United States Office of Water 

Environmental Protection (4606) July 2001 

Agency 

EPA 916-F-01-032 

Source Water Protection 

Practices Bulletin 

Managing Sanitary Sewer 

Overflows and Combined Sewer 

Overflows to Prevent 

Contamination of Drinking Water 

Sanitary sewer overflows (SSOs) are discharges of untreated sewage from municipal sanitary

sewer systems as a result of broken pipes, equipment failure, or system overload. Combined

sewer overflows (CSOs) are discharges of untreated sewage and storm water from municipal

sewer systems or treatment plants when the volume of

wastewater exceeds the system’s capacity due to periods of

heavy rainfall or snow melt. The untreated sewage can be

discharged directly into basements, streets, parks, and

surface waters including streams, lakes, rivers, or estuaries. 

This fact sheet focuses on the management of SSOs and

CSOs to prevent contamination of drinking water sources;

see also the fact sheet on storm water runoff. 

Sanitary sewer overflow 

OVERVIEW OF SSO AND CSO OCCURRENCE 

Most cities and towns started building sewer collection systems over 100 years ago and many of 

these systems have not received adequate upgrades, maintenance, and repairs over time. In 

addition, cities use a wide variety of materials, designs, and installation practices to construct 

sewer collection systems. Even well-operated systems may be subject to occasional blockages 

or structural, mechanical, or electrical failures. 

Sanitary sewer collection systems collect sewage and other wastewater and transport it to a 

facility for proper treatment and disposal. Sanitary sewer overflows occur when untreated 

sewage is discharged from the collection system due to pipe blockages, pipe breaks, infiltration 

and inflow from leaky pipes, equipment failures, and insufficient system capacity. 

Combined sewer systems are designed to carry 

sanitary wastewater and storm water in the same 

pipe to a sewage treatment plant during “dry 

weather.” In periods of rainfall or snow melt, 

however, the wastewater volume in a combined 

sewer system can exceed the capacity of the 

sewer system or treatment plant. For this reason, 

combined sewer systems are designed to 

overflow occasionally and discharge excess 

wastewater directly to nearby streams, rivers, 

lakes, or estuaries. 
WHY IS IT IMPORTANT TO MANAGE SSOs AND CSOs NEAR THE SOURCES 

OF YOUR DRINKING WATER? 

EPA estimates that there are at least 40,000 

SSOs and thousands of CSOs each year. 

The untreated sewage and wastewater from 

these overflows can contaminate our waters, 

causing serious water quality problems and 

threatening drinking water supplies. It can 

also back up into basements, causing 

property damage, and create threats to public 

health for those who come in contact with 

the raw sewage and wastewater. 

Combined sewer overflow SSOs and CSOs can carry bacteria, viruses, 

protozoa, helminths (intestinal worms), and inhaled molds and fungi directly into source water, 

and can cause diseases that range in severity from mild gastroenteritis to life-threatening 

ailments such as cholera, dysentery, infectious hepatitis, and severe gastroenteritis. People can 

be exposed to the contaminant from sewage in drinking water sources, and through direct 

contact in areas of high public access such as basements, lawns or streets, or water used for 

recreation. 

When sewage floods basements, the damaged area must be thoroughly cleaned and disinfected 

to reduce the risk of disease. Local health officials should be consulted to identify measures to 

be taken to remove the sewage and reduce health risks. Pesticides and other chemicals tend to 

be stored in basements. Where water from flooded basements that contain spilled chemicals is 

pumped or released to the ground outside the building, it may percolate through the soil and 

contaminate the ground water. 

Under the Clean Water Act, discharges from point sources into waterways are prohibited unless 

authorized by a National Pollutant Discharge Elimination System (NPDES) permit. NPDES 

permit requirements for municipal wastewater treatment plants must include limitations based on 

secondary treatment, including limits on oxygen-demanding pollutants and suspended solids, as 

well as any other more stringent requirements (such as disinfection) necessary to meet state 

water quality standards. Although CSOs are considered point sources, they are not subject to 

secondary treatment requirements; instead, NPDES permits for combined sewer systems are 

based on the provisions of EPA’s 1994 CSO Control Policy, which provides for implementation 

of minimum technology-based controls and long-term control plans to meet water quality 

standards. SSOs, on the other hand, typically are not permitted and are generally prohibited. 

EPA is considering how to better standardize NPDES permit conditions to clarify this prohibition 

and provide for better operation and maintenance of sanitary sewers, increased attention to 

system planning, and better notification to the public in the event of an overflow. 

AVAILABLE PREVENTION MEASURES TO ADDRESS SSOs AND CSOs 

A variety of nonstructural and structural prevention measures are available to address SSOs and 

CSOs. Nonstructural activities tend to be more general and applicable to most sewer collection 

systems. They include, but are not limited to, visual inspections, monitoring and maintenance 

programs, employee training, and public education. Structural activities tend to be more sitespecific and can be very expensive to incorporate. They involve upgrading the collection 

system, constructing wet weather storage facilities, or building a new sewer collection system. 

The most effective prevention plans encompass both structural and nonstructural activities. 

Please keep in mind that individual prevention measures may or may not be adequate to prevent 

contamination of source waters. Most likely, individual measures should be combined in an 
overall prevention approach that considers the nature of the potential source of contamination, 

the purpose, cost, operational, and maintenance requirements of the measures, the vulnerability 

of the source waters, the public’s acceptance of the measures, and the community’s desired 

degree of risk reduction. Some of the more conventional prevention measures are described 

below. 

Cities estimate that 60 percent of SSOs come from leaking service lines, and monitoring and 

maintenance programs are a key component in preventing them. Sanitary sewer collection 

system operators should monitor their sewer lines, service connections, and sewer line joints 

regularly to detect cracks and misalignments between joints that can cause leaks of untreated 

sewage. Service connections must remain tightly sealed to prevent additional leaks from 

occurring. Properly maintaining the sewer collection system allows parts of the sewer system to 

be repaired or replaced, if necessary, before they break and cause more serious and expensive 

problems. 

Maintenance programs should also include 

cleaning sewer lines, connections, and pumps. If 

trash and sediments build up in the sewer lines, 

they will block the sewage from flowing to the 

collection system or treatment plant. As the 

flow becomes blocked, the pressure on the lines 

increases and the system becomes surcharged 

leading to overflow of sewage out of manholes 

and into the street. Surcharging can also cause 

sewage backup into basements of homes 

connected to the line. In some cases, the lines 

may break and collapse, causing raw sewage 

Storm drain and wastewater to percolate through the soil to 

ground water. 

Employee training is an important tool for preventing contamination from sewer overflows. 

Employees should be trained on how to run the equipment, and shut it down, if necessary, to 

prevent overflows. Employees should have access to and knowledge of contingency and 

emergency response plans. They should be aware of any potential for overflow events and be 

prepared to take appropriate action to prevent sewage from entering the source water. 

Public education involves informing developers and the public of how sewer overflows occur, 

and what they can do to prevent them. Developers should be aware of the sewer collection 

design capacity, and plan accordingly. As new communities are developed, the additional 

sewage can overload the collection system. Developers should check to make sure the new 

sewer lines are compatible with the existing sewer system. If the lines do not fit the joints, then 

the sewage can leak out of the system, or rain water or snow melt can infiltrate the cracked 

lines and cause overflows. Developers should also make sure that sewer lines are not placed 

near trees; the roots can grow into the sewer lines and crack them. The community can help 

prevent overflows by conserving water and flushing only appropriate items. Citizens should also 

be aware that hazardous substances, pesticides, and fertilizers could be carried off in storm 

sewers and increase the deleterious effects of CSOs. 

Visual inspections of the surface and internal areas (pipelines and manholes) ensure that the 

equipment is running properly and efficiently. Operators should pay specific attention to sunken 

areas in the groundcover above a sewer line and areas with ponding water. Operators should 

perform these inspections on a daily or weekly basis at low flow times (e.g., overnight), 

depending on the system size or frequency of overflows, and log their findings. Inspection 

reports provide managers with pertinent information and keep them informed on how the system 

is running. This will help avoid equipment failure and resulting overflows. 
Incorporating system upgrades is another viable option, but this can be very expensive. As 

sewer systems become older, sewer lines and connections have to be repaired or replaced. 

Equipment also has to be replaced or updated as new technology becomes available. As new 

communities are developed, new sewer lines will be added to the collection system. Eventually 

the sewer system will reach its design capacity and will have to expand or a new collection 

system will have to be built. 

Adding a wet weather storage facility such as an overflow retention basin to a sewer collection 

system will reduce SSOs and CSOs by capturing and storing excess flow. The stored volumes 

of sewage and storm water are released to the wastewater treatment plant after the wet 

weather event has subsided and the treatment plant capacity has been restored. Retention 

basins are designed to control both flow rate and water quality. These basins can remove 

sediment and grit from the effluent before being released to the treatment plant. Retention 

basins can be constructed either on- or off-line from the sewer collection system. On-line 

basins are connected to the sewer system and retain excess flows when the inlet flow surpasses 

the outlet capacity. Off-line basins are connected in parallel to the sewer system and receive 

flows only during wet weather periods. Retention basins are typically earthen basins or covered 

or uncovered concrete tanks. Covered basins are more widely used because they are safer and 

provide better odor control and safety conditions. 

Eliminating direct pathways of sewage overflows to source water is an effective measure to 

prevent contamination. Regrading areas around pump stations and “vulnerable” manholes can 

divert overflow sewage from entering surface water directly. In addition, plugging storm water 

drainage wells (i.e., drywells used to discharge storm water underground) in the vicinity of pump 

stations and manholes would eliminate conduits for sewage overflow to enter the ground water. 

CSO control technologies include a number of engineering methods such as deep tunnel 

storage, in-system control/in-line storage, off-line near-surface storage/sedimentation (mentioned 

earlier), vortex technologies, and disinfection. In urban areas, where space constraints are 

severe, deep tunnel storage can be a viable option for managing CSOs. Large volumes of 

combined sewage can be diverted and stored in deep tunnels during a storm event. The stored 

combined sewage is then pumped out from the tunnel and conveyed to sewage treatment plants 

after the storm event subsides. Vortex separators regulate flow and cause solids to separate out 

from the combined flow, therefore allowing clarified flow to be discharged to surface water. 

Disinfection using liquid hypochlorite is the most common practice in controlling CSOs, and 

alternatives such as ultraviolet light, ozone, or gaseous chlorine are also available. 

FOR ADDITIONAL INFORMATION 

These sources contain information on sanitary sewer overflows and combined sewer overflows. 

All of the documents listed are available for free on the Internet. 

Earth Day Indiana Handbook 1997-98. Combined Sewer Overflows...what you should know. 

Retrieved February 15, 2001, from the World Wide Web: 

http://www.k12.in.us/earthdayind/handbook2.html. 

United States Environmental Protection Agency, Office of Wastewater Management. EPA’s 

CSO Control Policy– An Innovative Approach to Controlling Raw Sewage Discharges. 

Retrieved February 14, 2001 from the World Wide Web: http://www.epa.gov/owm/cso.htm. 

U.S. EPA, Office of Wastewater Management. Collection Systems O&M Fact Sheet: Sewer 

Cleaning and Inspection. Retrieved March 8, 2001, from the World Wide Web: 

http://www.epa.gov/owm/mtb/sewcl.pdf. 
U.S. EPA, Office of Wastewater Management. Combined Sewer Overflow O&M Fact 

Sheet: Proper Operation and Maintenance. Retrieved March 8, 2001, from the World Wide 

Web: http://www.epa.gov/owm/mtb/o&m.pdf. 

U.S. EPA, Office of Wastewater Management. Combined Sewer Overflow Management 

Fact Sheet: Pollution Prevention. Retrieved March 8, 2001, from the World Wide Web: 

http://www.epa.gov/owm/mtb/pullutna.pdf. 

U.S. EPA, Office of Wastewater Management. Combined Sewer Overflow Technology Fact 

Sheet: Retention Basins. Retrieved March 8, 2001, from the World Wide Web: 

http://www.epa.gov/owm/mtb/csoretba.pdf. 

U.S. EPA, Office of Wastewater Management. Combined Sewer Overflow Technology Fact 

Sheet: Screens. Retrieved March 8, 2001, from the World Wide Web: 

http://www.epa.gov/owm/mtb/screens.pdf. 

U.S. EPA, Office of Waste Water Management. National Pollution Discharge Elimination 

System (NPDES). Retrieved June 27, 2001, from the World Wide Web: 

http://www.epa.gov/npdes 

U.S. EPA, Office of Wastewater Management. Sanitary Sewer Overflows (SSOs). Retrieved 

February 14, 2001, from the World Wide Web: http://www.epa.gov/owm/sso.htm. 

U.S. EPA, Office of Wastewater Management. Sanitary Sewer Overflows– What are they 

and how can we reduce them? Retrieved February 20, 2001, from the World Wide Web: 

http://www.epa.gov/owm/ssodesc.htm. 

U.S. EPA, Office of Wastewater Management. The Nine Minimum Controls. Retrieved 

February 14, 2001, from the World Wide Web: http://www.epa.gov/owm/h-nmclis.htm. 

U.S. EPA, Office of Wastewater Management. Wet Weather. Retrieved February 14, 2001, 

from the World Wide Web: http://www.epa.gov/owm/wet.htm.