The Application Of Double-membrane Method In The Water Purification Plant To Treat Fluoride And Sulfate In The East Section Of The South-to-North Water Diversion Project

China's water resources problems are the shortage of water resources and water environment pollution.In order to solve the shortage of water resources in the north,especially in the North China Plain,China has implemented the South-to-North Water Diversion Project.The Datun water storage at the end of the eastern route of the South-to-North Water Diversion Project suffers from excessive sulfate and fluoride,which affects the safe supply of drinking water in the surrounding areas.Therefore,the Ruiyuan Water Plant in Wucheng County,Dezhou City,Shandong Province,whose water source is drawn from the Datun Reservoir,urgently needs to desalinate and remove fluoride on the basis of conventional water treatment.Methods to remove sulfate and fluoride mainly include adsorption,biochemistry,chemical precipitation,electrochemical,membrane separation,etc.Based on the advantage that the membrane separation method can remove fluorideand sulfate more thoroughly without producing other by-products.This paper mainly studies the application of ultrafiltration-reverse osmosis double membrane method in desalination and fluoride removal.The purpose of the research is to investigate the effect of ultrafiltration-reverse osmosis double membrane system on the removal of fluorideand sulfate and the operational stability of the system through a small test.The experimental exploration of the optimal operating parameters of the double membrane method can provide data support for the later actual project.This research has reached the following conclusions:First,the operating conditions of the ultrafiltration system were changed,and the effects of different membrane fluxes and filtration cycles on the trans-membrane pressure drop(TMP)and permeability of the ultrafiltration system were studied.The test results showed that when the membrane flux of the ultrafiltration system was 65 L/(m~2?h)and the filtration cycle was 40minutes,the sedimentation density index(hereinafter referred to as the SDI value)was less than 2.5,indicating that the effluent could meet the requirements of the next stage of reverse osmosis inflow.The operation effect is relatively good.Then,the ultrafiltration-reverse osmosis system was subjected to orthogonal experiments to change the operating conditions of the system,and the effects of different inlet water pressure,temperature and p H on the membrane flux,fluorine removal and salt rejection rate rate of the ultrafiltration-reverse osmosis system were studied.The optimal operating conditions for the removal of fluorideand sulfate by the dual-membrane system were determined:the temperature was 7?13?,the inlet pressure was 1.30 MPa,and the p H was8.0.Under these conditions,the mean flux of the water-producing membrane was 16.16L/(m~2·h),the mean fluorine removal rate was 96.18%,and the mean desalination rate was98.48%.The inlet water pressure was the main factor that affects the membrane flux,desalination rate,and fluoride removal rate.As the inlet water pressure increases,the desalination rate,fluorine removal rate,and membrane flux all increase,and the inlet water pressure was the maximum at 1.30 MPa.Finally,the results and data of the small-scale trial operation are used as data support for the treatment of sulfate and fluoride in the Ruiyuan Water Plant's dual membrane system,combined with the membrane manufacturers'years of experience,the"ultrafiltration system+reverse osmosis system"workshop was designed.During the operation of the membrane system,the effluent of the ultrafiltration and reverse osmosis systems is monitored,and the concentration of fluorideand sulfate effluent is mainly studied.The data showed that the fluoride ion removal rate is above 92%,the salt rejection rate of the reverse osmosis system is above 98%.The actual project operation effects good.