To protect the water supply for the Metropolitan Chicago area, the Illinois legislature, in 1889, created the Metropolitan Sanitary District of Greater Chicago. To insure the quality of the water supply, the Sanitary District collected and diverted the wastes of 750,000 people away from Lake Michigan. Today, the Sanitary District collects and treats the wastes from a domestic population of 5% million people and an industrial waste load equivalent to 4% million people. The District serves Chicago and 120 other cities and sub-urbs in an 858 square mile area.
The wastewater enters the treatment plant through underground sewers, some as large as 24′ in diameter. The wastewater passes through screens which remove coarse debris such as rags, branches, bits of wood, etc. before being pumped to ground elevation. After pumping, the wastewater flows by gravity thru grit tanks which remove grit and sand particles by sedimentation. After grit removal, the wastewater flows into a primary settling tank where from 40% to 50% of the organic and inorganic solids settle out and are pumped to anaerobic digesters. The wastewater then flows to aeration tanks where it is mixed with another flow containing microorganisms which feed on the organic matter remaining