Raw sludge is pumped to the primary digester where the contents are heated and mixed; most digestion occurs here. Anaerobic digestion reduces the organic solids contents between 50 and 60 % of what they were in raw sludge. Methane gas is produced in the digester. The methane gas can be used later to fuel the heat exchanger, which keeps the sludge heated, thereby reducing fuel costs. Excess methane gas is directed to the waste gas burner where it is vented and burned. Whenever raw sludge is added to the primary digester an equal amount of digested sludge is transferred to the secondary digester. There is no mixing in the secondary digester, this allows the heavier solids to separate from the lighter ones. After a period of settling the digested sludge at the bottom may contain about 3 to 6% solids. The digested sludge is relatively odor free and capable of being disposed of without causing a nuisance.