v>EPA
United States
Environmental Protection
Agency
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
DESCRIPTION
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.
FIGURE 1 COMBINED SEWER SYSTEM
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.
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• Reviewing the O&M program periodically
to up-date and revise procedures as
necessary.
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.
Training
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
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reviewed, and, if necessary, updated periodically.
Tide gates that require underwater inspection
should only be inspected by a certified diver.
ROUTINE MAINTENANCE ACTIVITIES
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
832-F-99-008).
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
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severely restricted, dry weather overflows can
result. In general, inadequate pumping capacity is
caused by:
• Mechanical, electrical, or instrumentation
problems.
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
weather.
• Modifying system piping to improve the
system
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
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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.,
1994).
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
buckets.
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
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system, which can cause the problems outlined
above.
COST
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..
REFERENCES
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,
CA.
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
Quality."
4. Byrd/Forbes Associates, Inc., 1995. Darin
Thomas, Byrd/Forbes Associates, personal
10.
11.
12.
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
Quality."
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
Quality."
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.
31.
U.S. EPA, 1989. A Compilation of
Significant References. Storm and
Combined Sewer Pollution Control
Program.
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.
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TABLE 1 NATIONAL SUMMARY OF MAINTENANCE COSTS
Identifier
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
$951-$46,9731
$695 -$1 9,831 1
$192-$9,0331
$0.3-$7,6161
$475 -5.2302
$1,000-$11,4502
Average Cost
$2,8233
$3,6261
$1,1851
$5121
$1,7001
$4,6001
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.
ADDITIONAL INFORMATION
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
U.S. EPA
National Risk Management Branch
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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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