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Wet weather sewer overflows are not a new problem for Southwestern Pennsylvania.

From the early 1800’s till around the 1920, sewers were constructed in City of Pittsburgh communities 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 communities had not yet constructed sewage treatment plants, engineers felt that stormwater helped flush the sewage solids away and one large pipe accommodating both sewage and stormwater was more economical to direct wastes to the rivers and streams. 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.

As early as 1930, the Pennsylvania Sanitary Water Board sought to compel Pittsburgh to submit a plan for a comprehensive sewage system.  In December 1937, Pittsburgh Mayor Cornelious Scully called a meeting of Allegheny County municipalities to discuss possible collective action. The onset of World War II delayed any action on sewage treatment.  In June l945, the Board issued the long-awaited orders to the City of Pittsburgh, 101 other municipalities and more than 90 Allegheny County industries to cease the discharge of untreated wastes into state waterways. 

In 1946, the Allegheny County Sanitary Authority (ALCOSAN) was formed to study the needs of the region and to develop and submit a treatment plan to the Pennsylvania Sanitary Water Board. Along with the construction of a treatment plant, more than 90 miles of very large pipes, called interceptors, were placed along the major rivers and streams in the 1950s. These interceptors were designed to receive wastewater from municipal sewer systems and "intercept" or redirect the sewage to the ALCOSAN treatment plant before it reached the waterways. ALCOSAN became fully operative as a primary treatment plant in 1959. 

Most of the municipal collection systems originally directly connected to the interceptors were 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 and mostly suburban communities chose to build separate pipes for sewage, called separate sanitary sewers and a set of pipes solely for stormwater. 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 ALCOSAN's interceptor system; in other cases, municipalities have connected directly to ALCOSAN'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 83 municipal combined and separate sanitary systems, which flow into each other before reaching the ALCOSAN sewage treatment plant, evolved in the region surrounding Pittsburgh. Much of the system over time became deteriorated and overloaded during wet weather, which resulted in frequent and illegal sewage overflows into our waterways, streets and homes.

In 2004, 83 ALCOSAN communities signed municipal consent orders committing to the assessment and repair of their sewage collection systems.  Work continues on what is considered the largest public works project Allegheny County has ever faced.