| Forward osmosis(FO)technology,which is a energy saving technology driven by osmotic pressure,has attracted more attention in recent years because of its high fouling resistance and wide range of foulant removal.However,organic membrane fouling is the main drawback that hinders the feasibility of this promising technology.Based on the domestic and international research on forward osmosis,this study proposes the concept of organic matter gradation,which refers to the filtration order of different molecular weights of organic foulants through the forward osmosis fouling experiments in order of molecular weight from large to small or from small to large.By changing the form of organic matter gradation,the effect of the model organics on the internal clogging of FO membrane porous support layer was investigated,and the efficiency of continuous filtration of FO membrane was explored accordingly.Further,based on the analysis of the characteristics of natural dissolved organic matter,the performance of FO membrane filtration under natural dissolved organic matter gradation was preliminarily investigated.The feasibility of applying forward osmosis to water treatment processes was explored,providing the scientific basis and data support for improving the continuous filtration efficiency of forward osmosis water treatment processes.In this study,fulvic acid,tannic acid,sodium alginate and bovine serum albumin were selected as module foulants,and the influence factors of initial flux level,initial TOC concentration,foulant type and ionic strength on the performance parameters of water flux,retention rate and adsorption during the operation of FO were explored.The experiments were also conducted to investigate the different organics characteristics within the FO membrane support layer under the influence of organic matter gradation and the sustained filtration efficiency of the FO membrane after gradation filtration.On this basis,a local reservoir water and a water supply plant backwash water were used as the target water bodies to analyze and characterize the organic matter in the raw water and coagulate the raw water through the ultrafiltration membrane treatment to further study the fouling characteristics of the organic matter gradation in the actual water bodies during the forward osmosis process.It was shown that the FO membrane had a good removal effect(above 97%)for all four foulants mentioned above.The higher initial flux level(36.49 L·(m2h)-1),higher initial TOC concentration(100 mg·L-1)and increased calcium ion concentration(1 mmol·L-1)resulted in a more significant decrease in pollutant flux,with the most severe decrease in water flux caused by tannic acid(about 62%)and sodium alginate water flux most affected by calcium ion.The retention rate of bovine serum protein,tannin and xanthic acid was above 80%,and the retention rate of large molecule sodium alginate was up to 93%.The permeation traction force caused by water flux played an important role in the adsorption of target organics in the support layer of the forward osmosis membrane,and small and medium molecules such as bovine serum protein,tannin and xanthic acid would enter the inside of the membrane support layer,while large molecules of sodium alginate were more adsorbed on the membrane surface.In organic matter gradation filtering experiments with foulants,it was concluded from forward gradation filtration(molecular weight from large to small)and reverse gradation filtration(molecular weight from small to large)that the contaminated water flux was reduced by 70%for forward gradation and 56%for reverse gradation,and the reverse gradation filtration method was able to obtain a higher water flux than forward gradation.The retention rate of foulant was above 87%for both gradation filtration,and the adsorption of foulants on the FO membrane was reduced.The experimental results of sodium alginate filtration by FO membrane after forward gradation filtration and reverse gradation filtration again showed that the FO membrane after forward gradation filtration had more foulant flux loss(about 25.9%)for sodium alginate filtration.However,the retention rate of sodium alginate by FO membranes after both gradation filtration was above 93%,which was slightly lower than that of sodium alginate when it was filtered singly.This indicates the advantages of FO membranes being able to filter large molecular weight dissolved organics again after gradation mode,and the higher sustained filtration efficacy of FO membranes after reverse gradation.Based on confocal microscopy observation of FO membrane after gradation fouling,the pore size and porosity of forward gradation and reverse gradation decreased significantly,and the foulants would be clogged deeper in the support layer,and forward gradation filtration would cause more serious membrane pore clogging than reverse gradation filtration.Natural surface water could be separated from medium molecular weight water samples and small molecular weight water samples after coagulation over membrane experiment.With natural organic matter as the gradation,the final water flux decreased by 5.5%under forward gradation compared to reverse gradation,indicating that the reverse gradation filtration method could achieve a higher water flux than forward gradation.The retention rate of natural organic matter by both gradation filtration was above 95%,and the filtration order had little effect on the retention of organic matter by the FO membrane.By XDLVO theoretical analysis,it was clear from the calculations of free energy of adhesion and free energy of cohesion that the decrease in fouling flux was more controlled by the free energy of cohesion between the foulants present in the feed solution and those already adsorbed on the membrane.According to confocal microscopy observations of FO membranes fouled with natural organic matter,natural organic foulants severely clogged the internal pore size of the membrane support layer,and the forward gradation caused more severe membrane pore clogging.The foulants within the support layer contained more proteins and polysaccharides but less algal cell.Hence,the reverse graded FO membranes had higher sustained filtration efficiency under natural organic gradation. |
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