Enhanced Removal Of Nitrogen And Phosphorus Using Modification Of The Two Sludge A~2/O-BAF System

China has achieved positive results in the treatment of urban sewage,and the eutrophication of water has been contained recently.However,the concentration of urban sewage has two general characteristics of low C/N and low C/P.A²/O process is used in more than 50%wastewater treatment plants in China.In this process,the nitrifying bacteria,denitrifying bacteria and phosphorus accumulation organisms?PAOs?have competitions and contradictions on carbon source and sludge rentention time?SRT?,and nitrates inflluence anaerobic phosphorus release,which results in the low efficiency of simultaneous enhanced removal of nitrogen and phosphorus.So it can essentially control the eutrophication of water and relieve the situation of water resource shortage.The modification of the two sludge A²/O-BAF system is to set the pre-anoxic in the front of A²/O-BAF to reduce the DO and NO₃^--N in the return sludge mixture,so the PAOs can fully release phosphorus in the anaerobic.Modified biological fillers in a fluidized state are used in BAF without backwashing.The system integrates activated sludge process and biofilm process,short SRT denitrifying bacteria and PAOs grow in the form of activated sludge in A²/O and long SRT nitrifying bacteria grow in biofilm in BAF.Thus the separation of time and space is achieved successfully,avoiding the confliction of SRT simultaneous ly and making them grow in the best environment.Denitrifying bacteria uses organic carbon source as electron donor in the A²/O anoxic stage and NO₃^--N as electron acceptor in the refluxing nitrification solution to execute denitrifying nitrate and phosphorus removal which achieves“one carbon dual-purpose and one nitrogen dual-purpose”.The modification of two sludge A²/O-BAF system is used to treat low C/N real domestic sewage,when the average influent concentrations of COD,NH₄⁺-N,TN and TP are 188.75,60.26,61.57 and 4.90 mg/L,the start up of combination process,and the effects of HRT and step feed on removal of nitrogen and phosphorus are investigated.The results are as follows:The A²/O reactor is started up under the low temperature?10¹6°C?in winter.When the volume ratio of the pre-anoxic,anaerobic,anoxic,and aerobic sections is 1:2:5:2,the TP removal rate is up to 80%,eliminating nitrifying ba cteria,achieving denitrifying phosphorus uptake successfully in A²/O.Inoculation combined with natural biofilm is used to start up BAF with the synthetic wastewater.When influent concentrations of NH₄⁺-N and COD are103.16 mg/L and 60 mg/L,respectively,the nitrification BAF performance is well and the light brown sticky film becomes thicken fulvous biofilm when the BAF is operated for the 25 days.The effluent concentrations of N H₄⁺-N and COD decrease to 36.17 mg/L and 6.66 mg/L,and the removal rates of them are 62.7%and 87.3%respectively.The effluent concentration of NO₃^--N reaches to 60.23 mg/L.Then the A²/O and BAF are combined,the effluent qualities have stabilized after 25 days.The start up and operation of the combination process has been realized successfully.After start up of the modification of the two sludge A²/O-BAF system,the effect of HRT on the removal of nitrogen and phosphorus is researched.When the SRT is 12 days,sludge return ratio is 100%,nitrate recycling ratio is 200%at the temperature of 18²0?,the effluent concentrations of NH₄⁺-N and TP meet the first class discharge standard?GB18918-2002?,but the removal of TN is not good.Short HRT leads to overload operation of nitrification BAF and the activity decrease of nitrifying bacteria in B AF.The activity and biomass of sludge in A²/O decrease and the removal efficiencies of nitrogen and phosphorus is destroyed with the decrease of HRT in system.In order to improve the utilization of carbon source,the utilization of carbon source is analyzed and evaluated under two step feeds?pre-anoxic zone and anoxic zone?according to the formula of material balance for COD,and the effects of step feed on denitrifying phosphorus and nitrate removal are studied simultaneously.The results show that when the ratio of step feed is 7:3,the utilization of carbon sources is up to 85.8%,and the material balance of COD is 92.4%.The effluent concentrations of COD,NH₄⁺-N,TN and TP are29.48,4.07,14.10 and 0.40 mg/L,and the removal rates are 82.1%,92.7%,75.5%and 91.2%,respectively.The ratio of denitrifying phosphorus accumulation organisms to phosphorus accumulation organisms?DPAOs/PAOs?is 98.8%.By optimizing step feed,the carbon source is utilized effectively,and the efficienc ies of nitrogen and phosphorus removal are improved simultaneously.Using low substrate and low C/N domestic sewage?The operation is on summer vacation?,the effect nitrate recycling ratio on removal of nitrogen and phosphorus is studied under the ratio of step feed is 7:3.The results show that when the average influent concentrations of COD,NH₄⁺-N,TN and TP are 146.29?50.02?53.68 and 3.23 mg/L,the effluent concentration of COD is bellow 50 mg/L,the effluent of NH₄⁺-N,TN,TP increase with the increase of nitrate recycling ratio.When the nitrate recycling ratio is 250%,the lowest effluent concentrations of NH₄⁺-N,TN,and TP are 2.71,11.36,and 0.31 mg/L,respectively and the DPAOs/PAOs is as maximum as 83.4%,which strengthens simultaneous denitrifying phosphorus and nitrate removal under low substrate operation.The results of this study have important guiding significance for the rapid start up,the improvement of carbon source utilization,and the enhanced removal of nitrogen and phosphorus with low C/N?low substrate?domestic sewage of the modification of the two sludge A²/O-BAF system.