Development of a Treatment Process for Electrodialysis Reversal Concentrate with Intermediate Softening and Secondary Reverse Osmosis to Approach 98-Percent Water Recovery

Abstract

The United States Army is constructing a new water-treatment facility for Fort Irwin/National Training Center in the Mojave Desert region of southern California to address existing regulatory requirements and to account for anticipated expansion at the installation. The proposed treatment, electrodialysis reversal (EDR), is anticipated to recover 92% of the influent water. The ultimate goal was to achieve 99% recovery, which required additional recovery of the EDR concentrate. This paper describes laboratory testing of conventional water-treatment methods to achieve water recovery beyond standard practice. The effectiveness of lime softening followed by secondary reverse osmosis (RO) was evaluated to treat the concentrate stream and recover additional water to approach 98%. Partial lime softening at dosages of 500−2,000 mg/L of hydrated lime was capable of removing hardness from simulated EDR concentrate. Adding magnesium chloride to the lime softening step increased silica removal, bringing SiO2 concentrations in the simulated EDR concentrate from 110 to 6.8 mg/L at room temperature. The resulting treated water was suitable for effective reverse osmosis with a standard seawater polyamide membrane. Rejection for all of the dissolved constituents was well above 90% with the exception of arsenic, which was reduced from 50 μg/L to levels on the order of 20 μg/L. To achieve 99% recovery, mechanical vapor recompression is being considered to further recover the concentrate from the RO unit, although this unit process was not evaluated in the research reported in this paper.