Experimental Study On Nitrogen Removal In Moving Bed Biofilm Multi Grade A/O Reactor

In recent years, water pollution in our country has become more and more serious,and lake eutrophication has become increasingly prominent. large-scale outbreak ofalgae has occurred in key lakes such as taihu chaohu dianchi. Due to the reduction offlow rate and the drop of water self-purification ability, the reservoir area has been alsofacing the risk of eutrophication after the completion of the Three Gorges Reservoir. Alot of studies have shown that excess accumulation of nitrogen in water is one of causeof eutrophication. So it’s very necessary to efficiently remove nitrogen in sewage.At the present stage, there are A/O process, A2/O process, oxidation process andSBR process which are widely used as denitrification process in wastewater treatmentplant. In the long-term production practice, these processes have played a huge role inthe removal of nitrogen in sewage. However, the practice also proved that the processeshad the disadvantages such as requirement of sludge recycle and Nitrifying liquidbackflow, long process and large area, the comples of operation and management.For multi-grade A/O process, one anoxic zone and one aerobic zone form a A/Ounit, number of A/O units then concatenate into a system, raw water flows into everyanoxic zone according to a certertain influent distribution ratio, denitrification andnitrification separately occur in anoxic zone and aerobic zone.. There is only sludgerecycle in system. Based on multi-grade A/O process, suspended carriers are added inaerobic zone in order to play the feature of biofilm. The combined process is calledmoving bed biofilm multi grade A/O process.In this study, the performance of denitrification of reactor was the main object. Afactorial test was employed to investigate the influence of influent distribution ratio,A/O volume ratio and suspended carrier filling ratio on the performance ofdenitrification of reactor, and to determine the optimum denitrification condition ofreactor. In order to develop the denitrification potential of reactor, simultaneousnitrification and denitrification study was carried out further.Results showed that:①Factorial test determined the optimum denitrification condition that influentdistribution ratio was5:3:2, A/O volume ratio was set on2:3, suspended carriers fillingratio was40%. According to ANOVA, the effect of each influencingfactor for nitrogenremoval was determined as: suspended carrier filling ratio＞influent distribution ratio＞ A/O volume ratio＞interaction between influent distribution ratio and A/O volumeration.②Under optimum condition, reactor had good treatment effect. The concentrationof COD, NH₄⁺-N, NO₃^—N, TN in effluent was lower than A level of first class ofNational Discharge Standard（GB18918-2002）. Especially, the concentration of TN ineffluent was10.7mg/L, the removal efficiency of TN could reach76.4%.③Under optimum condition, reactor player feature of maximum utilization of thecarbon source of raw water, so81.3%of total removed COD by biological effect wasdegraded by denitrification in anoxic zone, and the amount of TN removed in anoxiczone accounted for91.9%of the amount of biological nitrogen removal in the reactor.④DO concentration significantly effected the nitrogen removal of simultaneousnitrification and denitrification. With the increase of DO concentration, the performanceof simultaneous nitrification and denitrification became bad. For the small scale test ofthree stage, the optimum DO concentration was0.5mg/L.In addition, filling suspended carrier benefit the nitrogen removal of simultaneousnitrification and denitrification. There was big difference in the effect of nitrogenremoval among each stage, the first stage aerobic zone had the best effect, the secondstage was poor, the third stage was worst.⑤According to the consideration of various factors, in order to achieve best effectof simultaneous nitrification and denitrification, the DO concentration of each stageaerobic zone should be set as that fist stage was0.5mg/L, second stage was1.0mg/L,third stage was1.5mg/L. Under this condition, the concentration of COD, TN, NH₄⁺-Nin effluent was23.0mg/L,6.9mg/L,1.5mg/L. Aeration of the reactor was reduced by36%, meanwhile, denitrification capacity increased by21%, compared with no effect ofSND.