Influences Of Al³⁺ In PAC Coagulant On The Separation Performance And Fouling Of Ultrafiltration Membranes

With the rapid development of economy and society and the continuous improvement of production technology,a series of water environmental problems have been brought,at the same time,great challenges have been brought to drinking water treatment.As a new type of water treatment method,ultrafiltration has the advantages of simple operation and stable operation,so it is widely used.However,the problems such as membrane fouling in ultrafiltration process have been a huge obstacle to the development and application of ultrafiltration technology.Adding coagulation pretreatment process before ultrafiltration,with its advantages of high treatment efficiency and good effluent quality,has become a widely concerned technical means in drinking water treatment process.Nevertheless,the combination of coagulation and ultrafiltration has not only its unique advantages but also some challenges.For example,when coagulant is added to raw water,the coagulant will remain on the membrane surface and become part of the gel layer and cake layer on the surface of the membrane.Their presence will accelerate the membrane fouling rate,and even a part of them can not be removed by physical cleaning,thus forming an pollutant that can accelerate the aging rate and reduce the service life of the membrane.Taking PAC polyaluminium chloride coagulant and PES poly（ether sulfones）ultrafiltration membrane as an example,this paper explored changes of separation performance of ultrafiltration membrane caused by the addition of PAC coagulant in the process of coagulation-ultrafiltration.At the same time,this paper explored the interaction mechanism between Al³⁺and organic matters in raw water,the hydrolysis polymerization of Al³⁺itself,the main functional groups of residues remaining on the membrane surface that cannot be removed by physical cleaning and so on,so as to put forward some suggestions for the reduction of membrane fouling in the process of PAC coagulation-PES ultrafiltration.First of all,in order to test the membrane separation performance,the membrane flux,rejection rate,proportion of pollutants that can be removed by physical method and cannot be removed by physical method and membrane resistance analysis of the coagulation-ultrafiltration process were tested under the optimal coagulant dosage.The results show that with the reaction going on,the membrane flux gradually decreases,the rejection rate also decreases,but the extent is not large,and the proportion of substances that cannot be removed by physical method increases,and the membrane resistance gradually changes from the initial inherent resistance to the irreversible pollution resistance.Therefore,in the process of coagulation-ultrafiltration,the membrane separation performance gradually decreases with the progress of membrane filtration,and the membrane pollution degree gradually deepens.Secondly,in order to explore the state of Al³⁺hydrolysis and polymerization,Al-Ferron time-by-time complexing colorimetry experiment was carried out.It was found that the main entrapped products of ultrafiltration membrane were Al_bin the middle polymerization state and Al_cin the high polymerization state（Al>2）.The specific products could not be obtained due to the complex reaction process.In order to explore the functional groups retained on the membrane surface after the interaction of Al³⁺and organic matter,so as to put forward the methods to reduce the fouling degree of the membrane and prolong the service life of the membrane according to the research results,ATR-FTIR and XPS were used to analyze the membrane after filtration and cleaning in different periods.No characteristic functional group was detected in the infrared test results.A series of experiments suggested that the content of pollutants in raw water was too small to be detected.XPS analysis shows that the content of C element on the membrane surface is relatively reduced,and the content of O and Al element is relatively increased,which proves that the addition of coagulant does leave pollutants on the surface of ultrafiltration membrane that cannot be removed by physical cleaning and affect the use of membrane.