Alternating Aerobic and Anoxic Conditions to Eliminate Sludge Accumulation in the Oxidation Ditch System

The purpose of the present study is to investigate the performance of an upgraded oxidation ditch (OD) system, which was designed and implemented to solve the problem of sludge accumulation at the bottom as well as to get the best removal efficiency of total nitrogen (TN). The upgrading concept is based on dividing the operating volume of the upgraded OD to achieve interchanging between aerobic and anoxic circumstances in order to provide simultaneous nitrification and denitrification (SND). The obtained results indicated that the average TN removal efficiency was 60%, which could be obtained due to a highly efficient SND approach. In addition, the better TN removal efficiency corresponds to the lower sludge volume index (SVI), which reflects the efficiency of the upgraded OD in preventing the accumulation of sludge at the bottom. Effluent ammonium-nitrogen (NH4+-N) and nitrate (NO3--N) concentrations corresponding to a minimum SVI of 41.9 mL/g were 8.6 mg/L for NH4+-N and 8.6 mg/L for NO3--N, respectively. Furthermore, the upgraded OD successfully removes 5-day biochemical oxygen demand (BOD5), chemical oxygen demand (COD), and total suspended solids (TSS) below the permissible limit for final effluent of 60, 80, and 50 mg/L respectively.