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? How is sewage transformed into clean water?
Take a look below to learn more about wastewater collection, flow and treatment, and the product recovery and resuse incorporated into the process.
How It Works
System Entrance
Wastewater from your home or business is collected through your sewer pipes (sewer lateral), and stormwater drains into the storm drains on your street into the municipal sanitary sewer system. (1 of 4)
About the Sewer System
ALCOSAN’s service area includes 83 municipalities with varying types of sewer systems. Some municipalities have Separate Sewer Systems and some have Combined Sewer Systems. (2 of 4)
Separate Sewer System
In a separate sewer system, wastewater and stormwater flow into separate sewer lines. Starting in the 1940s only separate sewer systems could be built. (3 of 4)
Combined Sewer System
In a combined sewer system, wastewater and storm water both flow into a common sewer line. Older communities built before 1940s may have combined sewer systems. (4 of 4)
Wastewater Transfer
Water collected passes through your municipal sewer system and then flows by gravity or is pumped through ALCOSAN’s large trunk sewers to the treatment plant. (1 of 5)
Trunk Sewers
These large underground pipes are located along rivers or streams and carry sewage and storm water to ALCOSAN for treatment. (2 of 5)
Sewer Outfalls
When it rains or snow melts, excess water can overload some combined sewer systems resulting in discharges of sewage and other contaminants into the area’s rivers, streams and creeks. (3 of 5)
System Maintenance & Inspections
Regional Conveyance crews perform daily inspections and maintenance of the interceptor system and related structures, including five pumping stations on the Allegheny and Ohio rivers. Structures not readily accessible by land are maintained by ALCOSAN's boat crew. (4 of 5)
Sewer System Mapping
The sewer system is a complex network of pipes connecting 83 municipal systems to the ALCOSAN system and treatment plant. See the System Mapping (5 of 5)
Treatment Process
Wastewater treatment involves using physical, chemical and biological processes to remove harmful pollution – including raw sewage, industrial chemicals, runoff and more – from wastewater. (1 of 16)
Wet Well & Main Pump Station
Wastewater conveyed by the interceptors is collected in a deep wet well and is pumped into the treatment process at a rate of 128,000 gallons per minute. (2 of 16)
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. (3 of 16)
Aerated Grit Chambers
Wastewater passes into the grit chambers, allowing heavy inorganic particles such as rocks, pebbles and sand to settle out by gravity while aeration keeps lighter organic particles suspended. (4 of 16)
Primary Sedimentation
The flow is slowed significantly to allow minute particles which remain suspended to settle and collect at the bottom of the tanks. At the same time, floatables including scum and grease are cleared from the top of the tanks through a system of moving paddles, or flights. (5 of 16)
Secondary Aeration
Wastewater flows into aeration tanks where it is brought into contact with natural microorganisms, called activated sludge. These micro-organisms break down remaining organic matter and nutrients into harmless by-products. Compressed air is pumped through tiny diffusers to aerate the tanks and maintain a healthy colony of microorganisms. (6 of 16)
Secondary Clarifiers
The remaining “mixed liquor” flows to the secondary clarifiers where the activated sludge is settled out and either returned to the aeration tanks or pumped to the dewatering facility for disposal. (7 of 16)
Chlorine Contact Tank
In the disinfection stage, treated wastewater passes from the clarifiers to chlorine contact tanks where a potent sodium hypochlorite (bleach) solution is introduced for disinfection. (8 of 16)
Dechlorination
Prior to discharge, sodium bisulfite is added to neutralize chlorine by-products present in the effluent. (9 of 16)
Clean Water Outfall
ALCOSAN discharges approximately 140,000 gallons of treated wastewater per minute into the Ohio River. The water put back into the Ohio is cleaner than what's in the river. (10 of 16)
Sludge Dewatering
A liquid polymer is added to sludge pumped from the mixing tanks to thicken and condition the sludge for drying. The sludge is then fed into centrifuges which use high speed rotation to separate the moisture content from the sludge, similar to the spin cycle on a washing machine. (11 of 16)
Energy Recovery Facility
Approximately 55% of the sludge dried by the centrifuges is incinerated in ALCOSAN's two fluidized-bed incinerators. Super-heated air is added to a 5' bed of fluidized sand burning at 1500 degrees fahrenheit. Sludge is mixed into the sand where it is instantly incinerated. The hot "off-gas" from the incineration process is carried out and is reused to heat boiler water, to create steam for plant. (12 of 16)
Odor Control
Process odors are controlled at three points through the use of packed tower scrubbers, in which foul air is captured and chemically "scrubbed" prior to its release into the atmosphere. These scrubbers use both sodium hypochlorite and caustic soda to treat odors at the plant headworks, primary sedimentation and dewatering. (13 of 16)
Lime Stabilization
Sludge (biosolids) designated for beneficial reuse passes from the centrifuges to the lime loading facility, where pebble lime is added to raise the product's pH and destroy remaining pathogens (disease-causing organisms). The limed sludge is then loaded into trucks for transport to landfills or to land application projects. (14 of 16)
ALCOSOIL
ALCOSOIL is processed wastewater sludge, commonly called biosoilds, and is used beneficially to reclaim damaged land and as a fertilizer for pasture lands. Resembling crumbly wet dirt, ALCOSOIL is similar to soil humus in its formation and composition. In addition to naturally occurring minerals and elements, the process by-product is rich in organic matter and is limed stabilized to reduce pathogens. (15 of 16)
Laboratory Tests
ALCOSAN's in-house laboratory performs tests daily to confirm the quality of effluent being discharged to the river, to monitor the effectiveness of the plant's industrial pretreatment program and to ensure the quality of ALCOSAN's reusable biosolid product (ALCOSOIL). (16 of 16)
Quick Links
About Compliance
ALCOSAN must meet a series of requirements to comply with the Clean Water Act
Explore the Consent Decree
Follow the Flush
See the path wastewater takes to the ALCOSAN treatment facility from any address within the ALCOSAN Service Area
Flush ItFrom Wastewater to Clean Water
See How it Works