&EPA
United States
Environmental Protection
Agency
Wastewater Management Fact Sheet
In-Plant Wet Weather Peak Flow Management
INTRODUCTION
Managing sewage flows resulting from wet
weather events presents numerous challenges at
wastewater treatment plants (WWTPs). It
involves the integration of planning, design,
operation, and maintenance of not only the
treatment system but the collection system as
well (WEF Guide to Managing Peak Wet
Weather Flows, 2006). When wet weather events
contribute flows to a WWTP that exceed the
capacity of one or more treatment units at the
plant, inadequate treatment, operational
difficulties, and/or National Pollutant Discharge
Elimination System (NPDES) permit violations
can result. These problems may include
overflows, diversions in violation of the bypass
provision of the permit, and exceedences of
effluent limitations. A WWTP utilizing
biological treatment requires a certain
concentration of pollutants to operate effectively.
Therefore these treatment plants can be severely
impacted when dilute flows enter the treatment
train. This can impact operations beyond the
timeframe of the wet weather event. Addressing
such peak flow problems typically involves a
comprehensive solution that reduces flows in the
collection system, provides storage to dampen
peak flows, and increases the treatment capacity
at the plant. Wet weather events can impact both
combined and separate sewer systems. Though
the peak flows at combined sewer systems will
typically be greater, there are many similarities
in how a treatment plant with combined or
separate sewer systems can deal with the issue.
This fact sheet describes possible treatment
technologies and measures for managing
WWTPs during wet weather peak flow events.
DESCRIPTIONS
Several options are available for facilities that
pursue improved in-plant peak flow
management. These management approaches
generally involve either providing or increasing
storage capacity, decreasing the volume of water
entering the WWTP through rectifying
infiltration/inflow issues, improving the storage
of excess influent prior to primary clarification,
or increasing the capacity of the existing
treatment process by chemical and/or mechanical
means or constructing additional treatment units.
Prior to selecting a technology, it is important to
first characterize the existing conditions at the
WWTP.
Assessment and Planning
If peak flows are presenting difficulties at a
WWTP, it will be difficult to address those
issues without proper planning, development,
and evaluation of a peak wet weather treatment
strategy. It is first necessary to characterize the
existing conditions. This involves understanding
the existing service area, the flows generated
during peak events, operational techniques,
monitoring requirements, and the collection and
treatment systems. The characterization of
existing conditions involves gathering known
information, collecting missing information, and
conducting an analysis of the system and its
performance.
For the collection system, this analysis includes
defining the service area, quantifying flows, and
determining component capacity and limits.
Possible causes of collection system performance
difficulties include blockages in the system, high
levels of infiltration and inflow, overflows, and
infrastructure decay (pipe corrosion, etc.) (WEF,
2006).
For the treatment system, it is necessary to
understand peak flow intensity, volume, and the
corresponding wasteloads at the headworks of
the WWTP. Not all treatment plants perform the
same; therefore site specific information is
crucial in order to determine the course of action
for a particular WWTP. The treatment plant's
1
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ability to handle increased flows and loadings
vary depending on the facility's permitting
requirements, design preferences, and the levels
of operation and maintenance performed at the
facility (WEF, 2006).
A WWTP is comprised of numerous treatment
units that combine to form the treatment system.
It is necessary to understand the capacities of
individual units and how they relate to the
treatment system as a whole. It is also important
to understand the design performance of a
particular unit, the dry weather performance of
that unit, and the wet weather performance of the
unit. This can be a time and labor intensive task
but is vital in properly understanding how peak
flows impact the WWTP as a whole and which
individual units of the plant may be stressed in
these peak flow events.
A key component of the planning process is not
only determining present conditions, but
forecasting future needs. This can involve
predicting future growth in the service area,
assessing the impact of collection system
modifications, evaluating the timeframes for the
replacement of treatment units or collection
system components, or anticipating future
regulatory requirements.
Several techniques for addressing wet weather
peak flows are presented below. It is expected that
one or more of these practices or technologies will
be applicable to a particular WWTP and
understood that site-specific conditions will
determine the best course of action.
Assessment of Existing Capacity
According to the Water Environment Research
Federation's (WERF) report entitled Research
Priorities for Debottlenecking, Optimizing, and
Rerating Wastewater Treatment Plants (WERF
1999), the available capacity in WWTPs can be
increased as much as 20 percent due to existing
design and operation related inefficiencies. The
report identifies three types of opportunities to
achieve capacity gains within existing treatment
facilities. These are:
Debottlenecking: This involves the
identification and removal of individual
bottlenecks within a facility that can limit the
overall capacity of the WWTP.
Optimization: Reconfiguring an existing unit
process to boost its capacity, or otherwise
improve its performance, for example.
Rerating: Reassessing a unit's capacity based
on actual performance. Historical
information or stress testing is used to
redefine loading and flow capabilities.
Operational Procedures to Increase Capacity
of Existing Units
Use of Existing and New Storage
Storage or flow equalization is used to lessen
operational problems caused by flow rate
variations, to improve the performance of
downstream processes, and to reduce the size
and cost of downstream facilities. This can also
be an effective utilization of capacity which does
not require construction of additional treatment
capacity for infrequent events.
Storage is a means to reduce the magnitude of
peak flow events and to spread the loading to the
WWTP over a period of time. However, storage
does not lessen the volume of water that will
need to be treated. Flow equalization basins,
tunnels, and converted abandoned treatment
facilities are potential methods available to
attenuate peak flow loadings conveyed to
WWTPs.
Storage capacity should be evaluated for the
necessary volume needed based on storm
frequency, duration, and intensity. Determining
the total volume of water to be treated and
subtracting the design volume of the WWTP will
indicate the needed capacity. If possible, initial
removal of solids, mixing of the water, and
flushing of the basin post storm event will lessen
operational difficulties (WEF, 2006).
Storage can be applied upstream, midstream, or
downstream (Field & O'Connor, 1997). In-sewer
storage and use of unused treatment units for
water storage will be the least expensive.
Solids Removal
Solids removal can greatly impact treatment
plant performance. The performance of both
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primary and secondary clarification plays a large
part in the overall efficiency of the biological
treatment system at publicly owned treatment
works (POTWs). Optimizing the performance of
clarifiers during wet weather events will improve
the ability of the treatment systems to respond to
peak wet weather flows and improve the effluent
quality of the discharge. There are a number of
options that can be applied to both the primary
and secondary clarifiers. Some of these include:
Chemical Enhancement. Chemical
Enhancement involves adding coagulant and
flocculant chemicals to wastewater in order
to accelerate the process of separating and
removing solids. Coagulants are chemicals
that help to form larger, heavier particles.
Flocculants aid coagulants in the clarification
process by bridging and binding solids
together, thereby enhancing their settling
capabilities.
Baffle Installation in Clarifiers. Density
currents can channel solids through a clarifier
and over a weir, reducing effluent quality.
These density currents can occur in both
circular and rectangular clarifiers. Dye
testing can be used to identify if such density
currents exist and to assist in the placement
of baffles.
Operational Control
In addition to solids removal, modifications to
the biological treatment units can greatly
improve peak flow performance. For those
WWTPs employing suspended growth biological
treatment, possible operational approaches
include:
Biosolids Control. Maintaining the proper
mixed liquor suspended solids (MLSS)
concentration for the type of treatment
system being utilized can lessen the potential
for washout during peak flows. The goal is to
eliminate the build up of excessive solids
(MLSS) in aeration basins to lessen the
potential for washout and to maintain only a
MLSS concentration that results in adequate
treatment.
Return Activated Sludge Rate Control.
Regulating the rate of return of biosolids
from the secondary clarifier to the biological
treatment unit(s) can ensure proper solids
content in the aeration basins and sludge
blanket in the clarifier. This is vital to
ensuring that wash out of microorganisms
does not occur during peak flow events.
Aeration Control. A low level of dissolved
oxygen (DO) during periods of peak flow
can result in lower effluent quality and
promote filamentous growth which can affect
solids removal during secondary
clarification. The DO concentration can be
increased through surface aeration as well as
diffused air equipment.
Aeration Basin Configuration
Plant performance during peak flow events can
often be improved by modifying the flow of
water through the aeration basin with different
configurations, including:
Step Feed. The step feed process introduces
wastewater following primary clarification
into the aeration basin at several points in the
basin. Since this could lower removal
efficiencies for organics, it is recommended
this only be used during peak flow events.
The advantages to utilizing a step feed
approach during wet weather events include
equalizing the food-to- microorganism ratio
and dispersing shock loads to the biological
treatment system.
Contact Stabilization. Contact stabilization is
a modification of the traditional activated
sludge treatment process. The return
activated sludge is reintroduced to the
aeration basin downstream of the
conventional point. The return activated
sludge is aerated prior to being blended with
the influent to the basin. Again, this should
only be utilized during peak flow conditions
since it may reduce organic treatment
efficiencies. Use of contact stabilization
during peak flow conditions can help reduce
solids losses during hydraulic surge events
since the return activated sludge is
introduced to the aeration basin at a different
location than the direct influent flow.
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For those facilities using fixed film biological
treatment systems, there are also operational
modifications that can be made to better manage
peak flow conditions, such as the following:
Reduce/Stop Trickling Filter or Rotating
Biological Contactor Recirculation Flows.
The recirculation of wastewater in trickling
filters is commonly used to ensure adequate
wetting of the media. For rotating biological
contactors, recirculation is often used to
promote biological growth. During peak flow
conditions, recirculating water is not
necessary and can be temporarily reduced or
halted to allow for increased hydraulic
capacity throughout the treatment system.
Adjust Trickling Filter Distributor Arm
Speed. Some distributor arms for trickling
filters are hydraulically driven. During peak
flow events, this may result in arm speeds
that are problematic due to too much
rotational speed. Nozzles can be installed on
the distributor arm that discharges in the
opposite direction of the normal nozzles thus
slowing down the rotational speed of the
arms. The new nozzles can be capped to
return the arm to normal operation once the
peak flow has passed and operations return to
normal.
Chemical Disinfection
During times of peak flow conditions, the
contact time of wastewater to chemical
disinfectants such as chlorine or bromine may
not be adequate for disinfection. It is important
to ensure the influent that flows to the contact
chamber, and the chemical disinfectant, are
completely mixed before entering the chamber.
High rate disinfection processes that have been
shown to be successful include (Field and
O'Connor):
Increased mixing intensity;
Increased disinfectant concentration (Care
must be taken not to overdose, unless you
use UV or ozone. If disinfectants are used in
sensitive waters you have to disinfect or
chlorinate and dechlorinate with care, as
some disinfectants are toxic to aquatic life);
Two stage dosing; or
A combination of the above.
Advanced Physical - Chemical Processes
During peak flows, wastewater volumes can
exceed the capacity of treatment units at
WWTPs. These flows are typically intermittent,
of a generally short duration, high volume, and
may have a lower concentration of pollutants
than a dry weather flow entering the WWTP.
Physical-Chemical processes can help to mitigate
the impact of peak flow conditions.
Installing Parallel Treatment Processes
The installation of parallel treatment trains to
handle peak flow volumes can be a viable option
to WWTPs. A physical-chemical system has the
advantage of being able to be used intermittently
and handle more dilute concentrations of
pollutants.
Options for primary physical parallel processes
include microscreens, plate settlers, and
screening followed by dissolved air flotation,
among others (Field & O'Connor). Effluent from
the parallel process can either be discharged back
to the WWTP headworks or, if all NPDES
permit limitations and conditions are met,
discharged directly to the receiving water.
Chemically Enhanced Primary Treatment
(CEPT). CEPT is one type of chemical
enhancement process that employs coagulants
and flocculants in conventional primary
clarifiers.
CEPT allows the sedimentation basins to operate
at higher overflow rates, while still maintaining
high removal rates of total suspended solids
(TSS) and biochemical oxygen demand (BOD).
Hence the treatment infrastructure can be
smaller, which reduces capital costs.
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Table 1. Performance data summary for select supplemental treatment
technologies: Hydraulic Capacities and Removal Efficiencies.
Technology
Primary
Clarification
Screening
Coarse (5-25 mm)
Fine (0.1-5 mm)
Micro (less than
0. 1 mm)
Vortex Separation
Ballasted
Flocculation
Chemical
Flocculation
Source(s)
Metcalf and Eddy 1991;
NEIWPCC1998;
WEF1996
Metcalf and Eddy 1991
EPA 1996;
Boner et al. 1995;
WERF 2002
Radickef a/. 2001;
Scruggs et al. 2001;
Vick2000;
Poppe et al. 2001
Metcalf and Eddy 1991;
Moffa1997
Hydraulic
Capacity (gpd/ft2)
600-3,000
21,000-86,000
150-1,400
150-1,400
Up to and greater
than 100,000
Up to 90,000
Up to 20,000
BOD Removal
(Percent)
25-40
Not Available
Not Available
Not Available
Up to 55 a
65-80
40-80
TSS Removal
(Percent)
50-70
15-30
40-50
40-70
5-60
70-95
60-90
Table 2. Cost data summary for select supplemental treatment
technologies: Capital and O& M Costs.
Capital Costs
($/103m3/d)
Conventional Primary 3.1 4.2
Treatment
Conventional Primary Plus 9.1 9.8
Biological Secondary
Treatment
O&M Costs Total Costs
($/106m3) ($/106m3)
0.8 0.9 1.7 2.1
1.21.6 3.5 4.3
Chemically Enhanced
Primary Treatment
4.2 5.3
0.91.1
2.1 2.6
Additionally, CEPT provides the opportunity for
either reducing the size of subsequent treatment
units, or increasing the capacity of existing
conventional treatment plants, such as activated
sludge basins. The addition of metal salts and/or
a polymer will only require tanks for the
chemicals and injection equipment. Tables 1 and
2 present performance and cost data comparing
for several technologies and operational
measures.
Vortex/Swirl Separators
Vortex/Swirl Separators use centripetal force,
inertia, and gravity to send heavier solid particles
to the center and bottom of the swirling flow.
When configured to operate within a WWTP,
vortex/swirl separators help to remove solids
prior to primary treatment. This existing
technology has been successfully used at
treatment plants for many years for wet weather
flow treatment. Vortex/swirl separators are
compact systems that provide flow regulation
and some removal of solids and floatable
material. The flow in vortex/swirl devices
initially travels around the perimeter of the unit.
Flow is then directed into an inner swirl pattern
with a lower velocity than the outer swirl
(Figures 1 and 2).
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Figure 1. Onondaga Lake Improvement
Project (2005), New York, Schematic Diagram
Figure 2. Hydro-International (2007)
The concentrated underflow passes through an
outlet in the bottom of the vessel while the
treated effluent flows out of the top of the vessel
(i.e., "overflow water" flows down through
central pipe to outlet "effluent" at bottom).
Vortex/ swirl separators for wet weather flow
treatment typically would be installed offline and
would be empty at the start of a wet weather
event.
One technology that uses vortex-swirl systems
in wet weather applications is the Hydro
Stormwater Management System offered by
Hydro International (Portland, Maine). The
system consists of three basic phases:
a high-rate rotary-flow vortex separator to
achieve clarification of excess flow (an
example would be at a CSO point in a
combined sewer system);
an off-line storage system for clarified
effluent; and
controlling the flow from the catchment to
the collection system and on to the treatment
plant.
Vortex units have the primary advantage of
operating at surface-overflow rates ranging from
5,000 gallons per day per square foot (gpd/sf) to
6
100,000 gpd/sf. This technology removes
floatables and settleable solids by directing the
flow tangentially into a cylindrical tank, creating
a vortex (Figure 1). The vortexing action tends to
concentrate settleable solids towards the center
of the tank and removes the concentrated solids
through a foul sewer outlet located at the bottom
of the tank. The vortex separator has no moving
parts and is designed to operate under high
surface loading rates. Power is not required for
operation of the unit, although influent, effluent
and underflow pumping may be required
depending on the hydraulics of the specific
installation. Operation and maintenance
requirements are low since the majority of the
captured settleable solids and floatables are
flushed into the foul sewer during the storm
event.
Ballasted Flocculation. Ballasted flocculation is
a high-rate sedimentation process that introduces
coagulation and flocculation agents during high
speed mixing to promote settlement and enhance
solids removal (Figure 3). In the process, flow
enters the first zone of the facility where the
coagulating agent is added and mixed with
diffused air. The coagulating agents are typically
metal salts and/or polymer. The flow then enters
the second zone where the flocculating agent
together with a flocculating aid, either
recirculated sludge or sand, is added. In this area,
gentle mixing occurs to promote the formation of
suspended floe particles. The flow then enters
the settlement zone where the dense floes settle
out and are concentrated at the bottom of the
basin. Clarified effluent passes through a
lamellar settling zone to remove residual floe
particles and the final effluent is discharged. The
concentrated sludge is either recycled back to the
second zone or wasted. Sludge from
technologies utilizing sand as a flocculating aid
are conveyed through a separation process
whereby the sand is separated from the waste
sludge and recycled back into the process or
stored for future flow events (Onodaga Lake
Improvement Project, 2005).
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sludge
hydro cyclone
polymer
micro-sand & sludge to hydrocyclone
coagu-
lant
raw
water
INJECTION
COAGULATION MATURATION
TUBE
SETTLING w/ SCRAPER
Figure 3. Ballasted Flocculation
(Onondaga Lake Improvement Project, N.Y.,
2005)
Chemically Enhanced Clarification (CEC). also
known as High Rate Clarification, is another
type of chemical enhancement process that
involves using coagulant and flocculant media in
high-rate clarifiers such as DensaDegฎ and
Actifloฎ to form dense, high settling velocity
floes. Certain CEC systems will operate as either
chemical flocculation processes or ballast
flocculation processes to settle effluent solids.
Chemical flocculation relies on metal salt
coagulants and anionic polymers for solids
removal; whereas ballast flocculation relies on
metal salt coagulants and anionic polymers
coupled with materials such as microsand or
chemically enhanced sludge for solids removal.
ADVANTAGES & DISADVANTAGES
Storage
Storing effluent in either a flow equalization
basin or a converted, formerly abandoned
treatment process can provide WWTP operators
the ability to manage and store excess flows.
This helps maintain treatment efficiency and
allows for a greater volume of flow to receive
more treatment.
Availability for expanding other treatment
processes on the WWTP site should be taken
into account when considering constructing on-
site storage systems. In addition, restored
facilities used as on-site storage systems,
depending on their age, may deteriorate faster
than a newly constructed flow equalization basin.
Finally, on-site storage systems have finite
capacity, which may not be sufficient to prevent
combined sewer overflows (CSOs) or sanitary
sewer overflows (SSOs) from occurring. In
considering storage, factor in the impacts of
weak (diluted) influent on the capability of the
plant to consistently achieve permit limits.
Vortex/Swirl Separators
The major advantage of using vortex/swirl
separators is their ability to remove suspended
solids and floatables in effluent while dampening
volumetric surges. In addition, vortex/swirl
separators require little maintenance, as they
contain no moving parts. Furthermore, these
devices require only a small footprint for
placement and installation. Additional
advantages include no external power source
requirements and low system headless.
Limitations of the vortex/swirl separator are their
inability to: remove fine and soluble products:
disinfect excess wet weather flow (some newer
systems can provide disinfection capabilities):
process floatables during extreme wet weather
flows; and maintain efficiency when equipped
with sump systems.
Chemical Enhancement
Chemically Enhanced Primary Treatment, when
implemented, has the potential to increase TSS
removal efficiencies from a range of 55 to 65
percent to a range of 75 to 85 percent. Similar
improvement potential exists for BOD removal.
As a result, this technology may offer
downstream processes (i.e., aeration basins,
secondary clarifiers) greater latitude to operate
efficiently under increased flow conditions.
While CEPT offers greater removal efficiencies
of organic matter, its surface overflow rates
function under a similar range to that of
conventional primary clarifiers (3000 to 3500
gpd/ft2 [122 to 143 L/m2-d]). Consequently,
CEPT requires a footprint similar to
conventional primary clarifiers.
"Chemically Enhanced Clarification" systems
achieve TSS removal efficiencies in the range of
80 to 95 percent. In addition, CEC systems
operate at surface overflow rates 20-to-50 times
greater than conventional gravity settling, while
requiring a footprint that is typically only 5 to 15
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percent of the space required for installing
conventional primary clarifiers. In other words,
CEC systems remove solids at faster rates and
with a smaller footprint compared to
conventional clarification systems. On the other
hand, CEC systems have a limited ability to
remove soluble pollutants. The greater the ratio
of soluble to solid BOD, the greater the
likelihood of expecting reduced BOD removal
rates. In addition, CEC systems incur higher
operational costs compared to conventional
treatment systems due to cost of using and
disposing of chemicals and sludge.
"Ballasted flocculation" has been reported to be
capable of removing nearly 100% of settlable
solids, up to 84% of TSS, 54% of BOD, 25% of
TKN, and 90% of TP in CSO applications.
However, it has been reported that the process
requires approximately 10 to 30 minutes of
startup time in order to stabilize before it is able
to accomplish the above-stated pollutant removal
efficiencies. In addition, preliminary screening is
required before the flow is treated with ballasted
flocculation. The operation and maintenance
concerns associated with the system are high,
due to the requirements for screenings disposal,
chemical addition, sludge processing and
disposal and the relatively high consumption of
power.
The advantage of a ballasted flocculation system
is that it provides a high degree of treatment.
One disadvantage of this process is the physical
size of the facility required. Ballasted
flocculation facilities would cover approximately
twice the surface area of vortex facilities. Other
disadvantages are the time required to stabilize
the system and high operation and maintenance
concerns. As such, this technology is not
considered feasible for remote, unmanned CSO
treatment sites.
Unit cost data for these technologies is presented
in Table 3.
Table 3. Performance data summary for select supplemental treatment technologies:
Unit Capacities, Capital Costs, and Unit Costs
Technology
Primary Clarification
Screening
Vortex Separation
Vortex Separation with
Blending
Ballasted Flocculation
Chemical Flocculation -
Aluminum as Additive
Chemical Flocculation -
Ferrous Sulfate as Additive
Source(s)
Hufford 2001
EPA 1999
Sacramento 1999
Sacramento 1999
Wendle 2002
Hufford 2001
WERF 2002
Bremerton 2002
Hewing et.al, 1995
Hewing et.al. 1995
Capacity (MGD)
78
0.75-375
1.8-16.2b
0.71 -194
15
78
100
20
Not Available
Not Available
Estimated Total
Capital Cost a
$11.0 million
$40,800 -
$2.2 million
$10,000 -$50,000
$13,000 -$630,000
$5.5 million
$12.4 million
$20.0 million
$4.0 million0
$0.50 (cost per pound)
$0.17 (cost per pound)
Unit Cost a
(Per Gallon /
Day of Capacity)
$0.14
$0.01 -$0.05
$0.01
$0.01 -$0.02
$0.37
$0.16
$0.20
$0.20
$0.04 (per gallon
treated )d
$1.03 (per gallon
treated )d
Costs in 2002 dollars.
Vortex separator capacities are hydraulic capacities. Manufacturer recommended design capacities for optimal TSS
removal are generally 25 percent of the hydraulic capacities.
Includes costs for a 20 MGD Ultraviolet (UV) disinfection process. Cost for ballasted flocculation alone was not available.
Capital costs for chemical feed mechanisms not available. Treatment costs include chemical costs and sludge handling
costs.
Ferrous sulfate generates larger sludge volumes than aluminum, significantly increasing treatment costs.
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IMPLEMENTATION
Vortex/Swirl Separators
The City of Columbus, Georgia has installed
several treatment systems as part of an
evaluation and demonstration study conducted
through a Congressional appropriation and EPA
grant. Several technologies have been evaluated,
including vortex separators. Managing and
controlling flows upstream of the treatment plant
can lessen the need for collection system
infrastructure construction. That can result in
large cost savings for municipalities. The five
year program of operations and performance
testing of the full-scale facilities has shown that
the system demonstrated at full scale in
Columbus is equivalent to, or better than,
primary clarification. Cost savings can be up to
one-half that of primary clarification and occupy
one-tenth the footprint of conventional systems
(Andoh, et al. 2002).
Chemical Enhancement
In January 2006, the City of Tacoma,
Washington, began construction of a 75 mgd
ballasted flocculation process at the City's
Central Treatment Plant (CTP) to parallel
existing primary clarification systems at the plant
during wet weather events. The peak wet
weather flow component is a portion of a much
larger upgrade to the entire facility. The city
developed a plan for maintaining adequate
capacity at the CTP since the current separate
sanitary sewer system was capable of delivering
up to 110 MGD to the CTP. The existing
primary treatment and disinfection processes can
handle a capacity of 103 MGD and the existing
biological treatment processes can handle a daily
load of 78 MGD.
Cost became another factor in deciding to
supplement Tacoma's CTP with ballasted
flocculation systems and related wet weather
processes as its estimated construction costs
($50.7 million) were less in comparison to the
cost of expanding just the existing activated
sludge processes onsite ($130 million).
Pilot tests demonstrated effluent TSS
concentrations below 30 mg/L (with the
exception of the first day of testing) and TSS
removals ranging from 79 to 92 percent as well
as effluent BOD concentrations ranging from 20
to 42 mg/L and BOD removals ranging from 63
to 73 percent.
The commissioned ballast flocculation process
was scheduled to operate in parallel with the
existing processes employed at Tacoma's CTP
but only during wet weather events. The city
anticipates the commissioned system to operate a
maximum of 5.5 days in a row, 8 days in a
month, and 21 days per year.
Other Sources of Information
The Office of Wastewater Management has
developed additional fact sheets on storm water
technologies that are available. These fact sheets
can be found at http://www.epa.gov/owm/mtb/
mtbfact.htm.
ACKNOWLEDGMENTS
EPA acknowledges peer reviewers Richard Field
and Thomas O'Connor for their contributions to
this fact sheet.
OTHER INFORMATION SOURCES USED
City of Tacoma, Washington:
http://wspwit01.ci.tacoma.wa.us/es/ctp/inter/ctp
web site/default, asp
REFERENCES
Hydro International PLC.
www.hydro-international.biz/us
Metcalf and Eddy, Inc., 2003. Wastewater
Engineering: Treatment, Disposal, and Reuse,
4th ed. Boston: Irwin McGraw Hill.
Field, R. and T.P. O'Connor. "A Control
Strategy for Storm Generated Sanitary Sewer
Overflows." Journal of Environmental
Engineering, American Society of Civil
Engineering, Vol. 123, No. 1, pp. 41-46,
January, 1997.
Field, R. and T. P. O'Connor. "Optimization of
CSO Storage and Treatment Systems."
Journal of Environmental Engineering,
American Society of Civil Engineering, Vol.
-------
123, No. 3, pp. 269-274, March, 1997.
Field, R. and T. P. O'Connor. "U.S. EPA Urban
Wet-Weather Flow Research Program
Overview on Sanitary Sewer Overflow
Control Urban Wet-Weather Flow
Program." National Risk Management
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Professor Robert Y. G. Andoh*, Stephen P.
Hides** and Professor Adrian J. Saul***
&EPA
United States
Environmental Protection
Agency
EPA 832-F-07-016
Office of Water
September 2007
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