EPA-600/2-79-096
May 1979
EVALUATION OF FLOW EQUALIZATION
IN
MUNICIPAL WASTEWATER TREATMENT
by
J. E. Ongerth
Brown and Caldwell, Incorporated
Seattle, Washington 98119
Contract No. 68-03-2512
Project Officer
Franci s Evans, I I I
Wastewater Research Division
Municipal Environmental Research Laboratory
Cincinnati, Ohio 45268
MUNICIPAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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DISCLAIMER
This report has been reviewed by the U. S. Environmental
Protection Agency, Municipal Environmental Research Laboratory,
and approved for publication. Approval does not signify that
the contents necessarily reflect the views and policies of the
U. S. Environmental Protection Agency, nor does mention of trade
names or commercial products constitute endorsement or recommen-
dation for use.
For sale by tho Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20102
Stock Number 055-002-00170-0
ii
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FOREWORD
The Environmental Protection Agency was created because of increasing
public and government concern about the dangers of pollution to the health
and welfare of the American people. Noxious air, foul water, and spoiled
land are tragic, testimony to the deterioration of our natural environment.
The complexity of that environment and the interplay between its components
require a concentrated and integrated attack on the 'problem.
Research and development is that necessary first step in problem
solution and i.t involves defining the problem, measuring its impact,
and searching for solutions. The Municipal Environmental Research
Laboratory develops new and improved technology and systems for the pre-
vention, treatment, and management of wastewater and solid and hazardous
waste pollutant discharges from municipal and community sources, for the
preservation and treatment of public drinking water supplies, and to t
minimize the adverse economic, social, health, and aesthetic effects of
pollution. This publication is one of the products of that research;
a most vital communications link between the researcher and the user
community.
Variations in flow rate and composition are characteristics of
wastewaters treated at municipal treatment facilities. Improved efficiency,
reliability, and control of various physical, chemical and biological
treatment processes are believed possible at or near constant plant
conditions. This publication presents data gathered.from all sewage treat-
ment plants having equalization facilities that could be located throughout
the United States and analyses of plant operations are made. Analysis
proc dures and design principles are presented to cover the spectrum of
cor.-:,:ions to which equalization may be applicable.
Francis T. Mayo, Director
Municipal Environmental Research
Laboratory
ii i
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EXECUTIVE SUMMARY
The executive summary presents a detailed overview of the
major segments of the manual including purpose and scope, pro-
cedures, supporting data, and major recommendations.
Section Contents Description
• Section 1: Introduction; establishes the purpose and
intent of the manual. Basic problems are defined and
included with major problem subcategories. The purpose
of providing the manual is identified, and the scope of
subject material coverage is established.
• Section 2: Design and Operation Practices Recommenda-
tions; typical wastewater system situations requiring
evaluation for applicability of equalization are de-
scribed. Conditions favoring applicability of equaliza-
tion are summarized according to magnitude of input
variations, characteristics of sewer systems, size and
type of treatment facilities, etc. Recommendations are
made concerning size, type, location, and required appur-
tenances of equalization facilities appropriate for
typical sewerage and treatment system configurations.
• Section 3: Quantitative Methodology; presents and sum-
marizes established procedures for sizing equalization
facilities as a function of typical or critical influent
variations. Quantitative comparison is made between
flow and concentration smoothing afforded by in-line and
side-line equalization configurations. Methodology is
presented for evaluating collection system flow vari-
ations to determine when equalization is appropriate and
when collection system improvements are dictated. Pro-
cedures are outlined for using treatment plant operating
data to establish effects of equalization on unit process
and overall treatment plant performance.
• Section 4: Facilities Summary; contains results of the
nationwide survey of equalization facilities. Data pre-
sented define the characteristics of existing equaliza-
tion facilities and the nature of applications in terms
of basic sewerage system and treatment system features.
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* Section 5: Performance Evaluation; presents all avail-
able information collected from the literature, past
studies not published, and data from operating treatment
facilities that could be used to identify effects of
flow equalization on individual treatment processes and
overall treatment plant performance. Criteria for
selecting and evaluating operating data used are summa-
rized. Detailed data on the performance of individual
plants is presented and analyzed. General operating
performance of plants with equalization is compared to
plants not having equalization. Theoretical effects of
influent variations on unit process and treatment system
performance are summarized as a guide to evaluating
equalization performance.
• Section 6: Equalization Cost; unit costs of basic
equalization facility types and conventional appurte-
nances are presented as developed from the national
survey and Brown and Caldwell design files. Capital
costs and costs for ope'ration and maintenance are pro-
vided. Examples are given to illustrate cost comparison
of treatment facilities designed with and without equal-
ization. ./...'•
This report is submitted in partial fulfillment of Contract
No.^ 68-03-2512 by Brown and Caldwell, Inc., under the sponsor-
ship of the U.S. Environmental Protection Agency. This report
covers the period March - 7, 1977, to September 7, 1977.
v
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CONTENTS
Disclaimer ii
Foreword iii
Executive Summary . • • • • • iv
Figures ix
Tables xv
Acknowledgments xvii
1. Introduction 1
Background and Purpose 1
Background 1
Basic Problem Categories 2
Equalization Design Requirements .... 3
Purpose of the Manual 9
Scope of the Manual 9
Coverage of Subject Material 9
Information Sources Used ........ 10
Guide to the User 10
Table of Contents 10
Executive Summary 11
Section Contents Description 11
2. Conclusions and Recommendations 12
3. Quantitative Methods 14
Determination of Equalization Applicability . 14
Facility Planning 14
Design Flow Peaking Considerations ... 16
Stormflow Dominance Determination .... 20
Analysis of Storm Dominated System ... 24
Equalization Sizing Methods 26
Method 1—Simple Flow Balance 27
Method 2—Simple Concentration Balance . 37
Method 3--Combined Flow and Concentra-
tion Balance . 38
Method 4--Sine Wave Method 49
Method 5—Rectangular Wave Method .... 50
Specialized Analytical Methods ..... 52
Measurement and Evaluation of Equalization
Effects 53
Introduction 53
Performance Evaluation 53
Performance Requirements ........ 58
References 60
Vll
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4. Summary of Existing Flow Equalization
Facilities 62
Equalization Facilities Survey 62
Survey Scope 62
Survey Response 86
Equalization Facilities Summary 86
Treatment Plant Characteristics 86
Equalization Facility Characteristics . . 90
Equaliation Facilities Construction Costs . . 95
Construction Costs 95
Operation and Maintenance Costs 96
5. Equalization Performance Evaluation 107
Case Histories 107
Introduction 107
Activated Sludge Plants 108
Trickling Filters 167
Oxidation Ditch Plants 173
General Performance Observations .... 176
Equalized Versus Unequalized Plant
Performance 179
Activated Sludge Plants 179
Trickling Filter Plants 190
Effects of Equalization on Unit Process and
Treatment System Performance 190
Significant Flow Equalization Benefits . 197
References 211
6. The Development of Costs for Equalization and
Treatment Processes 213
Basis of Unit Cost Development 213
Construction Costs 214
Holding Tank and Basin Costs 215
Wastewater Pumping Costs 218
Aeration and Mixing Costs 218
Washdown and Solids Removal Costs .... 221
Additional Capital Costs 221
Operation and Maintenance Costs 221
Comparison of Equalization Costs and Treat-
ment Plant Peaking Capacity Costs 224
References 233
Vlll
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FIGURES
Number Page
1(a) Schematic flow diagram of equalization facilities:
in-line equalization 6
l(b) Schematic flow diagram of equalization facilities:
side-line equalization
Dependence of extreme flow ratios in municipal
3
4
5
6
7
8
9
10
11
12
13
14
15
Peaking effects on combined sewered areas
Daily peak flow monthly distribution
Wastewater flow variation before equalisation . . .
Flow and concentration iteration schematics ....
Wastewater flow and BOD variation before equali-
Examples 4 and 5 , flow and concentration balance . .
j
Example 6, excess volume for load equalization . . .
Equalization volume estimation by sine wave
Distribution of log daily average influent TSS con-
centrations, Ypsilanti Township, Plant 1, 1974 . .
Range difference for distributions of equal slope
_j_ /
19
21
23
28
29
30
40
43
46
48
51
56
56
IX
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16 Distribution of log effluent concentrations for
activated sludge plants designed to meet EPA
secondary treatment requirements 59
17 Flow equalization facility locations 87
18 Equalization facilities construction cost as a
function of equalization capacity . 100
19 Operation and maintenance cost as a function of
equalization volume 105
20 Operation and maintenance cost of equalization facil-
ities as a function of treatment plant capacity . 106
21 Distribution of weekly TSS concentrations, EPA
in-house study (P/A =1.5) Ill
22 Distribution of weekly BOD5 concentrations, EPA
in-house study (P/A = 1.5) 112
23 Distribution of weekly TSS concentrations, EPA
in-house study (P/A =2.0) 113
24 Distribution of weekly BOD5 concentrations, EPA
in-house study (P/A =2.0) 114
25 Distribution of weekly TSS concentrations, EPA
in-house study (P/A =2.5) 115
26 Distribution of weekly BOD^ concentrations, EPA
in-house study (P/A =2.5) 116
27 Distribution of 8-day TSS concentrations, Walled
Lake, Michigan/Novi, Michigan, 1974 119
28 Distribution of 8-day BOD5 concentrations, Walled
Lake, Michigan/Novi, Michigan, 1974 120
29 Distribution of TSS concentrations, Walled Lake,
Michigan/Novi, Michigan, 1974 (8-day) and
1976-1977 121
30 Distribution of BOD5 concentrations, Walled Lake,
Michigan/Novi, Michigan, 1974 (8-day) and
1976-1977 122
31 Distribution of 8-day TSS loads, Walled Lake,
Michigan/Novi, Michigan, 1974 123
32 Distribution of 8-day BOD5 loads, Walled Lake,
Michigan/Novi, Michigan, 1974 124
x
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33 Equalization tank, 1 million gallon capacity,
Tecumseh, Michigan - 126
34 Distribution of TSS concentrations, Tecumseh,
Michigan, 1970 and 1973 127
35 Distribution of 3005 concentrations, Tecumseh,
Michigan, 1970 and 1973 128
36 Distribution of TSS concentrations, Tecumseh,
Michigan, 1970 and 1976 129
37 Distribution of BOD5 concentrations, Tecumseh,
Michigan, 1970 and 1976 ............. 130
38 Distribution of TSS loads, Tecumseh, Michigan,
1970 and 1973 131
39 Distribution of BOD5 loads, Tecumseh, Michigan,
1970 and 1973 132
40 Distribution of TSS loads, Tecumseh, Michigan,
1970 and 1976 133
41 Distribution of BOD5 loads, Tecumseh, Michigan,
1970 and 1976 134
42 Distribution of TSS loads, Tecumseh, Michigan,
1973 and 1976 135
43 Distribution of BODs loads, Tecumseh, Michigan,
1973 and 1976 136
44 Distribution of TSS concentrations, Ypsilanti
Township, Michigan, 1974-1975 140
45 Distribution of BOD5 concentrations, Ypsilanti
Township, Michigan, 1974-1975 141
46 Distribution of TSS loads, Ypsilanti Township,
Michigan, 1974-1975 142
47 Distribution of BOD5 loads, Ypsilanti Township,
Michigan, 1974-1975 143
48 Distribution of TSS concentrations, Ypsilanti
Township, Michigan, 1976 144
49 Distribution of BOD5 concentrations, Ypsilanti
Township, Michigan, 1976 ..... 145
XI
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50 Equalization tank (background), 3 million gallon
capacity, Pontiac, Michigan 146
51 Distribution of TSS concentrations, Pontiac,
Michigan, East Boulevard Plant, 1973 and 1976-
1977 148
52 Distribution of BOD^ concentrations, Pontiac,
Michigan, East Boulevard Plant, 1973 and 1976-
1977 149
53 Distribution of TSS concentrations, Pontiac,
Michigan, Auburn Plant, 1973 and 1976-1977 .... 151
54 Distribution of BODc concentrations, Pontiac,
Michigan, Auburn Plant, 1973 and 1976-1977 .... 152
55 Equalization basin, 3 million gallon capacity,
Amarillo, Texas 153
56 Distribution of monthly TSS concentrations,
Amarillo, Texas, .1964 and 1966 ........... 154
57 Distribution of monthly 6005 concentrations,
Amarillo, Texas, 1964-1966 155
58 Distribution of monthly TSS loads, Amarillo,
Texas, 1964 and 1966 156
59 Distribution of monthly BODc loads, Amarillo,
Texas, 1964 and 1966 157
60 Equalization tank, 50 million gallon capacity,
Warren, Michigan 158
61 Distribution of TSS concentrations, Warren,
Michigan/Renton, Washington 160
62 Distribution of 6005 concentrations, Warren,
Michigan/Renton, Washington 161
63(a) Primary effluent BOD concentration distribution,
Newark, NY 164
63(b) Primary effluent TSS concentration distribution,
Newark, NY 165
64 Primary sedimentation removal efficiency distri-
bution, Newark, NY 166
65 Distribution of TSS concentrations, Palmyra,
New Jersey 168
xii
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66 Distribution of BOD5 concentrations, Palmyra,
New Jersey 169
67 Distribution of daily TSS concentrations, Midland,
Michigan/Bay City, Michigan 171
68 Distribution of daily BOD5 concentrations,
Midland, Michigan/Bay City, Michigan . 172
69 Distribution of monthly TSS concentrations,
Arlington, Washington/Dawson, Minnesota 174
70 Distribution of monthly 6005 concentrations,
Arlington, Washington/Dawson, Minnesota 175
71 Secondary and filter effluent TSS concentration
distributions, four equalized flow plants . .-. . 180'
72 Secondary and filter effluent BOD concentration
distributions, four equalized flow plants . . . .181
73 Distributions of average annual effluent TSS
concentrations, 31 equalized versus 46 unequalized
activated sludge plants 188
74 Distributions of average annual effluent BOD
concentrations, 31 equalized versus 46 unequalized
activated sludge plants 189
75 Distributions of average annual TSS concentrations,
8 equalized versus 65 unequalized trickling
filter plants 192
76 Distributions of average annual BOD concentrations,
8 equalized versus 65 unequalized trickling
filter plants 193
77 Classification of responses to loading variations
by wastewater treatment processes . . . . . . . . 199
78 Estimate on limits on soluble organics removal in
activated sludge . ..... 201
79 BOD removal as a function of organic loading for
activated sludge modifications 205
80 Effect of hydraulic loading on stone media trick-
ling filter BOD removal ...... 206
81 Effqct of hydraulic loading on plastic media trick-
ling filter BOD removal (35) 207
Kill
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82 Sedimentation tank effluent TSS versus solids
loading - normal hydraulic loadings 209
83 Construction cost as a function of concrete
holding tank size 216
84 Construction cost as a function of concrete lined
earthen basin volume 217
85 Capital cost as a function of pump station
capacity 219
86 Surface aerator capital cost as a function of
rated capacity 220
87 Equalization basin washdown system details 222
88 Equalization basin washdown system capital cost
as a function of system capacity 223
89 Equalization facility operation and maintenance
costs as a function of flow equalization facility
volume 225
90 Treatment systems used in equalized vs. unequalized
facilities cost comparison 227
91 Regions of cost effective equalization for cost
example, treatment system 5 232
92 Regions of cost effective equalization for cost
example, treatment system 9 232
xiv
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TABLES
Number Page
1 Example 2—Tabular Method 32
2 Example 3—5-Year Storm Effect ........... 35
3 Example 3—Tabular Method, First Case 35
4 Example 3—Tabular Method, Second Case 36
5 Beginning Flow and Concentration Iterations:
Example 4, In-Line Basin 45
6 Beginning Flow and Concentration Iterations:
Example 5, Side-Line Basin 45
7 Equalization Facilities Summary: " Plants Smaller
than 1 mgd—Treatment Plant Characteristics ... 64
8 Equalization Facilities Summary: Plants Smaller
9
10
11
12
than 1 mgd — Equalization System Characteristics. .
Equalization Facilities Summary: Plants Larger
than 1 mgd — Treatment Plant Characteristics ...
Equalization Facilities Summary: Plants Larger than
1 mgd — Equalization Facility Characteristics . . .
Plants with Equalization Information Incomplete . .
Construction Cost Data for Flow Equalization
Facilities
67
71
76
84
97
13" Operation and Maintenance Costs for Equalization
Tanks . 101
14 Operation and Maintenance Costs for Equalization
Basins 103
15 Treatment Facilities for Performance Evaluation . . 109
xv
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16 EPA Inhouse Study: Peak-to-Average (P/A) Flow
and Load Ratios 110
17 Effluent Discharge Requirements for Walled Lake-
Novi Plant 117
18 Mean BOD and TSS Load and Concentration Summary . . . 137
19 Effects of Equalization on Performance of Activated
Sludge Plants 177
20 Effects of Equalization on Variability of Activated
Sludge Plants 178
21 Activated Sludge Plant Performance 182
22 Trickling Filter Plant Performance 191
23 Summary of Alternatives in Improving Existing
Treatment Plant Performance 194
24a Equalization Cost Effectiveness Example 228
24b Equalization Cost .Effectiveness Example 229
24c Equalization Cost Effectiveness Example 230
xvi
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ACKNOWLEDGMENTS
This project was conducted by the staff of Brown and
Caldwell, Inc., Seattle, Washington, with important contri-
butions by:
J. Warburton, project manager
J. E. Ongerth, project engineer
R. V. Hermes, assistant project engineer
C. N. Anderson, assistant project engineer
E. Amoo, assistant project engineer
D. T. Merrill, technical consultant
M. S. Merrill, technical consultant
R. W. Stone, technical consultant
Report production was managed by Linda Henry with editing
by J. G. Dally, and the indispensable assistance of Shirley
Wilcox and H. R. E. Spouse.
\
The cooperation of sewage treatment plant operating per-
sonnel, city engineers, and consultants associated with treat-
ment plants listed in Tables 12 and 13 who provided information
and data summarized in Sections 3, 4 and 5 were instrumental to
the success and completeness of the study. Paul Blakeslee of
the Michigan Department of Natural Resources was most helpful
in locating pertinent treatment plant operating records.
The direction of study efforts provided by B. W. Lykins,
Jr., J. M. Smith, and F. L. Evans III of the U. S. Environmental
Protection Agency, Municipal Environmental Research Laboratory,
is gratefully appreciated.
xvi i
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SECTION 1
INTRODUCTION
BACKGROUND AND PURPOSE
Background
Equalization as the term is applied to wastewater treatment,
refers to facilities and procedures for minimizing variations in
the flow rate and composition of wastewater processed at munic-
ipal treatment facilities.
Variations occur characteristically in domestic wastewater
flow rate and composition as a result of cyclic activities of
the human population. Additional variations are commonly im-
posed by a combination of (1) random and cyclic activities in
the collective industrial-wastewater-generating segment of the
community and (2) by storm-related effects of infiltration and
inflow. In addition, the average wastewater flow rate at
typical municipal treatment plants may be expected to increase
by 25 to 100 percent or more over the design life of the facil-
ities. These variations and resulting problems are accepted in
wastewater treatment, and the vast majority of municipal treat-
ment plants today routinely operate under such conditions.
Operation of wastewater treatment plants at or near con-
stant conditions is commonly assumed to be advantageous. Im-
proved efficiency, reliability, and control of various physical,
chemical and biological treatment processes are.believed possible
under such conditions. Cost savings are assumed to result from
elimination of excessive peak treatment capacity and from re-
duced periods of operation under peaking conditions. Examples
supporting these assumptions are widespread in chemical process
industries and in water treatment where constant operating con-
ditions are maintained routinely and permit process optimization.
Equalization is not a new idea. Industrial applications
of equalization using lagoons, in-line sewer capacity and tanks
were discussed in early work by King, 1942 (1); Rudolfs, 1943
(2), 1946 (3); and Gurnham, 1955 (4), respectively. However,
municipal applications of equalization received little attention
until the advent of the Federal Water Pollution Control Act
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amendments of 1972. The Act has resulted in stricter water
quality standards requiring more extensive, more sophisticated,
and more reliable wastewater treatment. This in turn has con-
tributed to significant interest in the potential of flow equal-
ization for improving performance and reducing capital and oper-
ating costs at municipal wastewater treatment plants.
Basic Problem Categories
Equalization Definition—
For the purposes of this study, equalization will be de-
fined as any facilities and procedures for minimizing variations
in the flow through treatment plants, as long as the total flow
is ultimately processed through the treatment plant. Consider-
ation is limited to municipal wastewater treatment facilities.
The term "equalization" is applied most commonly to minimizing
diurnal variation at wastewater treatment plants regardless of
the source of variation. Minimizing variations during storm-
influenced periods only is commonly referred to as storm flow
retention, or as combined sewer overflow control where variations
result from the existence of combined sewers. These practices
will also be considered as equalization as long as storm flows
are ultimately processed by the normal treatment system, and not
bypassed receiving only partial treatment before disinfection
and discharge. Procedures for identifying storm flow character-
istics that can be accommodated by equalization are detailed in
Section 2. Temporary storage of industrial waste, or other
occasional sludge discharges that would be detrimental to normal
treatment, is also considered as equalization where the stored
flow is ultimately processed by the normal treatment facilities.
Applications of this type are common in communities with appre-
ciable manufacturing or food processing industry for example.
Equalization Applications—
Equalization may be applied either as a means of upgrading
existing facilities, or as an integral component of entirely
new facilities. Planning and design considerations for the
respective applications differ markedly. Upgrading of treat-
ment plants may be required for one or more of three major
reasons (5): first, to meet more stringent treatment require-
ments; second, to increase hydraulic and organic loading capac-
ity; and/or third, to correct or compensate for performance
problems resulting from improper plant design and/or operation.
Equalization may help a plant attain higher effluent quality by:
• permitting process optimization and improving
performance of existing treatment components;
*
• improving reliability by minimizing flow and load
peaking and/or reducing or eliminating bypassing;
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• reducing effects of shock loading of toxic or other
upsetting influent waste components.
Increased effective hydraulic and organic loading capacity may
result from allowing continued operation at constant average
flow in treatment units that have reached capacity under peak
flow conditions.
Equalization may help to overcome design deficiencies and
reduce operational problems by:
• compensating for one or more design deficiencies more
economically than correcting the deficiencies themselves;
* providing for simplified operation, thus minimizing
potential difficulties from operational error.
Equalization is one of many alternatives available for appli-
cation to each of these problem categories. Procedures for
identifying problem areas and evaluating available alternative
solutions, including equalization, are detailed in the U. S.
Environmental Protection Agency's (EPA) Technology Transfer
Manual on upgrading existing wastewater treatment plants (5).
Equalization may be included in new treatment facilities:
• to assist in achievement of effluent requirements;
particularly where effluent requirements are strict,
and where advanced and sensitive treatment processes
are included;
• to minimize peaking capacity of planned treatment
components;
• to permit process optimization, and to simplify.
operational requirements.
Application of equalization in the design of new facilities
is not constrained by existing physical facilities. Accordingly,
the most favorable application is permitted consistent with cur-
rent knowledge of hydraulic, siting, operation, and maintenance
factors. .
Equalization Design Requirements
The design of equalization facilities, requires evaluation
and selection of a number of features:
* type and magnitude of input variations
• required volume
• facility configuration
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• pumping/control mode
• type of construction
*. appurtenances; aeration, mixing, odor control, cover,
flushing
* cost and benefits
Design decisions must be based on the specific details and
unique requirements of each individual plant. Major influencing
factors will include the type and degree of treatment employed,
local site conditions, and cost relative to benefits in compari-
son to feasible alternatives.
Quantitative procedures are available for establishing
ability to equalize input variations according to their type
and magnitude, and for determining required equalization volume.
Criteria for establishing equalization benefits in terms of ef-
fects on unit process and treatment plant performance are pre-
sented in Section 2 along with illustrative examples. Costs of
equalization construction, operation, and maintenance have been
developed from an extensive survey of existing facilities and
are detailed in Section 5. Equalization facility configurations,
construction type, and appurtenance requirements are governed
by constraints of existing or planned facilities and conventional
design fundamentals.
Equalization Facility Configuration—
Equalization may be accomplished by an assortment of dif-
ferent kinds of facilities and procedures. The type of system
employed may be dictated to a significant degree by the nature
of existing collection and/or treatment facilities, by siting
conditions or constraints, or by required characteristics of
planned facilities.
Equalization may be accomplished in conjunction with ele-
ments of the tributary sewer system. Excess capacity in major
interceptors may be used to smooth flows to the treatment plant.
To justify development of the storage capacity available volume
must be a significant fraction of required equalization volume. ;
Such conditions are more likely to exist in combined sewer sys-
tems. Facilities required to take advantage of available stor-
age capacity may range from automatic flow regulating gates with
real time computer control, to manually controlled gates or in-
flatable dams. Variable speed pump stations in conjunction with
excess interceptor capacity or wet well volume, with suitable
controls may also be used to reduce flow peaking. The use of
an in-sewer storage remote from the treatment plant for equali-
zation will require regular procedures to prevent occurrence of
problems resulting from accumulation of solids. Nightly draw-
down of the storage system, with flushing where required, is
essential to prevent excessive discharge of accumulated solids
during daytime peak flow and loading periods.
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Equalization capacity may be provided in the form of off-
line storage tanks located at appropriate points in the collec-
tion system. This may provide economical relief for overloaded
collection system components in addition to equalizing downstream
flow. Location of such tanks adjacent to required pump stations
minimizes duplication of facilities. Tanks located within the
community remote from the treatment are generally relatively ex-
pensive due to required appurtenances to control solids accumula-
tion, vprovide for fail safe operation, and prevent odor and
aesthetic problems. ;
Currently the most common means of providing equalization
is through the use of specially designed basins at sewage treat-
ment plants. Tanks are located near the headworks to provide
equalization benefits for all downstream units. Location down-
stream of screening and grit removal eliminates the need for
handling accumulations of such materials. Location upstream of
primary clarifiers provides optimum conditions for clarifier
operation, but requires equipment to prevent excessive solids
accumulation and to maintain aerobic conditions. Either mechan-
ical mixing or diffused aeration, suitably designed, can satisfy
both requirements. Equalization located downstream of primary
clarifiers may be the most economical and troublefree applica-
tion. Although constant flow benefits would not be available to
the primary units, their performance is relatively insensitive
to flow peaking compared with secondary or tertiary treatment
components. Lower capital and operating costs, and reduced
maintenance requirements for storing primary effluent without
problems of solids accumulation may outweigh disadvantages of
operating with normal (unequalized) primary effluent variability.
Location of equalization basins following secondary treat-
ment may be justifiable in special circumstances. Where all or
part of a plant's secondary effluent is reclaimed or processed
by tertiary treatment components, particularly where influent
peak-to-average flow ratios are low, equalization of this type
may be practical. In such cases equalization provides protec-
tion against poor effluent quality due to upsets in the biologi-
cal secondary treatment.
Equalization basins may be designed as either in-line or
side-line units. In the in-line design, Figure l(a), all flow
passes through the equalization basin. This system uses the
pump station to provide essentially constant flow through the
plant. Since continuous pumping is required, design must be
coordinated with other influent pumping requirements to elimi-
nate costly duplication. In the side-line design, Figure l(b),
only flows greater than the daily average are diverted to the
equalization basin. Depending on unequalized plant pumping re-
quirements, this scheme may minimize additional pumping required.
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Raw
wjstewater
Bar scr
and/or
commi
»-
een
nutor
\
•*
^
Grit
removal
Eq
zat
has
jali-
on
in
Controlled-
flow pumping
station
Flow meter and control device
/
Primary
treatment
Secondary
treatment
Final
^"effluent
Sludge-processing
recycle flows
Figure 1(a)
Schematic flow diagram of equalization facilities:
in-line equalization
Final
effluent
Sludge-processing
recycle flows
Controlled-
flow pumping
station
Figure l(b)
Schematic flow diagram of equalization facilities
side-line equalization
Where equalization facilities are used to provide protection
against toxic or process upsetting materials, side-line facil-
ities would be required.
Provision for variable volume in key process units may
satisfy requirements for dry weather equalization. The acti-
vated sludge process is adaptable to this type of application.
Significant treatment capacity reductions, as well as improved
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performance, may be possible when applied to package type ex-
tended aeration treatment systems.(6) Application to an oxida-
tion ditch system has been shown to contribute to excellent
overall plant performance while requiring relatively low capital
expenditure.(7)
Pumping and Control Mode—
Addition of equalization to a wastewater treatment plant
adds to the total head required for plant operation. The head
required is equal to the sum of the maximum water surface level
variation and dynamic losses through the equalization system.
Additional head may be required for dewatering depending on the
equalization configurations. Very few treatment plant locations
afford sufficient head for operation without additional pumping.
Pumping requirements are established by plant and siting
constraints, and by the equalization configuration selected.
Required head may be developed either by pumping into or out of
equalization. Designing equalization tanks to fill by gravity
and empty by pumping permits filling even at excessive peak
rates and allows installed pump capacity to be minimized. In
order to maintain- desired constant flow effects, effluent pumps
generally require variable speed drives to accommodate day-to-
day and long-term changes in influent conditions.
Filling by means of pumps, and emptying by gravity requires
pump capacity to accommodate anticipated peak flows. This con-
figuration also generally requires variable speed pumping to
take full advantage of desired flow smoothing. Gravity dis-
charge systems require regulating controls for effluent flow in
addition to influent pump controls. Effective flow control
requires location of a flow measuring device downstream of equal-
ization to monitor final flows. Instrumentation and controls
should be provided to maintain preselected equalized flow rate
with a minimum of operator attention by automatic adjustment of
pump and valve settings.
Type of Construction—
Equalization basins can be provided through the construc-
tion of new facilities or by modifying existing facilities of
sufficient volume. Equalization may-be implemented with relative
ease in an upgrading plan that calls for the abandonment of
existing tankage. Facilities that may be suitable for conver-
sion to equalization basins include aeration tanks, clarifiers,
digesters, and sludge lagoons.
New basins may be constructed of earth, concrete, or steel.
Earthen basins are generally the least expensive. They can
normally be constructed with side slope varying between 3:1 and
2:1 horizontal to vertical, depending on the type of lining
used. Drainage facilities should be provided for ground water
-------
control to prevent embankment failure in areas of high ground
water. Precaution should be taken in design to prevent erosion
in large basins where a combination of aerator action and wind
forces may cause the formation of large waves. It is also
customary to provide a concrete pad directly under the equaliza-
tion basin aerator or mixer. The top of the dikes should be
wide enough to insure a stable embankment. For economy of con-
struction, the top width of the dike should be sufficient to
accommodate mechanical compaction equipment.
In-line basins should be designed to achieve complete mix-
ing in order to maximize concentration damping. Elongated tank
design enhances plug flow and should be avoided where concentra-
tion of load damping is desired. Inlet and outlet configura-
tions should be designed to prevent short circuiting. Designs
which discharge influent flow as close as possible to the basin
mixers are preferred.
Compartmentation—
Design of equalization should follow established sanitary
engineering practices, dividing required volume into two or more
compartments or basins. This permits dewatering for maintenance
and repair without interrupting service, and allows for opera-
ting flexibility. .Where equalization is designed to accommodate
wet weather flows, compartmented tankage allows dry weather
equalization using only a portion of the facilities, helping to
minimize maintenance requirements. When upgrading, existing
facilities tanks being considered for abandonment should be
analyzed carefully to determine possible suitability for equali-
zation.
Aeration and Mixing--
The successful operation of both in-line and side-line
basins may require mixing and aeration if placed upstream of
primary clarifiers. Mixing equipment should be designed to
blend the contents of the tank, and to prevent deposition of
solids in the basin. To minimize mixing requirements, grit
removal facilities should precede equalization basins wherever
possible. Aeration is required to prevent the wastewater from
becoming septic. Mixing requirements for blending municipal
wastewater having a typical suspended solids concentration of
approximately 200 mg/1 range from 0.02 to 0.04 hp per 1,000 gal-
lons of storage. To maintain aerobic conditions, air should
be supplied at a rate of 1.25 to 2 ft-^/min per 1,000 gallons of
storage.(8)
Mechanical aerators are one method of providing both mix-
ing and aeration. The oxygen transfer capabilities of mechani-
cal aerators operating in tap water under standard conditions
vary from 3 to 4 pounds C>2 Per horsepower-hour. Baffling may
be necessary to insure proper mixing, particularly with a
-------
circular tank configuration. Minimum operating levels for
floating aerators generally exceed 5 feet, and vary with the
horsepower and design of the unit. Low-level shutoff controls
should be provided to protect the unit. The horsepower require-
ments to prevent deposition of solids in the basin may greatly
exceed the horsepower needed for blending and oxygen transfer.
In such cases, it may be more economical to install mixing equip-
ment to keep the solids -in suspension and furnish the air re-
quirements through a diffused air system, or by mounting a sur-
face aerator blade on the mixer.
It should be cautioned that other factors, including maxi-
mum operating depth and basin configuration, affect the size,
type, quantity, and placement of the aeration equipment. In all
cases, the manufacturer should be consulted.
Purpose of the Manual
The purpose of this manual is to assemble and disseminate
all available information on equalization applications to munic-
ipal wastewater treatment developed to date. The manual is not
simply a compilation of data; rather, available information from
all sources has been reviewed and evaluated and, combined with
experience of the investigators, reasonable conclusions are pre-
sented. Procedures for evaluating equalization applicability
and performance are presented. Information on benefits and
costs of equalization have been analyzed. Recommendations con-
cerning equalization in the planning and design of municipal
sewerage systems are presented for consideration and use.
Information developed and presented in this manual rests
on experience gained through municipal applications of equaliza-
tion occurring almost exclusively over the last five years.
Many new and recently installed equalization systems in an in-
creasingly broad spectrum of applications will add significantly
to understanding equalization uses and utility. It may well be
that continuing developments in the field will require the re-
vision of this manual in the future. Knowledge of treatment
system performance related to equalization and equalization sys-
tem performance is not exhaustive. However, the body of current
knowledge is extensive and provides a firm basis upon which in-
tegrated treatment systems may be planned.
SCOPE OF THE MANUAL
Coverage of Subject Material
This manual presents information gathered from all sewage
treatment plants having equalization facilities that could be
located throughout the United States. Analysis and recommenda-
tions are based largely on this information. Analysis proce-
dures and design principles along with case examples are
-------
presented to cover the spectrum of conditions to which equali-
zation may be applicable. The vast diversity of individual col-
lection system-sewage treatment plant combinations prevents
exhaustive coverage of all possible conditions. Attention is
concentrated on equalization facilities or systems located at
treatment plants, because of the predominance of existing appli-
cations and the greatest potential effectiveness. Biological
secondary treatment plants and such plants with tertiary treat-
ment elements (most commonly nutrient removal and effluent fil-
tration) have received the most widely distributed application
of equalization. The bulk of the remaining applications are
those upstream of treatment plants including in-line flow con-
trol and pump station control applications. The entire spectrum
of system sizes is covered from package facilities serving
limited commercial or residential developments, to facilities
with major metropolitan service areas.
Information Sources Used
Information used in the preparation of this report
included:
• Current project reports and data accumulated and
supplied by the U. S. EPA Municipal Environmental
Research Laboratory.
* A national survey to identify existing and proposed
equalization facilities and to provide information on
their design, cost, and performance.
• Direct communication with operating personnel at
treatment facilities throughout the country.
• Private communication with investigators active in
the field.
• The general literature.
i:
• Experience of individuals involved in preparation of
this manual.
GUIDE TO THE USER
Table of Contents
The table of contents provides an overview of general
subject coverage provided in the manual.
10
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REFERENCES
1. King, V. L., et al. (1942), First Years Operation of the
Effluent Treatment Plant of the Calso Chemical Division,
American Cyanimide Co., Bound Brook, N. J., W&SW 14, 3,
660, March 1942.
2. Rudolfs, W. (1943), "Pretreatment of Acid Chemical Wastes",
Sewage Works Journal 15 1, 48. January, 1943.
3. Rudolfs, W. and J. N. Millar, (1946), "A Method of Acceler-
ated Equalization of Industrial Wastes", Sewage Works J.
18^ 4, 686, July, 1946.
4. Gurnham, C. F. (1955), Principles of Industrial Waste
Treatment, J. Wiley and Sons, New York,1955.
5. USEPA (1974), Process Design Manual for Upgrading Existing
Wastewater Treatment Plants, USEPA Technology Transfer
EPA 625/l-71-004a, October, 1974.
6. Speece, R. E., and M. LaGrega (1976), "Flow Equalization
by Use of Aeration Tank Volume", JWPCF 4_8:11, 2599,
November, 1976.
7. USEPA, (1977), "Effluent Treatment of Small Municipal Flows
at Dawson, Minnesota", USEPA Technology Transfer Technical
Capsule Report No. 2015, October, 1977.
11
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SECTION 2
CONCLUSIONS AND RECOMMENDATIONS
1. Flow equalization is seldom the only alternative for
dealing with real or anticipated performance problems at
a wastewater treatment plant. Care must be taken to demon-
strate that flow equalization is the most effective and
least expensive alternative.
2. Flow equalization benefits may be categorized as follows:
• Reduction of peaking requirements.
* Reduction of process overloads at existing plants under
some conditions.
• Protection against toxic upsets.
• Potential reduction of operational problems.
• Provides increasing benefits with increasing plant
complexity.
3. Feasibility of equalizing storm flows from infiltration/
inflow sources and in combined sewer systems should be
assessed, as would any treatment system component, using
conventional infiltration/inflow analysis procedures.
4. Where equalization is used to provide for treatment of
storm flow peaking in addition to equalizing diurnal flows,
sufficient treatment capacity must be provided to permit
emptying storage volume.
5. Side-line and in-line equalization provide approximately
equal degrees of BOD load equalization where storage volume
is provided to level only diurnal flow variations.
6. In-line equalization provides greater flexibility for in-
fluent waste load equalization using storage volume in
excess of the minimum required for daily flow equalization.
12
-------
7. Use of side-line equalization generally will not require
duplication of in-plant pumping. In-line equalization will
generally require an additional pumping station.
8. Placement of equalization following primary treatment mini-
mizes operation and maintenance, and minimizes requirements
for solids removal, aeration, and odor control equipment.
9. Application of flow equalization in an activated sludge
system will more than likely not reduce the soluble or-
ganics concentration of the effluent from the biological
process.
10. Although design factors considerably influence the accep-
table loading range of activated sludge sedimentation fa-
cilities, flow equalization may provide increased TSS and
organics (particulate fraction) removals when applied to
systems with peak hydraulic loading rates beyond about
1,000 gpd/ft2.
11. The removal of soluble organics in a trickling filter
would probably not be enhanced through application of flow
equalization ahead of the trickling filter.
12. Efficient removal of particulate materials in a trickling
filter sedimentation tank would probably not be signifi-
cantly improved by equalization of upstream flow.
13. The cost of the storage volume required for equalization
of the peak flows is likely to be less than the cost of.
the incremental treatment capacity in plants, of all
sizes, where the degree of treatment exceeds simple
secondary requirements.
14. Plow equalization should be considered as an alternative
to additional treatment capacity wherever influent P/A
ratios exceed levels of 1.3 to 1.5:1.
13
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SECTION 3
QUANTITATIVE METHODS
DETERMINATION OF EQUALIZATION APPLICABILITY
The applicability of equalization in any wastewater treat-
ment situation depends on its ability to function as an integral
element of the most cost effective treatment system available,
and as required to meet imposed discharge requirements.
Facility Planning
Equalization provides an alternative to building excess
plant capacity to accommodate input peaking; it also has addi-
tional benefits derived from plant operation under more stable
conditions and an ability to control intermittent plant upset-
ting inputs.
Conventional facility planning and design procedures formu-
lated to assess applicability of alternatives resulting in a
recommended optimum treatment scheme are as follows:
1. Identify treatment requirements (discharge permits);
2. Identify influent conditions;
3. Establish design flows for treatment;
4. Identify all feasible alternatives for meeting
treatment requirements;
5. Establish cost effective treatment scheme consistent
with requirements;
6. Detailed design.
Identifying influent conditions (item 2) may include conduct of
a Sewer System Evaluation Survey if the infiltration/inflow
analysis element of the facility plan indicates potentially ex-
cessive infiltration/inflow; or if the system is combined, re-
sulting in overflows of combined sewage, development of a pro-
gram to control the overflows will be required. The outcome of
14
-------
either or both of these analyses will impact item 3, the flows
that have to be accommodated at the treatment plant. The ap-
plicability of equalization is determined through this process
by three major factors: discharge criteria establishing the
degree and reliability of treatment required; the design capac-
ity of the treatment plant; and plant input variation and peak-
ing characteristics.
The degree of treatment required, and resulting feasible
treatment process schemes, establish the potential magnitude
of cost economy available by using equalization and minimum
peaking condition design. As the degree of treatment required
increases, the number of treatment processes with potential cost
savings from reduced design peaking increases. Thus, the ap-
plicability of equalization tends to increase with increasingly
stringent discharge requirements. It is essential in consider-
ing the potential of equalization to recognize the importance
of both specific effluent discharge requirements and convention-
al design criteria for affected treatment components. If dis-
charge requirements are all established on a 30-day or 7-day
average basis, then treatment components can be designed for
30-day or 7-day average conditions in the critical month of
operation. The success of such designs depends on better-than-
required performance occurring in below-average flow periods,
balancing the excess discharges occurring in the above-average
flow periods.
For example, the conventional activated sludge process,
with requirements only for carbonaceous BOD removal, may common-
ly be designed for average conditions in the most critical
month. In such a case, equalization is of limited benefit.
However, if discharge requirements specify absolute limits on
effluent components, designs must ensure acceptable performance
under peak loading conditions.
For example, activated sludge processes designed for
biological nitrification and for denitrification have been
shown (8) to require factors of safety equal to or greater than
the influent peak-to-average ratio to ensure desired performance,
In such cases equalization effectively reduces required design
capacity, and its applicability then depends on the relative
cost of equalization volume and treatment capacity. Specifical-
'ly, applicability must be determined in each individual case
according to design requirements for all unit processes in the
treatment scheme affected by equalization. Examples of cost
comparisons for treatment schemes with and without equalization
are provided in Section 5. The result of this process is iden-
tification of the most cost effective treatment scheme for
meeting imposed discharge requirements.
15
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The general range of treatment plant capacity is a major
factor in determining the type of treatment process to be
selected, and in establishing the unit cost of treatment com-
ponents. Generally, unit costs of treatment components decrease
with increasing size (economy of scale). In addition, peak-to-
average flow ratios typically decrease with increasing plant
size. These factors tend to reduce the economic benefit of
equalization as size increases. This may be balanced off by the
economy of scale factors involved in required equalization
volume. The relative costs of treatment and equalization for a
range of treatment plant capacities are illustrated by examples
in Section 5.
The magnitude of plant input variations and peaking charac-
teristics, and to a more limited extent, the source of those
variations, is a major factor in determining the applicability
of equalization. As the peak-to-average ratio (or other suit-
able measure input peaking characteristics) increases, the need
for peaking capacity in treatment components designed without
equalization increases. Thus, the advantage of equalization is
greater where input peaking is greater.
Where the source of peaking variations is due to ground-
water and storms, the determination of the most cost-effective
overall solution to reducing impacts of peaking on the treatment
facility will require evaluation of alternatives within the col-
lection system in addition to consideration of equalization
measures at the treatment plant site.
Design Flow Peaking Considerations
The flow received at a treatment facility reflects the
input to, and the characteristics of, the tributary collection
system. The influent flow characteristics in turn directly
affect the performance efficiency of the treatment facility.
Hydraulic and organic loading variations, outside the design
range of the treatment units, will result in variable effluent
qualities. From a national perspective, the most common ex-
treme loading variation encountered is rainfall influenced,
specifically if the tributary system has combined sewer ele-
ments. The major loading parameter of rainfall-influenced
flows is hydraulic, often accompanied by discharge of inorganic
solids carried into the sewer with the drainage flow. Flow
variations exceeding 100 times normal non-storm influenced
wastewater flows can result from storm effects, forcing storm
flows to become the controlling factor in determining the load-
ing parameter for plant design,.
In this section, guidelines are developed to identify those
systems that are storm-flow dominated. Flow equalization analy-
sis techniques for wastewater system optimization will differ
16
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depending on whether wastewater-influenced variations or storm
flow are the dominant design factors.
Factors Affecting Influent Flows—
The influent flow at a treatment plant is made up of
wastewater discharged by served customers, infiltration of
groundwater entering the collection system due to system defects,
and storm inflow entering the collection system from runoff dur-
ing rainfall and/or snowmelt. The location and characteristics
of the discharge from customers can easily be monitored, and in
most cases predicted; but the same cannot be said for infiltra-
tion and storm inflow. The impact of all these flows measured
at the treatment plant is dependent on the specific characteris-
tics of the collection system.
Sanitary Sewer Design--Conventional sanitary-only collec-
tion system' design is based on sizing sewers for the estimated
peak sewage flow, plus an allowance to accommodate infiltration
and inflow. As the number of tributaries flowing into a specif-
ic pipe section increases, the sewage peaking factor is reduced
nominally by the effect of gravity flow in the sewer. Flow
peaking and minimum flow variations from varying size communi-
ties, excluding impacts of industrial or storm flows, are shown
in Figure 2. Diurnal peaking flows of up to 5:1 can be expected
from small systems serving 1,000 people or less, reducing to
1.5:1 for populations of 1,000,000; decreasing in proportion to
increasing population.
o
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10
O.I
10 1000
POPULATION, THOUSANDS
1000
Figure 2.
Dependence of extreme flow ratios in municipal
sewers on population
17
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Combined Sewer Design—Sewers designed for sewage plus
storm drainage(combined sewers) are normally sized to accept
all sewage flows and runoff from a storm having a specific
intensity. The storm intensity is based on a storm of a
specific occurrence frequency. Storm flows that exceed the
design maximum are allowed to overflow at relief points through-
out the collection system. In combined sewers the major capac-
ity in the collection system is reserved for storm flows.
The hydraulic impact of combined sewers at a treatment
plant is dependent on a combination of factors, namely:
1. The proportion of combined sewers within the tributary
collection system;
2. Flow characteristics of the wastewater dischargers;
3. Development density;
4. Collection system size and hydraulic characteristics;
5. Rainfall patterns.
For the majority of collection systems that have tributary com-
bined sewers the treatment plant peak flow is determined by the
limits of the interceptor system transfer capacity. Flows ex-
ceeding collection system capacity overflow at relief points
within the system. Because of the large flows generated by
storm runoff, even limited areas of tributary combined sewers
can have a dramatic impact on treatment plant flows during
storm periods.
Assuming that no overflows occur a theoretical relation-
ship can be developed to determine the impact of combined
sewered areas on storm flow peaking characteristics for differ-
ent values of development density, runoff characteristics of
the combined sewered area and storm intensity. These factors
have been combined in a simplified way in Figure 3 and assume
a per capita sewage contribution of 70 gallons per day.
Figure 3 has been constructed using the following data as
the base condition:
Line VI - population density, 10/acre
Line V2 - combined sewered area percent impervious,
30 percent
Line V3 - rainfall intensity, 1.00 inch/hour
Line A, which joins the three variables VI, V2 and V3 together,
expresses the relationship between flow peak-to-average ratio
and percent combined sewer. For the above-assumed variables,
18
-------
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RATIO PEAK STORM FLOW TO AVERAGE DRY WEATHER FL
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PERCENT COMBINED SEWER
Figure 3. Peaking effects of combined sewered areas
(Assuming that no overflows occur)
this shows that with 10 percent of the tributary collection
system combined, a peak flow ratio exceeding 30:1 can occur.
To use the figure for differing variables of VI, V2 and V3,
•use the following procedure as illustrated in the following
example:
Assumptions: VI - 7 people/acre
V2 - 20 percent
V3 - 1.5 inches/hour
Step 1: On VI measure difference between 7 and Line A
Step 2: On V2 measure difference between 20 and Line A
Step 3: On V3 measure difference between 1.5 and Line A
Step 4: Sum the three values to determine net distance
from Line A
Step 5: Draw line parallel to Line A offset by the amount
determined in Step 4.
19
-------
The line drawn in Step 5 is the relationship between storm
flow peaking characteristics and percent combined sewer. This
graph indicates that even small percentage combined sewered
areas can produce storm flows far outside the hydraulic effi-
ciency range of wastewater treatment units.
Treatment Plant Influent Flows—Influent interceptor
capacity at a treatment plant is the critical peak flow control-
ling element of the sewerage system. Regardless of upstream
conditions, flow at the plant can never exceed the influent
sewer capacity. For the sanitary-only case of a system with
no storm influences, designed in accordance with conventional
criteria, peaking characteristics reflect the discharge to the
system. For the system that is totally combined the maximum
flow at the plant reflects the hydraulic limitations of the col-
lection system network. Regardless of what is the hydraulic
controlling factor, it is the flow characteristics at the plant
that impact performance. In establishing the balance point for
a specific system (whether it be storm flow dominated or waste-
water dominated) the plant tolerance to all flow variations and
not only the peaking characteristics of the plant influent has
to be considered.
Stormflow Dominance Determination
The determination of whether a system is stormflow domi-
nated requires the sequential consideration of the following:
1. Analysis of influent flow characteristics;
2. Determination of treatment plant process unit loading
sensitivity.
This determination is significant, for if a system is storm
dominated, then optimization of treatment requires the collec-
tion network to be included as an integral part of the analysis.
The analysis emphasizes the treatment facility in the case of
non-storm influenced systems.
Treatment Plant Influent Peaking Characteristics—
For any given treatment plant historic plant flow data
should be analyzed, and peak hydraulic and organic loading peak-
ing and volume characteristics identified. An example of a
plot of annual hydraulic loading peaking characterisitcs for
two similar size communities having measurable rainfall events
of 50 times per year is shown in Figure 4. Community A has
sanitary-only sewers; peak flows are contained within a small
range with the exception of a few heavy storm days. Community
B has tributary combined sewers; peak flows occur for all
measurable rainfall events, with maximum flows being controlled
20
-------
0
UJ
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UJ
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a.
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>-
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SANITARY!
ONLY
SYSTEM
RATIO PEAK FLOW TO AVERAGE DRY WEATHER FLOW
Figure 4. Daily peak flow annual distribution
21 -
-------
by collection system limitations. Similar plots can be made
for both peak organic and daily volume. The significance of the
influent peaking characteristics is dependent on treatment unit
sensitivity to peak loadings and the specific effluent quality
requirements. Where discharge requirements are based on absolute
limits, process units have to be sized to handle the peak load-
ing. For suspended solids and biochemical oxygen demand, dis-
charge requirements are normally based on monthly averages with
higher allowances for 7-day averages and one-day values. Thus,
process units can be sized for average conditions and intermit-
tent peak loading-induced effluent deteriorations can be accepted
even if they exceed the 30-day average values, but are within
the one-day maximum allowance.
To evaluate the case where only average values have to be
met requires a further plot by month of peak loading variation.
An example for a community with seasonal infiltration/inflow
for hydraulic peaking characteristics by month is shown in
Figure 5. This example has plots for each month of minimum
daily peak, peaks exceeded 7 times per month, 3 times per month
and the monthly peak values. Similar plots can be made for
organic and volume daily loadings depending on the treatment
unit critical loading parameter. This data is applied to the
performance characteristics of proposed treatment unit processes
to determine critical loading conditions as described below.
Treatment Unit Sizing—
Process units are normally sized using average dry weather
hydraulic and organic loadings. From analysis of the influent
characteristics information plotted, as described above, apply
the critical month loading data to the plant sized for dry
weather only loads. Then check the deteriorated effluent qual-
ities with the discharge requirements.
Case 1. If the discharge requirements can be met then
the system is not stormflow dominated. The optimization
' of equalization basin-treatment plant sizing can proceed
without further consideration of the tributary collection
system as discussed under the heading of EQUALIZATION
SIZING METHODS.
Case 2. If the discharge requirements cannot be met then
the system may be storm dominated, requiring the following
analyses.
Identify potential for operating the treatment plant on a
temporary basis in differing modes. Examples are:
a. Contact stabilization for activated sludge systems;
b. Chemical addition to improve process unit performance.
22
-------
MONTHLY PEAK VALUES
EXCEEDED 3/MONTH
EXCEEDED 7/MONTH
MONTHLY MINIMUM
PEAK VALUE
-------
Analysis of Storm Dominated System
For those systems that are storm dominated, optimal use of
wastewater facilities requires that the analysis go beyond treat-
ment plant/equalization basin capacity consideration, and include
the collection system. The objective of including the collection
system is to reduce the flows received at the treatment plant.
Literature has been published by federal and technical
groups on approaches to analysis of storm-influenced and combined
sewer systems. In most publications, equal emphasis is given to
the hydraulic and pollutant constituents of the stormwater con-
tribution. However, with respect to the analysis of storm flow
dominated systems, it is the hydraulic contribution that is the
significant parameter.
Infiltration and Storm Inflow—
An infiltration/inflow analysis is required for projects
financed using EPA grants (PL 92-500, Title II, Section 201(g)(3)).
In this analysis sources of infiltration and inflow are identi-
fied, quantified hydraulically, and deemed excessive or nonexces-
sive by economic evaluation. An infiltration/inflow source is
considered excessive if costs for its removal exceed transport
and treatment costs. Following conduct of the sewer system
evaluation survey, grant conditions require that all economically
excessive sources be removed.
When equalization is considered and found to be a cost-
effective treatment component, the lower unit cost of treatment
will result in shifting the treatment-sewer improvement balance
so that more infiltration and inflow will be treated. The
approach to be taken for analyses of infiltration and inflow in
noncombined systems is covered in the EPA Handbook for Sewer
System Evaluation and Rehabilitation, December 1975.(9)
For sanitary-only systems, and combined systems with no
overflows, the analysis requires that the most cost-effective
program be selected to address all infiltration/inflow. For
systems with combined sewer overflows, the level of control is
determined by a cost/benefit analysis. Specifically, EPA
Program Requirements Memorandum 75-34 (10) states that controls
will only be funded to a level where marginal benefits of over-
flow reduction exceed the marginal cost of the overflow control.
Implementation of the findings of an infiltration/inflow
analysis will result in reduced plant influent flows in cases
where excessive infiltration/inflow is eliminated.
Combined Sewer Overflow Control—
Where sewer systems include combined sewer service areas
the process is principally the same, although somewhat different
rules apply. Infiltration analysis is required, and is identical
24
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to that for separate sewers. If no overflows occur from the
combined system upstream of the treatment plant, then optimiza-
tion of treatment (including equalization) and sewer system
improvement is as described above. If combined system overflows
occur, then a separate cost effectiveness analysis for overflow
elimination must be conducted to determine the degree of over-
flow reduction that can be justified in terms of upgrading re-
ceiving water quality and restoring beneficial uses. Details
of the requirements for overflow reduction and justification
for the EPA construction grant allotment are specified in EPA
PRM 75-34. However, the analysis does not directly affect the
process of optimizing treatment with or without equalization
and sewer system improvement.
The analysis of combined sewer systems is complex, often
requiring the aid of computer-based mathematical modelling to
optimize system alternatives and varying control levels. Publi-
cations covering the analysis approach for combined systems are
referenced in the EPA published Areawide Assessment Procedures
Manual, Volume 1, Section 6.(11)
In general, where overflows occur from combined sewer
areas, overflow control evaluation, including equalization con-
siderations, can be summarized as follows: Recognizing that
overflows normally occur because of the limitations of the col-
lection system capacity for transport to the treatment plant,
four major alternatives are available for correction. The
object of the analysis will be to identify the least cost means
of reducing overflows as a function of frequency of occurrence.
The alternatives available include:
a. Source Control
1. Infiltration reduction by sewer system rehabili-
tation.
2. Stormwater inflow reduction by removal of storm-
water connections from the sanitary sewer, i.e.,
removal of downspouts, illicit drainage connections,
etc,
b. Storage
1. In-line storage by using existing sewer capacity
or enlarging specific sections.
2. Off-line storage using surface retention basins
and subgrade storage chambers.
Storage of overflow at individual or consolidated sources
is a form of flow equalization. However, location of equaliza-
tion facilities some distance from the treatment plant in order to
25
-------
accommodate storm flow peaks for a portion of the total collec-
tion generally does not satisfy requirements enabling treatment
influent equalization. However, this approach should be con-
sidered where applicable to the circumstances.
The last two alternatives are indirectly related to
equalization requirements in that the most economical means of
control may include one or the other in combination with equal-
ization.
The available alternatives must be evaluated to establish
the means of least-cost overflow control. An appropriate level
of control must then be established based on the requirements
of PRM 75-34. This process will ultimately establish the
amount of stormflow delivered to treatment. If applied only
to reduce existing overflows, none of the alternatives will
reduce existing plant flows. Remote treatment and discharge
will leave plant flows unchanged. Upstream holding will in-
crease the total quantity of wastewater to the plant without
increasing existing maximum flow rates. Increasing transport
capacity will increase peak treatment requirements proportion-
ately. This last case should be recognized as virtually the
same as the conventional infiltration-inflow analysis process.
This would result only if the combination of increased collec-
tion system capacity and increased optimum treatment system
(including equalization) capacity had been found to be less
costly than all other feasible control alternatives.
The sewer system evaluation, whether for separate or com-
bined systems, in combination with projections for expansion of
the service area during the design period, establishes design
flows and peaking characteristics; on which the design of all
individual treatment process components will be based.
EQUALIZATION SIZING METHODS
Numerous methods have been developed that may be used to
estimate the impact of equalization on normally varying waste-
water flows and concentrations. Each,of these methods requires
some amount of data, computation, and the making of assumptions
based on observations of the influent; methods differ greatly in
these regards. Various methods may be used to compute required
basin volume. Ability to estimate effects of equalization on
wastewater characteristics varies depending on the level of
detail employed in individual methods. For example, methods
taking into account influent concentration or mass loading in-
formation, and assumptions concerning mixing and reaction within
the equalization basin, can yield detailed information on con-
centration and mass loading in the effluent.
26
-------
The principal methods are:
1. Simple flow balance ("mass diagram")
2. Simple concentration balance
3. Combined flow and concentration balance
4. Sine wave method
5. Rectangular wave method
6. Batch dumping
7. Random concentration
Method 1 is simplest and directly applicable to municipal
wastewater treatment problems. Method 2 is oversimplified for
most applications and is presented mainly as an introduction to
Method 3. Method 3 is more detailed requiring greater computa-
tional effort. The method is, however, versatile and can be
used for more detailed and thorough problem analysis. Methods
4 through 7 may be useful in some municipal plants, but are not
expected to be as useful in this area as Methods 1 and 3. They
are described briefly, accompanied by references and a summary
of their applicability, advantages, and disadvantages.
Method 1—Simple Flow Balance^
Simple flow balances are easily performed either graphically
(mass diagram), or with a note pad and simple arithmetic. Only
two inputs are required; the flow entering the system as a
function of time, and either the discharge as a function of
time or the schedule of basin volume regulation. The output
results in the basin volume schedule if the discharge record is
given, or in the discharge record if the basin volume schedule
is given. These simulations may be used with any flow pattern,
and easily handle diurnal variations and storm flows. The
analyses may be applied to both in-line and side-line fbasins.
This method is useful as an operating tool, since flows and
volumes are easily measured.
A simple flow balance has two main drawbacks: First, no
information is provided on concentrations or mass flows (mass
flow being the product of concentration and flow rate). Second,
data is assumed to be known exactly, not just statistically.
As in any method, care must be taken to anticipate conse-
quences in cases where actual inputs are different from projec-
tions. For example, a typical design may provide an equaliza-
tion basin volume and treatment plant capacity sized to accom-
modate measured and predicted domestic, commercial and industrial
flows, with allowances for infiltration and inflow based on
infiltration/inflow analysis. Infiltration/inflow analyses are
frequently based on flow measurements taken during only moderate-
ly intense rainfall. However, flow measurements are seldom
available for relatively intense storms (e.g. the 5-10 year
27
-------
storm). Effects of larger storms on total plant flows may be
significant. Consequences on system design and performances
must therefore be considered.
The basic equation for application of the simple flow
balance is:
Qin At = AV + QQut At
(2-1)
where Qin
At
AV
= flow into the system, average rate during At,
I,3!"1
= time interval, T
= change in stored volume during At, L3
= flow out of system, average rate during At,
L3T-1
This equation is applied stepwise, or iteratively, for succes-
sive time intervals. Cumulative sums for all values up through
the last step are computed and recorded. For instance, if Q^n
and Qout are given, and basin volumes are desired, a record of
EAV must be generated. The record is comprised of the sum of
AV values up through the time step in question. The flow
balance equation (Equation 2-1) and the cumulative sum may be
recorded on a graph or in a table.
The relation between the physical system and system varia-
bles for this method is shown in Figure 6. Note that for a
side-line basin, the flow into the basin is Q . - Qj_ . (This
quantity is negative when flow is being discharged from the
basin.) Any consistent units may be used. For instance, if
Qin an<^ Qout are in millions of gallons per day, then At is in
days and AV is in millions of gallons. If Qin and Q0ut are in
liters per second, At is in seconds, and AV is in liters.
Q
0Ut
IN-LINE BASIN
SIDE-LINE BASIN
Figure 6. Simple flow balance schematics
28
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The following three examples demonstrate use of simple flow
balances, and illustrate some common phenomena in equalization.
Example 1—Graphical Solution, Diurnal Variation—
This example, drawn from the EPA Process Design Manual for
Upgrading Existing Wastewater Treatment Plants, (12) describes
the graphical solution for a flow balance applied to a diurnal
flow variation. The flow balance illustrated in the example is
used to determine the equalization volume required to exactly
balance diurnal variations, producing constant effluent flow.
The flow variation before equalization is shown in Figure 7.
This is a typical diurnal wastewater flow pattern, with a peak-
to-average ratio of 1.7 and a minimum-to-average ratio of 0.45.
There is one daily peak, at about 6:00 p.m.
The average flow rate for each hour interval is used to
compute the corresponding volume increment. Successive volumes
are plotted to form a hydrograph or mass diagram as illustrated
in Figure 7. A straight line drawn from the origin to the cumu-
lative volume at 24 hours (dashed line, Figure 8) has a slope
equal to the average flow rate over the day. In this case the
average daily flow is approximately 4.3 mgd.
To equalize the diurnal varying flow, tank volume must be
provided to accommodate flows in excess of the equalized flow
rate. For normal diurnal variations this volume typically
ranges from 10 to 20 percent of the average daily flow. The
volume required for equalizing flow variations in this example
is equal to the vertical distance (measured in millions of
gallons) between parallel lines of slope equal to the average
flow line (dashed line, Figure 8) and tangent to the extremities
of the inflow mass diagram. These lines are shown as A and B on
Figure 8. In this illustration, the required volume for equali-
zation is 740,000 gallons. This volume is approximately 17 per-
cent of the average daily flow.
FLOW RATE
0
MIDNIGHT
12
NOON
TIME OF DAY
24
MIDNIGHT
Figure 7. Wastewater flow variation before equalization
29
-------
BASIN FULL
REQUIRED EQUALIZATION VOLUME,
0,74 MILLION GALLONS
AVERAGE FLOW,
4.3 MOD
INFLOW MASS DIAGRAM
BASIN EMPTY
0 2
MIDNIGHT
4 6
-10 12 14
NOON
TIME OF DAY
16 18
20 22 24
MIDNIGHT
Figure 8. Hydrograph for Example 4
30
-------
A physical interpretation of the hydrograph is as follows.
At 8:00 a.m., the equalization basin is empty, as shown by the
tangency of the inflow mass diagram with the bottom diagonal.
At this point plant flow begins to exceed the average flow rate
and the tank begins to fill. Accordingly, inflow mass diagram
and the bottom diagonal begin to diverge at this point. At 5:00
p.m., the basin is full, as shown by the tangency of the inflow
mass diagram with the top diagonal. Finally, the tank is drawn
down from 5:00 p.m. to 8:00 a.m. on the following day, when the
flow is below average.
Importantly, the equalization basin volume used for actual
design must be greater than that obtained with the hydrograph
for several reasons: to provide freeboard, minimum depth for
aeration, and mixing equipment that might be used, and to pro-
vide for storm flows occurring in excess of normal diurnal
variations.
Example 2—Tabular Solution, Diurnal Variation—
This example illustrates a tabular solution (Table 1) of
the problem used in Example 1. Computations are summarized in
Table 1. A uniform time increment (At) of 2 hours is used as a
suitable value compatible with normal diurnal variations.
Variations from one hour to the next do not greatly affect the
equalization volume but variations over several hours are im-
portant.
The basin is assumed to be at a reference level of 0 at
midnight. For this example, the reference level must represent
at least 0.759 million gallons or the basin will not run dry
before it starts to refill. The total working volume required
is the maximum SAV minus the minimum EAV, or 0.786 million gal-
lons. This figure approximates the 0.74 million gallons of
Example 1. "
Note that the average flow estimated by the graphical
method (Example 2, Figure 8) is estimated to be 4.57 mgd. This
difference is due to limited accuracy of reading and plotting
the graphs. Totals for QinAt and QoutAt can easily be made as a
check of computational accuracy; these values should correspond
to the total flow over the day. The final £AV is very small,
as required for a periodic variation. The total of AV values
should be very small as in the example;- it is not identical to
the last AV because of round-off errors. The calculations
should be carried to three significant figures, the least count
corresponding to 1,000 gallons. The results, as always, cannot
be more accurate than the input values—5 percent at best.
Example 3—Tabular Method, Storm Inflow—
Example 3 describes the impact of storm flow variation
superimposed on the diurnal variation of Example 2. Examples
31
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TABLE 1. EXAMPLE 2—TABULAR METHOD
Time Interval
2400 - 200
200 - 400
400 - 600
600 - 800
800 - 1000
1000 - 1200
1200 - 1400
1400 - 1600
1600 - 1800
1800 - 2000
2000 - 2200
2200 - 2400
TOTAL
AVERAGE
(mgd)
3.0
1.6
1.6
3.0
5.1
5.6
5.9
6.3
6.6
6.5
5.4
4.2
54.8
4.57
p. At
in
(106 gal)
0.250
.133
.133
.250
.425
.467
.492
.525
.550
.542
.450
.350
4.57
Q ..At
xout
(106 gal)
0.381
.381
.381
.381
.381
.381
.381
.381
.381
.381
.381
.381
4.57
-
Av
(106 gal)
-0.131
- .247
- .247
- .131
+ .044
.086
.111
.144
.169
.161
.069
- .031
- .003
-
ZAV
(106 gal)
-0.131
- .378
- .625
- .756
- .712
- .626
- .515
- .370
- .201
- .040
+ .030
- .001
(final)
- .001
-
Q =4.57 mgd, constant
At = 2 hours = 0.0833 days
IAV = running total of AV values
6
Working volume required = 0.030 -(-0.756) = 0.786 10 gal.
32
-------
4 and 5, discussed under Method 3, illustrate equalization of
concentrations and loads.
For this example it is assumed that the Q^n in Table 1 rep-
resents the maximum wastewater flow from domestic, commercial,
and industrial sources, plus the highest seasonal infiltration.
In addition, storm inflow and storm-related infiltration are to
be considered. Assume that the effect of a 5-year storm is as
shown in Table 2, and that the storm is followed by another
storm, half as large, starting six hours after the initial storm
ends. This example illustrates computation of QOut anc^ basin
volumes to provide for flow equalization. The 5-year storm in
Table 2 is representative of a separate sewer system, with no
combined sewers and moderate amounts of infiltration and inflow.
This example demonstrates that additional flow could not be
handled in treatment components designed for equalized flow with
a capacity of 4.6 mgd, as in Example 2. The 4.6 mgd treatment
capacity held constant through the day is required for the non-
storm flows. Accordingly, no excess capacity is available for
storm flows. Treatment capacity must exceed 4.6 mgd to permit
discharge of stored storm flow.
Whether or not the storm effect in Table 2 is adequate for
design depends upon regulatory policy. When a larger storm than
the 5-year storm occurs, producing flows as assumed in Table 2,
it is likely that sewer overflows or bypassing will occur. Such
overflows or bypassing may be acceptable by virtue of the rarity
of occurrence. If not, system modifications would be required.
Collection system improvements to reduce infiltration and in-
flow or development of storage volume in the collection system
would reduce storm effects on existing treatment. Alternatively,
flows could be equalized at the plant or additional treatment
capacity provided. In the work that follows, the 5-year storm
hydrograph in Table 2 is used.
If no storage is provided, the required treatment capacity
may be calculated as follows:
6.6 mgd peak non-storm flow (Table 1)
+ 4.2 mgd peak storm effect (Table 2)
10.8 mgd peak flow
If equalization is located between primary sedimentation and
activated sludge, the primary sedimentation tanks will need
10.8 mgd peak capacity.
If downstream process components are sized for 6.6 mgd peak
flow, so that equalization is not required in dry weather, the
ability of equalization to handle the storm effect can be deter-
mined. The storm produces an average flow over 20 hours of 2.1
33
-------
mgd. The available capacity is 6.6 minus 4.6 mgd, or 2.0 mgd
during the storm itself. With 6 hours until the next signifi-
cant storm, the small remaining volume could easily be treated.
In fact, the peak treatment rate could be reduced slightly, as
discussed below. In that case, equalization would have to be
operated during dry weather.
Treatment capacity could be sized to accommodate peak dry
weather flow. Using 6.6 mgd for the maximum Qout' t^16 equaliza-
tion volume required to accommodate storm flows would be about
1.03 million gallons, as derived in Table 3. Note that the
storm has been assumed to occur at roughly the worst time, that
is, the storm peak will coincide with the non-storm peak at
about 6:00 p.m. This assumption is conservative. Also observe
that the basin is empty shortly before 10:00 a.m-. (ZAV is
slightly negative at 10:00 a.m.), so a subsequent storm could
occur at any time thereafter, and be of any size smaller than
the Table 2 storm without overloading the equalization basin
treatment system. Finally, note that QOut may be reduced in the
last 2 hours of the storm effect without causing any difficulty
in computation.
If treatment capacity is designed for the assumed peak dry
weather flow and only one million gallons of storage volume is
provided, it can be shown that flow equalization may still be
used in dry weather. The working basin volume required for the
storm (1.0 million gallons) is greater than the requirement for
diurnal variation (0.8 million gallons, from Example 2). There-
fore, if 1.0 million gallons are available, the flow may be
fully equalized in dry weather.
A smaller maximum Qout could not be used, with equalization
required in dry weather, using 1.0 million gallons of storage.
A greater storage volume is required is Q0ut cannot reach 6.6
mgd.
A smaller maximum Qout could be used -if storage volume were
increased. For instance, if QOut ^s limited to 5.5 mgd, the
solution is about 2.3 million gallons. As shown in Table 4, if
the basin is empty at noon on Monday, the maximum stored volume
will be 1.61 million gallons, occurring late Tuesday night
during the following storm. Note that the following storm is
significant in this case. Also.note that the 8:00 a.m. volume
is decreasing; since the storm peaks are more than 24 hours
apart, the flow from following storms will eventually be treated.
The tabulation was based on an empty basin at noon on Monday;
but by then the basin could easily have about 0.7 million
gallons. Therefore, a total working volume of about 2.3 million
gallons will be needed. To verify this estimate, a pattern of
preceding storms would have to be assumed, and a longer calcu-
lation would be needed. Finally, note that the basin must be in
34
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TABLE 2. EXAMPLE 3—5-YEAR STORM EFFECT
Hours after
storm begins
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Total
Average over 20 hr.
Storm effect
flow rate
0.9
2.0
2.8
3.5
4.2
3.8
3.5
3.4
3.2
2.7
2.3
2.1
1.9
1.6
1.4
1.1
.8
.5
.3
.2
(mgd)
42.2
2.1
i
TABLE 3. EXAMPLE 3—TABULAR METHOD, FIRST CASE
Time interval
1200- - 1400 -
1400 - 1600
1600 - 1800
1800 - 2000
2000 - 2200
2200 - 2400
2400 - 0200
0200 - 0400
0400 ,- 0600 -
0600 - 0800
0800 - 1000
1000 - 1200
TOTAL
AVERAGE
Qin (mg'd)
Non-storm
5.9
6.3
6.6
6.5
"5.4
4.2
3.0
1.6
1.6
3.0
5.1
5.6
(54.8)
4.57
Storm
0.9
2.8
4.2
3.5
3.2
2.3
1.9
1.4
0.8
0.3
0
0
(21.3)
1.78
Total
6.8
9.1
10.8
10.0
8.6
6.5
4.9
3.0
2.4
3.3
5.1
5.6
(76.1).
6.34
Qout
(mgd)
6.6
6.6
6.6
6.6
6.6
6.6
6.6
6.6
6.6
5.0
6.6
5.6
—
Q. At
win
(106 gal)
0.567
.758
.900
.833
.717
.542
.408
.250
.200
.275
.425
.467
6.342
«outAt
(106 gal)
0.550
.550
.550
.550
.550
.550
.550
.550
.550
.417
.550
.467
6.384
AV
(105 gal)
+0.017
.208
.350
.283
.167
- .008
- .142
- .300
- .350
- .142
- .125
0
-0.042
2AV
UO6 gal)
0.017
.225
.575
.858
1.025
1.017
.875 -
.575
.225
.083
- .042
- .042
-0.042 ,
—
35
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TABLE 4. EXAMPLE 3—TABULAR METHOD, SECOND CASE"
Time interval
Monday :
1200 - 1400
1400 - 1600
1600 - 1800
1800 - 2000
2000 - 2200
2200 - 2400
Tuesday:
2400 - 0200
0200 - 0400
0400 - 0600
0600 - 0800
0800 - 1000
1000 - 1200
1200 - 1400
1400 - 1600
1600 - 1800
1800 - 2000
2000 - 2200
2200 - 2400
Wednesday:
2400 - 0200
0200 - 0400
0400 - 0600
0600 - 0800
TOTAL
AVERAGE
Qin (mgd)
Non-storm
5.9
6.3
6.6
6.5
5.4
4.2
3.0
1.6
1.6
3.0
5.1
5.6
5.9
6.3
6.6
6.5
5.4
4.2
3.0
1.6
1.6
3.0
97.5
4.43
Storm
0.9
2.8
4.2
3.5
3.2
2.3
1.9
1.4
0.8
0.3
0
0
0
0.4
1.4
2.1
1.8
1.6
1.2
1.0
0.7
0
31.5
1.43 .
Total
6.8
9.1
10.8
10.0
8.6
• 6.5
4.9
3.0
2.4
3.3
5.1
5.6
5.9
6.7
8.0
8.6
7.2
5.8
4.2
2.6
2.3
3.0
130.4
5.93
°inAt
(106 gal)
0.567
.758
.900
.833
.717
.542
.408
.250
.200
.275
.425
.467
.492
.558
.667
.717
.600
.483
.350
.217
.192
.250
10.87
5.93b
0 ^At
out
(106 gal)
0.458
.458
.458
.458
.458
.458
.458
.458
.458
.458
.458
.458
.458
.458
.458
.458
.458
.458
.458
.458
.458
.458
10.08
5.50°
AV
(106 gal)
+0.109
+ .300
+ .442
+ .375
+ .259
+ .084
- .050
- .208
- .258
- .183
- .033
+ .009
+ .034
+ .100
-1- .208
+ .25R
+ .142
+ .025
- .108
- .242
- .267
- .208
•1-0.788
EAV
(106 gal)
0.109
.408
.850
1.225
1.483
1.567
1.517
1.308
1.050
.867
.833
.842
.876
.976
1.184
1.443
1.584
1.609
1.501
1.259
.993
.784
0.784
at*>t * 2 hours
b ,-
x j| - 5.93
X 41 - 5.50
0.0833 days; Q
out
5.5 mgd
36
-------
continuous use for at least 2 days (probably more), and is not
empty at the end of the calculation. That is why the total
Q-j_nAt is greater than the total QoutAt. This alternative does
not appear to be attractive, since a much smaller basin could be
used with a small increase in QOUf
Method 2—Simple Concentration Balance
This method is based on straight-forward principles, but
may require extensive calculation for some applications. It is
also limited to a constant flow rate, constant equalization
volume, and in-line basins.
For a completely mixed basin, and a -material that is un-
changed during storage, the concentration balance equation is:
Ac = ^At (cin-c) (2-2)
where Ac = change of concentration in basin during At
Q = flow rate, constant
V = basin volume, constant
At = time interval
C£n = influent concentration, average over At
c = concentration in basin, at beginning of At
Note that Q/V is the reciprocal of the basin detention time.
This equation is easily modified if necessary for a material
that decomposes during storage, provided that there is a good
model for the rate of decomposition as a function of c. For
example, if first order decay occurs, then the equation is:
Ac = ^At(cin-c) - cKAt - (2-3)
where K = decay coefficient
Units of Q, V and At must be consistent with each other, as in
Method 1. Units of Ac, Cin, and c must be the same, but they
need not be consistent with Q and V; for instance, c can be in
milligrams per liter, and V in millions of gallons. The units
of K must be the reciprocal of the units for At.
Equations 2-2 and 2-3 may also be extended to model an
equalization basin that is not completely mixed, but the equa-
tions will become more complex.
In these equations, an initial value for c must be assumed.
After a few detention times, this initial value has little im-
pact on c and its variations. Q, V, and values of cin as a
37
-------
function of time must be supplied to perform the solution; the
result is c as a function of time.
The time interval At should be much less than the detention
time V/Q. Also, At should be shorter than the time-scale of
variations in Cj[n.
In the case of Equation 2-2, fewer steps are needed if the
following equation is used:
Ac = [1 - exp(-2At)] (cin-c) (2-4)
where exp = the exponential function
This type of equation was developed by Reynold, Gibbon, and
Attwood. (13)
Method 2 is deterministic, not stochastic; that is, it is
assumed that Cin is exactly known at each time interval .
The previously listed equations require that Ac be much less
than c. This means that At must be small compared to V/Q and to
1/K; hence, many steps are required for the calculations, es-
pecially for Equations 2-2 and 2-3. For a larger At, so that
fewer steps are needed, c is appreciably different from the
average concentration during At. A better approximation to this
average concentration is c + Ac/2. By rearranging Equation 2-3,
the balance equation becomes:
(c-c) - cKAt
Ac = - AF~6 - (2~3a)
1 + ^(2 + K)
This equation is preferred to 2-3 if a relatively large At must
be used.
Method 3 — Combined Flow and Concentration Balance
This method is an extension of Methods 1 and 2 and is par-
ticularly useful for realistic applications. Any realistic
pattern of flow and concentration variations may be used, and
any realistic operating rule may be used for the equalization
basin. For instance, with this method a basin could be analyzed
that would produce a constant COD load in the outflow stream.
Like Method 2, Method 3 may be adapted to reactions within the
basin and to basins that are not completely mixed. The equa-
tions are fairly simple to understand. The method has one major
drawback: It requires a lot of arithmetic, particularly if
flows and concentrations fluctuate rapidly.
38
-------
The basic equations are balances on flow and load. Equation
2-1 from Method 1 is the balance on flow. For a completely mixed
in-line basin and no reactions within the basin, the load
balancing equation is:
Qin cin At = QQut cAt + (V+AV)(c+Ac) - Vc (2-5)
Mass inMass out Increase in stored mass
For reactions within the basin that are adequately represented
by first-order decay, the equation is:
Q. ci At = QoutcAt + (V+AV)(c+Ac) - Vc + VcKAt (2-6)
where K = decay constant
Similar equations may be developed for side-line basins,
but equations applied during basin filling will be different
than those applied during basin emptying. Also, the concentra-
tion in the basin is not the same as the concentrations in Qj_n
and Qout (as defined in Figure 9). This is true for a side-line
basin even if mixing is complete and there are no reactions,
unlike the case for QOut °^ a simple completely mixed in-line
basin. For a completely mixed basin with first-order decay, the
equations during filling are:
Qin > Qout <2-7>
AV = (Qin - QQut)At (2-8)
cout = cin (2~9)
AVc. = (V+AV) (cn + Ac-.) - Vc,-. + Vo-.KAt (2-10)
in & is D a
Mass into basin Stored mass increase Decay
where cout = concentration in outlet, average over At
CR = concentration in equalization basin at beginning
of At
Ac = change in CR during At
When the basin is neither being filled nor emptied, the
equations are:
Qin = Qout (2
AV = 0 (2-12)
39
-------
Out
AcB = -cBKAt
(2-13)
(2-14)
When the basin is being emptied, AV is negative and the
equations are:
Qin < Qout
AV =
At
Ac
B
GinQin
At
'B
CB(Qout - Qi^
'out
(2-15)
(2-16)
(2-17)
(2-18)
out
Figure 9 shows the application of the variables for com-
pletely mixed basins.
Q. C.
in, in
out, out
Qin, cin
Qout, cout
IN-LINE BASIN
SIDE-LINE BASIN
Figure 9. Flow and concentration iteration schematics
40
-------
The most convenient form of the equation will depend on the
items that are given, and those that are required. For instance,
suppose that an in-line basin is under study and the given quan-
tities are K, Qinr cin' anc^- Qout' plus initial conditions of c
and V, so that the desired outputs are c and V as functions of
time. Then Equation 2-1 may be written:
AV = Q±nAt - QQutAt (2-19)
and Equation 2-6 may be written:
0. c. At - c(Q^ At + AV 4- VKAt)
A -Lll -Lil ^J U. L- -/*"»«"* rt \
Ac = v + AV (2-20)
Equation 2-19 may be solved iteratively and its results
will supply the V and AV values for each step of Equation 2-20.
Each step of Equation 2-20 requires the previous Ac to generate
the new c, which is required to go on.
Equation 2-20 requires that Ac be much smaller than c for
much accuracy; otherwise Ac will tend to oscillate during the
calculations. This oscillation may be avoided in two ways. One
is to select a small At so that AV and Ac are small, which re-
quires more steps to the calculation. If a fairly large At is
required, better results will occur by refining the mass balance
equation 2-6 by using c + Ac/2 for the average concentration
during At, as shown in Method 2:
Ac Ac
Q-^C- At = Q_,,4. (C+-S5-) At + (V+AV) (c+Ac) - Vc + V(c+-^)KAt (2-21)
in in out 2 '
Mass in Mass out Increased stored Decay
mass
In this case , the iteration equation becomes :
Q,nc. At - c(AV + Q011+.At + VKAt)
Ac = in in - jrr - 2HE - (2-22)
V -f AV + ^ (QQut + VK)
This equation is based on a completely mixed in-line equaliza-
tion volume and first-order decay.
For a side-line basin, the necessary refinement depends on
the magnitude of KAt. If K is small, KAt can be small even
though At is appreciable. Thus, refinement only applies to
Equation 2-18, for concentration of the blended stream, which
becomes :'
41
-------
Ac
cinQin + to. + -A(Qout - Qin)
= - B2 - (2_23)
out
The other equations (Equations 2-7 through 2-17) are unchanged.
Equation 2-10 may be rearranged into an iteration equation:
V(c. - CR) - VcRKAt
Thus, equations are available for side-line fully mixed basins
with slow first-order decay. Equations for fast first-order
decay could be developed if needed.
These Method 3 equations may be worked using units as
described in Methods 1 and 2. For example, a possible set is:
V, AV (millions of gallons)
^in' ^out (mi11:i-ons of gallons per day)
At (days)
Ac, cin, c, cQut, cR (milligrams per liter)
K (per day)
These units, however, yield an odd combination for load rates.
The input load rate is:
Win = Qincin <2
The output rate for an in-line basin is:
Wout = Qoutc <2
The output rate for a side-line basin is:
Wout = Qout cout <2
The units of Win and Wout will be mgd-mg/£, which must be multi-
plied by 8.34 (pounds per million gallons) per (milligram per
liter) to be in the common units of pounds per day.
The initial conditions of c (or eg) and V affect the calcu-
lations. If Qin/ cin' and Qout are constant or periodic (e.g.
diurnally varying only) , c (or 03) and V will eventually reach
periodic variation, and the initial condition of c (or eg) will
42
-------
become unimportant. This will occur more rapidly as the initial
value of V.decreases; it also helps to start with a roughly
realistic value for the concentration in the basin.
A programmable calculator is very helpful for these calcu-
lations. The examples were performed on a calculator with eight
memories, "stack," and 50 program steps. The calculator's total
capacity was almost a necessity.
Examples 4 and 5 illustrate the use of these equations.
Example 4—In-Line Basin—
This example uses the flow variation in Example 2 (see
Method 1), a diurnal variation in input BOD concentration, a
decay rate, and initial conditions to calculate the output con-
centration.
The input variations in flow and concentration are shown in
Figure 10. This information is drawn from EPA's Process Design
Manual for Upgrading Existing Wastewater Treatment Plants.(12)
Note that mass loading is expressed in pounds per day, which
signifies a rate, not the duration of that loading rate. Other
input values are:
llj
•o
I 5
cc
o
_i
IL.
BOD CONCENTRATION
FLOW RATE
300
15.00O
o>
2OO
cc
I-
z
LU
o
10O o
o
o
m
10,000
5,000
z
Q
co
o
o
m
MIDNIGHT
12
NOON
18
24
MIDNIGHT
TIME OF DAY
BOD MASS LOADING:
PEAK: AVERAGE= 1.97
MINIMUM: AVER AGE = 0.14
PEAK: MINIMUM: 14.59
Figure 10. Wastewater flow and BOD variation before equalization
43
-------
V, 0.1 million gallons at 8:00 a.m.
c, initially assumed at 150 mg/1 at 8:00 a.m.
K, 0.1 per day
Q-out' constant, 4.57 mgd
This V of 0.1 million gallons at 8:00 a.m., coupled with the
basin operation from Example 2, requires a maximum V of 0.88
million gallons at about 10:00 p.m. The K value is roughly
correct for BOD in domestic sewage when the cell residence time
is shorter than the critical cell residence time for biological
growth. Much larger K values will apply if a biomass can
develop.
The first few steps of the calculation are shown in Table
5. The calculation was begun at 8:00 a.m. to minimize the im-
portance of the initial assumption for c; the calculation
reached within one percent of its eventual steady variation
after only two steps (4 hours). The calculation was carried
out over three full days to verify that c was the same (within
one percent) at each step from one day to the next.
The results are plotted in Figure 11 together with the
results from Example 5. The in-line basin produces a delay in
the peak concentration in the flow to treatment. But it does
not appreciably reduce the peak, because the volume in storage,
V, is small at 8:00 a.m. Thus, very little weak wastewater is
in the basin to dilute the stronger mid-day wastewater. If a
much larger V were maintained during the early morning hours,
there would be more concentration smoothing, as illustrated
below.
In-line storage reduced the average load from 7,415 to
7,185 pounds per day by virtue of BOD decomposition in the
basin; a reduction of approximately 3 percent. The peak-to-
average ratio for BOD mass loading was reduced from 1.92 to
1.48, and the minimum-to-average ratio was raised from 0.15 to
0.62, by in-line equalization. The overall peak-to-minimum
load ratio was reduced from 13.08 to 2.40.
Example 5—Side-Line Basin—
The same conditions as in Example 4 were applied to a side-
line basin. The first few lines of the calculations plus the
first transition from basin filling to basin emptying are shown
in Table 6. The results are plotted in Figure 11. The solu-
tion took 15 steps (more than a complete day) for the periodic
output to stabilize. Through the initial 15 steps the basin
concentration, CB^ is affected more than one percent by the
initial value for CB (150 mg/1). The calculation was carried
out for three full days (36 steps) to verify output stability.
44
-------
TABLE 5. BEGINNING FLOW AND CONCENTRATION ITERATIONS:
EXAMPLE 4, IN-LINE BASIN
Time interval
Qin (mgd)
cin (mg/1)
V (106 gal)
AV (106 gal)
(106 gal-mg/1)
QoutAt (106 gal);;
VKAt = 0.00833V
(106 gal)
QoutAt+AV+VKAt
(106 gal)
Ac (mg/1)
c (mg/1) '
wout (Ik/day)
0800 to 1000
5.1 . '
180
0.100
0.044
76.5
0.381
0.001
0.046
37.6 '..-
150
6434
1000 .to, 1200
'5.6
253
0.144
0.086
118.1
0.381
0..001
0.468
71.97
187.5
8522
12,00 to 1400
5.9
288
0.230
0.111
141.6 -
0.381
0.002
0.494
25.10
259.6
10372
1400 iio 1600 '
6.3
270
0.341
0.144
141.8
0.381
0.003
0.528
-12.57
284.7
10611
TABLE 6. BEGINNING FLOW AND CONCENTRATION ITERATIONS:
EXAMPLE 5.j SIDE-LINE BASIN
Time interval
Qin (mgd)
cin (mg/1)
V <106. gal)
Av (106 gal)
CB (mg/1)
AcB (m'g/lf
<=out (mg/lOi '
Qout (mgd)
wout db/day)
0800 to 1000
5.1
180
, 0.100
0:044
150
8.30
. 180
4.57
6,860
1000 to 1200 .
5.6
253
0.144
0.086
158..3
34.58
" .253
4.57
9,643
1200 to 1400
5.9
288
."0 . 230
0.111
192.9 .
29.88 "
288
4.57 . '
10,977
1400 to 1600
6.3
270
0.341 .
0.144
. . .222.8
12.72
\ 270
4.57
10 ,291
2000 to 2200
5.4
1.40
0.815
0.069
210.7
-7.14
140
4.57
5,336
2200 to 2400
4.2
153
0.884
-0.031
203.57
-1.70
157.03 .
4.57
5,985
45
-------
15,000
a
^
JO
ul
ce 10,000
z
a
5,000
BEFORE EQUALIZATION
AFTER SIDE-LINE
EQUALIZATION
AFTER IN-LINE EQUALIZATION
MIDNIGHT
10
12
NOON
14
16
18
20
O
g
CC
I-
UJ
-------
The results have some interesting features. The side-line
basin had a higher average concentration than the in-line basin
because the weak early morning wastewater did not enter the
side-line basin, and the decay was thus slightly increased. The
equalized flow had considerable fluctuation in concentration and
mass load during the night, as the equalized flow was composed
of different proportions of weak, entering wastewater and
strong, stored wastewater. As expected, the side-line basin was
not effective at reducing the daily concentration peak, but
neither was the in-line basin. For raising minimum concentra-
tions in the early morning, the side-line basin was decidedly
better. Generalizations in this regard are hazardous. It has
been assumed in previous reports (14) that in-line basins provide
concentration equalization, but side-line basins do not.
Examples 4 and 5 show that this is not necessarily correct, and
that in this case the side-line basin is more effective for
raising the minimum concentration.
The side-line basin affected the peak-to-average BOD
loading through flow leveling only, lowering this ratio from
1.92 to 1.53. The minimum-to-average BOD loading was raised
from 0.15 to 0.74.. The overall peak-to-minimum load ratio was
reduced from 13.08 to 2.05. Overall, given the input assump-
tions of this example, the side-line equalization configuration
produced load and concentration leveling performance in every
respect comparable to the in-line system.
Example 6—Excess Volume for Load Equalization—
Examples 4 and 5 have used equalization volumes calculated
by Method 1 procedures to just satisfy requirements for the
equalization of daily diurnal variations in plant influent flow
rate. Only a minimal-dead volume of 0.1 x 10^ gallons, or a
total volume equal to 113 percent of required volume, was used
to reflect conditions required for aeration of mixing equipment.
This example is provided to illustrate the potential for more
complete load and concentration leveling made possible by in-
creasing the excess equalization volume..
The same conditions used in Example 4 were applied to an
in-line basin. The basin volume used in this case included
1.6 x 106 gallons dead volume, or a total volume equal to 300
percent of that required for equalizing diurnal flow variations.
Equalized BOD loads and concentrations were computed as in
Example 4. The results are shown graphically in Figure 12,
along with input variations and the output from Example 4 using
only minimal dead volume. It can be seen that increasing the
total storage volume from 0.88 x 106 gallons to 2.34 x 106 gal-
lons, corresponding to 19 percent and 51 percent of the rated
daily average treatment plant capacity (4.57 mgd), reduced the
peak-to-average BOD load ratio from 1.92 to 1.20; compared with
the reduction from 1.92 to 1.48 realized using minimal dead
47
-------
15,000
UJ
< 10,000
o
tn 5.0OO
BEFORE EQUALIZATION
AFTER IN-LINE
EQUALIZATION
13% DEAD VOLUME
AFTER IN-LINE EQUALIZATION
200% DEAD VOLUME
10
12
16
18 20
22 24
o>
E
O
H
K
250
20O
150
100
\ \
AFTER IN-LINE
EQUALIZATION
13% DEAD VOLUME
BEFORE EQUALIZATION
\
MIDNIGHT
'AFTER IN-LINE
EQUALIZATION
200 % DEAD VOLUME
10
12
NOON
14
18
TIME OF 'DAY
20
22
24
MIDNIGHT
Figure 12. Example 6, excess volume for load equalization
48
-------
volume. The peak-to-average BOD concentration ratio was reduced
from 1.69 to 1.20, compared to the reduction from 1.69 to 1.51
realized by just meeting requirements for flow equalization.
The average load reduction due to BOD decay increased to approx-
imately 6 percent, compared to 3 percent for Example 4.
The average residence time in the larger basin (assuming
well mixed conditions) is approximately one half day compared
to approximately 4-1/2 hours for Example 4. The large dead
volume and longer residence time would warrant serious consid-
eration of aeration and/or mixing equipment to prevent odor
problems and solids accumulation. Increased costs for the
larger volume, and any additional equipment, should be justified
in terms of balancing cost savings in downstream. processes; such
as reduced peaking capacity in biological processes designed for
nitrification and denitrif ication.
Method 4 — Sine Wave Method
In Method 3, stepwise equations were presented, based on
conservation of volume of wastewater and on mass balance of con-
stituents. If influent concentration, flow, and outflow are
certain very simple functions, then it is not necessary to solve
the equations stepwise; a direct "solution is possible. Such a
direct solution requires little computation and little inlet
data, since simple functions are represented by very few
numbers.
A direct solution has been obtained assuming that both flow
and concentration may be represented by sine waves, in phase
with each other, and with a period, of one day. (15) The flow
equation used was of the form:
Qin (t) * QA " (QP "" QA> Sin 2lTt (2-28)
where Q. (t) = influent flow as a function of time
Q, = average flow
Qp = peak influent flow
t = time, in days
From such equations, simple and rapid estimates can be made of
certain equalization operations. For instance, if a constant
outflow is desired, Equation 2-28, minus the constant outflow,
may be integrated from t=0.5tot=1.0 day, yielding the
working volume of storage:
49
-------
Also, the rectangular wave approach could be used for concentra-
tion and mass loading.
Generally, the rectangular wave method has similar limita-
tions to the sine wave method. For peak-to-average input ratios
less than (ir-1) the rectangular wave model gave a somewhat more
conservative estimate; for ratios greater than (ir-l) the sine
wave model becomes rapidly more conservative (see Figure 13).
When a rough estimate is required and iteration is impractical,
the rectangular wave or sine wave methods may be used to esti-
mate volumes required to equalize daily diurnal variations.
Nevertheless, it should be recognized that iteration is more
flexible and provides reasonable accuracy.
50
LU
CC
UJ
e?
EC
LU
40
30
-------
1.0
V = ! (Q. - QA)dt (2-29)
0.5 ln A
>
QP " QA
V = -i—-—- (2-30)
Equations such as 2-30 are very easy to compute. Concentration
and mass loading fluctuations as above were considered,(15)
assuming completely mixed basins and first-order decay.
For comparison, Equation 2-30 was applied to the flow
variation of Example 2, Method 1. Equation 2-30 yielded a
working volume of 0.65 million gallons, whereas 0-. 79 million.
gallons were indicated in Example 2. Therefore, Equation 2-30
and similar equations must be used with great caution, because
they may give a lower answer.
The limitation of this approach is that municipal waste-
water variations do not follow simple sine waves. This was
recognized in the reported work because a rough approximation
was sufficient for the purpose of the example illustration.
Considering that iterative methods are quite workable for di-
urnal variations, that the sine wave method may give lower
answers, and that the sine wave method is applicable only to
diurnal variations, it should be considered as approximate and
used only to develop rough estimates.
Method 5—Rectangular Wave Method
The rectangular wave method is similar to the sine wave
method, except that flows are approximated by rectangular waves.
This method has been described (16) and applied assuming the
peak-to-average flow ratio to equal the average-to-minimum
ratio; which is a rough but reasonable approximation for many
sewer systems. With this assumption, and constant outflow,
the necessary volume is:
I --M2
V = CVX/; (2-31)
A(x2-l)
where V = equalization volume, working range
Q, = average flow
x = peak-to-average flow ratio = average-to-minimum
flow ratio
This equation is easily solved. A similar equation, not much
more difficult, could be developed without assuming that the
peak-to-average ratio equals the average-to-minimum ratio.
50
-------
Specialized Analytical Methods
Procedures described in Methods 1 through 5 are sufficient
for most routine analytical needs in planning and preliminary
design of equalization for typical municipal wastewater treat-
ment applications. However, not all wastewater treatment appli-
cations lend themselves to simple routine analysis. This sec-
tion briefly summarizes two additional, more sophisticated and
ultimately more powerful analytical methods for analyzing
specialized equalization requirements. Other methods include
auto-covariance and power spectrum. Application of these
methods to equalization analysis can be found in references
11, 19, 20 and 21.
Method 6—Batch Dumping Method—
For wastes produced in large numbers of batches (each batch
being of equa.1 size, but occurring randomly) , a method developed
by Beaudry is useful.(17) It requires no data besides the
volumes of the batches and of the equalization basin, and easily
computes a prediction for reduced scatter of the data due to
equalization. The method, however, appears to be of limited use
for municipal wastewater because the flows have periodic tenden-
cies; they do not occur at strictly random times. Beaudry"s
method, or a development of it, may be useful for very small
treatment systems serving, say, 20 families, where batch effects
may be noticeable. Also, the common fill-and-draw lift station
produces batches of wastewater, so a batch dumping method may be
useful for systems with many such lift stations.
Method 7—Random Concentration Method—
This method is simple to apply, but requires considerable
data and is limited in application. An autocovariance method
will require more computation, but will give more accurate
results from the same data, without such strict limits of
application.
The random concentration method is described in detail in
the Areawide Assessment Procedures Manual, Volume 1,(11) and in
more simplified form elsewhere.(18,19 and 20)
The basic basin formula is:
f®. - At (?
S± ~ 2t (2
where S = variance of effluent concentration
S- = variance of influent concentration
At = interval between samples
t = detention time of basin
52
-------
This equation is easily computed, yielding a statistical result.
However, it only applies to an in-line basin with constant flow
and volume; the basin must be fully mixed and no significant
reactions may occur in the basin. (Equations of reference 12
permit first-order decay.) To establish Si, it is necessary to
have a large number of samples collected at intervals of At.
At least 80, and preferably 150, samples should be taken,
covering at least ten times the length of the time scale of the
unacceptable effluent variation. Furthermore, the data must not
have any periodic tendencies, and At must be sufficiently long
that any autocovarience effect would be negligible. If there
are any appreciable periodic or autocovariant effects, Equation
2-32 will !show better equalization than will actually occur.
Nevertheless, At must be much less than t unless the equation
is made more complex. The random concentration method is ex-
pected to find very little use in municipal applications.
MEASUREMENT AND EVALUATION OF EQUALIZATION EFFECTS
Introduction
In the planning and design of treatment systems including
equalization, effects of equalization on the prospective treat-
ment system must be estimated to enable the making of reasonably
accurate comparisons between equalized and unequalized treatment
schemes. Final comparisons will be made on the basis of the
cost of alternative systems. Effects of equalization on unit
process and treatment system performance must be determined to
enable treatment component sizing prior to cost comparison.
The critical parameter for design of individual treatment com-
ponents will be a combination of influent flow rate, concentra-
tion, loading of typical BOD, and/or suspended solids. Avail-
able data projected for design conditions must be evaluated
accordingly.
Data for this study were collected from treatment plants
throughout the United States. Data collected consist primarily
of existing plant operating records. No new performance inves-
tigations were conducted. Extensive data from previous EPA
sponsored detailed studies of equalization performance were
also used. •
Performance Evaluation Method
In an effort to identify and assess the magnitude of equal-
ization effects on unit process and treatment plant performance,
two types of comparisons can be iriade: Between equalized and
unequalized periods of operation at the same plant (or between
equalized and unequalized parallel treatment plant sections
during the same period). And between a long-term average per-
formance of those treatment plants that have and do not have
53
-------
equalization facilities; possible comparisons could be based on
several years of operating data. Frequency distributions or
probability plots of average daily observations have been used,
since the available data is generally in the form of average
daily values of flow rate, concentration, etc., and to facili-
tate comparison of the large (annual) data sets. Performance
characteristics of unit process or treatment systems may be
identified by summarizing operating records in this form.
Probability Plot Characteristics —
A probability plot is a graphic presentation of the cumula-
tive frequency distribution of observations in a data set, pre-
senting the magnitude of individual or groups of observations
as a function of their frequency of occurrence. This method has
been used to summarize both influent and effluent characteris-
tics of municipal sewage treatment plants. (22) Data have
generally been found to be approximately log normally distri-
buted. (23) In such cases the data form a straight line on log
probability paper indicating that the logio of individual ob-
servations are normally distributed. The well established
characteristics of these distributions may conveniently be used
to describe both level and variability of treatment performance.
It should be recognized that these distributions apply only to
completely random observations. Thus, some approximation may be
involved in applying treatment plant influent and effluent char-
acteristics that have occasionally been shown to have signifi-
cant, periodic influence. (24) A plot of daily average influent
suspended solids concentration for one year of operation
(Figure 14) illustrates an observed log normal distribution.
The primary operating characteristics illustrated by proba-
bility plots are the level of performance and the variability
of performance. They are significant because they relate to
well established sewage treatment system capabilities, common
effluent discharge requirements, and anticipated effects of
equalizing input variations.
Level of operating performance' — The efficiency of unit
process or treatment system is interpretable from the median,
mean and range of a given distribution, or set of data. The
median may be seen at a glance from the midpoint or 50 percent
value of the log distribution. The mean (me) of the data set
can be computed from the median (m^) using the relationship:
m =
e
= md exp (2 .
where a, = the standard deviation of the log distribution,
"lO determined graphically as:
Iog10 (x) @ 50% - Iog10 (x) @ 50% + 34.15%
and where exp(x) = ex
54
-------
Variability of operating performance—The variability or
stability of treatment system performance on a daily average
basis can be interpreted from a slope and range of comparable
distributions. The slope of a distribution may be characterized
by the standard deviation of the logarithms of the observations.
Strictly speaking, since it is the logarithms of the observed
values that are normally distributed, and not the values them- '
selves, the standard deviation has no meaning in terms of the
values; concentra'tion for example. Nevertheless, the slope of
a distribution will provide a useful tool for evaluating the
variability of comparable data sets, if considered along with
the range of values of the distribution covered; for example,
by 80 percent (10 percent to 90 percent) or 98 percent (1 per-
cent to 99 percent) of the observations.
Simple visual comparison of distribution slopes should be
made with caution. If two distributions have identified median
values their slopes will be directly comparable. In this case
visual comparison is sufficient to identify differences in
variability.
If median values are significantly different the respective
ranges of observed values must be examined to assess relative
variability in average daily performance. For example, as shown
in Figure 15, two log normal secondary effluent BOD distribu-
tions having median values of 11 mg/1 and 18 mg/1 are parallel
by visual comparison. However, variability of the two data sets
differs by more than a factor of 2, with ranges of observed
values encompassing 98 percent of the data (from 1 percent to
99 percent) from 5 mg/1 to 23 mg/1, and from 8 mg/1 to 40 mg/1.
Application to Plant Operating Records—
A wide variety of process design, operating, and environ-
mental factors contributes to the establishment of these process
performance characteristics. Although observed differences
between individual data sets presented as comparable may appear
to be related to equalization effects, conclusions should be
drawn cautiously. Similar patterns observed to correspond to
each other in unrelated data sets should increase the proba-
bility that the effects are due to equalization. General un-
availability of data prevents use of more rigorous quantitative
analysis.
Care must be taken in examining the presented figures in
order that appropriate associations will be made between the
distributions and characteristics of the physical systems they
represent. For example, although distributions shown typically
provide an estimate of observed median influent and effluent
quality, observed median removal efficiencies cannot be deduced;
yet the value computed will be reasonably close to that computed
from the distribution of observed daily removal efficiencies.
55
-------
IUUO
800
700
600
500
400
v 300
o>
E
. 200
Z
o
<
tr 100
Z 80
UJ 70
0 60
§ 50
o 40
30
20
in
^
'"\^
" X,
^x
"^
X;
^
^
-
\
^.
^
•\
^
: ^N
O.I I 10 20304050607080 90 99
PERCENT OF VALUES GREATER THAN
99.9 99.99
Figure 14. Distribution of log daily average influent TSS
concentrations, Ypsilanti Township, Plant 1, 1974
o
E
O
cc.
UJ
o
z
o
o
100
80
70
60
50
40
30
20
10
8
6
5
4
3
2
,01 O.I I 10 20304050607080 90 99 99.9 99.99
PERCENT OF VALUE GREATER THAN
Figure 15. Range difference for distributions of equal slope
56
-------
Strictly speaking the difference between influent and effluent
median should be interpreted as just-that; or that the median of
observed effluent values Was a given percentage of observed in-
fluent values.
It should be remembered that the individual observations in
each distribution were daily average values, and that the range
is the range of daily average values observed in the period of
record used.
Influent and effluent distributions—Influent distributions
are used to illustrate comparability of conditions imposed on
the plants being compared, or of the different periods compared
for the same plant. Effluent distributions are used to illus-
trate observed performance at a given level of treatment (pri-
mary, secondary, or tertiary) that can be compared between
plants or different operating periods for the same plant. If
performance is improved by equalization, the median level of
performance would be expected to be lower than for a comparable
unequalized period or plant. If stability of performance is
improved by equalization, then observed effluent variability or
the slope and range of the distribution should be reduced.
There is no implication that effects of equalization on
plant unit processes will not result in lower effluent concen-
trations. Influent and effluent distributions expressed as
daily average loads allow the evaluation of effects of equali-
zation on the combined variations in concentration and flow at a
given plant. Flow equalization has been shown to have a greater
effect than concentration on leveling daily diurnal mass load
variations. Such effects are not reflected directly in the
probability plots because they are developed from daily compo-
site average observations. Loading variations expressed on this
basis will be affected by equalization insofar as daily average
concentrations are affected by beneficial influence on process
performance. In the case of comparing equalized treatment
operations concurrently supplied from the same waste source
(illustrated by observations of Ypsilanti Township and the MERL
pilot plant studies), it was shown that distributions of concen-
tration and loading observations should be in direct proportion
to one another.
Comparison of average annual performance—Using probability
plots the mean annual effluent BOD and TSS concentrations were
computed from available data for all plants with equalization
data compiled in this study. The distributions of average
annual effluent composition enable the making of a general com-
parison and distinction between equalized plant performance and
unequalized plant performance.
57
-------
Performance Requirements
Performance requirements are imposed on all municipal
sewage treatment plants by a combination of State and Federal
regulatory requirements. The Federal (EPA) minimum requirement
for meeting the definition of secondary treatment (25) is the
common denominator of performance requirements. State regula-
tory agencies commonly impose more stringent requirements to
meet the needs of local receiving waters.
Performance requirements are typically established in the
form of fixed limits on amount of pollutant materials acceptable
in a treated effluent. The EPA definition of secondary treat-
ment requires that:
(a) Maximum 30-day average effluent BOD and total sus-
pended solids (TSS) shall not exceed 30 mg/1 each.
(b) Maximum 7-day average effluent BOD and TSS shall not
exceed 45 mg/1 each.
(c) A 30-day average minimum of 85 percent removal of
influent BOD and TSS must be achieved.
The ability of a treatment plant to meet imposed perfor-
mance requirements depends on many interrelated factors per-
taining to its design, operation and general influent condi-
tions. A baseline for evaluating treatment plant performance
characteristics has been established by a recent statistical
study for 27 activated sludge plants in the United States.(26)
The plant sample was selected to provide a broad cross-section
of typical well operated plants ranging in size from 0.59 mgd
to 333 mgd. Considering the distribution of daily average,
7-day average and 30-day average effluent concentrations, sta-
tistical analysis was used to compute the distributions of
effluent BOD and TSS required for compliance with EPA secondary
treatment standards. The resulting distributions (Figure 16)
are based on daily operating data for one recorded year from
each of the 27 treatment plants. The significance of these
distributions as they pertain to evaluating equalization bene-
fits are as follows:
(1) Annual distributions of average daily effluent con-
centrations coincident with or below the calculated
distributions of Figure 16 will consistently comply
with secondary treatment requirements.
(2) The calculated secondary effluent concentration dis-
tributions were observed to be approximately log
normally distributed.
58
-------
(3) Activated sludge secondary BOD effluent performance
may be expected to be slightly more stable than TSS
effluent performance, as indicated by the relative
slopes of the distributions. This comparison may be
expressed numerically in terms of the log standard
deviations: log a^QD =0.21 and log aTss = 0.27.
For activated sludge treatment plants designed to
meet EPA secondary treatment requirements, mean
monthly BOD and TSS of 30 mg/1, the median annual
BOD5 concentration may be expected,to be slightly
higher than the mean annual TSS concentration
* 15 mg/1 BOD versus = 10 mg/1 TSS.
100
80
70
60
SO
40
30
20
o>
E
<
a:
LU
o
z
o
o
IO
8
6
5
SECONDARY
EFFLUENT
TSS
•SECONDARY
EFFLUENT
BOD
.01 O.I I 10 20304050607080 90 99 99.9 99.99
PERCENT OF VALUES GREATER THAN
Figure 16.
Distribution of log effluent concentrations for 22
activated sludge plants designed to meet EPA
secondary treatment requirements
59
-------
REFERENCES
8. Process Design Manual for Nitrogen Control, EPA Technology
Transfer, Brown and Caldwell, 1975.
9. Handbook for Sewer System Evaluation and Rehabilitation,
U. S. EPA Municipal Construction Division, Report MCD-19,
December 1975,
10. U. S. EPA, J. T. Deputy Asst. Admin., Program Requirements
Memorandum 75-34, "Grants for Treatment and Control of
Combined Sewer Overflow and Stormwater Discharges."
11. Areawide Assessment Procedures Manual, Volume 1, U. S. EPA,
MERL - OR & D, July 1976.
12. Process Design Manual for Upgrading Existing Wastewater
Treatment Plants, U. S. EPA, Technology Transfer, p. 3-7
to 3-10, October 1974.
13. E. L. Thackston, ed., "Equalization in Process Design in
Water Quality Engineering", Jenkins Publishing, 1972.
14. Process Design Manual for Upgrading Existing Wastewater
Treatment Plants, U. S. EPA, Technology Transfer, p. 3-2
to 3-10, October 1974.
15. R. Smith, R. G. Eilers, E. D. Hall, "Design and Simulation
of Equalization Basins", U. S. EPA, Advanced Waste Treat-
ment Research Laboratory, Cincinnati, Ohio, February 1973.
16. C.N. Click, F.O. Mixon, "Flow Smoothing in Sanitary Sewers."
17. A. T. Wallace, "Analysis of Equalization Basins", J. Sani-
tary Engineering Division ASCE, 94:1161, December 1968.
18. Process Design Techniques for Industrial Waste Treatment,
C. E. Adams, W. W. Eckenfelder, editors, Associated Water
and Air Resources Engineering, Inc., Enviro Press, p. 39-
50, 1974.
60
-------
19. V. Novotny, R. M. Stein, "Equalization of Time Variable
Waste Loads", J. Environmental Engineering Division, ASCE,
102:613, June 1976.
20. V. Novotny, A. J. Englande, Jr., "Equalization Design
Techniques for Conservative Substances in Wastewater Treat-
ment Systems", Water Research, 8:325, 1974.
21. D. M. DiToro, "Statistical Design of Equalization Basins",
J. Environmental Engineering Division, ASCE, 101:917,
December 1975.
22. G. M. Fair, J. C. Geyer, Water Supply and Waste-Water
Disposal, John Wiley & Sons, Inc., April 1963.
23. Effluent Variability from Wastewater Treatment Processes
and its Control; Progress in Water Technology; A Journal
of the International Association on Water Pollution
Research; 8:1, 1976.
24. R. V. Thoman, "Variability of Wastewater Treatment Plant
Performance", ASCE, J. Sanitary Engineering Development,
96:SA3, June 1970.
25. Water Programs-, Secondary Treatment Information, Part II,
U.^'S. Environmental Protection Agency, Federal Register,
Volume 38, No. 159, August 17, 1973.
26. W. H. Hovey, et al., Optimal Size of Regional Wastewater
Treatment Plants, California Water Resources Center,
University of California, Davis; Contribution No. 161,
January 1977.
61
-------
SECTION 4
SUMMARY OF EXISTING FLOW EQUALIZATION FACILITIES
EQUALIZATION FACILITIES SURVEY
A nationwide survey was conducted to identify communities
having flow equalization facilities as an integral part of their
wastewater system. And to determine design details, operating
practices and costs of the broadest possible range of equaliza-
tion facilities. Wastewater systems with equalization facili-
ties were located throughout the United States by contacting the
ten EPA Regional Offices, and then central and district office
personnel in water pollution control agencies in each of the 48
contiguous states. Some states', notably Michigan and New York,
maintain a comprehensive list of all municipal treatment systems
in the state coded to identify unit process components in each
treatment plant. Treatment plants including 'equalization fa-
cilities in those states were thus readily identifiable. In
other states central and district office personnel provided
plant references from their records and personal contacts. Ad-
ditional plants were located by references provided in direct
interviews with operating personnel of wastewater systems having
equalization facilities, and through articles in the current
literature.
Survey Scope
The survey of existing equalization facilities was conduc-
ted by phone interviews with plant operating personnel, by dis-
tribution of written requests, by interviews with state regula-
tory agency engineering staffs and inspection of state main-
tained operating records, and by direct site visits and inter-
views with design engineers and plant operating personnel.
Specific information requested from each wastewater agency
using flow equalization is summarized as follows:
1. Treatment and hydraulic capacity of wastewater treat-
ment plant, including plant efficiency and discharge
requirements.
62
-------
2. Flow range and frequency of occurrence of plant in-
fluent flow; including minimum, average, and peak dry
and wet weather flows.
3. Treatment plant type and schematic diagram.
4. Type and size of equalization structure installed.
5. Location of equalization structure within collection
system or sewage treatment plant.
6. Means utilized to fill and empty equalization
structure.
7. Aeration or mixing provided in equalization structure.
8. Methods used for scum and solids removal.
9. Odor control provided during operation of flow equali-
zation facility.
10. Type and quantity of flow to equalization facility.
11. Comments or problems associated with operation of the
flow, equalization facility.
12. Equalization facility cost information including
component costs, total construction cost, bid year
and month, and yearly operating and maintenance costs.
One hundred ten municipal and sewerage agencies owning and
operating 147 equalization systems responded in varying degrees
of completeness to the survey. Tables 7, 8, 9, and 10 summarize
the physical features and responses,of each respondent for fa-
cilities smaller and larger than 1 mgd, respectively. An addi-
tional 66 equalization facilities are identified for which no
detailed information was obtained. These are listed in Table 11.
Principal respondents to the survey included plant opera-
ting personnel, municipal engineering staff personnel, and con-
sulting engineers. One or more of these sources provided in-
formation for each contact resulting in detailed information.
Available time and budget limited practical survey procedures
to telephone and written correspondence in the majority of
cases. Reliance on these methods resulted in varying degrees
of detail obtained from plant to plant. In some cases informa-
tion received must be considered qualitative. This is particu-
larly the'case for plant flows reported and used in establishing
peak-to-average flow ratios for typical dry and wet weather
periods.
63
-------
TABLE 7. EQUALIZATION FACILITIES SUMMARY: PLANTS SMALLER
THAN 1 MGD—TREATMENT PLANT CHARACTERISTICS
Treatment Pl^nt
and Location
1. California Institute
for Women
I on tana, CA
2. Deer Creek STP
LI Dorado, CA
J. FUncho Bernardo STP
San Diego, CA
i. w»mt» STP
WilHts, CA
£•, Northeast KWTP
Oskalooska, IA
d. Southwest WWTP
CMkalooska, IA
". Boyne City HWTP
Boyne City, Ml
i, Dimondalc KWTP
Dimondale, MI
'), Eisexvillc STP
Csscxvillc, MI
:0r DflWSon KWTP
Dawson, MH
. ! . Bl*?)CHOod HWTP
Blackwcod, NJ
L. Borough of Palmyra
WWTP, Burlington
County, NJ
13. Cleiaenton STP
-lenc-nton, MJ
Plant .
^ d
Size,
ragd
Treat
(Hyd)
0.2
0.33
(0.5)
1.0
(1.75)
0.64
0.98
(3.3)
0.81
(2.5)
0.7
(2.3)
0.2
0.75
(1.87)
0.26
0.63
0.53
0.64
P/A
(D.W.)
1.50
3.0
1.66
1.56
1.15
1.4
1.43
1.14
2.0
1.28
1.75
1.71
P/A
(W.W.)
2.57
3.75
15.95
1.96
4.29
2.99
3.29
1.60
1.95
Plant
Type
Act
sludge
Act
sludge
Act
sludge
Act
sludge
Trickling
filter
Act
sludge
Act
sludge
Act
si udge
Trickling
filter
OX
ditch
Act
sludge
Trickling
filter
Act
sludge
Degree
of
Treat-
ment
S
S,P
S
S
S
S
S,P
S
S
S
S
S
S
When
Equali-
zation
Constrc
AP with
expansion
WP
AP with
upgrading
AP with
expansion
AP with
upgrading
WP
AP with
upgrading
AP
No other
change
AP with
upgrading
WP
WP
AP
No other
change
AP
No other
change
Location of
Equalization
Facility
Following
headworks
Following
headworks
Following
headworks
Following
headworks
Following
meter pit
Following
inlet struc
Following
headworks
bar screen
Following
headworks
Following
wet well
Following
headworks
Following
headworks
Following
pump station
Following
headworks
Type of
Structure
Tank
cone
covered
Basin
earthen,
lime
stabilized
PVC liner
covered
5 basins,
1 asphalt
lined,
4 unlined
Basin
earthen
lined
2 basins
unlined
Basin
clay
lining
Tank
cone
Tank
cone
Basin
cone
Tank
cone
Tank
cone
Tank
steel
(continued)
64
-------
TABLE 7 (continued)
Treatment Plant
and Location3
14. 'E. Windsor WWTP
E. Windsor, NJ
15. Service Area 3-S
N.J. Turnpike Auth
Cherry Hill Twp, NJ
16. Service Area 4-N
N.J. Turnpike Auth
,Mt. Laurel, NJ
17. Raniblewood STP
Mt. Laurel, NJ
18. Stratford Sewerage
Authority, Camden
County, NJ
19 . Chautauqua STP
Chautauqua Inst, NY
20. Pishkill WWTP .
Pishkill, NY
21. Hauppauge WWTP
Suffolk Co., NY
22. Mohawk WWTP
Colonie, NY
23. Oakwood Knolls
South S.D.
Wappinger, NY
24. Ravenwood WWTP
Colonie, NY
25. Strathmore Ridge
S.D. #8
Suffolk Co., NY
26. Waverly STP
Waverly, NY
27. Weather ford WWTP
Weather ford, OK
Plant
Size,
mgd
Treat
(Hyd)
0.20
0.08
0.80
0.5
1.0
0.84
(2.1)
0.40
(0.60)
0.10
(0.29)
0.025
(0.040)
0.20
(0.20)
0.038
0.05
(0.05)
0.6
0.52
(1.2)
P/A
(D.W.)
1.12
1.55
1.21
1.80
1.79
2.5
1.15
1.09
3.33
1.59
P/A
(W.W.)
1.59
2.73
1.36
3.6
2.62
!
1.72
j
Plant
Type
Act
sludge
Trickling
filter
Trickling
filter
Act
sludge
Trickling
filter
Act
sludge
Act
sludge
Act
sludge
Act
sludge
Act
sludge
Act
sludge
Rotating
disc
!
Act'
sludge
Degree
of
Treat-
ment13
S
' S,F
S
S
S
S
S,NR,
P
S,N,F
S
S
S,F
• S,P
' S
f
S
When
Equali-
zation
Constrc
AP with
expansion
AP with
expansion
AP with
expansion
AP
No other
change
AP with
upgrading
AP with
upgrading
WP
WP
WP
AP with
expansion
WP
WP
AP
AP
No other
change
Location of
Equalization
Facility
Following
pump station
Following
.headworks
Following
wet well
Before
'headworks
Following
headworks
Following
headworks
Before
wet well
Before
headworks
Before
wet well
Before
headworks
Following
headworks
Following
headworks
2 miles
upstream
of WWTP
Type of
Structure
Tank
Uf.
cone
. Tank
cone
Tank
cone-
Tank
eonc
Tank
cone
Tank
cone
Pipe
cone
Tank
cone
Tank
steel
Pipe
cone
Tank
co.no
Tank
cone
covered
Tank
cone
3 basins
unlined
(continued)
65
-------
TABLE 7 (continued)
Treatment Plant
and Location"1
So. pjyctte Twp
McAllicterville, PA
;-J. fngelsldc WWTP
Ingelside, TX
JO. Woodsboro WWTP
WoodsUjro, TK
31. Sslina STP
3alina, UT
32. shelburnc Fife Dist
HI WWTJ', Shelburne,
VT
•3. Lake Samish STP
Wlutcon County
Sewer Dist 112
Bellingham, WA
J4. Stevens Pass £
Yodel in WWTP
Stevens Pase, WA
J-%, [leteoaa WWTP
NeKooai. WI
36. northern Moraine STP
Glenbeulah, WI
37. Fort Edwards WWTP
fort Edwards, Wl
38. Valdcrs KWTP
Village of Valders,
MI
Plant,
Size,d
mgd
Treat
(iiyd)
0.13
0.50
0.16
(0.30)
0.30
(0.60)
0.38
0.23
0.062
(2.0)
0.498
(0.750)
0.60
(0.60)
0.56
(0.56)
0.15
(0.376)
P/A
(D.W.)
1.31
1.27
1.23
1.5
2.0
2.61
1.46
1.13
1.0
1.23
1.3
P/A
(W.W.)
2.29
3.18
2.31
1.5
7.2
1.46
1.29
1.67
1.68
2.63
Plant
Type
Act
sludge
Act
sludge
Ox
ditch .
Act
sludge
Act
sludge
Ox
pond
Act
sludge
Act
sludge
Act
sludge
Act
sludge
Act
sludge
Degree
of
Treat-
ment b
S
S
S
S
S
Inter-
mediate
S,P
S
S
S
S
When
Equali-
zation
Constrc
AP
AP
WP
WP
AP with
expansion
WP
WP
WP
WP
WP
WP
Location of
Equalization
Facility
Following
clarif ier
Following
pump station
Following
wet well
Following
primary
clarif iers
Following
headworks
Following
flow
splitting
structure
Upstream
10,000 ft
from WWTP
Following
wet well
Following
pump station
Following
wet well
Following
wet well
Type of
Structure
Tank
cone
Tank
cone
Basin
unlined
Basin
bentonite
lined
Tank
cone
2 basins
PVC liner
Tank
cone
covered
Basin
bentonite
Tank
cone
covered
Bentonite
Basin
asphalt
STP * ncwage treatment plant
KWTP » waotewater treatment plant
S = secondary
F = multi-media filtration WP = With primary
NR = Nitrogen removal
N = Nitrification
°AP = After primary Treatment plant capacities
listed are the design
capacity. Values in parens
are hydraulic capacity
where reported as different
from treatment capacity.
66
-------
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70
-------
TABLE 9. EQUALIZATION FACILITIES SUMMARY: PLANTS LARGER
THAN 1 MGD—TREATMENT PLANT CHARACTERISTICS
Treatment Plant
and Location
1. Cypress Creek WWTP
Florence, AL
2. S Mile Creek WWTP
5 Mile Creek, AL
3. Valley Creek WWTP
Birmingham and
Bessemer, AL
4. Central Contra
Costa San Dist
Walnut Creek, CA
5 . Chino Basin Muni
Water Dist
Cucamonga , CA
6. Laguna WHTP
Santa Rosa, CA
7. Pismo Beach Water
Reclamation Plant
Pismo Beach, CA
8 . Redlands STP
Redlands , CA
9. Rohnert Park WWTP
Rohnert Park, CA
10. Rossmore Sanitation
Inc., Laguna Hills,
CA
11. Sacramento Regional
WWTP, Sacramento,
CA
12. San Luis Rey WWTP
Oceanside, CA
13. Valley Community
Service Dist STP
Dublin, CA
- 14 . Water Reclamation
Plant, Livermore,
CA
15. Upper Thompson WWTP
Estes Park, CO
Plant
Size, a
mgd
Treat
(Hyd)
10.0 '
10
35
60
3.0
(3.0)
2.5
(15)
1.2
6.0
(12.0)
1.7
(12.0)
4.0
115
(240)
4.8
4.0
(12.0)
19.7
1.5
(2.0)
P/A
(D.W.)
1.40
1.31
1.60
1.60
1.60
1.43
1.78
5.0
1.36
1.75
2.00
1.8
1.2
P/A
(W.W.)
5.00
4.23
4.67
2.40
6.00
3.70
6.12
10.33
1.75
3.00
2.33
2.8
Plant
Type '
Act
sludge
Act
sludge
Act
sludge
Act
sludge
Act
sludge
Act
sludge
Trickling
filter
Act
, sludge
Act
s ludge
Act
sludge
Act
sludge
Act
sludge
Act
sludge &
trickling
filter
Act
sludge •
Degree
of
Treat-
ment
S
S
S,N
S
S
S
S
S
S
S
S
S,F
S,P
S,P
When
Equali-
zation
Constr
WP
AP with
upgrading
AP with
upgrading
AP
No other
change
WP
AP
No other
change
AP with
expansion
WP
WP
WP
WP
AP with
upgrading
WP
Location of
Equalization
Facility
Following
headworks
Following
headworks
Following
primary
sed tanks
Following
splitter
structure
Following
primary
sed tanks
Between
primary sett
tanks s aera-
tion tanks
Following
trickling
filter
Following
headworks
Following
control
Following
primary
sed tank
Following
headworks
Following
•primary
treatment
Following
headworks
Following
effluent
pumping
Type of
Structure
2 basins
unlined
Concrete
lined
pond
Basin
cone
lined
3 ponds
Basin
earthen
lined
Basin
cone
lined
Basin
gunite
lined
Basin
asphalt
lined-
Basin
gunite
lined
Basin
cone
gunite
lined
3 basins,
2 cone
lined, 1
earthen
lined
Tank
cone
covered
Basin
asph lined
Tank cone
2 basins,
gunite
lined,
earthen
lined
Tank cone
(continued)
71
-------
TABLE 9 (continued)
Treatment Plant
and Location
16. Broorafield WWTP
Brocmfield, CO
17. Clavey Road STP
Highland Park, IL
18. Ankeny WWTP No. 3
Ankeny, IA
19. Lawrence WWTP
Lawrence, KA
20. Manhattan WWTP
Kansas City, KA
21, Corrina STP
Corrina, MA
22. Paris Utility Dist
Plant, South Paris,
MA
23. Chapaton Pumping
Station, Detroit, MI
24. Dowagiac STP
Dcwaqioc, MI
25. Eaton Rapids WWTP
Eaton Rapids, Ml
26. E. Lansing WWTP
E. Lansing, MI
27. Grand Haven Spring-
lake WWTP, Grand
Haven, MI
23. Grand Rapids KWTP
Grand Rapids, MI
23. Hancock St. Pollu-
tion Control Pac
Saglnaw, MI
33. Jackson STP
Jackson, Ml
Plant
Size,8
mgd
Treat
(Hyd)
3.6
17.8
(36)
1.2
(3.0)
18
(45)
6.25
(12.5)
1.2
(4.32)
1.85
(10.9)
2.0
(4.0)
1.2
18.7
(48)
5.0
(7.0)
66
(150)
32
(120)
17
P/A
(W.W.)
1.03
1.22
1.30
1.22
3.10
2.42
3.24
N/A
1.40
2.0
1.4
1.43
1.75
1.13
1.2
P/A
(W.W.)
1.10
6.67
5.45
4.44
6.25
3.60
5.89
N/A
8.00
3.49
4.00
3.75
3.64
1.47
Plant
Type *
Act
sludge
Act
sludge
Act
sludge
Act
sludge
Act
sludge
Act
sludge
Act
sludge
Pumping
station
only
Act
sludge
Act
sludge
Act
sludge
Act
sludge
Act
sludge
CSO
Act
sludge
Degree
of
Treat-
ment
S
S,F.
S
S
S
S '
S
S,P
S,P
S,P,F
S,P
S,P,
N,F
Storm
pri-
mary
S,P
When
Equali-
zation
Constr
AP with
upgrading
AP with
upgrading
WP
WP
WP
WP
WP
WP
WP
AP
No other
change
AP with
upgrading
AP with
expansion
WP
WP
WP
Location of
Equalization
Facility
Following
headworks
Following
distribution
chamber
Following
flow splitter
Following
settling
basin
Following
primary
settling
basin
Following
headworks
Following
headworks
Before
WWTP
Follows
flow
diverter
Following
grit chamber
Following
grit
chamber
Following
headworks
Following
primary
treatment
At pump sta
5 miles up-
stream of
plant
Following
primary
treatment
Type of
Structure
Basin
earthen
lined
6 tanks
cone
covered
Basin
unlined
Basin
cone
lined
Basin
clay
lining
Tank
cone
Tank
cone
Tank
cone
2 basins,
asph lined
clay lined
2 tanks
cone
Tank cone
covered
Tank
steel
Basin
cone
lined
Tank
cone
Basin
cone
(continued)
72
-------
TABLE 9 (continued)
Treatment Plant
and Location
31. Lansing STP
Lansing, MI
32. Midland WWTP
Midland, MI
33. Mt. Clemens STP
Mt. Clemens, MI
34 . Pontiac WWTP
Pontiac, MI
35. Port Huron WWTP
Port Huron, MI
36. Southeastern Oakland
Co . , Red Run Drain
Madison Heights, MI
37. Tecumseh WWTP
Tecumseh , MI
38. Trenton WWTP
Trenton, MI
39. Walled Lake Novi STP
Novi, MI
40. Warren STP
Warren, .MI
41. Ypsilanti Twp. WWTP
Ypsilanti, MI
42. Brookhaven WWTP
Br ookhaven , MS
43. Greneda WWTP
Greneda , MS
44. Shoal Creek WWTP
Jopl in , MO
45. Warrensburg STP
Warrensburg, MO
46. Reno Sparks STP
Reno, NV
Plant
Size,3
mgd
Treat
(Hyd)
45
(50)
6.5
(13.25)
4.0
25.5
(50)
20
(58)
63
1.4
7.5
(17.5)
2.1
36
(50)
4.0
2.0
(5.0)
3.5
4.5
1.7
(3.4)
22
P/A
(D.W.)
1.02
1.67
1.07
1.4
2.36
1.31.
1.2
1.51
1.26
1:5
1.36
1.27
3.03
1.73
P/A
(W.W.)
1.76
2.08
-
2.5
3.14
1.31
^2.00
3.33
4.69
1.32
2.31
5.39
4.93
Plant
Type
Act
sludge
Trickling
filter
Trickl ing
filter
Act
sludge
Act
sludge
Storm
flow
control
Act
sludge
Act
sludge
Act
sludge
Act
sludge
Act
sludge
Act
sludge
Act
sludge
' Act
sludge
Ox
ditch
Act
sludge
Degree
of
Treat-
ment
S,P
S,P,F
S
S,P,F
S,P
S,P
S,P
S,P,F
S,P,F
S,P
S
S
S
S
S,P
When
Equali-
zation
Cons tr
AP with
expansion
AP with
upgrading
AP with
upgrading
AP with
expansion
AP
No other
mods
AP with
upgrading
WP
AP
No other
change
AP
No other
change
AP with
upgrading
AP
No other
change
AP
No other
change
WP
AP
No other
change
Location of
Equalization
Facility
Follows
primary
treatment
Following
flow splitter
1 mile
upstream
of plant
Following'
primary
treatment
Following
primary
sed tanks
12 miles
upstream
of WWTP
Following
headworks
1/4 mile
upstream
of plant
Following
wet well
Following
grit
chamber
'
Before
bar rack
Following
headworks
Following
, headworks
Parallel to
headworks
Following
wet well
Before
plant
Type of
Structure
Tank
cone
Tank
cone
Tank
cone
Tank
cone
covered
Tank
cone
Tank
cone
Tank
cone
covered
Basin
asph
lined
Tank
cone
Basin
cone
lined.
covered
Tank
cone
covered
Basin
earthen
lined
Basin
earthen
Alined
Basin
clay
lined
Basin
cone
lined
Interceptor
sewers
(continued)
73
-------
TABLE 9 (continued)
Treabaent Plant
and Location
47. Merriaack WWTP
Merrimack, NH
48. Elmwood STP
Mariton, NJ
49. Mew Providence STP
Hew Providence, NJ
50. Woodstream STP
Marl ton, NJ
51. Anherst STP
Amherst, NY
52. Delaware STP
Delaware, OH
S3. Hatfiold Twp WWTP
Colmar, PA
54, Oil City STP
Oil City, PA
55. Watcrtown STP
Water town, ED
56. Anurillo River Rd
NWTP, tearillo, TX
57. Duck Creek WWTP
Garland, TX
S3. Guadalupi-Blanco
River, Victoria, TX
59. Midland WWTP
Midland, TX
60. Odessa STP
Odessa, TX
61. QSO WWTP
Corpus Christ!, TX
62. Sandy Suburban STP
Sandy City, UT
Plant
Size,a
mgd
Treat
(Hyd)
5.0
(10.0)
1.5
4.5
(6.0)
1.25
<3.0)
15.0
2.5
(5-6)
(4.5)
8.0
(14)
4.0
(6.0)
12.0
(20)
11.5
(30)
3.0
(13.0)
6.0
(8.0)
6.0
(8.5)
12.0
(15.0)
1.5
P/A
(D.W.)
1.26
1.30
1.11
1.31
1.28
2.25
1.33
1.22
1.2
1.5
1.33
1.20
1.85
P/A
(W.W.)
1.05
3.89
1.88
2.22
4.00
4.13
2.00
3.43
4.00
2.55
4.09
2.04
Plant
Type
Act
sludge
Act
sludge s
trickling
filter
Trickling
filter
Act
sludge
Act
sludge
Act
sludge
Act
sludge
Trickling
filter
Act
sludge
Act
sludge
Trickling
filter
Act
sludge
Trickling
filter
Act
sludge -
Act
sludge
Act
sludge
Degree
of
Treat-
ment
S
S
S
S,F
S,P,F
S
S,F
S
S
S
S,P,
C,P
S
S
S
S
S
When
Equali-
zation
Constr
WP
AP
No other
change
WP
AP with
expansion
WP
AP with
upgrading
WP
AP with
upgrading
AP
with
expans ion
WP
AP with
upgrading
WP
WP
AP with
expansion
WP
WP
Location of
Equalization
Facility
Following
influent
pump station
Following
influent
pump station
Following
influent
pump station
Following
influent
pump station
Following
grit
chamber
Following
hoadworks
Following
raw sewage
pumps
Following
headworks
Following
control
house
Following
primary
clarif ier
Following
headworks
Following
wet well
Following
settling
basin
Following
primary
clarifiers
Before
plant
Following
primary
effluent
1
Type of
Structure
2 tank
cone-
Tank
steel
Tank
steel
Tank
stool
2 tanks
cone
Tank
cone
2 tanks
cone
Tank
cone
Basin
clay
lined
Basin
earthen
lined
Basin
cone
lined
Basin
unlined
Basin
cone
2 tanks
Interceptor
sewer
Basin
plastic s
cone lined
(continued)
74
-------
TABLE 9 (continued)
Treatment Plant
and Location
63. Lower Potomac Poll
Control Plant
Fairfax Co., VA
64. Moore Creek WWTP
Charlottesville, VA
65. Potomac Reg WWTP
Woodbridge, VA
66 . Roanoke WWTP
Roanoke, VA
67. Upper Occoquan
Sewage Authority
Manassas Park, VA
*
68. Lacy-Olympia-
Tumwater Thurston
County STP
Olympia, WA
69. Muni .of Metro
Plant
Size,a
mgd
Treat
(Hyd)
36
(68)
15
12.0
(32)
35
(90)
15.0
(30)
14
(19.2)
350
Seattle, West Point STP
Seattle, WA
70. Ada STP
Bluefield, wv
71. westside STP
Bluefield, WV
72. Chippewa Falls WWTP
Chippewa Falls, WI
73. Reedsburg STP
Reedsburg, WI
74 . Dry Creek WWTP
Cheyenne, WY
1.2
2.8
(3.5)
3.50
1.7
4.5
(9.0)
P/A .
(D.W.)
1.43
1.4
1.40
2.35
f
1.36
1.17
1.79
1.27
1.10
P/A
(W.W.)
2.04
5.0
3.45
6.00
2.71
4.0
9.5
1.43
1.10
Plant
Type
Advanced
waste
treat
Advanced
waste
treat
Advanced
Waste
Treat
Act
sludge
Act
sludge
CSO
Act
sludge
Act
sludge
Act
sludge
Act
sludge &
trickling
filter
Degree
of
Treat-
ment
S,P,C,
NR,F
S,P,
N,F
S
S,P,
N,F
S,P,C,
N,F
S
Pri-
mary
S
S
S
S
S
When
Equali-
zation
Constr
WP
WP
WP
WP
AP with
upgrading
WP
AP with
expansion
AP with
upgrading
AP with
expansion
WP with
upgrading
WP
Location of
Equalization
Facility
Following
secondary
chlorination
Following .
headworks
Following
secondary
chlorination
Following
primary
sed basins
3 locations: ,
a) Following
headworks
b) Following
2nd stage
recarb
basin
c) Before
filters
Following
headworks
Before
plant
2 miles
upstream
of STP
Following
lift station
Upstream
of plant
1600 ft
Following
wet well
Following
headworks
Type of
Structure
Basin
PVC
lining
Basin
Basin
PVC
lined
2 tanks
cone
a ) Emer g
pond
b) 2 ballast
ponds ,
concrete
lined.
2 backup
ponds
c) Filter
backwash
eq tank
2 basins
asphalt
lined -:
Interceptor
sewers
2 tanks
cone
2 tanks
Basin
asphalt
Basin
un lined
2 basins
earthen
lined
Treatment plant capacities listed are
the design capacity. Values in parens
are hydraulic capacity where reported
as different from treatment capacity.
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83
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TABLE 11. PLANTS WITH EQUALIZATION INFORMATION INCOMPLETE
Plant
Westside, AL
Pomona , CA
San Francisco, CA
Dover , DE
Gulf Gate, FL
University Shores, FL
Augusta, GA
Coosa River (Floyd Co.), GA
Gainesville, GA
La Grange, GA
Napa City, ID
Piano, IL
Sterling, IL
Fort Wayne, IN
Indianapolis (Belmont) , IN
Rockville, IN
Salem, IN
Southbend, IN
Bardstown, KY
Harrowsburg, KY
Lawrenceburg, KY
Mt. Sterling, KY
Richmond, KY
Herculaneum, MO
Springfield, MO
Exeter, NH
Freehold, NJ
Greenbrier, NJ
Leesfaurg, NJ (state prison)
S . Lakewood Co . , NJ
Stafford, NJ
Toms River, NJ
Akron, OH
Altus, OK
Duncan, OK
Meeker, OK
Muskogee, OK
Stillwater, OK
Coraopolis, PA
Derry Twp. , PA
Fairview, PA
Franklin, PA ' *
Borough of Grove City, PA
Size
(mgd)
30
0.5
5
7
0.05
5 (2)
5.0
0.050
Comments
No information
No information
Preliminary planning stage
No information
No information
No information
Step 1
Step 1
Step 1
Out for bid; Delaney w/state
Surge pond, storm runoff
Planning surge tank
Surge tank
Surge tank
Series of surge basins around
city
Jim Stantley w/state
Jim Stantley w/state
Unable to contact
Jim Stantley w/state
Jim Stantley w/state
Unable to contact
No information
No information
In operation; no information
No information
No information
No information received
Industrial
Storm water surge tank
Proposed; R. Beach (405)447-1950
Proposed; T. Lee (405) 252-0250
Emergency only, hot yet used
Proposed; E. Kernes (918)682-6602
Proposed; F. Louise (415)372-0025
Under construction
Town and industry under
construction
Race track in operation
Letter sent; no response
Construction not yet begun
(continued)
84
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TABLE 11 (continued
Plant
Size
(mgd)
Comments
Municipality of Hermitage, PA
Borough of Lamoyne, PA
Liberty Township, PA
Lower Salford, PA
Borough of Middleboro, PA
Milford Twp., PA
New Milford Twp., PA
Peddlers Village, Solebury Twp.,
PA
Pocono Country, Monroe Co., PA
Shohola Twp., PA
Oconee Co., SC
Beeville, TX
Borger, TX _
Ciblo Crk./Universal, TX
Longview, TX
Sequin, TX
Tiboli, TX
N. Bonneville, WA
0.007
0.007
0.026
0.05
0.05
0.075
5
Letter sent; no response
Letter sent; no response
Housing development in operation
Step 3 construction
High school in operation
Two schools under design (1975)
Housing development under con-
struction
Campground in operation
S. Hunt w/state; adding 2 new
plants; third to be used for
excess flows; (512)358-4641
No information
No information
No information
No information
S. Hunt w/state; (512)286-3313
No information
85
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Survey Response
Survey results provided information on individual flow
equalization projects located in 26 of the 48 contiguous states.
All major geographical sections of the United States are repre-
sented. Figure 17 shows the location of all treatment plants
identified with flow equalization facilities.
EQUALIZATION FACILITIES SUMMARY
Treatment Plant Characteristics
Applicability of equalization, in terms of the range and
degree of potential benefits, is dependent in varying degrees
on the particular characteristics of the specific treatment
plant.
The type of treatment used is generally related to how
sensitive plant performance is to influent variations and
peaking characteristics. For example, activated sludge systems,
including the secondary clarifier, are typically more sensitive
to flow peaking than comparable fixed film biological treatment
systems. This is largely due to differences in settling char-
acteristics of the respective biological solids and may be
partially compensated for by greater operational flexibility in
typical sludge systems.
The degree of treatment, in combination with the specific
processes used, establishes the magnitude of potential cost
savings by using equalization to minimize peaking capacity.
Potential cost savings increase as the number of unit processes
downstream of equalization benefitting from reduced peak flows
and/or loadings increases. Thus, a treatment plant incorpora-
ting activated sludge secondary treatment, designed for nitri-
fication, chemical addition for phosphorus removal, and efflu-
ent filtration, will find equalization of significantly greater
potential benefit than a comparably sized conventional activated
sludge plant with no additional treatment requirements.
The timing of equalization construction with respect to
treatment plant construction may be significant in terms of
potential design applicability and flexibility. Equalization
additions to existing plants have design constraints imposed by
the facilities in operation. The addition of equalization to
existing facilities may increase effective plant capacity and/or
reduce or eliminate existing operational problems.
Treatment Plant Type—
The distribution of basic treatment types at plants cur-
rently using equalization is as follows:
86
-------
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87
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Activated sludge 80
Oxidation ditch, activated sludge 3
Activated sludge with advanced
waste treatment* 3
Activated sludge with trickling
filters 3
Trickling filters 13
Rotating disc 1
Oxidation pond 1
Pump station 1
Combined sewer overflow control,
primary 3
The distribution is similar for both large and small plants,
with approximately 80 percent of equalization systems accompany-
ing activated sludge plants. This is due in part to the pre-
dominance of activated sludge plants throughout the country.
The smaller number of equalization installations at trickling
filter plants does not necessarily lower potential for benefits
at such plants. On the other hand, the small number of appli-
cations at advanced waste treatment plants is primarily a reflec-
tion of a small number of plants of this type currently in use.
Detailed assessment of costs and benefits must be made in the
specific context of each local sewerage system.
The pump station and combined sewer overflow control appli-
cations are a special case as far as this report is concerned.
These equalization applications exist almost exclusively to
accommodate peak storm flows. Many other such systems exist
throughout the country. However, the focus of this report is
on non-storm equalization, accordingly no effort was made to
include all such systems in the survey.
Degree of Treatment—
Survey response indicates that equalization currently used
at treatment plants, with degrees of treatment ranging from
primary to advanced waste treatment, is distributed as follows:
Primary 2
Primary with chemical coagulation 1
Secondary 69
Secondary with phosphorus removal 11
Secondary with nitrification 3
Secondary with effluent filtration 11
Secondary with phosphorus removal
and effluent filtration 6
Secondary with nitrification,
phosphorus removal and effluent
'filtration 3
*Any additional unit processes following secondary treatment
for removal of BOD, suspended solids, nutrients, etc.
88
-------
Secondary with phosphorus removal,
effluent filtration, and carbon
adsorption 1
Secondary with nitrification, .
phosphorus removal, effluent
filtration and carbon adsorption 1
Secondary with nitrogen and
phosphorus removal, effluent
filtration and carbon adsorption 1
Equalization systems in use at secondary treatment plants
account for 64 percent (69 of 108) of the facilities currently
in use. Equalization at secondary treatment plants, including
between one and four additional tertiary processes (nutrient
removal, effluent filtration, carbon adsorption), account for
nearly all of the remaining systems identified; 37 of 108 or
34 percent. Treatment plants with degrees of treatment higher,
than secondary accounted for 22 percent of equalized plants
smaller than 1 mgd compared to 42 percent of plants larger than
1 mgd. The distribution of equalized flow plants between secon-
dary and higher treatment, and between large and small plants,
is in reasonable correspondence with the proportions of the
respective categories in existence.
Timing of Equalization Construction— ^
Survey response indicates that approximately half of
equalization facilities currently in use were built as an in-
tegral part of a new treatment plant. The other half were
added to existing facilities. The distribution of construction
timing is as follows: ,
With treatment plant (WP) 51
After treatment plant with no other
plant changes (APm) 15
After treatment plant, as part of
plant upgrading (APu) 22
After treatment plant, as part of •
plant expansion (APe) 16
The distribution of construction timings is virtually the same
for both large ( >1 mgd) and small ( < 1 mgd) treatment plants.
Approximately 70 percent of new treatment plants including
equalization, in both large and small plant categories, are
designed for secondary treatment only. The large proportion
of plants being in this category is due largely to wet weather
peaking. Only 4 of 34 plants in this group report wet weather
peak to average flow ratios less than 3:1.
It is of interest to note the relatively small number of
treatment plants (15) to which equalization facilities have been
added as the only plant modification.
89
-------
Equalization Facility Characteristics
The basic features of an equalization system that define
its characteristics with respect to treatment plant performance
and overall cost include:
Volume
Type :
Location in treatment system
Physical details
Appurtenances
Equalization Volume—
Equalization volume is conveniently expressed as a percen-
tage of the design treatment capacity. Volumes of equalization
systems at 35 treatment plants smaller than 1 mgd capacity
range from 5 percent to 2,300 percent. The average volume of
29 systems with volumes less than 200 percent is 54 percent.
Twenty-five of the 34 volumes are encompassed by a band ranging
from 15 to 70 percent at 1.0 mgd. The band increases in width
with decreasing plant capacity to from 15 to 130 percent at
0.1 mgd. Three plants with equalization volumes less than 10
percent use either influent interceptor, or influent pump sta-
tion wet well volume for partial flow equalization.
Volumes of equalization systems at 68 treatment plants
larger than 1 mgd capacity ranged from 5 percent to 4,000 per-
cent. The average volume of 63 systems with volumes less than
500 percent is 69 percent. Considering the 56 systems with
volumes less than 150 percent, the average equalization volume
is 45 percent. Three out of four treatment plants larger than
1 mgd and having equalization facilities use equalization
volumes ranging from 10 to 100 percent of design treatment
capacity. Equalization volumes used in the majority of
equalized flow plants are significantly larger than minimum
volumes required for equalization of daily diurnal variations.
Response to the survey indicates that in a great proportion of
the cases where larger equalization volumes are used, the spe-
cific purpose is to accommodate peak flows occurring during
storm periods. A significant excess volume is also used at
plants expecting occasional spills of toxic or other process
upsetting materials.
Type and Location of Equalization—
Of the flow equalization facilities surveyed, approximately
39 percent have in-line equalization systems processing all flow
entering the treatment plant. An additional 56 percent of the
facilities have side-line equalization systems processing excess
(diurnal and storm/sanitary) flow over a set flow rate. Approxi-
mately 5 percent of the systems surveyed use up-stream equal-
izing; incorporating storage capacity in the collection system
90
-------
with suitable controls, with variable speed pumping equipment as
required to equalize influent flow to the treatment plant.
•• •' Depending on the specific characteristics and requirements
of individual -treatment plants, the type of equalization system
and its location within the treatment plant may have substantial
effects on overall system cost and operability. Available
storage capacity in oversized interceptor sewers may enable
significant flow smoothing at minimal capital expense. Pumping
requirements of in-line equalization are generally greater than
for side-line systems. However, in-line systems provide greater
flexibility for concentration and load smoothing, and more
positive protection against shock load effects. Side-line
systems provide the potential lowest cost in-plant equalization
capability, and may enable use of existing structures that would
otherwise be abandoned.
Of the 95 agencies responding to the survey, 91 of the 130
flow, equalization facilities pumped either in or out of the
equalization structure. Only 14 installations pumped both in
and out. Gravity flow operation (both in and out) of the
equalization-structure was reported in only 25 installations.
The distribution of those facilities Which pumped flow into the
equalization structure versus those which used gravity was
approximately 50 percent. Similarly, emptying the equalization
structure was evenly divided between pumping and gravity systems.
The location of the equalization structure within a collec-
tion system or sewage treatment plant has a significant impact
on the type of flow equalization facility components desired or
required. Of the 95 responding agencies (a total of 130 struc-
tures) the distribution of facilities by location is as follows:
In or adjacent to the.collection system 8
Upstream of the sewage treatment plant
headworks 18
Downstream of the headworks or influent
structure 92
Downstream of primary treatment 20 •
No information provided 1 .
Equalization at treatment plants smaller than 1 mgd capac-
ity is most commonly located at the headworks, receiving raw
sewage (31 of 37 plants, in this size range were of this type) .
A major reason for this apparent preference is that a large
proportion of smaller plants are designed as integrated "package
units, and not compatible with an intermediate equalization
stage. Many such plants are also designed without primary
sedimentation.
91
-------
Approximately 56 percent (36 of 64) of equalization systems
at treatment plants greater than 1 mgd capacity are located
before primary treatment. An additional 30 percent (19 of 64)
systems are located following primary treatment. Four systems
are located following secondary treatment to minimize peaking in
tertiary treatment components.
Physical Characteristics and Appurtenances--
Requirements for design of an equalization system, including
the type of structure and major equipment features, are highly
dependent on the details of the specific local situation.
The type of structure chosen depends on costs of construc-
tion associated with required volume, site constraints, whether
construction will be at or below grade, and the type of mechani-
cal equipment to be installed. Existing structures that would
otherwise be abandoned or made available for collection system
capacity must be evaluated in terms of compatibility with
equalization needs, and in comparison to new facilities.
Major equipment requirements—such as aeration and/or
mixing, solids removal, covering, and odor control—depend
largely on the characteristics of the wastewater to be stored
and on site requirements. Equalization storage of raw sewage
generally requires solids removal or mixing equipment to
sufficiently prevent solids deposition. Experience with
equalization of municipal wastewater following primary sedi-
mentation indicates that solids accumulation is very slight,
and that solids removal or mixing for solids suspension is not
necessary.
Requirements for aeration of wastewater in temporary
storage depend on particular characteristics of the incoming
wastes, length of storage, sensitivity of subsequent biological
treatment, and sensitivity to potential odors of the surrounding
area. Typical raw or settled municipal wastewater, unseeded by
active biological populations, may be stored in equalization
systems, with residence times of up to a half day, without
creating difficulties. However, the longer the residence time,
the more potential the system will have for generating odors,
floating sludge, and contributing to operational problems in
the following biological treatment process.
Requirements for covering and odor control depend on waste
characteristics, desired equalization conditions (including
whether or not aeration is provided), and the sensitivity of the
local environment. Treatment plants close to, or upwind of
residential areas will require closer attention to the problem
of odors.
92
-------
Type of structure—Survey response indicated the range of
conventional tank and basin types currently used to provide
required equalization volume. The distribution of types of
equalization as evidenced in 130 structures is summarized below:
Concrete pipe 8
Asphalt-lined earthen basin 9
Concrete-lined earthen basin 18
PVC-liried earthen basin 4
Clay-lined earthen basin 6
Gravel-lined or soil stabilized
earthen basin 2
Unlined earthen basin 26
Open concrete tank 40
Covered concrete tank 12
Steel tank 5
No information provided 17
Structure types at 38 plants with treatment capacity less
than 1 mgd include 24 tanks (63 percent) and 14 basins (37 per-
cent) . A basin is distinguished from a tank by the use of an
earth work for sides and bottom, as opposed to structural walls
and floor. Structure types at 70 plants with treatment capacity
larger than 1 mgd include 39 basins (56 percent) and 31 tanks
,(44 percent). Overall the distribution between basins and tanks
was nearly equal; including approximately 50 percent basins,
44 percent tanks, with the remaining 6 percent consisting of
pipe and wet well volume in the collection system. Approxi-
mately 40 percent of the basin systems are reported to be un-
lined. An additional 40 percent are equipped with a pavement
lining of either asphalt or concrete? the remaining 20 percent
have flexible plastic or rubber, clay, or stabilized soil
lining. It is interesting to note that several agencies with
unlined earthen basins indicated that they would prefer lined
basins to minimize operation and maintenance requirements.
Aeration, mixing, and solids removal—Overall, mixing or
aeration is provided as an integral part of the flow equaliza-
tion system in only approximately 50 percent of the facilities
surveyed. The distribution of methods, or combination of
methods used, is as follows:
No mixing or aeration systems 66
Mechanical mixing 14
Air diffuser system 24
Surface aerators 18
Combination of air diffusers and
surface aerators 1
Combination of mixer and air
diffusers 3
93
-------
Combination of mixer and surface
aerators 1
No information provided 3
Washdown or solids removal facilities in most of the instal-
lations surveyed are of the manually-operated type; the majority
of these use fire hydrants and hoses. The general distribution
of washdown facilities for the 130 equalization structures is
as follows:
No washdown or solids removal
systems 38
Manual system 74
Automated system 11
No information provided 7
Survey response concerning aeration, mixing and solids
removal equipment at plants with treatment capacity less than
1 mgd reflects the influence of the equalization facility's
location before primary treatment. Twenty-two of 31 plants with
equalization before primary treatment provide aeration for com-
bined requirements of oxygenation, and to prevent solids set-
tling. None of the plants smaller than 1 mgd are equipped with
mechanical aeration or mixing. Only two of the 31 plants have
mechanical solids removal equipment, but 21 of the 31 have a
manual system for basin flushing and cleaning. Seven plants
with equalization prior to primary treatment have no provision
for either aeration or solids removal. These plants are pre-
dominantly in the southwest and use lagoon type facilities for
equalization. Such systems function essentially as oxidation
ponds, and, therefore, can provide successful trouble-free
operation.
Survey response for plants with treatment capacity greater
than 1 mgd provides comparison of equipment requirements for
equalization facilities located before and after primary sedi-
mentation. Mechanical solids removal equipment is used in less
than 10 percent of all these facilities, regardless of location.
However, approximately 75 percent of all systems, both before
and after primary sedimentation, are provided with manually
operated flushing or washdown equipment.
Aeration and mixing equipment is used in 67 percent (24 of
36) of the systems located before primary settling. Only 16
percent (3 of 19) of the systems located after primary settling
use aeration. This is consistent with requirements in light of
differences between settled and unsettled sewage, and illus-
trates potential cost savings for equalization systems located
after primary sedimentation.
94
-------
Odor control, covers—The vast majority of flow equalization
facilities currently in use are not fitted with any type of odor
control system. Of those having this feature, by far the larger
percentage used some type of chlorination system. The exact
distribution by type is as follows:
No odor control system 101
Chlorination system 22 '
Ozonation system 2
No information provided 5
Three of the 24 equalization odor control systems are at
treatment plants smaller than 1 mgd; less than 10 percent of
the plants in this group. The remaining 21 odor control systems
accompany equalization at approximately 30 percent of the treat-
ment plants larger than 1 mgd. The higher proportion at larger
treatment plants is due to a combination of circumstances, in-
cluding more sensitivity to odor potential in larger communi-
ties, increasing potential for significant odors parallel with
increasing plant capacity, and a greater flexibility in the
design of larger facilities due to economies of scale.
As noted previously, 12 equalization systems (5 at plants
less than 1 mgd, 7 at plants greater than 1 mgd) are covered,
and may also be considered as having odor control. Only three
equalization systems currently in use are equipped with covers
and chemical odor control capability.
EQUALIZATION FACILITIES CONSTRUCTION COSTS
Typically, detailed breakddwn of construction costs by
equipment element incorporated in the various flow equalization
facilities is not available for the majority of installations
responding to the survey. In addition, costs associated with
equipment common to other areas of the wastewater treatment
plant are generally unavailable, or were impossible to separate
from the cost records of treatment plant and flow equalization
facility installation. Included in this category are items
such as pumping stations, electrical equipment and controls,
ventilation systems, yard piping, and chlorination equipment.
Construction Costs
No apparent correlation was noted between construction cost
figures received and the level of complexity of the individual
facilities. Based on this observation, it would seem that the
construction cost of an equalization facility is not solely
related to the size and type of equipment included. Local in-
fluences are more likely to play a significant role in deter-
mining such costs. Some of the more significant factors that
must be addressed when evaluating a given site or facility in-
clude the following:
95
-------
1. Location of the equalization facility in relation to
other unit processes or, alternatively, location within
the collection system.
2. Shared use of equipment and piping with unit processes
at the treatment plant.
3. Site conditions.
4. Labor and material costs and availability.
5. Competitiveness of construction and supplier market.
6. Time allotted for construction.
7. The existence of the equalization facility as an
addition to existing facilities or as an integral
part of a new treatment plant.
Table 12 summarizes construction cost data for existing
equalization facilities. To make these data comparable, all
construction cost information has been trended to a common cost
level—an Engineering News Record (ENR) construction cost index
of 2600. The construction costs listed in Table 12 do not in-
clude any allowance for engineering, legal, or administrative
costs. Cost information for this analysis was obtained from
72 of the 95 responding agencies. Figure 18 presents a log-log
regressional plot of the construction cost data.
Operation and Maintenance Costs
Operation and maintenance (O&M) costs for wastewater
treatment facilities can be separated typically into labor,
materials, energy, and chemical components. Cost data for O&M
provided by 34 of the 95 responding agencies were, for the most
part, incomplete and inconclusive. By far the great majority
of agencies surveyed had no breakdown of O&M costs for their
flow equalization facilities, and the information that was re-
ceived was not generally classified by individual O&M compo-
nents. Tables 13 and 14 summarize available data for equali-
zation tanks and basins.
The most prevalent response related to the yearly O&M cost
was "minimal." It was assumed that "minimal" cost was in rela-
tion to the yearly O&M cost of the associated sewage treatment
plant. Of course, if.plant O&M costs were high, the amount
spent on O&M for the flow equalization facility could be sub-
stantial. But because of the inconclusive nature of this type
of response, the agencies classifying such costs as "minimal"
were considered as non-responsive.
96
-------
TABLE 12. CONSTRUCTION COST DATA FOR FLOW EQUALIZATION
FACILITIES
Plant Name .
Ref No
Capacity <1 mgd Tables 7 &
Fontana , CA
Deer Creek STP, CA
Rancho Bernardo Pt. ,
CA
Willits, CA
Northeast/Oskaloosa ,
IA
Southwest/Oskaloosa ,
IA
Boyne City, MI
Dimondale, MI
Essexville, MI
Dawson. STP, MN
Blackwood , NJ
Borough of Palmyra,
NJ
Clemen ton, NJ
Windsor , NJ
NJ Turnpike Auth. Ser-
vice Area 3-S, NJ
NJ Turnpike Auth. Ser-
vice Area 4-N, NJ
Ramblewood , NJ
Stratford, .NJ
Chautauqua, NJ
Waverly STP, NY
Weatherford, OK
McAlisterville, PA
• Salina, UT
Shelburne, VT
Bellingham, WA
Stevens Pass, WA
Capacity >1 mgd Ta
Cypress Creek STP, AL
5 Mile Creek STP, AL
Valley Creek STP, AL
Central Contra Costa
San Dist, CA
. 1
2
3
4
5
6
7
8,
9
10
11
12
13
14
15
16
17
18
19
26
27
28
31
32
33
34
bias 9 S
1
2
3
4
Equal
Fac
Vol
(106 gal)
8
0.1
3.0
^0.2
16.0
4.07
4.07
74.5
0.138 ;
0.60
0.26
0.25
0.132
0.35
2.0
0.085
0.017
0.102
0.297
0.0371
0.12
8.15
0.039
0.5
. 0.15
0.202
0.076
10
2.0
30
5.0
164.0
Year of
Constr
ox Bid
1974-76
1972
1977
1972
1973
1976
1974
1969-70
1972
1970
1973
1973
1974
1972
1973
1968-69
1973
1976
1976
1968
1973-75
1976 ,
1977
1976
1976
1970-72
1975-77
1973-76
Constr
Cost (ID3 $)
3
' 153
34
300
58,
250
100
113
150
90
355
129
21
340
20
75
69
77
720
150
150
1,000
700
ENR
2600
Cost (103 $)
5 .
209
46
411
74
512
165
213
' , 171
124
486
190
29
465,800
77
79
850
162
282
1,180
959
Type
(tank)
(basin)
T
B
B
B
B
. B
B
T
T
B
T
T
T
T
T
T
T .
T
T
T
B
T
B
T
B
T
B
B
B
B
(continued)
97
-------
TABLE 12 (continued)
Plant Name
Chino Basin Muni W.D.,
CA
Santa Rosa, CA
Pismo Beach STP, CA
Redlands, CA
Rohnert Park, CA
Rossmoor San Inc . , CA
Sacramento , CA
San Luis Rey TP, CA
Dublin, CA
Livermore, CA
Estes Park, CO
Broomfield, CO
Highland Park, IL
Ankeny STP, IA
Lawrence STP, KA
Manhattan, KA
Corrina, ME
South Paris, ME
Detroit, MI
Dowagiac, MI
Eaton Rapids, MI
E. Lansing, MI
Grand Haven-Spring
Lake, MI
Grand Rapids, MI
Saginaw, Ml
Jackson, MI
Lansing, MI
Midland, MI
Mt. Clemens, MI
Pontiac, MI
Port Huron, MI
Southeastern Oakland
Co., MI
Tecumseh, MI
Trenton, MI
Walled Lake-Novi, MI
Warren, Ml
Ypsilanti, MI
Brookhaven , MS
Greneda, MS
Joplin, MO
Warrenburg, MO
Reno Sparks, NV
Merrimack, NH
Ref No
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
Equal
Fac
Vol
(106 gal)
1.2
17.0
0.38
1.25
0.75
2.5
207.0
1.08
2.3
20.0
0.238
21.0
4
6.7
0.25
0.40
0.21
28.0
4.9
0.07
5.0
0.80
10.0
3.6
12.5
4.0
3.25
30.0
3.0
5.7
63*0
1.0
13.5
0.315
50.0
0.624
81.0
72.0
12.2
0.53
0.956
Year of
Constr
or Bid
1977
1966-67
1975
1972
1970-71
1971-75
1976
1973-75
1972
1974
1975
1975
1973
1974-75
1975-77
1976
1969-71
1973
1968
1977
1976
1973
1972
1975-77
1972
1966
1971-72
1974
1974-75
1972-73
1972
1969-72
1971
1971-72
1969
1965
1965
1972-73
1977
1969-70
Constr
Cost (103 $)
200
138
495
188
959
408
180
491
120
8,700
125
380
252
190
5,875
310
1,935
674
7,167
750
73
4,560
2,500
25,000
525
150
5,102
314
80
197
ENR
2600
Cost (10 $)
206
351
930
354
1,035
560
266
634
154
12,876
161
448
267
262
15,747
365
2,864
1,267
8,457
1,410
204
5,882
3,700
47,000-
1,076
247
10,580
839
132
444
Type
(tank)
(basin)
B
B
B
B
B
B
B
T
B
B
T
B
T
B
B
B
T
T
T
B
T
T
T
B
T
B
T
T
T
T
T
T
T
B
T
B
T
B
B
B
B
B
T
(continued)
98
-------
TABLE 12 (continued)
Plant Name
Elmwood STP, NJ
New Providence, NJ
Woodstream STP, NJ
Amherst, NY
Delaware, OH
Colmar , PA
Oil City, PA
Water town , SD
Amarillo , TX
Garland, TX
Odessa, TX
Sandy City, UT
Lower Potomac Poll
Cont, VA
Moore Creek STP, VA
Potomac Reg. STP, VA
Roanoke, VA
Upper Occoquan Sewer
Auth, VA
Lacy-Olympia-Tumwater ,
WA
Seattle, WA
Ada STP, WV
Westside STP, WV
Cheyenne, WY
Ref No
48
49
50
51
52
53
54
55
56
57
60
62
63
64
65
66
67
68
69
70 •
71
74
Equal
Fac
Vol
(106gal)
0.258
0.426
0.299
0.623
1.0
0.22
0.52
10.0
3.0
11.8
2.80
0.70
14.3
4.6
14.3 •
30
10
2.5
0.4
1.35
Year of
Constr
or Bid
1973
1971
1976
1976-77
1973
1972
1976
1965-68
1974-76
1976
1961
1974
1976
1976
1973
1974
1974-75
1978-79
1975-76
Constr
Cost (103 $)
144
162
.72
1,310
32
131
142
,25
840
800
540
750
750
2,193
530
1,600
850
2.0 M
201
ENR
2600
Cost (103'$)
197
334 "
77
1,545
179
154
65
1,243
696
810
810
3,005
683
1,164
2.16 M
260
Type
(tank)
(basin)
T
T
T
T
T
T "
T
B
B
B
T
3
B
B
B
T
B
B
B
T
T
B
99
-------
(I2,88M)S e>(|5.75M) o(47.0M)
CTION COST, DOLLARS ((0°)
~ fo W Jh (A O N (rt * W C
1.05
8.04
03
.OZ
.01
X
x
X
x
,/
/
0
J
0
^
0-Q»
(,005I8)|
0 0
V
f^
\(!?_
— >q>^
y
§<
* o
o
\J^
• *^n
^
r
a
Ha
c
"(5"
X
k--
I
,/
'*i
o
/
ft
/
LC'
X
X
^X
? '
a
[i a
r-i
^0****^
D
a
1 1
a
U__
S
[
coi
ii
'°1
•i
^
i
1
^
$£
5
^
ft*
®TANK 33 ENTRIES
El BASIN 34 ENTRIES
**>
•^
»•
.ffl .02 J03;O4J05 ,1 ,2 .3 .4.5 1.0 2345 10 20 3O 40 50 IOO 200 3O040O IpOO
FLOW EQUALIZATION FACILITIES VOLUME (I06gal)
Figure 18. Equalization facilities construction cost as a
function of equalization capacity
100
-------
TABLE 13. OPERATION AND MAINTENANCE COSTS FOR EQUALIZATION
TANKS
Plant name
Capacity < 1 mgd
C.I.W., CA
Dimondale , MI
Essexville, MI
Blackwood, NJ
Palmyra, NJ
Clementon Sew Auth
Plant, NJ
E. Windsor, NJ
Cherry Hill, NJ
Mt. Laurel, NJ
Ramblewood, NJ
Stratford, Sew Auth,
NJ
Chautaugua , NY
Waverly, NY
Shelburne, VT
Stevens Pass, WA
Capacity > 1 mgd
San Luis Key TP, CA
Valley Comm. , CA
Upper Thompson, CO
Clavey Road, IL
Lawrence , KA
Manhattan , KA
Corrina Sew Dist Pit,
MA
Table
ref no.
Tables 7 & 8
- 1
8
9
11
12
13
14
15
16
17
18
19
26
32
34 '
Tables 9 & 10
12
13
15
17
19
20
21
Paris Utility Dist, MA: 22
Equal tank
Ind waste holding tank
Chapaton P . S . , MI
Eaton Rapids, MI
E. Lansing, MI
Grand Haven/Spring
Lake, MI
Hancock St. P.S., MI
Lansing, MI
Midland, MI ,
Mt. Clemens, MI
Pontiac, MI
23
25
26
27
29
31
32
33
34
Type
A
B
B
A
A
A
A
A
A
A
A
A
A
A
A
A
A
A
B
B
A
A
B
A
A
A
B
B
A
B
A
Equalization
facility
vol . , mg
0.100
0.138
0.600
0.248
0.132
0.350
0.200
0.085
0.017
0.102
0.297
0.036
0.110
0.150
0.076
1.160
0.150
0.238
21.000
5.000
2.900
,0.400
0.210
0.130
28.000
0.140
5.000
0.800
3.600.
4.000
3.250
30.000
3.000
Annual
O&M cost , $
Minimal
500-1,000
16,300
4,000
Minimal
Minimal
Minimal
Minimal
15,000
5,000
4,000
14,500
27,000
11,800
200,000
29,000
2,500
45,000
Minimal
(continued)
101
-------
TABLE 13 (continued)
Plant name
Table
ref no.
Capacity > 1 mgd, continued
Port Huron, MI
Southeastern Oakland
Co., MI
Tecumseh, MI
Walled Lake-Novi, MI
Ypsilanti, MI
Merrimack, NH
Elmwood, NJ
New Providence, NJ
Woodstream, NJ
Ambers t, NY
Delaware, OH
Hatfield Twp., PA
Oil City, PA
Odessa, TX
Roanoke , VA
Ada STP, WV
Westside STP, WV
Ely, MN
35
36
37
39
41
47
48
49
50
51
52
53
54
60
66
70
71
Type3
A
B
A
A
A
A
A
A
A
A
A
A
B
A
B
B
A
A
Equalization
facility
vol. , mg
6.000
63.000
1.000
0.315
0.624
0.952
0.258
0.426
0.299
1.250
1.000
0.220
0.476
2.800
22.000
0.400
0.055
Annual
O&M cost, $
4,186
800,000
Minimal
* 17,200
Minimal
56,000
Minimal
2,500
Not yet use
Minimal
Minimal
A * Equalization facility for diurnal flows
(in-line and side-line).
B = Equalization facility for storm flows.
102
-------
TABLE 14. OPERATIONS AND MAINTENANCE COSTS FOR EQUALIZATION
BASINS
Plant name
Capacity < 1 mgd
Deer Creek, CA
Rancho Bernardo
Willits, CA
Northeast Oskaloosa,
IA
Southwest Oskaloosa,
IA
Boyne City, MI
Dawson, MN
Weather ford, OK
Salina, UT
Lake Samish, WA
Capacity > 1 mgd
Cypress Creek, AL
5 Mile Creek, AL
Valley Creek, AL
Ref no-.
Tables 7 & 8
2
3
4
5
6
7
10
27
31
33
Tables 9 & 10
1
2
3
Central Contra Costa, CA 4
Chino Basin, CA
Laguna , CA
Pismo Beach, CA
Redlands, CA
Rohnert Park, CA
Rossmoor San Inc. , CA
Sacramento , CA
Valley Comm. , CA
Livermore , CA
Broomfield, CO
Ankeny , IA
Dowagiac, MI
Grand Rapids, MI
Jackson, MI
Trenton , MI
Warren , MI
Brookhaven , MS
Greneda, MS
Joplin, MO
Warrensburg, MO
Watertown , SD
Amarillo, TX
Duck Creek, TX
Sandy Suburban, UT
5
6
7
8
9
10
11
13
14
16
18
24
28
30
38
40
42
43
44
45
55
56
57
62
Type
A
A
B
B
B
A
A
A
A
A
B
B
B •
B
A
B
A
A
A
A
A
A
B
A
B
B
A
A
,B
A
A
B
A
A
B
A
A
A
Equalization
facility
vol . , mg
3.000
0.200
16.000
4.070
4.070
57.000
0.260
8.150
0.500
8.020
4.000
30.000
5.000
164.000
1.200
17.000
0.375
1.250
0.750
2.500
222.000 ,
2.300
20.000
2.000
4.000
4.900
10.000
12.500
13.500
50.000
81.000
72.000
14.660
0.53Q
2.000
3.000
11.800
0.70Q
Annual
O&M cost , $
Minimal
800
Minimal
Minimal
13,414
2,252
1,095
5,000
10,000
20,000
Minimal
Minimal
Minimal
33,435
1,872
Minimal
3,000
Under const
14,728
Minimal
Minimal
' Minimal
Minimal
Minimal
Minimal
32,500
(continued)
103
-------
TABLE 14 (continued)
Plant name
Ref no
Capacity > 1 mgd, continued
Lower Potomac, VA
Moore Creek, VA
Potomac Regional, VA
Upper Occaquan, VA
Lacy-Olympia-Tumwater ,
WA
Dry Creek, WY
63
64
65
67
68
74
Typea
A
A
A
A
B
A
Equalization
facility
vo 1 . , mg
14.00
4.60
14.30
45 & 10
2.50
2.50
Annual
O&M cost, $
50,000
25,000
20,000
Minimal
Not const
3,500
A = Equalization facility for diurnal flows
(in-line and side-line).
B = Equalization facility for storm flows.
A log-log regressional analysis of O&M cost data is shown
in Figure 19. An attempt was made in the analysis to develop
cost curves for both in-line facilities (used for either all or
excess daily flow) and side-line installations (used primarily
for storm inflow). No apparent correlation was observed between
type of use and complexity or size of the equalization structure,
The O&M cost data listed in Tables 13 and 14 were also com-
pared on the basis of size of the associated treatment plants.
A log-log regressional analysis of these data is shown in Figure
20. Although these data varied greatly, a more reasonable
pattern emerged that indicates that O&M costs associated with
equalization facilities are more closely related to the size of
the wastewater treatment plant than to the size of the equali-
zation facility installed.
A flow equalization facility must obviously produce some
additional O&M costs for a wastewater agency because of the
additional equipment and controls involved in such an operation.
Recorded observations at several plants previously studied by
others, and survey data received during this study indicate that
only a negligible amount of operator time is required on a day-
to-day basis for routine operation and maintenance procedures
for both in-line and side-line installations. Major cost fac-
tors were primarily for repair of equipment and control systems.
104
-------
V)
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Figure 19. Operation .and maintenance cost as a function of
equalization volume
105
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Figure 20.
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facilities as a function of treatment plant
capacity
106
-------
SECTION 5 • "
EQUALIZATION PERFORMANCE EVALUATION
CASE HISTORIES - - . - . .
Introduction
Increased treatment process stability and improved perfor-
mance are presumed benefits of equalization frequently cited in
favor of including it as a treatment component. Treatment pro-
cess design relationships and extensive qualitative reasoning
generally support these assumptions. * But current understanding
of the complex interactions between treatment process compo-
nents under typically variable diurnal loading conditions is
not sufficient to quantitatively predict the benefits of equali-
zation. Nevertheless, efforts to define its cost effectiveness
require quantitative information. Recent EPA sponsored equali-
zation studies and treatment plant operating records obtained
in this study have, therefore, been analyzed to determine quan-
titative effects of equalization on both performance level and
day-to-day variability of conventional wastewater treatment
facilities.
To evaluate effects of equalization on treatment process
and plant performance using only existing plant operating
records, a careful selection of the different types of data is
essential. A broad range of influent, environmental and
operational factors affect treatment plant performance. To
minimize the influence of extraneous variables, treatment plants
should be selected for analysis primarily if equalization has
been added to an existing plant as the only significant physical
or operational change during the period of interest. In such
cases existing plant operating records can be analyzed for the
years preceding and following the time equalization began.
Significant effects of equalization on average daily performance
may be identified, giving appropriate consideration to differ-
ences in influent conditions of the respective periods of
operations. Very few suitable treatment plants are in exis-
tence, and fewer yet have operating records adequate for thor-
ough analysis. In a few cases, where data are available, two
years of equalized flow performance data are presented in
separate comparisons. Before and after data is supplemented
107
-------
with data from additional treatment plants (with and without
equalization) having detailed operating records available for
analysis. The performance analysis includes treatment facili-
ties in a range of sizes, treatment types, and types of equali-
zation facilities (Table 15).
Performance characteristics of plants studied are estab-
lished by means of probability plots, as described in Section 2.
Data are analyzed uniformly for plant influent and secondary
effluent; where available, primary and tertiary effluent analyses
are included. Typically, operating records reflect daily average
flows and concentrations, with concentrations composited pro-
portional to flow. One year (365 consecutive days) of operating
data is used where possible to provide a description of typical
operating conditions. Performance characteristics are estab-
lished for each plant on the, basis of BOD and TSS concentra-
tions. Characteristics are also examined for some plants in
terms of BOD and TSS loadings to illustrate the significance of
differences between flow and concentration distributions over a
typical annual cycle.
In most cases, logarithmic transformations are found to
normalize both influent and effluent distributions. Comparisons
must be made in terms of the logarithmic standard deviations and
coefficients of variation in spite of the lack of physical sig-
nificance. In an effort to best describe observations of plant
performance characteristics and effects of equalization,
graphic presentations are used, permitting the reader to supple-
ment comparisons discussed. Effects on the performance level
are reflected by median and mean values of respective data sets.
Effects on day-to-day process variability are shown by the
slopes of the respective distributions.
The treatment plants for which data are analyzed have
widely varying characteristics, so comparison of plant perfor-
mance should be made with due caution. In spite of the variety
of characteristics among plants, and the wide range of design
and operating factors other than equalization (or the lack of
it), general patterns may be found that can be attributed to
equalization effects. The annual performance characteristics
required for compliance with secondary treatment standards
(Figure 16), as described in Section 2, provide a generalized
basis for evaluating performance of individual plants.
Activated Sludge Plants
EPA MERL Pilot Plant—
An in-house study of the effects of input variations on
activated sludge treatment was conducted by the EPA at the
Municipal Environmental Research Laboratory, Cincinnati, Ohio.
Facilities consisted of two independent, parallel, 20 gpm
108
-------
TABLE 15. TREATMENT FACILITIES
FOR PERFORMANCE EVALUATION
Plant type
and location
Activated sludge:
EPA Pilot Plant,
Cincinnati, OH
Walled Lake-Novi, MI
Tecumseh, MI,
Ypsilanti Twp, MI
Pontiac, MI
Amarillo, TX
Warren, MI
Renton, WA
Newark, NY
Trickling filter":
Palmyra, NJ
Midland, MI
Bay City, MI
Oxidation ditch:
Dawson , MN
Arlington, WA
Design
capacity
2 @ 20 gpm
0'.7 mgd
1 . 4 mgd
3 . 7 mgd ,
3 . 8 mgd
8.5 mgd
9 . 5 mgd
35 mgd
29 mgd
1 . 8 mgd
0.53 mgd
6 . 5 mgd
12 mgd
0.26 mgd
2 . 1 mgd
Equalization
type'
Constant flow
Side-line
In-line
In-line
unequalized
Side-line
Side-line
Emergency
Influent
regulation
In-line
In-line
Side-line
Variable
A . S . vo 1 .
Operating
data ..•'•-
Side -by-side
Equalized
Before/after
Side-by-side
Equalized
Before/after
Unequalized
Part.
equalized
Unequalized/
equalized
Before/after
Equali zed
Unequalized
Equalized
Unequalized
109
-------
capacity activated sludge treatment trains. Flow to one plant
was maintained at a constant 20 gpm, and the other was maintained
at a daily average flow of 20 gpm, with diurnal peak to average
variations of 1.5, 2.0 and 2.5:1 imposed during consecutive study
periods of approximately 3 months each. Since flow to the plants
was provided by pumping directly from the raw sewage source, no
concentration equalization was provided in the constant flow
system; as shown by values (Table 16) determined from diurnal
sampling during each of the peak-to-average flow periods. Unit
process loading rates were maintained as follows: primary
clarifier overflow rate = 1,200 gpd/ft2; aeration tank loading
* 35 Ib BOD/1,000 ft3; secondary clarifier overflow rate =
650 gpd/ft2.
TABLE 16. EPA INHOUSE STUDY: PEAK-TO-AVERAGE
(P/A) FLOW AND LOAD RATIOS
P/A Flow
1.5
1.5
2.0
2.0
2.5
2.5
2.5
2.5
Date
Feb
Apr
Jul
Oct
Jan
Jan
Feb
Apr
r74
'74
'74
'74
•75
'75
'75
'75
P/A Load
Constant
1.6
1.8
1. 7
1.4
l'.3-
1.7
1.8
(COD)
Flow
1 7
1. 55
(TSS)
P/A Load
Varying
2.1
1.6
2.4
2.2
2J4
2.3
2.7
(COD)
Flow
1 85
2. 1
(TSS)
Influent, primary effluent, and secondary effluent TSS and
BOD concentration distributions of the parallel constant and
diurnally varying flow pilot plants are shown in Figures 21 and
22 for a peak-to-average flow ratio of 1.5; Figure 23 and 24 for
a peak-to-average flow ratio of 2.0; and Figures 25 and 26 for a
peak-to-average flow ratio of 2.5. Differences between distri-
butions of primary effluent TSS and BOD concentrations and loads
at all three levels of peak-to-average flow were slight. In
addition removals observed were not typical of conventional ex-
perience. Accordingly, little emphasis is placed on this,in-
formation, and more detailed comparison will not be made.
Distributions of secondary effluent BOD and TSS at a peak-
to-average ratio of 1.5 were virtually the same for constant
and varying flow conditions. The pattern of similarity between
TSS and BOD distributions was observed to continue in the other
110
-------
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Figure 21. Distribution of weekly TSS concentrations,
EPA in-house study (P/A =1.5)
111
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.01 O.I I 10 20304050607080 90 99
PERCENT OF VALUES GREATER THAN
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Figure 22. Distribution of weekly BODs concentrations,
'EPA in-house study (P/A = 1.5)
112
-------
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Figure 23. Distribution of weekly TSS concentrations,
EPA in-house study (P/A =2.0)
113
-------
1000
800
700
600
500
400
300
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I 10 20304050607080 90 99
PERCENT OF VALUES GREATER THAN
99.9 99.99
Distribution of weekly BOD5 concentrations,
EPA in-house study (P/A = 2.0)
114
-------
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.01 O.I
I 10 20304050607080 90 99 99.9 99.99
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Figure 25. Distribution of weekly TSS concentrations,
EPA in-house study (P/A = 2.5)
115
-------
1000
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600
500
400
300
200
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.01 O.I I 10 20304050607080 90 99
PERCENT OF VALUES GREATER THAN
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Figure 26. Distribution of weekly BOD5 concentrations,
EPA in-house study (P/A = 2.5)
116
-------
two peak-to-average ratio periods. As the peak-to-average ratio
was increased, the difference between distributions for constant
and varying flow conditions is observed to increase. At a peak-
to-average ratio of 2.0, performance under constant flow was
better in terms of both TSS and BOD, with differences in mean
values of approximately 10 percent and 25 percent respectively.
Variability of BOD performance was slightly greater for constant
flow conditions. But variability of TSS performance was slightly
less for constant flow. At the peak-to-average ratio of 2.5,
TSS and BOD performances were again both better under constant
flow conditions, with differences between mean effluent concen-
trations each approximately 20 percent. The variability of BOD
and TSS distributions was observed to be greater under constant
flow conditions during this period.
«•
The data from this study suggest that the increasing
effectiveness of equalization, for improving process performance
runs parallel to the increase of influent variability. It is
not possible to distinguish, however, between the relative im-
portance of simple flow equalization arid a somewhat lower degree
of influent load equalization provided (Table 15).
Walled Lake-Novi, Michigan—
A study of flow equalization at the Walled Lake-Novi,
Michigan, wastewater treatment plant (27) was conducted from
January 1974 to February 1975. The facilities consist of a
0.7 mgd activated sludge "package" plant and effluent filters,
with a 0.34 million gallon sideline equalization tank. The
plant has stringent effluent discharge requirements (Table 17).
These discharge requirements dictated conservative design:
secondary clarifier overflow rates = 360 gpd/ft2; effluent fil-
tration rates = 1 gpm/ft-2. Complete details of plant design and
operation are given by Foess et al. (27) The aerated equalization
tank is operated to maintain essentially constant flow through
the plant on a daily basis. Average plant flow is estimated at
the beginning of each day. Controls then provide for either
pumping excess raw wastewater from the influent wet well to the
equalization tank, or for allowing stored wastewater to flow
back into the secondary process during deficient flow periods.
The plant does not have a primary clarifier.
TABLE 17. EFFLUENT DISCHARGE REQUIREMENTS
FOR WALLED LAKE-NOVI PLANT
Parameter
(mg/1)
BOD
TSS
NH3-N
Total-P
30-day average
10
20% of influent
7-day average
15
Daily maximum
10
2
117
-------
During the study period the plant was operated continuously
according to routine equalized flow procedures. For one week
(October 28 to November 3, 1974), the equalization tank was not
used, so that plant performance could be observed under un-
equalized flow conditions. The peak-to-average flow ratio under
typical dry weather conditions in 1977 is approximately 1.5.
Influent, secondary effluent and filtered final effluent
TSS and BOD concentration and load distributions for equalized
and unequalized flow periods during the 1974 study are shown in
Figures 27 through 30. Differences between the respective con-
centration and load distributions are attributable to differences
in the distribution of average daily flows experienced between
the equalized and unequalized periods studied. The influent
loading distributions (Figures 31 and 32) indicate the compara-
bility of loading conditions experienced in the respective
periods.
Comparison of distributions from equalized and unequalized
flow periods shows that some differences occurred both between
the level of performance and degree of performance variability.
The variability in .all distributions of the one-week unequalized
period may be artificially low compared to the rest of the one-
year equalized period. One week of consecutive average daily
concentration measurements benefits from the natural homogeneity
of conditions experienced in relatively short periods of time;
effects of the full range of typical annual influent conditions
are simply not represented.
Distributions of influent and effluent BOD (Figure 28)
indicate that secondary effluent in the equalized flow period
was both better and less variable than in the unequalized period,
and that filtered final effluent was better but more variable.
It may be noted that influent BOD loading conditions were some-
what lower and less variable during the equalized flow period.
However, because of the very light loading rates imposed on this
plant, the differences observed in influent conditions should
have had slight, if any, effect on overall performance.
Influent and effluent TSS concentration distributions
(Figure 27) indicate that, contrary to observed BOD performance,
secondary effluent in the equalized flow period was both of
poorer quality and more variable than in the unequalized flow
period. Filtered final effluent, on the other hand, was both
better and less variable under equalized conditions. Influent
TSS loading under equalized conditions was somewhat higher, but
had similar variability to that under unequalized conditions.
By virtue of the differences observed between respective influ-
ent distributions, conclusions about the effect of equalization
as opposed to other process variables are difficult to draw.
The uniformly better quality of filtered final effluent observed
118
-------
1000
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1974L
.01 O.I I 10 20304050607080 90 99 99.9 99.99
PERCENT OF VALUES GREATER THAN
Figure 27. Distribution of 8-day TSS concentrations,
Walled Lake, Michigan/Novi, Michigan, 1974
119
-------
.01 O.I
I 10 20304050607080 90 99
PERCENT OF VALUES GREATER THAN
99.9 99.99
Figure 28. Distribution of 8-day BODc concentrations,
Walled Lake, Michigan/Novi, Michigan, 1974
120
-------
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! ;ECONDARY EFFLUENT
.01 O.I
I 10 20304050607080 9O 99 99.9 99.99
PERCENT OF VALUES GREATER THAN
Figure 29.. Distribution of TSS concentrations, Walled Lake,
Michigan/Novi, Michigan, 1974 (8-day) and 1976-1977
121
-------
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(1976-1977)
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O.I I 10 20304050607080 90 99 99.9 99.99
PERCENT OF VALUES GREATER THAN
Figure 30. Distribution of 6005 concentrations, Walled Lake,
Michigan/Novi, Michigan, 1974 (8-day) and 1976-1977
122
-------
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igure 31. Distribution of 8-day TSS loads, Walled Lake,
Michigan/Novi , Michigan, 1974
123
-------
4000
3000
2000
1000
800
700
600
500
400
300
200
3
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80
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ENT
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(1974)
.01 O.I I 10 20304050607080 90 99
PERCENT OF VALUES GREATER THAN
99.9 99.99
Figure 32. Distribution of 8-day BOD5 loads, fAlalled Lake,
Michigan/Novi, Michigan, 1974
124
-------
during equalized operation suggests that equalization is a posi-
tive influence on effluent filter performance. These observa-
tions correspond closely to those reported in the original study.
Distribution of equalized influent and effluent TSS and BOD
concentrations for the year from August 1976 to July 1977
(Figures 29 and 30) are compared to the unequalized distribu-
tions from the 1974 study (Figures 27 and 28). The distribu-
tions of influent TSS and BOD concentrations for 1976-1977 are
comparable to those of the equalized -1974 period, and corres-
pondingly lower and somewhat more .variable than those of the
1974 equalized flow period.
The 1976-1977 distribution of secondary effluent TSS shows
improved performance compared to the equalized 1974 period, but
still slightly poorer performance than the corresponding un-
equalized period. The median 1976-1977 filter effluent TSS per-
formance is approximately 3 mg/1, with 80 percent of effluent
concentrations between 1 mg/1 and 7 mg/1. Corresponding 1974
equalized and unequalized filter effluent medians are 3.5 mg/1
and 5 mg/1, with 80 percent ranges of 1 mg/1 to 6 mg/1 and
1 mg/1 to 13 mg/1, respectively.
The 1976-1977 distributidn of effluent BOD concentrations
shows significantly better but more variable performance than in
the corresponding 1974 periods. Effluent filter BOD performance
during 1976-1977 is not quite as good as during the 1974
equalized period, but still substantially better than during
the unequalized period. This is in reasonable agreement with
patterns observed for TSS performance.
Tecumseh, Michigan—
In 1972 a 1 mgd equalization/emergency storage basin
(Figure 33) was added to the Tecumseh, Michigan sewage treatment
plant. No other physical or significant operational changes
were made at this plant during the period from 1970 through the
present. The facilities consist of a 1.4 -mgd contact stabiliza-
tion, activated sludge process with conventional headworks, and
primary and secondary clarifiers. The peak-to-average flow
ratio during typical dry weather conditions is about 1.3.
Average dry weather flow in 1976 was approximately 1.1 mgd.
The equalization basin is divided into two equal 500,000
gallon compartments. One compartment is normally kept empty to
provide emergency bypass storage from the headworks in the event
of industrial spill detection. Oils and plating wastes from
local manufacturing industry periodically contributed to severe
plant upsets before the equalization addition. The second com-
partment is routinely used for equalization of daily flow varia-
tions. Effluent from the primary clarifier flows by gravity to
the equalization basin. Flow to the activated sludge process is
125
-------
Figure 33. Equalization tank, 1 million gallon capacity,
Tecumseh, Michigan
pumped from storage at the estimated average daily flow preset
by the operator and maintained by wet well level pump controls.
Mixing and aeration are provided in the equalization tank by a
coarse-bubble, diffused air system.
Influent and effluent distributions of TSS and BOD concen-
trations and loads observed during the year before equalization
(1970) and two years following equalization are shown in Figures
34 through 43. Comparison of influent loadings for the three
years (Figures 38 through 41) show that loadings in equalized
periods are significantly higher than in the unequalized period.
Mean values of the various distributions (Table 18) illustrate
the_differences observed in influents and effluents during the
periods of record. In general, differences between loading
distributions for the periods compared were similar to those
observed between concentration distributions.
The distribution of effluent BOD concentrations was sig-
nificantly lower in the two equalized flow periods than in the
period before equalization. In 1973, the equalized effluent BOD
126
-------
O.I
10 20304050607080 90
99 99.9 99.99
Figure 34
PERCENT OF VALUES GREATER THAN
Distribution of TSS concentrations, Tecumseh,
Michigan, 1970 and 1973
127
-------
1000
800
700
600
500
400
300
200
o»
E
O
1
UJ
o
2
O
O
O
O
00
100
80
70
60
50
40
30
20
10
8
6
5
4
FdllALI7ED
Si gdNDARY EFF
-nmr
\
N
N
.01 O.I I 10 20304050607080 90 99
PERCENT OF VALUES GREATER THAN
99.9 99.99
Figure 35. Distribution of BOD5 concentrations, Tecumseh,
Michigan, 1970 and 1973
128
-------
2000
O.I
10 20304050607080 90
99 99.9 99.99
PERCENT OF VALUES GREATER THAN
Figure 36. Distribution of TSS concentrations, Tecumseh,
Michigan, 1970 and 1976
129
-------
2000
1000
800
700
600
500
400
^ 300
D>
E
Q
DC
UJ
o
z
o
o
f 200
100
80
70
60
50
40
03 30
20
10
8
6
5
4
FQUAI.I:
SECOND,
I NFLUENTtlff
RYOTLUCHT
\
1M
\
JEBLUEE414
\
975).
I TED
SONDARYCFFLJENr (»7(
.01 O.I I 10 20304050607080 90 99
PERCENT OF VALUES GREATER THAN
99.9 99.99
Figure 37. Distribution of BOD$ concentrations, Tecumseh,
Michigan, 1970 and 1976
130
-------
8000
7000
6000
5000
4000
3000
2000
1000
800
700
>> 600
-S 500
\ 400
.0
- 300
Q 200
<
O
_l
co 100
£ 80
H 70
60
50
40
30
20
IO
8
6
5
4c
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ISTI
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A
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IAL
IDA
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k
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RY
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iFFI
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UEN
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^
JENT (
\
N
•>
f U97(
.-r-INFLUEh
^.^ 13
1970) -^V
EQUALIZI
SECQNDA
S^(1973)
\
\\
o \
)-^\
^y
^k
^.
\
T (1973)
^\
D
RY fff\ 1 IF
V
X
X,
JT
M 0.1 1 10 20304050607080 90 99 99.9 99.
PERCENT OF VALUES GREATER THAN
Figure 38.
Distribution of TSS loads, Tecumseh, Michigan,
1970 and 1973
131
-------
10000
80OO
7000
600O
5000
4000
3000
2000
1000
800
700
600
500
400
•I*
I"
Q
O
CD
300
200
IOO
80
70
60
50
4O
30
20
IO
8
A
INFLUENT
(1973K1
INFLUENT
A V
(1970).
•UNEQOTI.IZED
(1970)
SECMARIIEELLENL
I ZED
JDABYEFF
1JEN1L
.01 0.1 I 10 20304050607080 90 99 99.9 99.99
PERCENT OF VALUES GREATER THAN
Figure 39. Distribution of BODc loads, Tecumseh, Michigan,
1970 and 1973
132
-------
2OOOO
IOOOO
8OOO
7000
6000
5000
4OOO
3000
2000
>• 1000
^ 800
^ 700
600
500
400
300
200
Q
<
O
CO
CO
100
80
70
60
50
40
30
20
!O
8
INFLUENT
(1970)-
FMT
5s,
(19'
\
.01 O.I
10 20304050607080 90
99 99.9 99.99
Figure 40.
PERCENT OF VALUES GREATER THAN
Distribution of TSS loads, Tecumseh, Michigan,
1970 and 1976
133
-------
1
a
o
CO
10000
8000
7000
6000
5000
4000
3000
2000
1000
800
700
600
500
400
300
200
100
80
70
60
50
40
30
20
10
8
INEL
1EM
US TO)-
SE(
7EI
:ON0ARY EFFLUENT (1976)
•INFLUEN1
1JNE3UM1ZEI1
SECiNMfiXIIELUEMI
•(1970)
(1976)
'.01 O.I I 10 20304050607080 90 99 99.9 99.99
PERCENT OF VALUES GREATER THAN
Figure 41. Distribution of BOD5 loads, Tecumseh, Michigan,
1970 and 1976
134
-------
20000
10000
8000
7000
6000
5000
4000
3000
2000
-g
1000
£ 800
700
600
500
400
o
o
to 300
200
SECON )ARY EFFLUEN ' U97
100 -
.01 0.1
Figure 42
I 10 20304050607080 90 99
PERCENT OF VALUES GREATER THAN
99.9 99.99
Distribution of TSS loads, Tecumseh, Michigan,
1973 and 1976
135
-------
10000
8000
7000
6000
5000
4000
3000
2000
>,
D
•O
O
<
3
§
CD
1000
800
700
500
400
30°
200
100
80
70
60
50
40
30
20
IO
8
illHJlEM
\
\
-£Q
JAU2ED-
CQNDARY EFFL
JEML
197: E
•INFLUENT
(1973)
(19761
iQtift.lZED
SFCniinARYFFFl
UFNT
.01 O.I I 10 20304050607080 90 99 99.9 99.99
PERCENT OF VALUES GREATER THAN
Figure 43. Distribution of BOD5 loads, Tecumseh, Michigan,
1973 and 1976
136
-------
TABLE 18. MEAN BOD AND TSS LOAD AND
CONCENTRATION SUMMARY
Item
1970
1973
1976
Mean BOD load, Ib/day:
Plant influent
Plant effluent
Mean TSS load, Ib/day:
Plant influent
Plant effluent
Mean BOD cone., mg/1:
Plant effluent
Mean TSS cone., mg/1:
Plant effluent
1,829
98
3,376
136
28
13
2,923
76
5,351
94
2,489
134
4,505
115
12
14
performance was less variable than before equalization. However,
although performance in 1976 was still significantly better than
before equalization, it was slightly more variable.
The distributions of effluent TSS concentrations for the
three years examined does not show a consistent pattern. In
the equalized 1973 period the mean effluent TSS concentration
was lower and slightly less variable than before equalization.
However, in 1976 the mean effluent TSS concentration was nearly
the same although slightly more variable than in the year before
equalization. '
No consistent relationship was apparent between effluent
quality and influent loading. When loadings were lowest before
equalization, effluent quality was poorest. However, during the
equalized flow periods effluent quality was generally best
during the period of higher loading. Although this suggests a
simple inverse relationship between loading and performance, it
is generally accepted that the opposite relation should exist
for plants such as Tecumseh operating at loadings approaching
the design capacity. Overall, with the exception of 1976 TSS
concentrations, effluent performance appears to have benefitted
from equalization. This is in agreement with observations of
plant operating personnel.
137
-------
Ypsilanti Township, Michigan—
A side-by-side comparison of equalized and unequalized
activated sludge treatment performance (28) was conducted from
June 1974 to July 1975. The Ypsilanti Township sewage treatment
facilities consist 'Of two parallel, but independent, activated
sludge plants on a common site. Separate interceptors serve the
two plants. Although influent sewage to the two plants is not
identical, characteristics are similar, and local factors
affecting sewage composition and influent variations are common
to both. Plant No. 1 is equipped with an in-line equalization
basin for diurnal flow smoothing; Plant No. 2 has no flow
equalization. Plant No. 1 has a capacity of 3.7 mgd rated at a
secondary clarifier overflow rate of 800 gpd/ft2, and has no
primary clarifier. Ferric chloride is added to the aeration
tank effluent for phosphorus removal.
*
Average daily flows to both plants under typical operating
conditions in 1974-1975 were about 4.0 mgd. Hourly average
flows ranged from 1.5 to 6.0 mgd, with raw sewage peak-to-
average ratios of about 1.5 under typical dry weather conditions,
During the study period, average influent BOD was approximately
200 mg/1, with average hourly concentrations ranging from 50 to
400 mg/1. Influent wastewater to both plants contains about
25 percent industrial wastewater.
The in-line equalization system consists of two converted
anaerobic digesters providing a total capacity of 624,000 gal-
lons. The tanks are aerated to prevent solids deposition and
to maintain aerobic conditions in the untreated wastewater.
Flow is pumped to the tanks from the plant influent pump sta-
tion. Constant gravity flow to the downstream treatment pro-
cesses is maintained by automatic control of the equalization
tank discharge valve.
During the experimental period, the unequalized Plant No. 2
was not altered from its normal operating routine. Data from
this plant provide a basis for comparing performance character-
istics for constant and varying flow conditions. Plant No. 1
was observed under three modes of operation: (1) equalized flow
at approximately design capacity, maintained for all but five
weeks of the June 1974 to July 1975 study period; (2) equalized
flow for hydraulically stressed conditions in the secondary
clarifier for brief periods between March 17 and April 17, and
from June 3 to July 3, 1975; and (3) unequalized flow conditions
were maintained from May 14 to June 14, 1975.
Operating data from the two plants for the calendar year
1976 are presented to provide an additional comparison of
equalized and unequalized performance.
138
-------
Influent and effluent BOD and TSS concentrations and load
distributions from the 1974-1975 study of equalized Plant No. 1
and unequalized Plant No. 2 are shown in Figures 44 through 47.
The obvious similarity between distributions of BOD and TSS con-
centration and BOD and TSS load results from the concurrence of
operating periods with nearly identical influent wastewater com-
ponents. Due to the similarity of the distributions only the
concentration figures will be discussed. Influent and effluent
BOD and TSS concentration distributions for 1976 are shown in
Figures 48 and 49.
The distribution of effluent BOD from equalized Plant No. T
was lower than the unequalized distribution; approximately 7 mg/1
difference in median values. The annual means can be calculated
as 15.8 mg/1 and 23.3 mg/1 respectively. Variability of the
equalized performance was slightly greater than unequalized per-
formance; approximately 20 percent difference in log standard
deviations. The observed difference between TSS performances
was approximately 20 percent in log standard deviations. The
observed difference between TSS performance levels of equalized
and unequalized plants was negligible; both population means
approximately 17 mg/1. In this case, performance variability
of the equalized plants was less than for the .unequalized plant;
approximately 27 percent difference in log standard deviations.
Considering the comparability of the plant loading conditions
over the study period, and the concurrent period of observation,
equalization appears to have contributed to improved plant per-
formance .
Observations of Plant No. 1 performance during alternating
consecutive 31-day unequalized, equalized, and unequalized
operating periods (from April 13 to July 14, 1975) should be
considered along with the comparison of Plant No. 1 and No. 2
above. The performance summary (28) indicates that little dif-
ference was observed between equalized and unequalized flow
periods. If anything, performance during the equalized flow
period was judged to be slightly better. However, differences
in flow and loading conditions during the respective periods
were large enough to have had as much or more influence on per-
formance as the existence or absence of flow equalization.
Comparing concentration distributions for the 1974-1975
study (Figures 44 and 45) and for 1976 (Figures 48 and 49)
reveals that plant performance was remarkably similar during
the two periods. Almost no difference can be seen between
effluent TSS distributions of the two plants during the two
periods. Median annual performance is approximately 13 to 15
mg/1; each with approximately the same level of variability.
139
-------
1000
800
700
600
500
400
300
200
o»
E
o:
ui
o
o
o
CO
to
100
80
70
60
50
40
30
20
10
I
6
5
4
SEONCARV E
FF
ENT
-LIE
INFLUEN
PLANT 2
(1974-1975
VOW ft«
PLANT
,,(1974-19:5^
o
"o.
ED
IN
.01 O.I I 10 20304050607080 90 99
PERCENT OF VALUES GREATER THAN
99.9 99.99
Figure 44. Distribution of TSS concentrations, Ypsilanti Township,
Michigan, 1974-1975
140
-------
IOOO
800
700
600
500
400
3OO
200
o>
E
2?
O
<
cc
h-
o
z
O
O
O
O
DQ
100
80
70
60
50
40
30
20
10
8
7
6
5
^S*""r iQUALJ
JZED
SECONDARY
5LANT I
1974-1975)
EFFL JEN F
JNEQUALIZEP
ECONDARY
f>LANT 2
1974-1975)
EFFLUEM
.Ol 0.1 I 10 20304050607080 90 99
PERCENT OF VALUES GREATER THAN
99.9 99.99
Figure 45. Distribution of BOD5 concentrations, Ypsilanti Township,
Michigan, 1974-1975
141
-------
30000
20000
10000
8000
7000
6000
5000
4000
3000
t
1
CO
2000
1000
800
700
600
500
400
300
200
100
80
70
60
50
40
30
.01
MR I
FNT
(1974-1975)
NFLUENT
'LANT 2w
EQUALIZED
SECONDARY
PLANT I
1974-1975)
EFFL-
UENT
UNEQUAl
PLANT
(1974-197&)
JZED
FFFI
I 10 20304050607080 90 99 99.9 99.99
PERCENT OF VALUES GREATER THAN
Figure 46. Distribution of TSS loads, Ypsilanti Township, Michigan,
1974-1975
142
-------
30000
20000
10000
8000
7000
6000
5000
4000
3000
-o
£
a
o
Q
O
GO
2000
1000
800
700
600
500
400
300
200
100
80
70
60
50
40
30
(1974-19'5)
EQUALIZED
SECONDARY
PLANT 1
(1974-1975)
£1
IN
(19
JENTi
H ENT
4-]
975
PLANT 2
•UNEQUAi
.PLANT 2
I ZED
SECOND/RY EFFLtfEfff
UW4-W7 f)
.01 O.I I 10 20304050607080 90 99 99J9 99.99
PERCENT OF VALUES GREATER THAN
Figure 47. Distribution of BOD5 loads, Ypsilanti Township, Michigan,
1974-1975 , - '
143
-------
1000
800
700
600
500
400
300
200
=c 100
I 80
70
£ 60
g 50
S 40
o
§
o
CO
CO
30
20
10
8
7
6
5
3
2
E
DUALIZED
SECOND A RY
o *
EQUALIZED
CONDARY EFFI
ANT 2 (1976)
IJEML
.01 O.I I 10 20304050607080 90 99 99.9 99.99
PERCENT OF VALUES GREATER THAN
Figure 48. Distribution of TSS concentrations, Ypsilanti Township,
Michigan, 1976
144
-------
1000
g
15
oc.
z
UJ
O
z
O
O
O
O
DQ
NEQUALIZED
FEONDARY FF
.01 O.I
I 10 20304050607080 90 99 99.9 99.99
PERCENT OF VALUES GREATER THAN
figure 49. Distribution of BOD^ concentrations, Ypsilanti Township,
Michigan, 1976
145
-------
Figure 50.
Equalization tank (background), 3 million gallon
capacity, Pontiac, Michigan
Pontiac, Michigan—
Two activated sludge plants, East Boulevard and Auburn,
treat wastewater from the City of Pontiac. Flow in a single
major interceptor passes the East Boulevard plant, where the
desired quantity is diverted for treatment. The remainder is
conveyed to the Auburn plant for treatment. The existing East
Boulevard activated sludge facilities have been in operation for
more than 30 years. A 3 million gallon side-line equalization
tank (Figure 50) was added in 1975 to limit peak daily flows to
the effluent filters, to stabilize operating conditions for
phosphorus removal, and to provide some protection against toxic
spills from extensive local manufacturing industry. No other
changes were made in the plant, but the capacity of the Auburn
plant was expanded at the same time, reducing flows to East
Boulevard. This reduced process loading at both plants, so that
performance characteristics before and after equalization are
not directly comparable. For example, average BOD loading to
146
-------
the East Boulevard plant was approximately 28 percent lower in
1976 than in 1973 (Figure 51).
The East Boulevard plant has a capacity of about 10 mgd.
Average dry weather flow to the plant in 1977 was maintained
between 8 and 9 mgd. The facilities consist of primary clarifi-
cation followed by equalization, activated sludge, and secondary
clarification. Ferric chloride and polymer are added to aera-
tion tank effluent for phosphorus removal. Final effluent is
piped to the Auburn plant for effluent filtration and disinfec-
tion.
The Auburn plant is also a conventional activated sludge
plant with primary and secondary clarification. It does not
have equalization facilities of its ;own. However, flows to the
plant are partially equalized by operation of the East Boulevard
and Auburn plants in series, equalization at East Boulevard, and
the location of the Auburn plant at the downstream end of the
system. Effluent filters and chlorine contact facilities at the
Auburn plant treat the combined flow from both plants. Total
average dry weather flow to the two plants in 1977 was about
20 mgd, with a peak-to-average ratio typically 1.3 to 1.4. Wet
weather flows may exceed 50 mgd.
During typical dry weather flow operating periods, primary
effluent is diverted to the equalization tank when flows at the
East Boulevard and Auburn plants exceed 8 and 12 mgd, respec-
tively. Stored wastewater is pumped to secondary treatment in
deficit flow periods. The stored wastewater is not mixed or
aerated.
Annual distributions of average daily influent and effluent
TSS and BOD concentrations at the East Boulevard plant are shown
in Figures 51 and 52. Data from 1973 are representative of
operation prior to the addition of equalization at East Boule-
vard and extensive additions to the Auburn plant downstream.
Data from September 1976 to August 1977 cover the year imme-
diately following commencement of equalization at East Boulevard.
The secondary effluent TSS distribution in the equalized
period indicates better and slightly less variable performance
than in the unequalized period. However, lighter influent and
primary effluent loadings in the equalized period could easily
have resulted in the observed secondary effluent differences.
Comparison of secondary effluent BOD distributions (Figure 52)
reveals very little difference in performance between the
equalized and unequalized periods. In fact, the secondary pro-
cess performance in the unequalized period could be considered
superior because of the substantially higher loadings .during
that period.
147
-------
1000
800
700
600
500
400
300
200
*-. 100
o>
e so
- 70
g 60
p 50
$ 40
O 20
o
C/5
CO
IO
8
SECONDARY
(1975)
-B
976)
:CON[ARVlEFHLUElNTi
f PRIMARY EFELUENT (V 76)
.01 O.I I 10 20304050607080 90 99
PERCENT OF VALUES GREATER THAN
99.9 99.99
Figure 51. Distribution of TSS concentrations, Pontiac, Michigan,
East Boulevard Plant, 1973 and 1976-1977
148
-------
O.I
I 10 20304050607080 90 99 99.9 99.99
PERCENT OF VALUES GREATER THAN
Figure 52. Distribution of BOD5 concentrations, Pontiac, Michigan,
East Boulevard Plant, 1973 and 1976-1977
149
-------
Annual distributions of average daily influent and effluent
TSS and BOD concentrations at the Auburn plant are shown in
Figures 53 and 54. The 1973 and 1976-1977 data used correspond
to those used to characterize operation prior to and following
equalization use at the East Boulevard plant. The major im-
provements in primary and secondary TSS and BOD effluent dis-
tributions are attributed to individual unit process loading re-
ductions resulting from expansion of the Auburn plant; including
primary clarifier, activated sludge, and secondary clarifier
capacity.
Comparison of Auburn and East Boulevard plant performances
for the 1976-1977 period does not permit any conclusions to be
drawn with respect to the significance of equalization. Secon-
dary effluent BOD performance of the Auburn plant is signifi-
cantly better than that of East Boulevard. Performance of the
two plants, with respect to BOD, in 1973 was virtually identical.
Amarillo, Texas—
A 3 million gallon equalization lagoon (Figure 55) was
added to the Amarillo River Road treatment plant in September
1965 to reduce peak flows, which were creating significant
solids loss from the secondary clarifiers almost daily. At that
time, the plant capacity was 7.5 mgd, but it was receiving an
average dry weather flow of 10.5 mgd, with afternoon peak flows
of 17 mgd. Secondary clarifier overflow rates during peak flow
periods were approximately 1,100 gpd/ft^. Consistent effluent
quality at this plant must be maintained because the reclaimed
wastewater is subsequently used for cooling water (after addi-
tional treatment) by a steam-electric power generating plant and
an oil refinery. The excess effluent is discharged to a dry
stream bed. No other significant changes were made in the plant
at that time, but some operational modifications were intro-
duced. Details of operating conditions and changes have been
reported in the literature. (29)
The River Road treatment facilities consist mainly of
primary clarifiers, plug flow activated sludge, and secondary
clarifiers. Chlorinated effluent is discharged to two 9 million
gallon effluent storage ponds before reuse or discharge. The
3 million gallon equalization lagoon receives primary effluent
that is pumped to storage during excess flow periods. Flow is
returned by gravity to the influent channel of the primary
clarifiers during deficit flow periods, with the desired rate
maintained by an operator preset, automatic flow control valve.
Distributions of influent and effluent BOD and TSS concen-
trations and loads observed during corresponding 3-month periods
(August-October) before (1964) and after (1966) instituting
150
-------
<
LU
O
z
O
O
-------
LI I 10 20304050607080 90 99 99.9 99.99
PERCENT OF VALUES GREATER THAN
Figure 54. Distribution of BOD$ concentrations, Pontiac, Michigan,
Auburn Plant, 1973 and 1976-1977
152
-------
"Figure 55
Equalization basin, 3 million gallon capacity,
Amarillo, Texas
equalization are shown in Figures 56 through 59. The concentra-
tion and loading distributions of both BOD and TSS show similar
patterns, attributed to the similarity of influent flow distri-
butions in the equalized and unequalized periods examined.
The distribution of effluent BOD concentrations was ob-
served to be lower and less variable during the equalized flow
period, with the difference in distribution means being approxi-
mately 8 mg/1 (or 20 percent), and approximately 20 percent
difference in the log standard deviations. Very slight dif-
ference was observed between distributions of equalized and un-
equalized TSS concentrations; equalized performance was approxi-
mately 5 percent lower but more variable. These observations
correspond generally to recollections of plant operating per-
sonnel. Although significant deterioration of effluent TSS
quality was reported almost daily, (29) plant records reflected
by Figure 56 indicate relatively small improvement. It is
possible that effluent sampling procedures permitted some bias
in favor of better effluent quality. Overall reported perfor-
mance and performance characteristics described herein indicate
a slight but positive influence of equalization on effluent
quality.
153
-------
1000
800
700
600
500
400
3OO
- ZOO
O
I-
LU
O
2
O
O
CO
CO
H-
100
80
70
60
50
40
30
20
10
6
5
4
INFLUENT (19
EQUALIZED
gFI"!(")Kir>APV
64)
1964!
11966)
JNEQ JALIZED
EFFLUENT
.01 O.I
10 20304050607080 90
99 99.9 99.99
PERCENT OF VALUES GREATER THAN
Figure 56. Distribution of monthly TSS concentrations, Amarillo,
Texas, 1964 and 1966
154
-------
1000
800
7QO
600
500
400
300
200
o>
E
O
h-
LU
O
•z.
O
O
Q
O
CD
100
80
70
60
50
40
30
20
10
8
6
5
4
(1966)
SEC ONI
JNJ OIL JJZE i
T9c»)
lARY
EFFLUENT
.01 O.I I 10 20304050607080 90 99
PERCENT OF VALUES GREATER THAN
99.9 99.99
Figure 57. Distribution of monthly BOD^ concentrations, Amarillo,
Texas, 1964-1966
155
-------
100000
80000
70000
60000
50000
40000
30000
20000
^ 10000
° 8000
7000
6000
5000
. 4000
3000
CO
CO
J-
2000
1000
800
700
600
500
400
300
200
100
NFLUI
:NT (1966)
LQ1IAI
3E(
W5T
1ZE1
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1964)
LUENT
EFFLUEN
.01 O.I I 10 20304050607080 90 99
PERCENT OF VALUES GREATER THAN
99.9 99.99
Figure 58,
Distribution of monthly TSS loads, Amarillo, Texas,
1964 and 1966
156
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100000
80000
70000
60000
50000
40000
30000
20000
10000
8000
=* 7000
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.01 O.I I 10 20304050607080 90 99
PERCENT OF VALUES GREATER THAN
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Figure 59. Distribution of monthly BOD5 loads, Amarillo, Texas,
1964 and 1966
157
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Figure 60.
Equalization Tank, 50 million gallon capacity,
(center, behind building, underground),
Warren, Michigan
Warren, Michigan—
The City of Warren, Michigan has a large activated sludge
treatment plant serving a major urban and industrial center in
southeastern Michigan adjacent to Detroit. The treatment plant
has a design treatment capacity of 36 mgd, hydraulic capacity of
80 mgd, and pumping capacity of 150 mgd. Flows as high as 50
mgd during storm periods can be treated successfully. Average
dry weather flow in 1977 was 32 mgd, with peak-to-average ratio
about 1.2:1. Nearly half of the waste flow is of industrial
origin.
Facilities consist of conventional headworks, two parallel
(essentially identical) trains of primary clarifiers, diffused
air activated sludge, and secondary clarifiers; all of which
comprise "east" and "west" plants, and effluent rapid sand
filters prior to chlorination and discharge. The plant also has
a 50 million gallon equalization/emergency storage tank (Figure
60) that is divided into 7 and 43 million gallon compartments,
and covered. The tank is currently used for temporary storage
of storm flows and influent waste when industrial spills are
detected. The tank can be filled by gravity directly from the
headworks or following primary sedimentation. Storm flows in
excess of 80 mgd are automatically diverted to the tank. The
plant is also equipped with continuous automatic cyanide moni-
toring for industrial spill detection. Flow during spills is
diverted, treated, and returned to the plant so as to avoid
process upsets. The plant is currently being provided with
158
-------
computerized control of pumping and valving that will permit use
of the retention tank to equalize daily variations in plant in-
fluent flow when completed.
Influent and effluent TSS and BOD concentration distribution
observed for the Warren plant in the calendar year 1976 are shown
in Figures 61 and 62. The plant can be seen to provide consis-
tent high quality performance. The somewhat high secondary
effluent TSS concentrations are not critical because of the sub-
sequent effluent filtration.
Renton, Washington—
The metropolitan area of Seattle, Washington is served in
part by a large activated sludge secondary treatment plant
located at Renton, Washington. This plant, from the headworks
through to the secondary clarifiers, has many similarities to
the Warren, Michigan plant. The Renton plant has a design
treatment capacity of 36 mgd, hydraulic capacity of 96 mgd, and
pumping capacity of 194 mgd. The plant is designed for treating
peak dry weather flows of 72 mgd; flows of this magnitude
averaged over 24 hours have been treated successfully in recent
months. Typical peak-to-average flow ratios during dry weather
are about 1.3 to 1.4:1; average daily flows of 50-60 mgd-have
been experienced for several days at a time during wet weather.
Slightly less than half of the waste loading is of industrial
origin.
Facilities at Renton consist of conventional headworks
followed by parallel arrangement of primary clarifiers, aeration
tanks and secondary clarifiers, similar to facilities at Warren.
The activated sludge system is designed to permit operation
ranging from plug flow to contact stabilization with a wide
range of step aeration, feeding, and solids return options in
between. The plant is not equipped with equalization facilities.
The influent interceptor, however, is designed for an ultimate
hydraulic capacity of 375 mgd, and therefore has excess volume
that can be backed up behind the influent pump station during
peak wet weather flows. Available volume at current wet weather
flow is between 4 and 6 million gallons. This volume is used
only during excessive flow conditions because of the problems
involved with solids deposition during storage. Very heavy
solids discharge occurs as storage volume is reduced to normal
flow levels, creating excessive solids loading and oxygen demand
conditions.
Influent and effluent TSS and BOD concentration distribu-
tions observed for the Renton plant for the calendar year 1976
are presented in Figures 61 and 62, along with data for the
Warren plant. The Renton plant provides consistent high quality
performance. Comparison of Renton and Warren performance shows
that the degree of secondary effluent variability is similar for
159
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Figure 61. Distribution of TSS concentrations, Warren, Michigan/
Renton, Washington
160
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Figure 62. Distribution of BOD5 concentrations, Warren, Michigan/
Renton, Washington
161
-------
the two plants. Average effluent TSS at Renton is significantly
lower than at Warren; the reverse is true for effluent BOD.
Several factors may contribute to these observed differen-
ces. Operation of the Renton plant is focused specifically on
maintaining low effluent TSS. This objective is used to estab-
lish desirable operating conditions in the activated sludge
system, and adjustments are continuously made in order to main-
tain optimum settling conditions in the secondary clarifiers.
The Warren plant, on the other hand, has effluent filters to
maintain desired effluent TSS, allowing aeration conditions to
be optimized for BOD removal. In addition, the Warren plant
uses alum addition to achieve required phosphorus removal. The
use of alum undoubtedly contributes to the observed differences
in secondary effluent quality.
Newark, New York—
The sewage treatment plant at Newark, New York was used for
a full-scale study of simulated flow equalization sponsored by
the EPA from March to July, 1971. (3.0) Newark has a conven-
tional activated sludge plant with an average flow (1971) of
approximately 1.8 mgd, with a peak-to-average ratio of about
1.4 under typical dry weather conditions.
Treatment facilities consist of conventional headworks
followed by parallel sets of primary clarifiers, aeration tanks,
and secondary clarifiers. The plant is not equipped with
equalization facilities. For the purposes of the study all
plant flow was processed through half of the parallel treatment
units, effectively doubling all process loadings. This was done
in an effort to generate operating conditions that would result
in effluent performance varying with flow rate. Under the high
loading study conditions the primary clarifier overflow rate was
approximately 1,000 gpd/ft^ at average flow of 1.8 mgd, or
1,700 gpd/ft^ at peak flows of 3.0 mgd during unequalized flow
conditions. The plant was operated for two consecutive periods:
the first with flows unequalized to establish plant performance,
with diurnal variation at the elevated flow rates; the second
with flows equalized, using the second aeration tank to estab-
lish plant performance under essentially constant flow condi-
tions.
Data generated in the study are somewhat controversial, and
difficult to interpret clearly. Bulking problems were experi-
enced during the equalized Phase 2 period, so that comparing
equalized and unequalized secondary performance is not possible.
Evaluation of equalization effects on primary sedimentation
using the study data is complicated by several factors: signi-
ficant increases in all waste constituents, including BOD, COD,
TSS, VSS, etc., were observed across the equalization tank;
samples taken were not composited proportional to flow; influent
162
-------
flows during the unequalized period were 15 to 20 percent higher
than in the equalized period. Due to the limitations of the
data, the original investigators relied primarily on the two
2-day equalized and unequalized flow sampling,periods, during
which bi-hourly sampling was conducted. Conclusions based on
these data are that primary sedimentation TSS efficiency was - -
59 percent during equalized flow, as opposed to-23 percent, .
during unequalized flow. Also the coefficient of the variation
of diurnal concentrations was approximately 50 percent less
during equalized flow.
Primary sedimentation performance data from the study have,
been reviewed and interpreted independently. Probability plots.
of the daily average TSS and BOD-data, adjusted by;appropriate
factors to approximate flow weighted composite concentrations, •
are presented in Figure 63. Distributions of TSS and BOD re-
moval percentages calculated from the same adjusted data are •-.:.
presented in Figure 64. Distributions of influent and; primary
effluent BOD concentrations (Figure 63) show that little dif-
ference was observed between equalized and unequalized periods.
This corresponds to original study conclusions. ...- , • :
Distributions of plant influent and primary effluent.TSS ,
concentrations (Figure 63) show that primary effluent concentra-
tions during unequalized flow were consistently below those
during the equalized period. However, primary sedimentation ,
performance during the equalized flow period should be deter^
mined with respect to primary clarifier influent concentrations
(equalization tank effluent concentrations). As shown, TSS
levels in the equalization tank were substantially higher than
in the plant influent, which resulted in the observed higher re-
movals across the primary tank during equalized flow. , The dis-
tributions of equalized and unequalized flow TSS removal percen-
tages (Figure 64) show mean TSS removal to be approximately 40
percent for the 6-week equalized flow period, and approximately
35 percent for the corresponding unequalized flow period.
Differences in day-to-day primary effluent variability-
observed between equalized and unequalized periods can be seen
in the TSS distributions of Figure 63. The broader range of
daily average primary effluent TSS concentrations observed
during the unequalized period appears to result largely from
the broader range of influent concentrations.
The conclusion of this analysis is that a modest improve-
ment in primary sedimentation TSS removal was observed during
equalized flow conditions. However, differences in influent
conditions between the two periods, and the undefined increases
in major waste constituents across the equalization tank prevent
development of general conclusions with respect to equalization,
effects on primary clarifier performance.
163
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Figure 63(a)
Primary effluent BOD concentration distribution,
Newark, NY
164
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Figure 63(b). Primary effluent TSS concentration distribution,
Newark, NY
165
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Figure 64. Primary sedimentation removal efficiency
distribution, Newark, NY
166
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Trickling Filters
Performance data for a recent year of operation from three
trickling filter secondary treatment plants are analyzed in this
section. The plant at Palmyra, New Jersey is the only trickling
filter plant located during the equalization survey that had
equalization added as the only plan modification at the time of
construction. The plant at Midland, Michigan is the only addi-
tional trickling filter plant with equalization located during
the survey for which reasonably complete operating records were
obtained. Performance data from the unequalized trickling filter
plant at nearby Bay City, Michigan are used to provide comparison
to Midland data.
The performance data in this section are presented as
available information of this type, but not necessarily repre-
sentative of performance at typical trickling filter plants.
Palmyra, New Jersey—
The Borough of Palmyra, New Jersey is served by an approxi-
mately 30-year old standard rate trickling filter secondary
treatment plant with .design capacity of 530,000 gallons per day.
The facilities consist of an influent pump station followed by
an in-line aerated equalization tank, primary clarifiers, stan-
dard rate trickling filter, secondary clarifiers, and chlorina-
tion pjrior to discharge.
The equalization tank, with a capacity of 130,000 gallons,
was added to the plant in 1975 to accommodate an anticipated
flow increase of .200,000 to 250,000 gpd from new tributary resi-
dential developments. Current (1977) flows during typical dry
weather periods average approximately 0.4 mgd, with peak flows
of 0.6 to 0.7 mgd.
Only secondary effluent data are available for analysis.
Unequalized data from the calendar year 1974 and equalized data
for equalized operation from the year 1976 are summarized in
Figures 65 and 66.
*
The annual distributions of-secondary effluent TSS (Figure
65) show that effluent in the equalized year is substantially
better than in the unequalized year. The equalized mean efflu-
ent TSS (50.6 mg/1) is 35 percent lower than the unequalized
mean effluent (78.5 mg/1). Equalized effluent TSS is also sig-
nificantly less variable. In the equalized year effluent TSS
is between 26 and 70 mg/1 for 80 percent of the time, compared
to a range of 24 and 160 mg/1 for 80 percent of the unequalized
period.
The annual distributions of secondary effluent BOD (Figure
66) show that equalized BOD performance is better than in the
167
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1976)
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Figure 65. Distribution of TSS concentrations, Palmyra, New Jersey
168
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500
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Figure 66. Distribution of BOD5 concentrations, Palmyra, New Jersey
169
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unequalized year. The equalized mean effluent BOD (76 mg/1) is
18 percent lower than the unequalized mean effluent (93 mg/1).
Secondary effluent variability is approximately the same for
both periods; with 80 percent of effluent BOD's between 35 mg/1
and 130 mg/1 in the equalized year, and between 40 and 150 mg/1
in the unequalized year.
Equalization appears to have significantly improved treat-
ment performance of the Palmyra facilities.
Midland/ Michigan—
Treatment facilities at Midland consist of two-stage, high
rate, plastic media trickling filters; primary and intermediate
clarification; chemical flocculation before secondary clarifi-
cation; and effluent filtration. The plant has a treatment
capacity of about 6.5 mgd, with hydraulic capacity about 13 mgd.
It is equipped with a 3.25 million gallon equalization tank.
Daily average dry weather flows are approximately 6 mgd, with
peak-to-average flow ratio ranging from 1.5 to 1.65. Average
daily flows during wet weather are typically 8 to 8.5 mgd, with
peak wet weather flows occurring in the range of 10 to 12.5 mgd.
The equalization tank is used to smooth daily diurnal flow
variations, and to supplement treatment capacity during wet
weather conditions. A summary of operating conditions has been
recently reported in the literature.(31)
Equalization was added to the existing secondary treatment
plant in 1972. Significant additional plant modifications were
made concurrently, including chemical addition for phosphorus
removal.
Bay City, Michigan—
Treatment facilities at Bay City are similar to those at
Midland, but they have ,no equalization. Treatment processes
include primary and secondary clarification, and single-stage,
standard-rate, plastic-media trickling filters. Ferric chloride
is added to second stage filter effluent for phosphorus removal.
Average daily flows are about 12 mgd, with a typical peak-to-
average flow ratio of approximately 1.3.
Distributions of influent and effluent BOD and TSS concen-
trations for the equalized 6.5 mgd trickling filter plant at
Midland, and the unequalized 12 mgd trickling filter plant at
Bay City (Figures 67 and 68), are presented primarily as back-
ground information. The distributions provide an example of
secondary effluent quality observed over one-year periods at the
respective plants. The similarity of the influent and effluent
distributions may be attributed at least in part to the simi-
larity of physical characteristics of the two plants, and to the
similarity of environmental factors. Their geographical proxi-
mity contributes to waste variations and influences process per-
formance .
170
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Figure 67. Distribution of daily TSS concentrations, Midland,
Michigan/Bay City, Michigan
171
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Michigan/Bay City, Michigan
172
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Oxidation Ditch Plants
Oxidation ditch activated sludge treatment plants have re-
ceived .widespread application recently. • Typically, applications
in predominantly rural areas are in plants of no greater capac-
ity than 5 mgd. A recent study (32) has surveyed oxidation use,
performance and costs, providing detailed reference information.
Another EPA sponsored study at Dawson, Minnesota (33) provides
information on the operation of an oxidation ditch treatment
plant with facilities for flow equalization. Performance data
from this study are compared to data from an unequalized plant
of similar size at Arlington, Washington.
Dawson, Minnesota—
Dawson, Minnesota is served by a 260,000 gallon per day
secondary treatment plant, with an oxidation ditch and activated
sludge facilities. The plant is designed to meet effluent re-
quirements of 5 mg/1 TSS and BOD, and 0.1 mg/1 ammonia nitrogen.
Following screening, raw wastewater is pumped directly to
the "oxidation ditch" aeration channel. The aeration channel is
designed to provide 83,000 gallons of equalized storage by vary-
ing the mixed liquor depth between a low level of 3 feet and a
high level of 4 feet. At design flows the mixed liquor depths
will correspond to aeration times of 17.7 and 25.4 hours. Use
of storage volume during initial operation is reported to have
reduced peak flows by approximately 31 percent. (34) Flows
averaged approximately 160,000 gallons per day. Following the
oxidation ditch, the Dawson plant has two secondary clarifiers
in series. Design overflow rates are 580 and 400 gpd/ft , re-
spectively. The plant has facilities for chemical addition and
flocculation between two clarifiers to insure desired solids
removal. Chemical addition has not been necessary during the
initial operation.
Performance data for the Dawson plant developed in an EPA
sponsored study (33) are summarized in Figures 69 and 70. Dis-
tributions of influent and final secondary clarifier effluent
TSS (Figure 60) indicate that the average performance is good,
but that a relatively high degree of effluent variability is
observed. Distribution of influent and secondary clarifier
effluent BOD (Figure 70) indicates excellent BOD performance,
and stable overall performance.
Arlington, Washington—
Arlington, Washington is served by an unequalized oxidation
ditch, activated sludge, secondary treatment plant. Design
capacity is 1.5 mgd; current flows (1977) average approximately
400,000 gpd. The Arlington facilities consist of conventional
headworks followed by the oxidation ditch aeration channel and
secondary clarifier.
173
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Figure 69. Distribution of monthly TSS concentrations, Arlington,
Washington/Dawson, Minnesota
174
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Figure 70. Distribution of monthly BOD5 concentrations, Arlington,
Washington/Dawson, Minnesota
175
-------
Annual distributions of secondary effluent TSS and BOD con-
centrations for 1976 are summarized in Figures 60 and 70, along
with corresponding distributions for Dawson, Minnesota. The dis-
tribution of effluent TSS was similar to that observed for
Dawson, with an annual mean of 19.1 mg/1 and variability slightly
higher than at Dawson. The observed mean effluent BOD (Figure
70) is approximately 9 mg/1, again with the level of variability
typical of other activated sludge plants although somewhat
higher than the high quality Dawson effluent.
General Performance Observations^
Inspection of influent and effluent distributions (Figures
21 through 70) reveals some general performance patterns.
Primary Sedimentation—
At the primary treatment level, mean BOD and TSS concentra-
tions and overall removals were better under equalized than un-
equalized conditions. As with secondary effluent, the pattern
of change in effluent variability was not consistent. Slightly
less variability was observed in equalized primary effluent con-
centrations in 1973; but in 1976 slightly more variability
occurred in the equalized primary effluent.
Secondary Processes—
Performance of the five full-scale activated sludge plants
was better almost across-the-board under equalized conditions
(Table 19). Greater differences were observed in BOD than in
TSS performance under equalized and unequalized conditions.
Also, differences in equalized and unequalized performances
tended to be greater for the smaller plants. Performance varia-
bility (Table 20), on the other hand, showed no consistent
pattern. A slight overall tendency was indicated toward less
variability of effluent BOD under equalized conditions. And a
similarly slight overall tendency toward greater variability of
effluent TSS was apparent under equalized conditions. The rela-
tively slight and inconsistent correspondence between influent
variability under equalized conditions and variability of sus-
pended solids concentrations in secondary clarifier effluent is
not surprising. But as described in detail in Appendix B, the
influence of the biological process directly preceding the
secondary clarifier is far greater than the influence of plant
influent. Although typical plant influent TSS concentrations
may range from 200 to 300 mg/1, secondary clarifier influent TSS
concentrations range from 1,500 to 4,000 mg/1. In addition,
settling characteristics are strongly influenced by conditions
in the biological process preceding the clarifier.
Effluent Filtration—
Mean effluent BOD and TSS concentrations of filtered secon-
dary effluent at Walled Lake-Novi were significantly better
176
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TABLE 19. EFFECTS OF EQUALIZATION ON PERFORMANCE
OF ACTIVATED SLUDGE PLANTS
Plant name
and
study period
Tecumseh, 70/73
BOD
TSS
Tecumseh, 70/76
BOD
TSS
Walled Lake-Novi, 74
BOD
TSS
Walled Lake-No'vi ,
74/76-77
BOD
TSS
Ypsilanti Township,
74/75
BOD
TSS
Ypsilanti Township,
76
BOD
TSS
Amarillo
BOD
TSS
Pontiac, 73/76-77
BOD
TSS
Average change, BOD
Average change, TSS
Secondary effluent,
mean concentration (mg/1)
Unequalized
28.4
12.8
28.4
12.8
13.7
21.5
14.2
9.5
24.7
17.0
22.8
17.0
34.4
17.9
12. 3:
13.6
—
—
Equalized
7.7
9.1
11.9
14.4
6.8
9.5
11.1
13.6
17.6
18.0
15.0
15.1
26.7
17,8
12,0
10.1
—
—
% Change
-73
-29
-58
+ 13
-50
-56
-22
+ 43
-29
+ 6
-34
-11
: -22
—
—
-26
-36
-8
177
-------
TABLE 20. EFFECTS OF EQUALIZATION ON VARIABILITY
OF ACTIVATED SLUDGE PLANTS
Plant name
and
study period
Tecumseh, 70/73
BOD
TSS
Tecumseh, 70/76
BOD
TSS
Walled Lake-Novi, 74
BOD
TSS
Walled Lake-Novi,
74/76-77
BOD
TSS
Ypsilanti Township,
74-75
BOD
TSS
Ypsilanti Township,
76
BOD
TSS
Amarillo
BOD
TSS
Pontiac, 73/76-77
BOD
TSS
Average change, BOD
Average change, TSS
Secondary effluent,
Iog10 standard deviation
Unequalized
0.43
0.36
.43
.36
.10
.18
.83
.97
.17
.33
.18
.28
.26
.17
.16
.25
—
Equalized
0.30
.31
.56
.32
.06
.28
.33
.25
.20
.24
.22
.28
.20
.21
.18
.25
--
% Change
-30
-14
+ 30
-11
-40
+56
-60
-74
+ 18
-27
+ 22
-23
+ 24
+ 13
-9
-6
178
-------
under equalized conditions. The variability of filter effluent
BOD concentrations was substantially greater under equalized than
unequalized conditions. In fact, during both equalized and un-
equalized periods filter effluent variability was greater than
the clarifier ef.fluent feed. This result may perhaps be due to
biological activity in the filters. The variability of filter
effluent TSS concentrations was less under equalized conditions.
Secondary effluent and final filter effluent distributions
for equalized flow plants are summarized in Figures 71 and 72.
As the peak-to-average flow ratio was increased, the mean
effluent BOD and TSS concentrations of the EPA MERL pilot study
of activated sludge systems under constant flow conditions was
observed to improve compared to the variable flow systems. No
consistent pattern of influence on effluent variability was
observed.
EQUALIZED VERSUS UNEQUALIZED PLANT PERFORMANCE
Average annual performances of equalized and unequalized
sewage treatment plants are compared in this section. Only a
small portion of equalized plants could be analyzed directly,
assessing the influence of equalization on individual plant per-
formance. However, comparison of average annual performance of
equalized and unequalized plants, at comparable levels of treat-
ment, provides an alternative means of assessing the effective-
ness of equalization. Performance data for 43 flow equalized
facilities are compared to data for unequalized periods at 16
equalized flow plants and 35 plants with no equalization.
Activated Sludge- Plants
Average annual influent and effluent BOD and TSS concen-
trations for 31 equalized and 46 unequalized activated sludge
plants are summarized in Table 21 and Figures 73 and 74. Dif-
ferences in mean annual effluent concentrations between the
equalized and unequalized plants in the sample are insignificant.
At the primary treatment level, the range of BOD and TSS perfor-
mance for unequalized plants is actually somewhat better than
for plants with equalization prior to primary treatment. At the
secondary treatment level the case is just the opposite. In
both cases it must be recognized that the broad range of design,
operating, and environmental factors contributing to performance
characteristics at each treatment plant may well have more sig-
nificance than would flow equalization. Accordingly, generali-
zations concerning the effects of equalization on plant perfor-
mance using the data in this section should be made with caution.
179
-------
-------
100
80
70
60
507
40
30
20
10
9
8
7
UJ
o
o
m
o
o
m
.9
.8
.7
.6
.5
4
_C
I
j—c
4^L
D
A
A
Equal. Sec. Eff.
Equal. Filter Eff.
Equal. Sec. Eff.
Equal. Filter Eff.
Equal. Sec. Eff.
Equal. Filter Eff.
Equal. Sec. Eff.
Equal. Filter Eff.
Warren (East)
Warren
Walled Lake-Novi
Walled Lake-Novi-
Pontiac Aurburn •
Pontiac
E. Lansing
E. Lansing
.Ol .05 . 1 .2 .5 I
2 5 10 2O 3040506070 80 90 95 9899
PERCENT OF VALUES GREATER THAN
99B
99.99
Figure 72. Secondary and filter effluent BOD concentration
distributions, four flow equalized plants
181
-------
TABLE 21. ACTIVATED SLUDGE PLANT PERFORMANCE
Plant
Equalized
Rossmoor ,
CA
Valley Community Ser-
vices District, CA
Bloomsf ield ,
CO
Freeport,
IL
East Lansing, MI
(north side)
East Lansing, MI
(south side)
Grand Rapids,
MI
Jackson,
MI
Lansing,
MI
Pontiac, MI
(Auburn Plant)
Pontiac, MI
(E. Blvd. Plant)
Port Huron,
MI
Tecumseh, MI
(1973)
Tecumseh, MI
(1976)
Trenton,
MI
Walled Lake/ttovi,
MI (1974)
Walled Lake/Novi,
MI (1976)
Warren,
MI
Ypsilanti Township,
MI (1974-1975)
Ypsilanti Township,
MI (1976)
Clementon,
NJ
Marlton, NJ (Elmwood
STP #2)
Influent
Freq
of
Data3
M
D
D
W
D
D
D
D
D
D
D
D
D
D
D
D
D
D
M
D
BOD 5
Cone
mg/1
213
298
202
142
142
114
91
162
80
82
53
303
273
318
263
165
113
200
189
# of
Data
Points
9
419
100
365
365
335
364
366
12
12
338
207 .
241
366
8
328
364
346
363
; TSS
Cone
mg/1
227
297
175
117
117
118
149
311
122
96
85
562
479
417
258
185
129
170
208
# of
Data
Points
9
422
100
365
365
355
360
366
12
12
366
187
241
366
8
303
366
357
362
Primary Effluent
BOD5
Cone
mg/1
163
120
73
70
90
97
125
49
48
73
129
135
279,
100
%
Rem
45
49
51
21
—
23
—
57
51
12
'
12
# of
Data
Points
420
8
324
333
342
364
366
12
12
322
176
231
366
364
TSS
Cone
mg/1
93
136
93
94
63
147
187
55
96
119
138
118
246
109
%
Rem
69
21
20
47
1
40
—
75
75
41
16
It of
Data
Points
422
8
324
333
357
362
366
12
12
365
192
231
366
363
(continued)
182
-------
TABLE 21 (continued)
Plant
Equalized
Ramblewood,
NJ
Woodstream,
NJ
Dept. of Environmental
Conservation, NY
Fishkill,
NY
Newark ,
NY
Wappinger,
NY
. Hatfield Township,
PA
Amarillo,
TX
Austin,
TX
Odessa,
TX
Influent
Freq
of
a
Data
D
W
M
M
M
M
M
BOD 5
Cone
mg/1
151
212
150
101
263
# of
Data
Points
12
23
40
21
15
TSS
Cone
mg/1
176
142
162
169
293
'# of
Data
Points
12
23
40
19
15
Primary Effluent
BOD5
Cone
mg/1
176
92
87
%
Rem
17
14
# of
Data
Points
24
40
20
TSS
Cone
mg/1
97
55
88
%
Rem
32
48
# of
Data
Points
23
40
17
M = monthly
D = Daily
W = weekly
(continued)
183
-------
TABLE 21 (continued)
Plant
Equalized
Rossraoor ,
CA
Valley Community Ser-
vices District, CA
Bloomsfield,
CO
Freeport,
IL
Cast Lansing, MI
(north side)
East Lansing, HI.
(south side)
Grand Rapids,
MI
Jackson ,
MI
Lansing,
MI
Pontiac, MI
(Auburn Plant)
Pontiac, HI
(E. Blvd. Plant)
Port Huron,
MI
Tecurasch , MI
(1973)
Tecumseh, Ml
(1976)
Trenton,
MI
Mailed L*ke/Novi,
MI (1974)
Walled Lake/Novi,
MI (1976)
Warren,
MI
Ypsilanti Township,
MI (1974-1975)
Ypsilanti Township,
MI (1976)
Clemen ton.
Ha
Marlton, NJ (Elmwood
STP 12)
Secondary Effluent
BOD5
Cone
mg/1
13
10
38
20
3
5
19
4
15
6
11
10
8
12
18
10
12
7
17
20
10
%
Rem
94
97
81
83
98
96
83
96
91
81
97
96
94
96
93
94
92
ft of
Data
Points
9
76
101
8
324
364
348
366
366
12
12
328
212
236
366 ,
8
358
361
335
12
64
TSS
Cone
mg/1
11
11
32
26
7
11
22
9
28
9
10
11
9
14
33
18
12
16
19
19
23
%
Rem
95
96
82
81
94
91
81
94
91
87
98
97
92
9
94
88
89
ft of
Data
Points
9
140
100
8
324
364
362
366
366
12
12
364
189
241
366
8
359
361
371
12
126
Filter Effluent
BOD 5
Cone
mg/1
3.2
2
2
4.4
4.3
4
1.6
14
%
Rem
99
99
99
98
98
99
93
# of
Data
Points
400
331
331
12
8
327
356
359
TSS
Cone
mg/1
2.2
4
4
1.4
4.2
3
1.4
14
%
Rem
99
91
91
98
98
99
93
# of
Data
Points
422
(
333
333
12
8
326
357
358
(continued)
184
-------
TABLE 21 (continued)
Plant- ••; •
Equalized
Ramblewood ,
NJ
Woods tr earn.
NJ
Dept. of Environmental
Conservation, NY
Fishkill,
NY
Newark ,
NY
Wappinger ,
NY
Hatfield Township,
PA
Amarillo ,
TX
Austin,
TX
Odessa,
Tx
Secondary Effluent . Filter Effluent
BOD5
Cone
mg/1
22
11
44
12
24
26
11
13
%
Rem
4
76
90
# of
Data
Points
87
41
40
20
15
12
12
TSS
Cone
mg/1
80
42
58
8
56
17
13
32
%
Rem
4
67
94
# of
Data
Points
97
66
40
20 ,
15
12
12
BOD5
Cone
mg/1
1.6
12
8.0
%
Rem
•92
# of
Data
Points
12
40
20
TSS
Cone
mg/1
1.4
8
8.7
%
Rem
95
# of
Data
Points
12
40
21
(continued)
185
-------
TABLE 21 (continued)
Plant
Unoquallzed
Lodi,
CA
District 26,
L.A., CA
District 32, L.A.,
CA (1973)
District 32, L.A.,
CA (1976)
Hyperion, L.A.,
CA
Long Beach, L.A.,
CA (1973)
Long Beach, L.A.J
CA (1976)
Los Coyotes, L.A.,
CA (1973)
Los Coyotes, L.A.,
CA (1976)
Pomona, L.A.,
CA
San Jose Creek, L.A.,
CA (1973)
San Jose Creek, L.A.,
CA (1976)
Valley Selling Basin,
L.A., CA
Whittier Narrows,
L.A., CA
Palo Alto,
CA
Rossmoor,
CA
Sacramento,
CA
San Jose-Santa Clara,
CA
Calumet, Chicago,
IL
Hanover Park,
Chicago, IL
Hazelcrest,
Chicago, IL
Secondary Effluent
BOD5
Cone
mg/1
15
7
3
8
5
8
9
8
11
9
4
10
18
48
10
26
19
10
16
%
Rem
# of
Data
Points
365
365
365
365
365
365
365
365
365
365
365
365
365
24
365
365
365
365
365
TSS
Cone
mg/1
21
12
8
2
6
9
5
12
14
7
8
2
23
12
24
53
15
33
18
10
23
%
Rem
# of
Data
Points
365
365
365
365
365
365
365
365
365
365
365
365
365
365
365
26
365
365
365
365
365
Filter Effluent
BOD 5
Cone
mg/1
%
Rem
# of
Data
Points
TSS
Cone
rag/1
%
Rem
# of
Data
Points
(continued)
186
-------
TABLE 21 (continued)
Plant
Unequalized
' Norths ide, Chicago,
IL
Indianapolis #1,
IN
Indianapolis #2,
IN
Pontiac, MI
(Auburn Plant)
Pontiac, MI
(E. Blvd. Plant)
Port Huron,
MI
Tecumseh ,
MI
Walled Lake/Novi,
MI
Ypsilanti Township,
MI (1974)
Ypsilanti Township,
MI (1976)
Lincoln,
NB
Newark, ,
NJ
Philadelphia,
PA
Amarillo,
TX
Leon Creek, San
Antonio, TX
Rolling Road, San
Antonio, TX'
Salado Creek, San
Antonio , TX
Pullman, WA
(1972)
Pullman, WA
(1974)
Pullman, WA
(1975)
Renton, WA
(1974)
Renton r- WA
(1976)
East Milwaukee,
WI
West Milwaukee,
WI
Winnipeg-Manitoba,
Canada
Secondary Effluent
BOD5
Cone
mg/1
13
28
17
9
12
10
28
14
24
25
66
63
32
9
11
6-
28
45
25
5
11
16
14
31
%
Rem
89
86
91
94
85
# Of
Data
Points
365
365
365
24
20
365
62
7
12
365
365
11
365
365
365
36
28
32
365
12
365
365
365
TSS
Cone
mg/1
16
25
12
20
14
~ 11
13
9.5
19
17
81
77
17
17
11
12
56
52
4
7
8
15
21
26
%'
Rem
85
87
97
96
94
# Of
Data
Points
365
365
365
24
20
365
60
7
12
365
365
11
365
365 '
365
36
32
32
365
12
365
365
365
Filter Effluent
BOD 5
Cone
mg/1
7.0
%
Rem
97
# of
Data
Points
7
TSS
Cone
mg/1
6.9
%
Rem
97
# of
Data
Points
7
187
-------
300
200
100
90
80
70
60
^ SO
6 40
z
2 30
tu
O
O
O
CO
CO
20
10
4
\
\
X
FILT
EQUALIZE!
ER EF
\
:LUE NT
PF IMARY
EFFLUENT
\
'EQLALI
-EFf
X
IUNEC
\
ttn
UAJ.IZED iECbNDARY
EFFLUENT
ZED SEJCOiNDARY
Wr-
.01 J05 .1 2 .5 I 2 5 10 20 30 40 50 60 70 8O 9O 95 98 99 99.8 99.99
PERCENT OF VALUES GREATER THAN
Figure 73. Distributions of average annual effluent TSS
concentrations, 31 equalized versus 46 unequalized
activated sludge plants
188
-------
200
100
u>
E
O
cc
LU
O
O
cF
O
CD
.01 -O5.I .2 ,512 5 10 20 304050607080 90 95 9899 998
PERCENT OF VALUES GREATER THAN
9999
Figure 74. Distributions, of average annual effluent BOD,
concentrations, 31 equalized versus 46 unequalized
activated sludge plants /
189
-------
Trickling Filter Plants
Average annual influent and effluent BOD and TSS concentra-
tions for 8 trickling filter plants with equalization, and 65
unequalized trickling filter plants, are summarized in Table 22
and Figures 75 and 76. Data on unequalized plant performance
was obtained from an EPA MERL report, "Upgrading Trickling Fil-
ters", 430/9-78-004, June 1978.
Comparison of secondary effluent distributions for the two
groups shows that the range of effluent BOD and TSS concentra-
tions is about the same. The differences between mean values
and slopes of the distribution appear to be a result of having
such a small sample of equalized plants.
EFFECTS OF EQUALIZATION ON UNIT PROCESS AND TREATMENT
SYSTEM PERFORMANCE
The theoretical benefits of flow equalization vary depend-
ing upon whether the plant is proposed (new) or existing. Flow
equalization at a new plant is advantageous in that unit pro-
cesses can be designed for constant, average rates of flow
and damped variations in input organics. Cost/benefit ratios
for such a situation are analyzed in Section 5.
For existing plants where treatment performance is to be
improved or treatment capacity increased, analyzing flow equali-
zation is more complicated. Factors such as the cost and
efficiency of correcting "bottle-necks" (i.e., insufficient
aeration capacity, limitation in hydraulics, etc.), design de-^
ficiencies (i.e., poor clarifier hydraulics, poor sludge settle-
ability due to excessive floe shear in aeration, etc.), and
site conditions (land limitation) all play a role in the
selection of the preferred alternative remedy for a given prob-
lem.
Table 23 summarizes a number of performance problems, and
alternative solutions for each situation. The purpose of Table
23 is to illustrate that flow equalization is one possible
remedy for a number of treatment plant performance problems.
This is not meant to be an exhaustive index of alternatives,
but an indication of other potential solutions that must be
considered. Most o'ften routine monitoring data required for
monthly NPDES compliance reports will not be sufficient to fully
define the nature of a given problem. For example, if the
effluent from an activated sludge plant is high in TSS (Table
23, Symptom II), it is first necessary to identify why the re-
moval of suspended material is not adequate. Is the system
hydraulically overloaded at peak flows? Are the sludge particles
heavy and well flocculated, or are they light and diffuse? Is
there some other difficulty? Details of this type must be pro-
vided before benefits of equalization can be assessed.
190
-------
CN
04
w
i-q
PQ
IS
Secondary Effluent
Primary Effluent
Influent
CQ
S3
in
Q
s
CO
W
EH
m
Q
en
£
in
8
«
w
4J
tH nl C
O 4-> -H
Id O
=8= Q ft
«
O i-l
c \
O tn
U g
ra
1 4->
i*H rO C
O -P -iH
Id O
* Q ft
(1)
* (U
O rH
0 Oi
U g
W
4J
-H
Id O
* Q ft
CD
<*> IA
U ^H
C \
8 g1
ra
jj
in is C
O -P -H
flj O
* Q ft
(1)
o\° («!
O rH
C \
O cn
o e
M
+J
Id
H
ft
OOOlCTt ^CM^DtNCN
m^^3rHrH
og n i— t m ro
,o^ r-
CD CO
OLOrH CNCNIOO^f
iHrHCN m,Hl/lrO^ ^
CM CO
CO «3
[^ CO
rH
a s a Q ' Q s s
-'O - -H
N'H^I'OCSOrd-H'a H
•H > o! c oSft!4 n ra c ns
M*!!l)!d4-)iHEHftl>1i
4-> (i
•H !H
°H I1*,
^S^S
ffl ft
>1 rH 43
rH A| 4-1
•rH
-------
800
700
600
5OO
400
300
|» 200
100
90
80
70
60
50
O
o
40
30
20
10
X
EQUA
V
-IZEI) EFFLU
s
EN]
EQUALIZED
INFLUENT
X
\
FLOKV
UNECUALI;:ED
^
\
EFf
LUENT
.01 .1 .2 .5 I 2 5 10 20 30 40 50 60 70 80 90 95 98 99 99-6 9999
PERCENT OF VALUES GREATER THAN
Figure 75. Distributions of average annual TSS concentrations,
8 equalized versus 65 unequalized trickling filter
plants
192
-------
O
1
UJ
o
o
o
a10
o
00
sou
800
7 OO
600
5OO
400
.300
200
100
90
80
70
60
50
40
30
2O
10
9
8
7
6
5
.0
x
•x
vj
i
^.^
^
I;.
r* 1
>
PN
-
i
-
X
X,
X
i
Xs
\
X
-^
/
X^
\
\
*l \
\
\
X
>
tfM
X
W J
/
\
^
1
!
UNE
^
EOL
\
>
ou;
AL
V
\
a
LI;
X
ZE
^>4
\
e
ED
D f
^
<
\
ro
FL(
^v
FFt
^-
%
\e
\
\
iw .IN
^
UENT
lUNEC
^iSL
>
t
0
•LV
^
UAL
X,
a
A
&
•NT
IZEO
\
&
&
^•^
EF
X
&
\v '
^v»
rLUEN
X,^
^
.- . - '
^
• •'
1 .1 .2 .5 1 2 5 10 20 3040506070 80 90 95 98 99 99.8 99.99
PERCENT OF VALUES GREATER THAN
Figure 76.
Distributions of average annual BOD concentra-
tions, 8 equalized versus 65 unequalized trickling
filter plants
193
-------
TABLE 23. SUMMARY OF ALTERNATIVES IN IMPROVING
EXISTING TREATMENT PLANT PERFORMANCE
Symptom
Possible Cause(s)
Possible Remedies
PRIMARY CLARIFIER
I. Carryover of settle-
able solids at peak
flows.
ACTIVATED SLUDGE SYSTEM
II. High secondary
effluent TSS
1. Hydraulic overload
of clarifier.
2. Clarifier design
deficiency -
hydraulic
1. Hydraulic overload
of clarifier @
peak flows.
2. Poor sludge settle-
ability; sludge is
"light" or diffuse.
3. Sludge blanket
rises up and
overflows at peak
conditions.
(continued)
a. Flow equalization ahead of
primary clarifier.
b. Add additional primary
clarifiers.
c. Add chemicals, particularly
at peak flows.
d. Design downstream processes
to accommodate solids carry-
over.
a. Modify inlet hydraulics
(e.g. extend or shorten
skirt in center-feed cir-
cular clarifier; provide
inlet baffles in rectangu-
lar clarifier).
b. Modify outlet hydraulics
(e.g. add weir length;
move weirs).
c. Add additional clarifiers.
d. Distribute flow equally to
multiple clarifiers.
e. Provide wind .screen to re-
duce effect of wind action.
a. Flow equalization to con-
stant flow.
b. Add additional clarifiers.
c. Add chemicals, particularly
at peak flow.
a. Add more air if DO<2 mg/1.
b. Add chemicals.
c. Reduce energy intensity of
aeration
(contributes to floe breakup)
d. Adjust F/M and MCRT for
improved SVI.
e. Check nutrient balance
(N and P).
f. Flow equalization.
a. Flow equalization.
b. Pace RAS rate with feed rate.
c. Install sludge blanket
controls; pace RAS with
sludge blanket.
194
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TABLE 23 (continued)
Symptom
Possible Cause(s)
Possible Remedies
4. Shock loads of
toxic materials.
5. Clarifier design
deficiency -
hydraulic
6. Clarifier design
deficiency - sludge
withdrawal
III. High secondary
effluent particu-
late BOD
d. Analyze sludge settle-
ability and densificatiqn ,
routinely; manually'set'..: '". ;
RAS ratesv .-. '. .-. • • .-•:,
f. Adjust F/M and MCRT. ' " -'•••
a. Flow equalization.
b. Add emergency storage basin
(if shocks can be detected
and diverted).
c. If activated sludge system
is plug flow, modify to
complete mix (depending upon
amount and concentration of
toxicant).
d. Add powdered activated
carbon to aeration basin.
e. Add "roughing" biofilter
upstream from activated
sludge.
a. (See .1.2.)
a. Modify sludge scraper
(e.g. add suction withdrawal)
b. Modify sludge hopper.
See Causes and Remedies under "II. High
Secondary effluent TSS".
IV.
High secondary ef-
fluent soluble BOD
1. High average daily
soluble BOD loading
2. High peak soluble
BOD loading.
a. Increase contact time by
adding additional aeration
volume.
b. Decrease F/M by increasing
MLVSS.
c. If aeration system is
"completely mixed", add
baffles to change residence
time distribution.
d. Add additional oxygenation
capacity if DO is low.
e. Add "roughing" biofilter
upstream from activated
sludge.
a. (See IV.1.)
b. Flow equalization
(volume required for smooth-
ing peak BOD loading would
probably be greater than
that required for flow smooth-
ing) .
(continued)
195
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TABLE 23 (continued)
Symptom
Possible Cause(s)
Possible Remedies
TRICKLING
FILTER SYSTEM
V. High secondary
effluent TSS
VI. High secondary
effluent particu-
late BOD
VII. High secondary
effluent soluble
BOD
3. Shock loadings of
toxic materials.
4. Shock loadings of
slowly degradable
materials.
1. Hydraulic overload
of clarifier at
peak flows.
2.
Clarifier design
deficiency -
hydraulic
a. (See II.4.)
a. (See II.4.)
b. Increase MLVSS.
c. Increase contact time by
adding additional aeration
volume.
a. (See II.1.)
b. Add tube settlers.
a. (See 1.2.)
b. Add tube settlers.
See Causes and Remedies under "V. High
secondary effluent TSS".
1. High soluble BOD
loading.
2. High peak soluble
BOD loading.
3. Shock loadings of
toxic materials.
4. Shock loading of
poorly degradable
materials.
a. If existing biofilter
is rock, change to
plastic media.
b. Increase media depth.
c. Add additional biofilter
units.
d. Add aeration basin down-
stream from biofilter
(upstream from clarifier).
e. Add or increase biofilter
recirculation.
f. Provide positive, continuous
ventilation of biofilter.
a. (See VII.1.)
a. Flow equalization.
b. Add emergency storage
basins if shocks can be
detected or anticipated.
c.(See VII.1.)
a.(See VII.3.)
196
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This report is not intended to provide information on the
broad range of alternative solutions to existing plant perfor-
mance problems. Information on alternative upgrading techniques,
for example, are provided in the USEPA Design Manual on Upgrading
Existing Wastewater Treatment Plants.(35)
Significant Flow Equalization Benefits
This section is intended to identify those circumstances
which separately, or combined, represent the most likely situa-
tions in which flow equalization will be useful and appropriate.
The categories have been developed from analysis of data on
existing flow equalization applications, and evaluation of
benefits theoretically attainable from typical wastewater treat-
ment unit processes operating under equalized flow conditions.
Flow equalization benefits may be categorized as follows:
• reduction of peaking requirements;
• reduction of process overloads at existing plants
under some conditions;
• protection against toxic upsets;
• potential reduction of operational problems;
• provides increasing benefits with increasing treatment
plant complexity; and
• reduction of plant recycle impacts from intermittent
side-streams such as batch sludge dewatering.
Peaking Requirements—
The term "peaking" in this context is intended to cover
peak hydraulic flow as well as peak mass loading of organics.
Planning and design methodology for reducing peaking conditions
by providing flow equalization is given in Section 2. Costs of
providing peaking capacity in wastewater treatment components
are compared to costs of providing flow equalization in Section
5.
Wastewater treatment processes are affected by peaks in
different ways. Effects vary from little or no change in pro-
cess performance to a complete loss of treatment capability.
In most wastewater treatment applications, unit processes are
installed consecutively to form the overall process train.
The effect of peaking on effluent quality must therefore be
evaluated on the aggregate of unit processes in a given system
because of the interactive effects of individual unit processes
on each other. Interactive effects of several unit processes
197
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can range from a dampening or elimination of upstream process
deterioration by downstream unit processes to complete system
failure. However, designing and operating unit processes in a
wastewater treatment system to handle highly variable loadings,
and the potential "cascading" effect of process deterioration
in upstream processes can be expensive. Therein lies the poten-
tial cost savings associated with flow equalization as it may
be applied to dampen loading variations.
The effects of peak loadings on wastewater treatment fall
into three general categories:
1. No effect
2. Gradual deterioration
3. "Threshold" effect
These three categories are described schematically in Figure 77.
In the first two categories, flow equalization does not provide
benefits in terms of process performance improvements. This is
because in category 1 (no effect) peak loadings do not influence
process performance. In category 2 (gradual deterioration) the
effects of process performance deterioration at peak loadings
are averaged out by increased process performance at loadings
below average conditions, with no net effect on average daily
performance. When a wastewater treatment unit process responds
to peak loadings according to the general category (threshold
effects) process performance at peak loadings deteriorates
significantly, and average daily performance is adversely
affected; sometimes significantly. In terms of process perfor-
mance, maximum benefits are obtained from the application of
flow equalization in situations where wastewater systems or
unit processes respond to peaking conditions according to
category 3, threshold effects.
Pretreatment and Primary Sedimentation—Pretreatment facilities
include prechlorination,bar screens,influent pumping, preaera-
tion, grit handling, and flow measurement. These processes are
conventionally sized for peak flow. Accordingly, reduction in
peak flow will reduce pretreatment processes cost. However, if
equalization facilities are located upstream of pretreatment,
operating problems may occur and excessive labor costs may be
required for process maintenance.
Primary sedimentation is intended to remove settleable
and floatable solids which include various amounts of suspended
solids, BOD, organic and ammonia nitrogen, and other constitu-
ents. In typical raw municipal wastewater, the settleable
solids contain approximately 35 percent of the total 3005; this
percentage can vary considerably, depending on the prescribed
industrial wastes and other factors. Primary sedimentation
198
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O
O
CD
CO
•• t
£ I
CO
-------
performs two basic functions: removal of solids with potential
for causing problems in downstream processes; and to provide
partial treatment of BOD and suspended solids to minimize the
size of more costly downstream processes.
From the standpoint of settleable solids removal, rather
high peak flows (peak overflow rates) are acceptable. Primary
sedimentation tanks typically provide effective settleable
solids removal at peak overflow rates of up to 2,000 gallons/
square foot/day (gpd/ft2) and higher. This assumes that the
overflow rate and not other factors (e.g., poor inlet baffling,
insufficient depth, etc.) is controlling process performance.
As overflow rates increase above the 2,000 gpd/ft level, loss
of settleable solids to the primary effluent will begin to
occur. However, this portion is that which is most easily re-
suspended? and it is the least significant concerning gross
solids in equalization basins receiving primary effluent.
As settleable solids removal drops, a corresponding drop
will occur in BOD removal efficiency in primary sedimentation.
However, if equalization is located between primary sedimenta-
tion and downstream processes, the drop in BOD removal efficiency
may be kept from affecting the downstream processes.
Activated Sludge Systems—The effects of peaking on biological
and sedimentation processes making up an activated sludge system
are discussed separately since peak loading effects can be dif-
ferent for each process. The basic function of the biological
unit is to convert dissolved and nonsettleable organics to
settleable biological solids. The sedimentation process serves
to separate these solids from the liquid stream, providing a
clarified effluent. Adverse effects of toxic or poorly degrad-
able substances are discussed in a subsequent section.
Peak hydraulic loadings in the biological portion of an
activated sludge system result in reduced residence times in
the aeration zone as compared to residence times at average
flow conditions. Such conditions reduce the period of time
available for microorganisms under aeration to remove organic
elements. Design practice for activated sludge systems pro-
vides considerably more residence time in the aeration zone
than the minimum needed for removal of organic constituents
(particularly soluble organics) from the wastewater feed. As
a result, no deterioration in effluent quality ordinarily re-
sults from reducing the residence time available in the aeration
cell at peak hydraulic loads. Figure 78 graphically depicts
the relationship between the influent concentration of soluble
organics (Co, expressed as COD, in mg/1), the suspended solids
concentration in the mixed liquor (X^o, a measure of the micro-
organism population, in mg/1), and the time required for the
microorganisms to remove the organics from solutions. These
200
-------
0.5
0.4
o
o
X
o
o
TROUBLE ZONE
( SOLUBLE ORGANICS BREAKTHROUGH
SAFE ZONE
( SOLUBLE ORGANICS REMOVAL)
WHERE:
Co = Influent Soluble COD, mg/l
Xao = MLSS, mg/l
TIME , HR
Figure 78. Estimate on limits on soluble organics removal in
activated sludge (36, 37)
201
-------
data, derived from batch tests, have been confirmed recently on
large scale pilot activated sludge units in Seattle.(35)
A large number of studies of existing, full-scale activated
sludge plants, as well as a number of well-controlled pilot
scale investigations, have shown that average effluent quality
of activated sludge units is seldom adversely effected by peak
loads..... Variations in effluent quality due to load changes-tend
to be "averaged out". These investigations were conducted over
a wide range of organic loadings, mixed liquor concentrations,
hydraulic loading rates, and activated sludge modifications. An
example of such data is given in Figure 79. All of these systems
were operating well within the range considered to be "safe"
according,to Figure 78, even though many were very heavily
loaded. Typical design criteria for activated sludge units will
result in operations well within the "safe" zone.
<•-, In summary, flow equalization for the purpose of reducing
hydraulic and organic peaks to the biological process in acti-
vated sludge systems cannot ordinarily be expected to result
in improved performance of the biological process. Except in
unusual cases, when C0/Xj^0 is high and the available residence
time is very low, the soluble organics content of the process
effluent will be unaffected by peak loading conditions.
Activated Sludge Sedimentation—Design and operation of sedimen-
tation processes in activated sludge systems have been imple-
mented in a wide variety of ways. Shapes of the tanks (circular,
rectangular), inlet and outlet configurations (center feed -
peripheral feed - peripheral withdrawal, etc.), sludge removal
mechanisms (suction, scraping, etc.), and other factors all
contribute to variations in the performance of existing or pro-
posed sedimentation units. The principal feature distinguish-
ing activated sludge sedimentation units from that for trickling
filter units is the large quantity and different settling rates
of solids that must be removed. Typical activated sludge units
process about ten times more solids through the sedimentation
tank as compared to a trickling filter system receiving compa-
rable influent loadings.
Available data on activated sludge sedimentation tank
performance indicate that the effect of peak hydraulic loadings
is small until a certain "threshold" is reached (see Figure 77).
This threshold can be defined in hydraulic terms (gallons per
minute per square foot, gpm/sf) or solids loading units (pounds
per day per square foot, ppd/sf), depending upon whether the
ultimate limitation in the sedimentation tank is related to dis-*
creet settling of individual particles (hydraulic effects) or
the ability to remove settled particles from the tank at an
adequate rate (solids flux effects) under peak loads. Sometimes
changing (increasing or decreasing) the rate of return sludge
202
-------
UJ
UJ
cc
h-
DC
O
o
UJ
CO
CO
CO
o
or
o
203
-------
from the secondary sedimentation tank, or pacing the return
rate with influent flow, can have the effect of reducing or
eliminating loss of solids at peak loadings. Simple observa-
tion of the sedimentation tank at peak loads can often confirm
the cause for excessive loss of solids. A number of possible
alternatives for reducing excessive solids loss from the second-
ary clarifier of an activated sludge plant are included in
Table 23.
Data was collected at the Ypsilanti Township (28) activated
sludge plant on rectangular secondary sedimentation tanks de-
signed for an average hydraulic loading in the range of 1,340
gpd/sf to about 1,500 gpd/sf. Performance deterioration ap-
peared to occur at the lower overflow rate (1,340 gpd/sf) if,
at the beginning of the test, a large quantity of sludge was
already present in the tank. Process performance appeared to
be adequate for periods of several hours at rates to about 1,500
gpd/sf when the sludge quantities in the sedimentation tank were
controlled with an adequate return rate. In two separate test
runs at 1,440 gpd/sf, performance was excellent and stable,
with a sharp decline in performance after about 12 hours. The
researchers in Ypsilanti concluded that the deterioration in
effluent quality of an activated sludge sedimentation tank can
considerably lag the increased flow rate.
At the activated sludge plant in Arnarillo, Texas (29) ,
secondary sedimentation tank overflow rates of 1,100 gpd/sf
were sufficient to cause a significant loss of solids over the
effluent weir. Design recommendations for activated sludge
secondary clarifiers contained in the USEPA Technology Transfer
Design Manual entitled "Suspended Solids Removal" (38) include
average overflow rates of 400-800 gpd/sf with peak rates in the
range of 1,000-1,200 gpd/sf. For new plants, tank sizing
should take into account both average and peak rate limitations,
with the tank actually constructed according to the criteria
resulting in the largest recommended size.
It appears, from the foregoing full scale test data and
design recommendations, that overflow rates beyond about 1,000
gpd/sf can cause effluent deterioration in an activated sludge
sedimentation tank. In some cases, peak sustained rates to
1/500 gpd/sf can be tolerated for a period of several hours
depending upon the design configuration of the tank, the quan-
tity of sludge in the tank at the start of peak loading condi-
tions, sludge settleability, and the ability of the sludge
withdrawal equipment to remove sludge at peak sludge loading
rates.
Flow equalization is one of the mitigating measures which
may be effective in improving activated sludge effluent quality
in existing systems when effluent quality deterioration is due
204-
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to loss of solids from the secondary sedimentation tanks at
peak flow rates. Flow equalization should be considered for
existing activated sludge systems when the hydraulic rate to
the clarifier exceeds about 1,000 gpd/sf for periods exceeding
4 hours per day where, effluent quality deterioration results.
Trickling Filter Systems—The removal of organics in the bio-
logical process occurs through flocculation and agglomeration
of the suspended and colloidal organic particles, and diffusion
of the soluble organics into the trickling filter biological
film. The overall removal (conversion to settleable form for
ultimate removal in sedimentation) of. organics in a trickling
filter is related to a number of factors; including the hydrau-
lic loading rate, the type and surface density of the trickling
filter media, media depth, temperature, and the composition of
the.waste stream.
Data from a number, of investigations are given in Figures
80 and 81., These data show that in several rock and plastic
trickling filter applications BOD removal was found to be re-
lated linearly to the hydraulic loading rate. The significance
of these data is that benefits from flow equalization on bio-
filter performance would be negligible since process efficiency
reductions at peak loads would be "averaged out" by efficiency
increases at loadings below the average rate.
The potential for increasing trickling filter performance
through use of flow equalization, therefore, does not appear
to be great. Subsequent discussions will focus on benefits
which may by achieved through flow equalization if a trickling
filter system receives shock loads of toxic or slowly degradable
materials from time to time.
Trickling Filter Sedimentation—The underflow from a trickling
filter typically contains biological solids and some undegraded
influent solids. The particle size of this material ranges
from colloidal and near-colloidal to large, readily settleable.
The large particles are easily removed in even the most rudi-
mentary sedimentation basins, but a large portion of the par-
ticulates are small and settle very slowly. Removal of the
particles on the smaller end of the particle size spectrum
frequently is a crucial factor in the ability of a trickling
filter plant to meet secondary effluent TSS and BOD standards-
of 30 mg/1.
Very little data is available to allow prediction of the
effects of peak hydraulic and solid loads on trickling filter
sedimentation tanks. For the average trickling filter sedimen-
tation basin, performance deteriorates gradually, and increases
at lower than average loadings tend to average out peak effects.
205
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ioo
80
ui 60
DC
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LU
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a:
ui
Q.
40
20
0
LEGEND
•»• McCABE & ECKENFELDER
• BURGESS ET. AL.
A GALLER & GOTAAS
* BENZIE ET. AL.
O NATIONAL RESEARCH COUNCIL
• BETHLEHEM, PA.
O.I
02
0.3
0.4
0.5
0.6
HYDRAULIC LOADING, GPM/FT2 (I NCLUDI NG RECYCLE)
Figure 80. Effect of hydraulic loading on stone media
trickling filter BOD removal (35)
206
-------
5
O
2
UJ
CC
O
O
00
UJ
O
o:
UJ
Q.
100
80
60
40
20
LEGEND
• DOW CHEMICAL CO. PILOT PLANT #1
A GERMAIN
• MOORE
O DOW CHEMICAL CO. PILOT PLANT f2
• DOW CHEMICAL CO. PILOT PLANT #3
A SEDALIA, MO.
I
I 23 4
HYDRAULIC LOADING, GPM/ FT2 (INCLUDING RECYCLE)
Figure 81. Effect of hydraulic loading on plastic media
trickling filter BOD removal (35)
207
-------
In such conditions flow equalization benefits would be minimal.
An example of such an observation is given in Figure 82. Data
are taken from a trickling filter system recently tested in
Seattle. Figure 82 shows a gradual change in effluent TSS from
about 12 mg/1 to 40 mg/1 as the feed TSS to the sedimentation
unit is increased from 100 pounds TSS/day/1,000 ft2 to 600
pounds TSS/day/1,000 ft2 of sedimentation tank surface area.
Also included in Figure 82 are data on the effectiveness
of tube settlers in reducing effluent TSS levels across the
same range of TSS loadings. Tube settlers were found to pro-
vide some improvement in effluent TSS after about 16 percent
of the sedimentation tank surface area was covered with the
settlers. This condition corresponded approximately to a tube
settler hydraulic loading rate equal to that of the uncovered
sedimentation tank. Beyond 16 percent coverage, the improve-
ment in effluent TSS with increasing coverage was dramatic.
As noted in Table 23, tube settlers are one important alterna-
tive to be considered if a trickling filter plant sedimentation
unit must be upgraded.
Plant and Process Overloading—
In many cases, the actual average loads received at a
particular existing treatment plant or wastewater collection
system are at or near design loadings; but the peak rates of
flow, organic or solids loadings are beyond the capability of
existing facilities. Obvious examples of such conditions would
be hydraulic overloads, which cause wastewater to overflow man-
holes and short duration dissolved oxygen deficiencies during
peak organic loads to activated sludge aeration basins. Flow
equalization should be one of the corrective alternatives in
such circumstances. Design of flow equalization to achieve a
given reduction in hydraulic or organic peak rates may be
carried out according to the criteria set forth in Section 2.
Toxic Upset Conditions—
The effects of toxic substances on wastewater treatment
performances may be reduced or eliminated through proper use
of flow equalization. Intermittent shock loadings of toxic
substances can adversely affect the viability of biological
processes for periods much longer than the actual exposure
period. Frequently the toxic substances may be diluted in
equalization facilities to harmless concentrations. Design
of such basins, however, requires that the magnitude and dura-
tion of the shock load be known or estimated, as well as the
time of occurrence. Design of equalization basins for dampen-
ing the concentration of toxic substances should be carried
out according to the procedures in Section 2; except that the
time and concentration factors associated with the toxic sub-
stance are used in the analysis rather than the flow and con-
centration factors indicated in the Section 2 discussion.
208
-------
o>
CO
V)
z
UJ
ID
U_
u_
UJ
CC
UJ
U.
CC.
o
60
50
40
30
20
10
0
NOTE: TUBE SETTLERS COVER 42% OF THIS CLARIFIER'S SURFACE
CORRELATION
COEFFICIENT = O.83
KEY:
O CONSTANT FLOW
• PACED FLOW
500 IOOO I50O 20OO ' 25OO
LBS. SOLIDS TO CLARIFiER / DAY/IOOO SQ.FT. SURFACE AREA
Figure 82. Sedimentation tank effluent TSS versus solids
loading - normal hydraulic loadings (36)
209
-------
Performance benefits can be significant when flow equali-
zation is applied in situations where toxic upsets are otherwise
known to occur. An example of the magnitude of such benefits
is available in data from Tecumseh, Michigan. Before equaliza-
tion, toxic upsets from the intermittent discharge of oils and
plating wastes from local industries were partially responsible
for a yearly average (in 1970) effluent BOD concentration of
28.4 mg/1. After equalization (in 1973), the annual average
effluent BOD had been reduced 73 percent to 7.7 mg/1. A
comparable, and sometimes greater, improvement in performance
following addition of flow equalization is attainable at other
plants, depending upon the severity of upsets resulting from
toxic substances and the type and volume of flow equalization
provided.
Operational Problems—
The benefit most frequently claimed for flow equalization
by operators of equalized plants was that the plant was easier
to operate (operational problems were reduced) following addi-
tion of flow equalization facilities. Though this benefit is
difficult to quantify, it is a factor which should be carefully
considered in the initial design or renovation of wastewater
treatment works.
Process Complexity—
The operational and cost benefits associated with flow
equalization increase as wastewater treatment plant complexity
increases. This is by virtue of the fact that the size, cost,
and operation of flow equalization facilities are essentially
independent of the number and type of treatment process(es)
downstream. Flow equalization benefits (such as reduced peak-
ing capacity requirements) increase as the number of affected
processes increases. A more complete analysis of the cost/
benefit factors associated with treatment plant complexity is
given in the cost analysis examples in Section 5.
210
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"REFERENCES
27. G. W. Fbess,, et al., "Evaluation of Flow Equalization at
a Small WastQwater, Treatment Plant", U. S. Environmental
Protection Agency, EPA-600/2-76-181, September 1576.
28. G. W. Foess, et al. , "Effects of Flow Equalization on the
Operation and Performance of an Activated Sludge Plant",
(in press) U. S,. Environmental Protection Agency, EPA
. Grant -No. S801985, X979, ..•','.."'".' "
29. V. U. Jordan, C. H. Scherer, "Gravity Thickening Techniques
at a Water Reclamation Plant", J. Water Pollution Control
Fed., 42:2, 180-9, February 1970. -
30. LaGregga, M., "A Study of the Effects»of Equalization of
Wastewater Flows", Ph.D. Dissertation, Syracuse University,
1972.
31. A. Maass,,0. L.: Dull'/ "Case History of Midland City
Tertiary Treatment", Water Pollution Control Fed. High-
" lights 14:3, 01-4, March 1977.
32. Ettlich, W. F., "A Comparison of Oxidation Ditch Treatment
Plants to Competing Processes for Secondary and Advanced
Treatment of Municipal Wastes", USEPA, MERL, OR&D Contract
No. 68-03-2186, 1977.
33. Grounds, H. C. and J. M. Bullert, "Nitrogen Removal in a
Flow Modulated Single Stage Oxidation Ditch", USEPA, MERL,
OR&D, Contract No. S-803067-01-1, 1977.
34. USEPA, "Efficient Treatment of Small Municipal Flows at
Dawson, Minnesota", EPA Technology Transfer Capsule
Report, 1977.
35. USEPA, Upgrading Existing Wastewater Treatment Plants,
Technology Transfer Design Manual.
36. Brown and Caldwell Pilot Studies on Activated Sludge
Treatment of Municipal Wastewaters, unpublished, 1978.
211
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37. McLellan, James C. and Busch, Arthur W., "Hydraulic and
Process Aspects of Reactor Design II - Response to
Variations", 24th Industrial Waste Conference, Purdue
University, p. 493-506.
38. USEPA, Suspended Solids Removal, Technology Transfer
Design Manual.
212
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SECTION 6
THE DEVELOPMENT OP COSTS FOR
EQUALIZATION AND TREATMENT PROCESSES
BASIS OF UNIT COST DEVELOPMENT .
Information required for preliminary estimation of con-
struction and operating costs for equalization facilities and
appurtenances is developed in this section. Relationships
between capacities, costs of construction and other initial in-
vestment costs of major components of flow equalization facili-
ties are. defined along with major operation and maintenance
costs. The data presented are appropriate for average condi-
tions and, when used with discretion for specific local situa-
tions, will permit development of preliminary estimates for
flow equalization facilities.
Construction costs can be expected to change with time, in
keeping with corresponding changes in the economy and regional
variations in materials and labor. A good barometer of these
changes is the Engineering News Record (ENR) Construction Cost
Index. It is computed from prices on construction materials and
labor and is based on a value of 100 for the year 1913. Experi-
ence has shown that the costs of municipal treatment works
follow quite closely the cost escalation reported in the ENR
cost index.
The estimates of construction costs presented in this re-
port for both existing cost data received and cost curves de-
veloped are representative of the national average price levels
(ENR Index = 2600) as of August 1977. If the estimating data
are to represent costs in a particular area at a different date,
they must be adjusted accordingly. The cost adjustment can be
based on the ENR index; which permits comparison of price levels
at differing locations and dates.
Cost curves have been developed for four major equalization
facility components: holding tanks or basins, wastewater pum-
ping stations, aeration/mixing systems, and facility washdown
systems (manual). Cost estimates for each component are related
to a single parameter for sizes of individual facilities ranging
in capacity from 0.1 to 100 million gallons. Use of these esti-
mating data is dependent on the design engineer establishing,
213
-------
on the basis of appropriate design criteria, the overall com-
plexity of facilities to be provided and the sizes of components
required.
These estimating data cannot in any way be used as a sub-
stitute for cost estimating based on detailed knowledge of a
particular local situation. Construction cost estimates must
take into account not only the constraints and design character-
istics of a particular equalization facility, but also such
items as labor and material costs and availability, time allowed
for construction, climate and seasonal factors, local site con-
ditions, and other variables such as whether the equalization
facility is to be added to an existing sewage treatment plant or
is to be part of a new facility.
The information on estimating both construction costs, and
operation and maintenance costs requirements is intended to be
applicable for average situations throughout the continental
United States. Of course, any estimating method having such
a broad applicability must of necessity be regarded as suitable
only for determining preliminary estimates. Based upon varia-
tions in construction costs, it is estimated that the costs of
individual components for any specific situation may vary in
excess of 100 percent from the average cost values shown in this
study. In particular cases this variation may be substantially
greater. Variations are due to differences in factors which are
not readily quantified or regulated, as discussed previously.
The user should recognize the inherent limitations of such esti-
mates and should develop applicable cost estimates as warranted
by local circumstances.
CONSTRUCTION COSTS
Data for estimating initial investment costs for wastewater
flow equalization facilities are presented here for four prin-
cipal components;
1. Holding tank or basin
2. Wastewater pumping station
3. Aeration/mixing system
4. Manual-type washdown or solids removal system
The estimated construction costs shown for each of the
components listed include all costs a contract bidder would
normally encounter in completing the specific facility compo-
nent named. Such costs include materials, labor, equipment,
electrical work, and normal excavation, etc.
Estimates of construction costs do not include extraordi-
nary costs associated with rock excavation, wet conditions or
site dewatering, and piling. Such costs cannot readily be in-
corporated into average cost estimates. If such conditions are
214
-------
encountered, the designer must include an additional cost allow-
ance for all extraordinary expenses involved, based on local ex-
periences.
Estimated quantities have been extended by a- material price
to develop an estimated construction price. The material price
used is a reasonably close average for the United States, ex-
cluding Alaska and.Hawaii. Typically, freight costs of mechani-
cal equipment are assumed at 1,500 miles transit to job site.
Sales tax is not. included in the construction cost estimate.
Construction equipment and labor prices have been averaged out
by multiplying manhours or equipment hours by crew, equipment or
composite rates (for the United States, excluding Alaska and
Hawaii) of the trades or equipment involved for a particular
component. The manhours projected or estimated for each com-
ponent of the' flow equalization facility are those which, under
average conditions, would be anticipated for any such project
built in any semi-urban area. The estimates also take into
account the workmanship experienced in building typical munici-
pal wastewater work projects.
- Cost estimates of flow equalization facilities may be de-
veloped by combining the basic structure, basin or tank, with
desired appurtenances. The system cost is determined as the
sum of all component unit costs.
Holding Tank and Basin Costs
Estimates of cost "for1 the basic structures are
based on a square concrete tank with 15-foot sidewater depth,
17-foot wall height, 15-inch average wall thickness, and 12-inch
average bottom slab thickness for facilities in the range of
0.1 to 10 million gallon capacity. A cost versus capacity
graph (see Figure 83) is presented for three conditions: 16-
foot, 10-foot and 4-foot installed depth. Excavation was assumed
for a 1 on 1 slope starting four feet from the outside wall.
Prices used for earth excavation assumed dry conditions. A
disposal site for excess excavation material was assumed to be
within a distance of one mile.
• For equalization facilities in the range of 0.5 to 100
million gallons, estimates were developed for a 3-1/2-inch
thick, gunite lined, earthen basin with a 6-inch thick concrete
bottom slab. Mechanical work items such as connecting piping,
overflow structures, and miscellaneous electrical systems were
estimated at 10 percent of the concrete and earthwork costs..
A typical cross section of the tank or basin is shown in Figure
84.
It may be noted that the cost of a simple basin or tank,
according to Figure 83 or 84, is significantly lower than
215
-------
CO
10
8
6
5
4
3
2
IX)
.8
£
.5
.4
o
0
~ .2
S JO
0 .08
.06
.05
m
.03
.02
jOI
^ZL
55
14
.2 .3 4 .5 .6.7 I 2 34567 K) 20 3O 40 50 100
CONCRETE HOLDING TANK (I06gol.)
Figure 83. Construction cost as a function of concrete
holding tank size
216
-------
10
8
6
1.0
0 -4
n"™" -3
o
o
.2
.10
.08
.06
.04
.03
.02
.01
^Z
y
n
17
EXIST. GROUND-^ ^-S"cONC.(TYP) ^—
EXIST. GROUND- 6"CONC.(TYR) — 3 I/2"GUNITE(TYP)
.5^.7.8.91 2 3 4 56789IO 20 30 4O50 100 2OO 400
CONCRETE LINED EARTHEN BASIN (I06gal.)
1000
Figure 84. Construction cost as a function of concrete
lined earthen basin volume
217
-------
comparable complete equalization systems identified in the
equalization facilities survey, Figure 18. The additional cost
of existing systems is accounted for in the cost of appurtenan-
ces. For example, 50 percent of all systems surveyed include
mixing or aeration, 70 percent include a pump station, and 65
percent include washdown facilities. Addition of such costs
from the cost curves for appurtenances, Figures 85 through 88,
results in estimated equalization system costs comparable to
those summarized in Figure 18.
Wastewater Pumping Costs^
Capital costs for pumping stations located downstream of
the sewage treatment plant headworks are shown in Figure 85.
The pumping volumes listed in the figure are based on a 24-hour
pumping rate. Typically, the pumping capacity of a flow equali-
zation facility should be sufficient to transfer the equaliza-
tion structure volume in or out within a 12-hour period. Cost
curves are for combinations of fixed speed, low lift, centri-
fugal pump installations without auxiliary or standby power
facilities. Costs include structure, pumping units, all elec-
trical and mechanical components, and control equipment.
Aeration and Mixing Costs
The construction costs of aeration equipment vary widely
depending on the type and size of the equipment used. The most
common types of equipment are surface aerators (fixed and
floating) and diffusion systems. Inasmuch as the type, size,
and cost of equipment will depend to a great extent on local
conditions, aeration cost curves developed in this report are
based on using floating, low speed, mechanical surface aerators.
The cost curve (see Figure 86) includes all mechanical and elec-
trical equipment and assembly necessary for a working installa-
tion.
Floating aerators with low water shutoff were used for the
typical flow equalization facility, because their operation is
complementary to a wide range of flow conditions. For existing
or planned equalization facilities accompanying a treatment pro-
cess system having diffused aeration equipment, it may be more
economical to share use of common equipment and install a dif-
fused air system.
Floating or fixed aerators are independent of all other
sewage treatment plant unit processes and equipment, and,
therefore, may provide more reliable data for a preliminary
estimation of the additional cost of supplying an aeration
component.
Aeration or mixing necessary to maintain oxygen levels
ranges typically from 0.015 to 0.04 hp/1,000 gallons. Aeration
218
-------
10
ao
6.0
4.0
2.0
10
»oa8
0.4
o
0
0.2
z:
0.1
.08
.06
.04
.02
.Ol
.1 ,2 .3 .4.56.7.891 2 3. 4 5678910 20 30 4O 6O IOO 20O
WASTEWATER PUMPING STATION (mgd)
Figure 85- Capital cost as a function of pump station capacity
219
-------
-------
required for complete mixing (to keep all solids in suspension)
ranges up to 2 hp/1,000 gallons. By virtue of this broad range,
it is felt that each designer must determine the amount of
horsepower required for each individual facility, and must
select the appropriate value; again depending on where the
facility is located, and the type and duration of flow stored.
These items will significantly affect the level of aeration or
mixing required. .
Washdown and Solids Removal Costs
Capital costs associated with two manual washdown systems
have been prepared. The first of these, suggested for use with
concrete tank structures, includes a piping system around the
inside of the top of the_tank structure, with spray nozzles
located on approximately four-foot centers. This system would
also include hydrant assemblies at strategic locations for use
in the washdown operation.
The second system, for use with concrete-lined earthen
basins, is provided with a piping network installed around the
periphery of the basin, and also across the basin at set inter-
vals (as required). This is fitted for use either with fire
boat-type hydrant assemblies, or fire hose connections. A
schematic illustration of each system is shown in Figure 87.
Within the suggested volume range of each holding tank or basin,
the additional capital cost required for a manual washdown or
solids removal system can be estimated from Figure 88.
Additiona1 Capita1 Co aits
No allowance for additional capital costs, such as con-
struction contingency costs, and legal and administrative costs,
has been included in the construction cost curves. For budge-
tary purposes, these costs are assumed to be 30 percent of the
total construction cost. Land costs also have not been included
because local land prices around the nation vary significantly.
These costs should be added to the preliminary construction cost
to arrive at a more meaningful estimate.
OPERATION AND MAINTENANCE COSTS
From an analysis of survey data and a review of the many
variables associated with the use and operation of flow equali-
zation facilities, it was concluded that no singular plot or
grouping of operation and maintenance values for each of the
individual equalization facilities components could reasonably
be developed with any acceptable level of responsibility.
Equalization facilities used as an integral component of the
daily operation of a sewage treatment plant would.have a wide
range of possible O&M costs depending on where in the plant it
221
-------
GUNITE SIDES
CONCRETE BOTTOM
QUICK COUPLING
FIRE HYDRANT NOZZLE
(OPTIONAL) AT FIXED
LOCATIONS AND QUICK
COUPLINGS AT OTHER
LOCATIONS
QUICK COUPLING (TYP)
SUPPLY LINE
(SIZE VARIES)
•SUPPLY LINE
HYDRANT
SPRAY NOZZLES
(TYP ALL AROUND)
Figure 87. Equalization basin washdown system details
222
-------
1000
100
CO
CE
Q
CO
Q
CO
O
fc
o
o
10
.2 .3 4 .5 .6 .7 .8.9 I 2 3 4 5 6 7 8 910 20 30 40 50 100
WASHDOWN SYSTEM (10 gal.)
Figure 88•
Equalization basin washdown system capital cost as a
function of system capacity
223
-------
is located, the complexity of the installation, and the duration
and frequency of flow bypassed to the equalization'structure.
Structures used as side-line flow equalization facilities would
have a still greater variance in yearly O&M costs,'and would, to
a great degree, be dependent on the number of times the facili-
ties were used. Therefore, any attempt to outline typical or
representative O&M cost figures is quite restricted. Addition-
ally, O&M costs associated with equalization facilities sharing
components of the treatment plant are difficult to quantify.
Operation and maintenance costs associated with aeration
components are based on using the equipment 12 hours per day at
a level of 0.02 hp/1,000 gallons of installed storage tank
capacity. Power costs are estimated at $0.02/kwh. If require-
ments and operating conditions vary significantly from
the assumed values, then the cost curve should be adjusted
accordingly.
Manpower costs included in the various O&M cost curves were
developed by assuming 6.5 hours of productive labor per man-day,
with the labor cost assumed at $13.30 per productive man-hour,
including fringe benefits.
Washdown or' solids removal operation and maintenance costs
are based on equalization facilities having some form of mixing/
aeration, and an assumed cleaning interval of twice per month.
The O&M cost relationships presented in this study were
derived from local experience, survey data from existing flow
equalization facilities, and from several EPA technical reports.
The O&M cost figures should be used only to establish the rela-
tive magnitude or range of O&M costs for each component. Based
on O&M data received as part of this survey, the cost curves
shown in Figure 89 would appear to be the upper limit of O&M
costs to be anticipated with flow equalization facilities.
COMPARISON OF EQUALIZATION COSTS AND TREATMENT PLANT
PEAKING CAPACITY COSTS
Sample calculations are summarized in this section to com-
pare the estimated cost of wastewater treatment plants designed
for peak flow conditions without equalization, and for average
flow conditions with either in-line or side-line flow
equalization. Costs are estimated for plants of 1 mgd, 3 mgd
and 10 mgd capacity, each for three moderate advanced treat-
ment: secondary treatment (activated sludge); moderate advanced
treatment (primary with alum, nitrifying activated sludge,
filtration); and high level advanced treatment (primary with
alum, high-rate activated sludge, nitrification-denitrification
by the "three sludge" process, and filtration). These three
treatment systems correspond to systems 5, 9, and 11 in the
Areawide Assessment Procedures Manual, (39)9
224
-------
IOOO
200
FLOW EQUALIZATION FACILITY VOLUME (10 gal)
Figure 89. Equalization facility operation and maintenance
costs as a function of flow equalization facility
volume
225
-------
as illustrated in Figure 90. Comparisons are made for peak-to-
average flow ratios of 1.3:1, 1.6:1 and 2.0:1. Selection of
these peaking ratios was based on preliminary calculations con-
ducted to identify the region of transition from cost effective
to not cost effective defined subsequently in Table 24 and
Figure 91.
Cost information for treatment systems is taken from the
Areawide Assessment Procedures Manual (39) using systems 5, 9
and 11. Construction costs are increased by 30 percent to allow
for site work, piping, electrical work, and instrumentation.
Also, in order to present data for mid-1977 cost levels, 5.05
percent is added; cost levels are based on an Engineering News-
Record Construction Cost Index of 2600. Cost differences due
to change in peak treatment flow are computed based on estimates
of the impact of equalization on each process in the plant; care
is taken to assure that equalization's benefits are credited
where appropriate, and not elsewhere. As noted in Section 4, a
simple and rigorous estimate of benefits is not possible in many
cases, but the best practical approximations are made.
Cost information for equalization facilities is taken from
unit cost curves in this section. Costs are estimated separate-
ly for side-line concrete tanks not requiring separate pumping,
and for in-line concrete tanks. To drain the side-line equali-
zation tank, the main plant influent pumps are assumed to suf-
fice. An additional pumping station with capacity for total
plant flow is required for in-line equalization.
Additional assumptions used in the cost comparisons
include:
1. Equalization volumes used are 9 percent at P/A = 1.3,
17 percent at P/A = 1.6, and 29 percent at P/A = 2.0.
2. Cost savings in equalized flow plants result from:
a. Treatment system 5: secondary clarifier, chlorine
contact, and aeration O&M.
b. Treatment system 9: aeration tank, secondary
clarifier, chlorine contact, filtration, chlorine
contact, and aeration O&M.
c. Treatment system 11: second-stage aeration,
secondary clarifiers, filtration, chlorine con-
tact, and first and second stage aeration O&M.
The selection of equalization volumes used, 9, 17 and 29 per-
cent of average daily flow, was based on mass diagram estimation
using hypothetical daily hydrographs corresponding to the re-
spective P/A ratios, 1.3, 1.6, and 2.0. The unit processes
226
-------
SYSTEM NO. 5
Alum
Primary I . Secondary
Lift Preliminary Clarifier I Activated Clarifier
Pumps Treatment /~^. T alua9f x^X Disinfect!.
ooo
Gravity Vacuum
Thickener Digestion Filter
Ultimate
Disposal
SYSTEM NO. 9
Alum
Primary
Clarifier
Activated
Sludge/
Nitrification
Vacuum Gravity
Filter Digestion Thickener
Disinfection
Filtration
Secondary
Clarifier
SYSTEM NO. 11
Alum
Nitrification
Clariftar
Gravity Vacuum
Thickener Digestion Film-
Ultima t»
' Dliposal
Polymer
j Denilrlf Ication
^MriW
90. Treatment systems used in equalized vs. unequalized
facilities cost comparison
227
-------
TABLE 24a.
EQUALIZATION COST
EFFECTIVENESS EXAMPLE
Treatment System: System No. 5
Plant
Size,
mgd
1
1
1
3
3
3
10
10
10
Peaking
Factor
for
Flow
1.3
1.6
2.0
1.3
1.6
2.0
1.3
1.6
2.0
Construction Costs - $ Millions
Plant
Without
Equali-
zation
2.1856
2.3222
2.5954
3.9614
4.5078
5.0542
9.2888
10.2450
12.2940
Equalized
Plant
2.1201
2.2106
2.4315
3.8520
4.2899
4.7263
8.9746
9.7396
11.4607
Equali-
zation
0.0702
0.0930
0.1313
0.1196
0.1872
0.2561
0.2561
0.4056
0.6604
Pumping
Station
0.1079
0.1079
0.1079
0.2210
0.2210
0.2210
0.4940
0.4940
0.4940
Net
Benefit
of
Equali-
zation
-0.0047
+0.0186
+0.0326
-0.0102
+0.0307
+0.0718
+0.0581
+0.0998
+0.1729
Net
Benefit
of
Equal i-
zation
w/P . S .
-0.1126
-0.0893
-0.0753
-0.2312
-0.1903
-0.1492
-0.4359
-0.3942
+0.3211
Treatment System: System No. 9
1
1
1
3
3
3
10
10
10
1.3
1.6
2.0
1.3
1.6
2.0
1.3
1.6
2.0
3.1418
3.4150
3.8248
5.8738
6.8300
8.0594
15.0260
16.3920
17.7580
2.9286
2.9721
3.2661
5.4599
6.0344
6.7713
13.4800
14.1108
14.0561
0.0702
0.0930
0.1313
0.1196
0.1872
0.2561
0.2561
0.4056
0.6604
0.1079
0.1079
0.1079
0.2210
0.2210
0.2210
0.4940
0.4940
0,4940
+0.1430
+0.3499
+0.4274
+0.2943
+0.6084
+1.0320
+1.2899
+1.8756
+3.0415
+0.0351
+0.2420
+0.3195
+0.0733
+0.3874
+0.8110
+0.7959
+1.3816
+2.5475
Treatment System: System No. 11
1
1
1
3
3
3
10
10
10
1.3
1.6
2.0
1.3
1.6
2.0
1.3
1.6
2.0
4.0980
4.4395
5.1908
7.9228
9.2888
10.6548
19.1240
21.8560
24.5880
3.8998
4.1157
4.6335
7.5390
8.5415
9.3873
18.1405
19.9983
21.2550
0.0702
0.0930
0.1313
0.1196
0.1872
0.2561
0.2561
0.4056
0.6604
0.1079
0.1079
0.1079
0.2210
0.2210
0.2210
0.4940
0.4940
0.4940
+0.1280
+0.2308
+0.4260
+0.264
+0.5601
+1.0114
+0.7274
+1.4521
+2.6726
+0.0201
+0.1229
+0.3181
+0.0432
+0.3391
+0.7904
+0.2334
+0.9581
+2.1786
228
-------
TABLE 24b.
EQUALIZATION COST
EFFECTIVENESS EXAMPLE
Treatment System: System No. 5
Plant
Size,
mgd
1
1
1
3
3
3
10
10
10
Peaking
Factor
for
Flow
1.3
1.6
2.0
1.3
1.6
2.0
1.3
1.6
2.0
OSM Costs - $ Millions/Year
Plant
Without
Equali-
zation
0.1575
0.1680
0.1890
0.2941
0.3362
. 0.3887
0.6825
0.7875
0.9450
Equalized
Plant
0.1507.
0.1560
0.1736
0.2309
0.3107
0.3496
0.6427
0.7164
0.8256
Equali-
zation
0.0057
0.0061
0.0063
0.0064
0.0071
0.0078
0.0078
0.0089
0.0098
>
Pumping
Station
0.0140
0.0140
0.0140
0.0185
0.0185
0.0185
0.0320
0.0320
0.0320
Net
Benefit
of
Equali-
zation
+0.0011
+0.0059
+0.0091
+0.0487
+0.0184
+0.0313
+0.0320
+0.0622
+0.1097
Net
Benefit
of
Equali-
zation
w/P.S.
-0.0129
-0.0081
-0.0049
+0.0383
+0.0001
+0.0128
-0.0000
+0.0302
+0.0777
Treatment System: System No. 9
1
1
1
3
3
3
10
10
10
1.3
1.6
2.0
1.3
1.6
2.0
1.3
1.6
2.0
0.2521
. 0.2626
0.2941
0.4412
0.5147
0.5985
1.0500
1.2600
1.4700
0.2356
0.2263
0.2396
0.3466
0.4262
0.4577
0.9386
1.0209
1.0776
0.0057
0 . 0061
0.0063
0.0064
0.0071
0.0078
0.0078
0.0089
0.0098
0.0140
0.0140
0.014O
0.0185
0.. 0185
0.0185
0.0320
0.0320
0.0320
+0.0108
+0.0302
+0.0482
+0.0882
+0.0814
+0.1330
+0.0334
+0 . 2302
+0.3826
-0.0032
+0.0162
+0.0342
+0.0697
+0.0629
+0.1145
+0.0015
+0.1982
+0.3506
Treatment System: System No. 11
1
1
1
3
3
3
10
10
10
1.3
1.6
2.0
1.3
1.6
2.0
1.3
1.6
2.0
0.3152
0.3467
0.3887
0.6093
0.7038
0.8194
1.4700
1 . 6800
2.1000
0.2876
0.2802
0.3100
0.5476
0.5763
0.6128
1.2462
1.2914
1.4286
0.0057
0.0061
0.0063
0.0064
0.0071
0.0078
0.0078
0.0089
0.0098
0.0140
0.0140
0.0140
0.0185
0.0185
0.0185
0.0320
0.0320
0.0320
+0.0219
+0.0604
+0.0724
+0.0553
+0 .1204
+0.1988
+0.2160
+0.3797
+0.6616
+0.0079
+0.0464
+0.0584
+0.0368
+0.1019
+0.1803
+0.1840
+0.3477
+0.6296
229
-------
TABLE 24c.
EQUALIZATION COST
EFFECTIVENESS EXAMPLE
Treatment System: System No. 5
Present Worth
Plant
Size
mgd
1
1
1
3
3
3
10
10
10
Peaking
Factor
for
Flow
1.3
1.6
2.0
1.3
1.6
2.0
1.3
1.6
2.0
•
P.W. of OSM
Net Benefit
$ Millions
+0.0124
+0.0676
+0.1048
+0.6515
+0.2110
+0 . 3590
+0.3668
+0.7140
+1.2577
P.W. of OSM
Net Benefit
w/P . S .
$ Millions
-0.1482
-0.0930
-0.0558
+0.4393
+0.0011
+0.1468
-0.0002
+0.3469
+0.8966
Overall Net Benefit
Construction + OSM
Total P.W.
of System
w/Equal .
$ Millions
+0.0086
+0.0862
+0.1374
+0.6413
+0.2417
+0.4308
+0.4249
+0.8138
+1.4306
Total P.W.
of System
w/Equal .
w/P.S.
$ Millions
-0.2599
-0.1823
-0.1311
+0.2081
-0.1914
-0.0024
-0.4361
-0.0473
+1.2117
Treatment System: System No. 9
1
1
1
3
3
3
10
10
10
1.3
1.6
2.0
1.3
1.6
2.0
1.3
1.6
2.0
+0.1239
+0.3464
+0.5528
+1.0117
+0.9337
+1.5255
+0.3834
+2.6404
+4.3884
-0.0367
+0.1858
+0.3923
+0.7995
+0.7215
+1.3133
+0.0172
+2.2734
+4.0214
+0.2669
+0.6963
+0.9802
+1.3060
+1.5421
+2.5575
+1.6733
+4.5160
+7.4299
-0.0016
+0.4278
+0.7118
+0.8728
+1.1089
+2.1243
+0.8131
+3.6550
+6.5689
Treatment System: System No. 11
1
1
1
3
3
3
10
10
10
1.3
1.6
2.0
1.3
1.6
2.0
1.3
1.6
2.0
+0.2512
+0.6928
+0.8304
+0.6343
+1.3810
+2.2802
+2.4775
+4.3552
+7.5886
+0.0906
+0.5322
+0.6698
+0.4221
+1.1688
+2.0680
+2.1105
+3.9881
+7.2215
+0.3792
+0.9236
+1.2564
+0.8985
+1.9411
+3.2916
+3.2049
+5.8073
HO. 2612
+0.1107
+0.6551
+0.9879
+0.4653
+1.5079
+2.8584
+2.3439
+4.9462
+9.4001
230
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realizing cost savings by design to meet average flow capacity
instead of peak flow have been identified as a design decision.
Flexibility of such decisions is necessarily limited in such an
example due to dependence on widely available cost curves for
treatment components (39). Nevertheless, such decisions must be
made in the design process in accord with details of individual
treatment conditions based on the knowledge and experience of
the design engineers.
The cost comparison of equalized and unequalized treatment
facilities is summarized in Table 24 (a, b, and c). In reviewing
the tabulated figures it is important to note the relative con-
tributions of construction and O&M costs, using O&M costs ex-
pressed in terms of present worth. The final two columns of
Table 24c indicate the overall net benefit of the respective
treatment systems, with equalization facilities compared to
unequalized treatment plants. This shows that, adopting the
assumptions used in developing the costs, for systems using the
side-line equalization without an additional pumping station,
the unequalized systems designed to accommodate full diurnal peak
flows. Systems using in-line equalization requiring an addi-
tional pumping station, however, are more expensive for the
simplest treatment system, and for the smallest size intermedi-
ate treatment system. Regions of plant size and peak-to-average
ratio, for which equalization is not cost effective for this
example, are summarized in Figures 91 and 92.
Examination' of this example enables some generalizations
to be made. Extending generalizations to the vast range of
potential applications differing from this example in important
details is, however, not possible. Costs and cost comparisons
are heavily dependent upon the underlying assumptions used in
their development. The range of required assumptions necessary
to allow a valid cost comparison for a given treatment plant is
very broad. It is therefore necessary that each potential
application of equalization be analyzed according to its own
situation in a manner similar to that above.
The example shows that, based on capital plus operation and
maintenance cost considerations, use of equalization facilities
should be considered in all but a few situations when new
treatment plants are being planned. Situations that do not
favor equalization are: small plant sizes (less than 5 mgd);
simple secondary treatment plants required to meet only 30-30
standards; low peak-to-average flow ratios with little wet
weather influence; and physical settings requiring in-line
equalization, including a pump station. Situations tending to
favor equalization include: large plants with high peaking
factors, plants with treatment in addition to simple secondary
required; and physical settings not requiring a duplicate pump
station.
231
-------
3.0
O 2,5
u.
Ul
o 2.0
o:
i
?
u
Q.
1.6
1.3
\
- -\
EQUALIZATION ^--v,
• NOT COST EFFFCTIVE
EQUALIZATION
COST EFFECTIVE
3 10
AVERAGE DRY WEATHER FLOW(MGD)
Figure 91. Regions of Cost Effective Equalization for Cost
Example, Treatment System 5
1 1
UL.
UJ
<
cc
UJ
1
o
H
i
^
-------
REFERENCES
39. Areawide Assessment Procedures Manual, Volume III, Appendix
H, U. S. EPA, MERL, ORND, EPA-600/9-76-014, July, 1976.
233
-------
TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1, REPORT NO.
EPA-60Q/2-79-096
3. RECIPIENT'S ACCESSION-NO.
4, TITUE AND SUBTITLE
Evaluation of Flow Equalization in Municipal
Wastewater Treatment Plants
5. REPORT DATE
6. PERFORMING ORGANIZATION CODE
7. AUTHORIS)
J. E. Ongerth
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Brown & Caldwell, Inc.
100 W. Harrison
Seattle, Washington 98119
10. PROGRAM ELEMENT NO.
1BC821, SOS #2. Task B.14
11. CONTRACT/GRANT NO.
12. SPONSORING AGENCY NAME AND ADDRESS
Municipal Environmental Research Laboratory, Cinti, OH
Office of Research & Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
13. TYPE OF REPORT AND PERIOD COVERED
14. SPONSORING AGENCY CODE
EPA/600/14
15, SUPPLEMENTARY NOTES
Project Officer: Francis Evans III
(513) 684-7610
16. ABSTRACT
This study was conducted to analyze the impact of flow equalization on the operation
and performance of municipal wastewater treatment plants. Objectives of the study
were: (1) establish the effects of flow equalization on plant performance; (2) sum-
marize current experience with design and operation of equalization facilities; and
(3) summarize unit costs of equalization facilities and appurtenances. A national
survey identified facilities and provided detailed information on design; operating
practices; and construction, operation, and maintenance costs. Quantitative effects
of equalization on plant performance were analyzed. Quantitative design methodology
is presented for the sizing and estimation of costs for equalization facilities.
This report was submitted in partial fulfillment of Contract No. 68-03-2512 by
Brown & Caldwell, Inc., under sponsorship of the U.S. Environmental Protection Agency.
The report covers the period from March 7, 1977 to September 7, 1978.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
SEWAGE TREATMENT: activated sludge process
trickling filters, packaged sewage plants;
SEWERS: combined sewers, sanitary sewers;
BASINS (containers); .FLOW CONTROL; FLOW.
MEASUREMENT; COSTS: cost effectiveness,
cost estimates, cost comparison, construc-
tion costs.
FLOW EQUALIZATION: in-
line, side-line; COSTS:
capital costs, operation
and maintenance costs;
SEWAGE FLOW: peak flows
flow prediction; TREAT-
13B
18. DISTRIBUTION STATEMENT
Release to Public
19. SECURITY CLASS (This Report)'
Unclassified
21. NO. OF PAGES
252
20. SECURITY CLASS (Thispage)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
234
ir US. GOVERNMENT PRINTING OFFICE : 1979 O—293-521
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