Stabilization of wastewater biosolids from the liquid stream process is performed by anaerobic digestion to Class B standards. The volatile solids content of the sludge is biologically reduced (stabilized) via heat, detention time, mixing, and the work of anaerobic bacteria. Digestion is performed seperately on primary sludge from the primary clarifiers and waste activated sludge from the secondary clarifiers.
Four anaerobic digesters are utilized for stabilization of primary sludge. Two digesters serve as primary digesters and two serve as secondary or storage digesters. Supernatant is decanted from the secondary digesters and is recycled to the head of the plant. Two anaerobic digesters are utilized for stabilization of waste activated sludge. The waste activated sludge is thickened to 4-5% solids via two prethickening centrifuges prior to digestion. The centrifuges separate the water from the solids by centrifugal force. The thickened sludge is then pumped to the two waste activated digesters via a progressive cavity positive displacement pump. The sludges are heated and circulated via a centrifugal pump. The byproduct of the sludge digestion is methane gas. The gas collects in the dome of the digester and is used as a fuel source for the boilers to heat the sludge.
When the digesters reach a certain operating level, the sludge is then pumped to a 2.5 million gallon liquid sludge lagoon via a piston pump. The sludge will stay in the lagoon until the contracted sludge haulers come out to haul it to the farm fields, where it is land applied. The District also has 41 sand drying beds. The sludge is pumped to the sand drying beds for dewatering. When the sludge dries, it is picked up with a front end loader and brought out to the farm fields. Currently, the sand beds are only used for a back up plain if the liquid lagoon is at full capacity. The drying beds as a whole can take 1 million gallons of sludge.