High Phosphate Removal Associated Phophate Recovery Of BNR System Subjected To Micro-Dissolved Oxygen Condition

Phosphorus is the key factor causing eutrophication in waterbody.As a result,the national standards of phosphorus concentration in the effluent from wastewater treatment plant enacted by each country are more stringent.Surveys of real municipal wastewater treatment showed that the aeration cost accounts for 50% to 80% of the total operating costs and the aeration in most wastewater treatment plants are excessive.The excessive aeration could not only increase aeration cost,but also cause the failure of the system.If the plant could be operated steadly under low DO concentration,the energy consumption of wastewater treatment plant will be reduced greatly.Meanwhile,as the migration of phosphate resources is one-way from the land to the ocean,phosphate will be exhausted without recovery.Therefore,its recovery and recycle has been widely concerned by the global society.The side-stream phosphate recovery is a techinique which extracts anaerobic phosphorus supernate by different side stream ratios to deprive phosphorus and other pollutants in the BNR system to take chemical reaction,then phosphate crystallizes,precipitates in a form of pure chemical sludge and is discharged from the system.During this process,COD/P ratio of the system increases,which could relieve the problem of deficient carbon source?COD?in the actual project performing biological phosphorus removal in wastewater.As a result,the combination of BNR system subjected to micro dissolved oxygen condition with phosphate recovery through extracting side stream could significantly improve the economic and environmental benefits.In this study,two sequencing batch reactors?SBRs?were operated with alternating anaerobic/oxic?An/O?for 127 days to investigate phosphorus removal performance of the BNR under different dissolved oxygen gradientin the first period.The results showed that the control reactor?R1?in which DO was not controlled and measured as high as 6 mg·L^-1during aerobic stage showed a stable phosphorus removal performance in the former 65 days during the operational period and the total phosphorus in effluent was less than 0.5 mg·L^-1.However,the phosphorus removal performance completely broke up until 97 day due to long-term excessive aeration.Throughout the operational period?0¹27 d?,only 39.4% of effluent TP met the national wastewater discharge standard A.For the experimental reactor?R2?in which DO was controlled under the level of 2,1,0.5,0.2,0.1 mg·L^-1 during aerobic stage,respectively showed a slight fluctuation of phosphorus removal performance at the beginning of each stage but increased to a stable state rapidly afterwards.Throughout the operational period,94.6% of effluent TP in R2 reactor could meet the national wastewater discharge standard A.The process performance was far better than that of R1.It was also found that the specific phosphate uptake rate was greatly influenced bythe low DO.And,it was found to be nearly the maximum under DO of 2 mg·L^-1.In addition,limited filamentous bulking under super-low DO level(0.1 mg·L^-1)was observed.Nevertheless,total effluent during this stage could meet the national standard,implying the phosphorus removal in micro-dissolved oxygen BNR system for saving energy was feasible.In the second period of the experiment,sequencing batch reactors?SBRs?were operated with alternating anaerobic/oxic?An/O?to investigate nitrogen and phosphorus removal performance of BNR subjected to different DO levels and phosphate recovery performance with side stream ratio of 0,1/4,1/3,1/2 anaerobic phosphorus supernate.Phosphate uptake properties of the sludge using different electron acceptors were also discussed.The results showed that the removal efficiency of COD was stable and is above 83.1%when DO was not controlled.NH₄⁺-N removal performance of the system was reduced in the three operational runs with the removal efficiency higher than 93.6% during all experimental stages and the effluent NH₄⁺-N concentration increased to 2.6 mg·L^-1.Meanwhile,the biological phosphorus rate was 95.9%,91.7% and 89.1%,respectively during the three runs.The phosphorus removal rate was stable and efficient in the first two runs.However,it began to fluctuate with the side stream ratio of 1/2 of the third-run and the phosphorus removal rate was the lowest with 54.2%.Besides,it was found that long-time extraction was beneficial for sludge reduction and had little impact on sludge settling performance.Moreover,long-time extraction resulted in PAOs losing phosphate releasing property gradually and the advantage of phosphate recovery through extracting phosphorus supernate consequently.Under the condition of DO=1 mg·L^-1,the removal efficiency of NH₄⁺-N was found to be relatively stable during all experimental stages.But effluent COD of the system during later period remained as high as 81.3 mg·L^-1.In the absence of side stream,the average phosphorus removal efficiency was 89.4% and increased to 98.5% and 99% when side stream ratio was1/4 and 1/3,respectively.However,phosphorus removal performance began to fluctuate with side stream ratio of 1/2 and the lowest phosphorus removal rate was 65.4%.During each experimental stage,phosphorus recovery rate improved with the increase of side stream ratio.It was also observed that TN removal rate decreased to 50.9% dramatically when side stream ratio increased to 1/2.Consequently,1/3 side stream ratio was proved to be the optimal condition in the whole experimental stage.Under the condition of DO=0.5 mg·L^-1,the removal efficiency of COD and NH₄⁺-N was found to be relatively stable during all experimental stages.When side stream ratio was 0,1/4,1/3,1/2,the average phosphorus removal efficiency was 92.1%,90.5%,95.2%,95.3%,suggesting that extracting side stream had little impact on the phosphorus removal efficiency in the micro-dissolved oxygen BNR system.In addition,the sludge multiplication decreased with the increase of side stream ratio under the long-term micro-dissolved oxygen BNR system.But the impact of sludgesedimentation is little.Under the condition of DO=0.1 mg·L^-1,the impact of COD removal efficiency was very little with the increase of side stream ratio.When side stream ratio was 0,1/4,1/3,NH₄⁺-N removal efficiency was stable but began to fluctuate with side stream ratio of 1/2.The lowest removal rate was 74.5%.In the absence of side stream,the average phosphorus removal efficiency was 87.4% and increased to 91.5% when side stream ratio was1/4.But the phosphorus removal performance began to fluctuate with side stream ratio of 1/3and 1/2.As a result,The BNR system under extra-low dissolved oxygen condition combined with phosphorus recovery through extracting side stream was feasible.By contrasting all experimental stages,anaerobic phosphorus release was found to decrease with the increase of side stream ratio.And phosphate release and uptake amount with DO=0.1 mg·L^-1was the lowest under all DO concentration gradients.Additionally,25mg·L^-1 NO2--N and NO₃^--N was added to the system as the electron acceptor respectively,to investigate the phosphorus uptake properties of the sludge when the reactor operation was stable during each experimental stage.It was found that reactor could use nitrate as electron acceptor for phosphorus uptake,but the phosphorus uptake efficiency reduced with the decrease of dissolved oxygen conditions adopted in the operation stage.The reactor could hardly use nitrite as electron acceptor for phosphate uptake with low efficiency,and secondary phosphorus release was observed after nitrite addition.