Effectiveness And Mechanism Of Combining Ultrasound And Microbial Electrolytic Cells To Degrade Excess Sludge

Rapid urbanization and the growth of waste-water treatment rates have resulted in the increased production of excess sludge in wastewater treatment plants.Excess sludge is ag-glomerated in a polymeric network formed by microbial cells and microbial extracellular polymeric substances.Because excess sludge typically contains a large number of organics,it creates significantly environmental risks and high disposal costs.Thus,it is significant to de-velop a highly effective excess sludge treatment technology for further engineering applica-tions.Due to ultrasonic disintegration(UD)is a sludge pretreatment method of high energy consumption,this study proposed a combined anaerobic hydrolysis(AH)-UD process for ex-cess sludge disintegration.The optimized solution involved AH for 3 days,followed by UD for 10 min.At this point,soluble chemical oxygen demand(SCOD)and protein concentra-tions reached to 3455.9 mg/L and 1713.3 mg/L,respectively.AH had an advantage in modi-fying excess sludge characteristics,reducing the difficulty of excess sludge disintegration,producing ethanol and organic acids et al.,enhancing the effect of ultrasonic cavitation,in-creasing excess sludge disintegration rate?and reducing UD energy consumption.Different density ultrasound could be occured in the same ultrasonic system because of the heteroge-neous density distribution of cavitation bubbles in ultrasonic reactors.Thus,the applyication of intermittent UD in the anaerobic digestion(AD)process was proposed in this study,to ad-equately use ultrasonic efficiency and improve the biochemical properties of excess sludge.The optimized application of intermittent UD was 3 min per 2 days during AD for 7 d.At the optimized operation levels,SCOD,protein and volatile solid(VS)concentration reached to 3482.5 mg/L,2316.7 mg/L and 2690 mg/L(VS removal was 41.6%),respectively.Thus,the intermittent UD technology in the AD process had a significant impact in increasing the ex-cess sludge AD efficiency and reducing energy consumption of UD in engineering applica?tions.This work has great significance for excess sludge disintegration by UD in further en-gineering applications.This study put forward to separate supernatant from treated excess sludge,and took the supernatant as high-concentration organic wastewater.The study also applied photosynthetic bacteria(PSB)and PSB-microbial electrolysis cells(MEC)to degrade supernatant from treated excess sludge to improve the efficiency of reduction of excess sludge.Under illumina-tion with anaerobic conditions,PSB could degrade the organic substance in supernatant,and the influence of the PSB proportion was very significant.When the PSB proportion of 0%,10%,20%and 30%in the MEC,SCOD removal rate in supernatant from treated excess sludge was 62.8%,72.2%?85.4%and 88.6%mg/L,respectively;protein in supernatant from treated excess sludge was 43.5%,49.2%?61.1%and 65.6%mg/L,respectively,after AD for 7d.The synergistic interaction between PSB and MEC was also very important.The low transparency of supernatant from treated excess sludge,low enzymes activity under higher temperature and relative stable MEC degradation did not lead to corresponding increase of degradation efficiency,instead of serious energy waste.Based on the above research,this study proposed an excess sludge disintegration and degradation process,which used UD to disintegrate excess sludge,applied an up-flow anaer-obic sludge blanket(UASB)reactor to degrade the disintegrated sludge,and used a MEC in-cluding electrode brush instead of a three-phase UASB separator to further degrade disinte-grated sludge.After 8 days of AD in the UD-UASB-MEC system,SCOD,protein and VS concentration in the UD reactor reached to 476.3 mg/L,345.9 mg/L and 1280 mg/L,respec-tively.Meanwhile,SCOD,protein and VS concentration in the effluent reached to 427.5 mg/L,309.1 mg/L and 320-380 mg/L,respectively.UD Time,temperature and voltage were key parameters in the UD-UASB-MEC system.UD time influenced sludge disintegration and three-phase separation.Influence of temperature on UASB unit and voltage on MEC unit were significant as well.Based on analysis pyrosequencing technology,the dominant species in the raw excess sludge were Proteobacteria,Bacteroidetes,and Firmicutes,taking propor-tions of 68.44%,16.14%,and 5.97%of the sample,respectively.After 8 days of operation,the dominant species in the sludge were Proteobacteria,Firmicutes,and Bacteroidetes,taking proportions of 31.47%,22.86%,and 20.18%of total bacteria,respectively.Meanwhile,high biodiversity of UD-UASB-MEC system could increase the ecological stability.In brief,UD-UASB-MEC system could potentially minimize the energy consumption during ultra-sound disintegration and simultaneously accelerate the excess sludge anaerobic degradation process,which could be a high-efficiency excess sludge degradation technology for further engineering applications.