&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
1.2—1.6 3.5 — 4.3
  Chemically Enhanced
  Primary Treatment
4.2 — 5.3
0.9—1.1
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.

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   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
   Research Laboratory, Cincinnati, OH.
   U.S. EPA, Edison, NJ.
Tacoma, City of, 2000. Ballasted Sedimentation
  Pilot Study Report and Feasibility Analysis,
  Central Wastewater Treatment Plant, City of
  Tacoma. Prepared by Parametrix, Inc.
  Sumner, WA: Parametrix.

Tacoma, City of, 2001. Facilities Plan: Central
  Wastewater Treatment Plant, City of Tacoma
  - Draft Final Edits. Prepared by Parametrix,
  Inc.  Sumner, WA: Parametrix.

U.S. EPA, 2004. Report to Congress, Impacts
  and Control ofCSOs and SSOs. EPA 833/R-
  04-001.

U.S. EPA, 2007. Emerging Technologies for
  Wastewater Treatment & In-Plant Wet
  Weather Management. EPA 832/R-06-006.

Water Environment Federation, 2006. Guide to
  Managing Peak Wet Weather Flows in
  Municipal Wastewater Collection and
  Treatment Systems. Alexandria, Va.: WEF.

Water Environment Research Foundation,  1999.
  Research Priorities for Debottlenecking,
  Optimizing, andRerating Wastewater
  Treatment Plants. WERF.

Onondaga Lake Improvement Project, Onondaga
  County Department of Water Environment
  Protection, New York, 2005.

9th International Conference on Urban Drainage,
  Portland, Oregon, USA, 8-13 September,
  2002. Improving Water Quality Using
  Hydrodynamic Vortex Separators and
  Screening Systems.

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
10

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