Nitrogen Removal Performance And Carbon Saving Improvement Of Partial Nitrification Denitrification SBR Treating Low C/N Ratio Wastewater

Partial nitrification denitrification（PND）is a new biological nitrogen removal process that has received much attention in recent years.It has many advantages over traditional denitrification techniques,and achieving stable nitrite accumulation is the key to the start-up and stable operation of PND.The effect of different temperature conditions on PND is still unclear.Reducing aeration and carbon source injection during nitrogen removal is of great significance for energy saving and carbon reduction in wastewater treatment plants.In this study,PND was operated in sequencing batch reactors（SBR）with simulated low C/N（COD/NH₄⁺-N<4）ratio wastewater,and the start-up of PND at high temperature and stable operation at room temperature were investigated;the performance of PND for nitrogen removal under two low temperature conditions,step-down and continuous low temperature,was compared,and the functional microorganisms for nitrogen removal and sludge bulking triggers were revealed;the conventional SBR was modified into an influent-anoxic/oxic step-feed SBR（IA/O-SFSBR）,aiming to enhance the carbon and energy saving of the PND wastewater treatment process.The main contents are as follows.Two PND-SBRs were started at 30±1°C,using a real-time control system for each operating process.Stable nitrite accumulation was maintained by dissolved oxygen（DO）,p H and redox potential（ORP）in concert with high temperature,low DO and short sludge residence time to continuously suppress nitrite oxidizing bacteria（NOB）.Single-stage PND at high temperature was achieved by domesticating the conventional nitrifying denitrifying sludge into PND sludge after 17 days.After that,the heating was turned off,and at room temperature（22.4±1.5℃）,the ammonia nitrogen removal rate was basically above 90%,and the nitrite accumulation rate reached up to 98.2%,realizing high-activity PND at room temperature.The operating conditions of high temperature followed by ambient temperature can be used as a general method for rapid PND start-up.The performance,sludge characteristics and microbial community of PND were evaluated under two low temperature conditions,stepwise cooling（28.2,24.5,20.6,17.0°C）and continuous low temperature（17.0°C）.The results showed that PND completed the start-up faster and achieved more than80%nitrite accumulation rate under gradient cooling conditions,while a longer start-up period was required under continuous low temperature condition.Higher DO limits should be used for real-time control at low temperatures to achieve complete ammonia removal.Fitting using Arrhenius equation,it was found that PND was not significantly affected by temperature at17-28°C.However,the denitrification activity was significantly inhibited by temperatures below 17°C.Bacteroidota phylum proliferated at low temperatures,while DGAOs proliferated only under stepwise cooling conditions,and they may contribute to efficient denitrification.Sludge bulking under sustained low temperature conditions eased with increasing PND activity,suggesting that sludge settling performance needs attention when activity is low.IA/O-SFSBR was successfully constructed to utilize the timing of the influent for denitrification.High efficiency PND was maintained with step feed and intermittent aeration strategy.PND nitrogen removal was achieved in 180 min,which greatly shortened the HRT,and NO_x^-removal was accomplished using only raw effluent organic carbon.At a fixed HRT,the effluent was divided into 1-5 aliquots for treatment.The TIN removal rate increased with the number of feed segments,reaching a maximum of 85.46%.The optimal operating strategy was the 5-step feeding（IA:O=6 min:30 min）,and further extension of the aerobic time（6min:40 min）could improve the ammonia nitrogen removal rate,but implies higher operating costs and the risk of switching to complete nitrification.IA/O-SFSBR has a strong economic benefit in terms of daily saving of CNY1,927 in aeration and carbon source supplement compared to conventional nitrogen removal process in one-step feeding.