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The plant uses the following processes for sludge handling: gravity thickening, two-
stage anaerobic digestion and belt press dewatering. The gravity thickening system
consists of two 8,265 gallon tanks and a self contained stainless steel decant box. Under normal operation, the waste activated sludge and pad cleaning solids are pumped into the tanks for thickening, storage and equalization of the sludge, before it is pumped to the primary digester.
The anaerobic digestion system consists of two digesters, one primary digester with a fixed cover and the other a secondary digester with a floating cover for gas storage. The primary digester has a volume of .333 MG with a hydraulic detention time of 20 to 25 days. The mixing system consists of two axial flow propeller pumps and a mechanical, recirculation nozzle in each digester which provides a tank turnover time of 1.8 hours and also can serve as a scum breaker. The heating system consists two horizontal centrifugal pumps, shell and tube heat exchanger, methane gas boiler and a natural gas boiler for backup. Hot water heat is provided for the primary digester and plant buildings. The primary digester has consistently reduced volatile solids by 40 to 45 percent.
The secondary digester has a volume of .515 MG and is used for storage of digested
sludge prior to dewatering and gas storage. The secondary digester can be used as
a primary digester. A waste gas burner is used to burn excess methane gas. Digester
gas production is metered and recorded. The digester control building is equipped with
a gas detection system which constantly monitors levels of oxygen and combustible
gas levels throughout the building.