Combined Sewer Overflow

Overview | History | How the Sewer System Works | Major Parts of the Sewer System | Understanding the Sewer Collection System | How to Be Notified of Overflows | Want to Be Involved? | CSO Map


The Pittsburgh region’s frequent rainfall brings an underground, out-of-sight problem into clear view. As little as one-tenth of an inch of rain-an average Pittsburgh rainfall is one-quarter inch-can cause raw sewage to overflow into our rivers and streams. Melting snow can cause the same effect.

During dry weather, the sewage collection system, which transports wastewater from thousands of homes to the wastewater treatment plant, operates effectively.

However, when it rains or snow melts, extra stormwater gets into the sewage collection system through direct connections or through leaky, cracked pipes. This extra volume of water overloads the sewage collection system pipes and raw sewage overflows at hundreds of locations before it reaches the treatment plant. Untreated sewage streams into waterways, overflows from manholes or backs up into homeowners’ basements.

And the effects of wet weather can last for days. During the recreational boating season, May 15-September 30, Allegheny County issues river advisories to warn individuals using the rivers to limit water contact when sewage overflows have likely contaminated the water with bacteria and viruses. Each time a river advisory is issued, it could last for several days after a rainfall.

Since the program began in 1995, river advisories issued by the Allegheny County Health Department have been in effect for nearly 50% (70 days) of each recreational season.

Sewage overflows present a public health risk. While exposure to disease-causing organisms, such as giardia or cryptosporidium, are not considered fatal for a healthy adult, they can be deadly for those with weaker immune systems, the elderly and small children. In addition, Pittsburgh’s three rivers serve as the main source of drinking water for 90% of Allegheny County residents. While the public water systems do an excellent job of purifying water before sending it to homes, source protection is the cheapest and most effective way to ensure drinking water quality.

Water is an important resource for the economic development of the Pittsburgh region and sewage overflows hinder growth. Because these overflows violate the Clean Water Act, regulatory agencies, such as the U.S. Environmental Protection Agency and PA Department of Environmental Protection, will not permit municipalities to add new connections to the existing sewer system until the overflow problem is fixed. This restriction limits the construction of new businesses and homes in many communities throughout the region.

Fixing the problem is going to require a substantial long-term investment. State and federal support may be available to help offset a portion of the bill, but municipalities must share resources and work cooperatively across geographic boundaries in order to significantly trim the total bill for ratepayers who will have to bear sewer rate increases in the coming years.


Wet weather sewer overflows are not a new problem for the Pittsburgh region.

Between 1889-1912, sewers were constructed in the areas adjacent the river to carry both wastewater and stormwater (in one set of pipes) away from streets, businesses and homes directly to the rivers to reduce disease and flooding. Because the collection systems carried both stormwater and wastewater, they were called combined sewer systems.

These combined sewers were the result of a debate between health and engineering officials. Since we had not yet constructed sewage treatment plants, engineers felt that stormwater helped flush the sewage away and one large pipe accommodating both sewage and stormwater was more economical.

Eventually, the rivers became so polluted with raw, untreated sewage that it became necessary to create a wastewater treatment system to reduce river pollution and improve the health and welfare of the region.

Most of the municipal collection systems directly connected to the interceptors are combined sewer systems, carrying both wastewater and stormwater. During dry weather, the interceptor pipes carry only wastewater to the treatment plant, but during heavy rains, the additional stormwater flowing into system often overloads the pipes. To help solve this problem, the original outlets, which dumped stormwater and sewage directly into the rivers in the early 20th century, were kept in place and regulators were built to control the flow to the plant. These outlets and regulators are called combined sewer overflow structures.

In the structures, overflow is controlled by a gate that releases a combination of diluted sewage and stormwater-a combined sewer overflow (CSO)-into the rivers when the interceptors are unable to transport the extra volume of water to the treatment plant. Combined sewer overflows were once thought to be a solution to a difficult problem. While they are not illegal today, the occurrence and volume of CSOs must be reduced to a minimum under the Clean Water Act.

In more recent years, newer communities chose to build separate pipes for sewage, called separate sanitary sewers. In order to transport sewage to the treatment plant, many municipalities had to connect their separate sanitary system to a neighboring communities’ combined sewer system, which then transports the wastewater to UAJSA’s interceptor system; in other cases, municipalities have connected directly to UAJSA’s interceptors.

Any overflows from a separate sanitary sewer system-called sanitary sewer overflows (SSOs)-are illegal under the Clean Water Act. Overflowing manholes and basement backups in a separate sanitary system are examples of SSOs.

Over time, a network of municipal combined and separate sanitary systems, which flow into each other before reaching the UAJSA sewage treatment plant, has evolved in the region. Much of the system today is deteriorated and overloaded during wet weather, which results in frequent and illegal sewage overflows into our waterways, streets and homes.


Within the UAJSA service area, the sewer system is a complex network of municipal collection systems all flowing to a single treatment plant. The collection system’s miles of underground pipes collect and transport sewage from each home or business to UAJSA for treatment and subsequent release back into the rivers.

To further complicate the wet weather problem, two types of sewage collection systems exist in the UAJSA service area-the combined sewer system and the separate sanitary system. Combined sewer systems are designed to carry both wastewater and stormwater in the same pipes. These systems are prevalent in older communities with collection systems built before the 1940s. Separate sanitary sewer systems are designed to carry only wastewater. Stormwater needs to be managed through a different collection system of pipes, culverts or ditches. Separate sanitary sewers were required for any new systems built after the 1940s.


In the UAJSA system, each municipality or municipal authority owns, operates and maintains its municipal satellite collection system. Whether in a combined or separate sanitary collection system, pipes in the system carry sewage from many individual homes to a large trunk sewer. The trunk sewer then carries the sewage from multiple municipal collection systems to UAJSA interceptor sewer. The interceptors are a series of pipes, some as large as 36 in diameter that transport the sewage on the final leg to the treatment plant.

In general, the sewage collection systems throughout the region transport sewage and stormwater by gravity flowing from higher geographic points to lower valleys and ultimately to the lowest point along the rivers where the trunk sewers connect to the interceptor system. In some parts of the system, a pump station is needed to transport the wastewater over a hilly area until gravity can take over again.

As described, the sewage collection systems follow the topography of the land; thus the pipes transporting wastewater to the treatment facility do not abruptly end at the boundaries of a community, but rather are integrated throughout municipalities in the UAJSA region. For example, an upstream community’s wastewater flows not only through its own collection system, but through its neighboring downstream community’s sewer system on its way to the treatment plant.

Throughout the UAJSA service area, overflow structures were designed and constructed to deliberately release excess stormwater and wastewater from the collection system when the flow exceeds the pipes’ capacity, usually during wet weather.


Inflow, Infiltration and Overflows

Unlike water pipes, always full because of the pressure used to deliver water into homes, sewer pipes are rarely full when wastewater is flowing from homes to the sewage treatment plant. When water pipes break, crack or are broken, they leak water out; sewer systems, on the other hand, allow groundwater and stormwater to leak in.

When groundwater or stormwater leak into the sewer system, it takes up extra space that could be carrying wastewater. If the pipes become overloaded, raw sewage may overflow at points throughout the sewer system before it reaches the treatment plant.

Inflow and infiltration are terms used to describe how stormwater and groundwater get into the sewer system.


Inflow is stormwater that is directly piped into a separate sanitary sewer system to control runoff. These connections, which may include storm drains in streets, parking lots and driveways and roof gutters, exist in a combined sewer system because it is designed to carry both wastewater and stormwater. Stormwater should never be connected into a sanitary system designed to carry only wastewater.

Some examples of the way inflow affects the sewer system:

  • In some cases, homeowners or contractors have illegally attached roof drain pipes and basement sump pumps to the sanitary sewer.
  • Streams can be directly piped into the sewer system. UAJSA has identified one stream and has already taken the steps to remove it from the system.
  • Hundreds of designed stormwater connections drain volumes of water into the combined sewer system.

Example of inflow: A downspout connected directly to the sanitary sewer system.


Infiltration is excess water that gets into the sewer system through open joints, cracks, and breaks in the pipes. These deficiencies may allow constant infiltration of groundwater. The average sewer pipe is designed to last about 20-50 years, depending on the material. In many cases in this region, collection system pipes and household laterals have gone much longer without inspection or repair and are likely to be cracked or broken.

Some examples of the way infiltration affects the sewer system:

  • Cracked or collapsed sewer pipes, caused by deterioration over time, or poor design, installation or maintenance, allow groundwater into the collection system.
  • Sewer lines are installed beneath a creek or stream because creeks are usually at the lowest point in the area, and it is more expensive to install pipes under a street. These sewer lines are therefore highly susceptible to infiltration when they crack or break. In some cases, broken lines have been known to drain entire streams into the local sewer system.

Example of infiltration: A deteriorated house lateral that allowed water to seep into the sewer collection system.

Inflow and infiltration play a significant role in the sewage overflow problem. During dry weather, about 40% of all flow that reaches the UAJSA treatment plant is due to inflow and infiltration of stormwater and groundwater.

Of course, wet weather magnifies the problem: Inflow and infiltration can add as much as 3,000 gallons of stormwater per person per day to the sewers, instead of the average daily 100 gallons per person of water use that is typical during dry weather. That’s an overload of 30 times more flow per day during rain or snow melt, which then causes sewage to overflow into creeks, streams and rivers at locations along the Allegheny River before reaching the sewage treatment facility.


When an overflow occurs in a separate sanitary sewer system, it is called a sanitary sewer overflow (SSO). This may occur at an overflow structure, into a street from a manhole cover or into the basement of homes. Overflow structures, which were legal at the time of construction, and unintentional SSOs both are illegal in separate sanitary sewer systems under the Clean Water Act.

In a combined sewer system, overflows are called combined sewer overflows (CSO). Because combined sewer systems are intended to carry stormwater and wastewater, they were designed with structures to deliberately release excess flow when the system becomes overloaded, usually during wet weather. While overflow structures in combined sewer systems are legal, municipalities must acquire a permit for each structure and very soon will need to dramatically decrease the number of CSOs that occur annually. CSOs may also occur at unintended locations, such as manholes and basements. Like SSOs, these types of CSOs are illegal.

While every community is likely to experience at least a few overflows in their sewer system, the older communities located in downstream valleys experience the most overflows and basement backups due to their low location in the watershed. The sewer collection systems in these communities not only carry their own sewage (and in many cases stormwater), but they also receive the wastewater flow from their neighboring communities upstream. The complex network of integrated sewer collection system pipes throughout the UAJSA service area makes it critical for all municipalities to collaborate on and share the responsibility for developing and implementing long-term solutions to the overflow problem.


The UAJSA has installed technology on its overflow structures that allow people to sign up and be notified of overflows in the system. It will send an email notification when the overflow begins and when it ends. This is important because overflows can often still be occurring after a rainstorm passes and the sun is shining. If you are planning a trip on the river or other activity that involves contact with the water then this system sends notice directly to your email inbox.

Even though you may not directly be in this area, this service could still help you plan river activities in the Pittsburgh Region because of the similarities to the overflow structures in UAJSA to those in the region. This means that if the structures in UAJSA are overflowing then it is likely that others may be as well.

To be put on the distribution list please email the Project Manager, Alison St. Clair, P.E. at with “CSO Notification System” in the subject line.


The UAJSA and its staff meet monthly to continue to work towards the most cost effective solution to end the overflows in a manner consistent with government regulations. Part of this effort involves getting feedback from the public on the various types of solutions that may be chosen.

If you would like to be part of a stakeholders committee then please email the Project Manager, Alison St. Clair, P.E. at with “Stakeholders Committee” in the subject line.