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.