7 - Sludge Digestion
Raw sludge withdrawn from the primary clarifiers, together with the scum
off the top of the primary and final clarifiers, is pumped to the primary
anaerobic digester. Here it is mixed continuously and heated to 35 degrees
Celsius. This stage provides the necessary environment for anaerobic digestion
of sludge, which is the decomposition of organic matter by micro organisms
in the absence of air. After a digestion period of 20 days, the sludge
flows to the secondary digester. Here the stabilized sludge is thickened.
Liquid from the top is returned to the plant. The sludge is then
transferred to storage lagoons for further thickening. The sludge is then
applied to farm land as a soil conditioner and fertilizer. During the
digestion process methane gas is produced. The gas is utilized to fuel
boilers and provide heat for the digesters and buildings.