The Application Of Loose Nanofiltration Hollow Fiber Membrane In Wastewater Treatment

Nanofiltration membrane bioreactor（NF-MBR）integrates NF membrane instead of conventional ultrafiltration/microfiltration membrane with biological reactors to obtain high-quality permeate,which is a novel technology to promote wastewater reuse.However,conventional NF membranes with tight surface structure result in the low membrane flux,high operating pressure and inevitable salt accumulation in bioreactor.Loose NF membrane with the relatively high salt permeation and high water permeability may be a good choice to alleviate these burdens of conventional NF-MBR.The development of advanced NF membrane,especially for the loosely structured NF hollow fiber membrane as the large specific surface area and high filling density,is highly desirable for the extended application of NF-MBR.However,the application of loose nanofiltration membrane in MBR for wastewater treatment is still lack of evaluation at present,and the process characteristics,membrane performance,the removal of typical micropollutants in this novel process need to be explored in detail.Based on this,a novel NF-MBR was developed based on a loosely structured nanofiltration hollow fiber membrane with low pressure and high permeability fabricated by an automatic coating technique in large area.The process performance,loose NF membrane fouling,removal performance and mechanism of typical pharmaceuticals and personal care products（PPCPs）,and the impact mechanism of the optimization of process operation parameters on the performance of the coupling system were investigated.The main research contents and conclusions are as follows:（1）The loosely structured nanofiltration hollow fiber membrane was successfully prepared by continuous interfacial polymerization.The thickness of the polyamide functional layer of the loose NF was about 120 nm and it had a molecular weight cut-offs（MWCOs）of 1500 Da with a mean effective pore radius of 1.62 nm and a high membrane permeability of 20.5±1.9 L m^-2 h^-1 bar^-1.The lower bacterial adhesion and protein adsorption mass of loose NF membrane was attributed to the formation of hydration layer on the surface of loose NF membrane with higher hydrophilicity.The critical flux of loose NF membrane in the short-term critical flux test was determined as 5.0 L m^-2 h^-1.If the loose NF membrane was immersed in Na OH solution（p H=13）for too long,the rejection capacity of the membrane would be slightly lost.Citric acid（p H=2）had no effect on the retention performance of loose NF membrane.（2）A novel nanofiltration membrane bioreactor（NF-MBR）was constructed by integrating a loose nanofiltration hollow fiber membrane into an aerobic activated sludge system.In order to verify the feasibility of loose NF membrane bioreactor,parallel experiments of conventional ultrafiltration membrane bioreactor（UF-MBR）and NF-MBR were conducted.The loose NF membrane showed a relatively high permeability at a level of around 10 L m^-2 h^-1 bar^-1(flux of 2.0 L m^-2 h^-1)under low operation pressure（10-30 KPa）.The results showed that NF-MBR achieved a higher removal efficiency of chemical oxygen demand（COD）and dissolved organic carbon（DOC）due to high rejection of loose NF membrane for biopolymers and part of the humic substances.During long term operation in NF-MBR,loose NF membrane displayed low salt rejection,resulting in no obvious salts accumulation in bioreactor.The average rising rates in TMP for the NF-MBR was 0.16 KPa day^-1,which was twice lower than that of UF-MBR.The results of fouling mechanism showed that reversible cake fouling was dominant for both NF and UF membranes,while the relatively small pore size of the NF membrane mitigated irreversible pore blockage.Further biofouling layer analysis demonstrated that humic substances,polysaccharides and proteins were the main organic foulants.The results of energy consumption revealed that NF-MBR had comparable total energy consumption compared with UF-MBR.（3）The novel moving bed biofilm reactor-nanofiltration-membrane bioreactor（MBBR-NF-MBR）with loose polyamide NF membranes were operated to treat roxithromycin（ROX）wastewater.Two types of loose NF membrane with different MWCO（1000 and 2000 Da）were operated at different membrane fluxes with fixed hydraulic retention time.Results showed that both MBBR-NF-MBRs achieved superior COD removal of 98.4%and 97.2%and excellent removal of ROX at 74.1%and 65.5%,respectively.The main membrane fouling mechanism was reversible fouling caused by the combination of abundant polysaccharides,proteins and Ca-P precipitates,which could be effectively removed by acidic cleaning.Sorption and biodegradation were the main removal routes of ROX in MBBR.Partial retention of loose NF membrane contributed to microbial metabolism and increased microbial diversity,especially the genera Hyphomicrobium in attached biofilm,which was reasonable for ROX removal.The cleavage of cladinose,demethylation,phosphorylation andβ-oxidation in macrolactone ring were the main biotransformation reactions of ROX.（4）The role of hydraulic retention time（HRT）on simultaneous removal of nitrate and typical PPCPs in secondary effluent from wastewater treatment plants in post-denitrification fixed biofilm reactor（PDFBR）was comprehensively evaluated.Results showed that PDFBR was favorable for nitrate removal（above 94%）.Prolonged HRT promoted the bio-utilization of nonaromatic soluble microbial products with low molecular weight and reduced biomass production.PPCPs were partially removed in PDFBRs（below 48%）.Microbial diversity increased along the extending of HRT and thus partially enhanced PPCPs removal.Batch experiments showed that changing HRT had no direct impact on the biodegradation rates of the selected PPCPs.Correlation analysis revealed that Dechloromonas,Terrimonas,and Phreatobacter were reasonable for simultaneous removal of PPCPs and nitrate.The abundance of nos Z gene had a rapid decrease under extreme HRT.The loose NF membrane could not completely retain the selected PPCPs with small molecular weight,but the corresponding combined process had a high operating flux.At the same time,enhancing the biofilm process could improve the removal of PPCPs in the combined process and delay membrane fouling.