| Nanofiltration?NF?can intercept organic and inorganic substances,and has higher transmittance to some ions which are necessary for human body,as well as has a high membrane flux.Therefore,NF is more and more widely used in drinking water purification and other fields.However,the problem of membrane fouling has always restricted the long-term effective operation of nanofiltration process,the most important of which is the organic fouling caused by a large number of natural organic compounds?NOM?in natural water.Therefore,it is helpful to promote the development and application of nanofiltration technology in practical production to explore the types of organic matters and resistances that contribute more to membrane fouling and are more difficult to clean,analyze the mechanism of organic fouling and cleaning of nanofiltration membranes,and carry out targeted optimization of cleaning agents.In this study,humic acid?HA?,bovine serum albumin?BSA?and sodium alginate?SA?were used to simulate the humus,protein and polysaccharides that constitute NOM in water.The self-made small cross-flow plate device was used to carry out the qualitative and quantitative organic pollution and preliminary cleaning experiments of single-component HA,BSA,SA,two-component HA+BSA,HA+SA,BSA+SA and multi-component HA+BSA+SA for nanofiltration membrane model NF-1812.The fouling experimental conditions were:temperature 25?,pressure 0.5 MPa,cross-flow velocity 4.5 cm/s;cleaning experimental conditions were:physical cleaning cross-flow velocity 9cm/s,cleaning time 0.5h,preliminary cleaning agent 0.1%NaOH+0.025%Na-SDS with a pH of 11.5,cleaning time 1.5h.The changes of membrane flux,organic matter interception rate,inorganic salt interception rate were studied,the size of membrane fouling index was investigated,the composition of membrane fouling resistance was analyzed,combining with the analysis of scanning electron microscopy,the effects and mechanisms of organic fouling and preliminary cleaning on the performance of nanofiltration membrane were clarified.Through systematic comparison of the effects of different components of organic fouling and preliminary cleaning on the performance of nanofiltration membranes,the types of organic matters and membrane fouling resistances,which contributed more to membrane fouling and were more difficult to clean,were explored,and the mechanism of membrane fouling and cleaning was analyzed.The results show that the attenuation degree of membrane flux is HA+BSA+SA>BSA+SA>SA>HA+SA>HA+SA>HA+BSA>HA>BSA>HA>BSA.Therefore,the higher the comprehensive concentration and viscosity of organic contaminated solution,the greater the decrease in membrane flux and the deeper the membrane fouling.Membrane fouling has little effect on the organic matter interception rate,which can be stabilized at 99.1%99.6%.In the membrane fouling resistance,in addition to the inherent resistance of the membrane,concentration polarization resistance is relatively large,about56.0%85.1%,can be easily removed by physical cleaning,is reversible fouling.Gel layer resistance and internal fouling resistance account for about 14.9%44.0%,which is difficult to clean.It must be eliminated by chemical cleaning and is irreversible fouling.After preliminary cleaning,the organic matter interception rate of nanofiltration membrane can be remained at 99.1%99.4%,the SEM image has good recovery effect.However,the membrane flux exceed the new membrane level,the internal fouling resistance is negative,and the MgSO4 interception rate decreas significantly,about 715 percentage points.This indicates that the agent used in the preliminary cleaning is too strong,and the nanofiltration membrane has a serious pores enlargement effect,and even membrane damage may occur,so the agent needs to be optimized.The optimization experiments are carried out by chemical alkali cleaning method and combined cleaning method.By adjusting the pH value and cleaning time of the agent,the optimum conditions of the two optimal cleaning methods are determined,and the cleaning mechanism is analyzed.The results show that the optimum conditions of chemical alkali cleaning for the preliminary cleaning agent are critical pH=10.0,and optimal cleaning time 50min.At this time,the membrane specific flux can be restored to 0.992,the organic matter interception rate is 99.2%99.5%,and the NaCl and MgSO4 interception rates are restored to24.26%28.53%and 80.28%90.62%,respectively.Adding 0.2%HCl to the preliminary cleaning agent is the combined cleaning method.The optimum conditions are the optimal cleaning time of alkaline cleaning 50 min,the pickling critical pH=3.0,and the optimal cleaning time of pickling 40 min.At this time,the membrane specific flux can be restored to0.999,the organic matter interception rate is 99.2%99.4%,and the NaCl and MgSO4interception rates are restored to 24.06%28.26%and 79.86%90.48%,respectively.Both of the two optimal cleaning methods can restore the performance of nanofiltratio n membranes.It is also confirmed that the nanofiltration membranes are not damaged.Further analysis of the cleaning mechanism shows that in addition to 30.55%of the inherent resistance of the membrane and 57.6%of the concentration polarization resistance which can be eliminated by physical cleaning,the chemical alkali cleaning method and the combined cleaning method can eliminate 11.60%and 11.81%of the gel layer resistance respectively,which is difficult to clean,only 0.25%and 0.04%of the internal fouling resistance is not eliminated.Therefore,the membrane fouling resistance elimination effect is good.The principle of the two optimal cleaning methods is analyzed.The results show that the essence of chemical alkali cleaning is a dyna mic balance between pores enlargement and blockage of membrane pores.Therefore,the cleaning effect of chemical alkali cleaning method is not thorough,and it is shallow cleaning.However,the essence of the combined cleaning method is a skillful combination of alkali-washing enlargement and acid-washing shrinkage for nanofiltration membranes.Therefore,the cleaning effect of it is thorough,and it is deep cleaning.According to this,a set of chemical cleaning schemes suitable for production and daily life is provided theoretically:multiple chemical alkali cleanings+one combined cleaning.In this way,the organic combination of shallow cleaning and deep cleaning can not only maintain the performance of nanofiltration membrane stab le and efficient for a long time,but also simplify the cleaning operation,save cleaning time and operation cost. |
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