Experimental Study On Domestic Wastewater Treatment By Downflow Sludge Recirculation Anaerobic Process

Upflow anaerobic activated sludge bed(UASB)was widely used in domestic wastewater treatment.An important advantage of UASB was the formation of granular sludge with excellent settling characteristic,high biomass concentration.While sludge granulation is a complex process,it is easily affected by numerous factors.The start-up of UASB reactor is difficult in the case of low-strength wastewater and low temperature condition.For conventional upflow anaerobic reactors(such as UASB reactor),there are shortcomings such as long start-up time,poor wastewater-sludge mass transfer,slow formation of granular sludge and low methane recovery efficiency in low-strength wastewater treatment.In this study,a downflow sludge circulation anaerobic reactor was constructed,and its operational effects and influencing factors were studied.In the experiment,the effects of downflow sludge circulation on mass transfer,granular sludge formation and methane recovery were analyzed to reveal the mechanism of sludge granulation and methane production and transport.Based on the theory of hydraulics and fluid mechanics,ANSYS Fluent flow simulation software was used to analyze the changes in flow pattern and sludge distribution in the reactor.In addition,high throughput sequencing analysis was used to determine the effect of downflow sludge circulation on microbial communities.In the lab-scale test,the results of the study showed that the application of downflow sludge circulation made the sludge-wastewater reverse flow,greatly improved the wastewater-sludge mixture in the reactor,and enhanced the wastewater-biomass mass transfer.Enhanced wastewater-biomass mass transfer and high shear force accelerated the formation of anaerobic granular sludge under low temperature and low load condition.After 58 days of cultivation,sludge granulation was achieved.The sedimentation performance of granular sludge was good,the settling velocity(SV)reached 48 m/h,and the specific methanogenic activity(SMA)reached 0.464 kg CH4 COD/kg-VSS-d.The reactor operated stably and the dissolved chemical oxygen demand(SCOD)removal efficiency exceeded 90%.The introduction of downflow sludge circulation significantly improved the liquid-gas mass transfer,promoted the conversion of dissolved methane from the wastewater into gaseous methane,and reduced the concentration of dissolved methane in the wastewater and the degree of supersaturation of methane.Methane recovery efficiency reached 79%.SEM analysis showed that the surface morphology and microstructure of the sludge changed significantly during start-up period.High throughput sequencing analysis showed a clear increase in microbial diversity at the end of start-up period.At phylum and genus level,the microbial community structure was significantly different,and the abundance of various microorganisms including Proteobacteria,Bacteroidetes,Tolumonas,and Desulfomicrobium changed significantly.Flow simulation analysis showed that the sludge was in suspension and evenly distributed in the reaction zone under the action of downflow sludge circulation.In the pilot test,the results of the study showed that the reactor had a high resistance to impact load and that the start-up was completed after 63 days of operation.The sedimentation performance of granular sludge in the reactor was good,and the SCOD removal efficiency reached 78.8%.However,methane production was lower and methane supersaturation(1.17-1.21)was larger under low influent OLR and low temperature,methane loss was 40.17-47.85%,which affected the recovery and utilization of methane.The results of the productive test showed that even in the low temperature environment(5-20 ?)in winter,the average SCOD removal efficiency exceeded 70%(up to 85.5%),showing good treatment effect on municipal wastewater.