completed with no significant deterioration of carbon
performance. Process results are discussed in detail along with costs.
The B. F. Goodrich Chemical Company performed extensive laboratory and
pilotplant investigations in order to competently design an industrial waste-
water treatment system for a proposed polyvinyl chloride manufacturing facility
in Salem County, New Jersey (122). Activated carbon adsorption was determined
to be impractical for this waste. Poor adsorption capacities were attributed
to the presence of water soluble long-chain organic soaps contained in the
waste stream.
Frohlich et al. (123) reported on results of a side-by-side pilot-scale
comparison of a conventional activated sludge system and a biophysical system
(activated sludge and powdered activated carbon) on a high-strength industrial
wastewater from a Pharmaceuticals organics chemicals producer. Both systems
appeared to have acceptable stability; however, performance of the biophysical
system was superior in terms of removing BOD, COD, color, odor, and nitrogen.
There was no observable long-term degradation of the biophysical system after
over 120 days of operation. Frohlich and his coworkers claimed the following
advantages for the biophysical system:
1. The weighting effect of the carbon makes possible the ability to
carry the active biomass at levels two to three times higher than activated
sludge and thus reduce the aeration basin size and hydraulic detention time.
2. The massive amounts of carbon present in the aeration basin tend to
serve as an "organic sink" for shock loads of toxic and refractory materials.
3. Oxygen transfer is improved—probably as a result of adsorption—
desorption from the activated carbon.
4. A larger portion of marginally degradable organics can be biologically
assimilated due to the long sludge residence time, enabling the carbon to
carry a higher load of truly refractory material.
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5. Nitrification is easily achieved with the long residence time.
6. Odor, color, and foaming problems are reduced.
Pilot-plant studies conducted at the DuPont Chambers Works Multi-
product Organic Chemical Plant revealed that addition of powdered activated
carbon to the aerator of an activated sludge system (76) would result in a
significant increase in performance over an activated sludge system alone
(124, 125, 126, 127, 128). Some additional improvement in performance
also was obtained by combined systems consisting of carbon columns either
preceding or following biological treatment. All three combined systems
achieved desired effluent quality for this particular waste stream during the
15-month test period, while the other two systems, biological treatment alone
or carbon column treatment alone, were found to be inadequate. The DuPont
workers determined the PACT system to be the least expensive system meeting
Chambers Works effluent criteria for BOD5, COD, TOC, color, and fish toxicity.
Toxicity values, expressed as 96-hour median tolerance limit (TL ), for
the raw effluent, primary effluent, and activated sludge effluent were 5, 16,
and 35%, respectively; there was no fish mortality at 100% for the PACT
effluent (125).
The effluent quality obtained by the PACT system is reported to be a
function of aeration basin temperature, sludge age, and powdered carbon dosage.
An inherent limitation of the PACT system is that for a given biological
system, the powdered carbon dosage which can be added is physically limited by
the solids-handling ability of the solids separation devices.
Flynn et al. (127) listed the following advantages of adding powdered
carbon to aerators of existing activated sludge plants:
1. Improved effluent water quality in a cost effective manner.
2. Enhanced treatability of wastewaters inhibitory or toxic to
biological treatment.
3. Expanded hydraulic capacity of existing plants without expansion
of unit processes.
4. Uniform plant operation and effluent quality.
5. Minimized operating problems; e.g., foaming, sludge bulking.
6. Eliminate ultimate disposal of secondary solids.
7. Compatible with existing plant operations.
DuPont has subsequently installed a 40-mgd secondary/tertiary
treatment system incorporating the PACT process at the Chambers Works
plant. The initial start-up phase of operation of the system was begun
in November 1976 (129). Operational data for the system will be published
in a forthcoming EPA report.
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SECTION III
DESIGN CONSIDERATIONS
There are a rather limited number of treatment processes which are capable
of removing refractory organic materials from wastewater. Adsorption on
powdered and granular activated carbon is included in these (21). Upon contact
with a wastewater containing soluble organic materials, activated carbon
selectively removes these materials by adsorption.
Although it has been claimed that the carbon adsorption process as a
secondary treatment step has some potential advantages over biological proces-
ses, Rizzo (130) reported that the activated carbon process, unfortunately,
cannot accomplish everything that a biological process can. Smith (131) re-
ported that unit costs for removal of biodegradable .compounds from wastewater
by carbon adsorption are generally more expensive than by biological treat-
ment. At the same time, however, adsorption is capable of removing nonbio-
degradable materials.
BASIC PRINCIPLES AND CONCEPTS OF THE CARBON ADSORPTION PROCESS
Adsorption is primarily a surface phenomenon whereby organic materials
are attracted and held to the surface of a solid material with which they come
into contact, because of forces of attraction at the surface. Activated
carbon is a particularly good adsorbent because it has an extremely large
surface area per unit volume. It has been noted that the carbon adsorption
process may be physical, chemical, electrical, or a combination of all three
(42, 98).
The concept of carbon adsorption and the basic principles involved have
been discussed by numerous workers. Some noteworthy publications that will
provide information on the nature, preparation, and properties of activated
carbon and the principles involved in its application as a method for removing
organic materials from wastewater are those by Ford (35), Hassler (36), Weber
(42, 132), Davies et al. (133), Rinehart et al. (134), and Westvaco Corporation
(135).
While each manufacturer has his own specific techniques for producing
activated carbon, the preparation involves two basic techniques—carbonization
and activation. Specific properties of activated carbons depend on the mate-
rial source and the mode of activation.
Appropriate consideration must be given to each of two different aspects
of adsorption when evaluating the suitability of adsorption as a unit of opera-
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tion for wastewater treatment or when designing a treatment system incorporat-
ing the adsorption process (136). The first of these two aspects is kinetics,
and the second is the ultimate capacity for adsorption or adsorption equilibria.
Weber and Morris (136) stated that efficient use of adsorbents can be obtained
only if the equilibria of adsorption are clearly understood and if the factors
that control capacities for adsorption are well defined.
The primary property for which one uses an activated carbon is its adsorp-
tion capacity (134). While various tests, e.g., iodine number, have been
developed to give relative removal capacities of activated carbon under specific
conditions, the best measure of adsorptive capacity is the effectiveness of the
carbon in removing the critical constituent from the actual wastewater in which
it is to be found (35).
Al-Bahrani and Martin (137) observed that, although adsorption on activated
carbon has been the only advanced process used to any significant extent for
removal of dissolved organics from wastewater streams, more research is needed
to study the various factors that influence their adsorption.
Ford (138) stated that the overall adsorption rate represents the combined
effects of diffusion through a laminar layer of fluid surrounding the constit-
uent, surface diffusion, and adsorption on the internal pore surfaces.
Essentially, there are three consecutive steps in adsorption of materials from
solution by porous sorbents such as activated carbon:
1. Transport of adsorbate through surface film of exterior of
adsorbent (film diffusion).
2. Diffusion of sorbate within pores of sorbent and/or along
pore wall surfaces (intraparticle diffusion).
3. Adsorption of solute on interior surfaces bounding pore and
capillary spaces of sorbent.
Forces that govern uptake kinetics of organic solutes from dilute solu-
tion by activated carbon are frequently quite different from those which
control the ultimate capacity of carbon for adsorption (136). The rate-
limiting mechanism generally is either film diffusion or intraparticle trans-
port, depending largely on the hydrodynamic character of the system in which
activated carbon is used. In contrast, the final portion of adsorptive
equilibrium is governed by the forces of adsorption, either chemical or
physical in nature. As a result of possible differences in the nature of
kinetic and equilibrium forces, factors that enhance rates of uptake may well
decrease the capability of activated carbon for certain adsorbates—the converse
may also be true. It is necessary, then, that these factors and their relative
significance to both rates and capacities of adsorption be'clearly defined.
Braus et al. (139) found that experimental adsorption data for granular
sorbents such as activated carbon could be explained by assuming that the
adsorption rate is controlled by rate of diffusion of the solute within the
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adsorbent particles. Morris and Weber (140), experimenting with activated
carbon adsorption of alkylbenzene sulfonate (ABS), also attributed control of
the adsorption rate to diffusion of the solute in the adsorbent pores.
Weber and Morris (141) carried out studies on factors affecting adsorption
with a view to using this method for the treatment of wastewaters. They
reported that the overall rate of adsorption appears to be controlled by the
rate of diffusion of the solute within the micropore structure of the granular
carbon. Relative reaction rates were found to vary reciprocally as the square
of the diameter of the individual carbon particles for a given weight of carbon.
The carbon particle is extremely porous—the pores being classified as
macropores and micropores. Macropores are generally regarded as playing little
part in adsorption, merely serving as an avenue into the micropores (133).
Therefore, for any adsorbate molecule the effective surface area for adsorp-
tion can exist only in the pores which the molecule can enter. Thus, the
equilibrium capacity for a given adsorbate will depend on the total internal
surface area and the pore size distribution. The pore structure and size
distribution of the pores of activated carbon are extremely important in
determining its adsorptive properties since adsorbate molecules are physically
prohibited from entering pores smaller than those molecules (135).
Recognizing the potential value of activated carbons for removal of water
pollutants, West (142) initiated a study for the general purpose of obtaining
information on the effect of porous structure of a carbon on that carbon's
activation. He found that the adsorptive capacity of an activated carbon is
determined by the inherent properties of the carbon and by its degree of
activation. The amount of surface area per unit mass of carbon, the accessi-
bility of that surface to the adsorbate molecules, and the concentration on the
surface of sites capable of attracting and holding specific adsorbates are
the keys to the adsorptive capacity of the carbon. The rate of uptake of
adsorbates is a distinct function of the porous structure of the adsorbent and
the character of the adsorbate.
Properties of carbon that are of prime importance in an adsorption opera-
tion include total surface area and pore size distribution (11). It is the
total area which determines the capacity of a carbon for adsorption, and it
is the pore size distribution which determines the accessibility of the surface
area to specific sorbate molecules. Carbon particle size primarily affects
rate of adsorption and not total adsorptive capacity of the carbon (21) . The
most important effect of particle size reduction is a decrease in time
necessary to reach adsorption equilibrium—that is, an increase in the rate of
adsorption (11). The most significant parameter in determining a powdered
carbon's performance for oil refinery wastewater treatment has been reported
to be surface area (102, 103). The pore size had no apparent effect on the
experimental results.
Since adsorption is an equilibrium phenomenon, capacity will also be
strongly dependent on concentration of the sorbate in solution around the
carbon particles (133). The rate of adsorption will be governed by particle
size, as it is dependent on diffusion of adsorbate molecules through macro-
pores so that in a smaller particle the center of the particle will reach
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equilibrium with the solution more rapidly than in a larger particle. It is
an advantage in liquid-phase applications to have a well-developed macropore
structure throughout the particle.
Because adsorption is a surface phenomenon, the extent and properties of
the surface will control adsorptive effects (134). For a particular type of
adsorbent, the greater the surface available the greater the amount adsorbed.
However, surface properties and conditions can affect the course and type of
adsorption. Because of its high porosity, activated carbon is one of the few
solids that can provide extremely high surface area per unit weight or unit
volume at relatively low cost. Activated carbons are prepared so as to
exhibit a high degree of porosity and an extensive surface area (132).
The surface properties of different carbons can have profound effects on
both the reate and capacity for adsorption (42). Chemical properties of the
surface are important in determining activity, or capacity for a specific
sorbate. Chemical properties of the surface depend on the starting material,
activation process, and conditions employed in activation (143). It is not
possible to determine activity or capacity from basic carbon properties, such
as surface area, or to relate activity for a reference sorbate, such as iodine
or methylene blue (42, 134). Activity or capacity must be determined directly
on the system of interest.
Information on the structure and function of activated carbon used in
water and wastewater treatment systems was reviewed by Snoeyink and Weber
(144) and correlated to provide a simple model to explain the properties of
activated carbon. The basic structural unit was examined, and raw materials,
method of preparation, activation additives, and noncarbonaceous impurities—
all factors affecting its sorptive properties—were discussed. Snoeyink and
Weber described the surface of activated carbon as a collection of functional
groups containing oxygen, occurring primarily at the edge of broken graphite
planes and basal planes, consisting of large, fused aromatic ring systems in a
graphite-like structure.
Anderson (145) discussed two classes of adsorption—physical adsorption
and chemisorption. In physical adsorption, impurities are held on the surface
of carbon by weak van der Waals1 forces; whereas chemisorption bonding results
from relatively strong bonding between the impurity and active sites on the
carbon surface. Thus, the efficiency of a carbon depends upon the accessible
surface area and the presence of active sites upon the surface. It should be
noted that physically similar sites on the carbon surface need not be chem-
ically similar. Highly adsorbent forms of carbon often contain substantial
amounts of hydrogen bonded to the surface carbon atoms. Some surface sites
may readily adsorb a molecule for which a nearby site shows little, if any,
affinity.
Snoeyink et al. (146) studied the sorption of phenol and p-nitrophenol
(PNP) from aqueous solution on particulate active carbon of two types. Equi-
librium measurements suggested a heterogeneity of active surface sites with
respect to the energy of adsorption, while desorption studies revealed signi-
ficant hysteresis effects when long equilibrium periods were involved. Although
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the two carbons tested had similar total surface areas, the coal-base carbon
sorbed phenol at low concentrations more extensively. Sorption on the
coconut-shell carbon was affected by strong acids.
Mattson et al. (147) investigated characteristics of the uptake of phenol
and nitrophenol by active carbon. The measurements of solution equilibrium
parameters as well as surface structural characteristics determined by infra-
red internal reflection spectroscopy are presented. A charge-transfer inter-
action is postulated which explains the observed adsorption and spectroscopic
characteristics on the basis of the electron densities in the sorbate molecule
as well as the types and concentrations of surface functional groups present
oh the carbon. All of the data support the mechanism of a donor-acceptor
complex with surface carbonyl oxygen groups with adsorption continuing after
these sites are exhausted by complexion with the rings of the basal planes.
Interaction of the aromatic ring with the surface of the active carbon must be
considered the major influence in these processes interacting through the pi-
electron system of the ring. There is considerable evidence in the literature
for the formation of donor-acceptor complexes between phenol and several kinds
of electron donors (148).
Coughlin and Ezra (148) observed that a change in the adsorptive capacity
for phenol and nitrobenzene took place upon oxidation or reduction, or both,
of the carbon surfaces. They observed that upon oxidation of the surface,
capacities for both decreased and with reduction, capacities for both increased.
The oxidation of the carbon surface increases the amount of strongly acidic
oxygen containing functional groups through the oxidation of carbonyl groups.
Mattson et al. (147) reported that many adsorption processes involving
organic molecules result from specific interactions between identifiable
structural elements of the sorbate and the sorbent. Different substitute
groups, even though of similar size, differ greatly in their influence on
adsorbability (36). The specificity so often encountered in adsorption often
depends on mutal relations between the lattice structure of the sorbent and
the configuration of the adsorbate molecule.
Every adsorptive power does not exist on all portions of the surface (36).
Specific affinities can reside in separate areas (active centers) of the surface.
Selective adsorption can be caused by qualitative characteristics of the surface.
Although each active center exhibits selectivity as to the kinds of substances
that can be adsorbed, the selectivity may not be limited to the adsorption of
only one molecular species.
DiGiano and Weber (150) illustrated PNP isotherms obtained for four
different activated carbons. Adsorption capacities indicated by these iso-
therms were significantly different. The adsorption capacity is a function
of the total surface area available to the PNP molecules but is also a function
of the type of activation treatment to which the carbon has been subjected.
Many activated carbons have comparable surface areas and pore size distributions,
yet do not exhibit similar PNP adsorption capacities.
Jain and Snoeyink (151) found that p-nitrophenol (PNP) and p-bromophenol
(PBP) compete for the same kinds of sites on the carbon surface. Both the
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nitro group and the bromo group are electron-withdrawing, and it is likely that
the mechanism of adsorption for both PNP and PBP on the carbon surface is
similar. These workers reported a very pronounced effect of PNP on the
adsorption rate of PBP. The magnitude of this effect is consistent with
strong competitive interaction. Further studies with PNP and benzenesulfonate
(BS) revealed that PNB and BS adsorb primarily at two different types of sur-
face sites without significant competition.
Coughlin (13) has shown by experiment that acidic surface oxides on
active carbon can profoundly influence the sorption of various organic mole-
cules from aqueous solution. Not only is the equilibrium sorption capacity
of the carbon affected, but the rate of sorption is also changed. However,
these changes are reversible, for removal of the acidic surface oxides can
restore the carbon to its original sorption capacity or beyond. In the cases
of sorption of phenol, nitrobenzene, sodium benzenesulfonate, and dextrose,
surface oxides reduced the sorption capacity of the carbon as well as the
speed of sorption. In the case of urea sorption, the sorption capacity of
the carbon was increased by the presence of acidic surface oxides. It appears
that the influence of these surface oxides depends on the relative strength
of their interactions with both the water solvent and the solute to be adsorbed.
As a general rule, organic compounds are adsorbable, but there are many
exceptions, and there are all degrees of adsorption (134). Two broad generali-
zations which may be helpful are:
1. The larger the molecule, usually the better the adsorption, provided
the pores in the activated carbon are of suitable size.
2. The greater the solubility of the adsorbate, usually the poorer the
adsorption.
Mass (152) presented some generalities relating to the types of materials
adsorbed by carbon:
1. Weak electrolytes are adsorbed better than strong electrolytes.
2. The more ionic a material is, the more difficult it is to adsorb.
3. Sparingly soluble materials usually are adsorbed better than highly
soluble materials.
4. High-molecular-weight materials may be adsorbed better than those of
low molecular weight.
Zuckerman and Molof (153) stated that the adsorbability of organic mate-
rial in a wastewater will be determined by molecular size or weight distribu-
tion of the soluble organic material. Davies et al. (133) observed that each
molecule will, for a given adsorbate, have an "adsorption potential." This
potential is a measure of the free energy of adsorption released when the
adsorbate molecule moves from solution to the adsorbed state on the carbon
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surface. The adsorption potential can be linked with molecular weight and
molecular structure. Generally speaking, the higher the molecular weight,
the more strongly a given material is adsorbed.
Weber and Morris (141) proposed that activated carbon adsorption of
soluble organic material in wastewater is controlled by intraparticle diffusion.
If this extrapolation is valid, then the molecular size and configuration of
individual soluble organic compounds will be factors of great significance in
determining the aggregate adsorption capacity of organic material in waste-
water.
There have been a number of published reports of evidence that molecular
weight is only one of many factors affecting adsorption of organic matter from
aqueous solution. Joyce and Sukenik (47) reported that adsorption decreases
with a decrease in concentration of organic material. Eskuty and Admundson
(154) postulated that rate of adsorption is controlled by rate of diffusion of
solute in the capillary pores of the carbon. Weber and Morris (141) found that
the rate of uptake was affected by the initial concentration of the solute
and by the molecular size and configuration of the adsorbate. Thus, the rate
of uptake decreases with increasing size of the molecule and with molecules of
highly branched structure.
According to Weber (132), molecular size is significant if the adsorption
rate is controlled by intraparticle transport, in which case the reaction
generally proceeds more rapidly the smaller the adsorbate molecule. He empha-
sized, however, that the rate depends on the class or series of molecules.
Large molecules of one chemical class may sorb more rapidly than smaller ones
of another if high energies (driving forces) are involved. Lang et al. (74)
found that a large portion of TOG remaining after carbon adsorption was made
up of low molecular weight organic compounds which are not readily adsorbed by
activated carbon (ethanol, acetaldehyde, acetone, formic acid, acetic acid).
The rate of adsorption of a solute at a solid surface may be expected to
vary with the surface area and, thus, for a constant weight of adsorbent, with
the particle size (155). In water, activated carbon has a preference for large
organic molecules and for substances which are nonpolar in nature (21). Forces
of attraction between the carbon and the adsorbed molecules are greater the
closer the molecules are in size to the pores. Best adsorption takes place
when pores are just large enough to admit the molecules.
In addition to size, studies by Morris and Weber (155) showed that molec-
ular configuration has an effect on the rate of adsorption—extensive branching
tends to reduce the rate of adsorption. Al-Bahrani and Martin (137) reported
that structure of the adsorbate molecule has a significant effect on its ad-
sorption.
Al-Bahrani and Martin observed the effect of molecular structure on 26 low
molecular weight aromatic pollutants from aqueous solution. Batch-type experi-
ments were used to investigate adsorption equilibria of each compound. Equi-
libria data for adsorption of the individual compounds studied were found to
33
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correlate with both the Langmuir and Freundlich equations for adsorption iso-
therms in the range of concentrations studied; however, two-step isotherms
were observed. They observed H bonding and steric hindrance to be significant
factors in the adsorption process.
Reimers et al. (156) reported that the principal factors influencing the
adsorption of organic solutes are the general molecular structure of the
organics, solubility of the organics, and degree of the organics ionization.
Some observations by these workers were:
1. Aromatics generally adsorb better than aliphatics.
2. Increased branching generally improves adsorption.
3. Both the position and type of group can influence adsorption of com-
pounds with substituted rings.
4. Increased solubility generally decreases the organics affinity to
adsorption on carbon.
5. With decreasing pH, organic acids, on a whole, will ionize less and
adsorb better, while the exact opposite phenomenon is usually observed with
organic bases.
Bartell and Miller (157) found that introduction of OH and NH2 groups to
organics acids had an adverse effect on their adsorption. Linner and Gortner
(158) found that a branched chain had little effect on the maximum adsorption
of an organic acid, while the introduction of hydroxyl and keto groups in-
creased, the adsorption markedly decreased.
Cheldrin and Williams (159) showed that the adsorption of 33 amino acids,
vitamins, and related substances fit the Freundlich adsorption isotherm. The
presence and position of polar groups and absence of aromatic nuclei were noted
as important factors in aqueous adsorption of organics by activated carbon.
Ford (35) presented a summation of the influence of molecular structure
and other factors on adsorbability:
1. An increasing solubility of the solute in the liquid carrier decreases
its adsorbability.
2. Branched chains are usually more adsorbable than straight chains. An
increasing length of the chain decreases solubility.
3. Substituent groups affect adsorbability:
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Substituent Group Nature of Influence
Hydroxyl Generally reduces adsorbability; extent of
decrease depends on structure of host
molecule.
Amino Effect similar to that of hydroxyl but some-
what greater. Many amino acids are not ad-
sorbed to any appreciable extent.
Carbonyl Effect varies according to host molecule;
glyoxylic are more adsorbable than acetic
but similar increase does not occur when
introduced into higher fatty acids.
Double Bonds Variable effect as with carbonyl.
Halogens Variable effect.
Sulfonic Usually decreases adsorbability.
Nitro Often increases adsorbability.
4. Generally, strong ionized solutions are not as adsorbable as weakly
ionized ones; i,e., undissociated molecules are in general preferentially ad-
sorbed.
5. The amount of hydrolytic adsorption depends on the ability of the
hydrolysis to form an adsorbable acid or base.
6. Unless the screening action of the carbon pores intervenes, large
molecules are more sorbable than small molecules of similar chemical nature.
This is attributed to more solute carbon chemical bonds being formed, making
desorption more difficult.
7. Molecules with low polarity are more sorbable than highly polar ones.
Giusti et al. (105) conducted an extensive investigation to quantify some
aspects of adsorbability of specific organics. The effects of functionality,
molecular weight and structure, aqueous phase, pH, solubility, polarity, carbon
surface structure, and adsorbate interactions were investigated. These workers
concluded that adsorbability, as reflected in constant dosage tests,, is favored
by increasing molecular weight, aromaticity, and degree of unsaturation. In-
creasing polarity, solubility, branching, and degree of dissociation tends to
severely limit the extent of adsorption. Typical graphs of adsorbate loading
versus molecular weight showed the favorable effect of unsaturation. Aro-
matics, even those containing polar groups (OH , W.~ , Cl ), proved quite
amenable to adsorption, while the polyfunctional oxygenated compounds (glycols,
glycol ethers) proved especially difficult to adsorb.
Hassler (36) presented a long list of factors influencing adsorption at
the carbon/liquid interface. The algebraic sum of all these forces is measured
35
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by the quantity of substance adsorbed by a given weight of carbon. The pub-
lished data on adsorption of organic compounds from solution provide ample
evidence that the architecture of a molecule is an important factor in adsorp-
tion phenomena. Some generalities presented by Hassler are:
1. Aromatic compounds are in general more adsorbable than aliphatic
compounds of similar molecular size.
2. Branched chains are usually more adsorbable than straight chains.
3. The influence of substitute groups is modified by the position
occupied; e.g., ortho, meta, para.
4. Stereoisomers show an inconsistent pattern of adsorption; fumaric acid
(trans) is more adsorbable than maleic (cis), but the trans form of hydro-
benzoin is less adsorbable than the cis form.
5. Optical isomers (dextra and levo) appear to be equally adsorbed.
Hassler also presents a good discussion of the influence of separate
factors on adsorption. Some of his most interesting observations are:
1. Although any change that causes in increase in solubility may be ac-
companied by a decrease in adsorption, even great solubility does not prevent
adsorption of a substance that is strongly attracted to the carbon surface;
e.g., the very soluble chloracetic acid is well adsorbed. Conversely, a
slightly soluble substance will be adsorbed only if and when an attraction
exists to the carbon surface.
2. lonization is usually adverse to adsorption by carbon. Undissociated
molecules of organic compounds are more adsorbabie than ions of dissociated
molecules; therefore, the acidity or basicity of the solution may be an impor-
tant factor. The optimum pH is specific for each solute; low pH.promotes ad-
sorption of organic acids, while high pH is favorable for adsorption of organic
bases.
3. While an increase in temperature may cause a decrease in adsorption,
the increase accelerated the velocity of adsorption by stepping up the diffusion
of solute molecules.
The extent to which the full surface area of an activated carbon can be
used for adsorption depends on the concentration of solute in the solution
with which the carbon is mixed (136). Temperature effects on adsorption equi-
libria generally are not felt to be significant, particularly over the range
of temperature encountered in water and wastewaters.
As adsorption reactions are generally exothermic and high temperatures
usually slow or retard the adsorption process, lower temperatures have been
reported to favor adsorption (36, 132). Very little information has been
presented, however, which documents significant shifts in adsorbability within
the temperature range 65-90 F.
36
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Snoeyink et al. (146) reported that because phenol sorption is apparently
exothermic, an increase in temperature would result in a decrease in sorption
of phenol. Lower temperatures should increase adsorption, but the effect in
aqueous solutions is very small (138). However, the rate of adsorption is
strongly temperature dependent, and generally higher temperature promotes
better adsorption.
The hydrogen ion concentration of a solution from which adsorption occurs,
for one or more of a number of reasons, may influence the extent of adsorption
on activated carbon (145). In general, adsorption of typical organic pollut-
ants from water increases with decreasing pH; thus, there is an increasing ad-
sorption of sulphonated alkyl benzenes on active carbon as the pH of the solu-
tion is reduced.
Ward and Getzen (160) reported on studies of the adsorption of three
herbicides over a wide pH range. Equilibrium adsorption data on these and
seven other structurally related compounds fit Langmuir isotherms in the range
of concentrations studied at all pH levels. There was a marked increase in
removal of all solutes from aqueous solutions on lowering the pH below 7.0.
Adsorption in the acid region was greater than expected from the molecular-
ionic ratio of the bulk solution. The effect was explained in terms of an
enhanced specific ion adsorption resulting from increasing the proton con-
centration as the pH was lowered and a subsequent alteration in surface
properties of the carbon. Maximum adsorption was attained near the point
where the pH = pK .
SL
Freundlich adsorption isotherms were obtained on 1,000-mg/liter solutions
of five different organic compounds using four different carbons at different
aqueous phase pH levels (105). The pH had a marked effect on the isotherm
adsorptive capacity of all carbons tested for butylaldehyde and ethyl acetate.
A detailed discussion of differences between the carbons tested and the effects
of carbon surface properties is presented.
Ford (138) reported that a change in ionization can drastically affect
adsorption. A low pH, for example, promotes adsorption of organic acids,
whereas a high pH would favor adsorption of organic bases. Phenol adsorbs
strongly at neutral or low pH, while adsorption of the phenolate salt at a
high pH is poor.
Wang et al. (161) reviewed and investigated the effect of the initial
hydrogen ion concentration on activated carbon adsorption mechanisms in single
and multicomponent organic aqueous systems. The adsorption mechanisms dis-
cussed adequately describe carbon adsorption of single organic compounds from
aqueous solution. The effect of pH upon activated carbon treatment of a
heterogeneous organic wastewater is complicated. Adjustment of the pH may
increase adsorption of one species while suppressing removal of another at
the same time. The optimum pH for activated carbon treatment of a specific
industrial waste must be determined experimentally because many competing
adsorption mechanisms are involved.
Zogorski and Faust (162) evaluated the chemical and physical parameters
involved in the adsorption of aromatic hydroxyl (phenolic) compounds by
37
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granular activated carbon. The rate of movement of the mass transfer zone
was independent of pH values between 2.5 and 6.4; on the other hand, pH values
greater than the pK value of the adsorbate significantly influenced the rate
at which the adsorblnt was utilized. Snoeyink et al. (146) studied adsorption
of phenol at various pH values and found maximum adsorption occurred at the
pH value of 7.5.
Giusti and his coworkers (105) indicated that a potential problem exists
in applying activated carbon to multicomponent wastewaters. It is not possible
to select a pH level that assures maximal adsorption of all wastewater constitu-
ents. The optimum pH is solute specific and must be determined for each waste-
water (138).
Decreasing temperature and pH act to increase both the capacity of the
carbon and the rate of adsorption. It seems evident, however, that the cost
of controlling either pH or temperature far exceeds any benefit that could be
obtained by this action (163).
Most wastewaters contain a myriad of compounds which may mutually enhance,
interfere, or act independently in the adsorption process (35, 132). Factors
which affect the overall adsorption of multiple adsorbates include the relative
molecular size and configuration, relative adsorption affinities, and the rela-
tive concentrations of the solutes (132). Factors affecting the rate and magni-
tude of adsorption include molecular structure, solubility, ionization and mix-
ture of solutes (35).
Jain and Snoeyink (151) reported on a study of competitive adsorption
from bisolute aqueous systems on active carbon in batch systems. Adsorption
of the two organic species without competition was consistent with adsorption
of each species on different sites. A model based on some amount of adsorption
without competition is proposed.
•r*' This study of competitive adsorption was performed to give insight into
some of the factors affecting selective uptake of organic compounds on active
carbon from bisolute aqueous solutions. Results of the study are consistent
with a number of phenomena which include: (a) adsorption with equal competi-
tion for adsorption sites, (b) adsorption of each of the two species on differ-
ent kinds of sites to an extent that only minor competitive effects were ob-
served, and (c) competitive adsorption of organic anions with electrostatic
repulsive forces between adsorbed species. Weber (164) has shown the signifi-
cant effect of competitive adsorption on the breakthrough curve, thus demon-
strating the importance of competitive effects in column design and operation.
The effects that can result from the presence of multiple organic solutes
can be quite specific, depending on relations existing among the various
ingredients (36). Some components diminish the adsorption of others; others
may cause a mutual increase; still others show no measurable change. Compounds
that show great adsorbability from pure solutions are often preferentially ad-
sorbed from a mixture, but there are many exceptions; the quantity adsorbed
does not always reflect the strength of the attachment to the surface. Conse-
quently, amounts adsorbed from the pure state should not be used to predict
the proportions in which ingredients will be adsorbed from a mixture. Mutual
38
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inhibition can be expected if the adsorption affinities of the solutes do not
differ by several orders of magnitude and there is no specific interaction
between solutes enhancing adsorption (42). Similarly, because the adsorption
of one substance will tend to reduce the number of open sites and, hence, the
"concentration" of adsorbent available, mutually depressing effects on rates of
adsorption may be predicted.
Lang et al. (74) stated that the adsorption of organic substances from a
mixed solution is a complex phenomenon, possibly owing to the effect of one
substance on another. This can manifest itself in preferential adsorption of
one substance over others, nonadsorption if a substance is only weakly adsorbed
at best, or the displacement of a weakly adsorbed by a strongly adsorbed sub-
stance. This selective or preferential adsorption must be kept in mind in the
evaluation of equilibrium adsorption data.
Davies et al. (133) stated that when considering adsorption from multi-
component adsorbate solutions, the solid-liquid system becomes more complex
but will tend to equilibriate at the lowest possible energy state. Hence, a
heavy molecular weight adsorbate will tend to replace a previously adsorbed
adsorbate of lower molecular weight. In the case of activated carbon, the
exact mechanism of adsorption on the surface is of less significance in prac-
tical terms than the sheer magnitude of available surface area.
Masse (152) stated that in general, however, the net effect of mixed
materials is not detrimental, and there may be an enhancement of the total
weight of materials adsorbed from mixtures.
ADSORPTION MODELS
There are several theoretical models which have been used to describe the
adsorption phenomenon, none of which have been shown to be universally appli-
cable to wastewater-activated carbon systems (58). This is not surprising
considering the extremely heterogenous nature of soluble organics in waste-
waters (molecular size, concentration, functionality, etc.) and of activated
carbon (pore size and distribution and functionality).
The analysis of steady state adsorption data may take several forms (18).
The most common form is the adsorption isotherm. Among the equations applied
to equilibrium adsorption data are the Langmuir equation and the Brunauer,
Emmet, Teller (B.E.T.) Model. These equations may be deduced from either
kinetic considerations or the thermodynamics of adsorption. The Langmuir
equation is valid for single-layer molecule adsorption, while the B.E.T. Model
reflects apparent multilayer adsorption. The familiar Freundlich isotherm
is of empirical origin but has since been derived by assuming a logrithmic
distribution of adsorption sites, a treatment valid only when there is no
appreciable interaction between adsorbed molecules. Burleson et al. (18)
stated that a mathematical representation of the adsorption characteristics
for unknown mixed solutes present in biologically treated industrial waste-
waters is difficult, if not impossible.
Adsorption results in removal of solutes from a solution and their con-
centration at a surface until such time as the amount of solute remaining in
39
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solution is in equilibrium with that at the surface (42). This equilibrium
is described by expressing the amount of solute adsorbed per unit weight of
sorbent as a function of the concentration of solute remaining in solution.
An expression of this type is termed an adsorption isotherm. Weber (42) stated
that the adsorption isotherm is useful for representing the capacity of an
activated carbon for adsorbing organics from a wastewater and in providing a
description of the functional dependence of capacity on the concentration of
pollutants. Theoretically, the sharper the rise of the isotherm to a given
ultimate capacity as the concentration increases, the more effective will be
the activated carbon in adsorbing organics from the wastewater. Experimental
determination of the isotherm is routine practice in evaluating the feasibility
of adsorption for treatment in selecting an activated carbon and in estimating
carbon dosage requirements.
Fornwalt and Hutchins (165) have outlined the isotherm procedure in detail.
They also discussed interpretation of the Freundlich isotherm. Qualitatively,
very steep isotherms, while indicating high ultimate adsorptive capacities,
also indicate the possibility that very low effluent concentrations cannot be
achieved without massive dosages of carbon.
The Freundlich isotherm is valid within the context of a batch test for
pure substances and some dilute wastewaters (35). Its application is limited
in certain cases when a significant portion of the organic impurities are not
amenable to sorption, resulting in a constant residual regardless of the car-
bon dosage.
The empirical Freundlich model has been widely used for wastewater-
activated carbon adsorption systems (47, 133, 135, 166). However, its complete
adequacy has not been fully established (58). Two major shortcomings are that
a maximum adsorption capacity (loading) or any unadsorbable soluble organic
fraction are not accounted for. When evaluating relative effects of carbon
type, pretreatment, and method of contacting, these shortcomings do not limit
utility of the Freundlich model.
Hutchins (65) stated that, since granular activated carbon is not suitable
for treating all types of wastewater, preliminary feasibility testing should
be conducted. He proposed the adsorption isotherm as a useful first step in
determining the feasibility of granular carbon for treating a given waste-
water. The isotherm provides a good idea of the effectiveness of the carbon
in adsorbing the impurities present in the wastewater and an indication of the
maximum amount of impurities that will be adsorbed by the carbon. The main
limitations of an adsorption isotherm are:
1. It provides no indication of the biological removal that may occur.
2. Lack of precision in measuring the parameters, e.g., BOD, COD, may
result in considerable scatter in the data points.
3. Adsorption isotherms are equilibrium tests, and they do not indicate
the actual performance of the carbon.
4. Granular carbon is not always completely exhausted in the adsorber.
40
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5. Powdered carbon has a significantly greater adsorption rate than
granular material.
Mathematical description of adsorption isotherms for a particular system
is also required for development of predictive models for design application
(16, 132). Details of conventional Langmuir and Freundlich isotherm expres-
sions are readily available in the literature (132). Laboratory techniques
and procedures for carbon adsorption isotherm studies and related analyses
are outlined by Eckenfelder and Ford (167).
Lawson and Fisher (107) reported that the use of "adsorptive capacity"
calculated from Freundlich adsorption isotherm in the design of continuous
systems is highly unreliable. Isotherm capacities are consistently higher,
often several times higher, than capacities achieved in columns. This
indicates the necessity for actual column tests to define the anticipated
performance. Batch isotherm tests are useful, however, in determining the
trend of organic removal with increasing carbon dosages. Data are presented
comparing adsorptive capacities calculated from Freundlich isotherms with
those observed in continuous column studies when the breakthrough curve was
taken to exhaustion.
Hager and Reilly (9) ran carbon isotherm studies on 11 different municipal
wastewaters. They concluded that standard isotherm testing is definitely in-
adequate to determine the dosage and feasibility of the adsorption process in
treating wastewaters. One phenomenon that has often manifested itself in
activated carbon studies has been the inability to match ultimate carbon
loadings found from isotherm tests with those experienced in continuous sys-
tems (105).
Lawson and Hovious (108) stated that isotherm tests provide a relative
indication of the amount of organic removal achievable by adsorption and the
ultimate adsorptive capacity of the carbon. Unfortunately, these data are
useful only in a relative sense—for comparing the relative merits of two
different carbons or for comparing the relative amenability of different
wastewaters to adsorption. Isotherm data are not suitable for designing
continuous granular carbon adsorption columns, since the dynamic effects and
interactions in a continuous bed differ too greatly from the batch equilibrium
situation in an isotherm.
Lawson and Hovious also reported that while pure component adsorption
data are useful in determining which waste streams are potential candidates
for activated carbon treatment, the data cannot be used to quantitatively
predict adsorption from multicomponent systems. Ford (138) maintains that ad-
sorption theory is rigorous for single solutes but becomes much less definitive
when applied to wastewaters containing multiple components with varying molec-
ular weights and chemical characteristics. He adds that batch isotherm studies
are not necessarily indicative of continuous flow carbon treatment systems.
Results of comprehensive carbon pilot testing of refinery wastewaters depict
an average organics removal significantly less optimistic than that indicated
in isotherm tests.
41
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Giusti et al. (105) examined interactions in multicomponent systems by
running adsorption isotherm tests on binary and four-component mixtures. The
total adsorptive capacity, as depicted by the isotherm results, was signifi-
cantly less than the sum of the pure component capacities. Thus, while pure
component data may be useful in determing which waste streams are potential
candidates for activated carbon treatment, the data cannot be used to quanti-
tatively predict adsorption from multicomponent systems. An additional phase
of the study consisted of operating continuous column granular carbon adsorbers
to exhaustion for comparison of ultimate capacities with isotherm capacities.
In all cases, the ultimate capacity (at exhaustion) was less than that predicted
by isotherms.
Ford (35, 138) observed that there are many factors which influence both
the rate and magnitude of adsorption, underscoring the difficulty in developing
predictive models which would apply to all complex wastewaters. He reported
that predictive models obviously require validation for complex wastewaters,
as extrapolation from investigations using synthesized wastes containing
controlled concentrations of selected adsorbates may not reflect all of the
interactions occurring in the waste. A summary of the more important factors
which potentially influence adsorbability is presented. Gulp and Gulp (21)
stated that for liquid adsorption systems, there is no precise method for pre-
dicting the performance of carbons founded on their basic properties or those
of the adsorbing materials.
Limitations of theoretical adsorption concepts relative to the practi-
calities of treatment requirements for refinery and petrochemical wastewaters
necessitate that comprehensive process simulation studies precede the finalizing
of design decisions (83). There is a tendency for investigators and equipment
developers to oversimplify the adsorption process adaptability for industrial
wastewater applications. The technical and economic justification for applying
carbon adsorption for the treatment of refinery and petrochemical wastewaters
at any point in a process sequence can be determined only after a thorough
investigation using continuous-flow pilot systems. Proper interpretation of
the results is then necessary to consummate the process evaluation, determine
the economies, and select the most appropriate treatment sequence.
A completely developed system of adsorption column design has been pre-
sented by Hiester and Vernuelen (168). They solved graphically the equation
for second-order reversible adsorption. Based on the Hiester and Vernuelen
reaction-kinetics solution for packed beds, Keinath and Weber (169) developed
a mathematical model for description of. mass-transfer processes in columns of
fluidized activated carbon. According to the described tests, this model pre-
dicts adequately the concentration-time profiles for systems in which flow
rate, particle size of the adsorbent, solute concentration, and depth of bed
varied.
Work was done by Weber and Morris (170) to define and evaluate the
characteristics of adsorption from aqueous solution in columns of fluidized
media. The kinetics of adsorption were investigated in terms of uptake pro-
files relative to the velocity dimensions, time and length, for selected values
42
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of these variables and for different sized particles of adsorbent. Thirteen
fluidized-bed studies were conducted. Postulated models show the kinetics of
the systems studied.
Bell and Molof (171) described a conceptual model of batch adsorption
kinetics. Intraparticle transport was proposed to occur in a series of dis-
tinct adsorption-desorption steps, each linear with respect to time.
Allen et al. (172) discussed some of the process engineering aspects of
adsorption columns for the purification of liquid wastes. These workers re-
ported that laboratory data on a liquid-phase adsorption system can be fitted
to an empirical equation which then can be used to study a wide range of plant
design and operation variables. A computer program for determining the rate
constant that gives the closest fit between experimental and calculated efflu-
ent concentrations has been developed by these workers and is summarized in
this paper.
Neretnieks (173) proposed a simple method whereby the film transfer co-
efficient and coefficient of diffusion in the particles may be determined from
finite bath adsorption experiments. The method also makes it possible to sep-
arate pore and surface diffusion. Under certain conditions, it is also possible
to determine the influence of the particle phase concentration on surface dif-
fusivity. The method is based on models describing the interstitial diffusion
in the solids. Data from six different adsorption systems were analyzed using
this method. It was found that in all systems surface diffusion was the
determining transport mechanism in the particles.
Westermark (7) presented an adsorption model which incorporates a linear
adsorption isotherm and accounts for both film and pore diffusion in the carbon
bed. Design of different modes of carbon contact were presented with the opti-
mization of contact time. Equilibrium constants and pore diffusivities were
evaluated for four different carbons. The theoretical model was used for the
development of design criteria and optimization criteria for continuous and
periodic adsorption plants.
Work by Andrews and Tien (174) represents the first attempt to separate
contributions of adsorption particle deposition and bacterial activity to
overall removal of TOG. By making several assumptions about mechanisms that
are described and their interactions, a mathematical model capable of predict-
ing TOG breakthrough curves was developed. Experimental work proved the model
basically sound. The authors are hopeful that it will be possible to develop
the model into a useful tool for the designer of wastewater treatment plants.
Hsieh et al. (175) concluded from their batch adsorption kinetic experi-
ments on multicomponent systems that the adsorption isotherm for the waste-
water activated carbon system is of the Freundlich type. Furthermore, the
internal diffusion model appears to be a more suitable one than the pore diffu-
sion model in describing the adsorption process, mainly because diffusion co-
efficients based on this model exhibit relatively smaller variations. The
internal diffusion model, in addition, leads to simpler mathematical formulations.
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Hutchins (176) presented a new calculation method called Bed Depth/
Service-Time (BDST) analysis that could speed up the design process by reduc-
ing the amount of preliminary testing. Basically, it is a means of predicting
effects of different feed concentrations, flow rates, or effluent compositions.
BDST analysis applies only to granular carbon systems which are not in equi-
librium. Hutchins cautioned, however, that even with good BDST data, design
should be tempered with experience and judgement.
Vanier and Tien (177) devised a mathematical model to simulate the adsorp-
tion and filtration of wastewater in an isothermal column packed with granular
activated carbon. The adsorption process is considered to be controlled by a
combination of liquid-phase diffusion and interparticle diffusion. In addition
to adsorption and filtration, effects of backwashing and regeneration are in-
cluded in the model. A computer program has been prepared and coded in Fortran
IV. A unique feature of this program is the clear separation of calculation
framework and model for the column behavior.
Weber and Crittenden (16) have developed a general modeling scheme termed
MADAM I (Michigan Adsorption Design and Application Model I). Based on numeric
solution techniques, MADAM I is not restricted to simplified rate and equilibrium
expressions to facilitate analytical solutions. Rather, it can accommodate
the dynamic aspects of fluid dispersion, solids mixing, multisolute interactions,
and biological growth on activated carbon surfaces—aspects which must be ex-
cluded because of mathematical complexity from models which are based on
analytical solution techniques. Mathews and Weber (178) have recently described
the use of a general, widely applicable, 3-parameter isotherm model for multi-
component waste streams.
Straightforward mathematical procedures for predicting breakthrough curves
of continuous flow columns, as described by Westermark (7) and Weber (132) may
be used for process design in certain cases when time or budget constraints
prohibit the conduct of column tests (179). These methods require prior knowl-
edge of such fundamental data as the equilibrium adsorption capacity, kinetic
parameters of the adsorption process, and the hydrodynamics of flow within the
columns. Various assumptions are necessary in order to facilitate the mathe-
matical solutions, and these further restrict their applicability to a limited
number of ideal systems.
Lukchis (180) discussed the mass-transfer-zone (MTZ) concept for use in
designing a fixed-bed adsorption system. Both rate and capacity considerations
confront the engineer designing a fixed-bed system. Mass-transfer coefficients
provide insight into the mechanism by which adsorption occurs but can be dif-
ficult to determine and tedious to use in designing problems. The MTZ concept
provides a simple, effective method for considering rate phenomena in fixed-
bed adsorption systems. The concept is particularly amenable to rapid deter-
mination and correlation of rate data and simple design procedures. Thus, a
simpler calculation of column height and diameter is possible.
An appraisal of the suitability of activated carbon for an untried opera-
tion involves two basic considerations (36):
1. Will activated carbon accomplish the desired objective?
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2. If so, does efficiency compare with that provided by other means?
MacCrum and Van-Stone (23) have suggested a method by which an industrial
plant can determine the feasibility of using a carbon process. The method
involves four initial fact-find steps: wastewater survey, pretreatment require-
ments, laboratory adsorption isotherms, and pilot studies.
PROPERTIES OF ACTIVATED CARBON
In addition to loading capacity, other properties must be considered
when choosing a carbon for a given application (74). These properties include
whether powdered or granular carbon is best, the rates of adsorption, the
friability of the (granular) carbon, handling problems, and cost. If a carbon
is to be used in a column contactor, it should be evaluated in continuous
column tests with the particle size most likely to be used in the future plant
scale of operation. The choice between powdered or granular carbon is usually
decided on grounds of economy and/or convenience (145). Both powdered and
granular forms of activated carbon are manufactured in a wide variety of grades
and sizes to accommodate the various liquid- and gas-phase applications (181).
Kelso and Lapp (181) have compiled information on various aspects of the manu-
facture and use of this material. Specific areas of interest were the raw
materials and methods of production, manufacturers, production capacities and
actual production, and consumption patterns of activated carbon.
Commercially available carbons vary widely in their characteristics and
more particularly in their effectiveness in removing dissolved TOG or color
from different effluent streams (74). By using a wide variety of commercially
available activated carbons in the laboratory program, it is possible to select
suitable commercial carbons for pilot-plant operation as well as for potential
full-scale application.
Activated carbons have specific properties, depending on the material
source and mode of activation. As a rule, granular carbons made from calcined
petroleum coke have the smallest pore size, largest surface area, and highest
bulk density (182). Lignite carbons have the largest pore size, least surface
area, and lowest bulk density. Carbons from bituminous coal have an average
pore size and surface area somewhere between that of petroleum coke and lignite
carbons and a bulk density equal to that of petroleum coke.
A comparison of activated carbons produced from lignite and bituminous
coal was presented by DeJohn (183). Lignite carbon gave higher organic load-
ings and during regeneration lost surface area at a slower rate. Also, larger
pore structure and ash content of lignite permitted lower temperature regenera-
tion. Comparative properties of these two carbon types are presented by
Swindel-Dressler (14).
DeJohn (184) reported that the best design data can be obtained by using
a long-service activated carbon which has been regenerated five or six times.
Since such carbon is not readily available and a virgin carbon must generally
be used, a lignite carbon will generally be the best alternative because it is
less susceptible to changes in properties and performance upon successive
regeneration than bituminous coal carbons.
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There is no adequate explanation of the selective action of different
carbons for different materials (145). This selectivity is best established
by means of experiments using the same waste material, the same conditions,
and the same carbons as proposed for the full-scale treatment systems.
Chaney et al. (143) summarized the general problem of the physical nature
and interrelationships of various forms of activated carbon. The type of car-
bon to be selected for a given purpose cannot be considered apart from the
development of the appropriate engineering methods for its employment. All
types of carbon do not respond similarly to changes in operating conditions;
consequently, the carbon that shows the most promise in the initial test may
not prove to be the best under operating conditions finally adopted (36).
Ford (35) reported that the selected carbon should:
1. Have the adsorption capacity to meet the effluent requirements.
2. Incur minimum losses during carbon transport and regeneration.
3. Have good hydraulic characteristics with respect to head loss or
pressure drop.
4. Represent the most cost-effective media to accomplish the prescribed
task.
These general areas of consideration should be augmented by test data and
process requirements developed from bench- or pilot-scale evaluations using
representative wastewater samples and selected carbons.
It is well known that carbons differ in their adsorptive capacity for a
given substance (36). Furthermore, carbons show highly specific interactions
with various substances. Thus, a carbon that has a high capacity for decolor-
izing molasses may have a relatively low capacity for removing phenol.
Large differences have been found between carbons, but no useful correla-
tion has been found between efficiency and either surface area or the ability
of the carbon to adsorb specific model substances (185). Therefore, index
numbers can be misleading, although they have merit for initial screening.
Bishop et al. (185) studied 12 carbons and sought to correlate adsorption
efficiency with other parameters that might be used as purchasing specifica-
tions. He stated that the superior performance of some carbons seems to be
related to their ability to remove a portion of the organic matter in suspension.
When an adsorbable solute molecule makes contact with a suitable unoccupied
space on the carbon surface, the molecule will adhere instantly; therefore,
time required for the adsorption process depends on the rate at which solute
molecules can diffuse to the carbon surface plus time to find the unoccupied
space (36). In carbon columns, initially many sites are available on the
exterior portions of the surface. Molecules arriving later must find sites
in the interior, which requires a longer time. After a brief, rapid initial
adsorption rate, the subsequent rate becomes much slower, especially for larger
molecules. Granular carbons may require many hours to utilize an appreciable
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portion of the total potential adsorptive capacity. With powdered carbon, if
the mixing is adequate, a major portion of adsorption is accomplished within
an hour, although adsorption may continue at a diminishing rate for days or
longer.
In comparing adsorption data of different investigations, one should be
aware that quantitative data (i.e., precise quantities of substance adsorbed)
are reproducible only when the studies are conducted under identical operating
conditions (36). A different temperature or concentration or use of a differ-
ent carbon can alter the amount adsorbed. Masse (11) reported that with a
given carbon and a specific wastewater, the product quality is a function of
the contact time and the amount of wastewater that has been passed through
the column.
The two most popular sizes of granular carbon for wastewater treatment
are 8 x 30 mesh and 12 x 20 mesh (21). Suggested specifications for granular
activated carbons to be used in wastewater treatment are also presented. It
is proposed that the best methods for evaluating and characterizing carbons
are: (a) quantifying the adsorption characteristics, (b) describing the
physical properties, and (c) conducting and interpreting pilot carbon column
studies.
The decision on the choice of carbon is not always easy since so many
factors must be considered (186). All granular carbons are not alike; and
differences exist among carbons from different manufacturers and, very likely,
for different batches from any single manufacturer. Before final selection
of a carbon, it behooves the consulting engineer to do some preliminary test-
ing, including some pilot work.
As a general rule, suppliers of activated carbon provide a table of quality
parameters for their products. Mattson and Kennedy (150) have reported that
most of this information is of little or no value to the individual whose
responsibilities include evaluating the economics of several competing acti-
vated carbons. They suggested that potential users undertake an evaluation of
each carbon in their own system and ignore the data provided by the suppliers.
Brunauer et al. (187) presented suggested specifications for granular
activated carbon for use in wastewater treatment. Hassler (36) stated that
two different grades of carbon should be employed when a single grade does not
have all the properties needed for purification.
GRANULAR CARBON SYSTEMS
In the general area of treatment by granular activated carbon adsorption,
the economics are such that the carbon columns are utilized best by driving
them to their equilibrium adsorption capacity and then regenerating and reusing
the carbon as many times as possible (150). An increase in the adsorption
capacity for any carbon can be economically offset by a corresponding decrease
in its ability to resist attrition. The ability of a granular carbon particle
to resist mechanical and hydraulic attrition plays the most important economic
role in selecting the carbon that will result in the lowest operating costs.
Carbon must be regenerable for these systems to work, and the regeneration
losses must be kept to a minimum.
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With the high first costs of carbon, it is essential in most applications
that the carbon be regenerated and removed after exhaustion of the initial
adsorption capacity to be economically feasible (188). The economic basis for
the use of granular activated carbon for removal of dissolved organics lies in:
(a) having a high capacity to remove residual organics and (b) the regeneration
of the carbon capacity for reuse in the treatment facility.
The most economical way to minimize operating costs in an established
granular carbon system is to use the carbon that gives the best performance
as a long-service carbon, provides the lowest loss, and has the lowest delivery
cost (189). Minimizing the dosage is the most effective method of minimizing
the effect of carbon-related factors on operating costs. Carbon loss or price
has a minor effect.
Whether bituminous coal or lignite carbons are used does not greatly
affect the operating costs of the system (189). Generally, the most economical
granular carbon system is the least expensive, properly designed system that
contains the least expensive, suitable granular activated carbon.
The technology required to employ granular activated carbon for adsorption
has been developed and available for several years. The commercial availability
of high-activity, hard, dense granular activated carbons made from coal, plus
the development of multiple-hearth furnaces for on-site regeneration of this
type of carbon, led to the early establishment of granular activated carbon as
an economically attractive unit process for wastewater treatment (21).
Joyce and Sukenik (47) and Joyce et al. (25) successfully demonstrated
that granular carbon can be reactivated thermally for reuse and a practical
operating capacity maintained. Thermal regeneration refers to the process of
drying, thermal desorption, and high-temperature heat treatment in the pres-
ence of limited quantities of oxidizing gases (15). Thermal regeneration using
a multiple-hearth furnace has been the most widely applied method in wastewater
treatment (35).
Reactivation of granular carbon by thermal reactivation has been discussed
in detail by Juhola and Tepper (190). The relative merits of directly and
indirectly fired furnaces for regeneration were presented. They concluded that
indirectly fired furnaces were capable of reducing regenerating losses to less
than 3%. Regeneration losses experienced in field tests have been found to be
from 5-20%, including mechanical attrition, for 8 x 30 and 12 x 40 mesh carbons
(150). Additional discussions of thermal reactivation of granular carbon have
also been presented by Gulp and Gulp (21), Schuliger and VanCrum (191), and
Juhola (192).
Himmelstein et al. (193) explored the technical and economic advantages
of in-situ regeneration of activated carbon. Operating experience from the
use of reactive regeneration and solvent regeneration is presented. Reactive,
solvent, and thermal regeneration are compared over a large range of concen-
trations. In-situ regeneration has the advantages of minimizing losses—first,
due to transport to and from the regeneration site and, secondly, due to the
avoidance of losses associated with thermal regeneration. An added attraction
is the opportunity to recycle products from the waste streams. As the concen-
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tration of the adsorbate increases, solvent and reactive regeneration systems
become more attractive than thermal regeneration. Several systems which have
been developed by the authors are presented as technological alternatives to
thermal regeneration.
Offline batch biological regeneration of spent activated carbon has been
shown to effectively maintain the carbon capacity near that of virgin carbon
(194). Recirculating aerated water through the contactor at incipient fluidi-
zation velocities proved to be a simple means of regeneration.
Carbon usage for treating industrial wastes may overshadow the municipal
use (186). It is in the industrial use of carbon where regeneration other
than thermal may be used. The recovery of product value from the adsorbed
material, in many cases, favors these approaches over thermal regeneration.
Johnson et al. (195) investigated the chemical regeneration of exhausted
carbon by the use of nine inorganic oxidizing agents. They concluded that the
economic feasibility of chemical regeneration was not promising. Friedman et
al, (196) reported that none of the chemical regenerants used restored the
original activity of the lightly loaded activated carbon upon which the tests
were performed. Generally, regeneration methods other than thermal will not
be effective enough if a complex mixture of organics have been adsorbed (197).
Loven (15) prepared a state-of-the-art report on carbon regeneration in
liquid-phase applications including historical development, recently developed
and experimental processes, and thoughts for further development. Attention
is directed to regeneration processes for spent carbons from wastewater treat-
ment systems utilizing both powdered and granular carbon processes. Loven
proposed increased research and development into the mechanisms of thermal,
chemical, and biological regeneration in order that better understanding of
these processes might be developed. He sees a clear need for continued growth
and improvement of carbon regeneration.
Extensive cost information on thermal carbon regeneration in a. multiple-
hearth furnace has been presented by Hutchins (176). To determine if spent
carbon should be regenerated on site for reuse, custom regenerated, or dis-
carded, operating costs must be compared. Carbon should be discarded at usage
rates lower than 350-400 pounds per day and thermally regenerated at higher
usage rates. If less than 580 pounds per day is used, it is generally more
economical to have the carbon custom regenerated than to regenerate it yourself
or to discard it. At higher rates, on-site regeneration will be more economical.
Cohen and English (186) reported that the minimum size physical-chemical plant
for which it is economical to provide on-site facilities for carbon regenera-
tion has not yet been established. In the future, small plants for which on-
site regeneration is not now economical may use jointly owned facilities or
may be able to use excess furnace capacity in some larger plant.
Lang et al. (74) reported that it is possible for the recovery of by-
products by adsorption-desorption cycles with activated carbon. The return of
such by-products could be credited against effluent treatment costs.
Because small pore surface area will decrease substantially as a result
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of thermal regeneration, column studies using carbon regenerated five or six
times should be conducted before a carbon system is designed (3). This is ex-
tremely important if the wastewater contains predominantly small molecules.
Joyce and Sukenik (198) reported that granular activated carbon in pagked-
bed column contactors 20 feet deep and operated at a flow rate of 4 gpm/ft
reduced the COD to an average of 18.5 mg/liter. The removal was not signifi-
cantly different from the removal obtained when operating the same columns at
10 gpm/ft . The cyclic saturation and regeneration of activated carbon through
16 cycles had an average carbon loss of 4.6%.
Design of carbon beds for removing organic colloids has also been dis-
cussed by Cookson (199). He illustrated how kinetic and equilibria data
could be used to predict the behavior of activated carbon beds. The variables
of flow rate, bed depth, void fraction, particle and bed characteristics, and
removal efficiency were evaluated by mass transfer theories. Mathematical
descriptions were used to predict the efficiency and bed capacity.
Gulp and Gulp (21) devote considerable attention to granular carbon ad-
sorption from selection of the carbon to design of full-scale treatment facil-
ities. The authors listed three controlling factors as most important in
determining effluent quality and carbon dosage: (a) contact time, (b) pre-
treatment, and (c) extent of the use of countercurrent principles. Bernadin
(200) listed the major design parameters as: (a) contact time, (b) carbon
usage rate, and (c) pretreatment requirements. He also discussed how to
determine whether activated carbon treatment is feasible and how to go about
collecting preliminary process design parameters.
Argaman and Eckenfelder (179) discussed design of activated carbon treat-
ment systems for industrial wastewaters, conduct of laboratory and pilot
experiments for specific applications, and .application of experimental results
for process design. Industrial wastewater is extremely variable in nature
and concentration and should be considered on a case-by-case basis. The most
reliable approach to design of a carbon adsorption system is to conduct pilot
column tests under conditions similar to those expected in the prototype.
Carbon column tests are often run in ways that give unreliable results,
e.g., testing at unrealistic flow rates or residence times, columns less than
1 inch in diameter, short runs, or unrepresentative feed (65). Obviously,
column tests should be based on the same conditions as those expected in the
actual plant system.
Anderson (145) highlighted the most important process engineering design
parameters for an activated carbon waste treatment system. The prime operating
cost is the expense of makeup carbon. The volume of makeup carbon can be
reduced to a minimum by careful attention being paid both to the chemistry of
the adsorption process, specifically to the treatment problem being considered,
and to the engineering design of the treatment system.
Cooper and Hager (8) reviewed the technical and economical feasibility of
waste treatment systems utilizing regenerable granular activated carbon.
Diagrams of the granular carbon reactivation cycle and the three basic adsorber
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design configurations (fixed beds in series, moving beds, and fixed beds in
parallel) are included. Design parameters are discussed; however, the infinite
variety of wastewaters from municipal and industrial sources makes any generali-
zation of design parameters difficult at best. Cooper and Eager stated that
operating costs of granular carbon systems reflect two major items of expense—
carbon makeup and equipment amortization. System optimization requires an
analysis of all factors as they vary with the treatment design.
Cover and Pieroni (33) summarized findings of the work on tertiary waste-
water treatment with activated carbon. The first phase consisted of a thorough
literature review of the data on tertiary wastewater treatment with a view to
generating sufficient basic data for process design of various schemes for car-
bon adsorption. The data extracted from the various sources should provide suf-
ficient information to make empirical designs of the type that have already been
demonstrated. The second phase consisted of preliminary economic evaluations
of several contacting schemes with a view toward finding which variables have
economic significance. Adsorbent cost and regeneration loss were found to have
a significant effect on total operating costs. Additional variables of impor-
tance are velocity, contact time, particle size, adsorber configuration, and
number of contacting stages.
Cover and Wood (201) presented the engineering design and cost estimates
of a 10-mgd plant for tertiary treatment of wastewater with granular activated
carbon. This report contains the process description, design bases, plant
operation, control rationale, equipment list, equipment specifications, and
cost estimate. The design incorporated reliability, economy, and then state-
of-the-art technology. The design was based upon the application of mature
engineering judgement to available data.
Cover and Wood (24, 202) presented detailed designs and cost estimates for
treating by carbon adsorption 1, 10, and 100 mgd of municipal wastewater from
biological treatment. These papers deal with the economic importance of several
design variables. Variables considered are plant size, velocity, contact time,
carbon loss, capacity, cost, type of contactor, number of contacting stages,
and certain combinations of these variables.
A process design manual for carbon adsorption was prepared for the EPA
Technology transfer by Swindell-Dressier Company (14). The manual includes
sections on general process considerations, process configurations, process
design parameters, equipment design, evaluation and selection of carbons, and
personnel requirements. Some of the more interesting observations presented are:
1. The carbon adsorption process readily lends itself to integration
into larger, more comprehensive waste treatment systems.
2. The greatest cost within the carbon treatment process is the cost
of carbon itself. Thermal regeneration of the spent carbon makes the process
economically feasible; the cost of the regenerating equipment, however, repre-
sents only a small fraction of the total capital cost.
3. The most important design parameter is contact time. Hydraulic load-
ing within the ranges used has little effect on adsorption.
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4. Data from laboratory and pilot tests, as well as experience from exist-
ing full-scale plants, must be carefully interpreted prior to the design of a new
plant.
Fornwalt and Hutchins (165) presented a method of evaluation that enables
the engineer to select the activated carbon that will economically yield the
product purity desired. It also describes how to get the data from which the
plant system can be designed. Fornwalt and Hutchins (203) also presented a
rationale for scaling up single-column breakthrough data to a full-scale,
multicolumn adsorption system utilizing carbon in a countercurrent fashion.
Contact time is held constant in this scale-up method.
Hager and Reilly (9) reported that the clarification-adsorption process
offers advantages over biological treatment which are difficult to quantify
in terms of dollars and cents but are of increasing importance to treatment
plant operators and the general public who see and use treated effluents.
The value of the isotherm data presented by Hager (19, 20) from his
Adsorption Isotherm Survey of Industrial Wastewaters is in its use as a refer-
ence when considering treatment alternatives available for a specified indus-
trial wastewater problem. The data provide preliminary indications as to the
utility of carbon adsorption for treatment of wastewaters from 104 manufactur-
ing operations. Hager (20) presented design parameters covering solvent fea-
tures of adsorption systems installed in 15 plants included in the 104 manu-
facturing operations. He proposed that adsorption must be considered with other
treatment processes to optimize economics in relation to desired treatment
objectives.
The design of granular carbon systems can be difficult and time-consuming
(65). The usual approach involves a four-step progression—powdered carbon
isotherms, laboratory column tests, pilot-scale tests, and finally design of
the commercial unit.
Zanitsch (204) discussed and described in detail adsorption system com-
ponents. He stated that the design engineer has considerable flexibility in
selecting the type of granular carbon system best suited for a specific situa-
tion. As a result, capital and operating costs of a system vary with the
design selected. Care must be taken to optimize the capital and operating
costs before a final decision is made.
Lukchis (205) discussed the factors influencing equipment design for carbon
adsorption systems. He stated that simplicity is the key to the design of com-
mercial beds. Adsorbers seem to perform best when the quantity and complexity
of vessel intervals are kept to a minimum. Vessel costs tend to increase dramat-
ically with the diameter. The minimum diameter of adsorber beds is set by pressure
drop limitations.
Design of the carbon column should be such as to ensure that the adsorptive
capacity of the carbon is utilized as fully as possible before regeneration
(145). Design factors include type of carbon, mesh size of carbon, length of
column, cross sectional area of column, and water flow rate. Flow rate and col-
umn length together determine contact time. Optimization of design parameters
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is essential; e.g., a. fine mesh carbon will provide a short contact time, but
this is best obtained by reducing the column length and thus reducing head loss.
Contact or residence time is the major design parameter for the adsorption
systems (65). The optimum residence time determines size of the adsorbers and
volume of the carbon bed. Regeneration equipment should be sized according to
the rate of carbon consumption required to maintain an acceptable product.
Hutchins (65) reported that minor design and operating parameters include linear
flow rate, impurity concentration, composition in feed product, pH, temperature,
and viscosity. Additional factors are properties of the adsorbent, such as,
particle size, adsorptive capacity, pore size distribution, and chemical nature
of the adsorbent surface. Some of these factors are interrelated; therefore,
both adsorption and regeneration sections of the system should be larger than
the optimum size based on estimated residence time and carbon consumption.
Lukchis (206) presented a simplified process design procedure for regen-
erative adsorption processes. Design of multicomponent systems is similar but
considerably more complex. Desorption is the key step in the performance of
regenerative adsorption systems. It involves simultaneous mass and heat trans-
fer passing through a packed bed of adsorbent under carefully controlled con-
ditions .
Carbon losses affect the operating costs of a granular carbon thermal
regeneration system (189). To minimize the amount of carbon lost as a result
of attrition, the adsorption, regeneration, and carbon handling system must
be adequately designed.
It has been noted in various treatment systems that long-term column
adsorption treatment studies generally result in developing carbon dosage
rates lower than that determined based on isotherm testing (89). Biological
degradation in carbon columns can reduce the carbon exhaustion rate in an
operating plant by as much as 30%. It was also noted that capital cost is
primarily a function of the amount of water to be treated.
The carbon capacity determined from continuous flow column studies is
10-80% greater than that predicted by the batch isotherm tests for the cases
studied (83). This difference is attributed to the higher concentration
gradient and biological degradation which prevail in columnar studies.
Weber et al. (38) reported that biological activity in carbon columns
results in partial regeneration of the carbon, thereby increasing the
capacity. The biological activity seems to enhance overall capacity for
removal of organics, thus affording longer periods of effective operation
than might be predicted. The biodegradable organics undergo anaerobic de-
composition inside the carbon pores and release low molecular weight inter-
mediate products typical to anaerobic processes. These products may be
further degraded aerobically if sufficient oxygen and aerobic biota are
present or else escape with the effluent. To eliminate excessive fouling of
columns due to aerobic biological growth, expanded-bed operation is advocated.
The continued adsorption activity of carbon well beyond its expected
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capacity has been observed by Perrotti and Rodman (207). This may be due to
the desorption effect produced by exo-enzyme activity. The mechanism of this
activity is still not fully understood, however.
Bishop et al. (208) conducted pilot-plant studies of a physical/chemical
process using granular activated carbon to treat raw wastewaters. In all
cases, a nonsorbable fraction was found to exist. The nonsorbable fraction
increased with the loading and the development of biological activity in the
columns.
Based on pilot study results, Weber et al. (38) and Hopkins et al. (39)
suggested that carbon columns become saturated rather quickly with some frac-
tion of the organic material which hinders total organic removal. Thus, the
main columns "leak" organics, and the polishing column is suggested as a pos-
sible solution. It evidently picks up organics which are only weakly sorbed.
Joyce et al (25) reported on a pilot-plant study to determine the eco-
nomic practicality of using granular activated carbon adsorption to treat
secondary effluents. The data indicated an organic residual averaging 15-20
mg/liter COD in the effluent. This supports the conclusion that a nonsorbable
material was present in the wastewater.
The leakage of organic matter through activated carbon columns was first
noted by Joyce and Sukenik (198). Such a leakage was also reported by Park-
hurst et al. (40), who observed that a certain fraction of the organic matter
of an activated sludge effluent was not removed in carbon columns. The pres-
ence of this nonadsorbable fraction has been illustrated in batch adsorption
studies (209). Increasing the amounts of activated carbon beyond a certain
dosage did not further decrease the equilibrium concentrations. Bishop et al.
(185) stated that low molecular weight compounds, e.g., sugars, glycols, amino
acids, hydroxyl acids, which may be present in the effluent, are too polar to
be adsorbed and would therefore pass through activated carbon columns.
Parkhurst et al. (40) also reported a leakage of organic compounds through
activated carbon columns. The exact nature of the leakage is unknown, but
both Joyce et al. (25) and Parkhurst et al. (40) reported a strong indication
that the leakage was comprised partially of nonadsorbable bacterial cell
fragments and partially of small organic molecules which were extensively
hydrolyzed in the biological treatment stage and thus rendered more soluble
and less subject to adsorption.
Pilot studies conducted at several refineries indicated a persistent
"leakage" of BOD organics when fixed-bed carbon columns were applied as a
single treatment system (83). Thus, the series of biological-carbon systems
will probably be the most prevalent application of carbon for refinery waste-
water treatment.
Bishop et al. (53) reported that after removal of colloidal substances by
clarification and filtration, carbon physically adsorbed most of the dissolved
organics. In continuous treatment, the buildup of soluble organic material on
the carbon rapidly promoted substantial biological activity within the adsorption
system and degraded the product effluent.
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Bishop et al. (185) reported that some, if not all, of the organic mate-
rial that penetrates carbon columns might be colloidal in nature, either
organic colloids such as fragments of bacterial cell walls and undigested
particles or low molecular weight organic substances adsorbed on mineral
colloids. Another class of organic substances refractory to carbon treatment
consists of strongly hydrophilic (polar) organic molecules, e.g., highly
oxygenated organic compounds. Although these strongly hydrophilic organics
tend to remain in the water phase during adsorption, most should be biodegrad-
able and, therefore, not present in well bio-oxidized secondary effluents.
The organic portion of the colloidal material or suspended solids in the
effluents should therefore constitute the major portion of organics refractory
to carbon adsorption.
In continuous 9-month pilot-scale studies, Friedman et al. (196) found
that aerobic expanded-bed activated carbon columns effectively and economically
treated a chemically coagulated and clarified primary effluent to produce a
high-quality final effluent. The primary objective was to study the effects
of biological activity in expanded beds of carbon-treating primary effluent.
More organics were removed with aerobic operation than anaerobic operation.
Furthermore, quality of the effluent was much more consistent in the aerobic
system. Anaerobic effluents consistently contained 8-13 mg/liter of hydrogen
sulfide, but effluents from aerobic columns were essentially free of hydrogen
sulfide. Effectiveness of the expanded-bed adsorbers was enhanced by occa-
sional air scrubbing and backwashing to remove excess biomass and to precipi-
tate coagulant from the column beds.
Culp and Shuckrow (210) presented a comparison of the major physical/
chemical treatment research efforts focusing on the identification of common
problems which exist. It is obvious from this review that more research is
needed where carbon columns are loaded with wastewaters having organic concen-
trations greater than 50-100 mg/liter TOC. Successful techniques for hydrogen
sulfide control must be demonstrated before physical/chemical systems with
granular carbon columns can reach full acceptance. This review also suggested
the need for additional research on carbon capacity and the relationship of
biological activity in carbon columns to that capacity.
Eckenfelder et al. (211) compared the performance of aerobic, anaerobic,
and sterile columns fed with a synthetic waste containing compounds of varying
biodegradability. They concluded that anaerobic growth causes a reduction in
carbon adsorption capacity, while aerobic growth may or may not enhance adsorp-
tion capacity depending on the biodegradability and the concentration of the
wastewater.
The presence of biodegradable organics can result in biological activity
in carbon columns, which can have beneficial or adverse effects on adsorption
capacity depending on the concentration and other parameters (179). In indus-
trial applications, the concentration of organics is generally rather high, and
operation time between successive regenerations is too short to allow a signi-
ficant buildup of active biota in the columns.
Rizzo and Schade (212) noted hydrogen sulfide generation in carbon columns
during pilot tests but reported that this could be controlled by frequent back-
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washing. Shuckrow (56) indicated hydrogen sulfide was a major problem in pilot-
plant operations treating a strong municipal wastewater. This study documented
the nonsorbable residual that might be present in wastewater and the difficul-
ties that may be encountered in controlling the hydrogen sulfide. Clearly,
based on this work, great care must be exercised in the design of physical/
chemical treatment systems. To proceed with such activities without extensive
pilot-plant testing risks substantial operational difficulties in full-scale
systems.
Suhr and Gulp (213) noted that under certain conditions granular carbon
beds provide favorable conditions for the production of hydrogen sulfide gas,
which has an unpleasant odor and which may contribute to corrosion of metals
and damage concrete. They presented a list of measures which could be taken in
plant design to provide flexibility for dealing with problems of hydrogen
sulfide production. They also presented some remedial measures available in
the operation of carbon facilities.
A laboratory study by Pressley et al. (214) showed that breakpoint chlori-
nation with proper pH control and mixing provided a physical/chemical method
for removing ammonia from wastewaters by oxidizing NH/+ to nitrogen gas.
Bishop et al. (53) reported that use of breakpoint chlorination for ammonia
removal produced an overall total nitrogen removal of 86% during their study.
The average residual total nitrogen of about 2.8 mg/liter contained approxi-
mately 1 mg/liter of ammonia-nitrogen. A second application of chlorine after
carbon adsorption could be employed to further reduce the total nitrogen to
about 2 mg/liter. Harr and Mastropietro (215) reported that breakpoint chlo-
rination followed by activated carbon filtration was an effective method for
the removal of ammonia nitrogen from wastewater.
In the treatment of aqueous wastes, granular carbon has been the form most
used with adsorber configurations being: fixed beds in parallel or series, or
moving beds with either, and upflow or downflow distribution system (8, 42,
145). Based on pilot-scale tests under field conditions, Hopkins et al. (39)
concluded that both packed and expanded carbon beds were equivalent in removal
of soluble organics from secondary effluents. Expanded beds had the advantage
of lower power and maintenance requirements, although suspended solids were
more effectively removed by packed beds.
Weber (164) clearly demonstrated the advantages of the fluid-column method
for contacting carbon with wastewaters over the packed-column method. Weber
reported that very high capacities, with resulting economies, can be realized
for activated carbon in fluidized columns under proper operating conditions.
Moving bed systems are finding wider application in continuous uses of
granular activated carbon because of their greater efficiency and reduced
labor requirements compared to fixed-bed systems (135). Movement of the carbon
countercurrent to the flow gives an extremely efficient use of the carbon
capacity.
Studies by Weber and Morris (136) on equilibria adsorption indicate, as
did previous kinetic studies by Weber and Morris (141), that column operation
is probably to be preferred to batch or nonflow processes. Column operation
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makes better use of the capacity of the carbon because much of the adsorption
in this type of process occurs at solution concentrations corresponding to the
plateau-like region of the adsorption isotherm; this is not true for batch
operations. These workers reported that a countercurrent column operation
would be even more satisfactory for removal of low levels of organics by ad-
sorption.
It has been shown that carbon particle size should be kept as small as
conditions of efficient operation allow so that high rates of adsorption may
be obtained (141). If filtration of the waste through a column of granular
activated carbon is contemplated, use of extremely small particles may result
in excessive head losses. Use of large particles requires large quantities
of carbon and long columns for providing time of contact sufficient for ob-
taining the desired effluent quality. Weber and Morris proposed fluidized-
column operation, in which waste flows upward through an expanded bed of fine
carbon, as one method of taking advantage of small particle size and yet
avoiding the problem of excessive head loss and difficulty of separating the
adsorbent from the solution.
Keinath (216) has shown that competitive adsorption, even in binary solu-
tions, can lead to chromatographic displacement effects in column adsorbers
treating wastewaters of varying composition. In this situation, the weakly
adsorbed solute is retained by the carbon for a time and then is eluted as a
rather concentrated "peak" upon prolonged contact with the feed wastewater.
He further showed that fluidized-bed adsorbers are much less susceptible to
this effect than more conventional packed beds—a finding that could have sig-
nificant impact on adsorber design.
It is obvious that any restriction of pore openings or buildup of ash or
other materials within the pore openings due to the presence of suspended or
colloidal materials and their accumulation on or in the carbon particles might
have an adverse effect upon the adsorptive capacity or service life of the
carbon (21). Therefore, the water applied to the carbon columns should be
pretreated to the highest practical chemical clarity.
In the treatment of wastes, the granular carbon system could benefit from
a pretreatment stage (145). This is especially so if it reduces the organic
loading to the carbon columns, removes colloidal material, and/or removes sus-
pended matter. Experience indicates that biological filtration and chemical
clarification are two systems which offer the most valuable pretreatment con-
tributions.
Harrison (217) indicated that removal of turbidity was needed be-
fore granular carbon treatment. Gulp et. al. (218) have shown that efficient
solids removal before carbon adsorption produces high-quality water from
secondary municipal effluents. A comparison of COD values in effluents from
clarified and unclarified secondary effluents subjected to carbon adsorption
indicates that organic breakthrough is associated with suspended solids.
Ford (138) stated that lack of pretreatment in removing certain constitu-
ents can have an adverse effect on activated carbon. Many of the negative
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aspects experienced in the activated carbon treatment of industrial wastewaters
have been attributed to poor pretreatment. Conversely, selected pretreatment
can enhance carbon adsorption performance.
Westrick and Cohen (219) investigated the influence of chemical pretreat-
ment on carbon performance by operating and evaluating three separate clarifica-
tion-carbon systems simultaneously. Results obtained are obviously specific to
the type of waste and conditions of operation discussed. Although slight dif-
ferences in the performances of the three systems were observed, all three
produced effluents in essentially the same range of quality. Carbon data
analysis illustrates that even slight differences in carbon performance may be
related to the economics for inclusion in the treatment process selection
procedure.
Zuckerman and Molof (153) reported on efforts to maximize the effectiveness
of activated carbon adsorption to yield high-quality water for reuse. The proc-
ess of chemical pretreatment followed by activated carbon treatment was found
to be qualitatively and economically superior to conventional tertiary treat-
ment.
Bishop et al. (185) reported that although clarification will reduce the
total organic load on a carbon column, it is unlikely that it will increase
substantially the volume that can be treated before breakthrough.
Weber (42) reported that although municipal wastewater treatment experi-
ence has indicated good treatment at contact times between 30-60 minutes, sig-
nificantly longer contact times are normally required for industrial waste
streams. This is consistent with generally higher organic concentrations.
Adsorption theory and practice indicate that treatment efficiency and economics
are favored by higher concentrations. Thus, at some industrial installations,
concentrated waste streams treated individually at their respective sources
would optimize overall treatment system design and economics.
POWDERED CARBON SYSTEMS
Although knowledge of the applicability of activated carbon to wastewater
treatment has been well established, limitations have been placed on the use of
powdered carbon forms because of the difficulty encountered in separating them
from solution and in regeneration (220). Rnopp and Gitchel (221) reported that
the application of powdered carbon systems to wastewater treatment has been
limited by the ability to economically regenerate powdered carbon. Shell (222)
observed that application of powdered activated carbon for the treatment of
wastewaters is dependent upon an efficient and effective method of regenera-
tion.
Interest in the application of powdered activated carbon (PAC) for treat-
ment of wastewaters is based on the fact that PAC has all the inherent process
advantages of a nonbiological treatment system, yet is less expensive than
granular activated carbon (GAG) and requires less inventory, and can be fed on
demand (222). Even with these basic process advantages, regeneration of spent
PAC is necessary for most wastewater treatment applications.
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Powdered activated carbon is a highly adsorptive material that can be
used to treat wastewater (194). The small structure size and highly porous
structure make it an ideal adsorbent. Shell (222) reviewed the advantages and
characteristics of powdered activated carbon in its application to wastewater
treatment and discussed various approaches to regeneration of PAC. He stated
that the basic process of regeneration is not different for PAC and GAG. The
carbon first drys, then bakes, and is finally reactivated. Disadvantages in
regeneration of powdered carbon compared to granular carbon lie in the diffi-
culty of dewatering the carbon before regeneration and in high losses during
thermal regeneration (74).
The development of thermal regeneration of spent powdered carbon has only
recently been vigorously pursued (28, 57, 163, 221, 222, 223, 224, 225, 226,
227). Results of these studies have indicated that thermal regeneration is now
technically feasible and may be economically justified. A transport reactor in
operation since 1971 now routinely regenerates 10 tons per day of powdered ac-
tivated carbon at less than one-third of what it would cost to buy it fresh (228).
Cost estimates are given. A multiple-hearth regeneration furnace has recently
been installed as part of the 40-mgd PACT treatment process at the DuPont
Chambers Works Plant (129).
Results of tests conducted by Bloom et al. (223) and Gitchel et al. (225)
indicated that water quality produced by using regenerated activated carbon
equaled the best obtained with virgin carbon. Davies and Kaplan (163) described
a method for reactivating powdered carbon in which the carbon was dewatered,
dried in an indirect steam dryer, and then fed to a regeneration furnace where
regeneration was effected in a steam atmosphere at 750 F. using a 1-hour deten-
tion time.
Bloom et al. (223) reported that because of its large surface area,
powdered carbon could be reactivated in a matter of seconds if the carbon were
heated in a steam atmosphere to 1400-1600 F. A process, called the dispersed
phase technique, was developed and demonstrated which takes the spent carbon
slurry and reactivates the carbon without drying or removing the bulk of the
water. The regenerated carbon has essentially the same activity as fresh
material.
Burant and Vollstedt (29) and Knopp and Gitchel (221) have reported that
the adsorptive properties of powdered carbon can be restored by partial wet air
oxidation without dewatering of the sludge. Wet air oxidation is reported to
regenerate the active carbon with less than 10% loss. Adams (72) reported that
the wet air oxidation process is, technically, the most attractive regeneration
method.
Fluidiged-bed systems for powdered carbon regeneration have been evaluated
on a pilot scale and appear to have promise (28, 227, 229). In this process,
a bed of inert granular material such as sand is fluidized by the upward flow
of hot gases, and the wet spent carbon is injected directly into the bed. Under
proper operating conditions, spent carbon can be restored to an active form as
effective as virgin activated carbon. Recovery of regenerated carbon has been
found to be about 85% per regeneration cycle.
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A new process, called the Atomized Suspension Technique (AST) for regen-
erating spent powdered activated carbon, has been developed and successfully
demonstrated (230). This process is the first to use an oxygen-free atmosphere
for regeneration. This eliminates the possibility of burning some carbon and
reducing the yield. Pilot-plant results show that AST reactivates the carbon
to about 95% of its original capacity with yields of better than 90%, which
compares favorably with regeneration results of granular activated carbon.
Flynn and Barry (231) have presented a methodology for evaluating the
choices between type of carbon (granular or powdered) and method of applica-
tion.. The choice between the various alternatives is dependent on waste charac-
teristics, thus requiring laboratory experimentation to define technical feasi-
bility, cost, and reliability.
An intensive laboratory investigation has been conducted using 11 commercial
and experimental activated carbons to evaluate their physical and adsorptive
properties, to select those best suited to treating municipal wastewaters, and
to gain insight into properties important for this application (209). Results
suggested that the best carbons for adsorbing organics from municipal wastes
have a broad spectrum of pore sizes so as to accommodate the wide variety of
molecules present in wastewaters. Mixtures of carbon of different pore struc-
ture characteristics might prove to be more efficient. As expected, rate of
adsorption of a powdered carbon was seen to be strongly influenced by its
particle size.
Obviously, properties of the powdered carbon used are of primary importance
(209). The carbon must have a high adsorptive capacity for the organic pollut-
ants, attain a close approach to equilibrium in a reasonable period of time, and
be readily removed by flocculation and clarification. Studies by 0'Conner et al.
(232) to determine which basic carbon properties are important for treating
wastewater have shown no strong correlation between COD adsorption ability and
BET surface area or ability to adsorb four specific model substances.
Burns and Shell (58) reported the following advantages of powdered acti-
vated carbon over granular activated carbon:
1. Cost of powdered carbon substantially less.
2. Powdered carbon adsorption will equilibriate with soluble wastewater
organics in a small fraction of the time.
3. Powdered carbon can be supplied on demand to meet varying feed organic
strength.
4. Powdered carbon systems require a fraction of the carbon inventory.
5. Powdered carbon contacting system is more amenable to control of
undesirable biological activity.
Soluble organic removal in a physical/chemical system treating municipal
wastewater is usually assumed to be primarily by a physical adsorption mecha-
nism. Burns and Shell (58) implied that a substantial amount of soluble organic
60
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removal was accomplished by anaerobic biological activity in a powdered activated
carbon physical/chemical treatment system. These workers also reported that
laboratory test results indicated no significant effect of chemical pretreat-
ment on organic removal.
Wallace and Burns (60) presented results from a 15-month pilot-plant
study indicating the effect of biological activity, pretreatment chemicals,
thermal regeneration, and countercurrent staging on soluble COD removal in a
powdered activated carbon physical/chemical treatment system.
Bishop et al (185) studied the removal of organic material from secondary
effluents using powdered activated carbon. Data from their studies suggested
the presence of a nonadsorbable organic fraction. The authors found that with
clarification and powdered carbon, treatment produced secondary effluent TOC
values of 2 mg/liter or less. They concluded that treatment studies with actual
wastewaters was the only dependable method of rating powdered carbon perform-
ance.
Davies and Kaplan (26) conducted studies on the effects of powdered carbon
size on the efficiency of organic removal and found that, although some improve-
ment in the adsorption rate could be obtained with finer particle sizes, the
added cost of sizing and the more difficult prospect of physical handling out-
weighed the improvements in adsorption. The workers concluded that it was not
physically possible to separate commercially available carbons to obtain par-
ticular particle sizes. They also found, in a laboratory study, that 90% of
the adsorption capacity of powdered activated carbon could be realized in less
than 5 minutes in most cases with turbulent mixing.
These studies by Davies and Kaplan also showed that powdered carbon could
be removed from treated effluents by flocculation and settling. The workers
tested a number of high-molecular-weight flocculating agents and found that
nonionic polyacrylamides were most effective. These studies also showed that
several high-molecular-weight polyelectrolytes were effective flocculants for
carbon at a dose of 1 mg/liter. Residual carbon in the settled effluent was
in the range of 10 mg/liter. Lime added in the range of 300-400 mg/liter
performed about as well.
The use of powdered activated carbon in the area of municipal wastewater
treatment has been reasonably well established (233). A high-quality effluent
can be achieved at a relatively low cost. The process was determined to be
well suited to automation and flexible in treatment of wastes of variable
strength and composition. Advantages of using powdered activated carbon in
treating industrial wastes are discussed. Also, several industrial wastewater
treatment designs employing powdered activated carbon and the developed associ-
ated costs of treatment are presented.
The addition of powdered activated carbon to the activated sludge process
is a recent innovation which has attracted attention because of its ability to
remove pollutants beyond the ability of ordinary secondary treatment at a cost
which is intermediate between secondary treatment and secondary treatment plus
carbon columns (30, 124).
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The principle advantage of the DuPont PACT process (76) is that it achieves
tertiary quality treatment in secondary treatment facilities. A mixing tank
and pump for feeding powdered carbon slurry to the aeration tanks is all the
equipment needed to upgrade an existing secondary treatment plant. Moreover,
by controlling powdered carbon concentration, one has a convenient way of
regulating the degree of treatment obtained (125). Lawrence and McCarty (236)
reported that at a given carbon dosage, performance of the PACT system could be
satisfactorily described by the use of conventional biological kinetic methods.
Data from laboratory tests indicated that addition of powdered activated
carbon to the aerators of an activated sludge process would not only provide
color and refractory organics removal in an effective manner but also would
add stability to the activated sludge process resulting in improved and more
consistent BOD removal (128). Subsequent research showed that bacteria tending
to grow around carbon particles in such a manner as to reduce the dispersion
of biological floes make the sludge more dense and improve settleability of
sludge in a clarifier. All of this would result in significant improvement
in the activated sludge process of wastewater treatment.
Commercial processes using powdered activated carbon to enhance activated
sludge wastewater treatment are available (234). Powdered activated carbon has
been shown to increase the sludge settling rate, decrease the sludge volume,
increase effluent clarity, and promote greater reductions in BOD and COD than
bio-treatment alone. Results presented by these workers indicate that the
carbon-assisted bio-unit could handle a greater flow rate or that a smaller
bio-unit could be used to process the same volume of wastewater as a conven-
tional activated sludge system.
Adams (62, 63) reported that both laboratory and full-scale evaluations
have demonstrated that addition of lignite powdered activated carbon to an-
aerobic digesters will provide many benefits, alleviating operating problems
and reducing sludge disposal costs. He stated that two properties of the lig-
nite powdered carbons which render them most effective are density and activity.
Adams suspects that by adsorbing organics and concentrating them for biodegrada-
tion, carbon helps to drive the reaction further toward completion.
Ferguson (236) demonstrated the capabilities of the PAC-biological contact
stabilization process. He also compared the effectiveness of two high-surface-
area powdered activated carbons with a low-surface-area PAC in the treatment
process. Powdered activated carbon addition to the contact stabilization process
improved removal of soluble organic matter at the PAC dosages tested. A dosage
of 25 mg/liter of either high-surface-area PAC was comparable to, or slightly
better than, a 75-mg/liter dosage of the low-surface-area PAC.
Thibault et al. (237) conducted a test program for the evaluation of
powdered activated carbon treatment of activated sludge. Results are presented
which show that low carbon levels (0-400 mg/liter) are ineffective in improving
contaminant removal and sludge settling characteristics. Some improvement in
clarity was noted. High carbon levels (1000-2000 mg/liter) may improve TOG and
COD removals during normal operation but do not affect ammonia removal. Based
on experience to date, these workers proposed that powdered activated carbon
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treatment would be most effective when used on a standby basis for low-cost
treatment or organic shock loadings.
Flynn (238) reported that a reliable methodology for use in the testing of
powdered carbons for use in the PACT system has been established. Not surprising,
those parameters which measure surface area available for adsorption seem to
correlate fairly well with effluent quality. Flynn stated that any comprehen-
sive design for a wastewater treatment plant based on powdered activated carbon
addition to the activated sludge process must consider the cost of the carbon
and ways of reducing it.
Flynn (239) developed a procedure for analyzing data to produce the re-
quired sludge age for a biological system with powdered carbon addition.
Removals of COD and TOC due to carbon alone can be computed and indicate that
the removals due to carbon apparently increase with sludge age.
Lang et al. (74) discussed and described a three-stage countercurrent
contacting system that uses a fine activated carbon of a size intermediate
between powdered and standard granular carbon. A preliminary engineering
assessment of this system (dubbed FACET—JFine Activated Carbon Affluent Treat-
ment) indicated that it combines some of the advantages of both conventional
systems. The process has been patented (75, 76). Carbon size and concentra-
tion are chosen such that contact time requirements are short, while the size
is large enough to provide fast settling without assistance of flocculation
agents. Therefore, FACET permits use of simple stirred tanks as contactor-
settlers and avoids need for the preclarification or frequent backwashing
commonly required in granular carbon systems to prevent column pressure build-
up. The suitable particle size range for FACET is 100-250 microns (140-60
mesh). This particle size is believed to be large enough to permit use of
the standard multiple-hearth furnace for regeneration. The workers discussed
advantages and disadvantages of FACET versus powdered and granular carbon
treatment.
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SECTION IV
DISCUSSION
An attempt has been made to review the literature pertaining to activated
carbon adsorption and its use in the treatment of petroleum refining, petro-
chemical, and combined municipal/industrial wastewaters. There is ample evidence
in the literature to suggest that activated carbon adsorption, using either granu-
lar or powdered carbon, should be considered when evaluating treatment alterna-
tives for these wastewaters. Activated carbon adsorption, however, should not
be thought of as a process which will satisfactorily remove all organic com-
pounds from wastewater (240). There are considerable variations in the adsorp-
tive behavior of organic compounds so that adsorption may not always provide a
suitable removal process.
Many classes of organic compounds (particularly oxygenated organics) are
not amenable to carbon adsorption and show up as residual BOD, COD, or TOC in
carbon column effluents (35, 138). This limits the overall process efficiency
of activated carbon when treating industrial wastewaters. Knowledge of the
chemical composition of wastewaters is essential for determining the applica-
bility of carbon adsorption and for the efficient design of adsorption facili-
ties (240). The final test of the applicability of a process will be not only
the efficiency but also the economy of the process or system (83).
The efficacy of utilizing carbon adsorption for the treatment of refinery
and petrochemical wastewaters at any point in a process sequence can be deter-
mined only after a thorough investigation (83). There has been a tendency for
investigators and equipment developers to oversimplify the process adaptability
for industrial wastewater applications. Specifically, translation of data
from carbon systems receiving domestic wastes into design criteria for indus-
trial utilization has limited validity, and the use of batch isotherm informa-
tion under any testing condition as a basis for process selection is imprecise.
Activated carbon treatment of industrial wastes, while promising, must be
carefully evaluated before process decisions are made and capital funds are
committed (35, 138). Breakthrough geometry and adsorption kinetics of multi-
component wastewaters are difficult to define; many organic compounds are not
amenable to carbon adsorption; and the effects of carbon regeneration are
variable and unpredictable. For these and other reasons, comprehensive testing
and technical reviews are a necessary prerequisite to process commitment.
There is no single wastewater treatment system which can be applied in
all cases. Each industrial operation has its own wastewater characteristics
and its own effluent requirements. Availability of land, complexity of opera-
tion, cost of treatment facilities, and variations in wastes all combine to
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make each wastewater treatment system unique. The optimum treatment system
can be designed only after careful study of the entire problem and preliminary
evaluation of several alternate designs.
To produce maximum results, a wastewater treatment system must be not only
well designed but properly operated. The simple adding on of a carbon system
will not make up for deficiencies in a biological system, regardless of whether
those deficiencies are from improper design or operation.
New truths pertaining to the subject of activated carbon treatment become
known on a continuing basis (138). However, when evaluating process concepts,
developing design bases, predicting effluent quality, and finalizing manage-
ment decisions in terms of constructing control systems with attendant capital
commitments, one must carefully base these judgements on the known process
capabilities and limitations of activated carbon.
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91
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TECHNICAL REPORT DATA
>Pkase read Instructions on the reverse before completing)
REPORT NO.
EPA-600/2-78-200
4. TITLE ANDSUBTITLE
REATMENT OF PETROLEUM REFINERY, PETROCHEMICAL AND
:OMBINED INDUSTRIAL-MUNICIPAL WASTEWATERS WITH ACTIVATED
IARBON - Literature Review .-.
7. AUTHOR(S)
John E. Matthews
3. RECIPIENT'S ACCESSIOI*NO.
5. REPORT DATE
September 1978 Issuing date
6. PERFORMING ORGANIZATION CODE
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
SAME AS BELOW
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
In-house
12. SPONSORING AGENCY NAME AND ADDRESS
Robert S. Kerr Environmental Research Lab. - Ada, OK
Office of Research and Development
U. S. Environmental Protection Agency
Ada, Oklahoma 74820
13. TYPE OF REPORT AND PERIOD COVERED
14. SPONSORING AGENCY CODE
EPA/600/15
15. SUPPLEMENTARY NOTES
16. ABSTRACT A review of the literature on activated carbon adsorption as a treatment con-
cept for petroleum refinery, petrochemical plant, and combined industrial-municipal
wastewaters is presented in this report. A total of 241 references are cited. These
references cover the various aspects of carbon adsorption and its application in the
treatment of industrial and municipal wastewaters.
There is ample evidence in the literature reviewed to suggest that activated carbc
adsorption, using either granular or powdered carbon, should be considered when eval-
uating treatment alternatives for industrial wastewaters. Successful applications of
this mode of treatment have been claimed at numerous municipal, industrial, and combine
municipal-industrial installations.
It must always be remembered, however, that there is no single wastewater treat-
ment system which can be applied in all cases. There are enough variations in the
adsorption behavior of organic compounds so that adsorption may not always provide
a suitable removal process. Each industrial operation has its own effluent character-
istics and requirements. Availability of land, complexity of operation, cost of treat-
ment facilities, and variations in wastes all combine to make each wastewater treatment
system unique. The optimum treatment system can be designed only after careful study
of the entire problem and preliminary evaluation of several alternate designs.
17.
KEY WORDS AND DOCUMENT ANALYSIS
a.
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS
COSATI Field/Group
Activated Carbon, Adsorption, Industrial
Waste Treatment, Refineries
Literature Review,
Carbon Regeneration,
Design Parameters.
Municipal Waste Treat-
ment, Petrochemical
Plants, Combined In-
dustrial-Municipal Treat
ment
68D
13. DISTRIBUTION STATEMENT
Release to Public
19. SECURITY CLASS (ThisReport)
UNCLASSIFIED
21. NO. OF PAGES
98
20. SECURITY CLASS (This page)
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (9-73)
92
ft U.S. GOVERNMENT PRINTING OFFICE 1978-657-060/1473
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