PROCESS DESIGN MANUAL FOR
SUSPENDED SOLIDS REMOVAL
For
ENVIRONMENTAL PROTECTION AGENCY
Technology Transfer
By
Burns and Roe, Inc.
700 Kinderkamack Road
Oradell, New Jersey 07649
Program No. 17030GNO
Contract No. 14-12-930
October, 1971
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The wention of trade nq . es or commercial products tn this manual is for illustration
purposes, and does not constitute endorsement or recommendation for use by the
Environmental ?rotection Agency.
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ABSTRACT
This manual comprises a compilation of information on the practice of suspended solids
removal from municipal wastewaters. General engineering considerations are cited with
respect to their impact on the design of treatment facilities. Specific processes utilized for
suspended solids removal are described, discussed and illustrated through the use of data
from installations which have employed these processes. Current technology and advanced
methods of treatment are stressed in order to provide usable information for
implementation in design of new treatment facilities Some aspects of the operation and
maintenance requirements of the described unit processes are delineated, along with the
overall estimated costs of construction and operation. The information and data provided
are presented in such a manner that they can be readily incorporated into practice.
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TABLE OF CONTENTS
Chapter Page
ABSTRACT iii
TABLE OF CONTENTS v
LIST OF FIGURES vii
LIST OF TABLES ix
FOREWORD Xi
INTRODUCTION 1-1
1.1 General 1-1
1.2 Wastewater Solids I-I
2 POPULATION AND DESIGN PERIOD FORECASTING 2-1
2.1 General 2-1
2.2 Population Growth 2-1
2.3 Economic Considerations 2-2
2.4 Physical Considerations 2-2
2.5 Wastewater Characteristics 2-2
3 FLOW EQUALIZATION 3-I
3.1 General 3-1
3.2 Clarifiers 3-2
3.3 High-Rate Settlers 3-2
3.4 Microscreens 3-2
3.5 Filters 3-2
3.6 Moving Bed Filters 3-3
3.7 Flotation 3-3
3.8 Solds-Contact Units 3-3
3.9 Summary 3-3
4 COAGULATION OF WASTEWATER 4-1
4.1 General 4-1
4.2 Coagulation Control 4-2
5 CHEMICALS AND FEEDING 5-1
5.1 General 5-1
5.2 Chemicals for Coagulation and pH Control
of Wastewater 5-1
5.3 Chemical Feeders 5-18
6 CHEMiCAL PROCESSES 6-1
6.1 General 6-1
6.2 Chemical Mixing and Flocculation 6-1
6.3 Solids-Contact Clarification 6-4
7 GRAVITY SYSTEMS 7-1
7.1 General 7-1
7.2 Primary Sedimentation 7-1
7.3 Secondary Sedimentation 7-7
7.4 Chemical Sedimentation 7-8
7.5 Flotation 7-8
7.6 High-Rate Settlers 7-1 1
7.7 Tube Settlers 7-1 1
7.8 Lamella Separator 7-14
V
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TABLE OF CONTENTS (Continued)
Chapter Page
8 PHYSICAL STRAINING PROCESSES 8-1
8.1 General 8-1
8.2 Microscreening S- I
8.3 Other Screens 8-1 1
8.4 Diatomaceous Earth Filters 8-11
8.5 Ultrafiltration 8-21
9 DEEP BED FILTRATION 9-1
9.1 General 9-I
9.2 Filter Design 9-1
9.3 Summary of Results of Filtration Studies 9-17
9.4 New Filtration Systems 9-20
10 OPERATION AND MAINTENANCE 10-1
10.1 General 10-1
10.2 General O/M Considerations 10-1
10.3 O/M Considerations - Individual Processes 10-3
10.4 Man-Hour Requirements 10-5
10.5 Job Descriptions 10-6
11 COST ESTiMATES 11-1
11.1 General 11-1
11.2 ProcessCosts 11-1
11.3 Data Analysis 11-5
11.4 Cost Estimation and Design 11-5
12 PLAN AND SPECIFICATION REVIEW — CHECK SHEET 12-1
12.1 General 12-1
12.2 Plan Review Data Sheet for Suspended
Solids Removal 12-2
APPENDIX A A-i
ACKNOWLEDGMENT
WRSIC FORM
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LIST OF FIGURES
Figure No. Page
4- 1 Jar Test Units with Mechanical (Top) and
Magnetic (Bottom) Stirrers 4-4
4-2 Six-Position Sampler 4-5
4-3 Settling Curves Frequently Obtained 4-6
4-4 Jar Test Results 4-7
4-5 Zeta Potential Apparatus 4-9
4-6 Coagulation of Raw Sewage With Alum 4-10
5-I Lime Requirement for pt-I 1 11.0 As A Function
of the Wastewater Alkalinity 5-1 5
5-2 Positive Displacement Pumps 5-20
5-3 Screw Feeder 5-23
5-4 Positive Displacement Solid Feeder-Rotary 5-23
5-5 Positive Displacement Powder Pump 5-24
5-6 Dry Chemical Dissolver 5-25
6-I Static Mixer 6-2
6-2 Impeller Mixer 6-2
6-3 Mechanical Flocculation Basin 6-3
6-4 Baffle Flocculation Basin 6-3
6-5 Solids Contact Clarifier without Sludge Blanket Filtration 6-5
6-6 Solids Contact Clarifier with Sludge Blanket Filtration 6-6
7-1 Functional Zones in an Idealized Sedimentation Basin 7-2
7-2 Idealized Settling Paths of Discrete Particle in a
Horizontal Flow Tank 7-2
7-3 Results of Salt-Injection Tests with Different
Types of Sedimentation Tanks 7 -3
7-4 Rectangular Settling Basin with Sludge and Scum Collector 7-4
7-5 Circular Settling Basin - Center Flow 7-5
7-6 Rim Flow Settling Basin 7-6
7-7 Pressure-Feed Flotation System 7-10
7-8 Tube Settlers - Flow Pattern 7-13
7-9 Inclined Tube 7-13
7-10 Module of Steeply Inclined Tubes 7-15
7-1 1 Tube Settlers in Existing Clarifier 7-1 6
7-12 Plan View of Modified Clarifer 7-16
7- 13 Lamella Settler 7-18
7-14 Lamella Separator 7-19
8-1 Typical Microscreen Unit 8-2
8-2 Typical Microscreen Unit, Cross Section 8-3
8-3 Microscreen Installed in Steel Tank at Factory 8-6
8-4 Large Microscreen Drums Prior to installation at
Chicago, illinois 8-7
8-5 Large Microscreens in Operation 8-8
8-6 Overall Dimensions for Four Sizes of Microscreeners 8-9
8-7 Laboratory for Determination of Filterability Index 8-10
8-8 Diatom ite Filtration System Detail 8-1 3
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LIST OF FIGURES (Continued)
Figure No. Page
8-9 Rotary Vacuum Precoat Filter 8-14
8-10 Vertical Leaf Vacuum Filter 8-15
8-1 1 Plate and Frame Pressure Filter 8-16
8-12 Cylindrical Element Vertical Filter 8-1 7
8-13 Vertical Leaf Pressure Filter, Vertical Tank 8-18
8-14 Vertical Leaf Pressure Filter, Horizontal Tank 8-19
8-1 5 Ultrafiltration Flow Diagram 8-22
8-16 “Storage Battery” Membrane Modules 8-23
8-17 Schematic Flow Diagram of the Pikes Peak
Treatment and Reuse System 8-24
9-1 Typical Rapid Sand Filter 9-2
9-2 Pressure Filter 9-2
9-3 Grain Size Curve 9-5
9-4 Media Comparisons 9-8
9-5 Filter Bottom and Underdrain Systems 9-I I
9-6 Nomogram for Selecting Wash Through Dimensions 9-12
9-7 Palmer Filter Bed Agitator 9-14
9-8 Automatic Gravity Filter, Single Compartment 9-16
9-9 Cross Section of Upflow Filter 9-21
9-10 Schematic Drawing of the Johns-Manville Moving Bed Filter 9-23
9-1 1 Hydromation in-Depth Filter 9-27
11-I Solids Removal by Coagulation and Sedimentation 11-2
11-2 Cost of Primary Sedimentation Tanks 11-3
11-3 Cost of Final Settling Tanks 114
11-4 Capital Cost - Tube Settler 1 1-7
11-5 Microscreening of Secondary Effluent 11-8
11-6 Capital Costs - Microscreening 11-9
11-7 Filtration Through Sand or Graded Media 11-10
11-8 Capital Cost - Multimedia Filter 11-11
11-9 Capital Cost - Moving Bed Filter 11-12
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LIST OF TABLES
Table No. Page
5- 1 Characteristics of Alum 5-1
5-2 Reactions of Aluminum Sulfate 5-3
5-3 Characteristics of Ferric Sulfate 5-7
5-4 Reactions of Ferric Sulfate 5-8
5-5 Characteristics of Fernc Chloride 5-9
5-6 Solution Characteristics 5-I 0
5-7 Characteristics of Lime 5- ! 2
5-8 Dry Polyelectrolytes 5-16
7- 1 Tube Settler Installations 7-17
8- I Microscreener Sizes, Motors and Capacities 8-1 1
8-2 Tertiary Treatment by Microscreeners 8-1 2
8-3 Vacuum Diatomaceous Earth Filtration of
Secondary Effluent 8-20
8-4 Pressure Diatomaceous Earth Filtration by
Secondary Effluent 8-20
8-5 Summary of Performance of the Dorr-Oliver Activated
Sludge-Ultrafiltration Plant Operations at Pikes Peak 8-25
9-1 Typical Multi-Media Design 9-7
9-2 Filter Gravel Design 9-10
9-3 Expected Effluent Suspended Solids from Multi-Media
Filtration of Secondary Effluent 9- 18
9-4 Design Parameters for Upflow Filters 9-22
9-5 Johns-Manville Moving Bed Filter Evaluation at
Bernards Township Sewerage Authority Treatment Plant 9-24
10-1 Man-Hour Summary 10-7
10-2 Manpower Requirements - Mixing and Flocculation 10-8
10-3 Manpower Requirements - Sedimentation 10-9
10-4 Manpower Requirements - Microscreening 10-10
10-5 Manpower Requirements - Filtration 10-Il
10-6 Manpower Requirements - Moving Bed Filter 10-12
Il-i Yearly Cost Indices 11-13
11-2 Sewage Treatment Plant Construction Cost Index 11-14
11-3 LaborCost lndex 11-15
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FOREWORD
The formation of the Environmental Protection Agency marks a new era of environmental
awareness in America. This Agency’s goals are national in scope and encompass broad
responsibility in the area of air and water pollution, water supply, solid wastes, pesticides,
and radiation. A vital part of EPA’s national water pollution control effort is the constant
development and dissemination of new technology for wastewater treatment.
It is now clear that only the most effective design and operation of wastewater treatment
facilities, using the latest available techniques, will be adequate to meet the future water
quality objectives and to ensure continued protection of the Nation’s waters. It is essential
that this new technology be incorporated into the contemporary design of waste treatment
facilities to achieve maximum benefit of our pollution control expenditures.
The purpose of this manual is to provide the engineering community and related industry a
new source of information to be used in the planning, design, and operation of present and
future municipal wastewater treatment facilities 1t is recognized that there are a number of
design manuals, manuals of standard practice, and design guidelines currently available in
the field that adequately describe and interpret current engineering practices as related to
traditional plant design. It is the intent of this manual to supplement this existing body of
knowledge by describing new treatment methods, and by discussing the application of new
techniques for more effectively removing a broad spectrum of contaminants from waste-
water.
Much of the information presented is based on the evaluation and operation of pilot,
demonstration and full-scale plants. The design criteria thus generated represent typical
values. These values should be used as a guide and should be tempered with sound engineer-
ing judgment based on a complete analysis of the specific application.
This manual is one of four now available through the sponsorship of the Environmental
Protection Agency to describe recent technological advances and new information m the
following subject areas.
Carbon Adsorption
Phosphorus Removal
Upgrading Existing Plants
Suspended Solids Removal
These manuals are the first edition copies and will be updated as warranted by the advancing
state-of-the-art to include new data as it becomes available, and to refine design criteria as
additional full-scale operational information is generated.
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CHAPTER 1
INTRODUCTION
1.1 General
This manual is designed to provide the consulting engineer and regulatory agencies with
up-to-date information on suspended solids removal from wastewaters. Much information
required for the design of advanced waste treatment plants is available for many of the
processes integral to the proper design of a facility. These data are included in many
publications, reports and manufacturers’ promotional and design literature. This available
information has been considered in the preparation of this manual.
The facilities included in the overall design of a waste treatment plant for the removal of
suspended solids should be commensurate with the degree of treatment and process design
required. The removal of suspended and/or colloidal solids from wastewater is of major
importance in any treatment system. The manual for suspended solids removal does not
cover in detail wastewater treatment processes which normally precede or follow the re-
nioval of suspended solids except to note the extent to which these processes affect the
suspended solids removal processes. In general, the removal of suspended solids from waste-
water is accomplished by the following broadly categorized processes:
a. Chemical Processes
b. Gravity Systems
c. Physical Straining
d. In-Depth Filtration
These process categories will be discussed in detail herein.
1.2 Wastewater Solids
Because the term “solids” is so encompassing, some discussion of definitions, assumptions
and relationships becomes necessary for proper understanding of the contents of this man-
ual. Wastewater solids are generally classified as total, settleable, suspended, colloidal and
dissolved. Total and settleable solids are self-explanatory and are discussed analytically
elsewhere (1). Suspended and dissolved solids are differentiated by a standard analytical
technique of filtration through a standardized filter (1). The relationship,
TS = SS+DS
where: IS = total solids
SS suspended solids
DS = dissolved solids
is the basis for characterization of wastes as to solid fractions. Any two of the three ana [ yses
will determine the relative fractions.
Colloidal solids are an entirely different matter, however. Technically, they can be defined
as those particulates with a size range of from I to 500 millimicrons (2). In fact, they
represent an intermediate fraction between suspended and dissolved solids with some flex-
ibility as to the exact upper and lower size limits. Standard methods of determination are
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not readily available as in the cases of the other solids fractions. References to the colloidal
matter are frequent in the literature in the contexts of discussions concerning coagulation
and turbidity of wastewater. Dean, et al. (3), have noted that bacteria, viruses, phages, and
other cellular debns fall into the colloidal fraction.
Solids of any of these physically-defined fractions can be further differentiated into organic
and inorganic (volatile and ash) or hydrophilic and hydrophobic fractions for example. The
need for such classification will vary with individual circumstances. Within the scope of this
manual, it is sufficient to note that a differentiation between total and soluble or dissolved
fractions of various parameters is of importance. The use of this breakdown is dependent
upon an understanding of the processes considered in this text. Certainly, a simple straining
or sedimentation design cannot be expected to remove dissolved or soluble pollutants.
Consequently, if the desired effluent concentration of such a pollutant is less than the
soluble concentration in the feed to a proposed process, one must consider using a process
or combination of processes which are capable of removing some of the soluble fraction.
This type of judgment becomes possible with proper analytical techniques, such as total and
soluble analyses.
The average suspended solids concentration of municipal wastewaters is 200 mg/I. This
number is highly variable, depending on a large number of specific circumstances. Contribu-
tions from individual homes, multi-family dwellings, or any other point source will often
exhibit suspended solids concentrations of a factor of two or more higher than the munic-
ipal average (4). Municipal wastewaters have been characterized as to the relative contribu-
tions of suspended and dissolved fractions by some investigators. For example, Rawn (5)
found that approximately one-third of the total biochemical oxygen demand (ROD) from a
subdivision was soluble, while Clark (6) noted less than one-sixth of the BOD from a
ski-lodge was dissolved. The residence time in the sewer and other factors which affect the
makeup of municipal wastewaters produce a significantly different picture at a central
treatment plant. Helfgott, et al (7), found the soluble-colloidal fraction of the chemical
oxygen demand (COD) of two New Jersey wastewaters to be 43 percent of the total COD.
Within the scope of this manual, an understanding of the above definitions and relationships
will be assumed as basic to all discussions All wastewaters and subsequent process effluents
will exhibit their own individual properties Any attempt to generalize process capabilities
must be treated as approximate and used only for guidance when approaching a specific
treatment problem.
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1.3 References
1. Standard Methods for the Examination of Water and Wastewater , 13th Edition, Ameri-
can Public Health Association, New York (1971).
2. Weiser, H.B, A Textbook of Colloid Chemistry , 2nd. Ed., John Wiley and Sons, Inc.,
New York (1949).
3. Dean, R.B., Claesson, S , Gellerstedt, N., and Bomari, N., “An Electron Microscope
Study of Colloids in Waste Water”, Env. Sci. and Tech., 1, 147 (Feb. 1967).
4. Watson, K.S., Farrell, R.P., and Anderson, J.S., “The Contribution from the individual
Home to the Sewer System”, Jour. WPCF,39, 2039 (Dec. 1967).
5. Rawn, A.M.. “Some Effects of Home Garbage Grinding Upon Domestic Sewage”, The
A merican City, 66, 110 (Mar. 195 1).
6. Clark, B.D., “Basic Waste Characteristics at Winter Recreation Areas”, U.S Dept. of
Interior, FWPCA, Report No. PR-7 (Aug. 1968).
7. Flelfgott, T.. Hunter, J.V., and Rickert, D., “Analytic and Process Classification of
Effluents”, Jour. SED-ASCE, 96, 779 (June 1970).
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CHAPTER 2
POPULATION AND DESIGN PERIOD FORECASTING
2.1 General
This chapter is a rather brief discussion of a highly complex subject concerning the con-
siderations and methods involved in determining proper design periods and flows. One of
the most difficult tasks in designmg a municipal wastewater treatment facility is the deter-
mination of the design capacity of the system. In order to determine design capacity, one
must make quantitative projections concerning future values of a number of pertinent
factors. Fair and Geyer (1) list the following variables for consideration:
a. Structure and equipment life expectancy and obsolescence.
b. Ease of expansion of facilities.
c. Anticipated contribution population, commercial and industrial growth.
d. Rate of interest on bonded indebtedness.
e. Anticipated inflation rate during retirement period.
f. Facility performance during the early years of underloaded operation.
2.2 Population Growth
Although engineering textbooks and manuals offer a variety of handy mathematical graph-
ical approaches to population forecasting, the most accurate estimate of not only popula-
tion changes, but also industrial and commercial growth and changes, can usually be found
in a city’s planning agency. Most urban centers are now required to have master plans and
planning agencies in order to obtam federal funding for various projects, most notably urban
renewal activities (2). The population projection method commonly used is the “cohort
survival model of population dynamics,” a most reliable form of forecasting This method is
based on well-established survival and reproductive patterns for all age groups. The major
uncertainty is the migration factor, but even this variable can be predictable within limits.
Included in the urban master plan are land use maps indicating the areas intended for
housing, types of housing, proposed changes in urban districts, and numerous other data
which can be useful to the designer of treatment facilities. The industrial and commercial
makeup of the local economy is usually established by an economic survey with predicted
changes due to city population, age, density and other factors which exert subtle influences
on this makeup.
Where master plans and their associated information are unavailable, the previously-
mentioned mathematical models and other pertinent information may be applied to census
data to compute some reasonable design population figures. Sources of information for
these calculations are numerous in the sanitary engineering literature (1), (3), (4).
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2.3 Economic Considerations
As previously noted, interest rates on bonds and anticipated inflation have a bearing on the
design period chosen All governmental bodies have been rated as to their financial status.
The corresponding bond rating roughly determines the amount of interest that the city or
county wilt have to pay for the required capital outlay for new facilities. Higher ratings will
usually result in lower interest rates Smith and Eilers (5) indicate that Grade Aaa and Bbb
securities had yield rates of 6.0 and 6.5 percent, respectively, in 1970. In essence, during
periods of low interest rates, longer design periods may be chosen. This economic analysis
may be further influenced by the projected rate of inflation, in that higher inflationary rates
mean repayment in dollars of lower value
2.4 Physical Considerations
When the type, size, and extent of the treatment facilities to be provided have been deter-
mined and before design work has physically begun, considerations must be given to the
actual positionmg of the vanous units on the available plant site. This aspect must be
considered carefully since the physical layout of the plant as a whole may be markedly
influenced by the prospects of future plant expansion, alteration or modification on the
basis of advancing technology and requirements. The matter of economy alone should not
dictate the building of a plant that will only serve to supply immediate needs. Neither is
there a valid reason for building a plant so compact that future expansion is virtually ruled
out. The basic structures (chemical feed storage and equipment, clarifiers, filters, labora-
tories, power supply lines and administrative area) will always be required and must, of
necessity, be considered as permanent, fixed installations.
Foresight seems to indicate that, with few exceptions, linear construction of a plant is
desirable if the facility will be enlarged to any degree in the foreseeable future. Although
this type of construction may appear to be excessive in its land requirements, most plants
usually find that ease of access to the clarifiers and the filters for the purpose of overhaul
and/or maintenance more than compensates for the added distances that must be covered
dunng the course of routine operations
In some cases, there will be uncertainty as to the precise treatment which is desirable,
leading to the suggestion that temporary treatment facilities or structures should be in-
cluded in the plant To find any structure ever abandoned, even the temporary ones, is
highly unusual. No purely temporary structure should ever be included in the design or
construction of a new plant unless short-term usage will make its removal or replacement
mandatory. Experience has shown that such structures become very difficult to dispense
with and will remain in use even though unsightly, inconvenient and inefficient.
2.5 Wastewater Characteristics
Wastewater flows, chemical and biological characteristics and treatability are necessarily
mcluded in treatment plant design procedures. Techniques and other established procedures
for accomplishing these tasks are thoroughly covered elsewhere (I ),(3),(4). It should suffice
for the immediate purposes of this manual to stress the importance of this aspect on the
overall plant design, as any failure in this activity can negate all other design considerations.
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2.6 References
1. Fair, G.M., and Geyer, J.C., Water Supply and Wastewater Disposal , John Wiley & Sons,
Inc., New York, N.Y. (1954).
2. Goodman, W.1., and Freund, E.C., Princjples and Practices of Urban Plannmg , Inter-
national City Manager’s Association, Washington, D.C. (1968).
3. Sewage Treatment Plant Design , Water Pollution Control Federation Manual of Practice
No. 8 (1963).
4. Design and Construction of Sanitary and Storm Sewers , Water Pollution Control Federa-
tion Manual of Practice No. 9 (1963).
5. Smith, R., and Eilers, R.G., “Cost to the Consumer of Collecting and Treating Waste-
water m the United States,” in press, R.A. Taft, Water Research Center, Cincinnati,
Ohio.
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CHAPTER 3
FLOW EQUALIZATION
3.1 General
The stream of wastewater entering most treatment plants varies significantly throughout the
day in rate of flow and contaminant concentration. This is caused by the habits of the
population served and the hydraulic characteristics of the sewerage system. The extent of
this variation is shown in References (1), (2) and (3) which report measurements made on
operating plants. As a result of this variation ni the plant in fluent stream, the quality of the
effluent streaim from processes also varies.
Measurement of peak flows versus 24-hour average flows in trunk sewers from residential
communities in Orange County, California, was reported in a study by Boyle Engineering
and Lowry and Associates (4). The data fitted the followmg relationship-
0.92
Peak Flow, MGD = 1.84 (Average Flow, MCD)
The peak to average ratio thus decreases as the average flow increases.
Generally, the design of almost all of the major structural elements comprising a treatment
plant is based on the avenge rate of flow. Other elements of the plant, such as conduits,
siphons and distributing mechanisms are designed on the basis of the extreme peak rate of
flow which is usually considered as three times the average rate of flow. Older plants, which
are supplied by combined sewer systems, must also take into account the effect of heavy
rainfall and storm flows. Modern collection systems utilize separate sanitary and storm
sewers, but must still consider contributions from ground water infiltration, illegal tie-ins
and extraneous surface water that may enter the collection systems. Similarly, consideration
of the minimum rate of flow is necessary in the design of certain elements of the total plant,
such as grit chambers, measuring devices and dosing equipment. For this purpose, the
average rate of flow for the 4-hpur minimum flow period is used. This is often assumed to
be 40 percent of the average hourly flow iate. Under normal conditions, the design of
individual treatment units should be based on the average hourly flow rate of domestic
sewage plus the average hourly rate of flow of mdustrial wastes dunng the maximum
significant period of the day. Where recirculation is employed through any individual unit,
or groups of units, the recirculated effluent must be added to the flow rate originally
considered.
Flow equalization, although commonly used in industrial waste treatment practice, has been
generally neglected in traditional municipal wastewater treatment facilities. There are two
major objectives in the design of flow equalization basins. The first of these is simply to
dampen the diurnal flow vanation that normally exists in typical municipal wastewater
collection systems. This is done to achieve a constant or nearly constant flow rate through
the subsequent treatment processes. In this type of system, little consideration is given to
controlling the quality changes that take place during storage. The major design factors are
to supply sufficient air to keep the basin aerobic and to provide adequate turbulence to
prevent solids deposition.
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The second objective of flow equalization is to provide the capacity to distribute shock
loads of toxic or process-inhibiting substances over a reasonable period of time so as to
prevent system failure and to minimize the adverse effects of periodic discharges of harmful
contaminants to the receiving stream or surface impoundment. Tithe-dependent concentra-
tion profiles and flow-through curves are normally used to analyze the flow characteristics
of these systems and to determine the effect of tank geometry, placement of effluent webs
and mixing regime on changes in contaminant concentrations through the basin.
Detailed discussions of the flow equalization design principles can be found elsewhere (2,3).
In essence, the justification for flow equalizing facilities is economic, although other factors,
such as solids settling and septicity, must be considered. The economic justification involves
the relative costs of providing equalization facilities as opposed to increasing design capaci-
ties of subsequent unit processes of the waste treatment facility to produce an effluent of
equal quality. Because of the different degrees of sensitivity inherent in each type of
treatment process, the economic balance is unique to each installation. Other factors which
affect such an analysis are related to esthetics, land values, areal limitations, and the eco-
nomics of size for each of the unit processes.
A short discussion of the effects of flow variation on several unit processes is presented in
the following sections.
3.2 Clarifiers
In general, the clarifier is usually of sufficient capacity to serve as a buffer against small
surges in flow. If, however, the clanfier influent flow is unusually large, exceeding the unit’s
design capacity, an overflow line from the parimary clarifier discharging to the influent wet
well or, ideally, a flow equalization basin may be necessary. However, if the flow is lower
than design capacity, the waste will be held longer with possibly improved clarification.
Since the overriding influence on clarifier efficiency is overflow rate, solids removal will be
affected by flow variations. If the basin is properly designed, these effects are minimal. The
allowance of excess capacity in the design provides an additional safeguard.
3.3 HighRata Settlers
Tube settlers are only slightly affected by variations in flow. At times of extreme high flow,
the detention time will be markedly decreased. At periods of low flow, the settling time will
be lengthened with some improvement in solids removal.
3.4 Microscreens
A microscreen unit is designed to maintain a constant level differential across the unit by
automatically increasing the drum speed when the debris concentration or the flow rate
increases and decreasing the drum speed when the debris concentration or flow rate de-
creases. Therefore, withm limits, the microscreen is unaffected by change in flow rate.
3.5 Filters
Flow variation can have a httle or a great effect on the operation of a conventional,
multi-media or up-flow filter, depending on the nature of the solids in the influent stream.
Usually, filters are operated at a constant rate of flow and should be protected from the
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flow variations which could damage product quality. Flow equalization ahead of this pro-
cess should be provided.
3.6 Moving Bed Filter
The units are installed with level controls in the head tank which increase or decrease the
sand pulse rate in direct proportion as the volume of the liquid feed changes. Similarly, in
the event of extreme low flow, the units are automatically shut down.
3.7 Flotation
Flotation, like sedimentation, is primarily responsive to the overflow rate. It generally
requires a relatively constant flow rate for proper performance. Flow equalization should be
provided ahead of these units.
3.8 Solids- Contact Units
Solids-contact clanfiers are very sensitive to flow changes To counteract this sensitivity,
sludge blankets must be kept low to allow for upsets. Sludge blanket control devices are
desirable.
3.9 Summary
If space is available) a flow equalization basin should be constructed when a tertiary
treatment system is added to an existing secondary plant, as the latter may be frequently
upset. Preferably) the flow equalization should be incorporated into the system. If space is
not available for a holding pond, variable speed pumps whose speed is controlled by the wet
well level can be used to deliver the secondary effluent to the tertiary treatment plant.
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3.10 References
1 Smith, R., and Eilers, R.G., “Simulation of the Time-Dependent Performance of the
Activated Sludge Process Using the Digital Computer”, in-press, R.A. Taft Water Re-
search Center, Cincinnati, Ohio.
2. Duttweiler, D.W. & Purcell, L.T., “Character and Quantity of Wastewater from Small
Populations”, Jour WPCF, 34, 962 (1962).
3. Ferguson, K.G., “Variation in Municipal Sewage Flow and Water Consumption in Wash-
ington State Cities”, Proceedings ASCE, 88, 19 (1962).
4. Boyle Engineering and Lowry and Associates, “Master Plan Trunk Sewer Facilities” for
County Sanitation District No. 3 of Orange County, California, (June, 1968).
3-5
-------
CHAPTER 4
COAGULATION OF WASTEWATER
4.1 General
Wastewaters contain a wide variety of organic and inorganic suspended solids. These solids
may have been present in the raw wastewater or may have been precipitated from solution
during previous stages of treatment. All must be removed if a high quality effluent is to be
produced.
A number of factors will influence the rates at which wastewater solids may be removed by
sedimentation. Particularly important is particle size. Small particles in the colloidal range
will not settle out in a practical detention time and must be agglomerated into larger
particles which will settle at reasonable rates.
Small particles have very large surface areas per unit weight of solid with associated forces
which tend to keep the particles separated or in a stable state. Effecting aggregation of the
solids is a matter of overcoming these stabilizing forces by application of selected chemical
coagulants. It is not the purpose of this manual to cover the subject of coagulation in detail.
Only such information which is useful in process control is provided. More extensive cover-
age can be found in a number of publications for those who desire further information (1 ),(2),
(3).
The principle stabilizing forces are electrostatic repulsion from electrical double layers sur-
rounding particles suspended in water and physical separation of particles by films of water
adsorbed on the particle surfaces. The electrical double layer is due to an imbalance of ions
near the particle surface and imparts an electrical charge to particles which is generally
negative for domestic wastewater particles (4). The particles with like charges then tend to
repel one another. The magnitude of the charge on the double layer must be reduced to
permit the particles to come together for agglomeration. Reduction of the charge is a
pnnciple function of the coagulant.
Principal natural destabilizing or aggregating forces are Brownian movement and Van der
Waals forces. Brownian movement is a constant random motion of small particles due to
collisions of the particles with thermally-agitated water molecules, while Van der Waals
forces are atonuc dipole interactions which exist between all atoms. Van der Waals forces,
which are always attractive, predominate at short distances from the particle surfaces. If
Brown ian movement causes two or more particles to approach within a short distance from
each other, and if the repulsive electrical forces have been sufficiently reduced, then the
particles will be held together by the Van der Waal forces. Other particles can be added to
the particle pair until the resulting aggregate or floc reaches such a size that it rapidly settles
out of solution.
These natural destabilizrng forces are generally not sufficiently rapid to permit effective
solids removal from wastewater,but must be augmented by addition of chemical coagulants
to destabilize and tie the particles together and by application of hydraulically-or mechanic-
ally-applied mixing to promote rapid collisions of the destabilized particles. A number of
mechanisms have been proposed to explain the action of coagulants in promoting particle
4-1
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aggregation. These include charge reduction, physical or chemical bridging of coagulant
molecular chains between particles and physical enmeshment of particles in a mass of
precipitated coagulant. All of these mechanisms probably play a greater or lesser role in
effective coagulation.
Coagulants used in wastewater treatment include those used in potable water treatment with
some additions These are. alum, sodium aluminate, ferric chloride, ferric sulfate, ferrous
chloride, ferrous sulfate, lime and organic polyelectrolytes. Other materials such as soda ash
or clays may be used as sources of alkalinity or weighting agents, respectively, to aid
coagulation. When added to water, salts of aluminum or iron react with the water or
alkalinity present in the water to form insoluble hydrolysis products. It is these hydrolysis
products which are the effective coagulating agents. These materials which are positively
charged in the neutral pH range adsorb on the negatively charged wastewater particles
reducing repulsive forces between the particles. The coagulant may also react with other
constituents of the wastewater, particularly anions such as phosphate and sulfate, forming
hydrolysis products containing various mixtures of ions. These various hydrolysis products
differ in their effectiveness as coagulants. The chemistry of the reactions is extremely
complex
For each combination of coagulant and wastewater there is an optimum dosage of coagulant
and an optimum pH range for coagulation. These are the two parameters which are generally
controlled in operation of advanced waste treatment plants for solids removal through
coagulation.
4.2 Coagulation Control
Because coagulation represents a group of complex reactions, laboratory experimentation is
essential to establish and maintain the optimum coagulant dosage and the effect of im-
portant variables on the quality of coagulation of the wastewater under investigation. With
hydrolyzmg coagulants three procedures may be followed for this purpose the jar test,
measurement of zeta potential, and measurement of phosphate content of the wastewater.
Lime is a special case Proper control of lime addition may usually be maintained by
measuring the pH or automatically titrating alkalinity after lime addition.
4.2.1 Jar Test
The single, most widely used test to determine dosage and other parameters is the jar test.
The equipment for this test and the directions for its proper performance have been pub-
lished(5),(6),(7),(8). The jar test attempts to simulate the full scale coagulation-flocculation
process and has rema med the most common control test in the laboratory since its introduc-
tion m 1918. Since the intent is to simulate an individual plant’s conditions, it is not
surprising that there has been no standardization of the test. The jar test as variously
performed does, however, have some elements of conformity. In its essentials, the jar test
simply consists of a series of sample containers, usually six, the contents of which can be
stirred by individual mechanically-operated stirrers. Water to be treated is placed in the
containers and treatment chemicals are added while the contents are being stirred. The range
of conditions, for example, coagulant dosages and pH, are selected to bracket the antici-
pated optima After a short, 1-5 minute, period of rapid stirring to ensure complete dis-
persion of coagulant, the stirring rate is decreased and flocculation is allowed to continue
for a vanable period, 1 0 to 30 minutes or more, depending on the simulation. The stirring is
then stopped and the floc are allowed to settle for a selected time. The supernatant is then
4-2
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analyzed for a variety of parameters. With wastewater the usual analyses are for turbidity or
suspended solids, pH, residual phosphorus and residual coagulant
If desired, a number of supernatant samples may be taken at intervals during the settling
period to permit construction of a set of settling curves which provide more information on
the settling characteristics of floc than a single sample taken after a fixed settling period. A
dynamic settling test may also be used in which the paddles are operated at 2 to 5 rpm
during the settling period. This type of operation more closely represents settling conditions
in a large horizontal basin with continuous flow.
Several six-position stirrers are available commercially for running jar tests; one from Phipps
and Bird, (Phipps and Bird, Inc., Richmond, Va.),another from Coffman Industries, (Coff-
man Industries, Inc., Kansas City, Ka.), are shown in Figure 4-1. Standard laboratory mixers
have also been used; however, it is difficult to obtain reproducible mixing conditions using
different pieces of equipment. Various types of containers, usually beakers or jars, are used
to hold the samples. Improved mixing may be obtained by adding stationary plates in the
containers as described by Camp and Conklin (8). The Coffman stirrer has an attachment
which makes it possible to add coagulant to all containers simultaneously, however, good
results can be obtained by rapidly adding coagulant from a large graduated pipette to each
jar in sequence.
A simple apparatus, shown in Figure 4-2, can be constructed from tubing, rubber stoppers
and small aquarium valves to permit rapid sampling of supernatant. The unit is placed next
to the sample jars at the beginning of the settling penod with the curved stainless steel tubes
dipping into the jars. At desired intervals the vent valve is covered with a finger, permitting
vacuum to draw samples into the small sample bottles. The needle valves are adjusted so that
supernatant is drawn into all the bottles at the same rate. When sufficient sample is ob-
tained, the vent is uncovered and the bottles are replaced with empties The maximum
sampling rate is about once per minute.
Figure 4-3 shows typical types of settling curves which may be obtained. Curve A indicates a
coagulation which produced a uniformly fine floc, so small that at the end of I to 2 minutes
settling, the supernatant had a turbidity equal to that of the starting water due, in part, to
the fine floc which resisted settling. Settling was slow and the final turbidity was excessive.
This coagulation would not be satisfactory in an advanced wastewater treatment plant.
Curve B represents the most common type of settling rate obtained. During the first 5
minutes, the settling rate was practically a straight line on a semilog plot. Settling was rapid
and clarification was satisfactory. The coagulation represented by curve C shows that a
mixture of large rapid settling floc and small, slow-settling particles was obtained. Settling
was rapid for the first two minutes, but with little further clarification after that. High
residual turbidity may also have resulted from incomplete coagulation. Curve D represents
the ultimate in coagulation. Practically all of the floc particles were so large and dense that
97% settled within three minutes. Sedimentation was essentially complete within that time
since only 0.5% additional hoc settled in the next 27 minutes. Final clanty of the super-
natant was entirely satisfactory. This coagulation was obtained with a coagulant aid.
Measurement of turbidity provides the most rapid indication of the degree of solids removal
obtained. The recommended procedure for turbidity measurement by light scattering is
given in the 13th edition of Standard Methods for Examination of Water and Wastewater;
however, other methods varying from simple visual evaluation to measurement of light
4-3
-------
Figure 4 -1. JAR TEST UNITS WITH MECHANICAL (TOP)
AND MAGNETIC (BOTTOM) STIRRERS
I
4-4
-------
TO VACUUM SOURCE
Figure 4-2. SIX-POSITION SAMPLER
SAMPLE
TUBE
MAIN CONTROL
PLUG
ING VALVES
4-5
-------
t t:
:I I1IfF
SETTLING TIME-MIN
1- 4 igure ‘i -i. LI !LflN i UUKVL 1-KLQUtJN!LY UIiIAIINtI )
c
—1—
lOd:
50
20
- =
z
-L ::ti- t - 4Th 2
- t ti - 4 I T 4 I t
10
::: ::
:
:
:
—
i =
:
:
EJI3E:
- -
—-I
5
c
2
- - H- - --
4 r
I — —
-4-
-5
4-6
-------
transmitted on a laboratory spectrophotometer can be used for purposes of comparison.
Measurement of residual suspended solids is the oniy procedure which gives the actual
weight concentration of solids remaining, but the procedure is too slow for purposes of
process control. Residuals of phosphate and coagulant in supernatant are usually of interest
and may be measured either manually or with automated equipment
A typical jar test might be run as follows Wastewater samples are placed in containers and
rapid mix is started at 100 rpm. Selected dosages of coagulant are rapidly added to the
containers covering the expected range of the optimum dosage and a timer is started. If a
polymer is to be used as a coagulant aid, it is added to each jar after two minutes and rapid
mixing is continued for one additional minute. The paddles are then slowed to 30 rpm and
mixing continues for 10 minutes. The paddles are then stopped and the sampling apparatus
previously—described is placed in position. At settling times of 1, 3, 5, 10 and 20 minutes
samples of supernatant are drawn for turbidity measurement. After the final turbidity
sample is drawn, a larger volume of supernatant may be decanted for more complete
analysis. Results are plotted as m Figure 4-4 for judgment as to the desired coagulant
dosage. The jar test may be repeated using a smaller dosage range around the observed
optimum to more closely locate the best dosage.
z
z
>
SETTLING TIME-MIN
Figure 4-4. JAR TEST RESULTS
100
50
20
10
5
2
Q —
\
\
\
I
I
—
.—.---D-—---
;
024 mg/I
I
Ferric Sulfate
—0—26mg/i
.‘— 28 mg/I
0—30 mg/I
N
—32mg/l
3
5
7
I
?
4-7
-------
If additional alkalinity is required to hold the coagulation in the optimum pH range, this
should be added to the samples ahead of the coagulant. Once an approximate optimum
dosage has been determined, it may be desirable to repeat the jar test with the optimum
coagulant dosage but using varying quantities of added alkalinity to give different pH values.
Experience in coagulating a given wastewater provides the best guide as to the best methods
for controlling the process.
4.2.2 Zeta Potential
Measurement of particle charge is another procedure which may be useful for control of the
coagulation process(9),(l0),(l 1). The total particle charge is distributed over two concentric
layers of water surrounding the particle, an inner layer of water and ions which is tightly
bound to the particle and moves with it through the solution and an outer layer which is a
part of the bulk water phase and moves independently of the particle. The surface, charge is
not measurable expenmentally, however, the zeta potential which is the reskluaL charge at
the interface between the layer of bound water and the mobile water phale ban be deter-
mined with a commercially-available instrument.*
In the zeta potential measurement procedure, a sample of treated water contaiping ficic is
placed in a special plastic cell under a microscope as shown in Figure 4-5. Under the
influence of a Voltage apphed to electrodes at the ends of the cell, the charged particles will
migrate to the electrode having a polarity opposite that of the paTticle. The velocity of
migration will be proportiohal to the particle charge and to the applied voltage. The particle
velocity can be calculated by observmg the time it takes a particle to travel a given distance
across an ocular micrometer. The zeta potential can then be obtained from a chart which
combines the particle velocity with instrumental parameters. Detailed operating instructions
are supplied with the instrument.
To control the coagulation by zeta potential, samples of water while being mixed are dosed
with different concentrations of coagulant. Zeta potentials are then measured and recorded
for floc in each sample. The dosage which produces the desired zeta potential value is
applied to the treatment plant. Zeta potentials of floc produced in the plant may also be
measured as a means of control The precise zeta potential which signals optimum coagula-
tion must be determined for a given wastewater by actual correlation with jar tests or with
plant performance as in Figure 4-6. The control point is generally in the range of 0 to -10
millivolts. If good correlations can be obtained between some zeta potential values and
optimum plant performance, then it is possible to make rapid measurements of particle
charge to compensate for major variations in wastewater composition due to storm flows or
other causes. Short term variations such as those due to sudden industrial waste dumps are
still beyond control with any present techniques because of the time lag between recogni-
tion of a problem with coagulation and adoption of a satisfactory change of coagulation
conditions.
4.2.3 Phosphate Monitoring
A third means of coagulant dosage control where the coagulant is being used to precipitate
phosphate as well as to remove solids is to automatically analyze the incoming wastewater
for soluble orthophosphate. The coagulant is then paced to maintain a selected ratio of
*A product of Zeta-Meter, Inc • N Y, N Y
4-8
-------
L
Figure 4 -5. ZETA POTENTIAL APPARATUS
•1
S
r T
-------
+10
0
10
• +
—20
-- .1
.1-
20
15
10
5
0
-
thE
1 iL1II III I
I
- - - -t
___
U
++
L
HE HI1±J
ff -t .t-i
• -
Et ±
- -
-
L
-.
-
f . f..
: ±H\
-H*i H t
IT
H- -
.4
200 300 400 500
ALUM DOSAGE, (mg/i)
Figure 4-6. COAGULATION OF RAW SEWAGE WITH ALUM
4-10
4-. .
iL
TI
I ± I ± H - ± H
t I ItJH- -
N
r u
—
I-
cI
- - Tt
• —
-—
-------
coagulant to phosphate either automatically or by frequent manual adjustment. The Techni-
con Auto Ana1yzer* which is commercially available has been adapted for this purpose.
Dow Chemical Co. has developed an automatic system which will add ferric chloride in
proportion to soluble orthophosphate with corrections for varying flow and concentration
of ferric chloride.
Coagulant dosages which produce good phosphate removal will also generally result in good
solids removal if polymers are used to aid in flocculating the fine precipitated matter. Under
normal conditions, the polymer feed rate should be varied with flow to maintain a constant
dosage; however, the polymer feed rate does not have to be changed with changes in dosage
of the inorganic coagulant.
A product of Technicon Corporation, Tarrytown, N Y.
4-11
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4.4 References
1. Black, A.P., “Basic Mechanisms of Coagulation”. JAWWA, 52, 492 (1960)
2 O’Melia, C R., “A Review of the Coagulation Process”, Public Works, tOO, 87. (May
1969) —
3. “State-of-the-Art of Coagulation”, A Committee Report to the AWWA Research Com-
mittee, JA WWA, 63, 99(1971)
4 Faust. S.D.. and Manger. M.C., “Electroniobihty Values of Particulate Matter in Do-
rnestic Wastewater”, Water & Sew. Wks. 111, 73 (1964)
5. Cohen, J.M., “Improved Jar Test Procedure”, JAWWA.49, 1425 (1957).
6 Black, A.P., Buswell, A M., Eidsness. F A., and Black, A L , “Review of the Jar Test”,
IA WWA, 49. 1414(1957)
7. Black, A P , and Harris, R H , “New Dimensions for the Old Jar Test”, Water & Wastes
Engrg .6,49 (Dec 1969)
8. Camp. T.R., and Conklin, G.F., “Towards a Rational Jar Test for Coagulation”,
JNEWWA, 8 325 (1970).
9 Black, A P & Chen. C , “Electrophoretic Studies of Coagulation and Flocculation of
River Sediment Suspension with Aluminum Sulfate”, JAWWA, 57, 354 (1965).
10 Riddick, T M.. “Role of Zeta Potential in Coagulation Involving Hydrous Oxides”,
TAPPI, 47, 171A (1964).
11 Riddick, TM , Control of Colloid Stability Through Zeta Potential , Vol. 1, Zeta-Meter,
Inc., 1720 First Avenue, New York, NY. 10028
4-13
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CHAPTER 5
CHEMICALS AND FEEDING
5.1 General
This chapter surveys the various available chemicals that are used for the purpose of solids
removal and provides pertment facts concernii g their handling, transport and storage. The
composition and physical properties of these chemicals are discussed, and a short review of
polymeric flocculant aids and their properties is included. A review of the various chemical
feeders and their application to specific coagulants is also included.
5.2 Chemicals for Coagulation and pH Control of Wastewater
5.2.1 Aluminum Sulfate
Ordinary ground alum is a mixture of rice-size material and fines. This grade is used by the
majority of water and wastewater plants. In general, ground alum is easy to feed and does
not bulk in hoppers. It is non-corrosive, requiring no protectLon for the hopper interiors.
(Specifications for filter alum may be obtained from the Amer. Wat. Wks. Assn., 521 Fifth
Avenue, N.Y.. N.Y. 10017.)
Rice grade alum is ground alum from which the fine material has been screened. Powdered
alum is the very fine material that is screened out to obtain the rice grade. This grade will
bulk in hoppers and can be extremely dusty and difficult to feed. Powdered alum is seldom,
if ever, used by wastewater plants Characteristics of the several forms of dry alum are given
in Table 5-I.
Table 5-1
CHARACTERISTICS OF ALUM (1)
Approx. Composition A 12 (SO 4 )3’ 14 H 2 0
A1 2 0 3 Content 17%(Min.)
Fe 2 0 3 075%(Max.)
Insolubles 0.5% (Max.)
Hygroscopic Very Slightly
Form Lump, ground, rice powder
Color Ivory White
Weight per cu ft
Lump 60-70 lbs.
Ground 63-76 lbs.
Rice 5 2-62 lbs.
Powdered 38-45 lbs.
5-1
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Table 5- I (Continued)
Angle of Repose (Approx.)
Ground 38-45°
Rice 33-38°
Powder 65°
Solubil ity
Lb Alum (17%A1 2 0 3 )
Temp. Per Gal. of Water
32°F 7.9
60°F 8.4
100°F 9.1
pH of Solution (1%) 3.4
AWWA Specifications for “purified” alum -Commercial “purified” alums vary froni 16.8 to 17.5%A1 2 0 3
For “unpurified” filter alum, AWWA specifications allow 10% insolubles. This type alum, sold under the
trade name of “Activated Alum”, contains 10 to 12 molecules of water of hydration, approx 18.5%
Al 2 03, 0 3% Fe 2 03, and 9% insolubles.
Lump alum ranges in size from 3/4 to 3”, ground alum passes 100% thru 4 mesh and 95% thu 10 mesh,
rice alum passes 100% thru 6 mesh and 5% thru 20 mesh, powdered alum passes 95% thru 100 mesh.
a) Handling and Storage
Commercial filter alum is usually shipped in 100 lb multi-wall paper bags, or in bulk, in box
cars or hopper cars for unloading by pneumatic or mechanical equipment.
Filter alum will cake when stored in bags in damp places, but it is usually free-flowing when
stored in dry concrete or steel bins. Storage bins are usually built with 600 slopes to the
hopper bottoms to insure complete emptying and to avoid arching.
Filter alum should not be handled in mechanical equipment that has close-fitting rotating
parts; it has the peculiarity of building up a flint-like coating due to the pressure and sliding
action of close fitting surfaces (For instance, star feeders having rotors that fit the housing
too closely will eventually build up such resistance to turning that shafts will be sheared or
motors stalled.)
b) Materials for Handling
Dissolving tanks, pipmg and pumps may be made of Duriron, Durimet-20, hard or soft
rubber, plastic, ceramic or lead. Asphalt and cypress lined dissolvers with acid bronze outlets
and Everdur are satisfactory. Dry feeders may be of standard construction, but any portion
of the equipment in contact with solutions of alum must be corrosion resistant
5-2
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c) Use of Alum
In treatment plants, alum is used as a coagulant to remove suspended solids. Reactions
between alum and the natural constituents of various wastewaters are influenced by many
factors, hence it is impossible to predict accurately the amount of alum that will react with
a given amount of natural alkalinity, lime or soda ash which may have been added to the
wastewater. Theoretical reactions can be written which will serve as a general guide, but, in
general, the optimum dosage in each case must be determined by laboratory jar tests.
The simplest case is the reaction of A 1 with OH ions made available by the ionization
of water or by the alkalinity of the water.
Solution of alum in water produces
3+
A 1 2 (S0 4 ) 3 .*-2Al + 3S0 4
Hydroxyl ions become available from ionization of water
H 2 O— -H + 0H
3+
The aluminum ions (Al ) then react.
3+
2Al + 6OH +-2Al (OH) 3
Consumption of hydroxyl ions in the water will result in a decrease in the alkalinity. Where
the natural alkalinity of the water is inadequate for the alum dosage, the pH must be
increased by the addition of hydrated lime, soda ash, or caustic soda. The reactions of alum
with the common alkaline reagents are shown in Table 5-2. While these reactions are an
oversimplification of what actually takes place, they do serve to indicate orders of magni-
tude and to point out that sulfate ion and CO 2 are byproducts of alum treatment.
Table 5-2
REACTIONS OF ALUMINUM SULFATE
A 12 (SO 4 )3 + 3 Ca(HCO 3 2 2 A 1(OH) 3 + + 3 CaSO + 6 CO 2 +
A 12 (SO 4 )3 + 6 NaHCO 3 2 Al (OH) 3 + 3 Na 2 SO 4 + 6 CO 2
2)3 + 3 Na 2 CO 3 + 3 H 2 0 2 Al (OH) + 3 CO 2
A 1 2 (S0 4 ) 3 + 6 NaOH 2 Al(OH) 3 + 3 Na 2 SO
A 12 (SO 4 )3 + 3 Ca(OH) 3 ) 2 Al (OH) 3 + 3 CaSO
5-3
-------
In terms of quantities, the reactions in Table 5-2 can be expressed as follows:
1 mg/I of alum reacts with:
0.50 mg/i natural alkalinity, expressed as CaCO 3
0.33 mg/i 85% quicklime as CaO
0.39 mg/I 95% hydrated lime as Ca(OH) 2
0.54 mg/I soda ash as Na 2 CO 3
These approximate amounts of alkali when added to water will maintain the alkalinity of
the water unchanged when I mg/ 1 of alum is added. For example, if no alkalinity is added,
I nig/l of alum will reduce the natural alkalinity by 0.50 mg/i as CaCO 3 , but alkalinity can
be maintained unchanged if 0.39 mg/i of hydrated lime is added. This lowering of natural
alkalinity is desirable in many cases to attain the pH range for optimum coagulation.
For each mg/I of alum dosage, the sulfate (SO ) content of the water wili be increased
approximately 0.49 mg/I and the CO 2 content o? the water will be increased approximately
0.44 mg/I. The general pH range, wherein the best coagulation occurs, is from pH 5.5 to
8.5. However, for a given wastewater, the optimum pH will likely fall within a relatively
narrow portion of this broad range. In tests with pure solutions, phosphate precipitation
with alum was optimum at a pH of 6.0 (2), but in wastewater alum is effective for phos-
phate removal over a wide pH range. A good summary of available data on alum coagula-
tion can be found in an article by Farrell, et al. (3).
d) Safety
Alum is irritating to the skin and mucous membranes because of its acidic nature. When
handling dry alum, be especially careful to protect against dust, as it can cause serious eye
injury. Wear a cap and bandana, dust mask, close-fitting goggles, and gloves. Wear loose,
denim quality, dustproof, long sleeve clothing. Tie trouser cuffs and sleeves to prevent dust
from entering. Exposed skin surfaces should be coated with a protective cream.
5.2.2 Liquid Alum
Liquid alum is aluminum sulfate in solution. One gallon of liquid alum weighs
approximately Il pounds and contains the equivalent of 5.4 pounds of dry aluminum
sulfate. Some water treatment plants prefer liquid alum because it is more easily fed.
All handling procedures for dry alum apply to liquid alum (4). In addition, the use of face
shields or fitted goggles, boots, gloves and a rubber apron to protect the operator from
splashes or sprays is recommended. Use of liquid alum is usually restricted to facilities
located within a radius of approximately 50-75 miles from the supplier, due to the dead
weight of solution water which increases shipping costs.
a) Characteristics
The strengths of the usual grades of liquid alum are as follows:
5-4
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Deg. Baunie’ A1 2 0 3
270 5.8%
32.2 7.2%
36.4 8.3%
Liquid alum (32.20 Baume) has a specific gravity of 1.285 at 60°F and weighs 10.7 lbs per
gal. Commercial liquid alum is a true solution containing less than 0.2 percent insolubles.
Storage at lower than freezing temperatures may cause crystal formation.
Since dry commercial filter alum contains at least 17 percent Al 2 0 (AWWA Specifica-
tions), then 32.20 Baume liquid alum containing 7.2 percent Al 2 03 is 7.2 17 or 42.3
percent of the strength of dry filter alum on a weight basis.
b) Handling and Storage
Customers’ storage tanks should have a capacity at least double that of delivery tank trucks
(2,000 to 4,000 gal) or tank cars (6,000 to 8,000 gal). There is no deterioration or loss by
storage of liquid alum.
Tank trucks are equipped with two-inch centrifugal pumps and rubber hose for pumping to
customers’ storage. Unloading time may vary from 20 mm to 2 hr depending upon the
pumping head. Tank cars are unloaded by air or gravity with equipment installed at receiv-
ing stations
Liquid alum should be transferred from storage to feeding equipment by centrifugal pumps.
C) Matenals for handling
Storage tanks for all strengths may be lead-lined steel, lead-lined Douglas Fir or rubber-lined
steel. Wooden tanks are suitable for weak solutions, but not for strong solutions because of
deligmfication of the wood.
Pumps may be Worthite, Duriron, Durirnet 20, Pioneer Metal, Aloyco 20 or other nickel-
chrome alloys.
Pipe should be lead-lined or rubber-lined steel, flanged and 125 lb standard test. Pipe
fittings should be lead or standard cast iron with lead or rubber lining, flanged and 125 lb
standard test. Gaskets should be rubber or rubber impregnated asbestos or graphited asbes-
tos. Rubber hose is also very satisfactory for conveying alum solutions.
d) Use of Liquid Alum
To convert AWWA Specification dry filter alum dosage to liqLiid alum dosage, multiply dry
alum requirements by the following factors, which are obtained by dividing the Al 2 03
content of AWWA Specification filter alum (1 7 percent) by the A 12 03 content of the three
grades of liquid alum (5.8, 7.2 and 8.3 percent).
5-5
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Strength
0 Baume’ Factor
27.00 2.93
32.2° 2.36
36.4° 2.05
5.2.3 Sodium Alum mate
Sodium aluminate (Na Al 2 04) is an alkaline material. Most of the experience with alu-
minate has been with its addition to the aeration tank of an activated sludge system to
precipitate phosphate. In these cases, the alkaline nature of the salt is neutralized by the
carbon dioxide produced by the organisms. If aluminate is used at some other point in the
process, it must be determined that the pH of the wastewater is not adversely affected.
As in the case of alum, the usual dose of alumin ate, on an aluminum basis, is one or two
times the phosphorus content of the wastewater. Aluminate is available in both dry powder
(32.91% Al) and liquid (2.27 lb Al/gal) forms.
Because of its caustic nature, both dry dust and liquid forms of aluminate are irritating to
the skin, eyes and mucous membranes. The dry powder must be stored in an area free of
water or excessive moisture. The shelf life is about six months, and the liquid form should
be protected from carbon dioxide absorption during storage.
a) Handling
Materials used for handhng dry or liquid aluminate must be resistant to alkali. Black iron or
stainless steel are recommended. Materials such as brass, bronze, copper and aluminum
cannot be used. Neoprene hose can be used for transfer lines. Liqwd aluminate is a very
viscous material and positive displacement pumps should be used for controlling dosage. At
low temperatures heating of the liquid aluminate to 70°F may be necessary for proper flow
characteristics.
b) Calculation of Dosage
To determme the amount of aluminate to be used, there are three operations to be accom-
plished: (1) determine the total daily amount of phosphorus, (2) the equivalent weight of
aluminate, and (3) adjust for available aluminum. A 10-million-gallon-per-day treatment
plant flow containing 10 milligrams of phosphorus per liter of sewage, measured as ele-
mental phosphorus and employing a dose of one milligram of aluminum per milligram of
phosphorus, can be used as an example. This would require 830 pounds of aluminum which
would be contained in 3,450 pounds of dry aluminate. If liquid aluminate were used, 500
gallons per day would be required.
5.2.4 Ferric Sulfate
Commercial fernc sulfate [ Fe 2 (SO 4 )3 X H 2 01 is marketed in 100 lb moisture-proofed
paper bags, 400 lb fibre drums, and in bulk in steel hopper cars (5). The general charac-
teristics of two commercial forms are shown in Table 5-3.
5-6
-------
Table 5-3
CHARACTERISTICS OF F ERRIC SULFATE
Trade Name
“Fern-floe”
“Fernc lear”
Approx Composition
Fe 2 (S0 4 ) 3 3 H O
Fe 2 (S0 4 ) 3 2
[ 120
Fe 2 (SO 4 )3 Content
68%
76% mm
Fe Content
F&’ Content
185%
3.0% max*
21%
33%
Alumina
1.0%
( ? )
Water Insolubles
Yes
Yes
Weight per cu ft
60 — 74 lb
78 — 90 lb
Color
Reddish-gray
Grayish-white
Hygroseopic
Slightly
Slightly
Form
Granular
Granular
*Usually less than 1%
The partially hydrated forms are only slightly hygroscopic and thus are generally easy to
transport, feed and dissolve. They do, however, tend to cake at higher relative humidities.
When stored in bulk concrete storage bins, a thin shell or crust may form over the surface of
the material in the bin, thus effectively sealing out nioist air from the rest of the material.
When these chemicals are shipped in bulk, they are usually handled by mechanical convey-
trig equipment such as screw conveyors, bucket elevators or bulk-flo type conveyors Air
conveying is practiced, but requires increased equipment maintenance, especially in damp
climates These chemicals are not corrosive when dry, but are actively corrosive in solution.
When handling these chemicals with air conveyors used for other chemicals, the system
should be carefully cleaned and the filter bags changed after each use. If the same conveying
equipment is used for both quicklime and ferric sulfate, precautions should be taken against
possible mixing of the two, because a mixture will heat and cake.
a) Matenals for Handling
Dissolvers, piping and pumps may be type 31 6 stainless steel, Duriron, hard or soft rubber,
plastic, ceramic or lead. The dry chemicals can be stored safely in ordinary steel bins of
chemical feeder hoppers.
5-7
-------
Dry feeders may be of standard construction generally, but parts in contact with the
solution, vapor or fumes should be of stainless steel.
b) Use of Ferric Sulfate
Ferric sulfate has certain advantages as a coagulant. The floe produced is denser than alum
floe and is precipitated over a wide pH range (3.5 up). It does not form a soluble salt at high
pH as does alum. On the other hand, ferric sulfate has certain disadvantages; it stains
equipment; it presents problems in dissolving; its solution is corrosive; and it may be re-
duced to the soluble form (ferrous) in the presence of organic matter or reducing agents.
The choice of a ferric sulfate chemical as a coagulant must be weighed against these factors.
Ferric sulfate reacts with the natural alkalinity of the water or with the added alkaline
materials such as lime or soda ash. The reactions may be written to show precipitation of
ferric hydroxide, although m practice, as with alum, the reactions are more complicated
than this. The reactions are shown in Table 5-4.
Table 5-4
REACTIONS OF FERRIC SULFATE
Fe 2 (SO 4 )3 ÷ 3 Ca(HCO 3 ) 2 — 2 Fe(OH) 3 $+ 3 CaSO ÷ 6 CO 2 +
Fe 2 (SO 4 )3 + 6 NaHCO 3 ”* 2 Fe(OH) 3 t + 3 Na 2 SO 4 + 6 CO 2 +
Fe 2 (S0 4 ) 3 + 3 Na 2 CO 3 + 3H 2 0-ø’-2Fe(OH) 3 t ÷ 3 Na 2 SO 4 + 3 CO 2 +
Fe 2 (SO 4 ) 3 ÷ 6 NaOH+-2 FE(O1J) 3 4 ’ + 3 Na 2 SO 4
Fe 2 (SO 4 )3 + 3 Ca(OH) 2 —*’- 2 Fe(OH) 3 4’ + 3 CaSO
In tests with pure solutions, optimum phosphate precipitation occurred in the 4.5-5 p1 -I
range (2); however, in wastewater good phosphate removals occur at least up to pH 8 with
ferric salts.
5.2.5 Ferric Chloride
Ferric chloride (FeC 13) is used in wastewater treatment as a coagulant, phosphate precipi-
tant and as a sludge conditioner. It is available commercially in the anhydrous, crystal
hydrated and liquid forms (6). Table 5-5 shows characteristics of the three forms of ferric
chloride.
5-8
-------
Table 5-5
CHARACTERISTICS OF FERRIC CHLORIDE
Anhydrous Crystal Liquid
Composition FeC 13 FeCI 3 6 H 2 0 3046° Be’
FeCI 3 Content 96-97% 60% 35-45%
Fe Content 33-33.3% 20.5% 12-15.4%
Form Granular Lump Solution
Color Green-black Yellow Yellow-brown
Weight per cu ft 85-90 lb 60-64 lb 11.2-12.4 lb
per gal
Hygroscopic Very Very
Melting Point 577°F 98.6-102 2°F l4°F ±
Heat of SoIn &
Hyd. Btu per lb 353 38.2 —
All forms of ferric ch londe are hygroscopic and will absorb enough moisture from the air to
spontaneously form solutions. The solutions are acidic and, therefore, highly corrosive.
Anhydrous ferric chloride is shipped in 150 and 350 lb non-returnable steel drums. Once
drums have been opened, they should be completely emptied or tightly resealed to prevent
absorption of moisture and liquefaction. Crystal ferric chloride is shipped in 100 lb kegs or
400 or 450 lb drums. Storage should be in a cool, dry place at less than 100°F to prevent
melting of the solids. Once opened, containers of crystal ferric chloride should be com-
pletely emptied. Liquid ferric chloride solution is shipped in rubber lined tank cars or
trucks. Because of freight costs, this mode of transportation is economical only if the point
of use is near the point of manufacture.
a) Handling and Feeding
Ferric chloride must be fed as a solution because of its hygroscopic nature. Positive displace-
ment liquid feeders should be used. Orifice-type solution feeders are not recommended due
to the varying viscosity and specific gravity of ferric chloride solutions with changes in
temperature. Solution characteristics are shown in Table 5-6.
5-9
-------
Table 5-6
SOLUTION CHARACTERiSTICS
Solution Table
Specific Gravity at 20°C (68°F)
9’ F C i
o e
15°C
59°F
20°C
68°F
25°C
77°F
30°C
86°F
lb per gal
water
lb per gal
solution
10
20
30
35
40
45
1.0867
1.1834
1.2935
1.3555
1.4205
1.4885
1.0851
1.1820
1.2910
1.3530
1.4175
1.4850
1.0853
11805
1.2885
1.3505
1.4145
1 0817
11786
1.2850
1.3475
1.4115
0927
2.06
3.73
4.50
5.55
6.75
0905
1.973
3.232
3.952
4.733
5.577
NOTES
To convert to other temperatures, multiply by (Sp.G at T/Sp.G at 20°C).
To convert Solution Table for use with FeC1 3 6H 2 0, (crystals), multiply by 1.666. The
rate of solution is slower for the crystal form, but any desired concentration of FeC 13 may
be obtained.
b) Dissolvers and Dissolving
Dissolving or solution tanks for either anhydrous or crystal ferric chloride should be sized to
hold a 24-hour supply in small plants In plants where there are three shifts, a supply of 8 to
12 hours should be provided so that the task of dissolving chemical need not be done more
than once a shift. These tanks should be designed so that an entire drum of the size used in
the plant can be emptied at one time. A rack or grid should be provided to prevent the
chemical from piling up in the bottom of the tank. Agitation must be provided to give
thorough distribution of the chemical in the solution.
Anhydrous ferric chloride has a high heat of solution and the temperature of the solution
will rise as the chemical is dissolved. This temperature rise is 3.75°F for each one percent
solution. Thus, a 40 percent solution (the maximum recommended) will show a temperature
rise of 150°F. This fact is important in the choice of materials for lining tanks and in safe
handling procedures. Crystal ferric chlonde (FeC 13 6 H 2 0) has a much lower heat of
solution, which presents no problem.
Hydrometer readings of ferric chloride solutions are not dependable indications of the
concentration (%FeC 13) because temperature and impurities are factors. Determine solution
strengths by chemical analysis or by adding known weights of chemical to known volumes
of water as indicated in Table 5-6. If desired, concentrated solutions may be diluted before
or during feeding.
5-10
-------
One satisfactory method of putting the anhydrous material into solution from the small
drums is as follows. Roll the drum over the solution tank, which has its top flush with or
slightly below the floor level, punch a hole in the top and another in the bottom of the
drum; insert a hose into the top hole and allow water to trickle through, and ferric chloride
solution will trickle out of the bottom hole. When the drum is empty, add water to solution
tank to make up desired concentration. The tank may be of concrete, it should be provided
with an agitator or stirring paddle. No grid is necessary in the tank, but a suitable support
must be provided for the drum over the tank.
Rubber or rubber-lined vessels and equipment are most common. Stoneware, glass, Bakelite
and other resinous plastics also may be used. Among the metal alloys, Hastelloy “C” or
Durich lor resist all concentrations at room temperatures. Rubber linings should be protected
against abrasion and high temperatures. Wooden grids and wooden tanks are slowly attacked
by a shrinking action of ferric chloride; grids may require replacement and tanks may
require tightening of the staves to prevent leakage. Concrete is frequently used without
coating, but is slowly attacked unless coated with plastic, asphaltic paint, rubber or acid-
proof brick.
If handled carelessly, anhydrous ferric chloride may produce dust which will irritate eyes
and throats of workmen. It may spatter when put into water thus causing undue hazard. In
concentrated form or solution, ferric chloride may be injurious, and all splashes on clothing
and skin should be washed off immediately with water. If splashed in the eyes, they should
be flushed with water, followed by a solution of boric acid and attention from a physician.
All forms of ferric chloride stain badly. Floors, walls, etc., should be protected by proper
paints. Workmen should wear rubber gloves, rubber aprons and goggles.
5.2.6 Ferrous Coagulants
Ferrous salts are effective as coagulants for solids removal and phosphate precipitation.
Granular ferrous sulfate may be used, however, the cheapest source of Fe+ 2 in many
localities is waste pickle liquor from the steel industry. Ferrous sulfate produced from
pickling with sulfuric acid has been the most common form but ferrous chloride from
hydrochlonc acid pickling is also available. The pickle liquor is quite variable in composition
depending on operation of the pickling process. Iron content of the ferrous sulfate liquors
ranges up to about 22% PeSO 4 and free acid ranges from 0.5% to 15%. A typical ferrous
chloride liquor used at Mentor, Ohio, contained 7 to 11% Fe and 0.4 to 0.6% free acid.
Special handling and feed procedures may be required with pickle liquor. Some ferrous
sulfate liquors must be stored in heated tanks or be diluted to prevent crystals of FeSO 4
from forming and blocking lines and feeders. Information for designing facilities for pickle
liquor should be obtained from the supplier.
Ferrous salts may be added to raw wastewater to improve removal of solids and phosphorus
in the primary, or they may be added to the aeration tanks of activated sludge plants. When
used in primary treatment it is generally necessary to add a source of alkalinity as a
conditioning agent to produce a settleable floe. Either lime or sodium hydroxide are suitable
for this purpose. A polyelectrolyte is also frequently necessary in this type of application.
The dosage of ferrous salts should also be controlled to avoid the appearance of excessive
iron residual in the effluent. If added to activated sludge, the applied oxygen oxidizes the
5-11
-------
excess Fet 2 to Fet 3 which precipitates excess iron as the insoluble Fe(OH) 3 . Plant scale
evaluations of ferrous salts have been run at Grayling and Lake Odessa, Michigan (7),
Mentor, Ohio (8), Texas City, Texas (9), and Milwaukee, Wisconsin (10).
5.2.1 Lime
Hydrated lime [ Ca(OH) 2 ] is used in wastewater treatment for coagulation of solids, precipi-
tation of phosphate, pH adjustment and sludge conditioning (11). Dolomitic lime, which
contains a substantial content of magnesium, may also be used for some purposes. Lime
may also be purchased as anhydrous quicklime, but is slaked before use. The slaking process
involves the addition of sufficient water in special equipment to hydrate the CaO or quick-
lime.
Table 5-7
CHARACTERISTICS OF LIME
Hydrated Lime
Composition Ca(OH) 2
Ca(OH) 2 82-99%
CaO Content 62-78%
(Std. 66%)
Mg(OH) 2 Content 0
Form Powder
(200-400 mesh)
Color White (tan)
lb/cu ft 30 to 50
Hygroscopic Slightly
Solubility - gm/i 00 ml 0.18 @ 0°C
0.16 @20°C
0.15 @30°C
pH of Sat. SoIn. 12.4
Angle of Repose Varies
NOTE: Angle of repose depends on packing pressure to which chemical is subjected, and
varies from 9Q0 to zero, as aerated hydrated lime will flow like water.
5-12
-------
Lime can be purchased in bulk in both car and truckload lots it is also shipped ut 50 and
100 lb multiwall moisture-proof paper bags Storage of bagged time should be indoors, in a
dry place. to avoid absorption of moisture Bulk lime is stored dry, inside or out, in air-tight
concrete or steel bins. To ensure emptying of the bins, agitation should be provided to break
arch formation. Bagged quicklime should not be stored more than 60 days due to moisture
absorption through the multiwalled bags and the resultant caking of the lime.
Bulk quicklime may be stored indefinitely if the bins are air-tight. Hydrated lime may be
stored for several months in a dry location.
Quicklime should be fed gravimetrically to slakers constructed of mild steel. Slaking time
should be about 3 to 5 minutes with the addition of slaking water in a weight ratio of 2:1 to
6:1 Generally, dissolvers for dry lime feeders are designed to provide 3 to 5 minutes
detention when forming a 6% slurry at the maximum feed rate required. Lime slurry is
easier to transport, gives better dispersion in water, and by wetting all the particles assures
that none will settle out in the treatment basin.
Lime slurry pumps should be of the open impeller centrifugal type with a cast iron body
and impeller, with bronze trim. Tanks should be mi]d steel with a mild steel propeller-type
mixer Rubber hose slurry lines should be supported in pipe channels or by angle irons Cast
iron or mild steel pipe may be used. Valves should be cast iron or mild steel Lime can be
transported to various locations in the plant by conventional dry powder conveying systems.
An efficient dust collecting system is strongly recommended at lime handling points. A dry
pickup vacuum cleaner should be employed for removing dust around and on unloading
equipment and slakers This vacuum cleaner should be emptied promptly after each use.
When quicklime and water are mixed, enough heat is generated to endanger nearby flamm-
able materials. Lime or quicklime is a strong caustic irritant affecting eyes, mucous mem-
branes, and tipper respiratory tract. When handling dry quicklime, proper clothing should be
worn. Recommended are those made of denim with long sleeves and a hat or bandana.
Trouser cuffs and shirt sleeves should be tied. Exposed skin areas should be covered with a
protective cream. Goggles or a face shield should be worn. A suitable dust mask should be
worn when handling lime as prolonged exposure to dust can cause lung damage Solutions of
hot lime suspension can cause severe bums and eye injuries. Workmen should take extreme
precautions and always wear goggles or face shields when inspecting and cleaning lime
s lakers.
Lime is somewhat different from the hydrolyzing coagulants. When added to wastewater, it
increases pH and reacts with the carbonate alkalinity to precipitate calcium carbonate. It
also reacts with orthophosphate to precipitate gelatinous calcium hydroxyapatute. If suf-
ficient lime is added, magnesium hydroxide will be precipitated to act as an effective
coagulant for solids. These reactions are as follows.
Cat 2 + HCO + OH CaCO 3 {+ H 2 0
5Ca t2 + 4 Ol-1 + 3HPO $ Ca 5 (OH) (P0 4 ) 3 f+ 3 I-l O
Mg+2 + 2 OH s - Mg(OH) 2
5 - 13
-------
The required lime dosage to reach a given pH is a function of the alkalinity of the waste-
water, as shown in Figure 5-1 (12).
In EPA’s pilot plant at the District of Columbia’s Blue Plains Plant (13, 14, 15, 16), the lime
precipitation system gives excellent removals of solids and phosphorus. It may employ
either a two-stage high lime process with lime addition to pH 11.5 in the first stage folLowed
by recarbonation to pH 9.5-10 with flocculant additives in the second stage, or a single stage
thgh lime process at pH 11 .5 with sodium carbonate added to water to reduce the excess
calcium ions. Poorer phosphorus removals of 77% occurred in the winter with single stage
operation. The two-stage process with flocculants maintained 90% removals of phosphorus
under winter conditions. With effective clarification followed by dual-media filtration at 3
gal/mm/sq ft, residual turbidities of approximately 0.5 units were maintained with filter
runs averaging 59 hours. Other locations where lime is being used for treatment of waste-
water are at South Lake Tahoe, California (17),and in South Africa (18).
Lime sludge may be thickened, dewatered and calcined to convert the calcium carbonate to
lime which is suitable for reuse. This is economical only in plants with flows greater than
about 10 MGD. The cost of recovenng lime is comparable in cost to purchasing new lime,
but there may be a considerable savings in sludge disposal costs. At the present time, lime is
the only coagulant which can be economically recovered for reuse.
5.2.8 Polymeric Flocculants
Addition of the hydrolyzing metal coagulants to wastewater often results in a small slow-
settling floc or precipitate of phosphorus. Additional treatment is required to produce a
water with low residual suspended solids Polymeric flocculants have proved to be quite
beneficial in aggregating the particles to a settleable size and increasing the shear strength of
the floc against hydraulic breakup.
Polymenc flocculants are high molecular weight organic chains with ionic or other func-
tional groups incorporated at intervals along the chains Because these compounds have
characteristics of both polymers and electrolytes, they are frequently called polyelectro-
lytes. They may be of natural or synthetic ongin.
All synthetic polyelectrolytes can be classified on the basis of the type of charge on the
polymer chain. Thus polymers possessing negative charges are called anionic while those
carrying positive charges are cationic. Certain compounds carry no electrical charge and are
called nonionic polyelectrolytes. Because of the great variety of monomers available as
starting matenal and the additional variety that can be obtained by varying the molecular
weight, charge density and ionizable groups, it is not surprising that a great assortment of
polyelectrolytes are available to the wastewater plant operator. A partial list is shown in
Table 5-8 (20). Use of any specific polymer as a coagulant aid is of necessity determined by
the size, density and ionic charge of the colloids to be coagulated. As other factors need to
be considered, i.e. coagulants used, pH of the system, techniques and equipment for dissolu-
tion of the polyelectrolyte, etc., it is mandatory that extensive jar testing be performed to
determine the specific polymer that will perform its function most efficiently.
It is now generally agreed that a bridging mechanism accounts for the flocculation behavior
of these compounds In its simplest form the theory postulates that the polymer molecules
attach themselves to the surfaces of the suspended particles at one or more sites and that
5-14
-------
Figure 5-1
LIME REQUIREMENT FOR pH 111.0 AS A FUNCTION OF THE
WASTEWATER ALKALINITY
I I I I
I I I I I I I I I I I I I I I I I I I I
‘I I
L4 H4L H .J
T t
L4 1.L.
4
I
. .i—t
1
iii
IF 1
1 I
-H4
H
I
H
- H
i E
H
H
J
T’TT T
i.ii i ç t
- —4-4- ’ .
T
:
:
.
.
HH H-
I I
I I
:::
Li
I I I I
H H
:
I
LLI .
I :
I ‘—__I_.._ ..._I_.___!__
.L
:
.j4 L
:
j_H ::
I
I I I i i —t
E _ fH
: :.r!
t .1__i
0 100 200 300 400 500
WASTEWATER ALKALINITY mg/I - CaCO 3
5-15
C
c)
E
q
C
500
400
300
200
100
i I
tt1
-------
Table 5-8 DRY POLYELECTROLYTES
Time to
Polyelectrolyte
Aquotloc 409’
Aquofloc 411’
Aquofloc 414
Aquofloc 418
Aquorud 49702
Colgon C-2256
Calgon C 2260
Calgon C-2270
Colgon C-2300
Colgon C-2325
Calgoru C-2350
Calgon C-2400
Colgon C-2425
Calgon W1-2cuOO
Colgon WT-2630
Calgon WT2660 (ST 260)
Colgan WT-2690
Calgan WT 2700
Calgon W1-2900
Calgon WT-3000
Homaca 196’
Hercalloc 810
Hercof loc 812
Hercofloc 818
lonoc NA-7i0
Jaguar Plus
Mognufloc 530C
Mognitloc 820A
Magnufloc 835A
Magnilloc 836A
Magnufloc 837A
Mognufloc 865A
Mognifloc 870A
Mognufloc 875A
Mognifloc SODA
Mognitloc 900N
Mognulloc 901N
Mognuflac 902N
Mognulloc 905N
Naftolyte 610
Nalco 633 HO
Nalco 636-HO
Nolco 635
Nalco 0-2339
Nalcolyle 675
Polymer F3
Palyf loc 1100
Palyfloc 1110
PolyiJoc 1120
PoJyf loc 1130
Polyfloc 1150
PolyfIoc 1160
Purifloc A 23
Superfloc 128
Tyckem 8024’
Tvcheun 8013
Zeta Floc C
Zeta Flac O
Zeta hoc K (+KMruO,)’
Zeta Flac S
Zeta Hoc WA’
Zeta Flux WN
lype Bulk Density
table lbfcui ft
3 Loose Pack Work
25 34 28 CNKL
42 53 45 CNKI
48 59 61 CNKI
DPKL
33 44 36 C 5
30 43 34 DLKP
24 35 28 DLKP
25 OLP
10 16 13 BLN
10 16 13 BIN
Ii 19 14 BIN
tO 16 13 ALKM
16 28 22 BLKN
23 34 27 OKLP
29 42 33 DKLP
27 39 31 DKLP
9 16 12 AIM
10 18 14 DIP
8 13 10 AKLN
16 29 22 AKIN
20 25 21 DLP
22 31 25 EKIR
21 31 24 EKIR
30 40 33 DKIN
38 47 40 CNI
31 40 22 FIPR
34 40 35 CIP
30 42 34 DKP
27 3 5 29 CKP
28 36 30 OLP
43 42 50 CIN
28
28
26
32 40 33 CIN
27 35 29 CIN
26 40 31 DKIP
42 68 32 DJIN
45 53 47 DIN
38 50 41 DIN
39 53 43 DJIS
35 50 40 DIN
32 40 34 DIN
34 40 35 CKN
35 42 37 CKN
36 42 37 CKN
35 48 39 DKP
33 45 36 FiR
34 40 35 ON
42 53 45 BKP
28 33 29 CIN
40
33 43 36 CIN
48 68 54 EKR
52 74 59 EJR
48 68 54 EKR
54 78 6! 8 S
54 78 61 F S
54 78 61 F S
1-2 I 2350 I 4-5
12 2 480 I 4-5
-2 2 660 I 7
1-2 2 23 I 24
12 7-9
½ 1 75 1 64
1 40 1 67
½ 1 35 1 73
¾- i 05 24 I 7
¾-i 05 275 I 7
025 425 1 7
¾-i 025 740 1 7
¾-i 025 800 1 7
½ I 38 1 4
½ 15 20 I 4
½ 05 20 I 4
½ 05 23 1 7
½ 025 80 1 75
½ 025 160 I 75
½ 025 250 1 75
½-I I <50 1 <6-7
1-2 05 400 1 <6-7
1-2 05 250 1 <6-7
12 05 1000 1 <8-9
½-i 02 ISO 1 5-6
1-2 I 800 1 8-9
2
2
2
15
15
15
05
05
025
025
025
1 5
15
05
05
0 25
025
025
2
0.5
05
05
T
05
D l
01
05
2
15
025
Os
o 25
4
05
05
05
05
05
10
disperse Percent
Flow unto a Solution—room temp ma, solution
table coil solution Vusc concentrat ion
4 hoer (s) PeTct cp Sp gr pH recommendef
AP
AP
NP
CP
CM
CP
CP
CP
NP
AP
AP
AP
AP
CP
CP
CP
NP
AP
AP
AP
S
CP
CP
AP
A l ’
CC
CP
AD
AD
AD
AD
AD
AD
AD
AD
ND
ND
ND
ND
P
CP
CP
AP
AP
“P
AG
AD
AD
AD
AD
AD
CD
CD
ND
ND
ND
BC ’
aNt
B C
BC t
BA 1
BN’
1-2
½-i
½- i
½-i
1½2
t4
¼
12
12
1-2
12
1-2
½-i
¾-i
½-i
½-¾
1-2
12
1 .2
12
12
½ 1
½-i
1-2
1-2
1-2
1-2
h ½
½½
½ t4
Y .-½
¼-½
1 160 1 42
Di 450 1 6-7
01 360 I 6-7
05 620 I 5
1 130 1 41
05 38 I 75
05 38 1 75
05 20 1 55
1 ISO 1 47
I 300 1 47
1 200 I 47
I 500 1 47
05 700 1 51
2 190 1 3
15 33 1 I
025 2500 1 85
03 2000 1 70
0252500 1 85
4 800 1 95
05 2000 1 85
05 2400 1 75
05 3500 1 88
i 3000 1 88
1 1000 I 6-7
1 1300 I 56
01 950 1 10
1 750 1 6-7
10 200 I 55-6
10 750 7 6
82
82
85
‘Approved by USfrtiS far potable water use
1 Alutnunum Silicate added
5-16
-------
Table 5-8 (Continued) LIQUID POLYELECTROLYTES
Type Solution
table strength
Polyelecirolyte 3 percent
Approved by USPHS for potable woter use.
‘Plus a primary coagulant
‘A linear honsopolyrner of diollyldimethyl ommonuum chloride.
‘Polyora ma tic.
A fl Oiiix
Slightly Anionic
Bentanite Cloy or Clay. natural, colloidal.
like t pe
b — plus Bentonite
C — Coiion.c
0 — Polyocrylomide. Synthetic. High MW, Paly.
electrolyte Polymer
E — Palyacrylanitiile. Synthetic Polyelectrolyte
F — Sutlonoted Polymer
G — Guar Gum, Polysocchoride. Natural Polymer
H — High MW . Organic Polymer
— Alkyl Guonidineamine Complex
K — Sodium Alginote or Aigin Derivative, Na-
tiiral Polymer
L — teguminous Seed Derivative. Natural Polymer
M — Palyamlne, Synthetic, High MW, Polyeiec ’
tralyta Polymer
N — Nonionrc
P — Synthetic High MW, Palvelectrolyte Polymer
R — Polyacrylomide and Corboxylic Group
S — Starch, derivative, mdaified, etc, Naturol
Polymer
T — Synthetic Polymer and Caustic Soda
U — Sodium Caiboxyrrethylcellulose, Natural
Palymer
X — Ethylene Oxide Polymer
V — Carbaxyl Polymer
2 — Biocollald + Inorganic Coagulant + Caustic
Soda
3 — Hydcapfsylic Colloid + Pregelotinized Starch
in ,Aliieloi
4 — Aluminum Hydroxide + Complex Organic
Palyrrhr
. 5 — Alumina -I Polymer 4 Caustic Soda
6 — Polyacrylic Acid or Polyacrylate of Sadlum
or Ammoniu’n
7 — Aluminum Hydrate + Caustic Soda
8 — Alkalol Concentrate + Metallic Ions
9 — Chemically Modified Natural Polymer
Sp
gr
ream temp.
(about)
Viscosity—cp
room temp
pH
(about)
F LOW
A Soft flakes. may hang up il packed excessively in a
confining area, otherwise free flowing Ucually will
not need aid (vibration or agitation)
B Pawderecf, soft flakes, hang up if packed escessively
in a canfininq area, may or may not need aid accord-
ing to rote af feed. etc
C Soft granules, sometimes fibrous or flattish. may hang
up if pocl ed excessively in a canfining area, otherwise
free flawing Usually will not need aid
D Powdered, soft granules, sometimes fibrous or flotiish,
hong up if packed excessively, rray ar may not need
aid, according to athor factors
E Granular, fluid powder, will arch f packed and can
be fluidired or is floadable Ito very floodable) Needs
aid and may need rotor, according to ate. etc
r Graiules and powder, will arch and can bo fluidiead
Needs aid and could need rator, atc
0 CohesIve powder and granules, will arch, but will
nat flood Needs aid
H. Coke up of room relative humidity
. 1 Tendency to cake Car moss) at hIgher relot lve humidi.
ty
K Coke at hIgher relative humidity
Moisture absorption, may lessen fiawobility
M Practically no dust
N Very little dust
P Some dust
R Dusty
S Very dusty
TRADE NAMES
Aquorid
—
Reichhold Chemicals
Calgon C
—
Calgon Corp
Calgon WT
—
Colgon Corp
Cot Floc
—
Colgan Corp
Hanica
—
A E Staley Monulaciuring Co
H ercaflac
—
Hercules Inc
lanoc NA.710
—
tonac Chemical
Natran
—
Nntianal Starch and Chemical Corp
Polyfloc
—
Bets Loharotories. inc
Palymer F3
—
Stein Holl
Tychem
—
Standard Brands (‘hem kid , Inc
Zeta Flac_
—
NaivonMinnigar ’dCliensicotCo
Dilution
Aquof loc 403
AH
Full
112
100
96
4-1
Aquof loc 405
CH
full
I 06
1.000
63
10 1
Aquafloc 407
NH
Full
1 00
10.000
43
10-i
Aquofloc 408’
AH
full
1 01
1,500
3-4
10 I
Aquoflac 410
CH
Full
103
i,00D
102
101
Aquaflac 412
CHi
Full
1 25
50
1 0
4-1
Aquailac 415
AH
Full
1 03
1.000
Ii S
10-i
Aquosid 49.700
CM
Full
ii
100-500
78
10 I
Aquarid 49-701
CM
Full
11
25 150
78
10 1
Aquarid 49-7Q3
AH’
Full
1 2
200 500
II 5
20-1
Cot-Floc (WT.2870)*
CH’
Full
1 025
2.000
42
<101
Mogniflac 521.C
CH
Full
115
225 325
45
-------
part of the long chain extends out mto the bulk of the solution. The free end of the
molecule is then able to adsorb onto another suspended particle when contact is made, thus
forming a bridge or link between two particles of suspended solids. The progressive linking
of more and more particles results in an ever-increasing size of floc whose eventual size is
limited by its ability to withstand the hydraulic shear gradient imposed upon it by agitation
or turbulence.
Anionic polymers have proved to be most effective when used with alum or iron coagulants.
The anion ics have also been used as sole additives to improve removals of solids in primary
treatment, but results have been marginal Cationic polymers when used alone will fre-
quently produce excellent clarification of raw wastewater, but generally require high
dosages. The cationics find their greatest application in conditioning of sludges for dewater-
ing.
Polymers are used in very small doses, usually less than 1 mg/I . The dosage range in which
the polymers are effective is usually limited. An overdose, in addition to the increased cost
of chemicals, will frequently restabilize the solids so that they cannot be settled out. A 10
MGD plant employing a 0.5 mg/I dose of polymer would use 42 lbs of polymer per day. At
typical polymer cost of $1 50/Ib, the cost of polymer for the plant would be $63.00/day
Polymers are usually shipped as dry powders and are converted to a liquid form at the plant
site, although some are available as liquids. During storage the dry powders must not be
allowed to pick up moisture. The dilute solutions of polymers are viscous and must be made
up according to the manufacturer’s directions. In general, the polymers are non-hazardous
and require only the usual protection from dust when handling the dry material. The dosage
of polymer may l)e accurately controlled by metering pumps.
5.3 Chemical Feeders
Chemical feed systems must be flexibly designed to provide for a high degree of reliability in
light of the many contingencies which may affect their operation. Thorough waste charac-
terization in terms of flow extremes and chemical requirements should precede the design of
the chemical feed system. The design of the chemical feed system must take into account
the form of each chemical desired for feeding, the particular physical and chemical charac-
teristics of the chemical, maximum waste flows and the reliability of the feeding devices.
In suspended and colloidal solids removal from wastewaters the chemicals employed are
generally in liquid or solid form. Those in solid form are generally converted to solution or
slurry form prior to introduction to the wastewater stream; however, some chemicals are fed
in a dry form. In any case, some type of solids feeder is usually required. This type of feeder
has numerous different forms due to wide ranges in chemical characteristics, feed rates and
degree of accuracy required. Liquid feeding is somewhat more restrictive, depending mainly
on liquid volume and viscosity.
The capacity of a chemical feed system is an important consideration in both storage and
feeding. Storage capacity design must take iiito account the advantage of quantity purchase
versus the disadvantage of construction cost and chemical deterioration with tinie (19).
Potential delivery delays and chemical use rates are necessary factors in the total picture.
Storage tanks or bms for solid chemicals must be designed with proper consideration of the
angle of repose of the chemical and its necessary environmental requirements, such as
temperature and humidity. Size and slope of feeding lines are important along with their
materials of construction with respect to the corrosiveness of the chemicals.
5-18
-------
Chemical feeders must accommodate the minimum and maximum feeding rates required.
Baker (I 9) indicates that manually controlled feeders have a common range of 20 1, but
this range can be increased to about 100 1 with dual control systems Chemical feeder
control can be manual, automatically proportioned to flow, dependent on some form of
process feedback or a combination of any two of these. The cost of the more sophisticated
control systems is relatively low if proper sensors are available. If manual control systems
are specified with the possibility of future automation, the feeders selected should be
amenable to this conversion with a minimum of expense. An example would be feeders with
external motors which could easily be replaced with a variable speed motor or drive when
automation is installed (19). Standby or backup units should be included for each type of
feeder used. Reliability calculations will be necessary in larger plants with a greater multi-
plicity of these units. Points of chemical addition and piping to them should be capable of
handling all possible changes in dosing patterns in order to have proper flexibility of opera-
tion. Designed flexibility in hoppers, tanks, chemical feeders and solution lines is the key to
maximum benefits at least cost (19).
Liquid feeders are generally in the form of metering pumps or orifices. Usually these
metering pumps are of the positive-displacement variety, plunger or diaphragm type. The
choice of liquid feeder is highly dependent on the viscosity, corrosivity, solubility, suction
and discharge heads, and mternal pressure-relief requirements (20) Some examples are
shown in the Figure 5-2. In some cases control valves and rotameters may be all that is
required. In other cases, such as lime slurry feeding, centrifugal pumps with open impellers
are used with appropriate controls (23). More complete descriptions of liquid feeder require-
ments can be found in the literature and elsewhere (20)
Solids characteristics vary to a great degree and the choice of feeder must be considered
carefully, particularly in the smaller-sized facility where a single feeder may be used for
more than one chemical. Generally, provisions should be made to keep all chemicals cool
and dry. Dryness is very important, as hygroscopic (water absorbing) chemicals may become
lumpy, viscous or even rock hard; other chemicals with less affinity for water may become
sticky from moisture on the particulate surfaces, causing increased arching in hoppers. In
either case, moisture will affect the density of the chemical and may result in under-feed.
Dust removal equipment should be used at shoveling locations, bucket elevators, hoppers
and feeders for neatness, corrosion prevention and safety reasons. Collected chemical dust
may often be used.
The simplest method for feeding solid chemicals is by hand. Chemicals may be preweighed
or simply shoveled or poured by the bagful into a dissolving tank. This niethod is of
economic necessity limited to very small operations, or to chemicals used in very weak
solutions.
Because of the many factors,such as moisture content, different grades and compressibility,
which can affect chemical density (weight to volume ratio), volumetric feeding of solids is
normally restncted to smaller plants, specific types of chemicals which are reliably constant
in composition and low rates of feed. Within these restrictions, several volumetric types are
available. Accuracy of feed is usually limited to less than 97%.
One type of volumetric dry feeder uses a continuous belt of specific width moving from
under the hopper to the dissolving tank. A mechanical gate mechanism regulates the depth
of material on the belt, and the rate of feed is governed by the speed of the belt and/or the
height of the gate opening. The hopper normally is equipped with a vibratory mechanism to
reduce arching. This type of feeder is not suited for easily fluidized materials.
5-19
-------
PLUNGER PUMP (Courtesy of Wallace & Tiernan)
i_ -,
/ 1 SUCTION VALVE
DIAPHRAGM PUMP
VI UHAKUE VALVE
SUCTION VALVE
DIAPHRAGM
POSITIVE DISPLACEMENT PUMPS
5-20
PLUNGER
DISCHARGE VALVE
(Courtesy of Wallace & Tiernan)
Figure 5-2
-------
Liq iid cylinder
Discharge manifold
Suction manifold
PISTON PUMP (21)
GEAR PUMP (21)
Figure 5-2 (Continued)
5-21
-------
Another type employs a screw or helix from the bottom of the hopper through a tube
opening slightly larger than the diameter of the screw or helix. Rate of feed is governed by
the speed of screw or helix rotation. Some screw-type designs are self-cleaning, while others
are subject to clogging. Figure 5-3 shows a typical screw-feeder.
Most remaining types of volumetric feeders generally fall into the positive-displacement
category. All designs incorporate some form of moving cavity of a specific or variable size.
In operation, the chemical falls by gravity into the cavity and is more or less fully enclosed
and separated from the hopper’s feed. The size of the cavity, and the rate at which the
cavity moves and is discharged, governs the amount of material fed. The positive control of
the chemical may place a low limit on rates of feed. One unique design is the progressive
cavity metering pump, a non-reciprocating type. Positive-displacement feeders often utilize
air injection to enhance flowability of the material. Some examples of positive-displacement
units are illustrated in Figures 5-4 and 5-5 (22).
The basic drawback of volumetric feeder design, i.e., its inability to compensate for changes
in the density of materials, is overcome by modifying the volumetric design to include a
gravimetric or loss-in-weight controller. This modification allows for weighing of the mate-
Hal as it is fed. The beam balance types measures the actual mass of material. This is
considerably more accurate, particularly over a long period of time, than the less common
spring-loaded gravimetric designs. Gravimetric feeders are used where feed accuracy of about
99% is required for economy, as in large scale operations and for materials which are used in
small, precise quantities. It should be noted, however, that even gravimetric feeders cannot
compensate for weight added to the chemical by excess moisture. Many volumetric feeders
may be converted to loss-in-weight function by placing the entire feeder on a platform scale
which is tared to neutralize the weight of the feeder.
Good housekeeping and need for accurate feed rates dictate that the gravimetric feeder be
shut down and thoroughly cleaned on a regular basis. Although many of these feeders have
automatic or semi-automatic devices which compensate to some degree for accumulated
solids on the weighing mechanism, accuracy is affected, particularly on humid days when
hygroscopic materials are fed. In some cases, built-up chemicals can actually jam the equip-
ment.
No discussion of feeders is complete without at least passing reference to dissolvers, as any
metered material must be mixed with water to provide a chemical solution of desired
strength. Most feeders, regardless of type, discharge their material to a small dissolving tank
which is equipped with a nozzle system and/or mechanical agitator depending on the solu-
bility of the chemical being fed. Insoluble materials, such as polyelectrolytes, may be
carefully spread into a vortex spray or washdown jet of water immediately before entering
the dissolver. Because this type of material does not truly dissolve, it is essential that the
surface of each particle become thoroughly wetted before entering the feed tank to ensure
accurate dispersal and to avoid clumping, settling or floating.
A dissolver for a dry chemical feeder is unlike a chemical feeding mechanism, which by
simple adjustment and change of speed can vary its output tenfold. The dissolver must be
designed for the job to be done. A dissolver suitable for a rate of 10 lb/hr may not be
suitable for dissolving at a rate of 100 lb/hr. As a general rule, dissolvers may be oversized,
but dissolvers for commercial ferric sulfate or lime slakers do not perform well if greatly
oversized. An example of a typical unit of this type can be seen in Figure 5-6.
5-22
-------
Figure 5-3 SCREW FEEDER
MOTOR AND
GEAR REDUCER
Figure 5-4 POSITIVE DISPLACEMENT SOLID
FEEDER — ROTARY (22)
HOPPER
ROTATING &
FEED SCREW
CHAMBER
LEVEL
JET MIXER
5-23
-------
Figure 5-5 POSITIVE DISPLACEMENT POWDER PUMP
(Courtesy of Robbins & Meyers)
P
FLUIDIZING HOPPER
FLUIDIZING
AIR INL
AIR INLET
SUCTION PORT
FLUIDIZING PAD
-------
r — — — OPTIONAL
I MIXING 1
FTJNNELç 1
Figure 5-6 DRY CHEMICAL DISSOLVER
WATER METER
MECHANICAL MIXER
DISSOLVING TANK
DAY TANK
5-25
-------
It is essential that specifications for dry chemical feeders include specifications on dissolver
capacity. A number of factors need to be considered in designing dissolvers of proper
capacity. These include detention times and water requirements, as well as other factors
specific to individual chemicals.
The capacity of a dissolver is based on detention time, which is directly related to the
wettability or rate of solution of the chemical. Therefore, the dissolver must be large enough
to provide the necessary detention for both the chemical and the water at the maximum
rate of feed. At lower rates of feed, the strength of solution or suspension leaving the
dissolver will be less, but the detention time will be approximately the same unless the water
supply to the dissolver is reduced. When the water supply to any dissolver is controlled for
the purpose of forming a constant strength solution, mixing within the dissolver must be
accomplished by mechanical means, because sufficient power will not be available from the
mixing jets at low rates of flow.
Specific factors as to recommended chemical feed rates per volume of water, detention
times required, and necessary materials of construction are available in the literature (24),
(25). Alum, lime and ferrous sulfate have been found to require about 5 minutes detention
time at about 0.5 lb/gal. Ferric sulfate requires longer detention times (20 to 30 minutes)
than the other granular chemicals. Hot water dissolvers are also available in order to decrease
the required tankage. Further practical experience with a number of these chemicals is
available in Culp and Culp (23).
The foregoing descnptions give some indication of the wide variety of materials which may
be handled. Because of this variety, a modern facility may contain any number of a variety
of feeders with combmed or multiple materials capability. Ancillary equipment to the
feeder also varies according to the material to be handled. Liquid feeders encompass a
limited number of design principles which account for density and viscosity ranges. Solids
feeders, relatively speaking, vary considerably due to the wide range of physical and
chemical characteristics, feed rates and the degree of precision and repeatability required.
5-26
-------
5.4 References
1. “Reference File No. I “, Water & Wastes Engineering, 6, 53 (November 1969).
2. USD1, FWQA, Water Pollution Control Research Series 17010 EKI 04/70, “Kinetics
and Mechanism of Precipitation and Nature of the Precipitate Obtained in Phosphate
Removal from Wastewater Using Aluminum (III) and Iron (III) Salts”.
3. Farrell, J.B., Salotto, B.V., Dean, R.B.,andTolliver,W.E., “Removal of Phosphate from
Wastewater by Aluminum Salts with Subsequent Aluminum Recovery”, Chemical
Engineering Progress Symposium Series, 4, No. 90, 232 (1968).
4. “Reference File No. 2”, Water & Wastes Engineering, 6, 46 (December 1969).
5. “Reference File No. 6”, Water & Wastes Engineering, 7, 61 (April 1970)
6. “Reference File No. 5”, Water & Wastes Engineering, 7, 65 (March 1970).
7. Wukash, R.F., “New Phosphate Removal Process”, Water & Wastes Engineering, 5, 58
(September 1968). —
8. “Phosphorus Removal by Ferrous Iron and Lime”, Final Report, EPA Grant 11010
EGO Lake County, Ohio (1971).
9. “Phosphorus Removal and Disposal from Municipal Wastewater”, Final Report,
FWPCA Grant WPD 223-01-68, University of Texas, Medical Branch,Ga lveston, Texas
(1970).
10. “Phosphorus Removal with Pickle Liquor in a 115 MGD Activated Sludge Plant”, Final
Report, EPA Grant 11010 FLQ, Milwaukee, Wisconsin (1971).
II. “Reference File No. 3”, Water and Wastes Engineering, 7, 53 (January 1970).
12. Mulbarger, M.C., Grossman, E , Dean, R.B., and Grant, O.L., “Lime Clarification, Re-
covery, Reuse, and Sludge Dewatering Characteristics”, JWPCF, 41, 2070 (1969).
13. O’Farrell, T.P., Bishop, D.F., and Bennett, S.M., “Advanced Waste Treatment at Wash-
ington, D.C.”, Chem. Eng. Progress Symposium Series 97, 65, 251(1969).
14. Stamberg, J.B., Bishop, D.F., Warner, H.B., and Griggs, S.H, “Lime Precipitation in
Municipal Wastewaters”, Presented at 62nd Annual Meeting of AICHE, (November
1969).
15. Bishop, D.F. O’Farrell, T.P., and Stamberg, J.B., “Physical Chemical Treatment of
Municipal Wastewater”, Robert A. Taft Water Research Center, Cincinnati, Ohio
(October 1970).
16. Bishop, D.F., O’Farrell, T.P., Stamberg, J.B., and Porter, J.W., “Advanced Waste Treat-
ment for Phosphate Control”, Presented at the 68th Annual Meeting of AICHE,
Houston, Texas (March 1971).
5-27
-------
5.4 References (Cont)
17. Slechta, A.F., and Cuip, G.L., “Water Reclamation Studies at the South Tahoe Public
Utility District”, JWPCF, 39, 787 (1967).
18. Stander, G.J., and Van Vuuren, L.R.J., “The Reclamation of Potable Water from
Wastewater”, JWPCF, 41, 355 (1969).
19. Baker, R.J., “Chemical Feed Systems Determine Plant Efficiency and Reliability”,
Water & Sew. Wks., 116, Ref. No , R-21 (November 1969).
20. Russo, F, and Can, R.L., Jr., “Polyclectrolyte Coagulant Aids and Flocculantr Dry
and Liquid, Handling and Application”, Water & Sew. Wks., 117, Ref. No., R-72
(November 1970). —
21. M. Rost & E.T. Visich, “Pumps”, Chemical Engineering, 76, 8,45 (Apnl 1969).
22. R.P. Lowe, “Chemical Feed Systems”, 10th An. Water Conference of Eng Soc. of W.
Penna., Oct. 17-19, 1949.
23. CuIp, R.L., and Cuip, G.L., Advanced Wastewater Treatment , Van Nostrand Reinhold
Company, New York (1971).
24. B.I.F. Industries, Water Treatment-Municipal.
25. Water Treatment Plant Design , American Water Works Association, Inc., New York
(1969).
5-28
-------
CHAPTER 6
CHEMICAL PROCESSES
6.1 General
Chemical processes which are considered for suspended solids and phosphorus removal
applications are usually of two types
a. Traditional separate rapid-mixing, flocculation and sedimentation.
b. Solids-contact clarifiers.
The purpose of these chemical processes is to promote ideal conditions for the chemical
reactions and for the separation of solids created as products of these reactions.
6.2 Chemical Mixing and Flocculation
Traditional water treatment systems have utilized rapid-mix and flocculation basins ahead of
sedimentation tanks for chemical clarification. The basic equipment used in wastewater
treatment is generally the same as that used in water treatment. The rapid mix is designed to
provide a thorough and complete dispersal of chemical throughout the wastewater being
treated to insure uniform exposure to pollutants which are to be removed. Detention times
vary, but 10 to 30 seconds are usual design figures with velocity gradients of about 300
se C ’ )(l). In-line blenders can be used as well as the traditional high-powered mixers which
may require as much as I horsepower/MGD. In essence, the rapid mix performs two func-
tions, the one previously noted (mixing) and a rapid coagulation. These functions are
enhanced by increased turbulence (2), (3). Hudson and Wolfner (4) recommend that mixing
equipment be of the variable-speed variety in order to promote optimum mixing conditions
for the particular conditions of application. Some typical mixers are shown in Figures 6- 1
and 6-2.
Flocculation promotes the contact, coalescence and size increase of coagulated particles (1).
Flocculation devices vary in form, but are generally divided into two categories. These are
mechanically-mixed and baffled flocculators Typical units are illustrated in Figures 6-3 and
64. Baffled basins have the advantage of low operating and maintenance costs, but they are
not normally used because of their space requirement, inability to be easily modified for
changing conditions and high head losses. Most installations utilize horizontal or vertical
shaft mechanical flocculators which are easily adjusted to changing requirements. These
units are compared and discussed by Hudson and Wolfner (4) and by Camp (5). The
location of these flocculators in series having progressively lower velocity gradients has been
shown to produce large rapid-settling flocs (1), (4), (5)
Theones of flocculation are available in the literature (2), (3), (6). It appears from these
studies that the criterion fqr proper flocculation lies in the dimensionless factor GCt, where
G velocity gradient (sec ), Cfloc volume concentration (vol floc/vol suspension) and
I time of flocculation (sec). Values available in the literature for good flocculation in both
solids-contact and conventional mixing-flocculation systems appear to fall in the range from
50 to 200 (6).
6-1
-------
Figure 6-1 STATIC MIXER
Figure 6-2 IMPELLER MIXER
flgIYE MECHANISM
MOTOR
SUPPORT BEAMS
IMPELLER
FEED
6-2
-------
cONTROl. VM.RC
ii
II II
II
I ! II Ii
/
U JI —,
H’ II
II
II II
Ii I LI
11 _ TL :
Figure 6-3 MECHANICAL FLOCCULATION BASIN
1
I
1-
I
- - — -
I
-L
-“
-
I
f
:
PLAN
- (—..‘ /—‘ (__ .
LLJ J
_______
SECT ION
Figure 6-4 BAFFLE FLOCCULATION BASIN
6-3
c rFt
FWENT
EFFLUENT-
TN OF
FLOW -O
-------
Cuip and CuIp (7) indicate their preference for the traditional rapid mix, flocculation,
sedimentation approach owing mainly to its amenability to control of each unit process. At
the South Tahoe plant, recycle of settled sludge to the head end of the system provides
additional flexibility for enhanced flocculation by providing for possible increases in the
floc volume concentration. Polymer addition is made after the wastewater has passed
through the flocculation basin.
6.3 Solids-Contact Clarification
Solids-contact clarifiers have become popular for water and advanced wastewater treatment
in recent years because of their inherent size reduction when compared to separate mixing,
flocculation and sedimentation basins in series. Their use in water clarification and soften-
ing was carned over to waste treatment when chemical treatment of wastewaters was initi-
ated. Theoretically, the advantage of reduced size accrues to their ability to maintain a high
concentration of previously-formed chemical solids for enhanced orthokinetic flocculation
or precipitation and their physical design, whereby three unit processes are combined in one
unit. In practice this amounts to savings in equipment size and capital costs. The basin
configurations and vertical flow patterns have been cited as conducive to reduced density
currents and short-circuiting when compared to horizontal tanks (8).
CuIp and CuIp (7) have expressed disfavor with the sludge-blanket clanfiers for reasons
which include possible anaerobic conditions in the slurry; lack of individual process control
for the mixing, flocculation and sedimentation steps; and uncontrolled blanket upsets
under varying hydraulic and organic loading conditions. The major allegation is the in-
stability of the blanket, which has presented operational problems in the chemical treatment
of wastewaters. Possibly the most effective method of control to date, other than close
manual control, has been to minimize the blanket height to allow for upsets (9). The
advantages of higher flow rates and solids-contacting are maintained, but the added ad-
vantage of the blanket is minimized. Another possibility which has not been fully evaluated
is the use of sludge-blanket sensors for automatic control of solids wasting.
Tesarik (10) classified commercial sludge-blanket clariuiers into four categories
a. Mechanically-agitated — central mixing with minimal fluidization of blanket due to
low overflow rates.
b. Hydraulically-fluidized — central mixing with full fluidization due to high overflow
rates.
c. Sludge circulation — slurry mixing with recirculation of settled solids to the slurry.
d. Unsteady discharge — pulsating feed of chemically-treated influent to a fluidized
blanket.
In the United States, most of the systems used are of type c, which can be termed “solids-
contact’ 1 clarifiers due to the intimate contacting of the influent stream with the recircu-
lating slurry. The high concentration of solids in the recycled slurry provides for a rapid
reaction. Typical solids-contact clarifiers are illustrated in Figures 6-5 and 6-6.
64
-------
TREATED WATER
EFFLUENT
CLEAR WATER
SEPARATION
I
•
RAPID MIXING AND RECIRCULATION
\
I \I 1 1
1 \I I L
Figure 6-5
SLOW MIXING AND FLOC FORMATION
/ CHEMICAL INTRODUCTION
/
CLARIFIED
WATER
RAW MATER
INFLuENr
$EDIM ENTAT1ON
SOLIDS CONTACT CLARIFIER WITHOUT SLUDGE BLANKET FILTRATION
Li
00 000
V I ii
L I—-_-_ t
‘I
•. y: 7
SLUDGE RECIRCULATION
SLUDGE REMOVAL
(Courtesy of Graver Water Conditioning Co.)
-------
COLLECTOR FLUME
Figure 6-6
PRECIPEFATOR DRAtN
SOLIDS CONTACT CLARIFIER WITH SLUDGE BLANKET FILTRATION
INFLUENT
N
SLUDGE AG ATOR
CONCENTRATOR ARM
F
M?XING BAFFLES
ZONE
(Courtesy of the Permutit Co.)
-------
Solids-contact clarifiers have been used for the treatment of secondary and primary efflu-
ents, as well as for the treatment of raw, degritted wastewater. Lime as the treatment
chemical has been used with overflow rates from 1200 to 1 700 gpdfsq ft in solids-contact
units, while iron compounds and alum have been used at lower values, usually between 500
and 1000 gpd/sq ft. All of these rates come from pilot studies of less than 1 MCD capacity,
and may be subject to change at a larger scale due to differences in hydraulics. Polymer
treatment can also influence the choice of overflow rates for design if their cost can be
economically justified when compared to the cost of lower overflow rates. Detention times
in these solids-contact basins have ranged from just over one to almost five hours. Sludge
removal rate is dependent on the solids concentration of the underfiow, which is a function
of the unit design as well as the chemical employed. These pilot plants have reported lime
sludge drawoffs from 0.5 to 1.5 percent of the wastewater flow at concentrations of from 3
to 17 peTcent solids. Alum and iron sludges have not been monitored extensively, but
drawoffs have been reported to be I to 6 percent of the flow with 0.2 to 1.5 percent solids.
Much of the design infonnation necessary for solids-contact clarifiers has been obtained
from water treatment experience. This is not surprising in that the principles of treatment
are identical. The characteristics of the solids that are formed and separated are the source
of differences. The organic matter contained in the chemically-created sludges causes the
sludge to become lighter and also more susceptible to septicity due to the action of micro-
organisms. The former condition suggests lower hydraulic loadings, while the latter suggests
higher ones, given a set physical design. Since sludge septicity is neither universal nor
uncontrollable, a lower design overflow rate may comprise much of the necessary adjust-
ment to waste treatment conditions from those of water treatment. As indicated previously,
design overflow rates from 1200 to 1700 gpd/sq ft for lime treatment and from 500 to 1000
gpd/sq ft for alum or iron treatment have been successful at less than I MCD capacity.
Good engineering practice would suggest designing for the lower values cited in order to
provide increased flexibility and reliability in the plant. Cold weather peak flow conditions
will probably constitute the limiting condition, as water treatment practice has shown that
overflow rates are reduced by as much as 50 percent at near-freezing temperature (11).
Wastewater will probably not reach such low temperatures in most areas, but the effects are
significant.
Design features of solids-contact clarifiers should include
a. Rapid and complete mixing of chemicals, feedwater and slurry solids must be
provided.
b. Mechanical means for controlled circulation of the solids-slurry must be provided
with a reasonably wide range of speeds. The maximum peripheral speed of mixer
blades should not exceed 5 ft /sec.
c. Means should be provided for measuring and varying the slurry concentration in the
contacting zone.
d. Sludge discharge systems should allow for easy automation and variation of volumes
discharged.
e. Sludge-blanket levels must be kept a minimum of 5 feet below the water surface.
f. Effluent launders should be spaced so as to minimize the horizontal movement of
clarified water.
6-7
-------
Tesarik (10) has derived the relationship that the minimum depth below the surface should
be one-half the horizontal distance between launders. If the 5-foot minimum sludge-blanket
depth (requirement e) is maintained, I 0-foot spacmg of the effluent weir should not be
significantly exceeded. Most of the above requirements are based on those cited in Water
Treatment Plant Design (1). Further considerations include skimmers and weir overflow
rates. Skimmers will probably be necessary when treating raw wastewaters due to the likely
presence of floatable solids and grease. Tertiary systems may possibly avoid the use of these
devices, but it may be good practice to mclude them m the design in case anaerobic
conditions develop. Weir overflow rates should conform to the requirements of other
clarification units.
6-8
-------
6.4 References — Chemical Processes
1. Water Treatment Plant Design , American Water Works Ass’n, Inc.. New York (1969).
2. Camp, T.R., “Floc Volume Concentration”. Jour. AWWA, 60, 656 (1968)
3. O’Melia, C.R., “The Coagulation Process Application of Research to Practice”, Report
submitted to ASCE (Oct. 1969).
4. Hudson, H.E., Jr., and Wolfner, J.P., “Design of Mixing and Flocculating Basins”, Jour.
AWWA, 59, 1257 (Oct. 1967).
5. Camp,T R., “Flocculation and Flocculation Basins”, Trans. ASCE, 120, (1955).
6. Ives, K.J., “Theory of Operation of Sludge Blanket Clarifiers”,Proc. ICE (Br.), 39, 243
(Feb 1968). —
7. CuIp, R.L., and Cuip, G.L., Advanced Wastewater Treatment , Van Nostrand Reinhold
Company. New York (1971).
8. Aitken, I.M.E., “Reflections on Sedimentation Theory and Practice - Part I,” Eff. and
Water Treatment Jour. (Br.) ,! , 4, 226 (Apr. 1967).
9. Burns, D.E., and Shell, G.L., “Physical-Chemical Treatment of a Munic ipal Wastewater
Using Powdered Activated Carbon”, Final Report of EPA-WQO Contract No.
14- i 2-585 (Apr 1971).
10. Tesarik, 1., “Flow in Sludge-Blanket CZarifiers”, Jour. SED-ASCE, 93, 105 (Dec.
1967). —
11. Eaton, C.D., “Influence of Water Temperature on Performance of Solids-Contact
Units”, TAPPI, 47, 10, lS8A (Oct. 1964).
6-9
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CHAPTER 1
GRAVITY SYSTEMS
1.1 General
Gravity separation of solids is based on the difference between the specific gravity of the
liquid (continuous phase) and the particulate matter (discontinuous phase). Obviously, only
two possibilities exist for separation. Heavier than water particulates will settle to the
bottom of the liquid and lighter than water particulates will float to the top. Sedimentation
is the simplest and most widely employed waste treatment process. An idealized sedimenta-
tion basin is shown in Figure 7-1. The basin is divded into four zones, inlet, outlet, settling
and sludge The inlet zone must dissipate the kinetic energy of the feed stream and provide a
uniform distribution of the flow entering the settling zone without creating excessive turbu-
lence. The relative importance of proper inlet conditions as compared to outlet conditions
has been shown (I). The outlet zone is less cntical to basin performance, but low approach
velocities to effluent weirs must be maintained for maximum sedimentation efficiency. The
difficulties in obtaining optimum inlet and outlet conditions vary with basin shape and flow
patterns (1).
The idealized rectangular basin shown in Figure 7-2 illustrates the situation which occurs
when a perfect inlet distribution of discrete particles occurs with no particle interaction and
no resuspension of particles from the sludge zone The resultant vector on the discrete
particle is then made up of its theoretical settling velocity, usually described by Stokes’
Law. and the superficial uniform velocity of the liquid. Numerous theoretical derivations
have been made on the assumptions of this diagram. The most notable is that of Camp (2).
In reality, uniform flow has been unattainable in the large tanks normally designed. Short-
circuiting, turbulence and density currents have been cited as causing the major deviations
from the ideal situation. Tracer studies have illustrated the extent of short-circuiting in
conventional basins. Most studies of this type have indicated that both vertical flow and
long horizontal tanks have better flow characteristics than circular tanks, particularly those
of the center-feed type (1). A salt 1racer study of four types of tanks produced the results
shown in Figure 7-3 (3).
The sludge zone serves the two-fold function of retaining the solids so that a minimum of
resuspension occurs and providing sufficient time for compaction, thereby minimizing
sludge pumping requirements. In most cases mechanical scrapers are utilized to slowly move
the sludge to the pump drawoff area. An alternative device uses suction to remove the
sludge at its deposition point, thereby eliminating the need for displacement prior to re-
moval. A number of different tank configurations are illustrated in Figures 7-4 through 7-6.
1.2 Primary Sedimentation
Primary sedimentation is the most widely used waste treatment process, whether alone or
preceding a secondary treatment process. Primary clarifiers are designed to remove the
settleable solids and their associated organic load from the wastewater. The basis of design is
the overflow rate, usually expressed in gallons/day/square foot (gpd/sq ft), which is equal to
the flow in gallons/day divided by the surface area of the clarifier in square feet. it is
obvious that overflow rate in itself does not totally determine the efficiency of a tank. In
7-1
-------
INLET OUTLET
ZONE /1 ZONE
EFFECTIVE
SETTLING
ZONE
:::_W_LLt L
— SOLIDS__REMOVAL__ZONE ____
Figure 7-1
FUNCTIONAL ZONES IN AN IDEALiZED
SEDIMENTATiON BASIN
SURFACE AREA A
DIRECTION OF FLOW Q IVO
L ____
Le
Figure 7-2
IDEALIZED SETTLING PATHS OF DISCRETE
PARTICLE IN A HORIZONTAL FLOW TANK
7-2
-------
Figure 7-3
RESULTS OF SALT-INJECTION TESTS WITH
DIFFERENT TYPES OF SEDIMENTATION TANKS
- j adial Flow hi Circular
T Tank.
Ibrirontal Aow i Wud.
Poctwiqular Tag.
I4oc.tpith l Flow in Narrow
RiaCtQA9ulOI’ T3nL
Vçticot Flow in Up(low
Dwiqn ci Tank.
A
0
z
A
z
-I
-l
0
z
-1
0
F-
-4
0
-------
Figure 7-4
8
______ - -______________
LINK.Bi COMPANY j- • Z t - 7 T -
T r
5tCTION AA
RECTANGULAR SETTLING BASIN WITH SLUDGE AND SCUM COLLECTOR
-------
/
j tNK-BEU COMPANY
L. ’/ T QN 1u u TAP J <
!_I ?... - 5 ,4g ii1T2 7 771
Figure 7-5
CIRCULAR SETTLING BASIN - CENTER FLOW
L Jt I D
ft. 7 fr,if1 PLOIJ t !*Y .4 -
f ’r4r4D # O4 4A/P,•
OCA r4 o a6 cP
7-5
-------
T1
C
T1
-------
addition to the previously noted influent and effluent zone effects, the basin shape and size
as well as the settling characteristics of each individual wastewater will influence the solids
removal efficiency of a given sedimentation tank. Seasonal temperatures can also exert a
significant influence on basin performance.
The Ten-State Standards recommend an overflow rate of 600 gpd/sq ft or less for plants
having a design flow of less than I MCD, but allow higher rates for larger plants (4). These
standards also require a minimum depth of 7 feet for primary tanks with mechanical
cleaning and a minimum length from inlet to outlet of 10 feet. Smith (5) has discussed the
use of excess capacity factors in design. American practice generally utilizes depths from 7
to 12 feet, which, when combmed with overflow rate, provide nominal detention periods
from I to 2 hours (6). European practice also considers mean flow velocities with a design
value for rectangular basins of about 2 feet/minute (fpm). This type of basin is designed
with depth to length ratios of 1/10 to 1/30. Widths generally vary from 20 to 30 feet, and
depths from 5 to 10 feet (7). Details of more subtle design considerations such as tank
slopes and ancillary devices can be found in the Water Pollution Control Federation (WPCF)
Manual of Practice No. 8 (6). Tube settlers, discussed elsewhere, are often installed in
existing sedimentation basins to increase total throughputs
As previously noted, the removal efficiency of primary sedimentation facilities will vary
widely. WPCF MOP No. 8 (6) shows a nearly straight line inverse relationship between
overflow rate and suspended solids removal, while the 10-State Standards show a similar
retatioriship for ROD removal (4). Fair and Geyer indicate ranges of 40 to 70 percent and 25
to 40 percent removal for suspended solids and ROD, respectively (8). Kalbskopf and
Metcalf and Eddy show a family of curves for different influent suspended solids concentra-
tions, the former versus horizontal velocity and the latter versusdetention time (7),(9).The
values of all these correlations fall in ranges of 35 to 75 percent and 25 to 40 percent
removal of suspended solids and ROD, respectively. Sludge volumes are generally less than I
percent of the total flow, and solids concentrations are usually between 2 to 7 percent of
the underflow. Skimmings are cited as 0.1 to 0.7 cu ft/MG (7).
7.3 Secondary Sedimentation
The principles governing design of secondary sedimentation tanks are significantly different
from those used for primary clarifiers. The major reason for the difference lies in the
amount and nature of the solids to be removed. While primary settlers are designed on the
basis of overflow rate alone, secondary clarifiers must be designed on the basis of overflow
rate and solids loading. The greater concentration and lighter nature of the mixed-liquor
suspended solids requires that the underflow concentration be considered in design. Settling
rates are slower, as hindered settling prevails instead of free settling which occurs in pnmary
basins. Solids loading, expressed as pounds of dry solids/day/square foot of surface area, has
been discussed in theoretical terms by Dick (10) and others (11). The information needed
for an accurate determination can be obtained from laboratory settling tests. After the
limiting solids loading has been determined on the basis of desired underflow concentration,
the required area of basin can be computed using design flow and mixed-liquor suspended
solids (MLSS) concentration. The area required by this calculation and the area required by
the overflow rate must then be compared, with the larger being the design size. A factor
which strongly affects the limiting solids loading is the sludge volume index (SW). As this
value increases, sludge settling and concentration become more difficult. Kalbskopf recom-
mends the use of a sludge volume loading parameter which would be the product of the
solids loadmg times the SVI (7).
7-7
-------
The 10-State Standards (4) do not consider solids loading, but recommend that overflow
rates not exceed 800 gpd/sq ft. They also require a minimum depth of 8 feet for activated
sludge systems. Metcalf and Eddy (12) recommend an overflow rate of 1000 gpd/sq ft (less
if MLSS exceed 2000 mg/ 1) along with a depth of 12 feet for clarifiers following a nitrifica-
tion system. They also suggest a 1200 gpdf sq ft maximum overflow rate for denitrification
facilities. Sludge volume storage requirements will result in deeper tanks. In many cases the
computed volume of tanks will be multiplied by an excess capacity factor of about 2,
including a surface area increase of 25 to 75 percent, to obtain a final design (5).
Inlet requirements are similar to those of primary tanks, although some European designers
have indicated the favorability of flocculation facilities in the inlet area (7). The influence of
the effluent weir overflow rate is greater in secondary clarifiers than in primary tanks due to
the nature of the solids, even though 10-State Standards (4) indicate the same maximum
weir loading for both. For plants of less than 1 MGD capacity, it is 10,000 gpd/ft and for
larger plants 15,000 gpd/ft. Detention times are generally longer than in the primary,
ranging from 2 to 3 hours. Scum removal is generally required for all clarifiers and is usually
integrated with sludge removal mechanisms. Activated sludge concentration in secondary
clarifier underflow is generally noted to vary from 0.5 to 2.0 percent solids, while trickling
filter humus tanks are found to vary from 3 to 7 percent solids in the resulting sludges.
The performance of secondary clarifiers is not generally cited, as they are considered integral
to activated sludge systems. However, they can be rated on the basis of effluent quality to
some extent. Usually, a large portion of the effluent organic content is associated with the
suspended solids. The success of treatment processes such as microscreens in treating
secondary effluent must be attributed to the shortcomings of secondary clarifiers. Although
no clarifier will completely remove all suspended solids, a properly designed facility which
results from the use of some of the available information, such as in the WPCF MOP No. 8,
should yield a good product if the biological process is working properly.
7.4 Chemical Sedimentation
Sedimentation of chemically-coagulated or precipitated wastewaters is similar in principle to
the other types noted above. Overflow rates from 450 to 1620 gpdfsf have been cited, for
plants where existing primary facilities were successfully used for sedimentation of iron and
polymercoagulated raw sewage( I 3),( 14). Design of sedimentation tanks for sewage coagula-
tion with alum or iron salts, with or without polymer addition, will generally require an
average overflow rate of not more than 1000 gpd/sf and a detention time of at least 2 hours
(6). The use of lime will probably allow a somewhat higher overflow rate than alum and iron
systems since water treatment experience indicates a settleability of these precipitates from
one to 4 times greater than alum flocs( I 5),( 16). Allowable weir rates from water treatment
are cited to be 11,000 to 29,000 gpd/ft, depending on the nature of the chemical solids
(15). Chemical sludges generated from these systems may vary from 0.5 to more than one
percent of the throughput volume and have solids concentrations from about 1 to 15
percent, depending on the chemical used and basin efficiency. Materials of construction for
tanks, piping, and other associated devices will require consideration of the chemicals used,
e.g., glass-lined or PVC pipes for lime sludge drawoff.
7.5 Flotation
In essence, flotation is a unit process whereby particulate matter is separated from a waste-
water by causing it to float to the liquid surface. The density difference which drives this
7-8
-------
separation is artificially induced by the addition of a third phase, a gas, to the system. The
gas, in the form of bubbles, becomes attached to the solid particulates to form a gas-solid
aggregate which has an overall bulk density less than the liquid, with resultant rise of the
aggregate to the surface of the fluid.
As a primary clarification device, flotation has been demonstrated to remove from 40 to
65% of the rnfluent suspended solids and their associated BOD(18),(19). Although it has
been suggested that flotation units can serve as a gnt removal device in its primary clarifica-
tion position, no data have been found to suppbrt an application of this concept (20),(21),
(22).The most obvious application of flotation in primary treatment is in the expansion of
clarification capacity by conversion of existing clarifiers as an alternative to additional
construction of more sedimentation basins(19),(22).No chemicals were utilized in any of
these installations.
The inherent quality of short detention periods for flotation units suggests that they should
be advantageous as secondary clanfiers. The dearth of data for this application appears to
point out that effluent quality is inferior to sedimentation, but highly concentrated sludges
(up to 10% solid) can be obtained. These results were obtained with units operating without
chemicals (22), (23).
As to the method of air dispersal in the liquid, flotation can be classified as dispersed-air or
dissolved-air. The former method involves introduction of air by mechanical means directly
into the flotation unit. For various reasons, discussed in the literature, this method is not
well-suited for municipal waste treatment operations. It is, however, widely used in the
mining industry and for some industnal waste applications where particularly high solids
concentrations exist. Dissolved-air flotation is the most commonly used for waste treatment.
Pressure systems are generaily preferred over vacuum types. In these systems influent or
recycled effluent is pressurized to 25 - 70 psig with concurrent addition of 5 to 8% air by
volume. The pressurized mixture is then held for a short period in a retention tank to allow
for effective gas transfer to the liquid and to eliminate excess or undissolved air. Sub-
sequently, the pressurized mixture passes through a pressure-regulating device which drops
the pressure to atmospheric prior to introduction to the flotation chamber. Wastewater
flotation generally involves pressurization of recycled effluent to avoid solids destruction.
Influent is blended with this stream following pressure release. Since pressure flotation is
widely used, the design of the ancillary equipment has been thoroughly discussed in the
literature (24),(25),(26).
The inlet design of the flotation tank is critical to the proper performance of that unit.
Baffles, walls, and other obstructive energy-dissipating devices tend to destroy aggregate
bonding with resulting loss in flotation efficiency. Also, turbulence in the region of the
froth will result in losses of floated solids. Ettelt (25) reported several different designs of
inlet structures in his prototype units. The tangential flow inlet appears to offer consider-
able promise where such designs are compatible with the entire structure.
The flotation tank is designed to permit aggregate rise with a minimum of interference in
the form of turbulence or obstructions and to provide for removal of floated froth, settled
sludge and treated effluent. Effluent ports must be sufficiently submerged to prevent inter-
ference with the froth on the surface. Sludge removal can be accomplished by gravity as well
as mechanical means. Froth removal occurs through the use of skimming devices which
carry the froth from its original areal position to the point of disposal. A typical flotation
tank is shown in Figure 7-7.
7-9
-------
+—===- =- 4:
4
END v(w
1 L EX CHAIN BELT INC. iILWAUKEE WISCONSIN 32OI
SUBJECT FLOAT-TREAT SLUDGE THICKENER
STEEL TANKS
Figure 7-7
PRESSURE-FEED FLOTATION SYSTEM
Sc,u.E:
SECTION AA
iP LET
SCREW
CO VE
FRONT VIEW
J
UTLCT
7-10
-------
The ranges of design criteria for floation tanks are quoted to be (21 ),(23),(24),(26),(27),(28)
Overflow Rate 1-6 gpm/sq ft
Depth: 4-9 ft
Detention Time: 10-40 mm
7.6 High-Rate Settlers
It has long been recognized that solids removal by settling could be accomplished within a
few minutes if shallow basins could be used,(29),(30), but early prototypes of such equip-
ment were not successful because of problems with flow distribution and sludge removal.
Improvements in design have been made, and now two basic types of high-rate settlers are
being applied in the treatment of wastewater. These are the tube settlers* which consist of
modules of inclined tubes with small hydraulic radii suspended in a basin, and the Lamella
Separator** which consists of suspended, inclined plates. Much information on the equip-
ment has been published or supplied by the manufacturers. However, operating data for
wastewater treatment application are limited. Satisfactory operation is indicated for most
installations, although there are reports of unresolved problems with buildup of slime
growths which may constrict the shallow passages through which the water must pass.
Periodic cleaning may therefore be necessary and equipment should be designed to permit
ready access to the settling units. The Lamella Separator is recommended by its manu-
facturer for wastewater solids removal only in conjunction with chemical coagulation.
The pnncipal advantages offered by high-rate settlers lie in their compactness. These devices
require only a small percent of the space of a conventional gravity settling unit. This
difference in space requirements has a number of benefits: a) the high-rate settlers may be
less costly to fabricate than a new conventional clarifier; b) the units are useful for upgrad-
ing an overloaded plant; c) where additional facilities are required for an existing plant, the
high-rate systems have the obvious advantage of requiring considerably less land, and d) for
the treatment of industrial wastewaters, it is feasible to place separate units at different
locations within the plant to permit optimum product recovery and/or wastewater disposal.
Factors such as short circuiting, turbulence and convection currents which adversely affect
conventional clanfiers are minimized. In some specific cases mechanical devices such as
sludge rakes may be eliminated because of the compact size of the high-rate settler.
7.7 Tube Settlers
In the basic tube settler system, wastewater carrying suspended solids is subjected to
clanfication by particle sedimentation as it moves from an influent well or distribution
chamber, upward through small tubes and into a collection gallery, clearwell or launder. The
basic configuration for the mstallation of tube settlers uses the steeply-inclined (45-60°)
tubes. When the tubes are installed in this position, continuous gravity drainage of the
settleable solids in the tubes is achieved. The incoming solids settle to the tube bottom and
Manufactured by Neptune Microfloc, mc, Graver Water Conditioning Co., Perinutit Co. and others.
• A product of the Axe! Johnson Institute for Industrial Research, Nynashamn, Sweden. Represented in the U.S. by
Parkson Corp, Ft Lauderdale, Fla.
7-1 1
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then exit by sliding downward as shown in Figure 7-8. In this flow pattern the solids settling
to the tube bottom are trapped in a downward flowing stream of previously settled and
concentrated solids. The tube cross section may be circular, square, rectangular, hexagonal
or other suitable shape.
The theoretical settling pattern of discrete particles in an ideal rectangular basin has been
descnbed as straight lines where all particles with identical settling velocities move in parallel
paths. A particle being carried forward by the velocity of the liquid flow through the tank
must settle at such a minimum rate that it reaches the bottom before the flow leaves the
tank. Thus, if V 1 is the velocity of the fluid, V$ the settling velocity of the particle, L the
length of the tank, and d its depth, then a particle at the influent will settle to the bottom
of the device only if:
V 1 max=V L/d
This means that the flow velocity at which a horizontal clarifying device may be operated
successfully is directly proportional to the length of the device, and inversely proportional
to the depth. Since this analysis applies to horizontal tubes and plates as well as horizontal
tanks, and height of the tubes is usually a few inches as compared to a height of several feet
in a horizontal tank, distances and required settling times can be dramatically reduced
provided that flow is kept laminar. Shallow settling devices become self-flushing if they are
inclined at an angle which exceeds the angle of repose of the settled material. When the
device is inclined, the particle no longer falls through distance d but some longer distance d’
as depicted in Figure 7-9.
Clearly, d is related to d by the relation
d’ — d
Cos 0
where e is the angle the device is inclined from the horizontal plane. In a case where
—in0 1 —
— .JV Cos 600 —
or the settling distance is twice the height of the device. The equation for the case of
inclined clan fymg devices becomes.
V 5 L V 5 Lcos9
V 1 max =
d/cose d
-l L
The above equation is limited to angles between 0 and tan —
Laminar flow is necessary for the efficient and effective operation of a tube settler because
the random connective currents present in turbulent flow redistribute any solids that tend
to settle. Laminar flow is maintained at high flow rates in small tubes by the increased
“drag” effect of the relatively large surface of the tubes. A purely theoretical analysis of
high rate settling has been done (32).
7-12
-------
DIRECTION
FLOW
OF
TO SLUDGE
COLLECTION
Figure 7-8
TUBE SETTLERS — FLOW PATTERN
Figure 7-9
INCLINED TUBE
7-13
-------
Tests to determine the optimum angle of inclination for tubes showed that tube efficiency
at 600 was comparable to that obtained at 50 (31). It appeared that, when the angle of
inclination exceeded the angle of repose of the settled sludge, additional flocculation oc-
curred as the heavier floc settled and collided with the smaller, upward moving floc, con-
tributing to increased efficiency. A continuing increase in angle eventually results in the
tube acting as an upflow clarifier.
For large scale plants where coagulation is to be used for clarification, steeply inclined,
self-cleaning settling tube modules, similar to the one shown in Figure 7-10 can be readily
incorporated into new clarifier designs in order to reduce areal requirements. The size of the
tube openings and tube length can be varied depending upon the particular application. The
most significant application of the tube clarifier concept is its application to secondary
clarification. Figure 7-1 1 shows how modules of steeply inclined tubes can be placed in
either a circular or rectangular secondary clarifier. The activated sludge mixed liquor or
trickling filter effluent is introduced beneath the tube modules. The flow is upwards
through the modules with the solids falling from the tube bottoms into the sludge collection
zone beneath the tubes. The clarified effluent is collected above the tube modules. Second-
ary clarification has been provided in basins, as shown in Figure 7-12, at surface loading
rates as high as 4,000 gpd per square foot, or about five times the loading rates used with
conventional basins.
Tube modules can be installed in an existing secondary clarifier to increase its capacity. An
installation of this type was carried out early in 1 968 near Pittsburgh, Pa. In this case, the
secondary clarifier in a 130,000 gpd contact stabilization plant was allowing 50-80 mg/ 1
suspended solids to be carried into the plant effluent. Tube modules were installed over half
of the clarifier surface with the remainder of the clarifier removed from service. The plant
effluent now contains only 5-30 mg/i suspended solids even though the surface loading on
the modified basin now approaches 3,000 gpd per square foot based on the total daily flow.
Other applications of tube settlers to wastewater treatment have been published (33),(34).
Results obtained with tube settler installations on wastewater are summarized in Table 7-i.
7.8 Lamella Separator
The Lamella Separator shown in Figure 7-13 consists of a nest of parallel inclined plates
through which the suspension is passed, each plate having an effective settling area equal to
its projection onto a horizontal plane. By putting the plates in very close proximity, it is
possible to obtain a high settling capacity in a very small volume. (The term “Lamella”
refers to the liquid layer between adjacent plates.) The basic difference between the Lamella
Separator and tube settling units, which also utilize the principle of multiple inclined sur-
faces, is that the Lamella Separator is fed from the top whereas the tubes are fed from the
bottom. Flow of the liquid is thus cocurrent with the sludge, so that the frictional drag of
the liquid and the force of gravity work in conjunction to transport the sludge down the
plates. In addition, with the liquid and sludge flowing in the same direction, less shear at the
interface and less probability for re-entraining sludge has been claimed.
Although the basic concept of the Lamella Settlers is not new, a number of problems had to
be overcome before the idea could be reduced to practice. Of particular importance, the
possibility of secondary flows has been reduced by the use of flow stabilizing devices in the
inlet to the lamella and by dividing the unit mto many compartments. The exit of the
clarified effluent is accomplished by a tube network which collects the liquid uniformly
7-14
-------
Figure 7-10
MODULE OF STEEPLY INCLINED TUBES
(Courtesy Neptune Microfloc, Inc .)
Ffo t*’
-------
TUBE SETTLERS IN EXISTING CLARIFIER
SUPPORT MODULE
TUBE SETTLER
MODULE S
Figure 7-12
PLAN VIEW OF MODIFIED CLARIFIER
Figure 7-1 1
7-16
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Table 7-1
TUBE SETTLER INSTALLATIONS
EXiSTING
TUBE DATA FACILITY
PLANT LOCATION TYPE SIZE TUBE O.R. EFF. SS O.R. EFF. SS
MGD LOC. (gpm/sf) (mg/I) (gpm/sf) (mg/I)
Philomath, Oregon TFa 0.15 Aeratora 33 43 — —
Philomath, Oregon TF 0.15 S. Clar. 3.3-4.6 60-70 0.6 60-70
Philomath, Oregon TF 0.15 P. Clar. 2.1-3.3 3 41 b 0.84
Hopewell Twp., Pa. AS 0.13 S. Clar. 2 - 3 27 0.34 60-70
Miami, Fla. AS 1.0 S. CIar. 1.7 33 1.3 500
Ontario, Canada AS — Aerator 1.0 28 — —
a Secondary clarifier converted to aerator + tubes
b Percent removal rather than concentration
across the width of the plate, with return through a center tube to prevent recontaminating
or disturbing the liquid being clarified These features are shown in Figure 7-14.
A Lamella Separator consists of one or more modules, each consisting of perhaps 10-100
plates. The plates are typically 1.5 in wide x 2.5 m long, are splaced 25-50 mm apart, and
are inclined at 25-45° to the horizontal. The total number of plates required can be deter-
mined by laboratory settling tests. Materials of construction can be of many varieties, but
where temperature conditions allow, PVC is usually the best material to use for the plates
from both a technical and an economic standpoint
The Lamella Separator is being used in Sweden at several locations to clarify lime-treated
municipal wastewaters. One unit operating at 0.8 MGD removes 96 percent of the solids
applied to it. The Lamella Separator has been successfully tested on a pilot scale for
clanfication of potable water, steel pickling rinse waters and pulp and paper mill effluents.
7-17
-------
Figure 7-13
LAMELLA SETTLER
7-18
-------
Figure 7-14
LAMELLA SEPARATOR (From Above)
—1
-------
1.9 References
I. Geinopolos, A., and Katz, W.J., “United States Practices in Sedimentation of Sewage
and Waste Solids”, in Water Quality Improvement by Physical and Chemical Processes ,
University of Texas Press, Austin, Texas (1970).
2. Camp, T.R., “Sedimentation and the Design of Settling Tanks”, Trans., Am. Soc. Civil
Engrs., 111, 895 (1946).
3. Aitken, I.M.E., “Reflections on Sedimentation Theory and Practice - Part 1”, Eff. and
Water Trtmt. Jour. (Br.), 7, No. 4, 226 (Apr. 1967).
4. Recommended Standards for Sewage Works , Great Lakes - Upper Mississippi River
Board of State Sanitary Engineers (1968).
5. Smith, R., “Cost of Conventional and Advanced Treatment of Wastewater”, Jour.
WPCF, 40, 1546 (Sept. 1968).
6. Sewage Treatment Plant Design , WPCF Manual of Practice No. 8 (1959).
7. Kalbskopf, K.H., “European Practices in Sedimentation”, in Water Quality Improve-
ment by Physical and Chemical Processes , Univ. of Texas Press, Austin, Texas (1970).
8. Fair, G.M., and Geyer, J.C., Water Supply and Wastewater Disposal , John Wiley & Sons,
New York (1954).
9. Metcalf, L., and Eddy, H.P., Sewerage and Sewage Disposal , McGraw-Hill Book
Company, New York (1930).
10. Dick, R.I., “Fundamental Aspects of Sedimentation - Part 2”, and Wastes Engrg., 6, No.
3,44 (Mar. 1969). —
11. Eckenfelder, W.W., and O’Connor, D.J., Biological Waste Treatment , Pergamon Press
Ltd., Oxford (1961).
12. Design of Nitrification and Denitrification Facilities , Metcalf and Eddy, Inc. Presented
at EPA - WQO Symposium on Design of Wastewater Treatment Facilities, Cleveland,
Ohio (Apr. 1971).
1 3. Green, 0., Eyer, F., and Pierce, D., “Studies on Removal of Phosphates and Related
Removal of Suspended Matter and BOD at Grayling, Michigan”, distributed by Dow
Chemical Co.
14. Hennessey, J., Jelinski, R., Beeghly, J.H., and Pawlak, T.J., “Phosphorus Removal at
Pontiac, Michigan”, Presented at EPA, WQO Design Seminar, Cleveland, Ohio (Apr.
1971).
15. Water Treatment Plant Design , American Water Works Association, Inc., New York
(1969).
7-21
-------
1.9 References (Cent)
16. Geinopolos, A., Albrecht, A.E., and Katz, W.J., “The Character of Suspended Solids
and Basin Hydraulics are Key Factors in the Clarification of Water and Wastewater”,
Industrial Water Engrg., 3, 10, 19 (Oct. 1966).
17. Burd, R.S., A Study of Sludge Handling and Disposal , USD1, FWQA Publication
WP-20-4 (May, 1968).
18. Chase, E.S., “Flotation Treatment of Sewage and Industrial Wastes”, Sewage & In-
dustrial Wastes, 30, 783-91 (June 1958).
19. Hay, T.T., “Air Flotation Studies of Sanitary Sewage”, Sewage & md. Wastes, 28,
100-7 (Jan. 1956). —
20. WahI, A.J., Larson, C.C., Neighbor, J.B., Cooper, T.W., Katz, W.J., Wirts, J.J., Sebesta,
S.J., and Hanlon, J.B., “1963 Operator’s Forum”, Jour. WPCF, 36, 401 (Apr. 1964).
21. WPCF Manual of Practice No. 8, Sewage Treatment Plant Design 1959).
22. Katz, W.J., “Solids Separation Using Dissolved-Air Flotation”, Air Utilization in the
Treatment of Industrial Wastes, University of Wisconsin (1958).
23. Mulbarger, M.C., and Huffman, D.D., “Mixed Liquor Solids Separation by Flotation”,
Jour. ASCE-SED, 96, 861 (Aug. 1970).
24. Vrablik, E.R., “Fundamental Principles of Dissolved-Air Flotation of Industrial
Wastes”, Proc. 14th Purdue md. Wastes Conf. (1959).
25. Ettelt, GA., “Activated Sludge Thickening by Dissolved-Air Flotation”, Proc. 19th
Purdue md. Wastes Conf. (1964).
26. Masterson, E.M., and Pratt, J.W., “Application of Pressure Flotation Principles to
Process Equipment Design”, in Biological Treatment of Sewage and Industrial Wastes,
11, Reinhold Pub. Co., New York (1958).
27. Rich, L.G., Unit Operations of Sanitary Engineering , John Wiley & Sons, New York
(1963).
28. Kalinski, A.A., and Evans, R.R., “Comparison of Flotation and Sedimentation in Treat-
ment of Industrial Wastes”, Proc. 8th Purdue md. Wastes Conf. (1953).
29. Hazen, A., “On Sedimentation”, Trans. Amer. Soc. Civ. Eng., 53, 45(1904).
30. Camp, T.R., “Sedimentation and the Design of Settling Tanks”, Trans. Amer. Soc. Civ.
Eng.,11l, 895 (1964).
31. Cuip, G., Hansen S., and Richardson, G., “High-Rate Sedimentation in Water Treatment
Works”, JAWWA, 60, 681 (1968).
32. Yao, K.M., Theoretical Study of High-Rate Sedimentation JWPCF 42, 218 (1970).
7-22
-------
1.9 References (Corn)
33. Hansen, S.P., Cuip, G.L., and Stukenberg, J.R., “Practical Application of Idealized
Sedimentation Theory in Wastewater Treatment”, JWPCF, 41, 8, 1421, (1969).
34. Culp, G.L., Hsiung, K., and Conley, W.R., “Tube Clarification Process, Operating
Experiences”, Jour. San. Eng. Div., Proc. ASCE, SA5, 829 (Oct. 1969).
7-23
-------
CHAPTER 8
PHYSICAL STRAINING PROCESSES
8.1 General
Physical straining processes are defined for the purposes of this manual as those processes
containing elements which remove solids by virtue of physical restrictions at their surface
and which have no appreciable thickness in the direction of the liquid flow. These restric-
tions may be from the straining device itself or may be due to a thin layer of previously-
removed solids deposited upon a relatively coarse substrate or fabric. Processes which fit
this category are rotary screens, vibrating screens, ultrafiltration and diatomaceous earth
filters, among others. Since the purpose of this manual is to provide information which will
be of value in designmg or modifying solids removal facilities, only processes on which
substantial reliable information of previous use is available will be discussed.
8.2 Microscreening
Microscreening has been a viable solids removal process for twenty years. Its use as a tertiary
unit process for filtering secondary effluent dates back to the early 1950’s when it was
installed at the Luton Sewage Works in England. Descriptively, a microscreener consists of a
rotating drum with a fine screen constituting its periphery, as illustrated in Figure 8-1(1).
Feedwater enters the drum through the open end and passes radially through the screen
with concomitant deposition of solids on the inner surface of the screen. At the top of the
drum, pressure jets of effluent water are directed onto the screen to remove the mat of
deposited solids. The portion of the backwash stream which penetrates the screen and the
dislodged solids are captured in a waste hopper, shown in Figure 8-2, and are removed
through the hollow axle of the unit.
Screens employed in microscreening have extremely small openings and are made from a
variety of metals and plastics. Individual manufacturers each have specific designs and sizes
for the peculiar needs of any potential installation. Some examples of microfabrics available
from various manufacturers areS
Opening 2
( Microns) No./ in Manufacturer
23 144,000 Crane Co., King of Prussia, Pa.
25 — Walker Equipment Co., Chicago, Ill.
35 80,000 Crane Co., King of Prussia, Pa.
35 120,000 Zurn Industries, Inc., Erie, Pa.
40 — Walker Equipment Co., Chicago, Ill.
60 58,500 Crane Co., King of Prussia, Pa.
The weave and shape of individual fabric wires are such that they allow the water from the
backwashing jets to penetrate and detach the solids mat which forms on the inside of the
screen during its passage through the feed stream. Bodien and Stenburg (2) have noted that
only about one-half of the applied washwater actually penetrates the screen. The rest flows
down the outer perimeter into the effluent section of the structure.
8-1
-------
Figure 8-1 TYPICAL MICROSCREEN UNIT
(Courtesy of Cochrane Division, Crane Co.)
00
-------
NORMAL OPERATING
LEVELS (Approx.)
NE I
f 1, ROM5flC ..— —
L *TC JM T ( S7 A Pd( — —
-• I
Figure 8-2 TYPICAL MICROSCREEN UNIT, CROSS SECTION
-------
Although the microscreens have small openings, they cannot account for the removal effi-
ciency of the unit. Actually, the mat of previously-trapped solids provides the fthe filtration
which charactenzes the unit performance. This being the case, Lynam, et al. (3), showed
quantitatively that the slower the rate of drum rotation, the better the product water.
Another factor which becomes important in light of this mat phenomenon is the nature of
the solids applied to the microscreening process. As an example, Lynam, et a!. (3), were
unsuccessful in filtering the resulting chemical floc when secondary effluent was coagulated
ahead of the microscreening unit.
As a section of microscreen passes through its cycle it becomes clogged rapidly as the solids
mat forms. The continuous cleansing afforded by the backwashing jets at the apex of its
travel must be augmented in some way to prevent the buildup of screen-clogging slimes over
a period of time. An ultraviolet lamp placed in close proximity to the screen has been
somewhat successful in slowing the development of these slimes. In general, however, units
must be taken out of service on a regular basis to have the metal screens cleaned with a
chlorine solution. In some instances, a similar cleaning with an acid solution may be occa-
sionally required to remove iron or manganese buildups on the metal screen. In cases where
oil and grease problems present themselves, a hot water and/or steam treatment can be used
to remove these materials from the microscreen.
Headloss through the microscreening unit, including inlet and outlet structures, is about 1 2
to 18 inches (1). A 6-inch limit across the screen is usually imposed at peak flows; head-
losses in excess of this value are prevented by bypass weirs, as illustrated in Figure 8-2.
Headloss buildup is reduced by increasing the rate of drum rotation and by increasing the
pressure and flow of the backwashing jets. These adjustments can be made manually or
automatically by controls based on liquid-level sensing systems. Two-pen influent and efflu-
ent level recorders can be obtained. The effluent level pen can be used to monitor flow
when an effluent weir is used. These and other control systems are available from equipment
manufacturers.
The continuous backwashing jets are usually found to require from 1 to 5 percent of the
total throughput volume. Higher values have been reported, up to 23 percent, during periods
of secondary clarifier upset (2). The source of backwash water is the process effluent in
most cases. Pressures change during operation, as in the Chicago installation which could
vary from 20 to 55 psi. The inline pressure strainer for backwash water has been found to
plug from grease accumulation (3). This problem was solved by bimonthly cleaning or
replacement of the strainers. Solids flushed from the screen and removed through the
hopper and hollow axle are generally returned to the head of the treatment plant. Charac-
terization of this stream has not been reported in the literature, except for one value of 700
mg/ 1 of suspended solids (4).
Other operating parameters include the hydraulic and solids loadings on the unit. Lynam, et
al. (3), found that the solids loading was the limiting factor in microscreening activated
sludge secondary effluent. Maximum capacity was found to be 0.88 lb/day/sq ft at an
hydraulic loading of 6.6 gpm/sq ft. Excessively high solids during upset periods can reduce
the hydraulic loading of the unit drastically from design levels. One of the advantages of
using microscreens is their low head requirement. It is feasible, therefore, and advantageous
to conduct secondary effluent without pumping to a tertiary microscreening installation in
order to minimize the shear forces imparted to the biological floc. Chlorination immediately
ahead of microscreening units should be avoided to protect the screens.
8-4
-------
The primary material of construction for microscreeners is carbon steel. The numerous
appurtenances, support members and fittings are made from a variety of materials. Smaller
units can be factory assembled in steel tanks (Figure 8-3), while larger units (Figure 8-4) are
generally placed in concrete substructures, constructed according to the manufacturers
specifications (Figure 8-5) (1). One manufacturer, Crane Co., King of Prussia, Pa., indicates
the overall dimensions of four standard microscreening installations in Figure 8-6. For those
same units the necessary drive and backwashing pump horsepowers are indicated in Table
8-1, along with the approximate capacities of these drum sizes. In larger installations, more
than one unit per tank may be employed.
The design of microscreening units has been based on an equation derived by Boucher. This
equation is:
H = mQCf (e) Q/S
where:
H = headloss across the strainer
Q = constant total rate of flow through unit
Cf = initial headloss through strainer
A effective submerged area
I = filterability index of influent feed
S = speed of strainer (rate of area exposure)
When H is expressed in inches, Q in gpm, Cf in feet, A in square feet and S in square
feet/minute, m is 0.0267 and n is 0.1337. The key to the successful use of this equation is
an accurate determination of the filterability index, I, which is defined as the volume of
water obtained, per unit headloss, when passed through a unit area of standard filter. This
can be determined accurately by a laboratory procedure (5) or roughly by a field testing
apparatus, as shown in Figure 8-7 (1). Critical values of headloss, drum speed and filtrabil-
ity index can then be employed in conjunction with the aforementioned equation to deter-
mine design requirements. One investigator has proposed a modification to the design equa-
tion for improved accuracy (6).
Microscreening devices have generally been applied as tertiary screens for the filtration of
secondary effluents from trickling filter and activated sludge treatment. Some attempts have
been made to utilize a microscreening unit in place of a secondary clarifier for trickling
filters in Great Britain, but these have been only partially successful because of the strict
dependence of the unit on the performance of the trickling filter, i.e., good trickling filter
performance yields good microscreening removals, while poor performance plays havoc with
the microscreener (4). Tertiary applications have been more frequent in Great Britain, but
interest and testing in the United States have been increasing with the advent of more
stringent requirements on treatment plant effluents. Published results have been generally
limited to a single manufacturer’s product, but a number of other equipment firms have
recently entered the market with similar devices.
8-5
-------
Figure 8-3 MICROSCREEN INSTALLED IN STEEL TANK AT FACTORY
00
(Courtesy of Crane Co.)
-------
Figure 8-4
LARGE MICROSCREEN DRUMS PRIOR TO
INSTALLATION AT CHICAGO, ILLINOIS
oc
S 1%a •:i
p -: v U
I
-------
Figure 8-5 LARGE MICROSCREENS IN OPERATION
(Photo Courtesy Zurn Industries, Inc.)
00
00
th.
r
—s . .S .i-C 4
-------
Figure 8-6
OVERALL DIMENSiONS FOR FOUR SIZES OF MICROSCREENERS
5dia. I’wide machine Sdpa.x 3wide maChine
76dia Sw de machine
IOdia IOwide machine
8-9
-------
Figure 8-7
:P
LABORATORY FOR DETERMINATION OF FILTERABILITY INDEX
(Courtesy Crane Co.)
C
-------
Table 8-I
MICROSCREENER SIZES, MOTORS AND CAPACITIES
Drive Sizes Motors Approx. Ranges
(ft) (BHP) of Capacity
Diam. Width Drive Wash Pump ( MGD )
5.0 1.0 0.50 1.0 0.1 to 0.5
5.0 3.0 0.75 3.0 0.3 to 1.5
7.5 5.0 2.00 5.0 0.8to4.0
10.0 10.0 5.00 7.5 3.0 to 10.0
A number of installations and some published results are shown in Table 8-2. Suspended
solids are the variable of primary interest as the BOD removals are merely a function of the
solids corn position.
From the table it can be seen that the finer screen produced somewhat better solids removal
averages, but direct quantitative comparison is not possible in this case.
8.3 Other Screens
Rotary screens, similar in principle and appearance to microscreeners, are available for more
gross solids removal. Also, vibratory self-cleaning screens are being used for similar purposes.
Their major use is generally in industrial waste treatment to remove the coarser solids prior
to disposal or further treatment. Recently, these self-cleaning rotary and vibratory screens
have been utilized for treatment of combined sewer overflows with some success. Their
advantage in such applications is owed to their compact size and their ability to operate on a
start and stop mode. Suspended solids removals for these systems have generally ranged
from 20 to 40 percent (7), (8).
8.4 Oiatomaceous Earth Filters
Diatomaceous earth (DE) filters have been applied for the clarification of secondary efflu-
ents at pilot scale. No full-scale installations have been characterized in the literature. DE
filtration utilizes a thin layer of precoat formed around a porous septum to strain out the
suspended solids in the feedwater which passes through the filter cake and septum. The
driving force can be imposed by vacuum from the product side or pressure from the feed
side. As filtration proceeds, headloss through the cake increases due to solids deposition
until a maximum is reached. The cake and associated solids are then removed by flow
reversal and the process is repeated. In the cases where secondary effluents have been
treated by this process, a considerable amount of diatomaceous earth (body feed) has been
required for continuous feeding with the influent m order to prevent rapid buildup of
headlosses and subsequent uneconomically short filter runs. Generally, the DE filtration
process is capable of removing suspended solids, but not colloidal matter. A schematic
thawing of a DE system during operation is shown in Figure 8-8.
8-11
-------
Table 8-2
TERTIARY TREATMENT BY MICROSCREENERS
Screen Size Plant Size S.S. BOD Backwash Manu-
Location ( Microns) ( MGD) ( Rem. %) ( Rem. %) ( % ) facturer
Luton, England 35 3.6 55 30 3.0 Crane
Bracknell, England 35 7.2 66 32 N.A. Crane
Harpendon, England 35 0.3 80 N.A. N.A. Crane
Brampton, Ontario 23 0.1 57 54 N.A. Crane
Chicago, Illinois 23 2.0 71 74 3.0 Crane
Lebanon, Ohio 23 Pilot 89 81 5.3 Crane
35 Pilot 73 61 5.0 Crane
Miami, Florida
(S. Gulf Utilities) N.A. N.A. N.A. N.A. N.A. Zurn
Islip, New York N.A. 0.165 99 96 N.A. Zurn
Murfreesboro, Tennessee N.A. N.A. N.A. N.A. N.A. Zurn
Howell Twp., New Jersey N.A. N.A. N.A. N.A. N.A. Walker
Columbia, South Carolina N.A. N.A. N.A. N.A. N.A. Walker
Macomb, Illinois N.A. N.A. N.A. N.A. N.A. Walker
N.A. Not available
A wide variety of diatomaceous earth (diatomite) grades are available for use. As might be
expected, the coarser grades have greater permeability and solids-holding capacities than do
the finer grades which will generally produce a better effluent. Fly-ash has been used as
body feed, also. Some grades of diatomite are pretreated to change their characteristics for
improved performance. A number of vessel configurations are also avaiiable,as illustrated in
Figures 8-9 through 8-14.
Design critena for diatomite filters have been discussed by Bell (10). The filtration cycle can
be divided into two phases,run time and down time. Down time includes the periods when
the dirty cake is dislodged from the septum and removed from the filter and when the new
precoat is formed. Most of the difficulties involved in these operations are dependent upon
the equipment design of the individual manufacturer. Run time commences when the feed is
introduced to the filter and ends when a limiting headloss is reached or an effluent quality
degradation occurs. The single most important problem with secondary effluent filtration
by DE filters is the amount of body feed required during the filtration or run time. A
ratio of 5 to 6 mg/i of body feed per JTU of influent turbidity was required at San Antonio
and the possible need for a higher ratio was suggested (11). Another pilot study used a
variety of ratios and filtration rates and used both pressure and vacuum systems for second-
ary effluent filtration (12). Results of this study are shown in Tables 8-3 and 8-4. Both
studies indicate that a precoat of about 0.1 lb/sq ft of filter area, high ratios of body feed to
influent turbidity and a filtration rate of approximately 0.5 gpm/sq ft of filter area should
be used.
8-12
-------
DIATOMITE FILTRATION SYSTEM DETAIL
Co
Figure 8-8
(Courtesy of Johns-Manville)
-------
Figure 8-9 ROTARY VACUUM PRECOAT FILTER
(Courtesy of Johns-Manville)
FLOW DIAGRAM
fliNt d ick
\\
a
a
prscsst
a
-------
CORRO QN PROOF
FiLTER ELEMENTS
FILTERED WATER
OUTLET
: .;c I
CORROSiON PROOF
TANK
C
(M
t s -
ffiM E
Vt ’
Figure 8-10 VERTICAL LEAF VACUUM FILTER
(Courtesy of Johns-Manville)
-------
Figure 8-11 PLATE AND FRAME PRESSURE FILTER
(Courtesy of Johns-Manville)
-------
• .••
Figure 8-12 CYLINDRICAL
(Courtesy of
ELEMENT VERTICAL FILTER
Johns-Manville)
8-17
-------
Figure 8-13
VERTICAL LEAF PRESSURE FILTER, VERTICAL TANK
(Courtesy of Johns-Manville)
•1
8-18
-------
Figure 8-14
VERTICAL LEAF PRESSURE FILTER, HORIZONTAL TANK
(Courtesy of Johns-Manvile)
0
-------
Table 8-3
VACUUM DIATOMACEOUS EARTH FILTRATION OF SECONDARY EFFLUENT
Precoat — 0.1 lb/ft’
Headloss at end of run — 18 in. Hg
*Jackson Turbidity Units
**product of Johns-Manville, New York, N.Y. 10016
Body-feed
Conc.
(mg/I)
Table 8-4
Turbidities
(JTU)*
Feed Product
*Jackson Turbidity Units
**product of Johns-Manville, New York, N.Y. 10016
Flow Rate
2
(gpm/ft )
Body-feed
Conc.
FilterAid (mg/i)
No. of
Runs
Run-length
(hrs)
0.53
0.75
1.0
Celite 545**
42
33
19
5
3
1
19.5
10.7
5.4
5.5
5 2
4.4
0.8
0.8
0.4
0.51
075
1.0
Celite 503**
36
29
21
2
1
1
8.0
3.0
1.2
10.5
9.2
8.0
3.3
0.7
0.75
0.54
0.75
Hyflo
Super-Cel **
35
21
6
2
12.7
4.4
4.6
5.8
0.85
0.8
PRESSURE DIATOMACEOUS EARTH FILTRATION OF SECONDARY EFFLUENT
Flow Rate
(gpm/ft 2 ) Filter Aid
No. of Run-length
Runs (hrs)
Turbidities
(JTU)*
Feed Product
0.50
0.75
0.81
1.0
1.0
Celite 545**
50
19
42
20
45
1
2
2
2
3
50.0
24.2
28.4
7.3
3!
8.2
5.7
8.3
6.4
‘7.5
3.1
2.5
3.9
2.1
3.0
0.76
Celite 503**
18
1
14.5
7.0
4.0
0.78
1 .2
Hyflo
Super-Cel **
21
29
1
1
22.3
9.7
8.1
6.5
4.9
3.8
Precoat — 0. 1 lb/ ft 2
Head loss at end of run — 35 psi
8-20
-------
8.5 Ultrafiltration
All membrane piocesses share certain operating and design aspects which can be discussed
before ultrafiltration is set into context (13). In order to separate dissolved materials from
water, a relatively “fine” membrane with relatively high pressures and low flow rates is
required. When particles in the colloidal and suspended range must be separated from water,
the membrane can be somewhat more “coarse” with correspondingly lower pressures; how-
ever, the flux, or flow through the membrane, may be substantially the same as for salt
separation. A problem common to both the salt and solids removal applications is that of
maintaining membrane flux in the lace of particulate or precipitate fouling of the membrane
surface. The flux tends to decline logarithmically with time.
The salt and solids removal applications have in common those aspects just discussed. in
every other respect they are different. It is therefore appropnate that two different process
names be applied to them. Salt removal is called reverse osmosis or hyperfiltration, while
solids removal is called ultrafiltration. 1-lyperfiltration has fluxes from S to 50 gpdf sq ft of
membrane area at pressures of 500 to 1500 psig Performance usually includes 90-99%
rejection of total dissolved solids and 100% of suspended solids. Ultrafiltration may exhibit
5-50 gpd lsq ft of flux, but requires pressures of only 10-100 psig and removes virtually
100% of the suspended material. Little operating information for ultrafiltration is available,
and it will be considered only briefly.
An ultrafiltration process flow diagram is shown in Figure 8-15. The two most important
elements of ultrafiltration are the membrane and the geometrical configuration into which it
is set. The exact operating and performance data depend upon the nature of the waste.
Given the current state of membrane-manufacturing technology, almost any set of desired
performance characteristics can be produced by a membrane. Specifications include mem-
brane flux and flux decline characteristics. Rather than discuss membrane capabilities and
formulas in detail, it is sufficient to note that a few of the leading manufacturers of
ultrafiltration membranes are Amicon Corp., Abeor, Inc. and Dorr-Oliver, Inc. Catalogues
offering a wide variety of membranes are available.
The geometrical configuration of the module containing the membrane is just as important
as the membrane itself. The single greatest problem of ultrafiltration or any other membrane
process is the fouling which causes membrane flux to decline. Such fouling is caused by
slimes, precipitates and organic and microbial deposits. The membrane can be cleaned
routinely with chemical or enzyme solutions, but prevention of this fouling is an important
design problem. The precise nature of the fouling depends on the nature of the feedwater.
Prevention of fouling may be accomplished by achievement of proper hydrodynamic con-
ditions within the module. The feedwater velocity past the membrane surface serves to con-
tinually scour that surfacc to prevent fouling.
Another aspect of module design concerns the magnitude of membrane surface area which
can be incorporated into a module. Because of the low membrane fluxes it is imperative to
design the module so as to maximize membrane surface area. One configuration adopted
solely for ultrafiltration is the storage battery configuration, as shown in Figure 8-16. The
membrane is cast on both external faces of a hollow plate, and many of these plates are
arranged in a parallel array. The edges of these plates face the incoming stream of solids, and
thereby function as a coarse screen which can be backwashed by reversing the approach
8-21
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Figure 8-15 ULTRAFILTRATION FLOW DIAGRAM
(Courtesy Dorr-Oliver, Stamford, Conn.)
COncentrate
lopor
membrane
modules
Feed
Pressurization
pump
U Itrafilt rate
Recirculätion pump I:
-------
Outlet
Rec I rcu
flow inlet
Ultrafiltrate
Figure 8-16 “STORAGE BATfERY” MEMBRANE MODULES
(Courtesy of Dor-Oliver).
00
Ultrafiltration
cartridge
Header
Cover
-------
POTABLE WATER SUPPLY
Figure 847
MEMBRANE
00
t J
a
SCHEMATIC FLOW DIAGRAM OF THE PiKES PEAK TREATMENT & REUSE SYSTEM
-------
direction of the feed. The influent passes through the membiane-plates and is collected by a
manifold connecting the interiors of the various hollow plates. This design is advantageous
for three reasons: (a) it is compact and incorporates a large membrane area into the module;
(b) the edges of the plates function as an easily backwashed coarse filter, and (c) the flow is
parallel to the membrane surface which induces scouring.
Ultrafiltration is currently competitive with other solids removal processes only in spe-
cialized situations. On account of its compactness, an ultrafiltration module can easily
replace a filter or settler in a package treatment plant where space is at a premium. A
package plant on top of Pikes Peak (14) treats 21,000 gpd by a high-solids activated sludge
and ultrafiltration process. The concentrate from the ultrafiltration step is recycled to the
aerator as shown in Figure 8-1 7. Table 8-5 shows the performance data collected during a
brief period in 1970 after the plant was installed. The plant had not yet reached peak
performance in some respects, but its chief merits are evident.
Table 8-5
SUMMARY OF PERFORMANCE OF THE
DORR-OLIVER ACTIVATED SLUDGE-ULTRAFILTRATION PLANT OPERATIONS AT
PIKES PEAK
AUGUST-SEPTEMBER, 1970
Influent Effluent Removal
Parameter (mg i I) (mg/I) (%)
BOD 285 1 99
COD 547 32 94
TOC 136 6.6 95
Turbidity (JTU) 47 0.33
Color (Units) 320 40
TSS 129 0 100
MLSS 3954
Coliform (Per 100 ml) 0 100
PO 4 -P 9.1 11.1
pH 7.9 5.9
Threshhold Odor Number 6
Pressure - .50 psig
Average flux - 21,000 GPD
8-25
-------
8.6 References
1. Diaper, E.W.J., “Tertiary Treatment by Microstraining”, Water & Sew. Wks., 116, 202
(June 1969).
2. Bodien, D.G., and Stenburg, R.L., “Microscreening Effectively Polishes Activated
Sludge Effluent”, Water & Wastes Engrg., , 74 (Sept. 1966).
3. Lynam, B., Ettelt, G., and McAloon, T., “Tertiary Treatment at Metro Chicago by
Means of Rapid Sand Filtration and Microstrainers”, Jour. WPCF, 41, 247 (Feb. 1969).
4. Truesdale, GA., and Birkbeck, A.E., “Tertiary Treatment Processes for Sewage Works
Effluents”, Wat. Poll. Control (Brit.), , 4, 371 (Aug. 1967).
5. Boucher, P.L., “A New Measure of the Filterability of Fluids with Application to Water
Engineering”, ICE Jour. (Brit.), 24, 4, 415 (1947).
6. Mixon, F.O., “Filterability Index and Microscreener Design”, Jour. WPCF, 42, 1944
(Nov. 1970).
7. Mason, D.G., “The Use of Screening/Dissolved-Air Flotation for Treating Combined
Sewer Overflow”, Paper presented at Seminar on Storm & Combined Sewer Pollution
Problems (Nov. 1969).
8. Cornell, Howland, Hayes and Merryfield, “Rotary Vibratory Fine Screening of Com-
bined Sewer Overflows”, U.S.D.1., FWQA Report No. 11023 EDD (Mar. 1970).
9. “Filtering with Diatomite”, Report No. FA-84A of Johns-Manville-Celite Division (June
1969).
10. Bell, G.R., “Design Criteria for Diatomite Filters”, Jour. AWWA, i4. 124! (Oct. 1962).
11. Wells, W.N., and Davis, D.W., “Filtration of Activated Sludge Plant Effluent”, Public
Works, 98, 4, 94 (Apr. 1967).
12. “Summary Report - Advanced Waste Treatment Research Program (1964-1967)”,
U.S.D.I., FWPCA Publication WP-20-AWTR-19 (1968).
13. Michaels, A.S., “New Separation Technique for the CPU’, Chem. Engrg. Prog., , 31
(Dec. 1968).
14. Kugelman, I.J., Schwartz, W.A., and Cohen, J.M., “Advanced Waste Treatment Plants
for Treatment of Small Waste Flows”, Advanced Waste Treatment and Reuse
Symposium, Dallas, Texas (Jan. 1971).
8-27
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CHAPTER 9
DEEP BED FILTRATION
9.1 General
With the exception of gravity sedimentation, deep-bed filtration is the most widely used
unit process for liquid-solids separation. Until recently its use was generally confined to the
treatment of municipal and industrial water supplies. The primary reason for its recent
adoption in wastewater treatment has been the need to upgrade effluents from conventional
treatment plants. Installations may use direct filtration of activated sludge or trickling filter
effluents, without the addition of chemical agents. Also, deep-bed filters are employed in
systems for phosphorus removal from secondary effluents and in physical-chemical systems
for the treatment of raw wastewater. In these latter cases chemical coagulation, flocculation
and sedimentation precede the filters as in water treatment plants.
In essence, the unit process of deep-bed filtration encompasses exhaustion of the bed
followed by a regeneration. Water containing suspended solids is passed through a bed of
granular material resulting in deposition of the suspended solids in the bed. Eventually the
pressure drop across the bed becomes excessive or the ability of the bed to remove sus-
pended solids is impaired. Thereupon filtration is stopped and the bed is cleaned prior to
being placed back in service.
9.2 Filter Design
At the present time virtually all deep bed filters utilized for waste treatment are “rapid
sand” type, i.e., downflow, static bed, with batch or semicontinuous operation. In this
section, design information on this traditional type of filter will be reviewed. In a later
section of this chapter, descriptions and design information for some new concepts in deep
bed filtration will be given.
Figure 9 -1 (3) is a cut-away view of a typical “rapid sand type” filter, illustrating most of its
components. In essence the filter is a box containing filter media, an underdrain system, a
backwash system, flow control systems and various conduits for bringing feedwater and
wash water to and conveying filtrate and used wash water away from the filter. Figure 9-2
illustrates a pressure filter. It can be seen that there is little difference between the pressure
and gravity flow filters except for the pressure housing. Because of size restrictions on
pressure filters they are equipped with a simpler wash water collection trough system than
gravity filters.
9.2.1 Design Parameters
Process design of a filter includes determination of:
a. type and size of filter media
b. depth of filter
c. rate, duration and timing of water backwash, air scour and surface wash
9-1
-------
Rate of flow and loss
Operating
floor
Pipe gallery
floor
Filter
Filter to waste
Wash line
Graded
from
tables
to filters
clear well
Drain
Figure 9-1 TYPICAL RAPID SAND FILTER
flLTIRED
WATER
OUTtET ,__
Figure 9-2 PRESSURE FILTER
STRAINER
l—__ STACK
0
DISH
UNDERDRA IP4
STRUCTURAL
Operatii
table
Filter bed wash -
troughs
filter
tank
Wash troughs
laterals
Cast-
manifold
p
RAW WATER
INLET —
RAFFLE
MANUAL
MU U sPORT
VALVE
BACKWASH LINE I
AND RATE SET VALVE
RINU LINE AND
RATE SET VAfl 1 E
I
(Courtesy of’ Perniutit Co., Paramus, N.J.)
9-2
-------
d. filtration rate
e. type of chemical pretreatment and dose requirement
f. expected duration of filter runs
Values for each of these parameters must be selected to yield a design which will produce an
effluent of the desired quality at a minimum cost.
The difficulty in arriving at an optimum process design is twofold. Many, if not all, of these
parameters are interdependent. Also, the present level of filtration theory can only semi-
quantitatively represent the interdependence of the process design parameters.
Process design information should be generated from pilot plant data. At best, present
filtration theory can only reduce the degree of pilot plant study required. All too often
process design of a filtration installation is performed according to “rule of thumb” ex-
perience. Use of such a procedure usually results in an overdesign (uneconomical) or an
underdesign (system failure to meet quality or quantity objectives.)
9.2.2 Pilot Plant Studies
A rather large number of mterdependent parameters must be considered in pilot plant
studies. In addition, the range of variation of each which may have to be investigated is
quite large. Thus, a field pilot investigation cannot generally be of short duration. Pilot
studies up to a year in length may be required, but it should be possible to decrease this
time period by utilizing experience as a guide in establishing limits of variation of the
parameters.
Pilot studies can be physically described as consisting of a series of filter columns using an
available supply of feedwater. Provisions must be made to measure effluent quality with
time, pressure drops over various sections of the bed and over the entire bed with time and
backwash rates required to achieve specified degrees of media expansion. The studies should
be conducted at several flow rates, with several media types and size ranges, to various
terminal headloss values and with a variety of chemical pretreatments where appropriate.
The data resulting from these studies should be utilized to eliminate from further considera-
tion those situations which obviously cannot meet effluent critena or cannot be justified
economically. Based on these analyses, a narrowed-down set of parameters can be used in a
second set of investigations. This second study may be more comprehensive than the first
series, including such variables as expected diurnal variation and slight changes in chemical
dose. This process may continue through several more rounds before an “optimum design”
is achieved. This final design should be given a lengthy test (one month or more) at con-
ditions as close as possible to those expected in the field. Even after this lengthy study, it is
probable that only a good approximation of field conditions will be obtained.
Specific details for conducting filtration pilot studies have not been established. The follow-
ing is a list of equipment and practices usually used.
a. Multiple filter tubes of transparent material with a minimum diameter of 2 to 3 in.
are utilized.
9-3
-------
b. The tubes are fitted for either gravity or pressure operation.
c. A false bottom underdrain is utilized with either a porous plate or strainer backwash
system.
d. Flow control is established by the use of a positive displacement pump or an effluent
throttle valve connected to a float
e. Pressure taps are provided above and below the media, as well as at other locations
within the bed. Tap locations are generally located near the top of each type of media
used.
Additional details of pilot filters are given in various references on filtration studies (1), (2).
Two areas of process design cannot be as adequately explored in pilot studies as the others
listed above. These are the parameters associated with cleaning the bed and the effect of
chemical treatment of the feed. Cleaning of the filter bed is difficult to simulate in pilot
scale because of the small surface area of the beds utilized. The small area makes it im-
possible to study surface wash and air scour. Results of water backwash may not be repre-
sentative because of the wall effect. Chemical treatment of the filter feed without floccula-
tion and sedimentation may be simulated in the pilot studies. If flocculation and sedimenta-
tion are employed after chemical treatment however, the performance of model flocculators
and clarifiers may not closely simulate that of full-scale field units. Thus the feed to the
filters may not be representative. Both areas of difficulty can be overcome by using large
pilot installations, i.e., minimum filter size of about 2’ x 2’ to study backwash, and mini-
mum clarifier size of about 10’ diameter to simulate full-scale sedimentation. However,
going to this scale is not usually economically justified. If studies are being conducted at an
existing plant, i.e., one that has full-scale clarifiers or filters, advantage should be taken of
the situation.
Although pilot studies give the most reliable information on which to design filters, it
should be remembered that filtration is a mixture of art and science. Excessive fine tuning
of the process design should not be attempted. The chosen process design should provide a
flexible system which will do the job with an adequate safety factor at a reasonabte cost.
Fine tuning of the process should be left to the plant operator.
9.2.3 Filter Media
Selection of the size, type and depth of the filtration media is the single most important
decision in the design of a filtration system. Unfortunately, it is impossible to specify
optimum media characteristics from theoretical considerations. Pilot studies should be con-
ducted with several sizes and types of media pnor to making a decision.
Sand has historically been the filtration medium most commonly used, but anthracite coal
and, to a lesser extent, garnet have been employed. These substances occur in nature and are
used in filters in a graded size range. Although use of a uniform size medium would have
certain advantages, it is not economical to excessively restrict the size range.
A typical grain size distribution curve for a naturally occurring filter medium is given in
Figure 9-3. This curve is often a straight line on log-probability paper. Historically, the two
points used to characterize a medium are the 10 percent size and the 60 percent size. These
9-4
-------
U.S. Standard Sieves
4 6 10
20 40 60 100 200
Grain Diameter in Millimeters
Figure 9-3 GRAIN SIZE CURVE
100
3” 2” 1” 3/4” 3/8”
V
>
‘P
(J
100 10 1.0 0.1
0.01
-------
are defined as the particle size of a distribution such that the weight of all smaller particles
constitutes the stated weight fraction of the whole. Usually the specification is given as the
effective size (10 percent size) and the uniformity coefficient (ratio of the 60 percent size to
the 10 percent size) in general, good correlation has been found between clean water
headloss and effective size (3). Fair and Geyer (3) present a method of calculating the size
fractions which must be sieved or washed out to convert from one size distribution to
another.
Until recently most filter designs called for a single filtration medium, either sand or coal.
Typical effective sizes range from 0.35 mm to 0.8 mm with uniformity coefficients between
1 .3 and 1 .7 (4). These size specification ranges can be utilized for preliminary selection of
sizes to be evaluated in pilot studies.
In coal and sand media of identical size, it has been found that thesolidsremovalwith coal is
somewhat infenor, but the rate of pressure drop build-up is lower (5). This finding is
explained by the greater angularity of coal with consequent greater porosity.
The relationship of medium size to filter performance can be generalized. Smaller media are
more effective in removing suspended solids at the expense of increased pressure drop or
headloss buildup rates, i.e., shorter filter runs. Larger media have reduced initial headlosses
and pressure drop increases during the filter run, but may yield a higher suspended solids
concentration in the effluent.
The purpose of pilot studies is to quantify the above experiences for the wastewater under
study. The quantitative relationships developed must then be used in conjunction with
economic and physical design factors to define the optimum design. The historic trend has
been toward coarser sizes of filter media in order to attain higher flow rates without
reducing the length of filter runs. In order to assure an adequate effluent quality with coarse
media and high filtration rates, chemical treatment of the feed may be required.
Another recent trend in filtration has been the adoption of the multi-media concept. Con-
ventional single medium filters have a fine to coarse gradation in the direction of flow which
results from hydraulic gradation during backwash. This type of gradation is not efficient as
virtually all of the removal and storage must take place in the upper few inches of the filter
with a consequent rapid increase in headloss. A coarse to fine filter gradation is much more
efficient as it provides for much greater utilization of bed depth, using the fine media only
to remove the finer fraction of the suspended solids.
One method of obtaining a coarse to fine filter gradation is the dual-media filtration con-
cept. This employs the use of a layer of coarse anthracite coal over a layer of fine sand. The
sizes of the anthracite and sand are chosen so that the coarser but lighter anthracite (specific
gravity 1 .6) will remain above the heavier (specific gravity 2.65) but smaller sand during
backwash. It is desirable to have the coal as coarse as possible to prevent surface blinding
and the sand as fine as possible to promote high degrees of removals. However, the disparity
in sizes cannot be too great lest overtopping of the coal by the sand would result. In general,
sand sizes much finer than 40 mesh are not utilized because the coal size required to prevent
overtopping by sand during backwash would be too small to allow high filtration rates. To
ascertain the degree of mixing which will occur during backwashing and its effects on
subsequent filter performance, pilot column studies are best utilized.
9-6
-------
An extension of the dual media concept is the mixed-media filter. Ordinarily this is a
tn-media filter of coal over sand over garnet (specific gravity 4.2). There is evidence that
judicious selection of the size of each medium will allow a degree of intermixing of the
media, such that a reasonable approximation of continuous coarse to fine gradation is
obtained. Figure 9-4 illustrates the conventional single medium filter, a non-mixed dual
media filter and an ideal coarse-to-fine filter. Typical designs of multimedia filter beds are
given in Table 9-I.
Table 9-1
TYPICAL MULTI-MEDIA DESIGNS (6)
Design
No.
Garnet
Sand
Coal
Size
(Mesh)
Depth
(Inches)
Size
(Mesh)
Depth
(Inches)
Size
(Mesh)
Depth
(Inches)
I
2
3
-40x80
—20x40
-40x80
8
3
3
-20x40
-10x20
—20x40
12
12
9
-lOx2O
-IOxl6
—10x20
22
15
8
Conley and 1-Isiung (6) present additional information on the design of multi-media filters
for a variety of applications.
Several studies have compared the performance of single medium, dual media and multi-
media filters (7), (8), (9). These studies indicated that in general the latter two types of
[ liters outperformed single medium filters. Better effluents were obtained at higher flow
rates, with longer filter runs.
9.2.4 Filter Bed Depth
Although it is known that single medium filters make effective use of only the top few
inches of the bed, the historical practice of designing deep beds (24 to 36 in.) has not been
abandoned. Dual and multi-media filter depths of the same magnitude are justified because
the full filter depth is utilized. In fact, deeper filters of this type may find utility.
9.2.5 Flow Rates
Traditionally, the design flow rate of rqost single medium filters was 2 gal/mm/ft 2 . Recently
these have been raised to 4 gal/mm/ft or greater when coarser media were employed along
with higher terminal pressi re drops. The multi-media filters have been successfully operated
at rates up to 8 gal/mm/ft on a continuous basis. Rates of this magnitude or higher can be
anticipated for design in the future. Of course, pilot studies are essential in determining the
design rates of a full-scale installation.
9-7
-------
Cross-Section Through
Single-Media Bed
Such as Conventional
Rapid Sand Filter
Cross-Section Through
Dual-Media Bed
Coarse Coal Above
Fine Sand
Cross-Section Through
ideal Filter
Uniformly Graded From
Coarse to Fine
From Top to Bottom
Figure 9-4 MEDIA COMPARISONS
Grain Size
Grain Size
I )t ‘1
•—‘ ..,-‘
, - --.- S. S \
.- ..
.-. \ •__‘\ ..—“
‘ -‘ — —-‘-- _: r : - ,)
Grain Size
9-8
-------
9.2.6 Filter Size, Layout arid Housing
Both gravity and pressure filter systems of standard sizes are provided by various filter
manufacturers. Specific size details can be obtained from manufacturers’ catalogues. Gravity
filters are usually constructed on site of concrete. Common feedwater and backwash water
supply header pipes generally travel down a central pipe gallery (see Figure 9-1) between
rows of filter elements. The gravity filters are usually enclosed in a filter building with
storage of finished water in the basement area. All pumps, motors, controls, etc., are housed
in the filter building.
Pressure units are more individualized than gravity filters. They are fabricated of steel and
are cylindrical in shape. Structural design is according to standard pressure codes. Vessels up
to 10 feet in diameter and 60 feet in length are available. It is essential that a manhole be
incorporated in the design to allow for maintenance. The filter should be designed with a
means for hydraulic removal of all the filter media. Sight glasses for observation of the bed
should also be incorporated in the design.
9.2.7 Filter Cleaning Systems
Termination of a filter run takes place when either of these events occurs
a. The effluent does not meet the quality criteria
b. The bed pressure drop is excessive
Either event indicates that the filter is excessively dirty and must be cleaned. The import-
ance of an effective cleaning system on the filter system performance has been reviewed in
several recent publications (10), (ii) If effective cleaning is not obtained, short filter runs
and poor effluent quality will result. Indeed in many respects, improper cleaning sets up self
perpetuating operational difficulties such as mud balls, filter media slime coating and media
cracking (5).
Backwashing the filter bed by flow reversal is the major, and in many cases, the only
method used to clean the bed. The sources of backwash water may include feedwater, filter
effluent or some other effluent downstream of the filter. It is virtually impossible to predict
the specific conditions required to insure cleaning of the bed by this technique. Experience
indicates that for a single medium filter provision should be made to backwash the bed for a
10- to 15-minute period at a rate which will insure fluidization of all the media. The
optimum requirements can only be determined by the plant operator once the plant is in
service. The plant operator should be instructed that excessive expansion of the bed is just
as bad as insufficient expansion. Optimum expansion varies with the size of the medium,
type of floc and penetration of Hoc.
The most important aspect of the design of the backwash system is to insure uniform
distribution of the wash water over the entire cross section of the filter area. Although the
function of the uriderdrain system of the filter is both to collect the filtrate and distribute
wash water, the latter function controls the design.
A variety of underdrain systems are available for use. The traditional system employs several
layers of graded gravel under the filter bed with a lateral header system positioned on the
9-9
-------
filter floor. The lateral header system is equipped with orifices and provides the preliminary
distribution of the wash water. The final distribution is accomplished as the water moves
upward through the gravel. Cuip & CuIp (12) provide rules for the design of a lateral header
system.
Several commercial systems are available which employ patented false bottom distributors
in place of the lateral header system. The gravel is then placed on top of the false bottom.
Diagrams of Leopold Block and Wheeler Filter Bottom systems are illustrated in Figure 9-5.
Some systems have been devised with the concept of eliminating the gravel layers. One
system utilizes porous plates over a false bottom, as shown in Figure 9-5. Another system
also utilizes a false bottom with strainers on 1 2” centers. The strainers are nozzles fabricated
of plastic and metal with many small openings.
Table 9-2 presents the usual specifications for gravel, as well as a new design suggested by
Baylis (5).
Table 9-2
FILTER GRAVEL DESIGN
Standard
Baylis
Depth
Size
Depth
Size
2—1/2”
3—1/2”
3—1/2”
2”
4”
6”
1 / 1 2”—1/8”
l/8”—1/4’’
l/4”—l /2”
l/2”—3/4”
3/4”-l-l/2”
I—l/2”—3—l/2”
5”
2”
2”
4”
2”
2”
4”
1” — 2”
1/2”—l”
1 /4”—l /2”
1 /8”—1 14”
1/4”-l/2”
l/2”—l”
— 2’’
It was found that with the old design the upper layers of gravel could fluidize if excessive
backwash rates were used. The Baybs design, which places a final heavy layer of gravel over
the finer gravel, prevents this fluidization.
After moving upward through the expanded bed the wash water is conducted out of the
filter by wash water troughs. In a pressure filter the inlet baffle serves as the wash water
collector. in a gravity filter separate troughs are used because of the large area which must
be serviced. In order to prevent non-uniform flow of wash water, the maximum lip to lip
distance between troughs cannot exceed six feet. The troughs are usually rectangular chan-
nels. A convenient width is assumed and the depth computed by a momentum analysis of
the backwater curve in the trough. Figure 9-6 is a nomogram for this solution. For example,
9-10
-------
1 ITII J 1T__
i11i ii2i
SECIION A-A
A Leopold-block underdrain, with glazed-tile blocks furnishing passages
to the water, instead of using laterial pipes.
The Wheeler-filter bottom, consisting of solid, inverted, truncated pyra-
mids with water connections at the apex of each pyramid, and the
pockets filled with cement or glazed earthenware spheres.
SECTION A-A
Porous-plate filter bottoms.
Figure 9-5 FILTER BOTFOM AND UNDERDRAIN SYSTEMS
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9-12
30:
25
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Figure 9-6
-------
an 18 inch wide trough carrying a flow of 3.33 ft 3 /sec will have a maximum water depth of
13.2 inches. The side wall depth should be increased by 2 to 3 inches to allow for freeboard.
Maintaining a clean bed by backwash alone has been found to be difficult when wastewater,
even after biological and/or chemical treatment, is the filter feed. Even periodic chlorination
during backwashing has not always been successful in preventing slimes. Consequently,
either surface wash or air scour should be employed in waste treatment filters. Multi- and
dual-media filters allow for greater floc penetration of the bed than single medium filters.
The use of surface wash and/or air scour is mandatory under these circumstances. In
essence, the function of air scour and surface wash is to loosen the accumulated deposits in
the filter. The normal backwash then flushes the deposits away.
A surface wash apparatus is illustrated in Figure 9-7. Either fixed nozzles or rotating pipes
fitted with nozzles are placed about I to 2 inches above the top of the bed. While the
surface wash is on, the backwash expansion is set at a lower rate than after the surface wash
is termlna!ed. Surface wash water is supplied at 50 to 100 psi at rates approximating 1 to 3
gal/mm/ft of bed.
Air scour is accomplished by injecting air into the underdrain system prior to initiating
water backwash. The following procedure has been recommended (12).
a. Stop influent and lower the water level to a few inches above bed.
b. Apply air alone at 2-5 ft 3 /min/ft 2 for 3-10 mm.
c. Apply water backwash at 2-5 gpm/ft with air on until water is within one foot of
wash water trough.
d. Shut air off.
e. Continue water backwash at normal rate for usual period of time.
f. Apply backwash for 1-2 mm. at a rate required to insure hydraulic classification of
the filter media.
Air backwash has the disadvantage of increased possibility for coal media losses. Although
small losses repeatedly occur due to air bubble attachment during normal backwashing
cycles, the danger of massive losses exists if air is applied during trough overflow periods.
Control is especially difficult in pressure filters as it is hard to observe the bed during
backwash.
9.2.8 Filter Control Systems
Recent improvements and advances have been made in the systems which are used to
control the operation of filters. As a result, where once a filter required a manual operation,
it is now possible to have a completely automated filtration plant. Not only have these
advances removed much of the drudgery from filter operation, but they have given the plant
operator powerful techniques for improving filter performance.
9-13
-------
Figure 9-7
PALMER FILTER BED AGITATOR
9-14
-------
Perhaps the most important of the recent advances is the use of automatic turbidimeters to
continuously monitor the filter feed and product. This allows the operator to anticipate
difficulties from changes in feed quality, and rapidly remedy process failures. In addition,
these devices allow the operator to rapidly evaluate the effects of changes in process vari-
ables and provide a continuous record of plant performance. All turbidimeters operate on
the pniiciple of measurement of scattered or transmitted light. A variety of commercial
instruments are available.
Automatic backwash, initiated either by terminal pressure drop across the bed or by excess
turbidity in the effluent, should be considered for all new plants. The cost of these systems
may be justified by a reduction in labor. Automatic backwash systems are available from a
vanety of manufacturers. It is important that these systems be equipped with delay mech-
anisms so that the wash water reservoir capacity is not exceeded if the wash cycles of several
filters overlap.
The automatic backwash system referred to above is essentially an electronics package
which operates valves, pumps, etc., by remote control Two other automatic backwash
systems based on a different concept are commercially available. An automatic gravity filter
is depicted in Figure 9-8. No data are available for waste treatment application of these
systems
A traveling backwash filter employs an Il-inch depth of sand supported on porous plates.
The bed is divided into many sections 8 inches wide. A traveling backwash assembly moves
from section to section as required to clean each section. Thus most of the filter is in
operation with only a small section being washed at any time.
In the past, one type of flow control system was utilized in most filters. This system
provides for constant flow with a constant water depth over the filter. Under this condition,
constant flow is maintained by varying the headloss downstream of the filter so that the
total headloss in the filter and downstream is constant. Headloss in the downstream line is
automatically adjusted by a throttle valve connected to a preset counterweight. Usually a
ventun with a variable opening diaphragm serves as the rate controller. Frequently main-
tenance of these rate controllers is troublesome.
An alternate system which does not require a rate controller to achieve constant flow has
recently been described (1 3). A weir in the inlet channel to the clear well provides a
constant back pressure on the filter. Flow into the filter box is over an inlet weir with the
crest set quite high above the bed surface. As headloss develops in the filter, the depth of
flow above the filter increases to maintain the filter flow constant. A disadvantage of this
method is the capital cost associated with building the filter box walls several feet higher
than required where a rate controller is used.
Another control concept is declining-rate filtration. This method is applicable only to
medium or large scale plants which utilize multiple filters. With this method the flow rate
through the filter is allowed to decline as the filter clogs. The filters are staggered in degree
of clogging so that the total production of the plant is constant. This procedure is claimed
to produce better effluent quality and longer filter runs because flow slows as the filter
clogs Thus, the rate of headloss increase decreases and the probability of floc breakthrough
is lessened.
9-is
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EFFLUEN1
AIR
BACKWASH WATER
ANTHRACITE
SAND
ROTO-SCOUR
UNDERDRAIN
Figure 9-8 AUTOMATIC GRAVITY FILTER, SINGLE COMPARTMENT
—
‘.0
INFLUENT
II
I
I
(Courtesy of Graver)
-------
A simple method of flow control which can be used with pressure filters is to pump at a
constant rate to each filter with a positive displacement pump. With this system, the pump
rate sets the flow rate and the pump discharge pressure rises as the bed clogs.
9.2.9 Chemical Pretreatment
Chemical pretreatment ahead of the filter is designed to:
a. Coagulate suspended solids to make them more amenable to sedinientation and/or
filtration.
b. React with soluble components which must be removed to form insoluble precipitates
for removal by sedimentation and/or filtration.
c. Adjust the strength of the floc to control the degree of penetration of solids into the
filter.
In order to ascertain optimum chemical doses to accomplish any of these objectives, pilot
studies are required.
While coagulants aid in removal of colloids. the floc formed may be relatively weak. The
major use of polymeric materials is to strengthen floc so that it will not penetrate through
the filter. The floc should not be excessively strengthened or it will not penetrate beyond
the filter surface, producing excessive pressure drop. A good rule to follow is that floc
breakthrough should coincide with the achievement of terminal headloss.
Chemicals are usually applied prior to sedimentation to remove the bulk of the solids before
the filter. Recent practice has employed further chemical addition just prior to filtration to
enhance the filterability of the feedwater solids.
9.3 Summary of Results of Filtration Studies
Rapid sand type filters have been used in three kinds of waste treatment systems direct
filtration of secondary effluent, filtration of secondary effluent after chemical treatment
and filtration of raw or primary wastewater after coagulation and sedimentation. Repre-
sentative results from field studies in each of these areas will be discussed below.
9.3.1 Filtration of Secondary Effluent
Early work on filtration of secondary effluent took place in Europe. Truesdale and Birkbeck
(14) reported on tests run between October, 1949,and May, 1950, at the Luton Sewage
Works. Beds of sand 2 feet deep, ranging in size from 0.9 mm to 1.7 mm, exhibited 72 to 91
percent removal of suspended solids 2 and 52 to 70 percent removal of BOD. Flow rates
ranged from 1.33 to 3.3Imp.gal/min/ft . Air-scour-aided backwash was used once per day to
clean the bed. Backwash flow rate was 11 .6 Imp. gal/mm/ft 2 .
Naylor, Evans and Dunscome (15) later reviewed 1 5 years of studies of tertiary treatment at
Luton. A 3-foot deep bed of-lU to +18 mesh sand consistently provided an effluent of 4 to
6mg/i suspendedsolidsatflowratesof3.3Imp.gal/min/ft . It was found best to wash the beds
every 12 hours.
9-17
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In the U.S.,most direct filtration work has been with activated sludge feed. At the Hyperion
Plant in Los Angeles, sand of 0.95 mm effective size was used in a shallow bed (11 inches
deep) traveling backwash filter. This study lasted for six months during which time 46
percent suspended sc ids removal and 57 percent BOD removal were obtained. Filtration
rate was 2 gal/mm/ft . Difficulty was encountered in cleaning the filters and performance
gradually deteriorated during the study. Use of a finer sand (0.45 mm effective size) in an
attempt to yield a better effluent was a failure due to very rapid clogging of the filter (16).
Much greater success utilizing the traveling backwash filter for activated sludge effluent
treatment was obtained by Lynam (17) in Chicago. The effective size of sand used in this
study was 0.58 mm. Suspended solids removal of 70 2 percent and BOD removal of 80
percent were obtained at flow rates of 2 to 6 gal/mm/ft . Terminal headloss was quite low
(11 inches of water.) The range of flows studied exhibited no significant difference in terms
of suspended solids removal.
A study of filtration of activated sludge effluent was pre ented by Tchobanoglous and
Eliassen (18). They found that flow rate (2 to 10 gal/mm/ft ) had little effect on perform-
ance, but effective size of sand in the range of 0.4 mm to 1.2 mm had a very significant
effect. It was also determined that virtually all suspended solids removal took place in the
upper six inches of the bed. it was concluded that the hoc strength of activated sludge is
quite high on the basis of the low degree of penetration even with very coarse media and
high flow rates. Poor removal of suspended solids (10 to 40 percent) was obtained in this
study, with filter depths greater than 6 inches producing no additional benefits. It was
found that a bimodal distribution of particle sizes existed for the activated sludge effluent.
Apparently, only the larger solids could be removed by the filters.
Cu Ip and CuIp (12) reviewed the work on plain filtration of secondary effluent with both
single medium and multimedia filters. They concluded that, with either type of filter, better
results would be obtained as the degree of self flocculation of the sludge increased. Thus, a
high-rate activated sludge effluent which contains much colloidal material should filter
poorly, while an extended aeration effluent should filter well. Multi-media filters exhibit a
marked supenority for filtration of activated sludge effluent because of the high volume of
floc storage available in the upper bed and the polishing effect of the small media. They
indicated the expected performance of multi-media filters for plain filtration of secondary
effluents, as shown in Table 9-3.
Table 9-3
EXPECTED EFFLUENT SUSPENDED SOLIDS FROM MULTI-MEDIA
FILTRATION OF SECONDARY EFFLUENT
Effluent Type Effluent S.S. mg/I
High Rate Trickling Filter 10 - 20
2-Stage Trickling Filter 6 - 15
Contact Stabilization 6 - 15
Conventional Activated Sludge 3 - 10
Extended Aeration I - 5
9-18
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9.3.2 Filtration of Chemically-Treated Secondary Effluent
Treatment of secondary effluent by chemical coagulation, sedimentation and filtration has
been conducted at a number of installations. The purposes of this treatment procedure have
often been twofold, suspended solids and phosphorus removal. Unfortunately, in many
cases, performance results have been reported for complete systems, not each individual
process. Thus, analysis of the filter performance is not possible. In most of these installa-
tions, the filter is viewed essentially as a polishing device to capture solids which escape the
sedimentation tank. As Culp and Culp (12) have indicated, this philosophy may be wrong,
as modern filters can absorb much greater solids loads than older designs.
in conjunction with the studies at Chicago (17), coagulation with alum followed by filtra-
tion was evaluated, it was found that the alum treatment had little effect on the effluent
quality, it is probable that this result is due to the weakness of chemical floc compared to
activated sludge floe.
An advanced waste treatment plant has been used to renovate step aeration activated sludge
effluent in Nassau County, New York. Alum at 200 mg/i is used to coagulate the waste-
water prior to sedimentation and filtration. The filters are dual-media containing 30 mches
of 0.9 mm coal over 6 inches of 0.35 mm sand. With the addition of 0.5 mg/I of an anionic
polymer, effluent turbidity is maintained below 0.4 JTU. Run length varies from 8 to 24
hours depending on the solids load from the clarifier.
At Lebanon, Ohio, treatment of the activated sludge effluent with lime has been investi-
gated (19). The lime dose averaged 300 mg/I Dual-media filters, consisting of 18 inches of
0.75 mm coal over 6 ii ches of 0.45 mm sand, followed clarification. The filters were
operated at 2 gal/m m/ft and were backwashed when the leadloss reached 9 ft of water.
Influent to the filters ranged from 13 to 36 mg/I of suspended solids (turbidity 4 to 10
JTU.) Filter effluent ranged from 0.07 to 0.14 JTU.
At Lake Tahoe, lime is used to coagulate activated sludge effluent prior to sedimentation and
filtration l2). Multi-media beds 3 feet deep are utilized at an average flow rate of 5
gal/m m/ft . Filter runs have varied from 4 to 60 hours. Polymers as well as alum have been
used to strengthen the floe. Normally, runs are terminated at headlosses of 8 ft of water.
Effluent turbidities are typically reduced to 0.3 JTU with correspondingly low values for
other parameters.
At the Environmental Protection Agency pilot plant at Washington, D.C., mineral addition
to the final phase of a step aeration activated sludge is practiced for phosphorus rejnoval.
After sedimentation the effluent is filtered through parallel filters at 2.4 gal/mm/ft . One
filter is a dual-media, while the other is a multi-media. it has been found that the multi-
media filter removes 5 to 10 percent more suspended solids than the dual-media filter. Filter
runs have been in the range of 24 to 32 hours. Typically, the filters reduce secondary
effluent suspended solids from 33 mg/ 1 to 8 mg/I (20).
9.3.3 Filtration Following Chemical Treatment of Primary or Raw Wastewater
Clanfication of raw wastewater followed by carbon adsorption is just emerging as a viable
treatment technology. This system employs filtration as part of the solids separation system.
Although several pilot installations employing this concept are in operation, filtration has
not been closely studied, thus data are sparse.
9- 19
-------
At the EPA pilot plant in Washington, D.C., two-stage lime treatment of raw sewage fol-
lowed by sedimentation, filtration and granular carbon adsorption is being studied (21).
Dual-media filters (18 inches of 0.9 mm coal over 6 inches of 0.45 mm sand) are employed.
Cleaning is initiated at a headloss of 9 feet of water. Cleaning is performed 2 automatically
with a surface wash rate of 3 gal/m m/ft and an upflow rate of 20 gal/mm/ft . Run lengths
averaged 50 hours dunng cold weather, but tn warm weather the growth of slirnes reduced
run lengths to less than 1 2 hours. Prechiorination of the filter feed was employed to restore
the run lengths to 50 hours. Filter effluent averaged 4.5 mg/I of suspended solids over a
6-month period, which represented a 70 percent efficiency for the filter. This plant operates
on a programmed diurnal 2 flow variation which produces a flow rate variation on the filter
from 1.7 to 4.3 gal/mm/ft
A similar system using single-stage lime treatment was run for several nonths at the EPA
installation in Lebanon, Ohio (22). Flow to the filters was 2 gal/mm/ft . Suspended solids
in the filter effluent averaged 10 mg I 1, which represented a 67 percent removal efficiency.
9.4 New Filtration Systems
The rapid sand type filtration system previously discussed has been a downflow, batch,
static bed system. Consequently, it suffers from a variety of process deficiencies including:
a. The need to stop the process periodically to clean the filter medium.
b. The limited ability to economically handle suspensions containing high concentra-
tions of suspended solids.
During the last decade a number of new filtration systems have been developed which are
aimed at overcoming these shortcomings. Several of these will be discussed below.
9.4.1 Upflow Filtration
As indicated previously, a major difficulty with downflow, single-media filtration is that
after backwash, the bed is graded fine to coarse in the direction of flow. If filtration is
conducted upilow, this difficulty is circumvented. However, once the headloss produced by
the upflow exceeds the buoyant weight of the filtration medium, fluidization with con-
sequent loss of filtration efficiency will result. One solution to this problem is to place a
restraining grid on or near the top of the filter medium to prevent fluidizatiori. A diagram of
an upflow filter is illustrated in Figure 9-9 (courtesy of DeLaval.)
The spacing between bars of the grid must be large enough to allow the bed to expand
dunng backwash, but must be small enough to prevent upward bed movement during
filtration. It would seem that these two requirements are directly contradictory; however,
arching of the grains takes place between the bars, allowing a reasonably large spacing. Space
between the bars is usually in the range of 100 to 150 diameters of the smallest grain size in
the beds. Dunng cleaning, air is first introduced to agitate the bed. After the air has broken
the arches, backwashing with water is started. Table 9-4 gives a summary of typical design
parameters for the upflow filters.
9-20
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COVER OPTIONAL
(FOR CLOSED SYSTEM) -
I I
I I
“GRID”
DEEP SAND LAYER
• GRAVEL LAYERS
INLET RAW WATER
• __
FILTRATE OUTLET
SAND “ARCHES’
I SPECIAL VENT
AIR FOR
4— SANDFLUSH CLEANING
CROSS SECTION OF UPFLOW FILTER
9-21
Figure 9-9
-------
Table 9-4
DESIGN PARAMETERS FOR UPFLOW FILTERS
Bed Material. Sand
Bed Construction: 60 in. 1 - 2 mm
lOin. 2-3mm
4in. 10-15mm
Flow RateS 2-3 gal/mm/sq ft
Backwash Rate To achieve minimum 20% expansion
Terminal Headloss: 6 to 20 feet of water
Boby and Alpe (23) reported on the performance of an upflow filter treating secondary
effluent at Totor, England. Average suspended solids removal was 85 percent, with the filter
effluent below 5 mg/I. These results were equal to or better than those obtained with
downflow filtration with the same size bed. It is claimed that this type of filter can absorb
higher loads of solids than a conventional filter.
9.4.2 Moving Bed Filter
The basic concept of a moving bed filter is the mechanical movement of the most heavily
clogged portion of the medium out of the zone of filtration with virtually no interruption of
the filtration process. The potential of such a process is operation at higher flow rates and at
much higher solids loadings than conventional systems. Superior cleaning of the filter media
should also be possible.
Johns-Manville Corporation has developed a moving bed filtration system. A diagram
illustrating the essentials of this system is given in Figure 9-10. Wastewater (A) flows
through the inlet pipe where chemicals, if required, are added at (B). The wastewater enters
the head tank (C) and then passes through the sand bed (D). The filtered water leaves
through the exit screens. When excessive headloss develops, the bed is pushed toward the
head tank by pressurizing a chamber separated from the bed by a flexible diaphragm. A
mechanical cutter (F) sweeps down over the face of the bed cutting off the top layers. These
then fall into the hopper (G) of the head tank. The sludge and sand are removed from the
head tank with the aid of an ejector using feedwater. The solids are hydraulically conveyed
to the sand washer (H) where filtered water or air and filtered water are used to backwash
the sand. Clean sand moves by gravity back to the base of the filter. The spent washwater is
sent to a sedimentation tank for removal of the wastewater solids. The operation of the
system is automated.
Under an Environmental Protection Agency contract (24), this system was evaluated by the
manufacturer for the treatment of raw wastewater, primary effluent and settled and
unsettled trickling filter effluents at the Bernards Township Sewage Treatment Plant. Alum
was used to precipitate phosphorus, and an anionic polymer was employed to prevent
excessive floc penetration. The results, given in Table 9-5, show excellent treatment
performance in these situations.
9-22
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A
WASTE
IN F LUE NT
WASH
WATER
RECLAIM
F’ CUTTER
(H) __
SAND
WASH ER
HEAD
TANK
HLTERED WATER
FOR
SAND WASHING”
‘ SAND
DRIVE
SYSTEM
FILTERED WATER
FOR SYSTEM REUSE
DISCHARGE
Figure 9-10
SCHEMATIC DRAWING OF THE JOHNS-MANVILLE MOVING BED FILTER
-------
The moving bed filter is being evaluated at full scale (2 MGD) at the Borough of Manville
Sewage Treatment Plant, Manville, N.J. At this site, unsettled trickling filter effluent is the
feed. This study is being partially funded by an EPA demonstration grant.
Table 9-5
JOHNS-MAN VILLE MOVING BED FILTER EVALUATION AT
BERNARDS TOWNSHIP SEWERAGE AUTHORITY TREATMENT PLANT
Parameter
Final Effluent
w/ o Chlorination
Unsettled
Trickling
Filter
Effluent
Primary
Effluent
Raw
Wastewater_____
(mg/I)
In
Out
%
In
Out
%
In
Out
%
In
Out
%
TotaiP
Filterable P
OrthoP
BOD 5
Suspended
Solids
Turbidity
( JTU)
9.37
8.03
7.80
65
50
33
0.51
0.11
0.10
12
15
7
95
99
99
80
70
79
19.1
14.9
12.4
55
86
39
0.99
0.62
0.53
3.8
7.1
3.4
95
96
96
93
91
91
14.6
13.2
9.8
67
77
53
1.13
0.58
0.38
12
11
3.7
93
96
96
82
87
93
21.5
18.6
13.2
115
156
123
2.16
0.79
0.57
19
27
16.7
91
96
95
84
83
87
Alum: 200 mg/i (commercial grade)
Polyelectrolyte: 0.5 mg/I anionic
The moving bed filter system is currently available in modules of 2-bed and 4-bed configura-
tions. These modules can be used singly or in combination to accommodate large flow
requirements as necessary.
a. The flow rate through any unit or combination of units is dependent on the quality
of the 2 incoming liquid and the discharge requirements. Flow rates up to 7.0 gal/
m m/ft of exposed filter area are possible.
b. Filter bed dimensions - 48-inch face diameter, 60-inch length from the face of the
unit to the center line of exit screen.
9-24
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c. Module dimensions -
2-Bed Unit 4-Bed Unit
Plan length 20’ 20’
Plan width 7’ 141
Height 17’ 17’
Weight, empty 22,000 lb 44,000 lb
operating 78,000 lb 156,000 lb
Plan floor space, 2 2
module 140 ft 2 280 ft 2
system 350 ft 720 ft
d. Power requirements -
Two-Bed System Four-Bed System
Function Conn. lIP Op. HP Conn. HP Op. HP
Diaphragm or
Sand-bed
Movement 3.0 3.0 6.0 6.0
Cutter 3.0 0.75 3.0 1.5
Sand Cleaning
System 3.75 3.75 7.5 7.5
Screen Wash 7.5 0.5 7.5 1.0
Chemical Pump
and Mixer 1.0 0.5 1.0 0.75
AirCompressor 1.0 0.5 1.0 0.75
19.25 9.0 26.0 17.5
e. Filter media - Hard sharp-grained quartz sand of filter grade quality (effective size 0.6
to 0.8 mm; uniformity coefficient 1.5)
£ Sand drive rate -
Linear - 12 in4our - maximum
Volume -25 ft /hour - maximum
Pump pressure - 75 psi - average
150 psi - maximum
g. Head tank - 30 minute detention time
h. Sludge settling tank - 30 minute detention time
i. Controls -
Low level and high level off-on
Differential pressure to actuate bed movement and cutter
Effluent turbidity to adjust chemical feed
9-25
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9.4.3 Radial Flow Moving Bed Filter
Recently,Dravo Corporation has introduced a radial flow moving bed filter to the American
market. At present, most applications of this system have been for industrial waste treat-
ment. In addition to the moving bed concept,the main feature of this system is the radial
flow concept. This geometric configuration provides more filter area per unit volume than
downflow or upflow systems. As the liquid flows radially from the central core, it slows
down, providing increased opportunity for solids removal.
9.4.4 Radial Flow-External Wash-Filter
The Hydromation Corporation has developed a new concept in filtration. A diagram of this
filter is illustrated in Figure 9-1 1. It is a batch-type radial flow filter with external media
wash. When a filter run is terminated, the media is pumped out of the filter and upward into
a scrubber. The flow velocity in the scrubber is 20 ftfsec, which produces a very high degree
of turbulence, assuring good cleaning of the media. The clean media circulates around the
backwash ioop to the radial flow bed.
This filtration system utilizes a polymer resin as the filter medium. It is claimed to have
superior dirt holding capacity compared to natural media. Because of the syperior dirt
holding character and special cleaning system, flow rates of 10 to 20 gal/mm/ft have been
purported.
9-26
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jr ;.
.4 I
Figure 9-11 HYDROMATION IN-DEPTH FILTER
9-27
-------
9.5 Aefarences
1. Kreissl, J.F., & Robeck, G.G., “Multi-Media Filtration. Principles and Pilot Experi-
ments”. Bulletin No. 57, School of Engineering and Architecture, University of Kansas,
Lawrence. Kansas (1967).
2. Summary Report, Advanced Waste Treatment . WP-20-AWTR-19, U.S. Dept. of the
Interior, FWPCA (1968).
3. Fair, G., Geyer, J., “Water Supply and Waste Water Disposal”. Chapter 24, John Wiley
& Sons, Inc., New York (1954).
4. Water Treatment Plant Design , American Water Works Association, Inc., New York
(1969).
5. Water Quality and Treatment , American Water Works Association, Inc., McGraw-Hill ,
Inc., New York (1971).
6. Conley, W.R., and Hsiung, K., “Design and Application of Multimedia Filters” Jour.
AWWA , 61,97 (Feb. 1969). —
7. Laughlin, J.E., and Duvall, T.E., “Simultaneous Plant-Scale Tests of Mixed-Media and
Rapid Sand Filters”. Jour. AWWA , 60, lOIS (Sept. 1968).
8. Westerhoff , G.P., “Experience with Higher Filtration Rates”. Jour. AWWA , 63, 376
(June 1971). —
9. Miller, D.G., “Rapid Filtration Following Coagulation Including the Use of Multi-Layer
Beds”. Proc. The Society for Water Treatment and Examination , 16,3, 197 (1967).
10. Hirsch, A.A., “Backwash Investigation of a Proposed Simple Uniformity Control”.
Jour. AWWA , 60, 570 (May 1968).
11. Johnson, R.L., and Cleasby, J.L., “Effect of Backwash on Filter Effluent Quality”.
Jour. San. Eng. Div., ASCE , 92, 215 (Feb. 1966).
12. CuIp, R.L., and Culp, G.L., Advanced Wastewater Treatment , Van Nostrand-Reinhold
Co., New York (1971).
13. Baumann, E.R., and Oulman, C.S., “Sand and Diatoniite Filtration Practice”. Water
Quality Improvement by Physical and Chemical Processes, University of Texas Press,
Austin, Texas (1970).
14. Truesdale, G.A., and Birkbeck, A.E., “Tertiary Treatment Processes for Sewage Works
Effluents”. Water Poll. Control Jour . (Brit.) 66, 371 (1967).
15. Naylor, A.E., E’wans, S.C., and Dunscombe, K.M., “Recent Developments on the Rapid
Sand Filters at Luton”. Water Poll. Control Jour . (Brit.) 66, 309 (1967).
16. Laverty, F.B., Stone, R., and Meyerson, L.A., “Reclaiming Hyperion Effluent”. Jour.
San. Eng. Div., ASCE , 87, 6, 1 (Nov. 1961).
9-29
-------
9.5 References (Cont)
17. Lynam, B., Ettelt, G., and McAloon, T.J., “Tertiary Treatment at Metro Chicago by
Means of Rapid Sand Filters and Microstrainers”. Jour. WPCF , 41, 247 (Feb. 1969).
18. Tchobanoglous, G., and Eliassen, R., “The Filtration of Treated Sewage Effluent”.
Proceedings of the 24th Purdue Industrial Waste Treatment Conference, 1323 (1969).
19. Berg, E.L., Brunner, C.A., and Williams, R.T., “Single Stage Lime Clarification of
Secondary Effluent”. Water & Wastes Engineering , 7, 3, 43 (March 1970).
20. [ -lais, A.B., Stamberg, J.B., and Bishop, D.F., “Alum Addition to Activated Sludge with
Tertiary Solids Removal”. Presented before AIChE National Meeting, Houston, Texas
(March 1971).
21. Bishop, D.F., O’Farrell, T.P., and Stamberg, J.B., “Physical-Chemical Treatment of
Municipal Wastewater”. Presented before the 43rd Annual Meeting, WPCF, Bston,
Mass. (Oct. 1970).
22. Villiers, R.V., Berg, E.L., Brunner, C.A., and Masse, A.N., “Treatment of Municipal
Wastewater by Lime Clarification and Granular Carbon”. Presented before ACS,
Toronto, Canada (May 1970).
23. Boby, W., and Alpe, G., “Practical Experiences Using Upward Flow Filtration”. Proc.
Society for Water Treatment and Examination , 16, 3, 215 (1967).
24. Phosphorus Removal Using Chemical Coagulation and a Continuous Countercurrent
Filtration Process , Final Report (17010 EDO), U.S. Dept. of the Interior, FWQA (June
1970).
9-30
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CHAPTER 10
OPERATION AND MAINTENANCE
10.1 General
This section presents some of the operation and maintenance factors that must be con-
sidered in the use of solids removal processes. Particular interest is paid to man-hour require-
ments for processes by consideration of the work required. First, general practices and
procedures that are common to several or all of these processes are presented. Operation and
maintenance (O/M) requirements peculiar to the individual processes will follow. Finally, an
estimate of man-hour requirements for proper operation and maintenance of these processes
will conclude this section. Only the more feasible processes on which sufficient information
is available are specifically discussed.
10.2 General O/M Considerations
Any process, insofar as it requires housing for protection of equipment and employees, will
also require regular custodial services. These include cleaning and washing of floors and
walls, the washing of windows and the disposal of refuse. These services are often over-
looked or disregarded in the operation of a waste treatment facility, since their performance
does not contribute directly to process operation and maintenance. However, proper atten-
tion to these services is essential to create a suitable working environment and a positive
mental attitude on the part of the employees. Areas that serve primarily as process en-
closures may be cleaned on a weekly basis. Administrative areas, laboratories and workshops
should be cleaned daily.
Those processes which employ exterior basins must be provided with means for rapid
dewatering to expose submerged equipment and structures for inspection, servicing and
repair. While the frequency for performance of these activities vanes considerably depending
on the number of process units and their size, an average frequency of at least twice a year is
recommended. At these times, the basin equipment should be thoroughly cleaned. All
exposed cracks in structural concrete should be filled and the exposed metal surfaces of
structure and equipment should be cleaned and painted or touched up to prevent corrosion.
Daily cleaning of weirs, channels, inlet boxes and outlet troughs is necessary to prevent
paper, dirt, stones and other assorted debris from clogging these facilities and interfering
with process efficiency.
A daily procedure for performing process monitoring and control functions should be
established. This should include frequent visits to each unit process to observe its operation.
Controls and gages should be checked weekly and calibrated on a regularly scheduled basis.
Also, a review should be made of the most recent operational records and reports in order to
stay abreast of the current operating problems. in addition, a daily sampling program for the
effluent of each process is required to maintain proper process control. This will involve the
collection of a series of composite and grab samples, either manually or by the use of
automatic sampling devices. These samples must also be analyzed and the results tabulated
on a regular basis to make the data available for reports and operational decisions.
10-1
-------
The establishment of a daily program of equipment inspection and repair is an absolute
operational requirement for the proper maintenance and operation of these processes. The
complexity of the formal preventive maintenance (P/M) program will depend largely upon
the size of the plant and the number and overall capabilities of assigned O/M personnel. The
most essential areas to be covered are the inspection and repair of electrical and operating
control systems; the greasing and lubrication of all mechanical wearing surfaces; the deaning
of pumps, valves, conduits, etc.; and the regular painting of all exposed metal or wood
surfaces.
Electrical and control equipment should be inspected regularly for tightness and to see that
moving parts are free, contact pressures firm, and shunts unfrayed. Controls should also be
checked to see that they are operating at rated voltage. It is most important to make certain
that these devices are clean and dry. Motors should also be cleaned and oiled on a regular
basis and starter contact points inspected for proper fit and corrosion. All indicated repairs
should be made at once.
Lubrication is one of the most important parts of a maintenance program. In this regard,
strict adherence to the manufacturers recommendations should be practiced. It is important
not to overlubricate, sipce this can also result, in motor failure.
A certain amount of time should be spent each day in the preparation of records, forms and
reports that describe the performance of the individual unit process and the overall treat-
ment system. It is recommended that records be kept on:
• treatment efficiency
• influent and effluent quality
• volume of wastewater treated
• results of chemical analysis
• other additional data as required by control agencies
• equipment repair history
• inventory control data
• status of P/M program
• cost data:
chemicals expended
purchase of supplies
man-hours expended
power consumption
10-2
-------
10.3 O/M Considerations- Individual Processes
The following is a list of operation and maintenance factors that are specific to the in-
dividual processes under consideration. Some of these processes are conventional and O/M
data from their use in the treatment of water or wastewater are published. Some of the
newer processes have not received large scale application at this time. The O/M factors for
these processes can only be surmised based upon an analysis of the mechamcs involved. This
generally involves an anticipation of potential problems rather than a recording of past
history.
10.3.1 Chemical Mixing and Flocculation
The major operational problems involved in this process are the receiving, storing, trans-
porting and feeding of the chemicals used. Chemical feed pumps and bottom sludge pumps
must be periodically disassembled and cleaned. As with any process that involves the agita-
tion of domestic wastewater, a foam can occasionally develop in the rapid mix tanks and
flocculators. This may be readily controlled by the application of surface sprays. Weir
plates, baffles and drains must be kept clean to maintain proper basin hydraulics. Weir crests
must always be kept sharp and clean. Motors, drive systems and paddles will require
servicing and repair.
10.3.2 Solids-Contact Clarification
The operation and maintenance functions for this type of unit are very similar to those for
the conventional process. The gain in spacial efficiency by the reduced size is balanced by
increased maintenance requirements and costs due to the more intensive use of mechanical
equipment. More frequent dewatering of the basins for inspection of submerged equipment
will be required. More constant attention to the sludge removal function and the repair of
these facilities is required to prevent excessive amounts of sludge from building up and
interfering with efficient operations.
10.3.3 Sedimentation
The operation of a standard sedimentation tank does not usually require a great deal of
time. Generally, the only piece of mechanical equipment employed will be the bottom
scraper and the surface skimming mechanisms. Care must be taken to remove surface
skimnungs regularly to avoid carryover of the floating solids to subsequent units. The sludge
collectors must be run often enough by manual or automatic timing devices to insure that
solids do not build up to a point where the mechanism will be damaged or the effluent
quality impaired. The side walls of the tank should be washed or scraped down on a regular
basis so that sohds do not collect on the surfaces and decompose causing an odor problem.
Regular cleaning of weirs, troughs, skimmers and scrapers are required for effective perform-
ance. The scrapers of circular tanks should be leveled regularly to maintain proper balance.
Weir crests should be kept sharp to maintain proper hydraulics.
Where more than one tank exists, care must be taken to be sure that the flow is properly
distributed to each of the units. Adjustment at the influent structure must be made on a
regular basis tobe certain that each unit is receiving its share of the load.
10-3
-------
Frequent samples of the sludge draw-off should be collected for laboratory analysis. A
common problem is excessive draw-off of sludge which results in process water being
pumped to the digesters with resultant upsets to that process. This can be controlled by
regulation of the sludge pumps.
10.3.4 High-Rate Settlers
The operation and maintenance requirements of high-rate settlers should be similar to those
of the conventional sedimentation basin. The size of the overall facility shouid be much
smaller, but additional maintenance results from the regular cleaning requirements of the
tubes themselves. Profuse biological growths occasionally occur in and on the top edges of
the tubes, which may often be controlled by water sprays. Frequent basin drawdowns may
be required if this method fails. Total man-hours required for operation and maintenance of
this system should be similar to conventional sedimentation.
10.3.5 Microscreening
The operation of a microscreen rnstallation requires regular checking of screen condition
and headloss through the screen. Periodic adjustment of the ultraviolet lamp system as well
as backwash pressure and flow rate are necessary. Periodic adjustment to the variable speed
drum drive is needed to meet changing flow conditions if no automatic controls are used.
The discharge weir must be raised or lowered to control screen submergence.
The major maintenance task is the cleaning and repair of the screens. If the unit must be
stopped, it should be carefully drained and the fabric cleaned prior to removal from service.
The drum should never be left standing for any length of time in a tank of dirty water. This
can lead to severe clogging of the microfabric. This condition may be difficult to treat
subsequently. Application of microscreening to chlorinated wastewaters can damage the
screens by chlorine attack Screen replacement should be performed on a regular basis and
as soon as any defect is noted. Minor repairs can be made without removing the screen from
the unit, and some screens have removable panels that can be replaced without affecting
adjacent panels. Major screen defects call for the removal of the entire screen for repair or
replacement on other models.
Other maintenance factors are the service required by the water spray heads and the ultra-
violet light system. The ultraviolet system is used to control microbial slimes that often clog
the screens. When the screen becomes heavily clogged, it is washed with a solution of
sodium hypoch lorite followed by a clean water nnse. This washdown will be needed
periodically and requires about 1½ hours to complete. If the waste contains manganese or
iron in solution, a film of iron or manganese oxide can be formed on the screen. This long
term effect can be controlled by application of an inhibited acid cleaner in a manner similar
to that with the hypoch lorite.
10.3.6 Filtration
The major operating requirement for rapid sand filtration is the supervision of the back-
washing operation. This operation must be closely controlled to be sure that the filtering
metha is kept free from dirt, hard spots, mud balls or other objectionable material. Often,
normal filter backwashing is not enough to maintain the filter bed under all conditions.
Regular inspection and maintenance are needed to insure that the beds remain loose, clean
10-4
-------
and free of incrustatioris. Frequent measurements of the depth of bed are taken and
compared with previous measurements to be sure that the bed is neither expanding nor
contracting excessively. This procedure may also help to determine media losses during
intervening backwashes. Periodic testing of the level, slope and elevation of the wash water
troughs is required to assure proper hydraulics. Regular painting of metal troughs is also
important.
If the backwashing system L5 insufficient for complete cleansing of the bed, wastewater
solids and chemical floes may combine with media to form clumps or balls during the
washing process. As their density increases, they sink to the bottom of the filtering media.
Improper filtering and backwashing can result. In severe cases, it may be necessary to
remove and replace the media.
10.3.7 Moving Bed Filter
The operation of a Moving Bed Filter requires continual surveillance because of the com-
plexity of the unit. The complete system includes pumps for adding coagulating chemicals,
advancing the sand filtering face, cleaning the discharge screen and operatiiig the sand
cleaning and feeding systems. The major maintenance tasks are servicing and repairing these
various pumps.
It is important to service, on a regular basis, the chemical tanks, pumps and chemical feed
lines. The sand cutting, washing and transporting systems and equipment have to be
checked, serviced and repaired continuously due to the fact that they are handling a very
abrasive material This system must be maintained in prime condition or the process may
break down and become inoperable. The diaphragm and pump assembly which actuates and
advances the sand bed represents a critical phase of the operation as the pump is of a high
pressure type with the attendant possibility of pipe and/or diaphragm failure. The pump
used to develop the pressurized stream for cleaning the outlet screen from the sand filter is
also of the high pressure type. Although used intermittently, this pump requires regular
servicing, along with inspection of its associated connecting lines, fittings and nozzles. The
air compressor and pump used in the agitation and washing of the sand require regular
servicing. The flow and level controls, which compensate for variations in waste flow and
composition, i’nust be inspected and serviced regularly.
10.4 Man Haur Requirements
In order to provide an estimate of the personnel required to perform the aforementioned
operation and maintenance tasks, it is assumed.
a. That each unit process is part of a larger treatment system and operation and
maintenance personnel can be shared with other units.
b. That most maintenance functions will be carried out by the operating agencies
staff, and a negligible amount will be performed by outside contractors. Small
plants may not be able to supply the complete range of specialties required. In
these cases a regional maintenance system is postulated. This system would provide
maintenance support to several plants.
c. That these processes are sophisticated enough to require 24-hour operation, seven
days per week.
10-5
-------
d. That maximum practical use be made of currently available automatic control
systems. Therefore, the amount of time that the operating personnel spend in
manual process control will be minimized.
e. That the basis of the detailed man-hour estimate will be to insure the efficient
operation of a plant with a flow of I MGD.
Within the limits of these assumptions, total annual man-hours will be set by consideration
of the aforementioned maintenance tasks, and the amount of time it should take reasonably
skilled personnel to do the work. This rational approach would be greatly assisted by repair
histories for these types of equipment from existing plants. However, because there are
relatively few existing installations for most of these processes, this kind of information is
not available. Because of the dearth of this information, these factors must be estimated
based on knowledge of the application of these processes in other areas. These estimates are
supplemented by manufacturers information. The following tables show the estimates of
man-hours required for the individual unit processes considered. Table 10-1 is an estimated
summary of man-hour requirements, showing a comparison between these estimates and
others, where available. The remaining tables (10-2, -3,-4,-5,-6) show, for each individual
process, the following:
• work categories
• job assignments
• performance frequency
• daily man-hours
• yearly man-hours
10.5 Job Descriptions
To aid in personnel recruitment, it is recommended that a set of job descriptions be pre-
pared for each new facility. These descriptions should be tailored to the specific location,
size and type of treatment process to be used. A list of standard job descriptions for
wastewater treatment plant positions will soon be published by the Environmental Protec-
tion Agency. The following is a list of occupations commonly found in wastewater treat-
ment plants:
Superintendent Mechanic I
Assistant Supenntendent Electrician 11
Clerk Typist Electrician I
Operations Supervisor Maintenance Helper
Shift Foreman Laborer
Operator II Painter
Operator I Storekeeper
Automotive Equipment
Operator Custodian
Maintenance Supervisor Chemist
Mechanical Maintenance Laboratory Technician
Foreman
Mechanic II
10-6
-------
Table 10-1
MAN-HOUR SUMMARya
By Process
Annual Annual
Man-Hours Man-Hours BaselineC
Unit Process Net Grossb Man-Hours
Coagulation 2540 3180 2060
Sedimentation 1240 1550 —
Microscreening 2420 3030 540
Filtration 3875 4810 2260
Moving Bed Filter 3615 4520 N.A.
a Based on I MGD capacity
b Efficiency factor of 0.8 assumed, based on time lost for lunch breaks and in
starting and finishing work.
C From curves in Reference (4), assuming 70% of direct O/M costs are for labor.
10-7
-------
Table 10-2
MANPOWER REQUIREMENTS
MiXING AND FLOCCULATION
Job Performance Daily Yearly
Task Assignment Frequency Man-Hours Man-Hours
Operation
Process Monitoring Operations Daily .75 275
Process Controlling Operations Daily 50 180
Sampling Operations Daily .50 180
Analysis Chemist Daily I .00 365
Chemical Handling Labor As Required — 400
0 — Subtotal 1400
Maintenance
Custodial Custodial Daily Neg.
Inspection & Testing Maintenance Daily .25 90
Minor Parts Adjustment
and/or Replacement Maintenance Daily .50 180
Grease & Lubrication Maintenance Daily .25 90
Major Overhaul and
Cleaning Maintenance 6 Mos. — 150
Clean. (Weirs, Drains,
etc.) Labor Daily .50 180
Painting Labor As Required — 100
Electrical Electrician As Required — 300
M — Subtotal 1140
Total 2540
10-8
-------
Table 1 0-3
MANPOWER REQUiREMENTS
SEDIMENTATION
Job Performance Daily Yearly
Task Assignment Frequency Man-Hours Man-Hours
Operations
Process Monitoring Operations Daily .25 90
Process Controlling Operations Daily .25 90
Sampling Operations Daily .50 180
Analysis Chemist Daily I .00 365
0 — Subtotal 360
Maintenance
Custodial Custodial Daily Neg Neg.
Inspection & Testing Maintenance Daily 25 90
Parts Adjust. and
Replacement Maintenance Daily 25 90
Grease & Lubricate Maintenance Daily .25 90
Major Overhaul Maintenance Daily 200
Cleaning (Weirs,
Drains) Labor Daily .50 180
Major Cleaning Labor 80
Painting Labor 50
Electrical Electrician 100
M — Subtotal 880
Total 1240
10-9
-------
Table 10-4
MANPOWER REQUIREMENTS
MICROSCREENING
Job Performance Daily Yearly
Task Assignment Frequency Man-Hours Man-Hours
Operations
Monitor Operations Daily .75 270
Control Operations Daily .75 270
Sampling Operations Daily .75 270
Analysis Chemist Daily 1.25 450
0 — Subtotal 1260
Maintenance
Custodial Custodia! Daily Neg. Neg.
Inspection & Testing Maintenance Daily .25 90
Parts Adjust. Maintenance Daily .25 90
Media Repair &
Replacement Maintenance As Required — 200
Major Overhaul Maintenance 6 Mos. — 400
Media Cleaning Maintenance Bi-monthly — 50
Control & Electric Electrician As Required — 150
M—Subtotal 1160
Total 2420
10-10
-------
Table 10-5
MANPOWER REQUIREMENTS
FILTRATION -
Job Performance Daily Yearly
Task . Assignment . Frequency . Man-Hours Man-Hours
Operations
Monitor Operations Daily 1.50 550
Control Operations Daily 1.00 365
Backwash Super. Operations As Required 2.00 730
Sampling Operations Daily .75 27.0
Analysis Chemist Daily 1.25 450
0 — Subtotal 2365
Maintenance
Custodial Custodial Daily .25 90
Inspection & Testing Maintenance Daily .: .50 180
Parts Adjust. Maintenance Daily .50 180
Major Overhaul Maintenance 6 Mos. . — 200
Bed Maint. &
Clean. Labor Daily . .75 280
Major Cleaning Labor . 6 Mos. — 80
Controls & Elect. Electrician As Required — 500
M—Subtotal 1510
Total 3875
10-Il
-------
Table 10-6
MANPOWER REQUIREMENTS
MOVING BED FILTER
Job Performance Daily Yearly
Task Assignment Frequency Man-Hours Man-Hours
Operations
Monitor Operations Daily 1.50 550
Control Operations Daily 1 00 365
Sampling Operations Daily .75 270
Analysis Chemist Daily 1 .25 450
Chemical Handling Labor As Required — 250
Makeup Sand Labor As Required — 100
0—Subtotal 1985
Maintenance
Custodial Custodial Daily .50 1 80
Inspection & Testing Maintenance Daily .75 270
Parts Adjust. Maintenance Daily .50 180
Major Overhaul Maintenance Monthly — 400
Control & Elect. Electrician As Required — 600
M — Subtotal 1630
Total 3615
10-12
-------
10.6 References
1. Garber, W.F., “Treatment Plant Equipment and Facilities Maintenance”, Jour. WPCF,
42,No. 10, 1740 (Oct. 1970).
2. CuIp, R.L., “The Operation of Wastewater Treatment Plants”, Public Works, (Oct.
1970)
3. Swanson, CL., “Unit Process Operating & Maintenance Costs for Conventional Waste
Treatment Plants”, presented at the Ohio Water Pollution Control Conference, Dayton,
Ohio (June, 1968).
4. Smith, R., “Cost of Conventional and Advanced Treatment of Wastewaters”, Jour.
WPCF, 40, 1546 (Sept. 1968).
5. Manual of Instruction for Water Treatment Plant Operators , New York State Depart-
ment of Health, Office of Public Health Information.
6. Manual of Instruction for Sewage Treatment Plant Operators , New York State Depart-
ment of Health, Office of Public Health Information.
7. Diaper, E.W.J., “Tertiary Treatment by Micro-Straining”, Water & Sew. Wks., 116,202,
(June 1969). —
10-13
-------
CHAPTEB 11
COST ESTIMATES
11.1 General
In general terms, suspended solids removal processes have an economy of size. In other
words, larger capacity units evince lower treatment costs per unit volume of wastewater
treated. This is due mainly to two factors, decreased capital costs per unit volume and
decreased operating costs owing to more efficient usage of land, personnel and time. Al-
though the actual values of these two factors varies for each process discussed, there is
evidence that the above generalization is valid. Thorough treatises of costs for the suspended
solids removal processes are available elsewhere (1), (2), (3). Most of the materials included
in this text are from those sources. Care must be taken to denote the difference in amortiza-
tion rates and penods and to adjust costs to current levels by use of available cost indices.
Even after these normalizing adjustments are made, most estimates are not directly com-
parable due to differing geographical, administrative and design problems and varying
assumptions. The most prudent use of the data provided is for guideline information on the
relative costs of the processes and the variation in costs which might occur.
11.2 Process Costs
Figure 11-1 illustrates the estimated cost of a chemical clarification system utilizing solids-
contact reactors (1). A recent estimate for two-stage lime clarification, excluding chemicals,
shows costs varying from 11 cents/l000 gallons at I MGD down to 2.7 cents/lOO gallons at
100 MGD (2). Evans and Wilson (3) indicates Tahoe costs for lime treatment, recalcination,
recarbonation and sludge disposal to be 10.8 cents/ 1000 gallons for their 7.5 MGD facility.
An October, 1970, estimate for two-stage lime clarification, recalcrnation, recarbonation
and sludge disposal varies from 25.3 to 5.9 cents/1000 gallons for 1 and 100 MGD, respec-
tively (4).
Capital costs for conventional sedimentation tanks (I) are shown in Figures 11-2 and 11-3.
Barnard and Eckenfelder (5) related the cost of primary sedimentation to surface area using
an overflow rate of 800 gpd/sf:
C = SAi ’ + 6.7
(SA)° 9 5
where.
C = capital cost in dollars
square feet
SA = surface area in 1000
Their estimate for secondary clarifiers (overflow rate = 750 gpd/sf) was:
16.2 + 69
C = SA 1 (SA)’ 3 ç
11—1
-------
Design Capacity, millions of gallons per day
C = Capital Cost, millions of dollars
A = Debt Service, cents per 1000 gallons (4-1/2% - 25 yr.)
O&M = Operating.and Maintenance Cost, cents per 1000 gallons
. T = Total Treatment Cost, cents per 1 000 gallons
Figure 1 1-1. SOLIDS REMOVAL BY COAGULATION & SEDIMENTATION
1 1-2
0
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0 2 4 6 8 10 12 14 16 18 20 22 24
Surfaee Area, Thousands of sq. ft.
Figure 11-2. COST OF PARIMARY SEDIMENTATION TANKS
January 1960 ENR Index = 812
11-3
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L L jILL Ii i Li. U.L • L J LLJL LL14 4
Surface Area, Thousands of Sq. Ft.
Figure 1 1-3. COST OF FINAL SETTLING TANKS
January 1960 ENR Index 812
11-4
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Tube settlers have been estimated to cost from $ 12 to $20/sq ft; with installation costs from
$5 to $ 15/sq ft by one manufacturer (6). Some other manufacturers estimates are shown in
Figure 1 1-4, representing capital cost of purchase and installation in existing basins (7), (8)
Smith and McMichae l (2) estimated the costs of microscreening of activated sludge second-
ary effluent, as shown in Figure 11-5. Some manufacturers’ estimates of capital costs of
microscreening, including installation, but not housing, are depicted in Figure 1 l-6(9),(lQ),
(11), (12). Lynam, et al (13), estimated the total cost of microstraining at 10 MGD capacity
based on their Chicago study to be 2.9 cents/bOO gallons.
One estimate of the cost of filtration through sand at 4 gpm/sq ft is illustrated in Figure
11-7 (2). A more recent estimate for mixed-media filters at the same hydraulic loading varies
from 8.0 to 1.6 cents/ 1000 gallons at I and 100 MGD, respectively (4). Lake Tahoe costs
have been computed at 4.1 cents/ 1000 gallons for mixed-media filters operated at approxi-
mately 5 gpmf sq ft Some estimates of installed costs by manufacturers, excluding the
enclosure facility, are illustrated in Figure 11-8 (6), (7), (8).
The movtng-bed filter capital cost has been estimated by the manufacturer as shown in
Figure 11-9.
11.3 Data Analysis
in order to normalize data presented, some of the necessary indices are presented in Tables
11-1 through 11-3. Table 11-1 shows a series of indices available for data analysis. Tables
11-2 and 11-3 provide monthly tabulations of the two indices which are most applicable for
normalization calculations. Regional indices must also be applied. Standard amortization
tables are available in a number of textbooks and handbooks.
11.4 Cost Estimation and Design
Some of the cost factors involved in the design or redesign of a treatment facility areS cost
of design studies, type and strength of waste to be treated, current and projected population
to be served, sewers, lift stations and transmission lines required, types of processes to be
used, current and projected costs of land, personnel funds and chemicals, maintenance and
service contracts and/or personnel costs, degree of treatment required and anticipated and
cost of equipment and construction. In addition to these more or less concrete values, there
are less predictable, but equally real, factors of political mood, funds, operating personnel
criteria and availability and increasingly stringent effluent criteria.
A complete cost estimate should generally begin with the statement of purpose and scope;
that is, “what is to be accomplished.” Estimated costs of the projected installation should
be broken down into process costs and ancillary facilities, with each unit process cost
evaluation developed separately. All cost estimates should be related to one area and time to
facilitate updating of those costs accordmg to suitable indices. All equipment should be
grouped according to function. Careful consideration should be given to installation cost
factors for each piece of equipment, such as jobsite handling, special installation equipment
or structures, assembly costs, etc. Associated work Items, such as foundations, monitoring
and control instrumentation and chemicals storage must be considered and included. Process
piping and electrical work, i.e., all necessary appurtenances required to interconnect the
vanous unit processes, should be considered separately.
11-5
-------
The cost of enclosing and support buildings and structures cannot be ignored in the overall
and total cost estimate. Cost factors such as lighting, low voltage power supplies, fire and
burglar alarms and additional line power supplies must also be included.
The capital cost estimate should be followed by an equally detailed annual operating esti-
mate if a true overall operating cost picture is to be generated.
Funds should be made available to have professional engineers handle all preliminary, design
and construction phases.
11-6
-------
10
C ,,
,& —
0
1.0
0.1
0.01
1.0 10
MGD
Figure 11-4. CAPITAL COST - TUBE SETTLER
100
11-7
-------
C = Capital Cost, millions of dollars
A = Debt Service, cents per 1000 gallons (4-1/2% - 25 yr.)
O&M= Operating and Maintenance Cost, cents per I 000 gallons
T = Total Treatment Cost, cents per I 000 gallons
Figure 1 1-5. MICROSCREENING OF SECONDARY EFFLUENT
1 1-8
1 A 10
1. J
I
7
I
I
0
0
L)
2
I
4
I
V
2
‘
•
E
Cost Adjusted to March, 1969 E
4
‘H
*iL _ ±
——
-H -+ ’ ...: . . . -.. —. .-. - .__ 4 ...1-. i ••
— — — — — .— -.—. . . 7 r - •— — — — —
1
! uiiw à Lrp
I L. E -I-
A
.
9
‘
j j
— -= -‘ L C — L t L. .-,-.L
:,
H :E :
H-frI _ ±LL_
rt ± +
C!I flIU
± ±±:
I
t
0.10
1.0
=
0
0
0
- _
0
L)
C l
- 0.10
7 I 9 10 0.01
.Lt.
TIIi
1 II
0.01 ‘,
mr
I i4
i ’
iq;
ii
•:.
.:
1
j
1.0
:j.
1
±tiiir. .
JT 11 Tr
‘
4i
j
.L
I
I 7 91O 2 3 4 5
T
10.0
Design Capacity, millions of gallons per day
100.
-------
10
MGD
Figure 11-6. CAPITAL COSTS - MICROSCREENING
0
1.0
0.1
0.01
1 10
11-9
-------
Design Capacity, millions of gallons per day
C = Capital Cost, millions of dollars
A = Debt Service, cents per 1000 gallons (4-1/2% - 25 yr.)
O&M= Operating and Maintenance Cost, cents per 1 000 gallons
T = Total Treatment Cost, cents per 1000 gallons
Figure 1 1-7. FILTRATION THROUGH SAND OR GRADED MEDIA - 4GPM/SQ FT
I 1-10
0
U
0
L)
a)
I
4 : j
S CostAdjusted toMarch, 1969 f irT r : T: Th
2
—t — - -. — —
T t I
LLL4’ W
-- IEE
- -4-k-- - * ---±HH-H — ±+r 4
‘j J. T
-- — - - --- - —.
$ r
2
4j III
$ -
F I* ± ± “- TT
2-- - ——- -—--
:± tr
[ ffi’IR Ti T Tm1
10.
1.0
0.1 1 _
4.0
1.0
c i
0
0
0
0
c i
Al
c i
.01
2
3 4
& $ 7 I 910
1.0 10.0 100.
I I
3
4 66
I 10
-------
10
MGD
Figure 11-8. CAPITAL COST - MULTIMEDIA FILTER
—
c o
1.0
0.1
0.01
1 10 100
11—11
-------
10
LU
0,-.,
c 0
0.1
0.01
0.1 10
MGD
Figure 11-9. CAPITAL COST - MOVING BED FILTER
1.0
11-12
-------
Table Il-I
YEARLY COST INDICES
Marshall & Stevens
installed—equipment
indices
Engineering
News—Record
Construction
index
Handy—Whitman
index for water
treatment plants’
Engineering
News—Record
Building cost
index
Chemical Engineering
plant instruction
index
Year
1926 = 100
1913 = 100
1936 = 100
1913 = 100
1957 — 1959 = 100
Large
Plant
Small
Plant
All
Industry
Process
Industry
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
209
225
229
235
238
237
239
239
242
245
252
263
273
285
294
206
224
228
232
237
236
237
238
241
244
252
256
270
282
292
690
724
759
797
824
847
872
901
936
971
1021
10432
11112
12062
13112
275
288
296
311
317
315
324
330
340
350
368
3802
276
289
296
309
317
315
322
327
336
346
362
2742
491
509
525
548
559
568
580
594
612
627
652
660
695
760
801
94
99
100
102
102
101
102
102
103
104
107
110
114
119
1232
1. Based on July of the year.
2. Based on January of the year.
3. Based on the first quarter of the year.
-------
—
—
—
yEAR
Table 11-2 SEWAGE TREATMENT PLANT CONSTRUCTION COST INDEX
ANNuAL
INDEX
Jan.
Feb.
Mar.
Apr.
May
June
July
Aug.
Sept.
Oct.
Nov.
Dec.
1957
98.04
1958
101.50
1959
103.65
1960
104.96
1961
105.83
1962
107.19
107.20
107.03
106.84
106.99
1963
106.80
107.05
107.08
107.11
107.22
107.78
108.07
108.52
108.58
109.54
109.51
109.60
108.52
1964
109.64
109.45
109.53
109.57
109.70
109.99
110.24
110.54
110.63
110.69
110.73
110.68
110.11
1965
110.82
111.04
111.07
111.12
111.15
111.83
112.31
112.57
112.70
112.82
112.87
113.09
111.95
1966
114.05
114.60
114.77
115.08
115.34
116.05
116.82
116.92
117.11
117.51
117.46
117.48
116.10
1967
117.76
118.08
118.11
118.22
118.34
119.11
119.63
120.28
120.59
120.89
120.91
121.01
119.41
1968
121.10
121.20
121.21
121.55
121.71
122.49
123.39
123.69
124.53
126.80
127.24
127.71
123.55
1969
128.68
129.50
129.84
130.03
130.03
131.11
132.44
135.34
135.46
135.85
136.61
136.86
132.65
1970
137.63
137.87
138.15
138.49
141.18
143.03
146.25
146.70
147.45
148.07
149.28
149.63
143.64
1971
150.60
150.89
153.34
155.41
157.29
Source: Sewer and Sewage Treatment Plant Construction Cost Index, FWPCA, Dept. of Interior, Dec. 1967
-------
Table I 1 3
LABOR COST INDEX
WATER, STEAM, AND SANITARY SYSTEMS NONSUPERVISORY WORKS, S/hr.
YEAR
Jan.
Feb.
Mar.
Apr.
May
June
July
Aug.
Sept.
Oct.
Nov.
Dec.
ANNUAL
AVERAGE
1958
1.96
2.00
1.97
1.96
1.96
2.00
2.00
2.00
2.02
2.03
2.08
2.05
2.00
1959
2.07
2.06
2.05
2.02
2.03
2.04
2.06
2.03
2.09
2.10
2.13
2.09
2.07
1960
2.11
2.12
2.12
2.14
2.13
2.15
2.18
2.18
2.18
2.21
2.23
2.22
2.17
1961
2.25
2.27
2.26
2.27
2.26
2.26
2.29
2.27
2.29
2.29
2.30
2.29
2.27
1962
2.28
2.30
2.30
2.32
2.31
2.33
2.34
2.32
2.34
2.33
2.37
2.36
2.33
1963
2.37
2.38
2.37
2.37
2.35
2.37
2.38
2.38
2.39
2.40
2.42
2.43
2.38
i964
2.42
2.44
2.43
2.44
2.44
2.44
2.44
2.43
2.46
2.45
2.48
2.47
2.44
1965
2.49
2.51
2.50
2.51
2.51
2.53
2.54
2.55
2.56
2.55
2.59
2.58
2.54
1966
2.62
2.65
2.63
2.67
2.66
2.65
2.69
2.67
2.70
2.72
2.74
2.74
2.68
1967
2.76
2.78
2.77
2.79
2.80
2.81
2.83
2.81
2.86
2.86
2.88
2.85
2.82
1968
2.89
2.91
2.91
2.95
2.97
2.98
3.03
3.01
3.05
3.08
3.11
3.00
2.99
1969
3.14
3.13
3.14
3.17
3.17
3.20
3.23
3.26
3.29
3.30
3.38
3.47
3.24
1970
3.47
3.44
3.44
3.46
3.48
3.48
3.52
3.53
3.56
3.58
3.62
3.59
3.51
1971
3.68
3.68
3.64
.
Source: U. S. Department of Labor, Bureau of Labor Statistics, Employment and Earnings Statistics for the
United States , 1909—67 (Bulletin No. 1312—5).
-------
11.5 References
1. Smith, R., “Cost of Conventional and Advanced Treatment of Wastewater”, Jour.
WPCF, 40, 1546 (Sept. 1968).
2. Smith, R., and McMichael, W.F., “Cost and Performance Estimates for Tertiary Waste-
water Treating Processes”, USD1, FWPCA Report No. TWRC-9, (June 1969).
3. Evans, D.R., and Wilson, J.C., “Actual Capital and Operating Costs for Advanced Waste
Treatment”, Paper presented at WPCF 43rd Annual Conference, Boston, Mass. (Oct.
1970).
4. Roesler, J.F., Private Communication.
5. Barnard, J.L., and Eckenfelder, W.W., Jr., “Treatment Cost Relationships for Organic
Industrial Wastes”, Paper presented at 5th International Water Pollution Research Con-
ference, San Francisco, California (July 1 970)
6. Neptune - Microfloc, Inc., Corvallis, Oregon.
7. Permutit Co., Paramus, N.J.
8. Graver Water Conditioning Company, Union, N.J.
9. Crane Company - Cochrane Division, King of Prussia, Pa.
10. Marske, D.M., “High-Rate. Fine Mesh Screening of Combined Wastewater Overflows”,
Jour. WPCF, 42, 1476 (Aug. 1970).
11. Walker Process Equipment, Aurora, Illinois.
12. Zurn Industries, Inc., Water and Waste Treatment Div., Erie, Pa.
13. Lynam, B., Ettelt, G., and McAloon, T., “Tertiary Treatment at Metro Chicago by
Means of Rapid Sand Filtration and Microstrainers”, Jour. WPCF, 41, 247 (Feb. 1969).
14. Johns-Manville, New York, N.Y.
11-17
-------
APPENDIX A
GLOSSARY
Adsorption — The taking up of a gas, vapor or dissolved material on the surface of a solid.
Aerate — To impregnate or saturate a liquid with air.
Aerobic — In the presence of oxygen.
Aggregate — Particles brought together; agglomeration.
Angle of Repose — That angle at which a material will just rest upon a surface without
internal shearing.
Alum — Aluminum Sulfate — Al 2 (SO 4 ) X H 2 0
Alumina — Aluminum oxide (A 1 2 0 3 )
Anaerobic — In the absence of oxygen.
Anhydrous — Dry or devoid of water.
Baume’ Scale — A measure of the density of a solution heavier than water; Degrees Baume’
‘°B = 145- 145
Specific Gravity
Biochemical Oxygen Demand — The amount of oxygen required for the biological oxidation
of the organic matter in a liquid over a stated period of time.
Brownian Moi.’ement — The random motion of colloidally dispersed particles as a result of
collisions with the molecules of the dispersing medium.
Chemical Oxygen Demand — The amount of oxygen required for the chemical oxidation of
organics in a liquid.
Coagulation — Process by which chemicals (coagulants) are added to an aqueous system for
the purpose of creating rapid-settling aggregates out of finely divided, dispersed matter with
slow or negligible settling velocities. Forces causing particles to repel each other are neutral-
ized by the coagulants.
Colloid — A dispersion of very small (I m p - 0.5 i i) particles suspended in a fluid.
Contact Clarification — A process consisting of the rapid mixing of chemically-treated waste
with previously settled floc, the settled floc constituting a surface for subsequent floc
formation.
A-I
-------
APPENDIX A (Continued)
GLOSSARY
Detention Time — The average period of time that a fluid element is held in a basin or tank
before discharge.
Dolomitic Lime — Calcium oxide plus variable quantities of magnesium oxide, typically
34-40% MgO.
Dual Media Filtration — A filtration process that uses a bed composed of two distinctly
different substances, such as anthracite coal and sand, as opposed to conventional filtration
through sand only.
Floc — An agglomeration of finely-divided or colloidal particles due to certain chemical-
physical or biological operations
Flocculation — A gentle agitation of waste water for a period of time so as to increase the
number of collisions between the smaller floc particles, causing them to stick together and
grow into large readily-settleable masses.
Heat of Solution — The amount of heat liberated or absorbed from dissolving a chemical in
water or other solvent.
Hydrated Lime — Calcium Hydroxide. Ca(OH) 2
Hydrophilic — “Water loving”; having a high affinity for water.
Hydrophobic — “Water hating”, having a low affinity for water
Hygroscopic — A solid with the ability to absorb moisture from the air without eventual
dissolution in that moisture.
Hyperfiltrarion — A high-pressure reverse osmosis process using a membrane which will
remove dissolved salts as well as suspended solids.
Ions — An atom or group of atoms with an unbalanced electrostatic charge.
Microscreening — A form of surface filtration using specially woven wire fabrics mounted on
the periphery of a revolving drum.
Millimicron — I O centimeters
Mixed Liquor Suspended Solids — (MLSS) - The concentration of suspended solids carried
in the aeration basin of an activated sludge process.
pH — Measure of acidity or basicity, expressed as the negative logarithm of the hydrogen ion
concentration.
A-2
-------
APPENDIX A (Continued)
GLOSSARY
Polye lectrolytes — Chemicals that consist of high molecular weight molecules with many
reactive groups situated along the length of the chain. Polyelectrolytes react with the fine
particles in the waste and assist in bringing them together into larger and heavier masses for
settling.
Precoat — The initial coating of the septum in a diatomaceous earth filter to provide the
initial straining medium.
Quicklime — Calcium oxide (CaO) or calcium oxide in natural associ tion with a lesser
amount of magnesium oxide.
Recalcination — A process or recovering lime for reuse by means of heating spent lime
sludge to high temperatures, thereby driving off water of hydration and carbon dioxide.
Recarbonation — The addition of carbon dioxide to lime-treated water to effect a down-
ward adjustment of the pH of the waste for further calcium removal and/or stabilization of
the water.
Soda Ash — Sodium carbonate Na 2 CO 3
Sludge Volume Index — The volume in milliliters of one gram of activated sludge in the
mixed liquor which has settled for thirty minutes; an indication of the settling characteris-
tics of the secondary sludge.
Specific Gravity — The ratio between the weight of a substance and the weight of an equal
volume of water at any stated temperature.
Turbidity — Optical measure of suspended matter in water.
Unit Process — A process which can be clearly distinguished from other processes in a
treatment system.
Weir Loading — An index of settlingbasin effluent hydrauhcs that is determined by dividing
the total daily flow by the total weir length.
A-3
-------
ACKNOWLEDGMENT
This manual was prepared by Burns and Roe, Inc. under the sponsorship of the
Environmental Protection Agency. The technical guidance and assistance of the
Environmental Protection Agency staff during the preparation of the manual is gratefully
acknowledged.
-------
CHAPTER 12
PLAN AND SPECIFICATIDN REVIEW
C l i EC K SHE El
12.1 General
The processes outlined in this design manual have been developed for suspended solids re-
moval from wastewaters. In some cases, these processes have had limited use in full-scale
design of wastewater treatment facilities. Design engineers and reviewing authorities may
not be completely familiar with the parameters to be considered in the preparation of plans
and specifications for a given project. The following plan and specification review check
sheet has been prepared to serve as a guide to the engineer in the design of proposed
facilities which use the process or processes outlined in this manual. It will also be used by
the Environmental Protection Agency, and may be used by State and local authorities in
their review of projects for approval.
As with the case of any check sheet, its purpose is to fully consider all possible parameters
for an individual process. For a given project all or part of the check sheet may be applicable
and should be used with this fact in mind.
12-1
-------
12.2 Plan Review Data Sheet for Suspended Solids Removal
Applicant_________________________________ Project No.
Design Engineer Address..........
Phone _____________
Review Engineer I st Review Date _____
Return for Revision ____________________________ Revision Receipt Date
Revision Approval Date
Date of State Approva’
Type of Work
Addition to Existing Facility Primary 0
New Facility Secondary Li
Tertiary 0
In the space provided below furnish a simplified flow diagram or a written description of the
plant units in the flow sequence. Include the method of sludge handling and ultimate
disposal.
12-2
-------
Sedimentation
No. tanks Shape of tanks ______________
Dimensions. Length Width ________ Diameter Depth -
Vol. each tank _______ cu ft Total vol ________________ cu ft
Ave. f1ow Present __________MGD; Design ________ MGD
Detention time. Present _________ ; Design
Overflow rate Present MGD/ft 2 Design gpd/ft 2
Length of weir __________________ Location
Weir loading Present gpd/ft, Design gpd/ft
Scum removal Method Disposition
Sludge removal Method _______________ Disposition
Sketch — Plan and/or section of tank
12-3
-------
High-Rate Settler
Type Tubes El Plates El
Unit in which installed __________________________________________________
Area of settling tank _________________________________________________________ sq ft
Surface area of high-rate settler __________________________________________________ sq ft
Length of tube or plate.
Sketch opening shape and dimensions: ___________________________________
Angle of inclination from horizontal: ____________________________________
Direction of feed versus sludge flow: El Cocurrent
El Countercurrent
Material of tube or plate:
Rate of flow _______________ gpm/sq ft (end area)
Volume throughput of high-rate installation _______ MGD
Chemical Clarification
a) Chemical Addition
Chemicals used: a. , b. _______ , c. _______
Chemical storage area: sq ft
Chemical storage volume: _______________ cu ft
a b c
Chemical day tanks: Material ________ ________ _________
Volume _______ _______ _______
Feed equipment: Manufacturer, size a ______________ gpd/ppd
and capacity b gpd/ppd
c gpd/ppd
Point of application: a __________________
b ____________________
C ______________________
12-4
-------
b) Mixing of Chcmicals and Wastcwater
Mechanical describe size and capacity
Hydraulic.
Location of mixing areaS Pipe
Tanks__________________________________________
Supplemental tank_________________________________
Tank size. Length _____Width _____ Diameter ____ Depth _____
c) Flocculation
Type of chamber: Baffle
Mechanical
Dimensions of tank. Diameter_____ Length ____Width ____ Depth
Detention time for floc growth
Number of baffles. V/H
Number of mixers
Type of mixer HP ____________________
Solids-Contact Clarifier
Number of tanks _____________ , Shape of tanks ________________________________
Tank construction
Dimension: Length Width _______Diameter ______ Depth
Volume each tank _______cu ft Total volume _______cu ft
Average flow/tank. Present MGD; Design ________ MGD
Average detention time. _________ , Max. ________ , Mm. ________
Overflow rate: _________________ gpd/sq. ft.
12-5
-------
Chemical feed application
Mixing and flocculating: Volume ___________________ cu ft
Mixer ____________ HP
Detention time ____________________________
Clarification area/volume: _________________ __________________ cu ft
Detention time
Solids recirculation
Sludge collector ______________________ ; Sludge thickener __________________
Head available: _______________________ ft
Maximum headloss ___________________
Flow inlet: _________in dia Flow outlet. __________ in dia
Collector trough (weir loadings): gpd/ft
Flow control
Microscreens
Number of units __________ ; Self-contained _______ ; Concrete structure —
Capacity of unit. MGD, Total capacity: MGD
Screen size _______________ ; Number of screen elements ____________________
Overall size of screen: Length , Diameter _______________________
Installation dimensions of single unit: Length Width Height
Flow rate _______ gpm/sq ft of submerged area
Drive motor ____________ HP ___________ Variable speed
Wash pumpS gpm; UP _______________________________
Waste water return pump (variable speed): _______ gpm HP __________
12-6
-------
Screening: ______________ ; Opening size in microns —
Valved overflow line Size _________________ Material.
Material of constructioiv __________________ Screen _______ ; Bearings
Pipe ; Backup , Waste collector —
Screen housing ; Shaft ________________ ; Splash guards
Inlet weir ; Outlet _______________________________________
Nozzles. No. ; Size _____________________________________
Instrumentation
Head differential indicator _____________________________________________
Level control indicator _________________________________________________
Automatic flow control Speed
Backwash
Recorder
High level alarm
Corrosion control and coating
Moving Bed Filters
Number of units 2 bed configuration
_________ 4 bed configuration
Total number of units ______________
Total flow filtering area. ____________ sq ft ___________________________________________
Flow/unit: gpm , Total flow: _________ gpm
Sand filtering media: ______________ e.s. , u.c. ____________________
Filter bed size: ____________________ Dia. ______________ ; Length
Tank material
Discharge screen material ___________________________________________________________
12-7
-------
Overall module dimension Length ________Width ________ Height
Head tank ___________ Dia. ________; Depth
Residence timeS _______ minutes _______________
Headloss ; Feed control ____________________
Level control
Sludge settling tank Diameter ; Depth
Detention time. _____ minutes
Volume. __________ cu ft
Collection equipment , Disposition
Chemical systemS Coagulant; Polyelectrolyte
Storage tank dia depth, material __________________________________________________________
Day tank dia depth, material
Feeders Rate and control ___________________________________________________________
Mixers
Sand washer tank: Dia ________ : Depth _______ ; Volume ______________
Air: cfm, Water. gpm
psi psi
Materials of construction
Tanks
Hopper bottoms
Diaphragm -
Sand educators
Pipe
12-8
-------
Pumps and motors Type: — Material; HP and capacity
Sand movement
Cutter _________
Sand cleaning
Screen wash
Chem. pump and mixer
Air compressor
Operational controls _____
Filters
Type Pressure ______________ ; Gravity
Dual Media ___________ ; Multi Media ________ ; Graded Sand _________________
Number of filters
Dimensions: Length ; Width/Diameter ; Depth
Volume each filter: _______cu ft Total volume: _________ cu ft
Average flow/unit. gpm
Total flow: ___________ gpm; gpd
Strainers ______________________ ; Retaining Plate ________________________
Sand _______________in. depth: Size __________ e.s. ____________ u.c.
Gravel _____________ in. depth; Size
Anthracite ___________in. depth; Size ___________ e.s ____________ u.c.
Garnet _____________ in. depth; Size ___________ e.s. ____________ u.c.
Control valves
Level control
12-9
-------
Chemicals: Coag. ; Poly.
Day tank: _________ Dia. _____ Depth _____ Dia. ______ Depth
Feeding equipment
Pt. of application
Backwash operation:
Surface wash _______ Air __________________
Backwash rate:
Collection of waste water
Point of waste water disposal
1 2-10
-------
. J Acce ,ion Number J 2 Subject Field & Group
SELECTED WATER RESOURCES A STRACT5
I 05D INPUT TRANSACTION FORM
J Organ izaUcu
Environmental Protection Agency
J Title
PROCESS DESIGN MANUAL FOR SUSPENDED SOLIDS REMOVAL
10 Author(&)
Burns and Roe, inc.
ii I Project Designation
17030 GNO
Note
Available from Technology Transfer
appropriate E.P A. Regional Office
22 J Citation
23_j Dc ’sr rip ora (Storred First.)
- Wastewater tieatment, *Treatnlent facilities, *Tertiary treatment, Chemical precipitation, Coagulation,
Diatomaceous earth, Filtration, Flocculation, Flotation, Membrane processes, Settling basins, Solids
contact process
25 :f ,e (Starrcd First) - - -
‘Design considerations, *Operation requirements. *Cost esti mates chemicals, chemical feeding,
high-rate settlers. microscreening
This manual comprises a compilation of information on the practice of suspended solids
removal from municipal wastewaters General enginecnng considerations are cited with
respect to their impact on the design of treatment facilities. Specific processes utilized foi
suspended solids removal are descnbed, discussed and illustrated through the use of data
from installations which have employed these processes Current technology and advanced
methods of treatment are stressed in order to provide usable information for
implementation in design of new treatment facilities. Some aspects of the operation and
maintenance requirements of the described unit processes are delineated, along with the
overall estimated costs of construction and operation. The information and data provided
are presented in such a manner that they can be readily incorporated into practice.
Ah fr. , br
James E ,J(r&issl_
I’. . JUL( l W
w .
— I Robert A. Taft Wa ter Research Center _____
scu. I ,, :o’- cr ooc.u, crir. . • .OLJ CES sc -ICNTIrIc NF FfliArlUN CL r R
I 5 L PA 1 1MLNT OF 1 1C Ni I fllQ
W1.S ING1 )N. D.C. .10240
* U S GOVERNMENT PRfl Tfl G OFFICE 1971 0 - 441-343
------- |