EPA-600/2-76-181
September 1976
EVALUATION OF FLOW EQUALIZATION
AT A SMALL
WASTEWATER TREATMENT PLANT
by
Gerald W. Foess
James G. Meenahan
J. Michael Harju
Johnson & Anderson, Inc.
Pontiac, Michigan 48056
Contract No. 68-03-0417
Project Officer
Ben W. Lykins, Jr.
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
LIBRARY
' ..,;.::.JAL rr.ra/noN AGr .,<
GS317
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DISCLAIMER
The Municipal Environmental Research Laboratory has reviewed this report
and approved its publication. Approval does not signify that the contents necessarily
reflect the views and policies of the U.S. Environmental Protection Agency, nor
does mention of trade names or commercial products constitute endorsement or
recommendation for use.
11
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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 it 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 prevention, 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 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.
This report provides a documentation of the operation and an evaluation
of the impact of flow equalization as it is applied to a small municipal wastewater
treatment plant.
Francis T. Mayo, Director
Municipal Environmental Research
Laboratory
111
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ABSTRACT
The primary objective of this project was to evaluate the impact of flow
equalization on the 0.092 m /sec (2.1 mgd) activated sludge plant at Walled Lake/Novi,
Michigan. Process streams were characterized for a twelve-month period under
equalized flow conditions with respect to BOD, total suspended solids and total
phosphorus. The effects of the equalization basin on final settling and filtration
were evaluated by conducting two intensive week-long studies, one with and one
without equalization of flow.
Flow equalization was effective in leveling influent flow variations but had
very little effect upon concentration leveling. Performance of the multimedia
filters was superior under the equalized flow, both in terms of average removal
efficiency and consistency.
This report was submitted in partial fulfillment of Contract Number
68-03-0417 by Johnson & Anderson, Inc. under the sponsorship of the U. S. Environ-
mental Protection Agency.
IV
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CONTENTS
Section Title Page
I CONCLUSIONS 1
II RECOMMENDATIONS 3
III INTRODUCTION 4
Need for Study 4
Scope and Objectives 4
IV PLANT DESCRIPTION 6
General 6
Design Criteria 6
Influent Characteristics 9
PI t Performance 9
Chemical Treatment 12
V EQUALIZATION SYSTEM 14
Selection of System 14
Design Considerations 16
General 16
Sizing the Basin 16
Air and Mixing Requirements 19
Pumping 21
Flow Metering 21
Operation and Control 21
VI EXPERIMENTAL PROCEDURES 23
Sampling 23
Analyses 23
Plant Operation 23
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Section Title Page
VII RESULTS AND DISCUSSION 2^
Performance of Equalization Basin 25
Flow Leveling 25
Concentration Leveling 25
Mass Leveling 25
Effects of Equalization on Influent Wastewater
Characteristics 25
Total Suspended Solids (TSS) 29
BOD5 29
Soluble Orthophosphate 29
Ammonia Nitrogen 30
Effects of Equalization on Plant Processes 30
Activated Sludge - Final Settling 30
Filtration 35
VIII COST CONSIDERATIONS 40
General 40
Equalization Basin 41
Pumping 41
Aeration 44
Chemical Feeding 45
Manpower Requirements 45
IX REFERENCES 46
VI
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LIST OF FIGURES
Number Page
IV-1 Treatment Schematic-Walled Lake/faovi, Michigan 7
V-l Schematic Flow Diagrams of Typical Equalization Systems 15
V-2 Cross-Sectional View of Walled Lake/Novi Equalization Basin 17
V-3 Typical Flow Variation - Walled Lake/Novi, Michigan 18
VII-1 Treatment Plant Flow Charts - Walled Lake/Novi, Michigan 26
VII-2 Effects of Equalization on Concentration Leveling 27
VII-3 Effects of Flow Equalization on Mass Leveling 28
VII-4 Mass Diagram - Final Clarifier Effluent 33
VII-5 Effect of Equalization on Secondary Effluent BOD,- and TSS 36
VII-6 Mass Diagram - Filter Effluent 37
VII-7 Effect of Equalization on Filter Effluent BOD^ and TSS 39
VII
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LIST OF TABLES
Number
IV-1 Summary of Design Criteria 8
IV-2 Discharge Concentration Limitations 9
IV-3 Summary of Raw Wastewater Characteristics 10
IV-4 Summary of Plant Performance ^
IV-5 Summary of Plant Operating Parameters 13
V-l Requirements for Aerating and Mixing Raw Wastewater 20
VII-1 Effects of Equalization on Wastewater Parameters 29
VII-2 Plant Performance Summary 31
VII-3 Summary of Operating Parameters During Testing Periods 32
VIII-1 Construction Cost - Walled Lake/Novi Equalization Basin ^2
VIII-2 Construction Cost - Walled Lake/Novi Wastewater
Treatment Plant ^3
Vlll
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ACKNOWLEDGEMENTS
The investigators at Johnson & Anderson, Inc., would like to express their
appreciation for the assistance and cooperation received from Mr. M. M. Corwin,
Superintendent of the Oakland County Water and Sewer Department, from personnel
at the Walled Lake/Novi treatment plant, and from Mr. David Blough who performed
the laboratory analyses.
Sincere gratitude is also expressed for the support and counsel of Mr. John M.
Smith and Mr. Ben Lykins, Municipal Environmental Research Laboratory, U. S.
Environmental Protection Agency, Cincinnati, Ohio.
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LIST OF ABBREVIATIONS
BOD 5-day biochemical oxygen demand
cf m cubic feet per minute
cm centimeter
cu cubic
ft feet
gal gallon
gpd gallons per day
gpm gallons per minute
kg kilogram
kw kilowatt
kwh kilowatt hour
1 liter
Ib pound
m meter
mm millimeter
mgd million gallons per day
mg milligram
min minute
ml milliliter
P phosphorus
SOP soluble orthophosphate
SRT solids retention time
sq square
SDI sludge density index
SVI sludge volume index
TSS total suspended solids
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SECTION I
CONCLUSIONS
1. The Walled Lake/Novi equalization system is highly effective in leveling
influent flow variations and providing a uniform flow rate to the process
units (Section VII, Page 25).
2. The Walled Lake/Novi equalization system is not effective in concentration
leveling; that is, both the raw and equalized wastewater exhibit similar
diurnal variations in strength. Some mass leveling occurs, but it is due more
to flow rate equalization than it is to wastewater blending (Section VII, Page
25).
3. For the underloaded Walled Lake/Novi secondary clarifier, peak overflow
3 2
rate = 20.4 m /m - day (500 gpd/sq ft.), variations in flow rate are apparently not
as important as other variables such as mixed-liquor settleability, wind and
density currents, etc., in determining effluent quality and percentage removal
efficiency. This was evidenced by (a) the similar removals of total suspended
solids (TSS) and BOD- that were obtained with and without equalized flow,
and (b) the lack of sensitivity of final clarifier performance to variations
in overflow rate (Section VII, Page 35).
4. Based upon intensive testing conducted during two week-long periods, filter
performance under an equalized mode was superior to performance under
a diurnal flow mode, even though the filter was operated at an average rate
3 2
of only 58.7 m /m day (1 gpm/sq ft.) during both conditions. Moreover
the better performance was achieved despite the fact that the filter influent
TSS concentration was higher and more variable during the equalized flow
period (Section VII, Page 39).
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and operating characteristics of the selected pumps (Section V,
Page 16).
5. Equalization tanks should be aerated to help maintain aerobic conditions
and to aid in conditioning the sewage for downstream processes. If sufficient
mixing is provided, sludge scraping mechanisms are unnecessary (Section
V, Page 21).
6. Manual control of diffused air flow to an equalization basin is unsatisfactory
because the variable liquid level in the tank affects the air flow rate, leading
either to inefficient air usage or to frequent adjustment of a control valve;
there is also the potential for surge or overload of the blower system if
the manual valve is not adjusted properly (Section V, Page 19).
7. Experience is necessary to accurately forecast the average daily flow rate
so that the equalization controls can be properly set. Even with experience,
adjustments may be necessary periodically to avoid overflowing or emptying
of the tank (Section V, Page 22).
8. Even if equalization is provided, it may be desirable to design chemical
feed systems with flow-proportional control to automatically compensate
for adjustments that are periodically made to the equalized flow rate (Section
IV, Page 12 and Section VIII, Page 45).
9. A theoretical analysis of power costs for pumping at the Walled Lake/Novi
Wastewater Treatment Plant led to the following conclusions:
a. Total power costs for pumping design flow with side-line flow equal-
ization would be only marginally (less than two percent) higher than
for the same plant operating without equalization.
b. Power rate schedules which penalize peak demands with a high demand
charge component may improve the cost effectiveness of flow equal-
ization (Section VIII, Page 41).
10. Based upon power usage data during the testing period, the presence of equal-
ization has very little impact on overall power consumption at the Walled Lake/Novi
Wastewater Treatment Plant (Section VIII, Page 41).
11. At Walled Lake/Novi, the presence of an equalization system does not appreciably
simplify plant operation, but neither does it add significantly to manpower
operational requirements, except for potential major repairs to pumps, controls,
or air compressors (Section VIII, Page 45 ).
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SECTION II
RECOMMENDATIONS
1. Flow equalization should always be evaluated for use upstream of granular
media filters which would otherwise be subjected to variable flows.
2. If a diffused aeration system is used in an equalization unit, preference
should be given to a centrifugal blower with an automatic control valve
tied to the basin liquid level (Conclusion 7, Page 2).
3. A diffused air input of 0.167 - 0.250 I/s/m is adequate for mixing an
equalization basin, but further studies are needed to establish the optimal
air requirement.
4. More information is needed on primary and secondary clarifier performance
under hydraulically stressed conditions with and without flow equalization.
This information is needed before the cost effectiveness of equalization
basins can be fully assessed.
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SECTION III
INTRODUCTION
Need for Study
Several benefits have been ascribed by various investigators (1,2,3,4) to
the use of flow equalization in wastewater treatment systems. The most notable
of these are the following:
1. Equalization improves performance of sedimentation basins by dampening
hydraulic variations and reducing peak flow rates; in so doing, it permits
smaller requisite clarifier sizes in new plant design or increases capacity
of existing units.
2. It benefits biological treatment by producing increased uniformity
in the concentration and flux of organics and nutrients in the waste-
water, as well as more stable retention periods in the aeration basin.
3. It simplifies manual and automated control of flow-rate dependent
operations, such as chemical feeding, disinfection, sludge pumping,
etc.
4. It improves treatability and provides some BOD reduction and odor
removal (if aeration is used for mixing in the equalization basin).
5. It provides a point of return for recycling concentrated waste streams,
thereby mitigating shock loads to primary settlers or aeration basins.
Despite the above advantages, flow equalization has not been widely applied
in the municipal pollution control field because of uncertainties in the areas of
cost effectiveness and operating problems. Additionally, there is a lack of plant
operating data to document the magnitude of beneficial effects. One pilot plant
study (5), for example, failed to show that equalization had any effect on the
24-hour composite BOD removal of an activated sludge system.
Scope and Objectives
This report describes the results of a brief study of an equalization system
at the Walled Lake/ Novi Wastewater Treatment Plant in Novi, Michigan. The
plant is a new 0.092 m /sec (2.1 mgd) tertiary facility which utilizes the activated
sludge process followed by multimedia tertiary filters to meet stringent effluent
4
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quality standards. Process streams were characterized from plant data for a
twelve-month period under equalized flow conditions with respect to BOD, total
suspended solids (TSS) and total phosphorus (TP). In addition, two intensive week-
long studies, one with and one without equalization of flow, were conducted to
determine the effects of the equalization basin on final settling and filtration
processes, and to provide data concerning performance of the equalization basin
itself.
The work reported herein developed as an offshoot of more comprehensive
studies which the authors are conducting at the Ypsilanti Township (Michigan)
Wastewater Treatment Plant. As such, this work should be considered supplemental
in nature; its limited objective is to provide additional information on full-scale
equalization systems so that the general applicability of flow equalization can
ultimately be determined.
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SECTION IV
PLANT DESCRIPTION
General
All incoming wastewater at the Walled Lake/Novi Wastewater Treatment
Plant flows into a wet well. Large solids are removed with a stationary bar screen
located at the entrance to the wet well. Process flow is pumped from the wet
well to the plant at the estimated average daily flow rate; raw wastewater flow
in excess of the average is pumped from the wet well to the equalization basin
from which point it eventually flows back to the wet well by gravity during low
flow periods. An automatic valve controls the discharge from the equalization
basin so as to maintain a uniform process pumping rate. From the grit chamber,
wastewater flows by gravity through the remainder of the plant treatment processes
which consist, respectively, of aeration, final settling, chlorination and filtration.
Alum and polyelectrolyte are added to the effluent of the aeration basins for
phosphorus removal.
Settled solids from the final settling tank are pumped to an aerobic digester.
The digested sludge is decanted, pumped to drying beds and ultimately trucked
to a landfill. Digester supernatant and filter backwash water are returned to
the influent wet well.
A schematic flow diagram of the plant is given in Figure IV-1.
Design Criteria
Plant design criteria are summarized in Table IV-1. Despite the inclusion
of flow equalization in the system, conservative process design loadings were
required by the regulatory agency to help ensure that stringent effluent standards
would be met .
It was known that initial flows at the plant would be small in relation to
the design value. Hence, the aeration basins and final settlers were designed
in three independent modular units to be constructed in stages.
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TABLE IV-1
SUMMARY OF DESIGN CRITERIA
WALLED LAKE/NOVI, MICHIGAN
Value
Item
Average Dry-Weather Flow
BOD Loading
TSS Loading
Equalization Basin (1 unit)
Volume
Maximum Water Depth
Applied Air
Aeration Basins
BOD Loading
(3 units)*
Volume per Unit
Detention Time
Flow Regime - Complete Mix
Final Settling Basins (3 units)*
Diameter
Overflow Rate at Average Flow
Depth
Multimedia Filters (4 units)**
Area per Unit
Loading Rate at Average Flow
Depth of Media
Media Composition (% by weight)
4.2 sp. gr. Material
2.6 sp. gr. Material
1.5 sp. gr. Material
Depth of Support Gravel
High Density Gravel
Silica Gravel
Backwash Rate
Rotary Surface Wash Rate
Metric Units
0.092 m3/sec
1630 kg/day
1905 kg/day
1,275.5 m3
4.57 m
0.015m3/min/m3
0.57 kg/m3 day
961 m3
8.7 hours
15.24m
14.7m3/m2 day
4.57m
18.2 m2
109.8 m3/m2 day
762 mm
15
30
55
76.2 mm
304.8 mm
880 m3/m2 day
41.1 m3/m2 day
English Units
2.1 mgd
3600 Ib/day
4200 Ib/day
337,000 gal
15 ft
15 cfm/1000
cu ft of basin volume
35 lb/1000
cu ft/day
254,000 gal
8.7 hours
50 ft
360 gpd/sq ft
15 ft
196 sq ft
1.86 gpm/sq ft
30 in
15
30
55
3 in
12 in
15 gpm/sq ft
0.7 gpm/sq ft
* Only two of the three aeration and settling units have been constructed
and only one unit was in use during the study.
** Only two of the four filter units were operated during the study.
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During the study, only one of the modular units was in use. Similarly, only two
of four filters were employed. Actual process loadings obtained during the study
period are discussed later in the report.
The effluent standards to be met by the plant, as given by the plant's NPDES
Permit, are summarized in Table IV-2 below:
TABLE IV-2
DISCHARGE CONCENTRATION LIMITATIONS
WALLED LAKE/NOVI, MICHIGAN Daily
Parameter 30-Day Average 7-Day Average Maximum
BOD5(mg/l) - - 10
TSS (mg/1) 10 15
NH3-N (mg/1) -- -- 2.0
Total P 20% of Influent
Cone.
Fecal Coliform 200/100 ml 400/100 ml
The stringent requirements for suspended solids removal are what dictated
the need for filtration of secondary effluent at the plant. Equalization was provided
principally to ensure a constant flow through the gravity filters; additional factors
related to selection of equalization are discussed in Section V.
Influent Characteristics
Walled Lake/Novi wastewater is primarily of domestic origin. A summary
of flow and strength variations over the 14-month period ending February, 1975,
is given in Table IV-3. Observation of the table reveals that flows were higher
during the months of January through May, indicating an infiltration/inflow problem.
However, there is also evidence that the City's infiltration/inflow correction program
is producing results. For example, peak flows during January and February, 1975,
were lower than during the corresponding period in 1974 despite an increased
service population. Moreover, sewage strength increased significantly during
the period, reflecting the reduced extraneous flows.
Plant Performance
Part of this study involved compilation and examination of existing plant
data to gain insight into the effects of equalization on plant processes. Some
of the observations from this examination are presented in this section. The results
of the detailed testing are presented in a later section.
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TABLE IV-3
SUMMARY OF RAW WASTEWATER CHARACTERISTICS
WALLED LAKE/NOVI, MICHIGAN
Month
January, 1974
February
March
April
May
June
July
August
September
October
November
December
January, 1975
February
Average Daily
Flow
(mgd)*
0.875
0.879
1.088
1.002
0.889
0.686
0.637
0.609
0.620
0.586
0.593
0.624
0.820
0.803
Peak 24-Hr.
Flow
(mgd)*
1.522
1.949
1.838
1.287
1.199
0.808
0.697
0.715
0.644
0.644
0.715
1.154
1.470
Avere
BOD,
(mg/1)
175
141
116
127
162
185
225
217
236
297
399
310
305
290
ige Concentre
TSS
(mg/1)
213
161
135
119
149
129
168
203
189
205
184
205
170
194
itions
Total-P
(rr,K/l)
8.4
6.5
5.2
5.9
6.1
7.3
8.0
10.0
10.3
10.3
10.2
91.
.4
8.0
7.5
* mgd x 0.04381 = m /sec
A summary of plant influent and performance data for the 12-month study
period is given in Table IV-4. During this time the plant was operated under equalized
flow conditions, with the exception of the one week testing program October 28
through November 3, 1974, when the equalization tank was not used. The data
indicate that both clarifier and filter performance were uniformly excellent through-
out the period, except for January and February when performance temporarily
deteriorated. Data which became available after the study period showed that
plant performance had again improved to earlier levels.
Closer observation of Table IV-4 reveals that removal efficiencies through
secondary clarification averaged 95, 94 and 82 percent for BOD TSS and P,
respectively. Removal efficiencies through final filtration averaged 97 percent
for BOD^ and TSS, and 84 percent for P. The latter figures indicate that the
multimedia filter produced a very marginal improvement in effluent quality.
Based upon the material that was applied to them, the filters removed only 39
percent of the BOD., 50 percent of the TSS and 13 percent of the applied TP.
However, the filter effluent quality was very similar to results reported in the
literature (6).
In interpreting the preceding results, it is instructive to examine the process
loadings and other operating parameters during the period. These values are sum-
10
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marized in Table IV-5 for a 12-month period. It is evident that all of the process
units operated well within conventional design loadings. It may be that the performances
of treatment processes at such low loadings are not sensitive to diurnal flow varia-
tions and that plant performance would have been equally high without equalization.
Thus, it is not possible to attribute the exceptional plant performance solely to
the beneficial effects of equalization.
Chemical Treatment
This plant is equipped with a chemical feed system for phosphorus removal.
The system was originally designed for flexible operation as follows:
1. Application of ferrous chloride to the aerated grit chamber;
2. Application of lime to the splitting chamber, located just upstream
from the aeration basins;
3. Application of polyelectrolyte either upstream of final sedimentation
or prior to filtration as a filter aid.
After a period of experimentation, it was decided to use liquid alum rather
than ferrous chloride as the primary coagulant for economic reasons. In addition,
the point of alum application was changed to the effluent trough of the aeration
basin, presumably to alleviate some foaming problems which had occurred in the
aeration basin. Application of the lime and polyelectrolyte has not been needed
and has not been carried out except for the initial trial.
All chemical feed pumps have variable speed drive and, despite the equalized
flow, are equipped with an automatic control system which paces the dosage to
flow rate. This feature eliminates the need for manual readjustment of the alum
feed rate when the equalized process flow rate is changed.
An A1:P weight ratio of about 1.6:1 is used to provide requisite phosphorus
removal. In order to maintain the proper ratio, the alum dosage is regulated based
on the influent phosphorus concentration.
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SECTION V
EQUALIZATION SYSTEM
Selection of System
Flow equalization was used at the Walled Lake/Novi Wastewater Treatment
Plant for the following principal reasons: (i) it was considered that constant rate
filtration would provide a superior effluent to filtration under diurnally varying
rates, for the same filter area; (ii) it was determined that the cost of providing
flow equalization would be nearly offset by the savings involved in sizing the filters
based on average, rather than peak, flow rates; (iii) it was felt, based upon information
available at the time, that equalization would have a beneficial effect upon the
activated sludge and final settling processes.
It was also decided to use a "side-line" equalization system for this plant,
as opposed to the "in-line" type. In the in-line design, all of the flow passes through
the equalization basin, whereas in the side-line scheme, only that amount of flow
above the average daily flow is diverted through the equalization basin. This basic
difference is illustrated schematically in Figure V-l,which also shows the gravity and
pumped alternatives which may be used for either in-line or side-line systems, depending
on plant hydraulics, site topography and subsurface conditions.
Side-line systems are generally, but not exclusively, applicable where flow
through the plant is by gravity. In this instance, the smaller pumping requirements
of the side-line scheme (i.e., only flows above the average daily flow) usually makes
it more economical. On the other hand, if a raw sewage pumping station is required at
the site anyway, an in-line system would minimize the flow to be pumped; in this
case, the basin can be inserted after the pumping station and have gravity discharge
to the process units, obviating the need for separate equalization pumps. However,
the raw sewage pump motors would have to be sized to account for the additional
head loss across the basin.
14
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K-0 (INFLUENT)
PRE- OVERFLOW
TREATMENT ~* STRUCTURE
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DEVICE
A) PUMPED FLOW FROM
EQUALIZATION BASIN.
B) GRAVITY FLOW FROM
EQUALIZATION BASIN.
IN-LINE EQUALIZATION
FIGURE V-l. SCHEMATIC FLOW DIAGRAMS OF TYPICAL EQUALIZATION SYSTEMS
15
-------
There are, of course, individual circumstances at each plant site which may
dictate selection of an equalization system apart from the generalizations mentioned
above. At Walled Lake/Novi, for example, a side-line system was selected even
though a raw sewage pumping station was required at the site. This selection
was made because analysis showed that overall pumping costs would be minimized
with the side-line system (even though the total amount of flow pumped would
be greater). In evaluating these pumping costs, the considerations were as follows:
1. In-line system: Pump incoming diurnal flows to the equalization basin
against a head of about 21.3 m (70 ft), with gravity discharge from
the basin to the process units.
2. Side-line system: Pump incoming flows above the average daily flow to the
equalization basin at a head of 11.6 m (38 ft), with gravity discharge
back to the wet well; pump incoming flows at the average daily rate to
the process units at an average head of 54 feet.
Another factor involved in selecting the type of equalization system to be
used is the degree of concentration dampening desired. In-line systems are more
effective for leveling variations in concentration because the entire flow passes
through the basin.
Design Considerations
General. A simplified cross-sectional view of the Walled Lake/Novi equaliza-
tion basin is shown in Figure V-2. It may be seen that the tank is equipped
with a diffused air mixing system and a sludge scraping mechanism. Considera-
tions involved in tank sizing, mixing and pumping are discussed in the following
paragraphs.
Sizing the Basin. The storage volume required for flow equalization is equivalent
to the cumulative flow above average occurring during the diurnal cycle. Various
methods for determining this volume have been described elsewhere (2, 4) and
will not be repeated here.
Figure V-3 illustrates a typical projected diurnal flow pattern at Walled
Lake/Novi and the method of computing the requisite equalization basin volume.
The computed storage volume of 1,230,125 1 (325,000 gal) represents 15.4 percent
of the design flow of 0.092 m /sec (2.1 mgd). As a rule of thumb, the authors
have found that equalization basins for plants in the 0.04-0.22 m /sec (1-5 mgd)
size range can be sized for 15 percent of the average daily volume flow, which
16
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FIGURE V-3. TYPICAL FLOW VARIATION - WALLED LAKE/NOVI, MICHIGAN
(mgd x 0.04381 = m3/sec; gal x 0.003785 = m3)
-------
is also the value used by Smith et al. (7). The correct volume will, of course,
depend upon individual diurnal flow patterns which vary with population and other
factors.
As constructed, the actual volume of the Walled Lake/Novi equalization
basin at the maximum water depth is about 1,275.5 m (337,000 gal). The nominal
working volume is about 1,192 m (315,000 gal) or 15 percent of the design dry-
weather flow.
Air and Mixing Requirements. Diffused or mechanical aeration systems
should be provided in an equalization basin to prevent septic conditions from developing.
Aeration can also serve the dual purpose of blending the contents of the tank
and preventing solids deposition, if these are desired objectives.
The minimum level of aeration required to prevent septicity and/or to provide
complete mixing for an equalization basin is not well established. Table V-l gives
some values for sewage aeration and mixing requirements as derived from the
literature. However, only reference (4) refers specifically to equalization basin
requirements. The authors' recent experience with the Ypsilanti, Michigan equalization
basin indicates that an air input of about 1.4 cfm/1000 gal of basin capacity
(0.0104 m /min/m3) is adequate.
The Walled Lake/Novi aeration system was designed to provide 2 cfm of air
per 1000 gallons of storage (0.015 m /min/m ) to the equalization tank. Based
on a maximum nominal storage volume of 1,192 m (315,000 gal) the air requirement
is 17.8 m /min (630 cfm). Unfortunately, actual air input to the basin is not monitored
separately and thus is not known.
Common compressors are used at the plant to serve not only the equalization
basin but all other air-using processes at the plant. Control of air flow to the
basin is accomplished by a manually operated valve. This system is not recommended
since varying liquid levels in the equalization basin affect the air flow rate and
frequent manual adjustment of the control valve is required to maintain a constant
flow rate. If the control valve is not adjusted properly, air usage will be inefficient
and there is the possibility of surge or overload of the blower system. Alternatively,
an automatic control valve, which is regulated by the liquid level in the basin,
can be used. Still another possibility is the use of a separate positive displacement
air compressor for the equalization tank.
19
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TABLE V-l
REQUIREMENTS FOR AERATING AND MIXING RAW WASTEWATER
To Maintain
Aerobic Conditions
1.25 to 2 cfm/1,000 gal.
of storage
0.006 to 0.01 HP/1,000 gal.
of storage*
5 to 8 mg O?/l-hr. (.04 to
.06 cfm/1,000 gal. of
storage)
To Prevent
Solids Deposition
0.02 to 0.04 HP/1,000 gal.
of storage
0.02 to 0.03 HP/1,000 gal.
of storage*
0.05 to 0.20 ft3/gal. of
storage
0.07 to 0.16 ffVgal.of
wastewater
0.1 to 0.4 ft3/gal. of
wastewater
20 cfm/1,000 cu ft of storage*
Reference
4
8
9
10
11
12
7
* These values pertain to mixed-liquor in an aerated pond rather than to
raw wastewater.
Notes: (1) cfm/1,000 gal x 0.0074 = m3/min/m3
(2) HP/1,000 gal x 0.1975 = kw/m3
(3) ft3/gal x 0.0074 = m3/!
(4) cfm/1,000 cu ft x 0.001 = m3/min/m3
20
-------
It may be observed in Figure V-2 that a sludge scraping mechanism was provided
in the Walled Lake/Novi equalization basin. Sludge collection is not necessary
if sufficient mixing is provided to prevent solids deposition.
Pumping. Pumping considerations are extremely important in the economic
design of a flow equalization system. As discussed earlier in this section, pumping
may dictate the choice of the type of system to be used.
The Walled Lake/Novi influent puming station houses six raw wastewater
pumps which include three process pumps and three equalization pumps. The
process pumps are designed to elevate wastewater at the average daily flow rate;
two variable-speed pumps and one constant-speed (stand-by) pump, each with
a capacity of 47.3 I/sec (750 gpm), are provided. The equalization pumps are
each of 75.7 I/sec (1200 gpm) capacity, with two being variable speed and one
constant speed (stand-by pump). These pumps are sized to elevate wastewater
in excess of the average daily flow rate. With two equalization pumps in operation,
up to 151.4 I/sec (2400 gpm) can be pumped to the equalization basin, which
is the estimated difference between the average and maximum daily rates of flow.
Flow Metering. Inclusion of equalization facilities in a plant may require
additional flow metering for control purposes. Whether or not additional metering
is required will depend on the particular method used to control flow rate to (or
from) the equalizaton basin. The Walled Lake/Novi plant has three magnetic
flow meters, whose functions are as follows:
1. A 254 mm (10-in.) meter measures the process flow; its function is
to indicate that the proper (preset) flow, equal to the average daily
flow, is maintained by the process pumps that pump from the influent
wet well to the grit chamber.
2. A 304.8 mm (12-in.) meter measures flows which are in excess of the preset
flow, such flows being pumped to the equalization tank; this meter has no
control function in this plant.
3. A 203.2 mm (8-in.) meter measures the flow from the equalization
tank back to the wet well; it is tied to a control valve and functions
to compensate for inflow deficiency during periods when the incoming
flow is less than average.
Operation and Control
Operation and control of the Walled Lake/Novi equalization facility may
be described as follows:
21
-------
Each morning the average flow expected over the following 24 hours is estimated
and process pumps are preset to maintain this rate. When the flow increases,
a controller starts an equalization pump to pump the excess flow to the equaliza-
tion basin.
When incoming flow diminshes to below average, the effluent control valve
opens and releases an amount of water from the equalization basin to the wet
well, to compensate for the deficiency of the influent. Even when shut, the control
valve allows a small portion of liquid to bleed constantly to the wet well to help
prevent accumulation of solids on the bottom of the tank.
The control system has functioned well since plant start-up and is capable
of maintaining a virtually constant process flow rate. Initially, the operators
had difficulty forecasting the average daily flow rate, necessitating overly-frequent
and excessively-large adjustments to the process pumping rate. Such adjustments,
of course, defeat the purpose of providing flow equalization. As experience has
been gained, however, this problem has been largely eliminated.
22
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SECTION VI
EXPERIMENTAL PROCEDURES
Sampling
The experimental program was designed to accomplish the following project
objectives: (i) evaluate the performance of the equalization tank with respect
to flow and mass leveling and (ii) compare the performance of the plant under
equalized and non-equalized conditions.
Budgetary restrictions limited the monitoring program to two weeks' duration
in the Fall of 1974*. During the first week (September 30 to October 7) the plant
was operated under equalized flow conditions, while during the second week (October 28
to November 3) the equalization basin was removed from service and normal diurnal
flow conditions prevailed.
Samples were collected every two hours from the raw wastewater, process
influent (i.e., following equalization), secondary clarifier effluent, and filter effluent.
All samples were refrigerated upon collection and analyzed the following day.
Analyses
Samples were analyzed for BODc, TSS, SOP, and NH,-N. All analyses were
performed in accordance with the 13th Edition of Standard Methods. Dissolved
orthophosphate analyses were performed using the automated colorimetric ascorbic
acid reduction method, while ammonia nitrogen measurements utilized the automated
colorimetric phenate method.
Plant Operation
Prior to the start of the equalized flow testing week, the equalization tank
was emptied to maximize the available storage volume. Thereafter, no further
adjustments were made to the flow rate control system and the tank level was
permitted to fluctuate freely. In this way, a virtually constant flow rate was maintained
throughout the 7-day testing period.
* Another week of intensive sampling was also conducted during the Summer
of 1974; however, because of operational difficulties, the data have not been analyzed.
23
-------
The multimedia filters were backwashed just prior to the start of each testing
period. No further backwashing was required during the duration of either the
equalized or the unequalized testing periods.
24
-------
SECTION VII
RESULTS AND DISCUSSION
Performance of Equalization Basin
Flow Leveling. The Walled Lake/Novi equalization system is highly effective
in leveling influent flow variations and providing a uniform flow rate to the process
units. Its effectiveness is documented by the plant flow charts; a smooth flow
curve is generated which, with proper system operation, typically shows a constant
flow rate throughout each day. Figure VII-1 gives representative portions of
actual plant flow charts from the testing period.
Concentration Leveling. Data illustrating concentration leveling effected
by the flow equalization system are presented in Figure VII-2. Concentrations
of various constituents are plotted as a function of time for both the raw wastewater
and the process (equalized) flow to the aeration basin. As may be noted, equalization
had marginal impact on the concentration of these wastewater parameters. Some
dampening of BOD5 and TSS concentrations is evident but, in general, both the raw
and equalized wastewater exhibited similar diurnal variations in strength.
In a side-line equalization system, concentration leveling is expected to
occur only during low-flow periods when the tank is emptying its stored contents.
At such times, the constituent concentrations in the released volume are likely
to be higher than in the raw wastewater, thereby raising the concentration in
the mixture. Conversely, during peak flow periods, very little flow is released
from the equalization basin and concentration damping cannot occur.
Mass Leveling. Side-line flow equalizaton necessarily accomplishes some
mass leveling even though it is fairly ineffective in concentration leveling. The
mass leveling occurs primarily because the flow rate is equalized, not because
any blending occurs. Figure VII-3 illustrates the mass leveling that occurred during
the testing period.
Effects of Equalization on Influent Wastewater Characteristics
It was not within the scope of the study to adequately characterize the changes
in wastewater characteristics produced by biological action in the equalization
25
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tank. The limited available data are summarized in Table VII-1, which is essentially
a mass balance for each of the constituents studies. The individual parameters
are discussed in the following paragraphs.
TABLE VIM
EFFECTS OF EQUALIZATION ON WASTEWATER PARAMETERS
Average Content During
Testing Period (Ibs/day)*
KawTUnequalizedr Process (Equalized)
Parameter Wastewater Wastewater % Change
TSS 1610 1420 -11.8
BOD. 1080 970 -10.2
36.3 36.9 +1.7
NH3-N 125 119 -4.8
^Testing Period was 9/30; 10/1, 2, 3, 4, 5, 7 (1974).
Ib/day x 0.4536 = kg/day.
Total Suspended Solids (TSS). The data indicate that the mass of TSS in
the equalized wastewater was 11.8 percent less than that in the raw Wastewater
over the testing period. A reduction in TSS is opposite to what would be expected
if growth of biomass occurred in the equalization basin. It is likely that solids
deposition occurred in the basin resulting in some accumulation of sludge. While
an attempt was made to avoid sludge accumulation by continuously bleeding a
small amount of flow from the bottom of the basin, there was no way of knowing
how successful this operation was. Unfortunately, long-term data are not available
because the equalized flow is not sampled by treatment plant personnel. We believe
such long-term data is necessary before meaningful conclusions can be drawn.
Biochemical Oxygen Demand. There was an apparent 10.2 percent overall
BODc reduction effected by the equalization tank during the 7-day testing period.
This percentage is higher than would be expected for a side-line equalization system
in which only about 15 percent of the flow is diverted through the equalization
basin. Most of this reduction can probably also be attributed to deposition and
accumulation of solids in the basin.
Soluble Orthophosphate (SOP). Over the 7-day period, the mass flow of
SOP for the equalized wastewater was almost equal to that for the raw wastewater.
29
-------
Because any change in SOP would be expected to be small, a longer sampling period
would be needed to establish a definitive trend.
NH-a-N. The data show that there was an average overall reduction in ammonia
nitrogen, but the change was generally negligible,
Effects of Equalization on Plant Processes
To determine the effects of equalization on plant treatment processes, intensive
testing was conducted on each process stream with and without flow equalization.
Table VII-2 presents a tabular summary of the results, which are discussed in detail
below, and Table VII-3 lists values for some of the pertinent operating parameters
for the study periods.
From Tables VII-2 and VII-3, some general observations can be made regarding
the experimental conditions that prevailed during the two week-long testing periods.
The average plant flow rates were virtually identical during each testing period
and the plant influent characteristics were likewise very similar, except that
the TSS concentration was 32 percent (80 mg/1) higher during the equalized week.
Aeration basin operating parameters were also very similar, except that the organic
loading (F/M ratio)was somewhat higher during non-equalized operation. Hydraulic
loadings for final clarification and filtration, respectively, were essentially the
same during the two testing periods; moreover, both of these processes were under-
loaded.
Activated Sludge - Final Settling. The mean performance values presented
in Table VII-2 reveal that secondary clarif ier effluent quality with respect to TSS content
was actually somewhat poorer during equalized flow than it was during the week
in which diurnal flow variations prevailed. For the other parameters, the secondary
clarif ier effluent quality was slightly better during the equalized flow period. These
differences can be more clearly observed in the graphical presentation of data in
Figure VII -4, in which cumulative mass in the clarif ier effluent during the testing
periods is plotted as a function of time for each of the parameters studied. Close
observation of the curves in Figure VII-4 show the following:
1. TSS - Over the respective 7-day periods, the total poundage of TSS
that escaped in the final clarif ier effluent during equalized flow was
240 kg (530 Ib) versus only 133 kg (293 Ib) during non-equalized flow.
Considering the low clarifier overflow rates in effect during the study
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TABLE VI1-3
SUMMARY OF OPERATING PARAMETERS
DURING TESTING PERIODS
WALLED LAKE/NOVI, MICHIGAN
1 2
Equalized Non-Equalized
Operation Operation
Aeration
Flow Rate (mgd) 0.574 0.585
MLSS (mg/1) 2,412 2,271
MLVSS (mg/1) 1,513 1,479
SDI 2.6 2.2
F/M 0.40 0.52
Sludge Age3(days) 6.8 5.1
D.O. in Tanks (mg/1) 4.5 3.7
Detention Time without recycle 10.6 10.4
(hrs.)
Fjnal Settling
Overflow Rate (gpd/sq ft) 292 298
(mVm9- day) 11.9 12.1
Filtration
Filter Rate (gpm/sq.ft.) 1.0 1.0
(m3/m2 day) 58.7 58.7
19/30; 10/1, 3, 4, 5, 6, 7 (1974) .
210/28, 29, 30, 31; 11/1, 2, 3 (1974).
O
^Computed as shown in Table IV-5
32
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DAY I DAY 2 DAY 3 DAY 4 DAY 5 DAY 6 DAY 7
FIGURE VI1-4.
MASS DIAGRAM - FINAL CLARIFIER EFFLUENT
(Ibs x 0.4536 = kg)
33
-------
periods, this difference is considered to be due to factors such as minor
differences in sludge settleability, lack of adequate control of sludge
blanket height in the final clarifier, wind and density currents, etc.
Based upon percentage removal, performance was actually very similar
for the two periods (95.3 and 96.5 percent for the equalized and non-
equalized flow periods, respectively).
2. BOD5 - 166 kg (366 Ib) of BOD5 escaped in the secondary effluent
during the equalized flow period as compared to 211 kg (466 Ib) during
the non-equalized flow period. It should be noted that BOD,- removal
was better during equalized flow even though TSS removal was worse
during the same period. This anomalous result indicates that the solids
escaping in the clarifier effluent during equalized flow were either
more stabilized due to lower F^M ratios or were of an inorganic nature.
3. SOP - With respect to SOP, 18 kg (39 Ib) carried over in the clarifier
effluent during the equalized flow period versus 21 kg (46 Ib) during
unequalized flow. Thus, soluble phosphorus removal was only marginally
better under equalized flow conditions, a result which is not unexpected
with a side-line flow equalization system which does not produce con-
centration dampening. This can be better understood by considering
what takes place when Al (or Fe) is applied in equalized versus unequalized
systems (assuming that in both cases chemical feeders are paced to
flow for dosage control):
a. Under diurnal flow conditions, the mass flow rate of applied
Al will vary diurnally, but the concentration of Al applied will
be constant through the day; this means that the weight ratio
of A1:P will necessarily vary diurnally since the P concentration
in the wastewater varies diurnally.
b. Under equalized flow conditions, the mass flow rate of applied
Al will be constant through the day as will the concentration
of Al applied to the wastewater; however, the weight ratio of
A1:P will vary diurnally just as in the unequalized case.
Thus, under either equalized or unequalized flow conditions, the A1:P
ratio will vary as the P concentration in the wastewater varies.
4. NH3-N - The curve shows that 10 kg (21 Ib) of NH3~N escaped in
the secondary effluent during equalized flow conditions as contrasted
-------
to 28 kg (62 Ib) during the non-equalized flow period. Expressed as
percentage removal, the results were 97.6 and 93.5 percent, respectively.
These high NH,-N removals in the plant are presumably due to the
use of a long solids retention time (SRT) although the data did not
permit computation of SRT's. The greater NH^-N removal during
equalized flow may be due to a greater SRT rather than to the effects
of equalization.
The preceding discussion is concerned primarily with comparing the average
values obtained during equalized flow with those obtained under non-equalized
conditions. It is also interesting to observe the "scatter" about the means. Figure
VII-5 shows frequency histograms for bi-hourly samples taken on secondary effluent
during the testing periods. These histograms indicate that TSS removal was not
only poorer but also showed greater variance under equalized flow conditions;
however, BOD,, removal was slightly better and somewhat more consistent with
equalized flow.
Ail of the preceding discussion suggests that, for the underloaded final clarifier,
variations in flow rate were not as important as other variables in determining removal
efficiency. Accordingly, TSS concentrations in grab samples of secondary effluent
did not correlate very well with the clarifier overflow rate at the time of sampling,
for samples collected under diurnal flow conditions. Thus, the sensitivity of the
activated sludge-final settling processes to flow rate variations may not become
apparent until they are stressed to higher loadings than occurred in this study.
Prior to the point at which a secondary clarifier is hydraulically stressed, factors
such as solids loading, wind and density currents, and bio-floe settleability may
dictate performance; settleability, in turn, is dependent on F/M, aeration basin
agitation, chemical dose (for P precipitation) and other factors.
Filtration. The data summary in Table VII-2 shows that filter effluent quality
was better under equalized flow conditions for each of the parameters studied.
Moreover, the higher TSS removal was achieved even though the filter influent
TSS concentration was higher and more variable during the equalized flow period.
A further comparison of filter performance under equalized and non-equalized
flow conditions is illustrated graphically in Figure VII-6. The curves for each
parameter are discussed individually below.
1. TSS - The total poundage of TSS that escaped in the filter effluent
during the non-equalized flow period was 93 kg (205 Ib) as opposed
35
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-AVERAGE2 9 (NON-EQUALIZED)
AVERAGE= 17 (EQUALIZED)
0 4 8 12 16 20 24 28 32 36
SECONDARY EFFLUENT TSS (mg/l)
AVERAGE2 10 (EQUALIZED)
AVERAGE = 14 (NON- EQUALIZED)
4 8 12 16 20 24 28 32 36
SECONDARY EFFLUENT BOD 5 (mg/l)
FIGURE VII-5. EFFECT OF EQUALIZATION ON SECONDARY EFFLUENT
BOD5 AND TSS
36
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CO
CO
o
QQ
LJ
>
< Q.
O
CO
ID
O
300
200
100
0
300
200
100
0
60
40
20
0
60
rO
x
20
0
O EQUALIZED FLOW
D NON-EQUALIZED FLOW
9 AM 9 AM 9 AM 9 AM 9 AM 9 AM 9 AM 9 AM
DAY I DAY 2 DAY 3 DAY 4 DAY 5 DAY 6 DAY 7
FIGURE VII-6.
37
MASS DIAGRAM - FILTER
EFFLUENT (Ibs x 0.4536 = kg)
-------
to only 51 kg (112 Ib) during equalized flow. The higher performance
under equalized flow is even more apparent when the results are compared
on a percentage removal basis. During equalized flow, 82 percent
of the TSS was removed across the filters versus only 33 percent during
the non-equalized flow period.
2. BOD^ - 60 kg (132 Ib) of BOD- carried through the filters during the
equalized flow period as compared to 103 kg (226 Ib) during the non-
equalized flow period. These values represented 61 and 52 percent
removal, respectively.
3. SOP - During equalized operation, 20 kg (43 Ib) of SOP penetrated the
filters, while the figure was 23 kg (50 Ib) under diurnal flow conditions.
Both of these values are slightly higher than the respective filter
influent content of SOP, but the change is insignificant. Also, the
monthly plant data, summarized previously in Table IV-4 shows that
there is an overall reduction in total P across the filters.
4. NH3-N - The data show that 8 kg (18 Ib) of NH3-N escaped in the
filter effluent during equalized flow conditions in contrast to 20 kg
(45 Ib) during the non-equalized flow period. There was very marginal
removal across the filter in both cases.
To observe the scatter in the filter effluent data, frequency histograms
were again constructed from the bi-hourly TSS and BOD5 data. The results are
depicted in Figure VII-7 which shows that filter performance was somewhat better
under equalized operation, both in terms of average removal efficiency and consistency.
The results of these experiments tend to support the generally held belief
that granular media filters should be operated at a constant rate. However, the
filters in these studies were run at the low average rate of only 58.7 m /m day
(1 gpm/sq ft), thus making it questionable whether the better performance under
equalized flow conditions can be attributed solely to the benefits of steady flow.
Further work is needed to evaluate the effects of transient hydraulic and solids
loading on filter performance. It is possible, for example, that some particles
removed during low flow-low shear conditions in a filter may be dislodged as the
flow rate increases, even at low average filter rates.
38
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-AVERAGE= 3 (EQUALIZED)
>
-AVERAGE = 6 (UNEQUALIZED)
4 6 8 10 12 14 16
FILTER EFFLUENT TSS (mg/l)
AVERAGE = 4 (EQUALIZED)
-AVERAGE2 7 (UNEQUALIZED)
4 6 8 10 12 14
FILTER EFFLUENT BOD (mg/l)
FIGURE VII-7. EFFECT OF EQUALIZATION ON FILTER EFFLUENT BOD5 AND TSS
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SECTION VIII
COST CONSIDERATIONS
General
The major cost components of an equalization system are those of the basin,
mechanical or diffused aeration system, pumping, and flow measurement. The
economic basis for bearing the additional costs of these components rests upon
the presumption that treatment units are indeed affected by diurnal flow variations.
If this presumption is true, then cost savings can result by providing equalization
and designing process units based on the average flow instead of the peak flow.*
The results of this limited research program have shown that BOD- and
suspended solids removals through the final settling process were not significantly
different with and without equalization, while the final settler was underloaded.
From the discussion in Section VII, any differences between equalized and unequalized
flow (through final clarification) appeared to be explainable in terms of factors
other than just the presence or absence of flow rate variations. However, it must
be emphasized again that the aeration basins and final settler at the Walled Lake/Novi
Wastewater Treatment Plant operated well within their design capacity throughout
the study. The maximum 2-hour overflow rate on the final clarifier, for example,
was less than 20 m /m day (500 gpd/sq ft) during the testing period. It is to
be expected that an underloaded clarifier would not be very sensitive to flow-
rate variations. If the clarifier had been stressed, the effect of diurnal variations
in flow might well have been significant.
On the other hand, filter performance in this study seemed to react
to flow rate variations. This result occurred even though the filters were not
stressed, indicating that the absolute value of the filter rate may perhaps be even
*It is not always recognized that diurnal flow variations are implicitly incorporated
into most of the loading factors used to size treatment units. Hence, the design
is actually based on the peak flow even though the average flow is used in the
computation.
40
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less important than whether or not there are variations in rate. Further research
is warranted to evaluate the effects of diurnal hydraulic and solids loadings on
filter performance.
Equalization Basin
The actual construction cost of the Walled Lake/Novi equalization basin
is given in Table VIII-1; construction costs for the wastewater treatment plant
as a whole are presented in Table VIII-2. It may be seen that the cost of the equaliza-
tion tank and appurtenances was $143,000 (EPA Index 172). This amount includes
$40,000 for sludge scraping equipment which has proven unnecessary. If the
cost of sludge scraping is excluded, the equalization system comprised between
6 and 7 percent of the total construction cost of this tertiary treatment plant.
Pumping
Plant pumping costs, including both the initial cost of the pumps and their
operating costs, will usually be higher if flow equalization is provided. However,
the difference may not be as great as imagined because: (i) constant flow conditions
reduce peak pumping demands and, hence, the requisite size of motors; and (ii)
pumps designed for constant flow can operate continually at a more efficient
point on their operating curve.
We have computed, for purposes of illustration, the theoretical power costs
for pumping at the Walled Lake/Novi plant with and without flow equalization.
The following assumptions were made:
1. Plant flow is at the design rate of 2.1 mgd, with diurnal variations
as shown in Figure V-3.
2. Power costs are $0.0124/kwh for the energy charge and $3.85/kw-month
for the demand charge (based on January 1975 rate schedule).
3. The combined pump and motor efficiency is 0.6 and is uniform at all
pumping rates.
Under equalized flow, it was found that the peak hourly power demand for
pumping would be 33.4 kw and the total daily energy requirement would be 647 kwh,
resulting in a daily power cost of $12.31. With the same flow, but without equalization,
the computed daily power cost is $12.10/day, based upon a peak hourly demand
of 37 kw and a daily energy requirement of 594 kwh. Thus, the additional power
cost due to equalization is only $0.21/day or less than $80.00 per year.
It is concluded that power rate schedules which penalize peak demands with
a high demand charge may improve the cost effectiveness of flow equalization.
41
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TABLE VIII-1
CONSTRUCTION COST
WALLED LAKE/NOVI EQUALIZATION BASIN*
Item .Cost.
Excavation $10,000
Mud Mat 2,000
Foundations 7,000
Slab 6,000
Form Walls 20,000
Pour Walls 6,000
Grout 2,000
Pumps, Motors & Controls 28,000
Instrumentation 15,000
Air Piping & Compressor 3,000
Exterior Piping 4,000
Sludge Scraping _40,000
TOTAL (with sludge scraping) $143,000
&&
TOTAL (without sludge scraping) $103,000
* Based on final construction cost breakdown; does not include land
costs, engineering fees, or interest during construction. The bid
opening was in January, 1971 ; construction was completed in mid 1972
(EPA Index 172).
** Sludge scraping equipment is not needed if sufficient mixing is
provided.
Note: Basin Storage Volume = 1,275.5 m3 (337,000 gal).
42
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TABLE VII1-2
CONSTRUCTION COST
WALLED LAKE/NOVI WASTEWATER TREATMENT PLANT*
Item Cost
Move-in & Cleanup $ 12,000
Comminutor 7,000
Process Equipment, Treatment Tanks &
Chlorine Contact Chamber 431,000
Influent Pumping Station 187,000
Equalization Tank** 93,000
Grit Chamber 23,000
Filters & Filter Building 265,000
Sludge Drying Beds 94,000
Process Piping, Fittings & Valves 152,000
Pumps, Motors & Controls 58,000
Instrumentation 83,000
Electrical 112,000
Administration Building 24,000
Fencing, Sidewalks & Landscaping 22,000
Miscellaneous Site Work 75,000
Access Road 43,000
Bond & Overhead 28,000
TOTAL $1,709,000
* Based on final construction cost breakdown; does not include land
costs or engineering fees. The bid opening was in January, 1971
construction was completed in mid-|972 (EPA Index I72),
** Cost does not include pumps, instrumentation, air supply or
exterior piping; costs for these components are included in other items,
43
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It is possible that, in some instances, power costs for pumping will be less with
equalized flow than they would be without it.
Aeration
Aside from pumping, power costs for aeration and mixing provide the major
operating expenditure for an equalization basin. At Walled Lake/Novi, 17.6 m /min
(630 cf m) of blower capacity is assigned to the equalization basin. This amount
represents 10.8 percent of the calculated air requirement for the total plant at
design flow.*
Theoretically, provision of flow equalization could reduce the peak aeration
demand in the biological unit by leveling the mass flow of BOD ~ to the unit and,
perhaps, by accomplishing some BOD~ reduction itself. This would mean smaller
requisite blower capacity for the biological unit and also reduce demand charges
for power. If automated dissolved oxygen control is provided for the aeration
basin, the savings in aeration costs imparted to the biological unit would offset
in part or in total the added costs of aerating the equalization basin.
Power consumption data recorded at the Walled Lake/Novi plant during
the two week-long test periods shows that total power usage was 35,400 kwh during
the equalized flow period and 34,560 kwh under unequalized flow conditions; thus,
total consumption was 2.4 percent higher during equalized operation. Similarly,
the peak daily usage was 5670 kwh during equalized flow and 5460 kwh during
unequalized operaton. These figures do not necessarily demonstrate that the
equalized mode has a greater power demand since no automated facilities were
available to optimize air usage during the study. In fact, there is some indication
that the biological unit was over-aerated during the equalized flow period since
daily dissolved oxygen measurements of the mixed liquor averaged 4.5 mg/1 versus
only 3.7 mg/1 during unequalized operation. Moreover, as mentioned previously,
the equalization basin at the plant is prone to use an excessive amount of power
because controls were not designed to reduce the air flow when the liquid level
in the basin drops. There, observations again indicate that automated control
are necessary to gain the full potential of flow equalization.
*Aeration capacity assigned to othec. units is as follows: Aeration basins, 104.2 m /min
(3720 rfm); aerobic digester, 47.0 m /min (1680 cfm); and aerated grit chamber,
2.1 m /min (75 cfm).
44
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Chemical Feeding
With regard to chemical feeding for phosphorus removal, experience at the
Walled Lake/Novi Wastewater Treatment Plant indicates that equalization may
not necessarily provide the commonly ascribed benefits of reduced capital costs
and simpler control. For example, the chemical feed system at Walled Lake/Novi
was designed to provide the same degree of automatic control that would have
been provided for a plant with diurnal flow. It must be remembered that a flow
equalization system does not guarantee uniform flow from one day to the next;
periodic adjustments in the equalized flow rate are necessary due to daily and
seasonal variations in flow. Thus, the ability to automatically pace chemical dose
to flow is desirable even with flow equalization in order to eliminate the need
for manual readjustment in chemical pumping rates whenever the equalized flow
rate is changed. In addition, the presence of equalization does not reduce the
operational requirements for checking influent P concentration (which can vary
from day to day) and adjusting the chemical dose according.
Manpower Requirements
It is clear that equalization must produce added manpower requirements
for operation and maintenance at a plant because of the additional equipment and
controls. It is the authors' observation that a negligible amount of operator time is
required on a day-to-day basis for routine maintenance and operation. The major
requirements will be for equipment repair. After three years of operation at
Walled Lake/Novi, the only significant equipment failure has been in the flow
metering system.
45
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SECTION IX
REFERENCES
1. La Grega, M. D., and Keenan, J. D., "Effects of Equalizing Wastewater
Flows," 3our. Water Poll. Control Fed., 46, 123 (1974).
2. Wallace, A. T., "Analysis of Equalization Basins, "Pro'c. Amer. Soc. Civil
Engr., Jour, of the Sanitary Engr. Div., SA6, 1161 (1968).
3. Spiegel, M., "Flow Equalization to Ensure Designed Wastewater Treatment
at Lowest Cost, "The Diplomat, Amer. Academy Environ. Engrs., p. 3-6
(May, 1974).
4. Flow Equalization, Technology Transfer Seminar Publication, U.S. Environ.
Protection Agency, 21 pp. (May, 1974).
5. Boon, A. G., and Burgess, D. R., "Effects of Diurnal Variations in Flow of
Settled Sewage on the Performance of High Rate Activated Sludge Plants,"
Water Poll. Control (Gr. Britain), 71, 493-522 (1972).
6. Process Design Manual for Suspended Solids Removal, Technology Transfer
Office, U. S. Environ. Protection Agency, Cincinnati, OH (January, 1975).
7. Smith, R., Eilers, R. G., and Hall, E. D., "Design and Simulation of Equalization
Basins," U. S. Environ. Protection Agency, Office of Research and Monitoring,
Advanced Waste Treatment Research Laboratory, Cincinnati, OH (Feb.,
1973.)
8. Eckenfelder, W. W., Water Quality for Practicing Engineers, Barnes and
Noble, Inc., New York (1970), p. 148.
9. Clark, J. W., Viessman, W., Jr., and Hammer, M. J., Water Supply and Pollution
Control. 2nd Edition, International Textbook Co., Scranton, PA (1971), p. 355.
10. Seidel, H. F., and Baumann, E. R., "Effect of Preaeration on the Primary
Treatment of Sewage," Jour. Water Poll. Control Fed., 33, 339 (1961).
11. Roe, F. C., "Preaeration and Air Flocculation," Sewage Works Jour., 23,
2, 127 (1951).
12. Wastewater Engineering, Metcalf and Eddy, Inc., McGraw-Hill Book Co.,
New York (1972).
46
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1 REPORT NO.
EPA-600/2-76-181
3. RECIPIENT'S ACCESSION»NO.
4. TITLE AND SUBTITLE
EVALUATION OF FLOW EQUALIZATION AT
A SMALL WAS TEW ATE R TREATMENT PLANT
5. REPORT DATE
September 1976(Issuing Date
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Gerald W. Foess, James G. Meenahan and
J. Michael Harju
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Johnson & Anderson, Inc.
P. 0. Box 1166 - 2300 Dixie Highway
Pontiac, Michigan 48056
10. PROGRAM ELEMENT NO.
1BB033
11. CONTRACT/GRANT NO.
68-03-0417
12. SPONSORING AGENCY NAME AND ADDRESS
Municipal, Environmental Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
EPA-ORD
15. SUPPLEMENTARY NOTES
16. ABSTRACT
The primary objective of this project was to evaluate the impact of
flow equalization on the 0.092 m3/sec (2.1 mgd) activated sludge plant
at Walled Lake/Novi, Michigan. Process streams were characterized for a
twelve-month period under equalized flow conditions with respect to BOD,
total suspended solids and total phosphorus. The effects of the equalization
basin on final settling and filtration were evaluated by conducting two
intensive week-long studies, one with and one without equalization of flow.
Flow equalization was effective in leveling influent flow variations
but had very little effect upon concentration leveling. Performance of the
multimedia filters was superior under the equalized flow, both in terms of
average removal efficiency and consistency.
This report was submitted in partial fulfillment of Contract Number
68-03-0417 by Johnson & Anderson, Inc. under the sponsorship of the
Environmental Protection Agency.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS C. COS AT I Field/Group
Waste Treatment, Wastewater, Flow
Control, Cost Comparison
Wastewater Treatment
*Flow Equalization
13B
18. DISTRIBUTION STATEMENT
Release to Public
19. SECURITY CLASS (ThisReport)
UNCLASSIFIED
21. NO. OF PAGES
57
20. SECURITY CLASS /This page)
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (9-73)
47
£USGPO: 1976 757-056/5412 Region 5-1
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