Upgrading a conventional activated-sludge plant using a multi-stage aerobic system with step sludge recirculation

The performance of a multi-stage activated-sludge plant with partial biomas s recirculation was examined. The system, which was designed to treat domes tic sewage, consists of four stages and has two recirculation flows: (a) di rectly into the anoxic stage, and (b) distributed over ail the aerobic stag es from the secondary settlement tank. Different values of recirculation Ro w to each aerobic stage were adopted during the experiments, and the optima l configuration for removing both ammoniacal nitrogen and carbonaceous subs trate was determined. The configuration which gave the best performance als o Sludge age (d) improved the sludge settleability.