Methane, carbon dioxide, and hydrogen sulfide gases are released by the microbes during the digestion process. The gases and excess water are removed from the digester while the stabilized sludges are pumped to a solids handling facility for dewatering. The sludges are now considered to be biosolids. Retention time through the two digester is in excess of 30 days. The biosolids are removed from the quiescent secondary digester and are pumped to 24 inch diameter, counter current, low speed centrifuges.
Polymer is added to condition the biosolids and aid in increasing the solids content.
The centrifuges rotate and the centrifugal forces cause the separation of the water from the solids. The water is pumped back to the head of the plant while the solids, know called sludge cake, are discharged from the centrifuge to a storage hopper. The centrifuges are typically operated once per day during the graveyard operations shift. Due to the limited storage capacity in the sludge hoppers and the quantity of sludge produced, they have to be disposed of once or twice per day. This is done by hauling the sludge to a sanitary landfill.