Ion exchange pretreatment for reverse osmosis desalination of brackish water
Water shortages affect 88 developing nations that are home to half of the world's population. Certain areas in developed nations also face water scarcity, such as the western states of USA, California and Arizona. In order to address the ever increasing demand for fresh water, amidst depleting reserves of fresh water, users are looking toward more saline waters, such as brackish groundwater or seawater. As seawater's salinity is between 33,000 to 37,000 mg/l and brackish water's salinity is between 2000 to 15,000 mg/L, they have to be desalinated to a specific purity level before they can be used for consumption or for other purposes. For example, EPA standard for drinking water is 500 mg/L. Reverse Osmosis (RO) is the commonly preferred method for desalination. ^ Reverse Osmosis desalination of brackish waters is plagued by problems such as membrane scaling and fouling etc., which cause the RO to operate at recoveries lower than optimal. RO operation at low recovery increases the amount of brine, disposal of which adds to the product water cost. This necessitates the need for pretreatment of feed water. Detailed simulations and analysis and an extension of the Ion Exchange (IEX) pretreatment with regeneration using concentrated water from RO process (Vermeulen and Klein) were done. An IEX pretreatment helps remove the the Calcium and Magnesium ions to limit precipitation and scaling of salts on the membrane. ^ As the composition of brackish waters can have huge variations, a case study was done for two brackish waters, a Calcium rich brackish water in which the relative composition of different ion mimics a real brackish water source, and a Calcium poor brackish water in which the relative composition of different ions mimics sea water. ^ It was observed that for the Calcium rich feed considered (TDS = 12000 ppm), RO operation without IEX or anti-scalant pretreatment resulted in 36% recovery (operation at higher recoveries causes precipitation in RO). In comparison, IEX pretreatment helped us operate RO at recoveries as high as 90%. RO operation at higher recoveries with IEX pretreatment decreased the cost per m3 of product water significantly by 78% when the brine disposal costs are high and by 22% when brine disposal costs are low. It was observed that IEX pretreatment was not beneficial for the Calcium poor feed (TDS = 12000 ppm).^
Phillip C. Wankat, Purdue University.