Membrane Fouling Characteristics And Persulfate Backwash Performance Of Anammox MBR

Anaerobic ammonium oxidation（Anammox）can use nitrite as an electron acceptor under anoxic conditions to oxidize ammonia nitrogen to nitrogen directly,which has the advantages of economical environmental protection,energy saving and high efficiency,low sludge yield and no need for external carbon sources.However,anammox bacteria grow slowly and have low cell yield,which makes their enrichment difficult.Membrane bioreactor（MBR）,which can effectively prevent the loss of functional bacteria through the interception of membrane,is an ideal device for enriching anaerobic ammonia-oxidizing bacteria.However,MBR often faces the problem of membrane fouling,which affects the stable operation of the reactor.Microorganisms and their secretions are the main factors leading to membrane fouling.Therefore,clarifying the characteristics of anammox bacteria and their secretions is the premise of controlling membrane fouling.Chemical backwashing is a common method to control membrane fouling.At present,strong oxidizing reagents such as Na Cl O are widely used.Although the membrane fouling control effect is great,it has a negative impact on the microbial flora.Sulfate radical(SO₄^·-)produced by persulfate(S₂O₈^2-)activated through ultraviolet light,transition metal and other means has the characteristics of high oxidation potential,wide p H range,long half-life and little influence on functional flora.It is an ideal reagent for chemical backwashing to control membrane pollution.In view of this,this study firstly studied the correlation of the characteristics of anammox MBR membrane pollutants,bacterial flora biodiversity and key genes,which can contribute to reveal the intrinsic mechanism of anammox MBR membrane fouling.And then we explored the feasibility of controlling anammox MBR membrane pollution by Na₂S₂O₈chemical backwashing,and revealed its control mechanism.The main content and research results are as follows:（1）The membrane fouling characteristics of anammox MBR were revealed.Protein（PN）is the main component of extracellular polymer（EPS）and humic acid（HA）is the main component of dissolved microbial products（SMP）in anaerobic ammonium oxidation membrane pollutants,which play different roles in different stages of membrane fouling.When membrane filtration resistance is lower than 20×10¹²/m,SMP is the main component of membrane pollutants.Meanwhile,HA and SUVA（the ratio of absorbance at 254 nm to dissolved organic carbon concentration）are the main contributors to membrane filtration resistance.When membrane filtration resistance is between 20 and 35×10¹²/m,the contribution of EPS to membrane fouling gradually increases.It controlled the increase of membrane filtration resistance together with SMP.When the degree of membrane fouling is higher than 35×10¹²/m,EPS almost completely replaces SMP as the main component of membrane fouling,among which PN was the key factor for the continuous increase of membrane fouling.（2）)The relationship between the characteristics of membrane pollutants and the flora in anammox MBR was explored,and the key genes in membrane pollutants were determined.There are 10 kinds of bacteria closely related to SMP-HA,among which unclassified＿d＿Bacteria contains the most related genes.Among the genes closely related to SMP-HA,gly A and met H are the most closely related genes,which belong to genes of amino acid biosynthesis.Among the bacteria closely related to SMP-SUVA,Candidatus＿Uhrbacteria has the most genes.Meanwhile,bacteria closely related to SMP-HA and SMP-SUVA contain much QS and QQ genes,which control SMP-HA and SMP-SUVA together with amino acid biosynthesis genes and extracellular polysaccharide biosynthesis genes.In addition,only one kind of amino acid biosynthesis gene（thr C）in Cadidatus＿Jettenia is closely related to EPS-PN.（3）The mechanism of UV/Na₂S₂O₈chemical backwashing to control membrane fouling was elucidated.Taking pure water as the control group,the chemical backwashing properties of Na Cl O,UV/H₂O₂,HCl and Na OH were compared and studied.The results show that the relative cumulative operating time after SO₄^·-chemical backwashing is the longest,reaching 18 days;the membrane water production is larger,which is comparable to the pure water group and the HCl group and the filtration resistance recovery rate is slightly larger than that of the Na Cl O group,much greater than that of the other groups.The content of the washed out substances is second only to the Na Cl O group,and PN occupies the main part.The adhesion of foulants on the membrane surface after backwashing was similar to that of the Na Cl O group,and the roughness was small,but the bactericidal effect on the anammox sludge system was the weakest,and the proportion of normal cells（72.7%）was second only to the control group（78.8%）,much higher than the Na Cl O group（61.9%）and the H₂O₂group（57.9%）.In order to reveal its control mechanism,the molecular weight distribution and ultraviolet spectrum of membrane foulants were detected,and the results showed that SO₄^·-can convert substances above 10000Da into small molecular substances below 1000Da,and tend to crack the C=C of the aromatic ring converted to C=O,thereby reducing the aromaticity of membrane pollutants.27 figures,8 tables and 122 references.