United States
                      Environmental Protection
                     Off ice of Water
                     Washington, D.C.
EPA 832-F-99-039
September 1999
Combined  Sewer  Overflow
O&M  Fact Sheet
Proper  Operation  and  Maintenance

Combined sewer  systems  (CSSs), as shown in
Figure 1, are single-pipe sewer systems that convey
sanitary wastewaters (domestic, commercial and
industrial) and storm water runoff to a publicly
owned treatment works.  During periods of heavy
rainfall, however, the sanitary  wastewaters and
storm waters can overflow the conveyance system
and discharge directly to surface water bodies. This
is called a combined sewer overflow (CSO).

CSOs may contain high levels of suspended solids,
biochemical oxygen demand (BOD), oil and grease,
floatables,  toxic pollutants,  pathogenic
microorganisms  and other pollutants.   These
pollutants can exceed water quality standards and
 Source: U.S. EPA, 1989.

                    pose risks to human health, threaten aquatic species,
                    and damage the waterways.

                    Because of the pollution potential from CSOs, EPA
                    issued the CSO Control Policy on April 19, 1994.
                    This policy states that permittees with CSSs that
                    have CSOs  should be able to provide, at  a
                    minimum, primary treatment and disinfection, when
                    necessary, to  85 percent of the volume captured in
                    a CSS on an annual average basis. The policy also
                    includes nine minimum control  requirements for
                    inclusion in the CSO discharge permit.  One of
                    these minimum controls is proper operation and
                    regular maintenance (O&M) programs for the sewer
                    systems with CSOs.

                    KEY PROGRAM COMPONENTS

                    Proper O&M of  combined sanitary sewers and
                    overflows is not significantly different from that of
                    sanitary sewer systems, with the objective being to
                    maintain   maximum  flow  to  the  wastewater
                    treatment  plant and to maximize either  in-line
                    storage  capacity  or detention upstream  of the
                    system inlets. There are several key components of
                    an O&M  program that a  municipality/authority
                    must provide to ensure proper O&M and to meet
                    the minimum control requirement. These program
                    components include:

                         Scheduling   routine   inspections,
                          maintenance and cleaning of the CSS,
                          regulators and outfalls.

                         Developing O&M  reporting and  record
                          keeping   systems   with  maintenance
                          procedures and inspection reports.

                         Providing training for O&M personnel.

      Reviewing the O&M program periodically
       to  up-date  and  revise  procedures  as

These components are further described below.

Operational Review

Prior  to  developing  an  O&M program,  the
municipality  should undertake  an  operational
review of its system to inventory and assess existing
facilities,  operating  conditions  and  maintenance
practices.  The municipality should have a complete
plan of the collection system, showing all sewers
and points where CSOs and outfalls are located.
This plan should reference streets and other utilities
to enable  the maintenance crews to  locate the
structures  and CSOs quickly.  This plan may also
aid in scheduling and planning the inspection and
maintenance of the CSS system and overflows; for
example,  the regions or  areas that  are prone to
flooding   or  premature  overflows  should  be
inspected first after a major storm.

The  nine  minimum  CSO  control  requirements
include conducting a characterization of the CSS.
This characterization should include documentation
of overflow  occurrences and correlation of these
events with rainfall patterns (e.g., volume, intensity,
duration).  The results of the CSS characterization
are critical to designing an  O&M program that is
effective in optimizing system operations.  As part
of these studies, it is important to measure actual
system flows and the response to various operating
and wet weather conditions. This information will
be critical during  the  development of specific
operation and maintenance procedures that will be
part of the O&M program.

Municipalities may eventually be able to use data
from  their  Long-term  CSO  Control  Plans  to
supplement their O&M programs.  As part of these
plans,  a  system may  conduct  modeling  of the
integrated system (sewers, regulators, and treatment
plant) to analyze operational improvements. These
modeling  efforts typically  identify operational
modifications that maximize storage and transport,
provide improved treatment in the existing system,
and decrease untreated  CSO discharges.  Because
many municipalities  will implement their O&M
programs  before  their Long-term  CSO Control
Plans are completed,   the  results of the CSS
modeling  may not be available during the early
phase of the O&M program.  However, the O&M
program should be updated periodically to address
this type of additional information.

Record Keeping System

The O&M program should include a record keeping
component.   The record keeping system should
document  maintenance  procedures  through
inspection reports.  These reports should include
information about when the system was inspected,
and, if applicable, what maintenance  action was
taken,  including  the equipment used  and the
personnel  involved.   Geographical information
systems (GIS) and desktop mapping may be useful
in storing O&M data on the CSO system, as well as
in developing a database of problem areas.

System Operating Procedures

Each municipality should have written  policies,
procedures,  or  protocols  for  training  O&M
personnel and should conduct periodic reviews and
revisions   of  the   O&M   program.   Some
municipalities have reported that alternating crews
between O&M and  other functions  has proven
beneficial because it reduces the tedium of the work
by making it less routine, and it promotes the cross-
training of employees. Other municipalities prefer
devoting personnel strictly to O&M  because it
keeps the work assignments simple.


The  O&M  Program  should  have   established
training goals, procedures, and schedules.  Training
should provide the maintenance personnel with an
understanding of the CSS operations and system
characteristics.  Hands-on training  illustrates the
specific  O&M   procedure  to  those  directly
responsible for performing these  activities.  In
addition, the nature of the O&M work may require
employees to work in confined spaces  or to be
exposed to dangerous gases.  Providing proper
safety training, in accordance with Occupational
Safety  and  Health  Administration  (OSHA)
standards, is imperative. Safety programs should be

reviewed, and, if necessary, updated periodically.
Tide  gates  that  require underwater inspection
should only be inspected by a certified diver.


Proper operation of the CSO  system begins with
proper operation and maintenance of the individual
components - the regulators, tide  gates, pump
stations,  sewer  lines,  and  catch  basins;  and
implementation of an organized plan that provides
regular, consistent, andresponse-orientedO&M.  In
addition,  operators   must  develop  plans  for
determining where CSOs occur, and for conducting
system-specific repairs to prevent future CSOs.

Regulator/Tide Gate Maintenance

Because of the debris normally present in combined
sewage, regulators are particularly susceptible to the
accumulation of materials that cause clogging and
blockages.  Trash blockages at the entrance to the
orifice of the interceptor increase  the  headloss
through the orifice and  causes  the  majority  of
unnecessary overflows in passive regulators. Other
causes of unnecessary  diversions at regulators
include weir plates or dams that are improperly set,
damaged, or broken off. Similarly, tide gate failure
can often be attributed to trash or debris becoming
lodged in the gate, or corrosion of the gate  or
deterioration of the gate gaskets. Tide gate failure
allows the receiving water  to  enter the CSS,
reducing the storage and flow capacity.  For more
information on solids and floatables control, refer to
the EPA's CSO Technology Fact Sheets on Screens
(EPA 832-F-99-027) and Floatables Control (EPA

Frequent inspection of CSO  regulators  and  tide
gates  for the  problems outlined  above,  and
subsequent   program to implement corrective
measures (such as cleaning or repair of the regulator
or tide gate) will ensure maximum storage or flow
capacity.  Inspection of tide gates  is most easily
performed during dry weather and at low tide, when
most installations are above the water level of the
receiving water.   Tide valves that  are below the
level of the receiving water at all times may require
a diver to perform the inspection. Regulators which
have proven to be problematic should be inspected
after every rainfall event.

There are many different ways of determining if an
overflow has occurred at a regulator or tide gate,
how  long  it  lasted,  and  what  volume  was
discharged. For instance, some municipalities have
installed switches on their tide gates that  sense
when the gate  is  open; others  have  installed
instrumentation in the discharge line upstream of
the tide gate that senses when there is water in the
line.    In  both  cases,   the  signal  from  the
instrumentation is sent to the operating municipality
via telemetry to alert the operator  of a possible
overflow.  This type of system may be especially
useful if the  tide gate is inaccessible or difficult to
inspect. These types of systems should be regularly
tested to ensure proper operation.

An inexpensive way of passively determining if an
overflow occurred at the  CSO is to place a  small
wooden block on the static weir; if the block is not
present after a rainfall event, then it was carried off
with the overflow. If the wooden block disappears
after a  period  of  dry weather flows, then  the
overflow structure needs to be recalibrated.  Base
sanitary flows can increase over time as a result of
changes in the  drainage basin, (e.g., more paved
areas), higher sanitary flows, and increased I&I. An
increase in  base sanitary flow could cause  dry
weather overflows that need to be identified and
eliminated.   Another inexpensive method to
determine overflows is to install a portable water
level or depth gauge (e.g., sonic meter or bubbler)
in the combined sewer  line and to  check  dry
weather head relative to overflow control structure
elevation.  This method can quickly determine if the
overflow weir or other device needs to be adjusted.

Pump Station Maintenance

Pump stations should be maintained to operate at
the design  conditions.   Wet wells  should be
routinely cleaned because grit and solids deposition
in the wet well  can damage or restrict the flow of
wastewater into the pump.

Inadequate or improper pump station operation can
lead to reduced storage  and hydraulic  capacity
during wet weather, and, if the pumping capacity is

severely  restricted, dry weather  overflows can
result.  In general, inadequate pumping capacity is
caused by:

      Mechanical, electrical, or instrumentation

       Changes in the upstream drainage area that
       cause storm runoff to exceed the original
       design basis.

       Changes in the discharge piping (e.g., tying-
       in or manifolding  with  another pressure
       system) that creates more headloss in the
       discharge system.

If conditions upstream of the pump station (such as
development) increase the flow above the design
values, steps should be taken to upgrade the station
to meet  the  increased flowrate.   Pump station
upgrading may include such items as:

      Installing new pumps and motors.

      Changing out impellers.

      Upgrading/changing  pump   controls  to
       maximize  use of all pumps during wet

      Modifying  system  piping to improve the

       head curve.

       Installing additional force main piping for
       wet weather pumping.

Depending  on the  complexity  of the system,
changes to the downstream discharge conditions
that may affect the system head curve may require
extensive study and should be evaluated on a case-
by-case basis.
Sewer Line Maintenance
Sewer line maintenance can be broken down into
two main components, which include the use of
diagnostic methods to identify potential trouble
spots in the line; and actual physical inspections of
the lines for cracks, breaks, or blockages.

The  use  of diagnostic methods  allows system
operators to predict where problems may occur in
the lines,  thus allowing a more efficient use of
O&M resources.  Proper maintenance of a sewer
system requires  a  knowledge  of the  system,
including information about the age of the system,
the drainage areas served,  the  elevations of the
drainage structures, and slopes of the sewer lines.
Adequate knowledge of the age of the sewer system
is  crucial  because  many  older  systems  are
constructed of weaker materials (such as clay pipe)
that are prone to cracking and collapsing. Cracked
and collapsed sewers can pose significant problems,
such as infiltration of the sewer  flow into the
groundwater and the introduction of sediment into
the system. This may lead to hydraulic restrictions.
Knowing which  sections of the sewer system are
the oldest or identifying sections that are made of
less sturdy materials will allowthe system operators
to track the most likely trouble spots in the system.

Information regarding the elevations of the sewer
system is important for determining the likelihood
of sediment accumulation in the line. The slope of
a sewer line is directly proportional to the  line
capacity  and velocity.   When the wastewater
velocity in the  line is below  the self-cleaning
velocity of 2 feet per second, solids tend to settle
out, creating a flow restriction.  Oversized sewers
placed on very flat gradients are especially prone to
conveying the wastewater at low velocities, and, as
a result, filling with sediment.  Small- and mid-
sized storms are of significant concern because the
flow velocity from these storms may be below the
self-cleaning velocity.  Therefore,  areas that are
prone to deposition should be inspected frequently.
Sewer lines with a history of sediment deposition
and blockages should be identified  and  scheduled
for routine cleaning.

Modeling a  sewer  to evaluate  the  need for
improvements can  be  especially  beneficial in
avoiding future problems. For instance, increasing
the flow in an upstream sewer can create problems

downstream if the downstream sewer does not have
the capacity to handle the increased  flow.  Other
problems, such as flow backing up into basements,
may appear as a result.  In cases  where there is
concern about back-ups into basements, abackflow
preventer may be warranted. Modeling will help to
determine how raising a weir will decrease CSOs.
Methods  of increasing the  flow through  sewers
include  increasing the pumping  rate from the
upstream pumping station and injecting polymer to
reduce the sewer roughness coefficient (Field et al.,

Determining whether an overflow occurred in  a
discharge sewer is important in understanding how
the system works and for requirements on reporting.
An  inexpensive  method  for   determining  the
maximum depth of flow in the discharge line is to
draw a chalk line around the inner circumference of
the discharge sewer.  The overflow water will
dissolve this substance to the maximum depth of
flow.    More   advanced   techniques  include
employing instrumentation that measures the flow
in a discharge  and relays  this information via
telemetry to the municipality.

The  second part of a sewer  line  maintenance
program  is physical inspection  of the lines.  If
possible,  CSSs and CSOs  should be inspected
regularly to ensure peak performance. Sewers are
commonly inspected by television  cameras, but if
the sewers are large enough and flow conditions are
low enough, manual inspection may be possible. If
manual  inspection is the  chosen  method, the
inspector must follow the OSHA  confined space
entry guidelines.

Inspections should  be used to identify blockages,
cracks, or other problems in the  lines. Blockages
are typically  the  result  of sediment  and  grit
accumulating  in  the sewer system,  although
dislodged vegetation and debris restrict flow as
well. Another common cause of sewer blockages is
tree roots, which can grow through cracked sewers.
System blockages in sewer  systems  can decrease
both the  hydraulic capacity of the sewer  and its
effective  storage capacity. This can cause flow to
back up and overflow the sewer system.
Once  these  problems  have  been  identified,
maintenance crews must be dispatched to correct
them. Crews should ensure that all lines are cleared
of all lodged debris. They should check and empty
any in-line grit chambers or flushing stations where
sediment routinely causes blockages in the system.
Cracked sewers should be repaired and collapsed
sewers should be replaced to restore the system
capacity and prevent infiltration.

Catch Basin and Grit Chamber Maintenance

Catch basins and grit chambers are inlet chambers
that provide sumps for the retention of sediment,
grit, and debris.  These basins should be cleaned on
a routine basis to prevent grit and sediment from
filling the structure and passing untreated flow into
the  CSS.   Cleaning  methods  include  utilizing
vacuum trucks, jet sprays, submersible pumps that
can handle grit and slurry mixtures, and clamshell

Sediment Control

As sediment is a significant source of the problems
in combined sewer systems, control of sediment
from the source can prove beneficial. An  example
of  source  control  includes  implementing and
maintaining effective erosion control practices for
construction in the drainage area. These practices
will prevent sediment from being transported to the
sewer inlet during a rainfall event.  Frequent street
sweeping has also proven effective in decreasing
the sediment load to the sewer system.

Infiltration & Inflow

Sewer system evaluation studies (SSES), such as
smoke testing  and  television  inspection,  are
effective  methods of determining infiltration and
inflow of groundwater into the sewer system.  This
is the  result  of structural failure of the piping
system that allows groundwater into the piping
system and is a common problem in older sewer
systems. Often, tree roots will grow into the broken
piping system, causing more blockage problems in
the sewer. This problem is a serious one  not only
because it introduces additional flow into the sewer
system which can lead to surcharges and overflows,
but also because it can introduce sediment into the

system,  which can cause the problems  outlined


The cost of operating and maintaining CSOs and
CSSs is especially difficult to determine because it
is a function of many different factors, including the
age  of  the  system, the  type(s)  of overflow
structure(s), the size of the system (both  in linear
footage and in the diameter of combined sewer),
and  the  drainage  areas.   Cost data  for  key
components of proper O&M of CSO  systems is
summarized in other EPA Fact  Sheets, including
"Sewer Cleaning and Inspection" (EPA 832-F-99-
018) and "Catch Basin Cleaning" (EPA 832-F-99-
011). For example, average costs for catch basin
cleaning can range from $8-$ 16 per catch basin
depending  on  whether  the cleaning  is  done
manually or with a vacuum sweeper.  Table  1
summarizes average national cost data for cleaning
and inspecting sewers, another key component of
proper CSO system O&M..


1.     Arbour, R. and K. Kerri,  1997.  Collection
      Systems:  Methods  for  Evaluating  and
      Improving Performance.   Prepared for the
      EPA Office of Wastewater Management by
      the California State University, Sacramento,

2.     Black  & Veatch,  1998.   Optimization of
      Collection  System   Maintenance
      Frequencies  and System Performance.
      Prepared for the EPA Office of Wastewater
      Management under a cooperative agreement
      with American Society of Civil  Engineers.

3.     Burgess, E.  H. et al., 1994.  Operational
      Plan for CSO Abatement in Indianapolis,
      Indiana.     Presented   at  the  Water
      Environment Federation Conference "A
      Global  Perspective for  Reducing CSOs:
      Balancing Technologies,  Costs, and Water

4.     Byrd/Forbes Associates, Inc.,  1995.  Darin
      Thomas, Byrd/Forbes Associates,  personal
communication with Parsons Engineering
Science, Inc.

Despault, R., L. Gohier, and A. Perks, 1994.
CSOs: A Fresh Look at Combined Sewer
Operations.    Presented  at the  Water
Environment Federation  Conference  "A
Global Perspective for Reducing CSOs:
Balancing Technologies, Costs, and Water

Field, R., T.P. O'Conner, and R. Pitt,  1994.
Optimization  of  CSO  Storage   and
Treatment Systems. Presented at the Water
Environment  Federation  Specialty
Conference on CSOs.

Gross, C. E. et al., 1994.  Nine Minimum
Control Requirements for Combined Sewer
Overflows.    Presented  at  the  Water
Environment Federation Conference,  "A
Global Perspective for Reducing CSOs:
Balancing Technologies, Costs, and Water

Louisville Metropolitan  Sewer District,
1995.    Derrick   Guthrie,  Louisville
Metropolitan Sewer  District,   personal
communication with Parsons Engineering
Science, Inc.

Southeastern Wisconsin Regional Planning
Commission (SEWRPC),  1991.  Cost of
Urban Nonpoint Source Water Pollution
Control Measures, Technical Report No.

U.S.  EPA,  1989.   A Compilation of
Significant   References.     Storm   and
Combined   Sewer   Pollution  Control

U.S. EPA, 1993. Combined Sewer Overflow
Control Manual.  EPA 625-R-93-007.

U.S. EPA Federal Register [FRL-4732-7]
Part VII, April 19, 1994. Combined Sewer
Overflow Control Policy.

Total O&M cost/mile*year
Labor (cost/mile/year)
Fringe Benefits (cost/mile/year)
Chemicals (cost/mile/year)
Hydroflush Cleaning (cost/mile)
Television Inspection (cost/mile)
Preventive Maintenance
Range of Costs
$695 -$1 9,831 1
$475 -5.2302
Average Cost
63% of Total Maintenance Costs (excludes depreciation)
 Source: 1 Water Environment Research Foundation, 1997.
        2 Arbour and Kerri, 1997.
        3 Black & Veatch/ASCE, 1998.
13.    U.S.  EPA Storm  &  Combined  Sewer
      Pollution Control Program, 1995. Richard
      Field, U.S. EPA Storm & Combined Sewer
      Pollution   Control   Program  personal
      communication with Parsons Engineering
      Science, Inc..

14.   Water Environment Research Foundation
      (WERF), 1997. Benchmarking Wastewater
      Operations - Collection,  Treatment,  and
      BiosolidsManagement.  Project 96-CTS-5.


Byrd/Forbes Associates, Inc.
Tom Jones
2315 Southpark Drive
Murfreesboro, TN 37128

Center for Watershed Protection
Tom Schueler
8391 Main Street
Ellicott City, MD 21043

Jefferson County Metro Sewer District
Dan Knowles
700 West Liberty Street
Louisville, KY 40203
Metropolitan St. Louis Sewer District
Bernie Raines
Environmental Compliance
10 East Grant Avenue
St. Louis, MO 63147

National Risk Management Branch

Office of Research and Development
Richard Field
2890 Woodbridge Avenue
Edison, NJ 08837

The  mention of  trade  names  or  commercial
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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