REVERSE-OSMOSIS TREATMENT OF MULTICOMPONENT ELECTROLYTE-SOLUTIONS

The treatment of multicomponent electrolyte solutions with two high-pressure thin-film composite reverse osmosis membranes was investigated. Experiments were performed with small membrane coupons and spiral-wound elements placed in series in a closed-loop test apparatus. The feed solutions investigated were representative of a pretreated agricultural drainage return water containing small quantities of nitrate and selenium, and large quantities of sulfate, chloride, magnesium, and sodium ions. Parameters affecting membrane performance included feed pressure, feed solution composition and concentration, concentration-polarization, membrane compaction, and the density of charged chemical groups contained in the active layers of the membranes. It was found that a charged RO membrane acted as an ion-exchange medium as well as a barrier to neutral ion-pairs. In general, the ability of both membranes to reject inorganic electrolytes were comparable in terms of solute rejections. However, each membrane gave superior rejections for some types of trace components. One membrane commonly gave better rejection of the nitrate ion ( > 99 percent), and the other membrane performed better with biselenite ( > 99.7 percent rejection). Both membranes removed selenate to about the same degree ( > 99.94 percent rejection).