Pilot Study On Step-feed Anoxic/oxic Activated Sludge Process

Secondary treatment of domestic wastewater was conducted in pilot scale at Beishiqiao Wastewater Purification Center, Xi'an, China, using step-feed anoxic/oxic activated sludge process(SAOASP) for the organic matter removal ,the nitrogen removal situation and the water quality variety of the system . In this study, this process, with four influent feed locations, nine-hour hydraulic retention time (HRT), 50% rate of return activated sludge (RAS), and twenty-day sludge retention time(SRT), was operated for more than one year. The results indicated as follow:1. The organic matter degradation and the nitrogen removal situation : (1) When the flow split to each anoxic tank was 25%, 25%, 25%,and 25% of the influent flow respectively and the volume in each pass was equal and volumetric ratios of anoxic and oxic zone in each pass was 1:3, the effluent TN was about 11.49 mg/L and effluent NH3-N was not exceed \mg /L. The average removal rate of TCOD, NH₃-N and TN reach 92.3%, 97.8% and 67.6%, respectively. (2) When the ratio of influent provided to each anoxic tank were changed to 25%, 35%, 30%, 10%,respectively,and volumetric ratios of anoxic and oxic zone in each pass were adjusted to 1:2,1:1,1:1,and 1:1,from passes 1 through 4,respectively, the average removal rate of TN was increased from 67.6% to 73.6% while TCOD and NH3-N removal was reduced to 90.2%and 82% respectively. Therefore, SAOASP has achieved high-efficiency organic mater and nitrogen removal. It is suggested that SAOASP has potential for domestic wastewater treatment.2. MLSS, organic matter and nitrogen variety respectively of reactor (1) The concentration of MLSS in the early passes of SAOASP was more than that in the later passes ,while the average concentration of MLSS of system was about4A6mg/L .(2) The average concentration of SCOD of anoxic zones was less than that of oxic zones in each pass .(3) the NH3-N concentration of the oxic tank was not exceed \mg/L, indicating that the system approach to be complete nitrification . The NO3-N concentration of anoxic and oxic tank was increased from passes 1 through 4 respectively, while the NO3-N concentration of the first anoxic tank was about 0 mg/L, indicating that it approach to be complete denitrification. The TN concentration of system was increased from passes 1 through 4 respectively, while the NO3-N concentration of the oxic tank was more than that of anoxic.3. In dynamic experiments, maximum specific nitrification rate (SAUR) of 4.55 NH3-N/gMLVSS ' h and maximum specific denitrification rate (SNUR) of 19.68 NO3-N/gMLVSS ' h ,which have been compared with SAUR and SNUR values by nitrogen balance calculation, were obtained, and maximum specific oxygen uptake rate (SOUR) of 75.7mgO2/gMLVSS h was observed.