United States
                      Environmental Protection
                      Off ice of Water
                      Washington, D.C.
EPA 832-F-99-036
September 1999
Combined  Sewer Overflow
Technology  Fact Sheet
Maximization  of  In-line Storage

This fact sheet describes technologies designed to
maximize the in-line storage capability of combined
sewer systems, thus reducing the frequency and/or
severity of Combined Sewer Overflows (CSOs).

Sewer systems that convey  both sanitary sewage
and storm water through a single pipe are referred
to  as combined sewer systems (CSSs).  In dry
weather, the CSS is able to convey all flows to the
wastewater treatment facility.  During periods of
heavy rainfall, however, the capacity of the CSS
may be exceeded and an untreated combination of
sewage and storm water may discharged directly
into a nearby receiving water.  These overflow
instances are called Combined Sewer Overflows, or

CSOs contain untreated domestic, industrial, and
commercial  wastes.   Contaminants  include
suspended solids, biochemical oxygen  demand
(BOD), oils and grease, toxics, nutrients, floatables,
pathogenic microorganisms and other pollutants.
CSOs  often contribute to exceedances  of water
quality standards and can result in threats  to public
health, aquatic species, and aquatic habitat.

Under the U.S. Environmental Protection Agency
(EPA)'s CSO Control Policy, discharge permits
issued to communities with  CSSs  are expected to
include "nine  minimum controls." The  nine
minimum controls are measures that can reduce the
magnitude, frequency and impacts of CSOs without
significant construction or expense. Maximization
of storage in the collection system is one of the nine
minimum CSO controls.
                     In-line storage capacity is  typically  obtained
                     through the use of control measures downstream
                     from a point where excess capacity exists. Typical
                     control measures include:

                        Inspection of the collection system and removal
                        of obstructions.

                        Maintenance, repair and replacement of tide and
                        control gates.

                        Installation and adjustment of regulators.

                        Reduction/retardation of inflows.

                        Upgrade/adjustment of pumps.

                        Real time monitoring.

                     These control methods are discussed in more detail

                     Collection system  inspection and removal of

                     Collection system inspection involves investigation
                     of pipes and inflow points to determine where
                     sediments, debris,  and other obstructions are
                     preventing the system from operating as designed.
                     Removal of obstructions can reduce CSO volume
                     by increasing the amount of storage capacity in the
                     system. Sediments and small obstructions can be
                     removed by sewer flushing; large obstructions are
                     often  removed manually.   Routine  inspection
                     ensures that obstructions are identified and removed

                     Tide and control gate maintenance, repair and

Tide  gates  are  designed to reduce  or  prevent
receiving water from flowing back into the sewer
outfall during high tide.   Proper maintenance is
essential for the tide gate to function as designed.
Leaks  and cracks  that allow flow to  pass alter
design performance and  reduce  available in-line

Flap style tide gates are commonly used to resist
tidal influences.  These gates are typically hinged
stainless steel doors attached to the outfall, opening
and  closing dependent on the relative pressure
applied by CSO flows or tidal influence..  While
simple in  construction and operation, typical flap
gates are subject to fouling and sticking and require
appreciable hydraulic heads to operate against their
own weight (WEF,  1989). Commonly encountered
problems with typical flap gates include warpage,
corrosion, and the tendency to become stuck in one
position (U. S. EPA, 1993).

Elastomeric tide gates may  be considered  when
installing or replacing a tide gate.  An elastomeric
tide gate is a positive  pressure device that forces
CSO flows to open an otherwise collapsed rubber-
based valve.  Elastomeric tide gates can overcome
many of the problems encountered by typical flap
gates. They are designed to open with smaller head
requirements and to close over larger debris.  A
typical elastomeric check valve appears in Figure 1.

The  use of flap  gates and elastomeric tide gates
allows greater retention volume in the collection
system by minimizing tidal inflows. The benefits
of these  devices  are greater  at outfalls with
significant tidal influence.

Regulator Installation and Adjustment

Regulators  control the  amount  of  flow  to  a
downstream point and provide an outlet for flows in
excess  of the sewer capacity.  Adjustment  of
regulator settings, proper regulator maintenance and
increasing a regulator outlet to the interceptor are
control  measures that can ensure optimal system
performance and maximize in-line storage.

A vortex  valve is  used to discharge  high  flows
through a spiral action valve while leaving low
flows untouched. Vortex valves have been used to
Source: WEF, 1989.

                CHECK VALVE

divert flows to CSO treatment facilities, control
flow  out of storage facilities,  and replace failed
mechanical regulators (U.S. EPA, 1993). Since the
flow through the vortex value is constant during
storm  events,  it  serves  to  dam  high  flows.
Obstructions to flow such as trash and debris pass
through the vortex, reducing  maintenance costs
over traditional valves.

Inflatable  dams are popular  control measures
whereby a rubberized fabric device is inflated and
deflated to control flows and maximize storage in
designated points in the combined sewer system.
Inflatable dams are usually activated by automatic
sensors that measure flow levels at specified points
in the system. Generally, very little maintenance is
required for inflatable  dams.  The air or water
supply used to inflate the dam, however, should be
inspected  regularly.   A typical  inflatable  dam
appears in Figure 2.

Many other mechanical regulators are designed to
operate under the control of gravity. These devices
are  semiautomatic  and  mechanically variable,
depending on flow.  Because  these systems are
activated by float mechanisms or pressure plates,
they require little maintenance, and they eliminate
the need for power sources.

As conditions change with system demands  over
time, regulator  adjustment  ensures an  optimal
routing  of flows in the  combined sewer  system.
Regulators  in areas of  growth may need to  be

                       To Interceptor
Source: WEF, 1989.

adjusted  to allow additional flows  beyond  the
intended  discharge (U. S. EPA, 1995). Increasing
regulator outlet to the interceptor can be allowed if
sufficient  capacity  exists in  the  downstream
interceptor or the wastewater treatment plant.

Reduction/Retardation   of  Inflows  and

The  reduction or  retardation  of inflows and
infiltration (I&I) lowers the magnitude of the peak
flow passing through the collection system, helping
to prevent the occurrence of CSO events.

Inflows are wet weather flows resulting  from  the
entry  of  storm water  directly  into the  CSS.
Examples of  inflows include  parking  lots and
surface roads containing storm water inlets, and
roof and sump drains connected to  storm  water
inlets,  either directly or  from sheet flow over
impervious surfaces. Infiltration is that flow that
enters the CSS through joints, cracks and manholes
as a result of wet weather events.  Infiltration can
also enter the system from leaking tide gates.

Inflows can be reduced by detaining or infiltrating
storm water runoff at the point of generation (e.g.
onsite ponds, tanks,  infiltration trenches, porous
pavement), in regional detention facilities, and by
disconnecting  or blocking roof, sump pump and
perimeter drains.  Commercial devices that retard
the  rate  of inflow can also be installed.  They
include storm water inlet flow restriction grates and
piping, and hydrobrakes.  Before installing such
devices,  care  should  be taken to  ensure that
sufficient  drainage   is  provided   to   prevent
upgradient flooding.

I&I studies  can be performed  to determine  the
extent and  severity  of CSS capacity reduction
impacts.  Manhole inspection, smoke testing and
television inspection  are common  methods  of
measuring  I&I.   Infiltration  can be corrected
through sewer repair, relining and replacement, and
root control in areas  with  mature or aggressive

Upgrade and Adjustment of Pumps

Upgrading/adjusting a  lift station pump can result
in additional in-line storage in upgradient portions
of the system, if sufficient hydraulic downgradient
capacity  is  available.  Effects  of the additional
pumping rates  on the ability of the  wastewater
treatment plant to treat increased flows must first
be determined.

Raising Existing Weirs and Installation of New

Raising existing weirs and installing new weirs
utilizes in-system storage by controlling discharge
in the overflow conduit. By controlling discharge,
the CSO volume and frequency are typically
reduced.     The  installation  of  weirs   can  be
implemented in stages so as to reduce the risk of
surcharging. Masonry bricks, concrete blocks and
stop logs are commonly used media for weir

System of Real-time Monitoring/Network

A computerized system to control regulators can be
set up to maximize in-line storage.  In 'real-time'
systems,  flow and precipitation data are measured
by sensors throughout the system.  The collected
data is transferred immediately from the measuring
device to a central control computer through  a
communications system. The controlling computer
uses the data as input for system operations.  The
control center then automates  control of gates,
pumps and other regulators based upon the analysis.
The process allows for the instantaneous change of
control  settings  in   response   to  changing

precipitation and flow conditions throughout the
service area.


Implementation of control measures that maximize
in-line  storage can  result  in  the lowest  cost
additional storage  alternative.  In-line storage  is
appropriate for systems with limited space for other
types of controls.  Construction  disruption  is
minimized, and available storage can include any
part  of  the  system  with  potential to  contain
additional CSO volume.  Potential for the greatest
performance  improvements will  be found  in
systems  where trunk and interceptor lines can
handle  additional  flow  volumes.   Additional
treatment of combined flows can be realized via
settling of sediments within the system. Access to
areas where increased detention and settling  is  to
occur is  important as sediments must periodically
be removed.

Monitoring and modeling systems are frequently
used tools to determine available storage locations.
Hydraulic  monitoring of the  system  provides
necessary information on flow characteristics.
Computer modeling of rainfall and flow data can
simulate system response to event conditions and,
thereby,  assist planners in determining the most
appropriate areas for maximizing in-line storage.


Limitations of in-line storage include: the potential
for backing flows (surcharging) into basements  or
streets; the amount of additional flows the system
can  detain;   and  increased   operation  and
maintenance requirements.

Before applying an in-line storage control measure,
the hydraulic grade line must be evaluated in order
to determine the effect of the proposed measure on
the  system.   The hydraulic  grade line is an
imaginary line obtained by adding the force of the
flow (pressure head) and the  elevation of the flow
relative to the outfall discharge (elevation head) at
a given  point in the collection system. A serious
potential for surcharging flows into basements  or
streets exists without proper hydraulic evaluation of
proposed in-line storage measures. If a computer
model is used, the maximum height of the hydraulic
grade line should be  specified so that control
measures designed to maximize in-line storage do
not cause surcharging of the system.

Combined sewer systems must maintain minimum
flow velocities in order  to transport  solids and
sediments to the wastewater treatment facility. This
may limit the increase in the volume of flows that
can be detained with additional  control measures.
Effects of each in-line storage measure should be
evaluated before implementation to determine the
effect on the conveyance  of dry weather flows to
the wastewater treatment facility,  and on  other
existing and proposed controls.

Most control  devices have routine  operation and
maintenance requirements. With increased volume,
additional solids and debris will collect in the trunk
and  interceptor  lines. Therefore, maintenance
requirements for the system usually increase with
control measures designed to  maximize in-line

By reducing  the volume of CSOs  released to
receiving water,  maximization of in-line storage
reduces the adverse impact of BOD, TSS, nutrients,
and other contaminants.

Surcharging of the collection system is a possible
limitation of maximizing in-line storage.   The
likelihood of surcharging, and the associated public
health risks, must be evaluated before implementing
control  measures  to  maximize in-line storage.
Flooding problems can be avoided through effective
monitoring efforts, hydraulic modeling, and/or
physical barriers such as disconnection of basement
sump pumps and perimeter drains.


The performance of maximizing in-line storage for
CSO control can be examined through a series of
case study examples outlined below.

Cleveland, Ohio Area

The Northeast Ohio Regional Sewer District began
using an automated control system for its combined
sewer system in 1972 and has expanded the system

over time. The current system comprises sensing
elements, field control elements, communication
equipment, and a central control center. A network
of 25 rain gauges, 24 remote level monitors, 56
remote flow monitors, and local  level monitors at
the regulators comprise the sensing system.  Data
collected from these devices are sent via modem or
radio to a central control facility. The entire system
is connected by a central computer which operates
on a local area network  of personal computers.
Twenty-nine automated regulators, all which have
distributed   control  with programmable  logic
controllers (PLCs), are used to control the amount
of flow in various portions of the system.  This
system was augmented by physical in-line changes,
whereas fixed weirs were replaced by gates and
inflatable dams.  The PLCs use  the sewer levels,
gate  positions, and dam pressures as variables to
compare  with fixed  points.  By comparing the
variables and fixed points, the amount of available
storage and CSO discharge is determined.  In-line
storage  is maximized by adjusting  controlling
regulators based on current system conditions.

Results of the Cleveland area real time monitoring
and control system have proven  to be efficient in
achieving  significant reductions in  overflow.
Interceptor  capacities have  been maximized by
utilizing in-line storage, and reductions in BOD(5)
and TS S discharges to the environment have proven
to  be  cost  effective through the  operation  of
automated regulators.   Less  than one percent of
capital improvement funds since 1972 have  been
spent on real time control systems (Hudson, 1994).

On average, the automated system has prevented
about 2 MGD of untreated flow from entering the
environment every day.   In  1994 approximately
950,00 Ibs. of BOD(5) and 6.7 million Ibs.  of TSS
were captured.  The operating cost for storage of
these parameters were $0.15 and  $0.16 per pound,

Boston, Massachusetts Area

In  1993,  the Massachusetts Water Resources
Authority   (MWRA)  completed  a   System
Optimization Plan (SOP) for Boston and three other
nearby CSO communities.  The SOP identifies
relatively   simple  structural   or  operational
modifications that can reduce CSO frequency and
volume. The SOP projects are designed to be low in
cost and easy to implement.

Recommended improvements included raising weir
elevations,  repairing  regulators,   constructing
regulators and  weirs, plugging and  abandoning
certain overflow pipes, and replacing or repairing

The MWRA recommended SOP measures at 103
locations.  As of June 1997, approximately 95% of
SOP projects have been completed (Parker, 1997).
The MWRA SOP analyzed the projected volume
and percent reduction  as  modeled for  each
optimization  project.   It  also  analyzed  the
cost/benefit ratios for each project. The MWRA
estimates that SOP improvements would reduce the
volume of CSOs (including treated CSOs) by over
25% (MWRA, 1993).

Norwalk, Connecticut

The City  of Norwalk, CT, eliminated untreated
CSO discharges in 1980 by separating some outfalls
and bringing all remaining flows to the wastewater
treatment plant (WWTP). Like many old combined
sewer  systems, Norwalk  has  large  diameter
connecting and interceptor lines. Utilization of the
maximum capacity  in the  system allows for the
greatest treatment of CSS flows at the WWTP.
This is accomplished through regulation of the
plant's main gate. Existing in-line storage capacity
is utilized to hold as much flow as possible until the
storm event passes.  Operators use rain gauges and
monitoring of a large siphon at the main interceptor
junction to assist in regulating the plant's main gate.
Regular maintenance and cleaning of sewer lines
after storm events ensure that the greatest possible
volume of flow is stored in the collection system.

During  certain storm  events,  excess flow not
handled by the WWTP  enters a CSO treatment
facility  that  provides  micro  screening   and
disinfection. Flows entering the micro screening
units pass through spacings of 33 microns up to 256
microns, depending  on volume of flow. All flows
are then routed through the WWTP chorine contact
chamber  for disinfection. The  CSO treatment
facility averages 30 per cent BOD(5)  and TSS

removal, with some smaller storm event removals
of up to 50 per cent.

The Norwalk WWTP is undergoing an upgrade to
biological  nutrient removal (BNR).   The plant
treatment capacity will be increased from 20 MOD
to 30 MGD per day. As this facility comes on-line,
maximization  of   in-line storage will allow
maximum treatment of wet weather events to the
BNR facility.  Since the existing  CSO treatment
facility provides only the  equivalent of  primary
treatment,  significant water quality improvements
will be obtained by  maximizing treatment to the


Techniques to  maximize in-line storage are most
cost-effective in trunks and interceptors with mild
slopes and large storage capabilities.

Capital and O&M costs vary considerably based the
on  characteristics of the system and the type of
control measure implemented. Ancillary costs may
include traffic  rerouting and traffic control plans,
proper  ventilation  requirements,  and  post
construction monitoring.

Some in-line  storage measures do not  require
capital  improvements.   In  Norwalk,  CT,  for
example,  no  capital  costs  are  involved with
regulation of the main control gate.  Expensive
replacement of drum screens and associated O&M
at the  CSO treatment  facility is reduced by
maximizing in-line  storage.    O & M  for  the
collection  system does increase as a result of the
operational modification.

A comparison of costs   for  control  measures
implemented by  various communities appears in
Table 1.


1.     Association  of Metropolitan  Sewerage
       Agencies (AMSA), 1994. Approaches to
       Combined  Sewer  Overflow  Program
      Development. Washington, D.C.
2.      Hudson, D.  M.,  1996.   Protecting  the
       Waters of Lake Erie through Real Time
       Control of CSO's in the Cleveland Area.
       Proceedings, Urban Wet Weather Pollution
       Control Sewer Overflow and Storm Water
       Runoff,  Water  Environment Federation,
       Quebec City, Quebec.

3.      Massachusetts Water Resources Authority,
       1997.  David Parker, Massachusetts Water
       Resources Authority personal communication
       with Parsons Engineering Science, Inc.

4.      Metcalf & Eddy, 1993. System Optimization
       Plans for CSO Control.  Prepared for the
       Massachusetts Water Resources Authority.
       Wakefield, MA.

5.      Northeast Ohio Regional  Sewer District,
       1997.  Daniel  Hudson,  Northeast Ohio
       Regional   Sewer  District,  personal
       communication with Parsons Engineering
       Science, Inc. regarding Construction Cost
       for Auto Regulations.

6.      City of Norwalk, CT, 1998. Fred Treffeisen,
       City   of   Norwalk,   CT,   personal
       communication with Parsons Engineering
       Science, Inc.

7.      U. S. EPA. 1993.  Manual: Combined
       Sewer Overflow Control. Washington D.C.
       EPA 625/R-93-007.

8.      U.S.   EPA,  1995.  Combined  Sewer
       Overflows:  Guidance for  Nine Minimum
       Controls. Washington D.C. EPA 832/B-95-

9.      Water  Environment Federation,  1989.
       Combined  Sewer  Pollution  Control
       Abatement.  Manual of Practice FD-17.
       Alexandria, Virginia.


Massachusetts Water Resources Authority
David Parker
100 First Avenue
Boston, MA 02129

Capital Cost
Annual O & M
 (Inflatable Dams)
 Pump Installation

 Install Raised
Washington D.C.   per dam  $1,219,0001
San Francisco,
Boston, MA Area
 Real Time           Cleveland, OH
 Monitoring System    Area
 Collection System
 Tide Gate
Philadelphia, PA
Norwalk, CT
gpd      $0.10-$0.11 gpd2

per weir   $3,250 brick & mortar3

         $12,050 formed concrete3

         $18,100 stop logs3
per unit   $264,000 auto regulators4

         $12,900 remote level
system   $1,906,0005
Boston, MA Area   per gate  $24,1003


                   automated regulator

                   rain gauge $2,8303

                   flow and level monitor
                   reduction in O & M for
                   microscreening devices;
                   increase in O & M for
                   collection system
ENR = 5750

1 Average cost per dam of 12 dams installed at 8 sites; complete cost includes all monitoring equipment
2 Used to convey flows from CSO treatment facilities to WWTP as conditions allow
3 Typical cost; costs vary by site
4 Average bid cost of automated regulators (usually inflatable dams) in Southerly WWTP district; costs vary by site
5 System = 1 central computer, 8 computer controlled regulators, level measuring devices at 53 regulators, 22 rain gages, flow
metering at 18 stations, several other level devices
Sources: AMSA 1994, Hudson 1996, Metcalf & Eddy 1993, Northeast Ohio Regional Sewer District 1997, Treffeisen 1997.
City of Nashville, Tennessee
Mrs. Lyn Fontana, P.E.
Metropolitan Government
1600 2nd Avenue North 4th Floor
Nashville, TN 37208

Northeast Ohio Regional Sewer District
Daniel Hudson
Environmental and Maintenance Services Center
4747 East 49th Street
 Cuyahoga Heights, OH 44125

City of Norwalk, Connecticut
Fred Treffeisen
Malcom Pirnie, Inc.
60 South Smith Street
East Norwalk, CT 06855

City of Philadelphia, Pennsylvania
Gene Foster
                                 Philadelphia Water Department
                                 Fox and Roberts Streets
                                 Philadelphia, PA 19129

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                                 products  does  not  constitute  endorsement  or
                                 recommendation  for  the  use   by  the   U.S.
                                 Environmental Protection Agency.

                                                   For more information contact:

                                                   Municipal Technology Branch
                                                   U.S. EPA
                                                   Mail Code 4204
                                                   401 M St., S.W.
                                                   Washington, D.C., 20460
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