Optimization of a cyclic activated sludge system for norfloxacin wastewater treatment under low-load conditions

A sewage treatment station was used to treat the pharmaceutical intermediate wastewater of norfloxacin. The designed influent was 480 m(3) d(-1), but the actual influent was only 100 m(3) d(-1). Thus, the cyclic activated sludge system (CASS) was under low-load conditions, which demonstrated poor operation. In this experiment, the optimal operating parameters of the CASS under low-load conditions were studied by adjusting the inflow duration, return sludge ratio, aeration time, dissolved oxygen (DO) concentration, and mixed liquid volatile suspended solids (MLVSS) concentration to improve pollutant removal. The results showed that the optimal inflow duration was 4 h, the return sludge ratio was adjusted from 100% to 133%, the aeration time was 6 h, the DO concentration was adjusted from the designed parameter of 2.0 mg L-1 to the range of 2.8 similar to 3.0 mg L-1, and the MLVSS concentration was adjusted from the designed parameter of 2,500 mg L-1 to the range of 2,300 similar to 2,500 mg L-1. Under these optimal operating conditions, the entire effluent could meet the local indirect discharge standard of Henan Province. Furthermore, the guaranteed rate of the effluent reaching the standard exceeded 95%.