Development of an additional step to current CO 2-based CaCO 3(s) dissolution post-treatment processes for cost-effective Mg 2+ supply to desalinated water

Abstract

New water quality standards, specific for desalinated water, have been recently approved in Israel. To satisfy the new quality criteria a novel post-treatment process has been developed, aimed at cost-effectively meeting the new standards as well as supplying Mg 2+ ions, required for both health and agricultural reasons. The paper introduces a modification to the original process. In the modified process calcite is dissolved using CO 2 (instead of H 2SO 4, as in the original process), and/or a combination of CO 2 and H 2SO 4. An additional presented feature is the option to elevate product water pH by CO 2 stripping rather than NaOH dosage. The modified process can be implemented as an add-on to existing CO 2-based calcite dissolution post-treatment systems or in cases where a restriction is posed on the total hardness value in the product water, since the modification extends the flexibility in the product water quality, with respect to the ratio attained between total hardness and alkalinity concentrations. Cost estimation reveals that upgrading a conventional post-treatment process based on CO 2 dissolution of CaCO 3 to the modified process for attaining [Mg 2+] of 10 mg/l, can result in an increase in the operational costs of between 0.15 and 0.69 $cents per m 3 of product water, depending on the cost of chemicals.