Main-stream Nutrient Removal Enhanced By Side-stream P-recovery

The severe water pollution,such as black and smelly water and eutrophication,lead to the increasingly stringent wastewater treatment discharge standards.As a result,the existing wastewater treatment plants?WWTPs?have to be upgraded and new technologies are urgently needed.In China,WWTPs rely on the addition of carbon source and chemical agents to achieve efficient nitrogen and phosphorus removal due to the low carbon content of municipal sewage.In this study,a new process of bio-P removal complemented by chemical phosphorus removal is adopted.This combination makes full use of the characteristics of high PO₄^3--P content in anaerobic supernatant,suitable for chemical phosphorus removal in addition to bio-P removal.With dosage of a small amount of chemical agents yet without organic carbon,P-recovery and bio-P removal can be achieved simultaneously.A lab-scale 5-stage modified UCT process was built,and an anaerobic supernatant side-stream P-recovery unit was designed.In this study,the mathematical model was also used to assist the laboratory experiment,the simulation results were used to guide the experimental operation while the experimental results were fed back to correct the model parameters.After stable operation,the lab-scale reactor had excellent N and P removal performance(the effluent TN<10 mg N/L,TP<0.5 mg P/L,PO₄^3--P<0.1 mg P/L).The simulation results and the experimental results also fitted well.The optimum operating conditions of the lab-scale reactor were:Q_A=1.5Q_IN,Q_B=2Q_IN,Q_C=0.The simulation and experimental results showed that when the COD in the synthetic wastewater was less than 220 mg/L?COD/N=4.4,COD/P=44?,the effluent TP and PO₄^3--P were higher than 0.5 mg P/L and 0.1 mg P/L respectively.It indicated that the carbon source for bio-P removal was insufficient.At this time,the side-stream P-recovery of anaerobic supernatant was enabled with keeping the influent quality unchanged.The simulation and experimental results showed that when the Q_SN/Q_IN increased from 10%to 30%,the COD/P was indirectly increased to 77 when the supernatant after P-recovery?accounting for 58%of the influent TP load?was refluxed to the main-stream process,so that the effluent TN and TP could meet the standards again.As a result,P-recovery was realized,and the addition carbon source was also omitted.The oxidation and reduction potential?ORP?in anaerobic tank has an obvious impact on P release and bio-P removal.The experiment results showed that the anaerobic tank ORP should be maintained below-200 mV in order to obtain a better bio-P removal performance.Anaerobic tank ORP also had a certain impact on the composition of PAOs.16sRNA detections showed that when the anaerobic tank ORP=-100 mV,PAOs were almost comprised of Accumulibacter,while when the anaerobic tank ORP=-300 mV,Tetrasphaera was found yet in a very low level?0.0013%?,which indicated that anaerobic tank ORP control was very important.