Dynamic simulation of a WWTP operated at low dissolved oxygen condition by integrating activated sludge model and a floc model

While an aeration tank in an activated sludge process is often operated with high dissolved oxygen (DO) concentration to ensure organic degradation and nitrification, it may be operated at low DO concentration to reduce energy consumption and achieve desired denitrification. The ASM1 (Activated Sludge Model No. 1) can be used to describe the activated sludge process if the nitrification and denitrification occur either during different phases or in different tanks, but it may encounter problems in simulating the denitrification phenomenon caused by low DO concentration in the aeration tank. In the present work, we developed a model integrating the ASM1 kinetics and a biofloc model to account for the actual anoxic and aerobic rates. Oxygen was assumed the only substrate of both bio-kinetically and flux limiting in the flocs and its dispersion coefficient was estimated as 1.2 × 10−4 m2 day−1 by using a set of measured effluent qualities of a full-scale wastewater treatment plant (WWTP) operating at low DO concentration (∼0.80 mg L−1) for 60 days. Simulation studies predicted the optimal DO level of 0.36 mg L−1 which would lead to minimum total nitrogen of 15.7 mg N L−1 and also showed the insignificance of the addition of carbon source for nitrogen removal for the operation under study. The developed model may be helpful for process engineers to predict the plant behaviors under various configurations or operating strategies.