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Wastewater Treatment: How it Works

Where does water from your bathtub, toilet and sinks go once it goes down the drain?  Where does rainfall or melting snow go after flowing down the storm drains along the curbs on your street?  How does a combined sewer system work?

Water that drains from your home or is collected in the storm drains on your street passes through municipal sewer lines and flows by gravity or is pumped through large interceptor sewers to ALCOSAN's treatment plant.  In a combined sewer system, wastewater and storm water both flow into a common sewer line that may overflow during periods of heavy rain or snow melt.

Interceptor Sewers
These large underground pipes are located along rivers or streams and carry sewage and storm water to ALCOSAN for treatment.  ALCOSAN maintains 90 miles of interceptor sewers within its service area.

Wet Well & Main Pump Station
Wastewater conveyed by the interceptors is collected in a 120 ft. deep wet well and is pumped into the treatment process at a rate of 128,000 gallons per minute.   There are six main sewage pumps with a total pumping capacity of 435 million gallons per day (mgd).  Typically only 2 or 3 pumps are necessary to achieve ALCOSAN's permitted flow of 250 mgd.

Bar Screens
Wastewater passes through 3/4" bar screens which collect leaves, styrofoam, rags, and other solids from the raw sewage.  This accumulated debris is automatically raked from the bar screens and is sent by conveyor to a compactor for landfill disposal.

Aerated Grit Chambers
Wastewater then passes into the grit chambers where its velocity is reduced, allowing heavy inorganic particles such as rocks, pebbles and sand to settle out by gravity while aeration keeps lighter organic particles suspended.  Settled grit is removed from the bottom of the grit chamber for landfill disposal.

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3300 Preble Avenue
Pittsburgh, PA 15233
(412) 766-4810