Mechanism And Control Of Membrane Fouling In Anaerobic Osmotic Membrane Bioreactors

Anaerobic osmosis membrane bioreactor?AnOMBR?is superior to the traditional anaerobic membrane bioreactor?AnMBR?owing to its better effluent quality,lower fouling tendency and lower energy consumption.However,low water flux and serious membrane fouling are two problems restricting the development of AnOMBRs.At present,few studies focused on the mechanisms and alleviation of membrane fouling in AnOMBRs,and correspondingly the understanding of fouling mechanisms and the fouling control methods are not clear.Based on these facts,this study focused on the impacts of draw solution concentration on FO membrane fouling in AnOMBRs,and subsequently clarified the fouling mechanisms of active layer of FO membrane and proposed the corresponding fouling control methods.In addition,the RO was integrated with the AnOMBR for recovering the draw solute.During this process,the accumulation of contaminants in the draw solution and its impacts on the fouling of FO membrane support layer were investigated.Based on those results,the fouling mechanisms of support layer of FO membrane and its control methods were discussed.The main contents and results are listed as follows:?1?The variation of FO membrane water flux could be divided into two stages,the rapid decline stage and the stable stage.Compared with the stable stage,the rapid decline stage had a more contribution to the FO membrane fouling.In addition,an increase of draw solution concentration could enhance the water flux of FO membrane.However,when the draw solution concentration exceeded the critical value,the water flux would significantly decrease owing to serious membrane fouling.?2?Based on the significant contribution of the rapid decline stage to FO membrane fouling,controlling the driving force in a two-stage pattern was applied for alleviating membrane fouling.The two-stage pattern of driving force was realized by decreasing the draw solution concentration at the first stage and then increasing the draw solution concentration to a normal level.As a result,the two-stage pattern of driving force effectively mitigated active layer fouling of FO membrane and achieved a higher water flux and water production.It could be attributed to the fact that a lower driving force in the first stage alleviated the membrane fouling and thus leading to a loose and porous fouling layer.?3?The RO unit was combined with the AnOMBR for achieving the recovery of draw solute.The high rejection of FO membrane enhanced the product water quality of the AnOMBR,while it resulted in an accumulation of TOC?NH₄⁺-N and metal ions such as Ca²⁺and Mg²⁺in the draw solution.A serious support layer fouling of FO membrane led to a larger flux decline rate in the AnOMBR-RO system compared with that in the AnOMBR.?4?Adding EDTA into the draw solution and chemical cleaning by NaClO were applied for controlling the support layer fouling of FO membrane.Periodically cleaning with NaClO could effectively remove the biofoulants in the support layer and thus enhancing the water flux of FO membrane.