Mitigating Organic Fouling Of Forward Osmosis Membrane Via Directly Extracting Calcium From The Deposited Foulants On Membrane Surface

Forward osmosis（FO）has drawn increasing interests in the field of wastewater treatment and reclamation owing to its excellent water quality,low fouling tendency and energy consumption.Although the fouling tendency of FO membrane is less than that of microfiltration（MF）or ultrafiltration（UF）membranes,membrane fouling is inevitable especically during the long-term operation and becomes one major obstacle hindering FO operation.Organic fouling is a major fouling type of FO membrane treaing wastewater.Moreover,metal ions（e.g.,calcium ions and magnesium ions）can bridge the organic foulants,which makes the fouling layer more compact and subsequently enhances the organic fouling.Thus,in order to alleviate the calcium-enhanced FO membrane fouling,the concept of alleviating membrane fouling via in-situ extracting Ca²⁺from the deposited membrane foulants was proposed in this manuscript.Sepcifically,effects of the diffusive gradients in thin-films（DGT）technology and the self-made ethylenediaminetetraacetic acid（EDTA）-based adsorption layer on extracting organic-bound calcium from FO membrane surface and mitigating FO membrane fouling were investigated,and then the better extracting method was selected and further applied in the FO membrane for real wastewater treatment.The main results and conclusions are summarized in the following.（1）The feasibility of DGT technology for in-situ extracting Ca²⁺from the FO fouled membrane surface and alleviating membrane fouling was investigated.The results indicated that the DGT could effectively adsorb the Ca²⁺binding with the sodium alginate via the chelation of the Chelex functional groups,and its Ca²⁺adsorption efficiency increased with the rise of Ca²⁺concentration in the feed solution.However,the Ca²⁺adsorption efficiency decreased when Ca²⁺concentration increased to 35 mM due to the maximum adsorption amount of the DGT.Owing to the extraction of Ca²⁺from the fouling layer by the DGT,the FO membrane fouling was effectively mitigated evident by significant enhancement of water flux,and at the same time,foulants became easily removed by physical cleaning.The alleviation of FO membrane fouling by the DGT could be attributed to the fact that the structure of the fouling layer became more porous and looser after in situ removing Ca²⁺.（2）The feasibility of in-situ removing Ca²⁺from the FO fouled membrane surface by the EDTA-based adsorption layer was investigated.The results suggested that the EDTA-based adorption layer can effectively remove the Ca²⁺binding with sodium alginate,and its adsorption amount correspondingly increased as a function of Ca²⁺concentration in the feed solution.The maximum adsorption efficiency of the EDTA-based layer for Ca²⁺was 93.5%.Owing to the effective extraction of Ca²⁺from the fouling layer by the EDTA-based adsorption layer,water flux was significantly enhanced,and the remaining foulants became easily removed by physical flushing.Mitigating FO membrane fouling by the EDTA-based adsorption layer could be attributed to the fact that the fouling layer structure became more porous and looser and some deposited polysaccharides deteached from the membrane surface owing to the destruction of the structure of alginate-Ca²⁺gel networks by removing Ca²⁺.（3）The EDTA-based adsorption layer was applied in extracting FO membrane in anaerobic osmotic membrane bioreactor（AnOMBR）in order to evaluate the feasibily of mitigating FO membrane fouling for real wastewater treatment.The results showed that the EDTA adsorption layer could effectively extract the Ca²⁺from the FO membrane surface in the AnOMBR,and its adsorption efficiency was up to 71.54%.After adsorption,the Ca²⁺deposited on the FO membrane surface decreased from 1.30 mg/m² to 0.37 mg/m²,and the biofoulants on the FO membrane surface significantly decreased,especially the polysaccharides with a reduction of about 79%.The fouling layer structure became loose due to the destruction of the sodium alginate-Ca²⁺gel network and the separation of the deposited polysaccharide,thus membrane fouling was controlled and the water flux of FO membrane correspondingly increased 62.5%.