(514a) Selective Desalination of Brackish Water By Electrodialysis for Irrigation Use

Given that the ionic composition of desalinated seawater is inappropriate for irrigation because of the high sodium adsorption ratio (SAR) and the lack of essential minerals (calcium, magnesium, and sulfate), brackish water is added back into the desalinated seawater to provide these minerals. However the chloride added back with the minerals increases plant irrigation water consumption. Applying electrodialysis to the partial desalination of brackish water allows partial removal of the chloride. If appropriate ion exchange membranes are chosen, this can be done without losing too much of the divalent ions.

In order to upgrade marginal waters (e.g. saline ground water) for irrigation, it was necessary to gain a better understanding on the ED separation process between beneficial ions (i.e., Calcium and Magnesium) and other ions such as chlorides and sodium. The ions separation study was carried out progressively by examining three types of commercial monovalent selective ion exchange membranes (MIEM); Neosepta CMS/ACS by Astom Co., Selemion CSO/ASA by Asahi Glass, and MVK/MVA by PCCell using two ED cell units (PCCell ED200 and PCCell ED 64). Primarily, the ED experiments focused on ED performance under various operating conditions of current density and feed ionic strength. The influence of current density was studied on Selemion CSO/ASA membranes by operating the ED stack at currents level on the scale between 0.3-1.4 of I/I_lim, where Ilim refers to limiting current based only on sodium ion content of the feed solution. The effect of feed water initial ionic strength was studied on the Neosepta CMS/ACS by assessing separation and membrane selectivity using an artificial solution with significantly higher ionic strength. Furthermore, membrane permselectivity was compared by means of separation efficiency of different pairs of ions namely Cl versus SO₄, and Na versus Ca and Mg.

ED was successfully implemented in an automated batch system (0.2 m² cell pairs) equipped with monovalent selective membranes to partially desalinate the brackish water from the Mashabe Sade well field, reducing sodium chloride levels while retaining almost all of the sulfate and significant amounts of magnesium and calcium. Choosing appropriate ion exchange materials and conditions resulted in reducing the SAR of treated well water from 12.3 to 2.4.