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.
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.
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.
In a separate sewer system, wastewater and stormwater flow into separate sewer lines. Starting in the 1940s only separate sewer systems could be built.
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.
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.
These large underground pipes are located along rivers or streams and carry sewage and storm water to ALCOSAN for treatment.
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.
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.
Wastewater treatment involves using physical, chemical and biological processes to remove harmful pollution – including raw sewage, industrial chemicals, runoff and more – from wastewater.
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.
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.
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.
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.
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.
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.
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.
Prior to discharge, sodium bisulfite is added to neutralize chlorine by-products present in the effluent.
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.
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.
Approximately 32% 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 use and to generate electricity.
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.
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.
ALCOSOIL is processed wastewater sludge, commonly called biosoilds, and is used beneficially to reclaim damaged land and as a fertilizer for feed crops. Resembling crumbly wet dirt, ALCOSOIL is simlar to soil humus in its formation and composition. In addition to naturally occuring minerals and elements, the process by-product is rich in organic matter and is limed stabilized to reduce pathogens.
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).