Research On Membrane Fouling Mitigation In Electro-Membrane Bioreactor Through Applying Low Frequency Alternating Current

Membrane bioreactor（MBR）combines conventional activated sludge system and membrane separation technology together,presenting many advantages such as superior effluent quality and small footprint,and has been widely used.However,the problem of membrane fouling during the filtration process is inevitable.Membrane fouling increases the energy consumption caused by membrane cleaning and the operating cost required for membrane replacement,which is a bottleneck problem that limits its further widespread application.In order to depress membrane fouling,this study prepared carbon nanotube hollow fiber membranes（CNTs-HFMs）with good conductivity and used them as the basic separation unit to construct alternating current（AC）electrically enhanced aerobic and anaerobic membrane bioreactor（AC-EMBR）.A certain period（T）of low-frequency AC was applied to the membrane surface,so that CNTs-HFMs alternately served as anode and cathode.Since the AC-enhanced membrane combined the electrical repulsion and the electrooxidation between the cathode/anode membrane and the pollutants,AC-EMBR exhibited excellent separation ability and anti-fouling performance during operation.The main research contents and results are as follows:In this study,CNTs-HFMs with excellent properties were prepared by wet spinning method.CNTs-HFM’s electrical conductivity was about 189Ω/cm,the contact angle was 73.8°,and the pure water flux can be as high as 1.73×10³ L/（bar·m²·h）.Then filtration experiments were set up to compare the effects of different AC period（T）on membrane fouling.The results showed that under the enhancement of AC with a period of 120 s,CNTs-HFMs exhibited the strongest anti-fouling ability.The membrane flux of CNTs-HFMs with AC（120 s）dropped from 1.21×10³ L/（bar·m²·h）to 882 L/（bar·m²·h）after 180 min,which only dropped 27.0%.While the membrane flux of the control group without electrical enhancement decreased by73.0%.Subsequently,AC（+1.00 V/-1.20 V,120 s）was applied to the CNTs-HFMs to construct an aerobic AC-EMBR reaction system.In the 73-day operation,the AC reinforced membrane showed good anti-fouling performance,and the TMP average growth rate of AC-EMBR was only 1.70×10^-2 bar/d.From the SEM image,the thickness of the filter cake layer on the AC membrane surface was around 8.00μm,which was only 1/3 of the unpowered CNTs-HFMs.In addition,the membrane regeneration data revealed that the pollutants on the AC enhanced CNTs-HFMs were mainly reversible pollution.After hydraulic and chemical backwashing,the flux recovery rate could be as high as 90.0%.The EPS_P/EPS_C ratio of the sludge on the membrane surface decreased under AC,AC reduced the microbial flocs’hydrophobicity and restrained their adhesion ability,indirectly alleviating the membrane fouling.Moreover,the AC-EMBR system also exhibited excellent wastewater treatment performance,with a COD removal rate of up to 96.0%,and the effluent NH₄^--N was less than 2.00 mg/L.In addition,by comparing the TMP growth rate of MBR,A-EMBR,C-EMBR and AC-EMBR and the regeneration capacity of the CNTs-HFMs,it was found that the anti-fouling ability of the AC reinforced membrane was higher than that of the direct current（DC）reinforced membrane.In general,the sequence of anti-fouling effect was:AC enhanced membrane>cathode membrane>anode membrane>no electrical enhanced membrane.Finally,AC（+1.00 V/-1.20 V,120 s）was applied to the CNTs-HFMs to construct an AC-enhanced anaerobic membrane bioreactor（AC-An EMBR）.TMP data showed that AC had stronger anti-pollution ability than DC.After hydraulic and chemical back-cleaning,the AC membrane flux can be restored to 86.0%at the most.showing that the membrane fouling degree of the anaerobic reactor was more serious the aerobic reactor from sides.The COD removal rate of AC-An EMBR exceeded 90.0%,and the CH₄ yield was up to 218 m L/d,which was slightly lower than the negative bias enhanced anaerobic membrane bioreactor（C-An EMBR）and higher than the positive bias enhanced anaerobic membrane bioreactor（A-An EMBR）.The16S r RNA gene sequencing results showed that whether it was the sludge on the membrane surface or the sludge deposited in the reactor,after the AC-An EMBR operation,the community distribution diversity and community abundance were reduced compared with the initial sludge.Among them,the growth of Chloroflexi and Planctomycetes was easily inhibited by the electric field,especially the relative abundance value of Chloroflexi dropped from the initial 35.0%to about 3.00%.Firmicutes,Bacteroidetes and Proteobacteria were enriched under the stimulation of electric field and became the dominant genus.In addition,the interception effect of the membrane and the adhesion of contaminants on the membrane surface caused the continuous accumulation of different kinds of microorganisms on the membrane surface,resulting in a higher diversity of bacterial communities on the AC membrane surface than the sludge deposited inside the AC-An EMBR reactor.In addition,the dominant genus of sludge on the membrane surface of the three reactors are different.Ethanoligenens,Lactococcus and Prevotella become the dominant bacteria genus of AC membrane surface.According to the operating performance of AC（+1.00 V/-1.20 V,120 s）enhanced aerobic and anaerobic AC-EMBR,AC had greatly improved the membrane fouling while showing high-efficiency separation capabilities.In terms of anti-fouling ability,AC enhanced membrane>cathode membrane>anode membrane>no electrical enhanced membrane.At the same time,the AC electric field did not cause obvious damage to the sludge activity inside the reactor and on the membrane surface,which proved the feasibility of AC to alleviate MBR membrane fouling.