Study On Treatment Efficiency And Mechanism Of Domestic Sewage In Southern China With UASB

It is urgent to improve the efficiency of nitrogen and phosphorus removal and reduce the energy consumption of wastewater treatment.In recent years,as a new autotrophic biological nitrogen removal technology,the partial nitrification-anammox process has become a research hotspot.However,excessive carbon sources flowing into the system will have a negative impact on autotrophic nitrogen removal.Therefore,when this treatment process is adopted,other processes need to be used for decarbonization before.Upflow anaerobic sludge bed(UASB)can be used as a carbon removal technology in the front section,which can effectively remove organic matter and generate methane.However,anaerobic digestion is often used for the treatment of high concentration organic wastewater,and rarely used in the treatment of low concentration domestic sewage.In this study,UASB was successfully started with simulated domestic sewage at 30℃and anaerobic granular sludge was cultivated.Subsequently,research was carried out on the process and mechanism of treating simulated domestic sewage at ambient temperature and the efficiency of treating actual domestic sewage in southern China.The influence of HRT and temperature on UASB treatment effect,as well as the change rule of methane production and sludge were investigated.The reaction kinetics of UASB in treating domestic sewage was analyzed.The process mechanism of UASB treatment of domestic sewage was clarified by analyzing the morphological structure of granular sludge and microbial community,combined with the pollutant removal rate and the transformation law of pollutants along the reactor.The operation strategy of UASB in treating domestic sewage in southern China was proposed.The following main conclusions are drawn:The UASB inoculated with anaerobic digestion sludge was started with the simulated domestic sewage with COD of 300 mg/L.The HRT=48 h was set at the beginning of the start-up,and HRT was gradually shortened when the effluent COD removal rate was stabilized at>80%.The start-up was completed at 120 d.At this time,the effluent COD was below 40 mg/L at HRT=3 h and the methane production rate was 0.121 L/g COD.The mass proportion of sludge with particle size>0.25 mm in UASB accounted for about 49%.The UASB that had been started up treated simulated domestic sewage at ambient temperature(17.3～27.9℃)and there were no obvious fluctuations in COD removal rate,p H and VFA within this temperature range.The average COD removal rate was 90.15%and91.82%at HRT=5and 3 h,respectively,but when HRT decreased to 2 h,the average COD removal rate decreased to 62.21%and the sludge flowed out of the reactor.The optimum HRT was 3 h at which the average effluent COD was 24.54 mg/L with a removal rate of 91.82%and the average methane production rate was 0.076 L/g COD.UASB had a certain impact resistance capacity,and COD load and p H shock in a short time would not cause too much damage to the reactor.Granulation of anaerobic sludge continued during the operation at ambient temperature.In the end,the mass proportion of sludge with particle size greater than0.25 mm accounted for about 74.0%.The degradation of COD in UASB was in accordance with the Grau second-order kinetic model.The main limiting factors were temperature and HRT.The mechanism of UASB in treating domestic sewage includes:hydrolytic fermentative bacteria,hydrogen-producing acetogen and methanogen cooperate to complete the hydrolysis,acidification and methanation of organic pollutants;through the winding of filamentous bacteria,a stable granular sludge structure is formed to reduce the mass transfer restriction of microbial population;the mixing and lifting effect of water and gas makes the anaerobic granular sludge with high concentration show gradient distribution along the height,which enhances the mixing and mass transfer.When UASB treated actual domestic sewage in southern China at ambient temperature(23.2～29.0℃),the influent COD,TN,TP and SS were 127.46,26.67,1.01and 90.75 mg/L on average,and the effluent COD and SS were 53.33 and 43.33 mg/L,but the removal of nitrogen and phosphorus was poor,with an average removal rate of only 6.44%and 9.90%,respectively.When HRT was 3 h,the average COD of effluent was 40.83 mg/L,and the methane production rate was 0.065 L/g COD.The amount of small particle sludge(particle size less than 0.5 mm)and large particle sludge(particle size greater than 2.0 mm)gradually decreased while the amount of particle sludge in the middle particle size(particle size between 0.5 and 2.0 mm)gradually increased when treating the actual sewage.The stability of UASB in treating actual domestic sewage was good.Therefore,in actual daily operation,it is not necessary to add chemical agent to adjust the p H,and it is suitable to operate at a short HRT(3-8 h)while maintaining the influent COD load above 0.35 kg COD/(m~3·d).This study provides a technical and theoretical basis for the research and application of UASB in treating domestic sewage in southern China.