| step-feed biological nutrient removal (BNR) has long been proposed as one of the most effective processes for the treatment of wastewaters from municipality and industries, Although several laboratory and pilot scale studies in USA and Europe demonstrated that the process is cost-effective for old wastewater treatment plant upgrading and new plant building, there is still a shortage in plant design and operation in China.In this study, a pilot scale plant was employed and operated for more than one year to investigate the process performance, process stability and operating parameters with a 50% of return activated sludge (RAS) under 9 hours of hydraulic retention time (HRT) and 20 days of sludge retention time (SRT). The experiment included two phases. In phase I , the tank layout is symmetrical and the volume is equal, the feed split is equal (40L/h), and VA/Vo is 1:3 in each pass. In phase II, each pass flow rate is changed to 40, 56, 48, 16L/h accordingly, and the VA/Vo of the first pass was changed to 1:2. The average removal efficiencies of TCOD, SCOD, NH3-N, TN and TP could reach as high as 92.3%, 89.5%, 97.8%, 73% and 75% respectively in phase I , and reach 89.8%, 85.0%, 80%and 73.6% respectively. In phase II. The results obtained showed that step-feed BNR is efficient and cost-effective for nitrogen and phosphorous removal from municipal wastewater.The nitrification rate reached 0.81-4.44 rngNH3-N/gMLVSS·h and the denitrification rate reached 3.0-10.5 mgNO3-N/gMLVSS·h in the rector. The maximum specific oxygen uptake rate is 75.96mgO2/gMLVSS·h, ammonium uptake rate is 4.55mgNH3-N/gMLVSS·h, nitrate uptake rate is 19.68mgNO3-N/gMLVSS·h in batch experiment. The results showedthat the microorganisms in the system are highly active.In conclusion, the step-feed BNR is an alternative and effective technology of nutrient removal for municipal wastewater treatment. It is feasible and practical using step-feed BNR in treating Xi'an municipal wastewater.
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