Identification Of The Membrane Fouling Mechanisms In Forward Osmosis Membrane Bioreactors

Forward osmosis membrane bioreactor(OMBR) has attracted increasingly attentions due to its lower energy consumption, less membrane fouling and better water quality than the traditional membrane bioreactor(MBR). However, the OMBR is also with some drawbacks such as membrane fouling, the internal concentration polarization(ICP) and salinity accumulation in the bioreactor, which leading to the serious flux decline of OMBR. Furthermore, the membrane fouling mechanism in the OMBR is significantly different from that in the traditional MBR due to the high salinity environment of OMBR. Based on the above facts, it is very essential for comprehensively understanding forward osmosis(FO) membrane fouling mechanisms in order to develop the OMBR technology. However, until now, the studies on the FO membrane fouling especially biofouling in the OMBR are still limited. In this study, a testing method of multiple fluorescence labeling and confocal laser scanning microscopy was firstly established in order to analyze the formation and growth process of biofouling on the FO membrane surface in the OMBR, and then the formation and development of biofouling in the laboratory-scale OMBRs treating synthetic wastewater were analyzed. In addition, the two common FO membranes were investigated for obtaining their impacts on the performance and membrane fouling of the OMBR. The main research contents and results are as follows:(1) The testing method of multiple fluorescence labeling and confocal laser scanning microscopy(CLSM) was firstly established, and then was applied for analyzing the fouling FO membrane collected from a long-term operating OMBR. Based on the results, it could be concluded that the established method was fully applicable to the FO membrane fouling and could achieve the purpose of getting the fouled composition and morphology of the biofouling layer without destroying the fouling cake layer. In addition, the structural parameters such as fluorescence intensity of each fouled layer, biological volume and average thickness of the biofouling layer were calculated by the softwares of PHLIP and Image J.(2) Three identical laboratory-scale OMBR systems for the treatment of synthetic wastewater on days 3, 8 and 25 in sequence were operated under the same environment. The formation and growing rule of the biofouling layer of the FO membrane in the three OMBRs were in-situ characterized by the testing method of CLSM. CLSM images indicated that the variations in abundance and distribution of polysaccharides, proteins and microorganisms in the biofouling layer during the operation of OMBRs were significantly different. Before the 8th day, their biovolume dramatically increased. Subsequently, the biovolume of β-D-glucopyranose polysaccharides and proteins continued increasing and leveled off after 8 days, respectively, while the biovolume of α-D-glucopyranose polysaccharides and microorganisms decreased. EPS played a significant role in the formation and growth of biofouling layer, while the microorganisms were seldom detected on the upper fouling layer after 3 days. Based on the results obtained in this study, the growth of biofouling layer on the FO membrane surface in the OMBR could be divided into 3 stages.(3) Impacts of FO membrane materials on operating performance and membrane fouling were investigated in flat-sheet OMBRs for synthetic wastewater treatment by CTA-FO membrane and TFC-FO membrane. The results showed that the membrane fouling tendency of TFC-FO membrane was more serious than that of the CTA-FO membrane due to its rougher surface. Furthermore, the biological foulants composition of the two FO membranes was identical; however, the TFC-FO membrane had more intense and porous fouling layer, and was with bigger inorganic particles. Thus, compared to the TFC-FO membrane, the CTA-FO membrane was more suitable for the operation of OMBR based on the membrane fouling.