Operating Performance And Membrane Fouling At Different COD/SO₄^2- Ratios In AnMBRs For Wastewater Treatment

Anaerobic biological treatment of sulfate wastewater has always been a research hotspot,and the influence of sulfate on the process of anaerobic microorganisms has not been understood clearly.Previous studies on anaerobic membrane bioreactors（AnMBRs）have mainly focused on the effect of sulfate on methanogenic performance,but less attention has been paid to the process or mechanism of membrane fouling under sulfate stress.The effect of sulfate on the properties of sludge mixture and the micro-particles composition of supernatant and its correlation with membrane fouling behavior are still unclear.Therefore,three identical AnMBRs were set up and operated stably under the same condition.Then R1 as the control group（no sulfate was added in the influent）,different sulfate concentrations were added into R2 and R3,respectively.As such,the reactor operation was then continued under the different COD/SO₄^2-ratios of>10,2.5 and 0.5,respectively.In this study,the effects of different COD/SO₄^2-ratios on the nutrient removal,gas production and membrane fouling in the AnMBR system,and the effects of influent sulfate on sludge properties,the generation and composition of sub-visible micro-particles（0.45-10μm）in the supernatant,microbial community structure and interaction,and their correlations between membrane fouling behavior were comprehensively investigated.The results were showed as follows:（1）When the COD/SO₄^2-was 2.5 and 0.5,the concentrations of COD,polysaccharides and proteins in the supernatant and effluent increased greatly,and correspondingly the removal rates of COD and proteins decreased,but the removal of polysaccharides was not affected;When the ratio of COD/SO₄^2-was 2.5,a part of the organic matter was used for sulfate reduction,and the volume ratio of H₂S increased（about 9000 ppm）,but the methanogens were not inhibited well,and the methane yield decreased slightly.When the ratio of COD/SO₄^2-was 0.5,the ratio of H₂S was as high as about 16000 ppm,the methanogens were severely inhibited,the methane production was significantly reduced,and the biogas could not be even produced at the later stage of operation.With the decreased of COD/SO₄^2-ratios,the properties of the sludge in the reactor changed greatly.The sludge concentration of R3 decreased,the average floc size decreased,the EPS of the sludge increased,and the proportion of dead cells increased.（2）After the addition of sulfate,the growth rate of the transmembrane pressure（TMP）in R2 and R3 increased distinctly,and the organic fouling resistance of the membrane increased,which was undoubtedly related to the increasing concentration of organic matter in the supernatant and the changing sludge properties;SEM and fluorescence microscopy images showed that a large number of filamentous bacteria were intertwined in the cake layer of R2 and R3,and the proportion of dead cells was higher,indicating that under the long-term operation at the lower COD/SO₄^2-ratios of2.5 and 0.5,the proliferation and death of filamentous bacteria would aggravate the membrane fouling of AnMBRs;through the dead-end filtration test of various components（sludge flocs,supernatant and SMP）of sludge mixture in different reactors,it was found that the sludge mixed solution of R3 showed the worst filterability at the same TSS concentration,followed by R2 and R1,which was consistent with the results of the increasing trend of TMP in the reactors;in addition,the membrane filtration behavior of the supernatant was almost the same as the sludge mixed solution,indicating that the supernatant had a significant contribution to the membrane filtration performance of the whole sludge mixture;the supernatant of different reactors was further taken and kept at the same COD concentration for dead-end filtration.It was found that membrane fouling of R3 was the most serious,the filtration rate was the slowest and the membrane resistance was the highest,which further confirmed that the supernatants played an important role in membrane fouling.（3）With the decreased of COD/SO₄^2-ratios,the particles size distribution of supernatant in AnMBRs had a big change,and the proportion of small-size particles in the supernatant greatly increased.Both the size-fractionation and chemical quantification analysis showed that the concentration of fine particles（0.45-10μm）in the supernatant of R3 and R2 was significantly higher than R1;the proportion of SMP,and fine particles in the range of 0.45-5μm in the cake layer was higher than that in the supernatant,indicating that these micro-particles in the supernatant had a higher accumulation potential on the membrane.On the other hand,the cracking or death of sludge cells in the system under the stress of high hydrogen sulfide conditions could be another main reason for the increasing SMP and fine particles,thus severer fouling.（4）High-throughput sequencing analysis found that when the COD/SO₄^2-was 2.5,the addition of sulfate increased the microbial richness in the sludge,but when the COD/SO₄^2-was 0.5,the excessive sulfate concentration would decrease the microbial richness.Proteobacteria was affected greatly by the ratios of COD/SO₄^2-in sludge and supernatant.With the addition of sulfate,the relative abundance of sulfate-reducing bacteria（Desulfovibrionaceae and Desulfobacteraceae）in the phylum of Proteobacteria increased,and Firmicutes and Bacteroidetes also increased.The methanogens（Methanosaetaceae and Methanoregulaceae）in the phylum of Euryarchaeota were inhibited by high concentrations of hydrogen sulfide,which further explained the decreases of methane production.In addition,with the decreasing COD/SO₄^2-ratio,the filamentous bacteria（SJA-15 and Anaerolineaceae）of Chloroflexi were greatly poisoned or inhibited in the supernatant,and their live/dead cell ratio was greatly reduced.Similarly,the filamentous bacteria of Chloroflexi in the cake layer still had a relatively high proportion,although it was not the dominant phylum;while the abundance of Proteobacteria and Bacteroidetes increased,and they accumulated in the cake layer,indicated their high fouling potential.（5）The correlation analysis of microbial community and membrane fouling confirmed that,Desulfovibrionaceae and Syntrophobacteraceae of Proteobacteria,D8A-2 and Ruminococcus of Firmicutes and Lentimicrobiaceae of Bacteroidetes in sludge and supernatant were all positively related to membrane fouling rates and their relative abundances increased with the addition of sulfate in the system.In addition,SJA-15 and Aminicenantales were also positively correlated with the fouling resistance of the cake layer,and their abundances was much higher in the cake layer.Microbial source tracking analysis of the cake layer further confirmed the important membrane fouling role of the microbial communities of the supernatant.Overall,this study identified and characterized the changes of sludge properties and the composition and characteristics of fine particles in the supernatant of AnMBRs under sulfate stresses,and constructed the correlations between microbial communities in the sludge and supernatant and membrane fouling behaviors,revealed the important membrane fouling behaviors of sulfate-reducing bacteria and glycolytic acid-producing bacteria in the supernatant,which all could provide theoretical basis for further regulation of membrane fouling in AnMBRs.