Dewatering is a physical/mechanical process used to reduce the moisture in digested sludge (biosolids). There are several reasons for dewatering sludge. In general, it is more economical to dispose of the dewatered sludge than it is to pump or haul liquid sludge to disposal sites because by reducing the moisture content, the sludge volume and weight are reduced.
The CAWD plant uses a belt filter press to dewater the digested sludge, with sludge drying beds for temporary storage in the event of emergency belt filter shutdown. The belt filter press consists of two endless belts that travel over a series of rollers. The sludge is pre-conditioned with polymers, then applied to the “free water drainage zone” of the filter belt where most of the free water is allowed to drain through the filter. The partially dewatered solids are then carried to a point where they are trapped between the two endless belts and dewatered further. This part is known as the “press” or “dewatering zone.” As the solids travel through this zone, they are subjected to shearing forces, and the water is forced from between the belts into filtrate trays. The retained solids are scraped from the belts when they separate at the discharge end of the press. The two endless belts then travel through washing chambers for removal of fine solids that may increase the chance of plugging.
The original belt press installed in 1984 was capable of dewatering 80 gpm of 2.5% digested sludge. A newer belt press capable of dewatering 80 gpm was installed in 1999 and the older unit is now on standby.
The dewatered sludge in the form of a “cake”, with a solids content of approximately 18%, is disposed of by trucking to Kings county where it is composted and land applied to non-food crops. Total annual biosolids production is approximately 2,000 to 2,200 wet tons.
The District continues to explore alternative biosolids disposal options.