Novel Methods For Membrane Fouling Control Based On Photocatalytic Technology And Their Application In Membrane Bioreactors

Membrane bioreactors（MBRs）,which combine activated sludge processes and membrane separation technology,are not only paid much attention in conventional biological wastewater treatment and aerobic nitrogen-removal processes,but also increasingly applied in emerging anaerobic nitrogen-removal processes,such as anaerobic ammonium oxidation（Anammox）.However,membrane fouling is a major bottleneck to the application of MBRs for wastewater treatment processes.Among numerous antifouling strategies,membrane modification is considered to be one of the most effective and feasible approaches.However,some conventional membrane modification methods with modifying agents of bactericides or hydrophobic chemicals inevitably result in secondary pollution,environmental risk,and extra treatment costs.According to the deficiencies of the conventional modified membranes,novel environmentally-friendly and cost-effective antifouling methods were developed and applied in the long-term operation of Anammox MBRs in this study based on photocatalytic technology.The membrane antifouling effect was systematically evaluated and the potential antifouling mechanisms were revealed simultaneously.Firstly,visible-light-driven photocatalysts of Cd S/MIL-101 and Bi₂Mo O₆/Cu S were fabricated,and two photocatalytic membranes modified with Cd S/MIL-101 and Bi₂Mo O₆/Cu S were prepared by blending modification.By characterization and comparison of the modified membranes with different addition content of photocatalysts,the photocatalytic membranes with the optimum pore size,porosity,hydrophilicity and water permeability were determined.Under visible light irradiation,the inactivation rates of the photocatalytic membranes,represented by the 1 wt%Cd S/MIL-101 modified membrane C2,against both Gram-negative and Gram-positive bacteria,represented by the model bacteria（Escherichia coli and Staphylococcus aureus）,reached 93%and 89%,respectively.In the model foulants filtration process,C2 showed better antifouling capabilities,lower flux declining rates,and higher foulants rejection rates compared with the pristine membrane.The antibiofouling mechanisms of the C2 membrane were that,the heterojunction formed between Cd S and MIL-101 effectively enhanced the visible-light photocatalytic capability of Cd S/MIL-101,promoted the generation of reactive species（RSs,such as OH,e^–and h⁺）on the modified membranes under visible light irradiation,and realized the bacteria inactivation and organic foulants degradation on the membrane surface.The two abovementioned photocatalytic modified membranes were applied in the operation of Anammox MBRs under in situ irradiation of visible-light LED lights for membrane biofouling control.During long-term operation,the average fouling cycle of the 1 wt%Cd S/MIL-101 modified membrane C2 extended to 212.5%of the pristine membrane C0;the average fouling cycle of the 1 wt%Bi₂Mo O₆/Cu S modified membrane B2 extended to 228.5%of the pristine membrane B0.In the antifouling process,the total nitrogen removal efficiencies of the Anammox MBRs loaded with photocatalytic membranes（C2 and B2）and with pristine membranes（C0 and B0）were comparable and maintained at 84%–90%.The energy against the membrane fouling resistance was saved about 18.4%by filtering with B2.The generated·OH,h⁺and e^-on photocatalytic membranes were responsible for the alleviated membrane fouling.The organic foulants（especially extracellular polymeric substances,EPS）were degraded,and the bacteria were inactivated effectively by the photocatalytic reactions of the Cd S/MIL-101 on C2.Compared with B0,the RSs generated on B2 brought about47.7%lower organic foulant by decomposing protein,polysaccharide and lipid;and contributed to 76.1%lower total bacteria,51.0%higher necrotic cells,and 29.1%higher esterase-inactivated cells.For addressing the problem of cost increase in the membrane separation technology caused by membrane replacement of above-mentioned photocatalytic modified membranes after a working lifetime,the antifouling method combining photocatalytic optical fibers（POFs）with membrane separation was further developed to avoid the direct modification of membrane material with photocatalysts.To prepare aerobically and anaerobically applicable POFs,novel Zr-MOFs/r GO/Ag₃PO₄ Z-scheme heterostructure photocatalysts were constructed and coated on the surface of optical fibers.By evaluation of the antibiofouling capability of the POFs during membrane-filtration of bacteria and foulants under visible-light irradiation,the disinfection efficiencies of the POFs against Escherichia coli and Staphylococcus aureus reached 95.7%and 92.4%,respectively,under aerobic condition;and reached90.3%and 85.5%,respectively,under anaerobic condition.For the inactivated bacteria,cells membrane and membrane-associated functions were destroyed,accompanied by antioxidant enzyme decomposition,loss of cell respiration and ATP-synthesis capacity,and leakage and oxidation of protein,lipid,potassium,DNA and RNA.During membrane-filtration of foulants,the POFs significantly alleviated the membrane-flux decline by foulants disintegration.By qualitative and quantitative detection and quenching tests of RSs,aerobically generated·O₂^-and·OH and anaerobically generated·OH from the POFs played dominant roles in the antibiofouling process.By anchoring the POFs on the membrane module and loading in the Anammox MBR system,the membrane antifouling effect and mechanisms of the POFs were investigated in long-term operation.After 202 days operation,the average membrane-fouling-cycle of the M_P reactor,which was loaded with the POFs,extended by 137%with 18.1%electricity consumption saved,compared with the M_C reactor without the POFs loading.Moreover,the total nitrogen removal efficiencies（85.3%–90.4%）of the two MBRs were comparable.By adding RSs scavengers in the flux-recovery tests,the·OH generated from the POFs was proved to be the key RSs for membrane fouling control.The amounts of the foulant and total bacteria on the PM membrane surface,which was loaded with the POFs,were significantly lower than that on the control membrane CM.Aromatic nucleus and conjugated bonds in each component of the EPS were destroyed,resulting in the decomposition of macromolecular organics into micromolecular organics,and further photocatalytic degradation.The photocatalytic oxidative effect of the POFs significantly inactivated the bacteria on the PM.The cytomembrane was firstly destroyed,resulting in the inflammation,the inhibition of metabolic activity of cells,and the cellular apoptosis.The cells membrane was then destroyed,leading to the loss of cellular integrity,and the leakage of cellular content.The RSs entered into the bacteria also destroyed the cellular antioxidant system,and brought about irreversible oxidative damage on the cellular content（including genetic materials）and organelles.This study combined the theory of membrane fouling control with photocatalytic technology,and attamptted to realize bacteria inactivation and foulant degradation on membrane surface simultaneously with renewable light energy.Novel methods for membrane biofouling control were developed,and obtained excellent antibiofouling effect in the membrane filtration system represented by the Anammox MBR,while the antifouling mechanisms were systematically revealed.The achievements will provide effective antifouling methods for the application of membrane separation technology in the wastewater treatment,promote the theory and application innovation of membrane-fouling-control technologies,and estabilish the foundation for further development of membrane separation technology in the field of wastewater treatment.