Characteristics Of Low-intensity Ultrasound Enhancing Anaerobic Baffled Reactor For Treatment Of Low-strength Wastewater

In this paper,low-intensity ultrasound was used to enhance the anaerobic baffled reactor（ABR）for the treatment of low-strength wastewater aiming at the problems of poor sludge activity and low treatment efficiency in the anaerobic biological treatment of low-strength wastewater.Combined with operation characteristics of ABR in treatment of low-strength wastewater under the enhancement of low-intensity ultrasound,the characteristics of sludge in the compartments of ABR were thoroughly analyzed and the changes of microbial community in the compartments of ABR were systematically studied,in order to reveal the mechanism of the coupling effect between low-intensity ultrasound and the reactor.The main contents and results are as follows:In order to improve the treatment effect of low-strength wastewater by ABR,after the reactor was successfully started,low-intensity ultrasound（ultrasonic frequency 20 k Hz,ultrasonic density 0.1 W/m L,irradiation time 10 min,irradiation cycle 24 h,irradiation sludge proportion 10%）was used to irradiate the anaerobic sludge in each compartments.The results show that the removal effect of organic matter can be improved,and the operation was stable.Under the condition of high chemical oxygen demand（COD）removal rate obtained at start-up,when the average COD concentration of influent water was 609.12 mg/L and hydraulic retention time（HRT）was 8 h,the average COD concentration of effluent water of control group was 62.46 mg/L and the removal rate was 89.75%.The average effluent COD concentration in the ultrasound group was 30.54 mg/L,and the removal rate was 94.99%,which is 5.24%higher than the control group.The COD concentration of the effluent can reach the first-class A standard in the Pollutant Discharge Standard for Urban Sewage Treatment Plants（GB18918-2002）.During the stable operation of ABR,the characteristics of sludge in each compartment under the irradiation of low-intensity ultrasound were investigated.After periodic ultrasound,the total suspended solids（TSS）and volatile suspended solids（VSS）of each compartment of the ultrasound group were lower than the control group,but the VSS/TSS ratio was higher.The extracellular polymeric substances（EPS）increased in the ultrasound group,including the loosely bound EPS（LB-EPS）and tightly bound EPS（TB-EPS）.The protein（PN）content increased but the polysaccharide（PS）content decreased.The dehydrogenase activity（DHA）of each compartments in the ultrasound group was 26.43,23.43,21.87,19.55 mg TF/（g VSS·h）and that in the control group was 18.13,17.01,13.56,9.90 mg TF/（g VSS·h）,respectively.The content of coenzyme F₄₂₀ in each compartments of the ultrasound group was 0.82,0.73,0.69,0.65μmol/g VSS,while that in the control group was 0.65,0.53,0.52,0.49μmol/g VSS,respectively.Low-intensity ultrasonic irradiation of anaerobic sludge can greatly increase the activity of sludge dehydrogenase and the content of coenzyme F₄₂₀ in each compartment of the ultrasound group,and enhance the ability of sludge to degrade organic matter to produce methane.The methanogenesis first-order kinetic fitting X∞in each compartment of the ultrasound group was 106.00,129.73,55.71,41.89 m L/g VSS respectively,while that in the control group was 92.29,95.27,52.33,29.72 m L/g VSS respectively,which also indicated that the ultrasonic irradiation improved the methane production capacity of sludge.At the same time,the particle size of the sludge is reduced and the specific surface area is increased to facilitate mass transfer,however the types of functional groups on the surface of the sludge remain basically unchanged.Finally,the effects of low-intensity ultrasound on microbial community structure in the compartments of ABR were studied.According to the high-throughput sequencing technology analysis,low-intensity ultrasound reduced the richness and diversity of bacteria and archaea in each compartment,and long-term ultrasonic irradiation played a role in screening anaerobic sludge microorganisms.Low-intensity ultrasound changed the bacterial community structure and improved the acid production capacity of each compartments in the ultrasound group,which was mainly reflected in the substantial increase in the relative abundance of Proteobacteria,Bacteroidetes,Actinobacteria,and Acidobacteria at the phylum level.Ultrasonic irradiation changed the community structure of Archaea,and some methanogens became dominant microorganisms.At the class level,low-intensity ultrasound mainly promotes the enrichment of Methanomicrobia.And at the genus level,low-intensity ultrasound mainly increases the relative abundance of Methanothrix.In summary,low-intensity ultrasound can improve the treatment effect of ABR in treating low-strength wastewater,change some characteristics of anaerobic sludge in the compartments of ABR,and play a role in screening and enrichment of microorganisms.