The Digester

Sludge from the thickener is pumped to the digester where heat and bacteria decompose the solids to further process it for land application. The digester is tested for pH, acidity, alkalinity and the temperature is also monitored. Solids tests are done on the raw and digested sludge to make sure the reduction of volatile solids is adequate for the land application.

Sludge is land applied at the City owned airport in the spring and fall. During application, a sample is tested for arsenic, cadmium, chromium, copper, lead, mercury, molybdenum, nickel, selenium, and zinc, of which there is a maximum cumulative loading of lbs. / acre. Sludge can only be applied up to 2 tons / acre dry weight per year. A detailed sludge report is sent to IaDNR and EPA yearly.