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solids and liquids.
The flow, now slowed, next enters a grit chamber so smaller inorganic items, such as gravel, glass, and metal, can be removed. All this inorganic material goes to the landfill. Pumped-in air helps in further separating the liquids from the solids, and the whole flow enters large, circular concrete tanks for “primary treatment."
“Primary' treatment” consists of sweepers skimming off floating scum, such as fats and oils, while solids settle to the bottom of the tank and liquids are allowed to stand, permitting further settling to take place. Scum and solids are diverted to a solids processing line where they are spundried, treated, and converted into “cakes" that reenter the environment, some as sludge put on farmland.
“Secondary treatment" takes the liquid in the tanks and moves it into biological reaction tanks, also circular and of large size. The liquid is sprayed a few feet into the air to gather oxygen. Bacteriacontaining “activated” sludge is added, containing microorganisms and more oxygen, and some chemicals are inserted. More solids settle out.
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