Fouling Mechanisms And Its Controlling Methods Of Forward Osmosis Membrane In Anaerobic Osmotic Membrane Bioreactors

As a new membrane bioreactor technology,anaerobic osmotic membrane bioreactor(AnOMBR)has gained increasing interests in recent years.In the AnOMBR,forward osmosis(FO)membrane was applied for retaining the anaerobic biomass instead of the microfiltration(MF)membrane commonly used in anaerobic membrane bioreactors(AnMBRs).Compared to the conventional AnMBR,AnOMBR has better effluent water quality due to the high rejection of FO membrane for organic matters,nitrogen and phosphorus.In addition,the FO membrane is considered as a low fouling membrane due to its lack of hydraulic pressure and lower flux condition.However,the AnOMBR also has some drawbacks such as salt accumulation and membrane fouling.Therefore,this study is focused on the FO membrane fouling in the AnOMBR.Specifically,a novel AnOMBR combined with the MF membrane(AnMF-OMBR)for alleviating the salt accumulation was firstly constructed,and then the contribution of the membrane fouling to the decline of FO membrane and the fouling mechanisms were investigated,finally the physical and chemical cleaning of fouled FO membrane were applied.The main research contents and conclusions are listed as follows:(1)Based on the fact that the soluble salt can pass through the MF membrane,the MF membrane was integrated into the AnOMBR(AnMF-OMBR)for controlling the salinity build-up,and the performance of the AnMF-OMBR including the FO membrane fouling and its contribution to the flux decline was investigated.The results indicated that the salinity was effectively controlled at the level of 2-3 mS/cm with the help of MF membrane.The TFC-FO membrane had an excellent rejection for total organic carbon(TOC)and total phosphorus(TP)(98.0% and 100%,respectively).However,its NH4+-N rejection was much lower(nearly 0%),which was mainly due to the Donnon potential between the TFC-FO membrane with the NaCl as the draw solute.The TFC-FO membrane had a serious fouling,which resulted in the flux decline of FO membrane from 7.99 LMH to 2 LMH.The membrane fouling was composed of organic fouling,inorganic fouling and biofouling.Compared to the inorganic fouling,organic fouling and biofouling played a more important role in the fouling of TFC-FO.The analyses of confocal laser scanning microscope(CLSM)indicated that total cells,proteins and?-D-glucopyranose polysaccharides were the dominant foulants.(2)The physical cleaning methods including water flushing,ultrasonic cleaning,backwashing and periodic cleaning were applied for cleaning the fouled TFC-FO membrane in the AnMF-OMBR.The results showed that water flushing could not remove the foulants from the TFC-FO membrane,and the TFC-FO membrane had the risk of being destroyed by ultrasonic cleaning.Moreover,backwashing and periodic cleaning could not effectivelyrecover the permeability of the TFC-FO membrane.(3)The chemical cleaning reagents including citric acid,HCl,NaOH,SDS,EDTA-2Na and H2O2 were applied for cleaning the fouled TFC-FO membrane in the AnMF-OMBR.The results implied that citric acid,HCl,NaOH,SDS and EDTA-2Na could not effectively remove the foulants from the TFC-FO membrane with a severe fouling,while H2O2 can effectively recover the permeability of the fouled TFC-FO membrane.The further study indicated that the most effective cleaning procedure was used 0.5% H2O2 at 25 ? for 6 h.Moreover,when the cleaned TFC-FO membrane was put into the AnMF-OMBR and re-operated,it has the same flux decline tendency and rejection for TOC,TP and NH4+-N as a virgin membrane.It implied that the cleaned TFC-FO membrane after cleaning by H2O2 has completely recovered its characteristics as a virgin membrane,and can continue to use in the AnMF-OMBR.