Pilot Study On The Suitability Of UF-NF Combined Process For The Treatment Of Xijiang River Source Water

The pollution situation of water resources in China is still grim,and there are also organic micro-pollution and sudden pollution problems in the source water of the Xijiang water system.With the increasingly strict drinking water quality standards and people’s demand for high-quality drinking water,more drinking water plants are choosing suitable advanced treatment processes produce high-quality water to meet new drinking water quality requirements.Aiming at the requirements for producing high-quality drinking water in a water plant in Zhongshan City,this paper mainly verifies the suitability of the ultrafiltration-nanofiltration combined process to treat the source water of Xijiang water system through a pilot plant,and provides technical support for the improvement of drinking water plants.Firstly,the type and dosage of coagulant and pretreatment form in the coagulation pretreatment before ultrafiltration membrane were optimized through small tests.The results show that the effluent turbidity of the direct ultrafiltration process is less than 0.1 NTU,the removal rates of COD_Mn and UV₂₅₄ are 13.60% and7.44%,during the filtration process,the membrane flux decline is the most significant,and the membrane resistance is the largest at the end of filtration.The use of coagulation pretreatment can effectively improve the removal effect of organic matter and further alleviate membrane fouling.The removal effect of coagulant using polyaluminum chloride on organic matter and the alleviation effect on membrane fouling is better than that of polyferric sulfate.Increasing the dosage of coagulant will improve the removal effect of the ultrafiltration combination process on organic matter,but excessively high dosage of coagulant is not conducive to alleviating membrane fouling.The mitigation effect of coagulation sedimentation-sand filtration pretreatment on membrane reversible pollution is superior to coagulation sedimentation pretreatment,and the effect of two pretreatment methods on organic matter removal is basically the same.Then,the important operating parameters in the ultrafiltration and nanofiltration processes were optimized through pilot tests.The results show that in the ultrafiltration process test,the membrane flux,filtration cycle and backwash time have no significant effect on the removal effect of the ultrafiltration process on turbidity,particulate matter and organic matter.And reducing the membrane flux,shortening the filtration cycle and extending the backwash time can alleviate ultrafiltration membrane fouling to a certain extent.In the nanofiltration process test,increasing the operating pressure and reducing the recovery rate can improve the removal effect of the nanofiltration process on organic matter,ammonia nitrogen,and inorganic salts,increaseing removal rate is in the range of 4～20%.However,increasing operating pressure and recovery rate will aggravate nanofiltration membrane fouling to a certain extent.In this test,due to the good raw water quality,the quality of the nanofiltration concentrated water tested in each test group complies with the “Standards for Drinking Water Quality”（GB 5749-2006）.The nanofiltration concentrated water can be considered for reuse or direct discharge.Finally,using the conventional treatment process and ozone-activated carbon process as a reference,the characteristics of the ultrafiltration-nanofiltration combined process to treat the source water of the Xijiang water system and the ability to deal with sudden pollution were studied.The results show that the coagulation sedimentation-sand filtration process has a good removal effect on turbidity,organic matter and ammonia nitrogen in water,but the removal ability to deal with metal and benzene series pollution is limited.The addition of ozone-activated carbon process can further remove turbidity,organic matter and ammonia nitrogen in the water,and can also deal with metal and benzene series pollution to a certain extent.The addition of ultrafiltration-nanofiltration process has more advantages than the addition of ozone-activated carbon process,which can improve the biological and chemical safety of drinking water.The effluent turbidity of the ultrafiltration-nanofiltration process is basically stable at 0.023 NTU,the total number of particles is basically maintained at 0 CNT/m L,the total number of colonies,total coliforms and aluminum are below the detection limit.And the average removal rates of COD_Mn,UV₂₅₄,TOC,ammonia nitrogen,chloride,fluoride,total hardness and sulfate by ultrafiltration-nanofiltration process are 74.54%,99.58%,68.34%,32.51%,10.38%,35.94%,35.98% and 96.10%.At the same time,it can also deal with metal and benzene series pollution to a certain extent.However,the concentration of nitrate nitrogen in the effluent of the ultrafiltration-nanofiltration process will increase slightly,but it is still below the national standard limit.The ultrafiltration process uses air flushing+water backwashing+water positive flushing and the nanofiltration process uses positive flushing physical cleaning methods to effectively alleviate membrane fouling.After the restorative chemical cleaning of the ultrafiltration membrane and nanofiltration membrane that have been in stable operation for one month,the ultrafiltration membrane pressure difference and the nanofiltration membrane flux basically returned to the initial value.