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UNDERSTANDING THE SEWER COLLECTION SYSTEM
History
Wet weather sewer overflows are not a new problem for the Pittsburgh
region.
Between 1889-1912, 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 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.
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 in 1959.
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 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, has evolved in the region surrounding Pittsburgh.
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.
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