Flow
Equalization
EPA Technology Transfer Seminar Publication
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FLOW EQUALIZATION
ENVIRONMENTAL PROTECTION AGENCY* Technology Transfer
May 1974
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ACKNOWLEDGMENTS
This seminar publication contains materials prepared for the
U.S. Environmental Protection Agency Technology Transfer Program
and has been presented at Technology Transfer design seminars
throughout the United States; it appeared originally as chapter 3 of
the Technology Transfer Process Design Manual for Upgrading
Existing Wastewater Treatment Plants, as revised by Metcalf & Eddy,
Inc., New York, N.Y.
NOTICE
The mention of trade names or commercial products in this publication
is for illustration purposes, and does not constitute endorsement or recom-
mendation for use by the U.S. Environmental Protection Agency
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CONTENTS
Page
Introduction and Concept 1
General 1
Variations of Flow Equalization 1
Benefits of Dry Weather Flow Equalization 5
Impact on Primary Settling 5
Impact on Biological Treatment 5
Miscellaneous Benefits 6
Determination of Equalization Requirements 7
Determination of Required Volume 7
Impact of Equalization on Diurnal Concentration Variation 10
Basin Construction 10
Air and Mixing Requirements 11
Pump and Pump Control Systems 12
Miscellaneous Considerations 13
Costs 15
Performance and Case Histories 17
Ypsilanti Township, Mich 17
Fond du Lac, Wis 17
Walled Lake-Novi, Mich 18
Novi Interceptor Retention Basin, Oakland County, Mich 19
References 21
111
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INTRODUCTION AND CONCEPT
GENERAL
The cyclic nature of wastewater flows in terms of volume and strength is well recognized.
Nearly all municipal wastewater-treatment plants today are processing unsteady wastewater flows.
However, improved efficiency, reliability, and control are possible when physical, biological, and
chemical processes are operated at or near uniform conditions. For this reason, flow equalization is
employed in the field of water supply and in the treatment of some industrial wastewater. At
present, the advent of more demanding water-quality standards is stirring interest in the application
of flow equalization to municipal wastewater treatment.
The primary objective of flow-equalization basins for municipal wastewater plants is simply
to dampen the diurnal flow variation, and thus achieve a constant or nearly constant flow rate
through the downstream treatment processes. A desirable secondary objective is to dampen the
concentration and mass flow of wastewater constituents by blending the wastewater in the equal-
ization basin. This results in a more uniform loading of organics, nutrients, and other suspended
and dissolved constituents to subsequent processes.
Through achieving these objectives, flow equalization can significantly improve the perform-
ance of an existing treatment facility, and is a useful upgrading technique. In the case of new plant
design, flow equalization can reduce the required size of downstream facilities.
VARIATIONS OF FLOW EQUALIZATION
Equalization of municipal wastewater flows may be divided into three broad categories.
• Equalization of dry weather flows
• Equalization of wet weather flows from separate sanitary sewers
• Equalization of combined storm and sanitary wastewater
This publication is primarily concerned with equalization of dry weather flows. This proce-
dure provides a technique for achieving normal operation of a treatment plant under near ideal
loading conditions. Its relatively low cost makes it attractive for upgrading an overloaded plant.
Increased wet weather flows in sanitary sewers is the sum of two components, infiltration and
inflow. In some cases, it is feasible to equalize stormwater inflow, depending on its magnitude and
duration. Infiltration from high ground water tables can seldom be equalized. Equalization of wet
weather flows from combined storm and sanitary sewers usually requires very large storage basins.
The design of equalization basins to deal with these types of flow requires a special knowledge of
the collection system, precipitation patterns, topography, and other factors not directly related to
wastewater treatment. Strictly speaking, wet weather and combined sewer flow equalization cannot
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be considered as a wastewater-treatment upgrading technique, and the design of such a facility is
beyond the scope of this publication. However, the concepts presented for dry weather flow equal-
ization are generally applicable to equalization of wet weather and combined wastewater flows.
In some instances, large interceptor sewers entering the treatment plant can be effectively used
as storage basins to dampen peak diurnal dry weather flow variations. In such cases, velocity be-
comes of critical importance to avoid deposition and subsequent abnormally heavy "first flush"
loads. Nightly or weekly drawdown of the interceptor system is necessary to flush out solids which
may have been deposited during the previous storage period.
Although the use of influent sewers for equalization should not be ignored, the most positive
and effective means to maximize the benefits possible with equalization is through the use of
specially designed equalization basins. These basins should normally be located near the head end
of the treatment works, preferably downstream of pretreatment facilities such as bar screens, com-
minutors, and grit chambers. Adequate aeration and mixing must be provided to keep the basins
aerobic and prevent solids deposition.
It is sometimes desirable to locate the equalization basin at strategic locations within the
collection system. This offers the added advantage of economically relieving trunk sewer overload
during peak flow periods.1 However, it does result in the need for a pumping facility and therefore
is best located where a need for pumping already exists.
Equalization basins may be designed as either in-line or side-line units. In the in-line design
shown on figure la, all the flow passes through the equalization basin. This results in significant
concentration and mass flow damping. In the side-line design shown on figure Ib, only that amount
of flow above the daily average is diverted through the equalization basin. This scheme minimizes
pumping requirements at the expense of less effective concentration damping.
For new construction and for upgrading large plants, it is desirable to construct compartmen-
talized or multiple basins. This feature will allow the flexibility to dewater a portion of the facility
for maintenance or equipment repair while still providing some flow equalization. Where a basin
is designed for storage and equalization of wet weather flows, compartmentalized tanks will allow
the utilization of a portion of the basin for dry weather flow equalization.
Single basin installations may be used for upgrading small plants, but must have the provision
to be dewatered while maintaining complete treatment. This will require a bypass line around the
basin to allow the downstream portion of the plant to operate unequalized when the flow equali-
zation facility is out of service.
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Raw
wastewater
Bar screen
and/or
comminutor
\
] A
^
Grit
removal
*—
Equali-
zation
basin
Controlled-
flow pumping
station
Flow meter and control device
/
Primary
treatment
Secondary
treatment
Final
*" effluent
(a)
Sludge-processing
recycle flows
Bar screen
and/or
comminutor
Overflow structure
Raw
wastewater
(b)
Sludge-processing
recycle flows
Control led-
flow pumping
station
Figure 1. Schematic flow diagrams of equalization facilities: (a) in-line equalization; (b) side-line equalization.
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BENEFITS OF DRY WEATHER
FLOW EQUALIZATION
Flow equalization has a positive impact on all treatment processes from primary treatment to
advanced waste treatment.
IMPACT ON PRIMARY SETTLING
The most beneficial impact on primary settling is the reduction of peak overflow rates resulting
in improved performance and a more uniform primary effluent quality. Flow equalization permits
the sizing of new clarifiers based on equalized flow rates rather than peak rates. In an existing
primary clarifier that is hydraulically overloaded during periods of peak diurnal flow, equalization
can reduce the maximum overflow rate to an acceptable level. A constant influent feed rate also
avoids hydraulic disruptions in the clarifier created by sudden flow changes, especially those caused
by additional wastewater lift pumps suddenly coming on line.
LaGrega and Keenan2 investigated the effect of flow equalization at the 1.8-mgd Newark,
N.Y., Wastewater Treatment Plant. An existing aeration tank was temporarily converted to an
equalization basin. They compared the performance of primary settling under marginal operating
conditions, with and without equalization. The results are shown in table 1.
It has been demonstrated3 >4 that preaeration can significantly improve primary settling. Roe3
concluded that preaeration preflocculates suspended solids (SS), thereby improving their settling
characteristics. Indications are that this benefit may be realized by aerated equalization basins.
This benefit may be diminished when the equalized flow is centrifugally pumped to the primary
clarifier, due to the shearing of the floe.
IMPACT ON BIOLOGICAL TREATMENT
As contrasted to primary treatment or other mainly physical processes where concentration
damping is of minor benefit, biological treatment performance can benefit significantly from both
concentration damping and flow smoothing. Concentration damping can protect biological
Table 1 .—Effect of flow equalization on primary settling, Newark, N. Y.
Item
Primary influent SS mg/l
Primary effluent SS mg/l
SS removal in primaries percent
Normal flow
136.7
105.4
23
Equalized flow
128
68
47
Note.—Average flow slightly higher in unequalized portion of study.
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processes from upset or failure from shock loadings of toxic or treatment inhibiting substances.
Therefore, in-line equalization basins are preferred to side-line basins for biological treatment
applications.
Improvement in effluent quality due to stabilized mass loading of BOD on biological systems
treating normal domestic wastes has not been adequately demonstrated to date. It is expected that
the effect will be significant where diurnal fluctuations in organic mass loadings are extreme. This
situation may arise at a wastewater-treatment plant receiving a high-strength industrial flow of short
duration. Damping of flow and mass loading will also improve aeration tank performance where
aeration equipment is marginal or inadequate in satisfying peak diurnal-loading oxygen demands.5
The optimum pH for bacterial growth lies between 6.5 and 7.5. In-line flow equalization can
provide an effective means for maintaining a stabilized pH within this range.
Flow smoothing can be expected to improve final settling even more so than primary settling.
In the activated-sludge process, flow equalization has the added benefit of stabilizing the solids
loading on the final clarifier. This has two ramifications:
• The mixed-liquor suspended solids (MLSS) concentration can be increased thereby decreas-
ing the food-to-mass ratio (F/M) and increasing the solids retention time (SRT). This may
result in an increased level of nitrification, and a decrease in biological sludge production.
It may also improve the performance of a system operating at an excessively high daily
peak F/M.
• Diurnal fluctuations in the sludge blanket level will be reduced. This reduces the potential
for solids being drawn over the weir by the higher velocities in the zone of the effluent weirs.
MISCELLANEOUS BENEFITS
In chemical coagulation and precipitation systems using iron or aluminum salts, the quantity
of chemical coagulant required is proportional to the mass of material to be precipitated. Damping
of mass loadings with in-line equalization will improve chemical feed control and process reliability,
and may reduce instrumentation complexity and costs.
Flow smoothing will reduce the surface area required and enhance the performance of tertiary
filters. A constant feed rate will lead to more uniform solids loadings and filtration cycles.
The equalization basin provides an excellent point of return for recycled concentrated waste
streams such as digester supernatant, sludge-dewatering filtrate, and polishing-filter backwash.
Some biochemical oxygen demand (BOD) reduction is likely to occur in an aerated equaliza-
tion basin. A 10- to 20-percent reduction has been suggested3 for an in-line basin equalizing raw
wastewater. However, the degree of reduction will depend upon the detention time in the basin,
the aeration provided, wastewater temperature, and other factors. For an existing treatment plant,
a simple series of oxygen uptake studies on a representative sample of wastewater can determine the
BOD reduction that will occur.
Roe3 observed that preaeration may improve the treatability of raw wastewater by creating a
positive oxidation-reduction potential, thereby reducing the degree of oxidation required in
subsequent stages of treatment.
aDr. Robert E. Baumann, private communication, Iowa State University, Dec. 11, 1972,
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DETERMINATION OF EQUALIZATION
REQUIREMENTS
The design of an equalization basin requires the evaluation and selection of a number of
features as follows:
• In-line versus side-line basins
• Basin volume
• Degree of compartmentalization
• Type of construction—earthen, concrete, or steel
• Aeration and mixing equipment
• Pumping and control concept
• Location in treatment system
The design decisions must be based on the nature and extent of the treatment processes used, the
benefits desired, and local site conditions and constraints.
It may not be necessary to equalize the entire influent flow where high flow or concentration
variations can be attributed to one source, such as an industry. In these cases the desired benefits
can be achieved by simply equalizing the industrial flow. This can be accomplished through
construction of an equalization basin at the industrial site or through in-house industrial process
modifications to effect an equalized wastewater discharge.
DETERMINATION OF REQUIRED VOLUME
Two methods are available for computing equalization volume requirements. One procedure
is based on the characteristic diurnal flow pattern. In this case, the function of the basin is to store
flows in excess of the average daily flow and to discharge them at times when the flow is less than
the average. The required volume can be determined graphically through the construction of a
hydrograph. The second procedure is based upon the mass loading pattern of a particular con-
stituent. This method computes the volume required to dampen mass loading variations to within
a preset acceptable range.6 >7
Since the prime objective of flow equalization in wastewater treatment is to equalize flow, the
determination of equalization volume should be based on the hydrograph. Once the volume has
been determined for flow smoothing, the effect on concentration and mass load damping can be
estimated. The required volumes for side-line and in-line basins will be identical. The hydrograph
procedure is discussed below.
The first step in design involves the establishment of a diurnal flow pattern. Whenever possible,
this should be based upon actual plant data. It is important to note that the diurnal pattern will
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O
BOD mass loading:
. Peak: average = 1.97
Minimum: average = 0.14
Peak: minimum = 14.59
300
,. 600
- 200
100
O
cc
h-
z
UJ
O
O
o
Q
O
OQ
- 400
i- 200
2
Q
O
C/3
Q
O
CO
"- 0
1?
Midnight
Figure 2. Raw wastewater flow and BOD variation before equalization.
vary from day to day, especially from weekday to weekend, and also from month to month. The
pattern selected must yield a large enough basin design to effectively equalize any reasonable dry
weather diurnal flow. Figure 2 depicts a typical diurnal flow pattern. The average flow rate is 4.3
mgd. For purposes of this example, the average flow is used as the desired flow rate out of the
equalization basin. The diurnal peak and minimum flow rate for this example are 1.7 and 0.45
times the average, respectively.
The next step involves the actual construction of the hydrograph. The hydrograph for this
example is shown on figure 3. The inflow mass diagram is plotted first. To do this, the hourly
diurnal flows are converted to equivalent hourly volumes, and accumulated over the 24-hour day.
A line is then drawn from the origin to the end point on the inflow-mass diagram. The slope of
this line actually represents the average flow for the day.
Enough tank volume must be provided to accumulate flows above the equalized flow rate.
This normally requires a volume equivalent to 10 to 20 percent of the average daily dry weather
flow. To determine this volume, the inflow mass diagram must be enveloped with two lines parallel
to the average flow line and tangent to the extremities of the inflow mass diagram. These are
shown as lines A and B on figure 3. The required volume is represented by the vertical distance
between these two lines. In this illustration, the required volume for equalization is 740,000
gallons, which represents approximately 17 percent of the average daily flow.
The physical interpretation of the hydrograph is simple. At 8 a.m. the equalization basin is
empty, as signified 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. This is
represented by the divergence of the inflow mass diagram and the bottom diagonal. At 5 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 p.m. to 8 a.m. on the following day, when the flow is below
average.
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o
LU
4,000 -
3,500 -
3,000
2,500 -
2,000 -
1,500 -
1,000 -
500 -
Required equalization
volume, 740,000 gal Ions
12 2 4 6 8 10 12 2 4 6 8 10 12
Midnight Noon Midnight
TIME OF DAY
Figure 3. Hydrograph for typical diurnal flow.
The actual equalization-basin volume must be greater than that obtained with the hydrograph
for several reasons, including
• Continuous operation of aeration and mixing equipment will not allow complete drawdown.
• Volume must be provided to accommodate anticipated concentrated plant recycle streams.
• Some contingency should be provided for unforeseen changes in diurnal flow.
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The final volume selected should include adequate consideration of the conditions listed above and
will also depend on the basin geometry. For the example presented herein, a basin volume of
approximately 1 million gallons is adequate.
IMPACT OF EQUALIZATION ON DIURNAL CONCENTRATION VARIATION
At this point, it is appropriate to examine the impact of flow equal ization on mass loading
and concentrations. As previously mentioned, side-line equalization has a minimal effect on diurnal
concentration variations. The following discussion is therefore limited to in-line basins.
An hourly concentration plot for raw wastewater BOD is plotted with the diurnal flow pattern
on figure 2. Note that low BOD concentrations occur at night with low flows, and high BOD con-
centrations occur during the daytime with high flows. This is a typical pattern for dry weather
flows and BOD's. Because of this characteristic, the mass loading rate of raw wastewater BOD,
shown on figure 2, exhibits even greater fluctuations. If this wastewater is equalized in a 1 million
gallon in-line basin, the equalized flow will exhibit the characteristics shown on figure 4, provided
• The basin is designed to provide complete mixing.
• There is no BOD reduction in the basin.
This damping effect would be similarly beneficial for all concentration variables including SS,
nitrogen, phosphorus, and toxic constituents.
On figure 4, the changes in BOD concentration are most pronounced during periods of
minimum wastewater volume in the equalization tank. If desired, increased damping can be
achieved by increasing the active volume of the tank, i.e., the volume in excess of that obtained
from the hydrograph.
BASIN CONSTRUCTION
Equalization basins can be provided through the construction of new facilities or through the
modification of 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
which may be suitable for conversion 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. To prevent embankment failure in
areas of high ground water, drainage facilities should be provided for ground water control. In
large basins where a combination of aerator action and wind forces may cause the formation of
large waves, precaution should be taken in design to prevent erosion. It is also customary to
provide a concrete pad directly under the equalization basin aerator or mixer. The top of the dikes
should be wide enough to insure a stable embankment. For economy of construction, the top
width of the dike should be sufficient to accommodate mechanical compaction equipment.
10
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9
3
7
6
"S,
m- 5
LLJ
<
oc
s 4
3
"- 3
2
1
Q
BOD mass loading:
~~ Peak: average =1.22
Minimum: average = 0.61 -
_ Peak: minimum = 2.01
BOD concentration
\ _-- -x
,''''
— X S -A
\ /' ^
^x^ ^X f ^'^ / 'X
'X x. / ^- Flow rate
'x^ **--* .'
A" x^ /
T " x— j —
BOD mass loading
-
I I! 1 i l l 1 l i i i 1 1 i 1 i l l 1
300 n 600
200 z-
0
H
£
fe
LU
U
z
100 8
Q
O
m
0
-400 -.
13
Z
Q
O
_l
CO
^
-200 5
Q
S
_ n
12 6 12 6 12
Midnight Noon Midnight
TIME OF DAY
Figure 4. Raw wastewater flow and BOD variation after equalization.
In-line basins should be designed to achieve complete mixing in order to optimize concentra-
tion damping. Elongated tank design enhances plug flow and should be avoided. Inlet and outlet
configurations should be designed to prevent short circuiting. Designs which discharge influent
flow as close as possible to the basin mixers are preferred.
To continue the previous illustration, an earthen basin has been selected for the equalization
facility. A square plan has been chosen to effect optimum mixing. A section view of the basin
with appropriate dimensions is shown on figure 5. The volume requirement computed from the
hydrograph is provided in the upper 8 feet. Note that the minimum required operating depth lies
above the minimum allowable aerator operating level.
AIR AND MIXING REQUIREMENTS
The successful operation of both in-line and side-line basins requires proper mixing and
aeration. 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 a municipal wastewater having an SS
concentration of approximately 200 mg/1 range from 0.02 to 0.04 hp per 1,000 gallons of storage.
To maintain aerobic conditions, air should be supplied at a rate of 1.25 to 2 ft3/min per 1,000
gallons of storage.8
Mechanical aerators are one method of providing both mixing and aeration. The oxygen
transfer capabilities of mechanical aerators operating in tap water under standard conditions vary
from 3 to 4 pounds O2 per horsepower-hour. Baffling may be necessary to insure proper mixing,
particularly with a circular tank configuration. Minimum operating levels for floating aerators
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153'
3.0' freeboard
/' "' ' 'X T
'"-'^XT Maximum surface level
Floating aerator
El. 15.0
Minimum required
operating level
Minimum allowable _-
operating level to
protect floating aerator"
— - 45'
-* Concrete scour pad*
Volumes.
El. 0.0 to El. 7.0 approximately 260,000 gallons
El. 7.0 to El. 15.0 approximately 740,000 gallons
Total volume = 1,000,000 gallons
"These dimensions will vary with aerator design and horsepower
Figure 5. Earthen equalization basin.
generally exceed 5 feet, and vary with the horsepower and design of the unit. Low-level shutoff con-
trols should be provided to protect the unit. The horsepower requirements 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 equipment to keep the solids in suspension
and furnish the air requirements through a diffused air system, or by mounting a surface aerator blade
on the mixer.
It should be cautioned that other factors, including maximum operating depth and basin con-
figuration, affect the size, type, quantity, and placement of the aeration equipment. In all cases, the
manufacturer should be consulted.
PUMP AND PUMP CONTROL SYSTEMS
Flow equalization imposes an additional head requirement within the treatment plant. As a
minimum, this head is equal to the sum of the dynamic losses and the normal surface level variation.
Additional head may be required if the basin is to be dewatered to a downstream location. It may
be possible to dewater the basin upstream (e.g., ahead of raw wastewater pumps) by gravity.
Normally, the head requirement cannot be fulfilled by gravity, thereby requiring pumping
facilities. The pumping may precede or follow equalization. In some cases pumping of both raw
and equalized flows will be required. Influent pumping will require larger capacity pumps to satisfy
diurnal peaks.
Gravity discharge from equalization will require an automatically controlled flow-regulating
device.
12
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A flow-measuring device is required downstream of the basin to monitor the equalized flow.
Instrumentation should be provided to control the preselected equalization rate by automatic adjust-
ment of the basin effluent pumps or flow-regulating device.
MISCELLANEOUS CONSIDERATIONS
The following features are considered to be desirable for the design of the equalization facility:
• Equalization should be preceded if possible with screening and grit removal to prevent grit
deposition and rag fouling of equipment in the basin.
• Surface aerators should be fitted with legs to support and protect the units when the tank is
dewatered.
• Facilities should be provided to flush solids and grease accumulations from the basin walls.
• A high-water-level takeoff should be provided for withdrawing floating material and foam.
• An emergency overflow should be provided.
13
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COSTS
The development of alternatives for any plant upgrading program should include at least one
which incorporates flow equalization. In all cases, the added cost of flow equalization must be
measured against the savings in cost of modifying downstream processes to accept diurnal variations
and the improved performance that can be achieved by operating downstream processes under
relatively constant loading conditions.
The cost of flow equalization will vary considerably from one application to another, depend-
ing on the basin size, construction selected, mixing and aeration requirements, availability of land,
location of facility, and pumping requirements. Some judgment must be made on the distribution
of pumping costs. Pumping costs for an equalization basin used to upgrade existing facilities should
be charged to the basin.
Capital costs for equalization facilities have been estimated by Smith et al.9 and are listed in
table 2. The costs for earthen basins include plastic liner and floating mechanical aerators. The
costs for the concrete basins include diffused aeration facilities. Pumping costs are based on the con-
struction of a separate equalization basin effluent pumping station. The costs were developed in
conjunction with activated-sludge treatment-system designs, and therefore include a proportional
amount of the engineering fees and interest during construction.
The construction cost for the earthen equalization basin on figure 5 is estimated at $80,000.
The cost includes excavation, plastic liner, sand subbase, concrete scour pad, dike fill, underdrain,
and a 40-hp floating aerator. The costs do not provide for pumping costs, land costs, engineering
and legal fees, nor interest during construction.
Table 2.-Cost of equalization facilities (EPA Index 175}
Plant
size
mgd
1
3
10
Basin size,
millions
of gallons
0.32
.88
2.40
Earthen basin
With
pumping
$124,000
170,000
318,000
Without
pumping
$ 72,300
84,000
134,000
Concrete basin
With
pumping
$175,000
333,000
779,000
Without
pumping
$124,000
247,000
595,000
15
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PERFORMANCE AND CASE HISTORIES
Little full-scale operating data are currently available to compare the performance of wastewater-
treatment plants with and without flow equalization. However, an increasing number of plant designs
are incorporating the use of equalization facilities for upgrading existing plants and construction of
new plants. The following case histories are presented as examples of equalization basin design.
YPSILANTI TOWNSHIP, MICH.
A flow-equalization project at the Ypsilanti Township Sewage Treatment Plant is currently under
way. The treatment facility consists of two adjacent activated-sludge plants recently upgraded from
7.0 mgd to treat a total flow of 9.0 mgd. Two 350,000-gallon digesters have been converted to
equalization tanks. Data will be collected over a 2-year study period for each plant. The flow will
be equalized to one plant the first year while background data are collected for the remaining plant.
The situation will be reversed the second year, with the flow being equalized to the second plant
while unequalized flow performance data are collected on the first plant. Comparison of these data
will be made to determine the beneficial effects of flow equalization on each plant.
FONDDU LAC,WIS.
This case illustrates a situation in which only a portion of the flow is equalized. The city of
Fond du Lac, Wis., presently employs a single-stage trickling-filter plant to treat combined municipal-
industrial wastes. Placed in operation in 1950, the plant was designed to treat an ultimate dry
weather flow of 8.0 mgd and a BOD loading of 12,500 Ib/d. The facility is presently treating an
average of 7.1 mgd with a BOD loading of 24,000 Ib/d, and hence is organically overloaded. This
condition is aggravated by the fact that the waste discharges from a major industrial contributor (a
tannery) are preo^ntly concentrated during daylight hours. The tannery discharges wastes to the
the treatment plant via a separate force main. It accounts for about 35 percent of the BOD and
50 percent of the SS into the plant, and about 15 percent of the influent flow, resulting in a widely
fluctuating BOD and SS diurnal load profile.
The wide fluctuations in organic loading are resulting in reduced performance of the trickling
filters. This, in conjunction with the advent of more stringent treatment standards, has rendered this
facility inadequate. Plans are presently under way to upgrade the treatment plant.
This case represents an ideal situation for employing partial equalization in the upgrading scheme.
The volume of the wastes from the tannery is relatively small compared to the total volume of flow
received at the plant, whereas the organic contribution is significant. Therefore, a relatively small
volume equalization tank is all that is required to attain effective organic load equalization. In addi-
tion, because the tannery discharges to the treatment plant via a separate force main, equalization
may be accomplished at the treatment-plant site. The effect of equalizing the tannery flow over
24 hours is illustrated on figure 6.
Located at the plant site are six abandoned square anaerobic digesters, each measuring 50 feet
by 50 feet by 17.5 feet deep. Four of the units have fixed covers and two have floating covers. The
utilization of these tanks for equalizing the tannery flow was investigated. The investigations indicate
17
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2,000
12 midnight
12 noon
TIME OF DAY
12 midnight
Existing load profile
— — Equalized load profile
Figure 6. Effect of tannery flow equalization.
that the four fixed-cover tanks would be adequate for equalization for all but a few days each year
when the use of the two additional tanks would be necessary because of high flows or maintenance.
The conversion of the abandoned digesters to equalization tanks entails complete modification
of the four fixed-covered tanks and only minimal modification of the two tanks which have floating
covers. The four fixed-covered tanks would each require the installation of a mechanical mixer to
maintain solids in suspension, including structural modifications in order to support the mixers. A
ventilation system would be required for the covered tanks to insure the safety of plant personnel
who may enter the tanks for purposes of inspection or maintenance. Minor structural repairs and
waterproofing of all six tanks would be necessary to insure their structural integrity and watertight-
ness. The two floating covers would be removed and the pipe gallery would be converted to a pump
station.
The cost for converting these units to equalization tanks is estimated at approximately $440,000.
This cost includes process pumping equipment and piping, four mechanical mixers, tank ventilation
system, instrumentation, electrical work, structural renovations and alterations, and engineering fees.
At present, additional studies are under way to evaluate the feasibility of equalization of tannery
wastes at the tannery in lieu of equalizing these wastes at the plant site.
WALLED LAKE-NOVI, MICH.
The Walled Lake-Novi Wastewater Treatment Plant is a new 2.1-mgd facility employing side-
line flow equalization. The treatment plant was placed into operation in 1971. It was designed to
meet stringent effluent quality standards, including a summertime monthly average BOD2n of 8 mg/1,
a wintertime monthly average BOD20 of 15 mg/1. and 10 mg/1 of SS. The facility utilizes the
activated-sludge process followed by multimedia tertiary filters. Ferrous chloride and lime are added
ahead of aeration for phosphorus removal. Sludge is processed by aerobic digestion, and dewatered
on sludge-drying beds. A schematic diagram of this facility is shown on figure 7.
18
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Influent
• Effluent
Process
pumps
Comrninutor and
aerated grit
chamber
Filter backwash
Figure 7. Walled Lake-Novi Wastewater Treatment Plant.
A major factor in the decision to employ flow equalization was the desire to load the tertiary
filters at a constant rate. The equalization facility consists of a 315,000-gallon concrete tank which
is equivalent in volume to 15 percent of the design flow. The tank is 15 feet deep and 60 feet in
diameter. Aeration and mixing are provided by a diffused air system with a capacity of 2 ft3/min
per 1,000 gallons of storage. Chlorination is provided for odor control. A sludge scraper is installed
to prevent consolidation of the sludge.
The equalization facility is operated as follows:10 The process pumping rate is preset on the
pump controller to deliver the estimated average flow to the treatment processes. The flow delivered
by these pumps is monitored by a flowmeter which automatically adjusts the speed of the pumps to
maintain the average flow rate. When the raw wastewater flow to the wet well exceeds the preset
average, the wet well level rises, thereby actuating variable speed equalization pumps which deliver
the excess flow to the equalization basin. When the inflow to the wet well is less than the average,-
the wet well level falls and an automatic equalization basin effluent control valve opens. The valve
releases enough wastewater to the wet well to reestablish the average flow rate through the plant.
Since this is a new plant as opposed to an upgraded plant, no comparative data exist. However, the
treatment facility is typically producing a highly treated effluent with BOD and SS concentrations
less than 4 mg/1 and 5 mg/1, respectively.8
NOVI INTERCEPTOR RETENTION BASIN, OAKLAND COUNTY, MICH.
This case11 illustrates the utilization of an equalization basin within the wastewater collection
system.
A portion of the wastewater collection system for the city of Novi, Mich., discharges to the
existing Wayne County Rouge Valley Interceptor System. Due to the existing connected load on
the Wayne County system, Novi's wastewater discharge to the interceptor system is limited to a
maximum flow rate of 4 ft3/s. This rate was matched by the existing maximum diurnal flows from
the city. In order that additional population could be served, it was decided to equalize wastewater
19
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flows to the interceptor system. By discharging to the interceptor continuously at an average rate
of flow, the total wastewater flows from the city of Novi to the Wayne County Rouge Valley Inter-
ceptor System could be increased by a factor of 2.6.
An 87,000-ft3 concrete basin was constructed for equalizing flows. The tank has a diameter of
92 feet and a depth of 10.5 feet. Aeration and mixing are provided by a diffused air system with a
capacity to deliver 2 ft3/min per 1,000 gallons of storage.
An 87,000-ft3 concrete basin was constructed for equalizing flows. The tank has a diameter of
92 feet and a depth of 10.5 feet. Aeration and mixing are provided by a diffused air system with a
capacity to deliver 2 ft3/min per 1,000 gallons of storage.
A manhole located upstream of the equalization basin intercepts the flow in the existing Novi
trunk sewer. The intercepted wastewater flows into a weir structure which allows a maximum of
4 ft3/s to discharge into the Wayne County system. The wastewater in excess of the preset average
overflows into a wet well where it is pumped to the equalization basin. When flows in the inter-
ceptor fall below the preset average, a flow-control meter generates a signal opening an automatic-
valve on the effluent line of the basin, allowing stored wastewater to augment the flow.
20
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REFERENCES
1C. N. Click, "The Feasibility of Flow Smoothing Stations in Municipal Sewage Systems,"
USEPA Project No. 11010 FDI, Contract No. 14-12-935, Aug. 1972.
2M. D. LaGrega and J. D. Keenan, "Effects of Equalizing Sewage Flow," presented at 45th
Annual Conference of the Water Pollution Control Federation, Atlanta, Ga., Oct. 1972.
3F. C. Roe, "Preaeration and Air Flocculation," Sewage Works J., 23, No. 2, 127-140, 1951.
4H. F. Seidel and E. R. Baumann, "Effect of Preaeration on the Primary Treatment of
Sewage," J. Water Pollut. Cont. Fed., 33, No. 4, 339-355, 1961.
5A. G. Boon and D. R. Burgess, "Effects of Diurnal Variations in Flow of Settled Sewage on
the Performance of High Rate Activated-Sludge Plants," Water Pollution Cont., 493-522, 1972.
6P. R. Bradley and J. Y. Oldshue, "The Role of Mixing in Equalization," presented at 45th
Annual Conference of the Water Pollution Control Federation, Atlanta, Ga., Oct. 1972.
7 A. T. Wallace, "Analysis of Equalization Basins," J. Sanit. Eng. Div., ASCE, SA6, 1161-1171,
1968.
8 J. M. Smith, A. N. Masse, and W. A. Feige, "Upgrading Existing Wastewater Treatment Plants,"
presented at Vanderbilt, Sept. 18, 1972.
9R. Smith, R. G. Eilers, and E. D. Hall, Design and Simulation of Equalization Basins, U.S. En-
vironmental Protection Agency, Internal Publication, Feb. 1973.
10 Johnson & Anderson, Inc., Operation and Maintenance Manual for Wastewater Treatment
Plant, Walled Lake Arm, Huron-Rouge Sewage Disposal System, Oakland County D.P.W., Oakland
County, Mich., June 1973.
1 * Johnson & Anderson, Inc., Operation and Maintenance Manual for Sewage Retention
Reservoir, Novi Trunk Extension No. 1, Huron-Rouge Sewage Disposal System, Oakland County
D.P.W., Oakland County, Mich., Sept. 1973.
21
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METRIC CONVERSION TABLES
Recommended Units
Description
Length
Area
Volume
Mass
Time
Force
Moment or
torque
Stress
Unit
metre
kilometre
millimetre
micrometre
square metre
square kilometre
square millimetre
hectare
cubic metre
litre
kilogram
gram
milligram
tonne or
megagram
second
day
year
newton
newton metre
pascal
kilopascal
Symbol
m
km
mm
;jm.
m2
km2
mm2
ha
m3
1
kg
9
mg
t
Mg
s
d
year
N
N-m
Pa
kPa
Application
Description
Precipitation,
run-off.
evaporation
River flow
Flow in pipes,
conduits, chan-
nels, over weirs.
pumping
Discharges or
abstractions,
yields
Usage of water
Density
Unit
millimetre
cubic metre
per second
cubic metre per
second
litre per second
cubic metre
per day
cubic metre
per year
litre per person
per day
kilogram per
cubic metre
Symbol
mm
m3/s
m3/s
l/s
m3/d
m3/year
I/person
day
kg/m3
Comments
Basic SI unit
The hectare (10000
m2) is a recognized
multiple unit and
will remain in inter-
national use.
The litre is now
recognized as the
special name for
the cubic decimetre.
Basic SI unit
1 tonne = 1 000 kg
1 Mg = 1 000 kg
Basic S! unit
Neither the day nor
the year is an SI unit
but both are impor-
tant.
The newton is that
force that produces
an acceleration of
1 m/s2 in a mass
of 1 kg
The metre is
measured perpendicu-
lar to the line of
action of the force
N. Not a joule.
of Units
Comments
For meteorological
purposes it may be
convenient to meas-
ure precipitation in
terms of mass/unit
area (kg/m3).
1 mm of rain =
1 kg/m2
Commonly called
the cumec
1 l/s = 86.4 m3/d
The density of
water under stand-
ard conditions is
1 000 kg/m3 or
1 000 g/l or
1 g/ml.
Customary
Equivalents
39.37 m.=3.28ft=
1.09yd
0.62 mi
0.03937 in.
3937X 103=103A
1 0.764 sq ft
= 1.196sqyd
6 384 sq mi =
247 acres
0.00155 sq in.
2471 acres
35.314 cu ft =
1.3079 cu yd
1. 057 qt = 0.264 gal
= 0.81 X 10 4 acre-
ft
2.205 Ib
0.035 oz = 1 5.43 gr
0.01 543 gr
0 984 ton (long) =
1.1023 ton (short)
0.22481 Ib (weight)
= 7.233 poundals
0.7375 ft-lbf
0.02089 Ibf/sq ft
0.14465 Ibf/sq in
Description
Velocity
linear
angular
Flow (volumetric)
Viscosity
Pressure
Temperature
Work, energy,
quantity of heat
Power
Recommended Units
Unit
metre per
second
millimetre
per second
kilometres
per second
radians per
second
cubic metre
per second
litre per second
pascal second
newton per
square metre
or pascal
kilometre per
square metre
or kilopascal
bar
Kelvin
degree Celsius
joule
kilojoule
watt
kilowatt
joule per second
Symbol
m/s
mm/s
km/s
rad/s
m3/s
l/s
Pa-s
N/m2
Pa
kN/m2
kPa
bar
K
C
J
kJ
W
kW
J/s
Comments
Commonly called
the cumec
Basic SI unit
The Kelvin and
Celsius degrees
are identical.
The use of the
Celsius scale is
recommended as
it is the former
centigrade scale.
1 joule = 1 N-m
where metres are
measured along
the line of
action of
force N.
1 watt = 1 J/s
Customary
Equivalents
3.28 fps
0.00328 fps
2.230 mph
15,850 gpm
= 2.120 cfm
15.85 gpm
0.00672
poundals/sq ft
0.000145 Ib/sq in
0.145 Ib/sq in.
14.5 b/sq in.
5F
- -17.77
3
2.778 X 10 7
kw hr =
3.725 X 10'7
hp-hr= 0.73756
ft-lb = 9.48 X
10'4 Btu
2.778 kw-hr
Application of Units
Customary
Equivalents
35.314 cfs
15.85gpm
1.83X 10 3 gpm
0.264 gcpd
0.0624 Ib/cu ft
Description
Concentration
BOD loading
Hydraulic load
per unit area;
e.g. filtration
rates
Hydraulic load
per unit volume;
e.g., biological
filters, lagoons
Air supply
Pipes
diameter
length
Optical units
Unit
milligram per
litre
kilogram per
cubic metre
per day
cubic metre
per square metre
per day
cubic metre
per cubic metre
per day
cubic metre or
litre of free air
per second
millimetre
metre
lumen per
square metre
Symbol
mg/t
kg/m3d
m3/m2d
m3/m3d
m3/s
l/s
mm
m
lumen/m2
Comments
If this is con-
verted to a
velocity, it
should be ex-
pressed in mm/s
(1 mm/s = 86.4
m3/m2 day).
Customary
Equivalents
1 ppm
0.0624 Ib/cu-ft
day
3.28 cu ft/sq ft
0.03937 in.
39.37 in. =
3.28ft
0.092 ft
candle/sq ft
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