Operation and Optimization of the Parallel AN/AO Process

The parallel AN/AO process, first proposed by the authors to efficiently use denitrifying phosphorus removing bacteria, was briefly introduced in the paper. Key operational parameters i.e. influent feed ratio of AN unit vs. AO unit, internal mixed liquid suspended solids (MLSS) return ratio (IMRR), external sludge return ratio (ESRR), sludge retention time (SRT) and hydraulic retention time (HRT) were optimized and determined as 12:8, 200%, 25%, 15 days and 6 hours respectively. Mass balance of main pollutants such as chemical oxygen demand (CODcr), total nitrogen (TN) and total phosphorus (TP) were conducted to demonstrate environmental fates of pollutants and to evaluate optimization potential of different compartments. The process operated under optimal parameters achieved pollutants removal efficiency of 74%, 99%, 68%, and 83% in COD, NH3-N, TN and TP respectively. Effluent COD, NH₃-N, TN and TP concentrations were 49.1 mgL-1, 0.4 mgL-1, 14.8 mgL-1, and 0.8 mgL-1 comparing to influent concentrations 186.7 mgL-1, 37.8 mgL-1, 46.2 mgL-1, and 4.7 mgL-1 respectively. Effluent of the process can meet the Discharging Standards for Chinese Urban WWTPs (GB18918-2002) first grade (B) stably.