Study On Anti-pollution Performance Of Ozonation Coupled MnO₂/PVDF Hybrid Membrane In MBR

Membrane bioreactor（MBR）has been widely used in the treatment and reuse of domestic sewage and industrial wastewater due to its advantages of high volume loading,excellent effluent quality,less area occupancy and low residual sludge.However,the problem of membrane fouling is an obstacle to the further development of MBR.In this study,the modified polyvinylidene fluoride（PVDF）membrane by nano manganese dioxide particles was applied to MBR,and then coupled with in-situ ozonation to study the anti-fouling performance and reveal its mechanism.In this study,α-crystalline nano-MnO₂ was prepared by aqueous precipitation method,and used as a hydrophilic additive for preparing MnO₂/PVDF hybrid membrane.In addition,MnO₂/PVDF hybrid membranes were prepared by using polyvinylpyrrolidone as porogen and N,N-Dimethylacetamide as solvent.The results showed that the prepared hybrid membranes had a rich microporous structure and high hydrophilicity,and the electronegativity of the membrane surfaces was reduced.The modified membrane had the largest permeability（351.69±8.10 L/m~2·h·bar）when the content of nano-MnO₂ was 0.75 wt.%.To further explore the anti-fouling performance,the modified PVDF membranes with nano-MnO₂ additions of 0wt.%,0.75 wt.%and 1.00 wt.%were put in the long-term MBR operation,which were named as R1,R2 and R3,respectively.Results showed that,the three reactors had similar removal effects on various pollutants.The removal rates of total organic carbon and 3-chlorophenol were very high.When the reactors were stable,the removal rates of total organic carbon and3-chlorophenol almost reached 95%and 100%,respectively.However,all the reactors didn’t show an excellent removal rate of ammonia nitrogen.The transmembrane pressure in reactors showed a typical two-stage rise.The fouling rate of the modified PVDF membrane with the addition of nano-MnO₂ of 1.00 wt.%was only 0.67 k Pa/d,and the cleaning cycle was 1.4times longer than that of the pristine PVDF membrane,showing an excellent anti-fouling performance.Through pre-ozonation experiments,the optimal coupling ozone concentration was determined to be 4.64 mg/L.In addition,it was found that the in-situ ozonation（<6.50 mg/L）effectively improved the filtration performance of the mixed liquid.When the ozone dosage increased to 13.33 mg/L,the activated sludge cells would rupture,leading to the increase of the dissolved organic matter content in the sludge mixture.The membrane was immersed in deionized water oxidized by ozone at a concentration of 4.64 mg/L for 6 hours,and the morphology of the membrane didn’t change significantly.Ozone had great influence on the electronegativity of the pristine membrane surface,and had no obvious influence on its hydrophilicity.However,the surface electronegativity of the modified membrane was basically unaffected by ozone and the hydrophilicity was significantly improved.In the different operation cycles of MBRs,ozone with a concentration of 4.64 mg/L was coupled with the MnO₂/PVDF hybrid membrane to further explore whether the membrane pollution could be alleviated.The results showed that the membrane cleaning cycle of R3 with ozone in the second operating cycle was 1.5 and 1.7 times longer than that of R1 and R2 in the same period.The membrane fouling of R1 was not significantly alleviated after ozone was added in the third operating cycle.Therefore,in-situ ozonation coupled with nano-MnO₂ membrane could effectively control membrane fouling.One possible reason was that manganese dioxide catalyzed ozonation could directly remove contaminants on the membrane surface in-situ.Secondly,the hydrophilicity of the membrane surface has been improved,reducing the enrichment rate of extracellular polymer on the membrane surface.The catalyzed ozonation also could directly degrade the protein-like extracellular polymer in the sludge mixture.In addition,through the analysis of the biological community structure of the reactors,it was found that catalytic ozonation increased the abundance of Pseudomonas and Hydrogenophaga in R3,which could effectively degrade protein.The change of the biological community structure enhanced the biodegradation of protein-like extracellular polymers.