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WHAT HOMEOWNERS NEED TO KNOW
Understanding the Sewer Collection System
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 4,000 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 represent 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
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.
Major parts of the sewer system
In the ALCOSAN collection 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
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.
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 83 municipalities in the ALCOSAN
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 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.
Next: Inflow, Infiltration
and Overflows
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