About Combined Sewer Systems

What is a CSS?

A Combined Sewer System (CSS) is a wastewater collection system that conveys both sanitary sewage and stormwater in a single pipe to a WWTP.  These systems were considered a “state of the art” design back in the 1930s when the majority of the BSA’s sewer and interceptor system was designed and constructed. As part of this design, regulators were built into the collection system as relief points during high wet weather flows to convey dry weather flows to the treatment plant and divert excess wet weather flows to under-capacity parts of the system or directly to receiving water bodies via an overflow. An illustration of how a CSS system works is shown below.

What is a CSO?

A Combined Sewer Overflow (CSO) is a designed discharge point for release of the combined wastewater when the capacity of the CSS is exceeded during wet weather.  CSOs can occur during heavy rain or sudden snowmelt that causes the capacity of the combined sewers to be exceeded. When this occurs, sanitary sewage can mix with runoff from buildings, streets and parking lots, and flow untreated into the local receiving waters. At the time that they were designed, CSOs were constructed to prevent overloading of the system and also prevent basement and street flooding. Today, CSOs are being eliminated or minimized, pursuant to the federal government’s Clean Water Act, because the discharges may impact water quality, the aquatic environment and human health and safety.

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