Study On Membrane Fouling Potential Assessment Method Based On Gradient Filtration

Pressure-driven membrane technologies for water treatment in terms of microfiltration（MF）,ultrafiltration（UF）,nanofiltration（NF）and reverse osmosis（RO）have the advantages of stable and excellent permeate quality,flexible design and assembly of membrane modules,compact structure and small footprint,high degree of automation,and etc.Full-scale engineering applications have been gradually achieved at home and abroad in recent years.Efficient prevention and control of membrane fouling is the key to improve its technical and economic features.Accurate and rapid assessment of influent membrane fouling potential is of great significance for effective membrane fouling prevention and control.Based on the analysis of single and combined membrane fouling models,a quantitative evaluation method for membrane fouling potential via gradient filtration（MF→UF→NF）was initially established by optimizing the gradient filtration tests of model pollutants with different concentrations,and then it was preliminarily applied to the evaluation of membrane fouling potential of reall industrial wastewater in this study.The main research results are as follows:The average rejection rates of turbidity,DOC and UV₂₅₄ of raw surface water and coagulated surface water by UF were 98.6%and 89.1%,3.8%and 3.0%,55.5%and 52.2%,respectively.Coagulation mainly removed the turbidity-causing substances in the water.Four single membrane fouling models（complete/standard/intermediate blocking,cake layer）in surface water ultrafiltration process fit well（R²>0.96）,while only the combined complete blocking and cake layer model fit well（R²>0.99）,indicating that single membrane fouling models might produce statistical illusion and thus could not truly describe the real surface water ultrafiltration process in which multiple fouling mechanisms occurred simultaneously.Therefore,the membrane fouling potential assessment should be based on the combined membrane fouling models.In gradient filtration,the rejection rates of MF membrane（pore size of 0.45μm）,UF membrane（molecular weight cut-off of 100 k Da）and NF membrane（molecular weight cut-off of 300 Da）for kaolin solution,mixed solution of sodium alginate and bovine serum protein,humic acid solution were 90.0%,69.0%,98.3%,respectively.In addition,the concentration of pollutants was positively correlated with the value of fouling index in the single model.Therefore,the membrane fouling potential of suspended particles,organic colloids and dissolved organics could be quantitatively evaluated by MF,UF and NF,respectively.From combined membrane fouling model analysis for the gradient filtration of 5mg/L and 10 mg/L model pollutants,cake layer was not the dominant fouling mechanism in MF process,this may be related to the interaction between contaminants（such as non-covalent interaction and spatial effect）in microfiltered aqueous solution..Cake layer was the dominant fouling mechanism in UF and NF process.The value of membrane fouling index and the corresponding water quality index in gradient filtration was positively correlated.Thus,a membrane fouling potential assessment method based on the gradient filtration was successfully established.In the gradient filtration of high/low DOC concentration MBR superniter of printing and dyeing wastewater,the cake layer is the dominant fouling mechanism except for the low DOC MF of printing and dyeing wastewater.The UV₂₅₄ of the high/low printing and dyeing wastewater is 1.76 cm^-1 and 1.11 cm^-1,the UV₂₅₄ rejection rate of the UF membrane is 9.9%and 5.8%,and the UF fouling index of the MF effluent is 15.05 and 7.64 m^-1,respectively.The DOC of UF effluent of printing and dyeing wastewater with high and low DOC concentration by gradient filtration is 28.55 mg/L and 23.22 mg/L,respectively.The rejection rate of DOC by NF membrane is 57.8%and 81.2%,respectively.The corresponding NF fouling index of UF effluent is 3.75 and 2.80 m^-1 respectively.It can be seen that in the process of gradient filtration of different printing and dyeing wastewater,there is a significant positive correlation between the fouling index of cake layer of different membranes and the inlet water quality index.Similarly,similar conclusions are also found in the gradient filtration of different electroplating wastewater.The feasibility of the above method has been proved preliminarily.Moreover,This method is suitable for reverse osmosis process,and can also be applied to the assessment of influent membrane fouling potential in NF and UF processes.