Simulation of ion exchange water softening pretreatment for reverse osmosis desalination of brackish water

Abstract

Prevention of membrane scaling in reverse osmosis (RO) desalination may require operation at low recoveries, which increases the amount of brine, increasing brine disposal costs and creating environmental hazard. Ion exchange (IEX) pretreatment removes Ca2+ and Mg2+ ions to limit scaling of salts. Use of concentrated water from RO process for regeneration of the IEX column eliminates the need for regeneration chemicals. Detailed simulations of the process were done. Optimal IEX/RO configurations were developed to avoid precipitation and minimize brine disposal cost.Simulations were done for Ca2+ rich and Ca2+ poor brackish water, in which relative composition of different ions mimics real brackish water source and sea water respectively. For the Ca2+ rich feed considered (TDS=12,000ppm), RO operation without either IEX pretreatment or anti-scalant predicted 36% recovery before precipitation occurred. In comparison, RO simulations after IEX pretreatment predicted recoveries as high as 90% with no precipitation in RO modules. IEX/RO operation at higher recoveries decreased the cost of product water by 75% when brine disposal costs are high and by 20% when brine disposal costs are low. Simulations showed IEX pretreatment was not beneficial for the Ca2+ poor feed, which did not precipitate.