Within the ALCOSAN service area, the sewer system is a complex network of 83 municipal collection systems all flowing to a single treatment plant. The collection system's 4,100 miles of underground pipes collect and transport sewage from each home or business to ALCOSAN for treatment and subsequent release back into the rivers. Laid end to end, these pipes would stretch the distance from Pittsburgh to Phoenix, Arizona and back.
To further complicate the wet weather problem, two types of sewage collection systems exist in the ALCOSAN service area—the combined sewer system and the separate sanitary system. Combined sewer systems were 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 set of pipes, culverts or ditches. Separate sanitary sewers were required for any new systems built after the 1940s.
Major Parts of the Sewer System
In the ALCOSAN 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 ALCOSAN's 90 miles of interceptor sewer. The interceptors are a series of pipes, some as large as 12 feet in diameter that transport the sewage on the final leg to the treatment plant. The interceptor system, buried up to 120 feet deep under the rivers, is sometimes referred to as the deep tunnel system.
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.
Because the sewage collection systems follow the topography of the land ultimately flowing to the lowest point along the rivers, the pipes transporting wastewater to the treatment facility do not abruptly end at the boundaries of a community, but rather are integrated throughout 83 municipalities in the ALCOSAN region. 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 ALCOSAN service area, 317 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. Of these structures, 265 are within combined sewer systems and 52 within separate sanitary sewer systems. Additionally, approximately 200 overflow structures have been indentified throughout the municipal collection system.
Once wastewater (and some stormwater from combined sewer systems) reaches ALCOSAN’s 59-acre treatment plant, it goes through an eight-step treatment process before it is released back into the Ohio River at a rate of approximately 140,000 gallons per minute. ALCOSAN processes up to 250 million gallons of wastewater daily in dry weather.