The contact stabilization activated sludge process (CSASP) has been adopted and applied in full scale plants treating domestic sewage. Acquiring smaller volume thereby reducing costs and achieving the same treatment efficiency as the conventional activated sludge plants are the main advantages of the CSASP. This activated sludge modification is especially ideal for small to mid-scale plants where influent wastewater contains a high fraction of particulate COD. The simulation results reveal that the same amount of mass sludge can be retained in the system with almost 30% volume reduction compared to conventional activated sludge plants. It is also found that the CSASP treatment efficiency increases when the particulate COD fraction of an influent domestic wastewater increases. The most important process component in the design of CSASP is the sludge distribution factor (α) which directly affects the effluent characterization Having most of the biomass in the stabilization reactor (meaning low α values) decreases the nominal hydraulic retention time (θHN) of the system. However the sludge distribution factor must be high enough to ensure an acceptable effluent quality.

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