Study On Hydrolytic Acidification Of Refinery Wastewater And Microbial Characteristics

Refinery wastewater is large in volume,complex in composition and contains a large amount of organic matter that is difficult to biodegrade.Hydrolytic acidification can effectively improve the biochemical properties of wastewater,and was widely used in the treatment of refinery wastewater.However,the understanding of hydrolytic acidification performance and microbial characteristics of refinery wastewater is still unclear.In this study,hydrolytic acidification of refinery wastewater was investigated under different hydraulic retention time（HRT）and temperature.The performance of acid and gas production and the changes of organic composition were analyzed.The analysis was carried out by using macrogenomic studies to characterize the structure and function of the hydrolytic acidification microbial community,the core microorganisms and the metabolic pathways of typical pollutants.（1）The COD removal rate of wastewater increased with the extension of HRT（8～16 h）at 25℃,but the biodegradability showed a trend of first increasing and then decreasing.At HRT of 12 h,the effluent BOD₅/COD reached the maximum value of0.39 and the highest VFA yield was obtained with an average of 12.52 mg g^-1COD.CHO and CHOS compounds were the main polar organic compounds in refinery wastewater,among which O2,O3S1 and O4S1 compounds were of high content.The type of organic matter increased and molecular weight reduced by hydrolytic acidification.Long HRT was benefit for the conversion from refractory components of O3S1 and O4S1 to organic acids.Hydrolytic acidification increased the diversity and abundance of the microbial community.KEGG functional annotation showed that the abundance of metabolism-related genes was highest.Mycolicibacterium and Desulforegula,which can utilize refractory organic matter and long-chain alkanes,respectively,had higher abundance under optimal HRT（12 h）condition.Xenobiotics biodegradation and metabolism including ko00362（benzoate degradation）,ko00633（nitrotoluene degradation）,ko00626（naphthalene degradation）,ko00984（steroid degradation）,ko00642（ethylbenzene degradation）and ko00624（PAH degradation）pathways promoted the degradation of petroleum contaminants.Meanwhile,the high abundance of oxidoreductases might be the key role enzymes in the bioconversion of pollutants during the hydrolysis acidification of refinery wastewater.（2）Under HRT of 12 h,hydrolytic acidification at 35°C was more favorable for COD removal,biodegradability improved to 0.53 of BOD₅/COD,and the average VFA yield was 1.64 mg g^-1COD,which was higher than that at low temperature（15°C）.The molecular weights of organic matter in the effluent were all reduced,and the organic species increased under mesophilic condition.Mesophilic was more favorable for microbial growth,with higher abundance of organic degrading bacteria of unclassified＿p＿＿Chloroflexi and hydrolytic enzymes secretion bacteria of unclassified＿o＿＿Bacteroidales.Ko00643（styrene degradation）,ko00633（nitrotoluene degradation）,ko00622（xylene degradation）,ko00364（fluorobenzoate degradation）,ko00642（ethylbenzene degradation）and ko00365（furfural degradation）promoted the degradation of aromatic and nitrogenous pollutants,thereby improved the biodegradability of effluent.Meanwhile,the higher abundance of hydrolytic enzymes facilitated the hydrolysis of pollutants and promoted the hydrolytic acidification at mesophilic condition.（3）The core microorganisms of hydrolytic acidification are Pseudomonas,unclassified＿p＿＿Chloroflexi,unclassified＿c＿＿Betaproteobacteria,Desulfovibrio,unclassified＿c＿＿Deltaproteobacteria,unclassified＿p＿＿Proteobacteria,unclassified＿p＿＿Acidobacteria,unclassified＿p＿＿Planctomycetes,Mycolicibacterium,Hyphomicrobium,unclassified＿d＿＿Bacteria,unclassified＿o＿＿Bacteroidales,unclassified＿c＿＿Alphaproteobacteria,and Desulforegula.There was a correlation between environmental factors and core microorganisms.Operating conditions（HRT,Temperature and OLR）were the main factors affecting microbial community structure.Some genus were significantly correlated with environmental factors.Correlation network analysis showed that Proteobacteria were the most important bacteria influencing the microbial community structure.The complete polycyclic aromatic hydrocarbon degradation pathway（ko00624）and benzoate degradation pathway（ko00362）were detected,which are the key pathways of VFA-production for hydrolysis acidification of typical refractory pollutants.