| Membrane fouling is considered as the one of the key problems for the large-scale application of nanofiltration?NF?technology.In this research,the fouling layers'structure and composition of several NF membranes and pretreatment elements in drinking water treatment were analyzed.Then,the controlling effect of different pretreatment elements on NF membrane fouling,and the pollution's influence on the retention effect of NF membranes were investigate.Finally,the fouling mechanisms of four inorganic pollutants were studied.The results of membrane fouling revealed that there were many amorphous pollutants on the pretreatment elements,while the pollutants on the NF membranes were mainly crystals of calcium carbon.The fouling of NF membranes were affected by influent water quality.Some NF membranes were fouled by microorganism and organic matter.Experiments indicated that in the single-stage NF membrane process as“5?m polypropylene cotton of brand A/5?m polypropylene cotton of brand B/1?m polypropylene cotton of brand B and ultrafiltration?UF?membrane+granular activated carbon+activated carbon+NF+post-activated carbon”,1?m polypropylene cotton of brand B and UF membrane were effective for NF membrane fouling control.In consideration of flux decline,removal rate of DOC and inorganic ions,and fouling controlling of NF membranes,1?m polypropylene cotton of brand B was the most suitable pretreatment element for groundwater treatment and surface water treatment.The pollution had a similar effect on the flux decline for five single-stage NF membrane processes,but enhanced the removal rate of dissolve organic carbon?DOC?for groundwater treatment.As for surface water treatment,the more seriously fouling of the pretreatment element,the faster the flux decline,and the pollution had similar influence on single-stage NF processes with pretreatment element.Besides,the pollution increased the removal rate of Na+and Cl-for both groundwater treatment and surface water treatment.Compared with single-stage NF membrane processes,the anti-fouling ability of multistage NF membranes processes as“carbon polypropylene filter+3NF menbranes”was improved remarkably.In addition,the removal rate of DOC of multistage processes were higher than single-stage processes.In summary,multistage processes were better than single-stage processes.Pollutants were concentrated on the carbon polypropylene filter,which consisted of silicon,carbon,oxygen and iron elements.It is unlikely that H4SiO4,Fe?OH?3,CaCO3 and kaolinite could induce fouling alone.When mixed with each other,the resistance of H4SiO4 and Fe?OH?3 mixtures increased distinctly,because there existed synergistic effects between H4SiO4 and Fe?OH?3.Concentration and ionic strength jointly affected the resistance of silicic acid.The high ionic strength and high concentration promoted the agglomeration of silicic acid,so that the particle size became larger,resulting in the forming of fouling layer on the membrane surface,which blocked the water channel.On the other hand,for the solution of low concentration and low ionic strength,film holes could be seen with the help of SEM.Therefore,the resistance of silicic acid on the condition of high ionic strength and high concentration increased obviously.It was found that the resistance increased obviously if Fe?OH?3 was added to silicic acid,because a gel layer was formed on the membrane surface.In addition,under the influence of Fe?OH?3,silicic acid particles aggregated and the size of them turned larger.Consequently,the CPF in the multistage NF membranes processes failed to remove silicon element at the very beginning,but the iron accumulated in the CPF promoted the precipitation of silicic acid so that the retention rate of silicon increased after a period of time. |
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