Research On Phosphorus And Nitrogen Removal By Modified A~2/O Process Based On Biological Selector

Along with the increasing of eutrophication and serious environmental problems it caused, nitrogen and phosphorus are restricted severely in our country, so nitrogen and phosphorus removal has become a hot and difficult problem in the field of sewage. All the sewage treatment plants are faced with the advanced removal of nitrogen and phosphorus. Now at home and abroad A~2/O process is the mainstream process. However, the traditional A~2/O process is hard to realize efficient simultaneous nitrogen and phosphorus removal due to the inherent contradiction, such as competition of carbon source, contradiction of SRT, the nitrate interferes with phosphorus release.Based on the in-depth analysis of various contradictions of the traditional A~2/O process, and draw lessons from the previous academic research results, we proposed a modified A~2/O process. In order to prevent the nitrate in the returned sludge flows into the anaerobic zone, a small selector zone was added to the traditional A~2/O process after the anaerobic zone to accept the returned sludge. The nitrate in the returned sludge is consumed through denitrification in the selector zone. And in the anoxic zone, the organic carbon substrate can be used simultaneously for both nitrogen and phosphorus removal through the denitrifying dephosphatation.Test results show that the technology has good nitrogen and phosphorus removal, and the denitrifying phosphorus removal bacteria are successfully enriched in the anoxic zone. The average removal rates of TP,TN,COD are 95%,77.6%,89.2% separately,the average effluent concentration of TP,TN,COD are 0.34mg/l,11.6 mg/l,29 mg/l separately, which is qualified to Standard A of "Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant" (GB18918-2002). The anoxic phosphorus uptake in anoxic zone is 4.6 mg/l, which is about 34% of the total phosphorus uptake. Ammonia nitrogen concentration in the effluent is under detection limit, The effluent TN is dominated by nitrate.To analyse the function of the selector shows that the setting of the selsctor changes the function of anaerobic, the function of anaerobic from denitrification and phosphorus release changes to phosphorus release only. Besides, the selector can prevent sludge bulking well.The change of operation parameters of the system have great influence on the nitrogen and phosphorus removal, the results show that the suited inflow C/N ratio is 4 ~ 7, and when inflow C/N ratio is 6, the nitrification liquid return ratio is 200%, the mixed liquid ratio is 150%, the sludge return is 100%, the system can get the best nitrogen phosphorus removal.The enhanced biological phosphorus removal of the system was studied in the aspects of anaerobic phosphorus release and anoxic phosphorus uptake.The anaerobic zone has very good phosphorus release effect, the average anaerobic phosphorus release content is 22.04 mg/l. The anoxic phosphorus uptake in anoxic zone is clearly. The average anoxic phosphorus uptake is about 4.39 mg/l, which is about 35.96% of the total phosphorus uptake. The test results shows that when the inflow C/N ratio is 6,the nitrification liquid return ratio is 200%, the mixed liquid ratio is 150%, the quantity of phosphorus released and the anoxic phosphorus uptake ratio are both high, so the phosphorus removal rate is high.