Novel Phased Isolation Ditch System for Enhanced Nutrient Removal and Its Optimal Operating Strategy

Abstract

Phased isolation ditch system with intrachannel clarifier is a simplified novel oxidation ditch system enhancing simultaneous removal of biological nitrogen and phosphorus in municipal wastewater. The system employs two ditches with intra-clarifier, and eliminates external final clarifier, additional preanaerobic reactor, and recycle of sludge and nitrified effluent. Separation of anoxic, anaerobic, and aerobic phases can be accomplished by alternating flow and intermittent aeration. Its pilot-scale system operated at HRTs of 10–21 h, SRTs of 15–41 days, and a cycle times of 2–8 h showed removals of BOD, TN, and TP in the range of mixed liquor temperature above 10°C as high as 88–97, 70–84, and 65–90%, respectively. As the SRTs became longer, the effluent TN decreased dramatically, whereas the effluent TP increased. Higher nitrogen removal was accomplished at shorter cycle times, while better phosphorus removal was achieved in longer cycle times. Optimal system operating strategies maximizing the performance and satisfying both the best nitrogen and phosphorus removals included HRTs ranged 10–14 h, SRTs ranged 25–30 days, and a cycle time of 4 h at the mixed liquor temperature above 10°C. Thus, complete phase separation in a cycle maximizing phosphorus release and uptake as well as nitrification and denitrification was accomplished by scheduling of alternating flow and intermittent aeration in the simplified process scheme. Especially, temporal phase separation for phosphorus release without additional anaerobic reactor was successfully accomplished during anaerobic period without any nitrate interference and carbon-limiting.