Frequently Asked Questions about Combined Sewer Overflows (CSOs)


Heavy rainfall can cause water quality issues for several reasons.   First, while most modern communities have separate collection and conveyance systems for sewage and stormwater, some of CT’s older wastewater collection systems, primarily located in its urbanized areas, have systems that were designed over a 100 years ago.  These systems still collect and convey both sewage and stormwater in one pipe (combined sewer systems).  During certain rainfall events, these combined sewer systems discharge untreated sewage into CT’s waterbodies at points known as combined sewer overflows or CSOs.  Second, stormwater runoff from impervious or agriculturally impacted areas can carry bacteria and other pollutants into waterbodies. As a result of climate change, the intensity of rainfall is increasing and causing more severe weather events leading to more surcharging of and discharges from the state’s combined sewer systems as well as increased stormwater runoff carrying pollutants. 

What is a combined sewer?

Sanitary sewers are pipes which carry wastewater from homes and businesses (i.e wastewater from toilets, laundry, bathing, dishwashing).  Storm sewers are pipes that carry rainwater from storms.  A pipe which is designed to carry both types of flow is called a combined sewer. 

The advantage of a combined sewer system is that most of the time; both stormwater and wastewater are treated to meet water quality standards prior to discharge into a water body. The disadvantage of a combined sewer system is that during heavy rains, untreated stormwater and wastewater may be discharged at CSO locations.

CSO system on a dry day
CSO system on a dry day
 CSO system on a rainy day
CSO system on a rainy day
What is a Combined Sewer Overflow (CSO)?

When combined sewer systems were designed over 100 years ago, it was an appropriate way of managing both wastewater and stormwater.  Wastewater and stormwater were conveyed together to larger bodies of water, with little or no treatment provided prior to discharge.  At the time, it was expected that dilution in a larger body of water would be adequate to protect human health and the environment.

Most of the time, the combined sewer system was adequate to convey the wastewater and stormwater.  Occasionally, however, higher intensity storms could overload the carrying capacity of the pipes. To provide relief from this overload, combined sewer overflows (CSOs) were designed into the systems to allow excess flows to discharge to nearby streams.  This served to protect the cities from back-ups of raw sewage into homes, reduced the potential for street flooding, and protected the pipes and treatment systems from damage due to overloading.

Where are the combined sewer overflows in Connecticut?

There are six municipalities in Connecticut that still have CSOs.  View a map showing locations of the CSOs

Are CSOs a new problem?

No. As already pointed out, combined sewers have been in existence for well over 100 years.  There are far fewer CSOs today, because of construction projects that have been performed to control these overflows.  Unfortunately, because combined sewers are in older, densely populated urban areas, sewer separation projects are complex to construct and are therefore very expensive. 

What actions are being taken to reduce/eliminate untreated or partially treated sewage discharges?

CSOs can and do occur with less intense precipitation events.  The type of precipitation event that triggers a CSO(s) is very specific and dependent on the duration and intensity of an event, the level of saturation of ground conditions during the event, and the capacity and structural condition of a specific pipe or segment of a municipality’s combined sewer system.

CT municipalities and sewer authorities have been working for many years to modernize their infrastructure to minimize and eliminate CSOs. Current CSO efforts are focused on CSO elimination, where feasible or alternatively, on CSO reductions through improvements and upgrades of the conveyance and treatment infrastructure to collect and treat up to a 1-yr or 2 -yr rainfall event (a metric related to the recurrence frequency of precipitation events). For comparative purposes, Tropical Storm Elsa (in August, 2021) was considered in 100-yr rain event in portions of the state where over 5” of rain fell over a 6-hour period.   An engineering control standard of a 1-yr event and 2-yr event (equating to approximately 2” or 2.5” of precipitation, respectively, over the same time period) would prevent the occurrence of CSOs from many routine, high-frequency precipitation events.;

Billions of dollars to separate combined sewer systems to improve the state’s water quality has been allocated at the municipal and state levels.  The Clean Water Fund for wastewater projects, administered by CT DEEP, has provided critical support for advancing CSO mitigation projects.  In the last 20 years, a total of approximately 1.2 billion dollars have been spent on statewide CSO reduction or elimination projects, of which approximately 470 million dollars consisted of state grants through the Clean Water Fund.

Reduction of CSOs to a 1 or 2-yr design storm as well as projects to fully eliminate CSOs statewide will likely take several decades to plan, design, construct, and finance additional CSO projects with additional cumulative costs in the billions.  CT DEEP continues to prioritize CSO projects to achieve environmental improvements and meet the state’s water quality goals.

How can CSOs be eliminated?
There are three general approaches to eliminating CSOs:
  1. Sewer Separation: Build a second system of pipes to keep stormwater separate from wastewater.  By removing stormwater from the sanitary sewer, pipes will no longer overflow during heavy rains.

    Building another piping system is challenging and expensive, especially in densely populated urban areas.  The construction for a sewer separation project will impact utilities and traffic for an extended period of time. Multiple utilities exist in some roadways, and some utilities may need to be relocated or modified for the new stormwater piping.
  2. Increase Storage Capacity:  Store excess combined wastewater during heavy rains and slowly release it to the treatment plant over time.

    Underground tanks and larger pipes like interceptors and tunnels can be used to store combined wastewater and stormwater, but they usually have to be large enough to hold several million gallons of combined wastewater and stormwater.  Finding enough space to fit a tank or pipe of this size can be more difficult than installing another pipe in the roadway.
  3. Treatment:  Transport the combined wastewater and stormwater to a treatment plant before discharging to a body of water.

    Sending more water to the municipal wastewater plant or a satellite treatment facility will increase the volume of water receiving treatment.  The treatment plant may need to be expanded to manage the additional flow. Like most CSO Long Term Control Plans throughout the country, the Long Term Control Plans proposed in Bridgeport, Hartford, New Haven, and Norwich contain a combination of different strategies, and at least one of the strategies indicated here.
What health & safety precautions are taken to protect people from CSOs/SSOs?

DEEP advises residents from recreating in waters known to receive CSOs/SSOs. View a map showing locations of the CSOs

Also, to ensure the health and safety of its beaches, CT DEEP collects samples at 22 state-owned and managed swimming areas from Memorial Day to Labor Day on a weekly basis.  Samples are analyzed for indicator bacteria, which are not disease-causing pathogens, but are one of the tools used by public health and environmental protection officials to evaluate the potential contamination of waterbodies.  CT DEEP closes swimming areas when data indicate poor/unsafe quality. Additional information on beach monitoring can be found at Beach Monitoring (

Local towns and health districts have their own protocols on managing and testing their swimming areas.  Some perform direct water quality testing like CT DEEP, others use rainfall thresholds to close swimming areas.  Swimming areas and town and state beaches, parks and forests may close during or after storms for reasons unrelated to water quality as well, such as fallen trees, power lines or washed out roads.

How does the public know when CSOs are active?

The “Sewage Right-to-Know law in Section 22a-424a of the CT General Statutes requires the Department of Energy and Environmental Protection (DEEP) to post a map showing active sewage spills and combined sewer overflows.  The current map is available at Public Sewage Spill Notification Map.

The six municipalities in Connecticut containing CSOs are also responsible for posting permanent signs near all CSO outfalls indicating their presence.  These signs warn people to avoid swimming or fishing in water that may be impacted near sewer pipe overflows. Bacteria and chemicals from CSOs can increase the risk of getting sick from swallowing water or eating fish in the area.

Where is it safest to swim?

It is best to swim in a designated swimming area after checking to make sure that it is open for swimming.  Water quality is routinely monitored from Memorial Day to Labor Day at 23 state-owned and managed swimming areas, and the CT DEEP Beach Status posts the most up to date water quality report at these facilities or type from any web accessible device.  This information can also be accessed by phone (866) CTPARKS or (866) 287-2757, option #5, and from the Connecticut Department of Public Health.

DEEP advises swimmers to take simple precautions that can help make your beach visit safe by following Connecticut Department of Public Health Advice to Swimmers.  Please contact your local health department for a list of designated swimming areas in your area.  Boating, swimming and fishing, popular recreational activities in Connecticut, benefit from clean and safe water.  Every person out on the water has a role in keeping our waters clean.  Contact your DEEP Watershed Manager to get involved with a local watershed management effort or find more information at: On The Water Recreation in Connecticut.

What will happen if I go in the water near a CSO sign?

If you do go in the water near a CSO location during or after a heavy rain, the greatest health risk would come from getting untreated overflow water in your mouth.  If you accidently go in the water, near a CSO location, during or after a heavy rain, avoid getting water in your mouth, and wash yourself with hot water and soap as soon as possible. Most people will not get sick by just getting untreated overflow water on their skin.

There is a chance that pathogens (bacteria and viruses) in untreated overflow water could make you sick, especially if you are already sick or have lower immunity. In general, young children and elderly people may have a higher risk of getting sick from untreated overflow water.

What if my dog goes in the water near a CSO sign?

Animals are usually not affected, but if your pet does go in the water near a CSO location during or after heavy rains, bathe the animal with hot water and soap as soon as possible

If your pet is very young or old, it could be at higher risk for getting sick by being exposed to untreated overflow water.  If your animal develops diarrhea, you should withhold food, and consult your veterinarian

Can I eat fish or crabs collected from water contaminated with sewage?

Finfish and Crabs:
Evidence suggests that eating finfish and crustacean shellfish such as crabs taken from water contaminated with sewage will not result in any known health effects as long as standard hygiene measures are followed. This includes washing your hands with warm water and soap after handling fish or crabs, thoroughly washing fish with clean water before cooking, and cooking using correct times and temperatures. Take care when cleaning the fish by keeping internal organs away from fillets or meat of the fish. Wash fillets or meat with clean water after cleaning the fish.

Fisherman may want to avoid fishing in areas where sewage overflows have occurred due to the potential for incidental contact with the contaminated water. If you do decide to fish in an area that has been posted with warning due to a sewage overflow, consider wearing gloves when handling fish taken from the water. Also try to avoid other contact with the water or shoreline sediments and wash your hands after any contact.

Shellfish: Clams, Oyster, and Mussels:
In contrast to the recommendations for finfish and crustacean shellfish, molluscan shellfish, such as clams, oyster, and mussels taken from water contaminated with sewage DO have the potential to make people sick if consumed.  Bivalve molluscan shellfish are filter-feeding organisms that pump large quantities of seawater through their bodies as a part of the normal feeding process.  As a result, any microorganisms, including human pathogens present in the growing area, can become concentrated in shellfish meats by as much as 100 times that found in the water column.  Sewage contamination is the main source of human pathogens in shellfish growing waters and the correlation between sewage pollution and disease has been well demonstrated.  In order to ensure the safety of shellfish for human consumption, shellfish growing areas are classified based on evidence of contamination. CSO discharges are considered a direct source of pollution which requires that these waterbodies be classified as Prohibited to shellfishing at all times, not just after a CSO event.  Shellfish harvested from CSO areas are never safe for human consumption.

The classification of waters for the harvest of shellfish such as clams, oysters and mussels is regulated by the Connecticut Department of Agriculture Bureau of Aquaculture, however, each municipality within the State sets its own shellfisheries regulations including species, size restrictions, harvest limits, closures and fees.  In Connecticut, you should only harvest shellfish recreationally from areas that you have a permit for, and individuals should always check the status of these areas prior to harvesting to make sure that the areas are open.  Again: Any waterbody that has a CSO outfall is considered prohibited to recreational shellfishing at all times.

For more information on recreational shellfishing, please visit the Connecticut Department of Agriculture Bureau of Aquaculture website

For information on which towns have recreational shellfishing programs, and how to get a permit, please see the CT Sea Grant 2016 Guide to Recreational Shellfishing Along the Coast of CT

How much rain does it take for a CSO discharge to occur?

It varies among CSO locations, depending on how the system was designed and built. Some locations will have CSO discharges during a moderate storm, while other locations will only overflow during the heavier and longer storms.

How long does water stay impacted after a rain event?

After rain stops, it can take up to 48 hours (2 days) for the water near the CSO to return to baseline conditions.  During this time, the bacteria die off and the currents dilute the germs and chemicals in the water.

Are CSOs the only potential source of pollution to our receiving water bodies?

No.  There are two general types of pollution sources to surface water bodies: point sources and non-point sources.

What are Point Sources?

Point sources are discrete pipes that discharge directly to water.  There are several different types of water discharge permits that the Department of Energy and Environmental Protection issues:

DEEP’s Bureau of Materials Management and Compliance Assurance, Water Permitting and Enforcement Division administers a permit program to regulate discharges to waters of the state, including all surface waters, ground waters and Publicly Owned Treatment Works (POTW) (i.e., wastewater treatment plants) to protect water quality.

DEEP uses both individual and general permits to regulate discharge activities.  Individual permits are issued directly to an applicant, general permits are issued to authorize similar minor activities by one or more applicants.  Authorization of an activity under a general permit is governed by that general permit.

DEEP issues discharge permits in three major categories. While the process for each is similar, specific application requirements may vary:

  • The Surface Water Discharge Permit Program, also known as the National Pollutant Discharge Elimination System (NPDES) under federal law, regulates discharges into surface waters. 
  • The Ground Water Discharge Permit Program regulates discharges to ground water from any source, including but not limited to large septic systems, agricultural waste management systems, and all waste landfills. 
  • The Pre-treatment Permit Program regulates discharges to a sewage treatment plant through municipal sanitary sewer systems, or through combined storm and sanitary sewer systems.
Examples of permitted but treated discharges to rivers or the Long Island Sound include: municipal wastewater treatment plant effluent, permitted industrial wastewater discharge, and CSOs.

What are Non-Point Sources?
A variety of activities that take place throughout Connecticut’s landscapes have the potential to pollute Connecticut’s waters.  That diffuse pollution is called nonpoint source, which means it comes from many different points of origin.

Pollution sources, such as failing septic systems, pet and other animal waste, drainage from outdoor washing activities, or stormwater running off of roofs and pavement often contribute pollutants to the runoff that may end up in surface waters.

If pollutants from these nonpoint sources are in high enough concentrations or flows, the surface waters may become impaired. Pollutant levels, or loadings, from many nonpoint sources are closely linked to rainfall, snowmelt or other weather conditions which cause stormwater runoff.  In Connecticut stormwater runoff from urban areas and construction activities are two of the most significant categories of nonpoint source pollution.

DEEP's Nonpoint Source Pollution webpages provide more information on nonpoint source pollution.What can I do to keep local water safe and clean?

Remember that what goes down drains may go into lakes, streams, or the Long Island Sound. We can all help keep the water clean by keeping paints, oils, and pesticides out of storm drains, fixing leaks from vehicles, and taking cars to a commercial car wash, where the water is often recycled before going to the treatment plant.

  • If you wash your own car:  Wash it with biodegradable soap over grass or gravel, use a bucket and a hose nozzle to limit the amount of soap and water used, and only the outside of the car, not the engine.
  • If you have roof leaders or a sump pump connected to the sewer system:  Disconnect them from the sewer system and redirect the flow to your lawn or a rain garden.
  • If you use fertilizers on your lawn or garden:  Follow the instructions on the packaging regarding when and how much fertilizer to apply, and how to store fertilizers until you need them
  • If you have a septic system:  Keep it properly maintained.  Septic systems typically need to be pumped every 3 to 5 years depending upon system size and usage.  Don’t dump toxic chemicals or pharmaceuticals down the drain.  For more information on septic system maintenance, contact your local health department.

Low impact development (LID) is an alternative way of developing land and managing stormwater that is aimed at minimizing the impacts of urbanization on natural habitats and hydrology.  LID accomplishes this by controlling runoff close to the point of generation and retaining more water on the site where it falls, rather than funneling it into pipes that drain into local waterways.;

DEEP has developed Low Impact Development outreach materials for municipalities including a series of Brochures for Municipalities and homeowners who wish to learn more about implementing innovative stormwater controls and Low Impact Development.  These brochures can be printed on legal size paper, double sided in color, and quad-folded for distribution.

Save the Sound has created a website which also contains information on Low Impact Development, which is sometimes referred to as Green Infrastructure

Content last updated October 19, 2021