Before discharge, the treated wastewater moves through a final stage of treatment, which consists of Tertiary Treatment (filtering to remove fine particles) and Disinfection (chemical treatment to remove pathogens).
Tertiary Physical Filtration
One of the ways water is purified in nature is by being filtered through the sand and fine gravel particles at the bottom of streams and rivers. Ground water is also purified by passing through layer after layer of sand, gravel and rock under the surface of the earth. As the water passes through these materials, fine particles are trapped and left behind.
Tertiary treatment consists of passing the wastewater through a bed of material with a rough surface to remove any fine solids that might still be in the water. The tanks in which this is done are called “sand filters.”
A project to rehabilitate the tertiary filter building is under way at this time.
Next all the cleaned wastewater is fed into the “chlorine contact tank” in which sodium hypochlorite (bleach) is mixed with the water to disinfect it. This kills off any harmful organisms that may have escaped the treatment process.
Excess chlorine that might prove harmful to aquatic life is then removed from the water by treating it with sodium bisulfite. This leaves an effluent that can be safely discharged to the South Branch of the Kishwaukee River.
Effluent Quality Assurance
Before entering the river, effluent is collected for lab tests. Effluent quality is monitored constantly and it is stringently tested for harmful organisms, or unhealthy levels of any substance.
This testing is monitored by the United States Environmental Protection Agency under the directives of the Clean Water Act of 1972 and controlled via the District’s National Pollution Discharge Elimination System (NPDES) Permit.
We are regulated as to the limits of total suspended solids, biological oxygen demand, fecal coliform, pH level, ammonia level, and other discharge parameters.
Treatment time has been about 10 to 13 hours for wastewater that was treated in the Trickling Filter/BioDisc facility and 6 hours for wastewater that was treated in the Activated Sludge facility.