A Study of a Combined Anaerobic/Aerobic System: Enhanced Nitrogen Removal by Alternating Aeration Modes in the Aerobic Digestion System

A combined anaerobic/aerobic digestion system was studied to test its solids and nitrogen removal efficiencies. The anaerobic digester was operated under constant conditions throughout the study at a 15-day solids retention time (SRT) at 37°C. The focus was on variations in the SRT and aeration pattern in the aerobic phase. In the first part of study, the aerobic system was aerated continuously and operated at retention times from 2 to 5 days. More solids and ammonia reduction was observed from the aerobic system operated at the longer retention time but higher effluent nitrite/nitrate was also measured. In the second part of study, various alternating aeration modes (15/15, 15/30, 15/45, 20/20, 20/40 and 30/30 air off/air on in minutes) and retention times (3 to 5 days) were applied to the aerobic system to determine the optimum aeration on/off condition for solids and ammonia removal along with effluent nitrite/nitrate. Among alternating aeration modes, the 20 off/40 on mode for a 4 day SRT was determined as the optimum setup, which achieved about 7% additional volatile solids reduction, 90% ammonia removal and a nitrite/nitrate discharge of 2.6 mg-N/L. Better nitrogen removal efficiencies in the aerobic systems operated for 4 day retention time seem to be associated with more even relative abundant but diverse microbial community distribution along with greater similarity of the community composition between different aeration modes (i.e. time for aeration off or on).