Study Of Forward Osmosis In Seawater Desalination

With the increase of population and economic development,water shortage has become one of the important factors restricting human development.Due to the ocean contains over 97%of global water,seawater desalination is considered one of the promising technologies to produce freshwater.The reverse osmosis(RO)process is a physical membrane separation process without phase change,and its energy consumption for seawater desalination is much lower than that of traditional thermal desalination technology.In recent decades,seawater reverse osmosis(SWRO)technology has become the most widely used desalination technology.70%of desalination plants desalinate seawater by the RO system in China.However,in the actual SWRO process,RO systerm requires high-pressure operation and the specific energy consumption(SEC)is still very high.In addition,the current RO membrane has poor rejection of the trace element boron in seawater.The boron concentration of the 1st pass RO permeate cannot meet the requirements of use standard.Forward osmosis(FO)is a membrane process in which water diffuses from the low-concentration side to the high-concentration side with the osmotic pressure difference between both sides of the semipermeable membrane as the driving force.Since the FO process is driven by osmotic pressure and does not require high-pressure operation,the energy consumption of water production is extremely low.In addition,the FO process also has the characteristics of high retention performance,light membrane fouling and simple operation,which has great potential for application in seawater desalination.This study mainly explored the feasibility of FO process in seawater desalination.The FO-NF-RO seawater desalination process using FO for pretreatment was proposed to reduce the energy consumption of seawater desalination.After clarifying the effectiveness of NF-like FO membrane for seawater pretreatment,the economic and technical feasibility of the FO-NF-RO process was systematically evaluated.The RO-FO process,which used the FO process for advanced treatment of 1st pass RO permeate,was proposed to reduce the boron concentation of the produced water.After systematically exploring the effect of FO process operating conditions on the boron removal performance,the feasibility of FO process as an alternative to 2nd pass RO was further clarified.The main research conclusions are as follows:(1)In the FO-NF-RO process,the FO process with NF-like FO membrane could effectively reject dissolved organic matter(DOM)and divalent ions;the subsequent NF process could reject divalent ions and recover DS;RO process could intercept NaCl and produce freshwater.The produced water TDS was 120-510 mg/L,which lowed 1000 mg/L and satisfied the drinking water quality standards of China.(2)In the FO-NF-RO process,the FO SEC mainly depent on the water flux and water recovery rate.The use of ERD could greatly reduce the SEC of the NF and RO process,which could be reduced by about 35%.The SEC of the FO-NF-RO process was mainly derived from the high-pressure NF and RO processes.After using ERD,the total SEC of the process reduced by about 30%.(3)In the FO-NF-RO process,the minimum SEC of the FO-NF-RO process was 3.02 kWh/m3,which was equivalent to the conventional SWRO SEC(3.10 kWh/m3);the minimum specific water cost(SWC)was 5.47 yuan/m3,slightly lower than the SWRO average SWC(7.17 yuan/m3).In summary,the FO process with NF-like FO membrane was an effective pretreatment method for seawater desalination,and the FO-NF-RO seawater desalination process had practical application feasibility.(4)In the RO-FDFO process,the operating conditions of the FDFO process had varying degrees of influence on its boron removal performance and SEC.The pH mainly affected the boron removal by changing the form of boron in the solution.Other operating conditions mainly affected the boron removal performance by changing the boron external concentration polarization(ECP).Operating conditions mainly affected the water flux to further change SEC.(5)In RO-FDFO process,the best operating conditions for the FDFO process is:DS pH=11.0,the volume ratio of FS to DS is 1:1,the operating flow rate is 15 cm/s,the operating temperature is 25?,and AL-FS mode.Under this experimental condition,the minimum boron concentration of diluted DS was 0.4 mg/L,and the minimum SEC was 0.17 kWh/m3,which was equivalent to RO-RO seawater desalination process.These results indicated that the FDFO process had potential as an alternative of the 2nd pass RO.