A necessary by-product of the activated sludge (AS) process is the excess biomass produced, which is removed from the system as waste activated sludge (WAS). While conventionally viewed as a “waste” product, the WAS shares the same composition as the “useful”
activated sludge in the aeration tank, and can profitably be used to affect additional pollutant removal from wastewater. This paper presents details of a new process, the WASAC (Waste Activated Sludge Anaerobic Contact) process, that uses WAS from an AS process to treat wastewater in an anaerobic
contact process. The WASAC process configuration consists of two separate process streams, a “donor” stream consisting of an AS process configured for BPR, and a “receiving” stream that consists of an anaerobic contact (AC) process. The BPR-AS process is used to cultivate
a population of phosphorus accumulating organisms (PAOs) capable of taking up soluble BOD under anaerobic conditions. The WAS from the BPR-AS process is introduced into the anaerobic contact tanks of the AC process, where it is mixed with untreated wastewater. The unaerated conditions prevailing
in the AC tanks facilitate the uptake of soluble BOD by the PAOs (introduced via the WAS). Adsorption of particulate pollutants also occurs, resulting in a substantial removal of BOD without aeration. The solids separation step following the AC tank separates out the biomass and other particulate
matter, leaving an effluent largely free of both soluble and particulate organic pollutants. If further polishing of the AC effluent is required, it can be redirected through the AS process. Verification of the WASAC process concept was conducted using both process modeling and pilot testing
at a full-scale operating AS plant. Results show that the WASAC process can offer significant advantages over conventional AS, including a major reduction in aeration costs, and increased energy recovery from anaerobic digestion. The WASAC process can be applied to meet a variety of discharge
requirements, including carbonaceous BOD removal and nutrient removal.
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