Analysis Of Influence Of Typical Drinking Water Treatment Process On NF Membrane Operation

Nanofiltration（NF）membranes have come a long way since it was first recognized in the late 80’s.Due to its low operating pressure,Excellent treatment effect and convenient operating conditions,its application and research in water treatment at home and abroad have been increasing year by year.In 2020,with the realization of the goal of building a moderately prosperous society in an all-round way in China and the improvement of China’s economic level and people’s happiness index,people’s requirements for drinking water quality are also gradually rising.Jiangsu,Zhejiang,Shanghai and other places have issued their own local drinking water standards based on GB5749-2006.Strict control of water quality means higher requirements for water treatment process,and NF process also further stands out in the treatment of high-quality drinking water.However,NF process has certain requirements for the quality of influent water.When the pretreatment method is different,the operation effect of nanofiltration membrane will be different.In this study,three typical water treatment processes on a pilot-scale were used as the pretreatment process for NF operation:conventional treatment process（coagulation,precipitation,filtration）,conventional+O₃/BAC advanced treatment process and ultrafiltration（UF）treatment process.The operating effect of NF and membrane fouling under different pretreatment processes were analyzed and studied.In the experiment,the improvement of water quality and the guarantee performance of water quality safety by different technologies were analyzed through the determination of various indexes of conventional water quality and precursor of disinfection by-products.High performance size-exclusion chromatography（HPSEC）and three-dimensional excitation emission matrix（3D-EEM）were used to analyze the effects of different processes on the properties of organic compounds,combined with parallel factor algorithm and area integration method.According to the change of nanofiltration TMP and analysis of chemical cleaning fluid,the influence of organic matter on the pollution of nanofiltration membrane was explored.The main conclusions of the paper are as follows:（1）During the experiment,the turbidity,water temperature,algal cell density,and organic matter concentration of raw water in Taihu Lake changed regularly with the seasons.With the increase of algal density,the content of organic matter,especially aromatic family proteins with benzene ring structure,which are easy to be metabolized by microorganisms,increased obviously.The dissolved organic carbon（DOC）in Taihu Lake water was mainly small molecular organic matter（79.9%）with a molecular weight of 0.2-5k Da,followed by medium molecular weight organic matter（13.1%）with a molecular weight of 5-60k Da,which was mainly due to the metabolism of algae,plankton and aquatic animals and plants.（2）O₃/BAC+NF process has the best effect on the removal of organic matter,and the removal rates of DOC,UV₂₅₄and COD_Mnare 89.3%,93.5%and 87.7%,respectively,especially for fluorescent organic matter,and the average fluorescence integral removal rate of the total area is 93.2%.Compared with the ultrafiltration process,the conventional process has a better removing effect on each water quality index,but the removing effect of UF+NF is better than that of the conventional+NF process.（3）Through the determination of trihalomethane precursor（THMFP）and haloacetic acid precursor（HAAFP）.The content of Br-tribromomethane precursor was low,but increased with the operation of the process,and could not be reduced even by using O₃/BAC and NF process;Conventional and UF processes have limited effects in removing THMFP and HAAFP,and the concentration of THMFP and HAAFP in the effluent increases due to the retention of large molecules;Nanofiltration has a good removal effect on chlorinated THMFP and HAAFP.The O₃/BAC+NF process is the most effective,and the removal rate can reach 78.4%and 71.5%.（4）Through HPSEC and 3D-EEM analysis,it was found that conventional and UF processes had little effect on the removal of DOC in the molecular weight distribution range of 0.2-60KDa,which was the main source of DBPs.NF has a good removal effect on this part of organic matter,and it is more inclined to remove UV-responsive organic matter;Cl-DBPs have a good correlation with the concentration of medium molecular weight（5-60KDa）organic matter（R²>0.8）,and Br-DBPs have a good correlation with the proportion of small molecular weight（0.2-5KDa）organic matter（R²>0.7）.（5）It is found that the temperature has the greatest influence on the TMP of NF membrane.At the initial stage of NF membrane operation,the relationship between temperature and TMP is in accordance with the Exp Dec1 equation,and R²>0.99;When the water temperature of the O₃/BAC+NF process is 4.8℃lower than that of the conventional+NF process,the operation time of the O₃/BAC+NF process is 150 hours longer than that of the conventional+NF process.The temperature of UF+NF is below 10℃during operation,when the temperature is too low,the viscosity of the water increases,the pores of the NF membrane shrink,and the organic matter is more likely to adhere to the membrane surface,and the TMP limit is reached after only two days of operation.（6）The chemical cleaning solution analysis of NF membrane showed that small molecules（0.2-5KDa）fluorescence organic matter produced weak filter cake layer pollution in the early stage of NF operation,medium and small molecules（0.2-60KDa）organic matter produced membrane pore pollution in the middle stage,and further produced filter cake layer pollution and large molecules（>60KDa）surface adsorption contamination of organic matter.Small molecule organic matter is the main factor causing membrane pollution in UF+NF process,while the synergistic effect of inorganic matter,organic matter and biological pollution is the reason for the increase in TMP of conventional+NF and O₃/BAC+NF process.