Membranes can be installed in the clarifier (or aeration tank) of an existing activated sludge plant to enhance the biomass separation function of the system, thereby effectively overcoming any operating constraints associated with sludge settleability. The resulting upgraded plant can be operated at high biomass concentrations (10–20 gMLSS/L), leading to an increase in its treatment capacity. The membranes also ensure a treated water consistently free of suspended solids and a superior disinfection performance. The system offers an enhanced operating flexibility, and allows to operate at high sludge ages leading to a low excess sludge production.

Such an immersed membrane activated sludge process (BIOSEP®) has been developed and applied to the treatment of raw sewage. When treating screened raw sewage with this process, with a sludge concentration of 15 gMLSS/L and a volumetric loading of 1.2 kgCOD/m3/d, a 96% COD reduction and a 95% Total Kjeldahl Nitrogen (TKN) reduction have been obtained. The disinfection performance of the system was over 6 Log removal for fecal coliforms. The resulting production of sludge was 0.20 kgMLSS/kgCOD.

Two desk case studies are given for 900 m3/day upgraded plants. In one case, the primary objective was to increase the treatment efficiency and develop nutrient removal for the original plant, while in the other case the primary objective was to increase the capacity of the original 460 m3/day plant.

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