A different approach for brackish-water desalination, comprising acidification of the feed-water and CO2(aq) reuse for alkalinity, Ca2+ and Mg2+ supply in the post treatment stage

An alternative approach is presented for the operation of the pre- and post-treatment stages of desalination plants fed by brackish waters characterized by a high carbonate concentration. A strong acid (either HCl or H2SO4) is dosed to the feed water with the aim of converting HCO3− to CO2(aq) thereby eliminating potential precipitation of solids and scale inhibitor requirement. CO2(aq) is a small, uncharged molecule, which readily passes RO membranes. Consequently, since the CO2(aq) concentration in the feed/brine water closely equals the CO2(aq) concentration that develops in the permeate water, the corresponding negative calcium carbonate precipitation potential values in the permeate is further utilized in the post treatment stage for enhancing CaCO3(s) dissolution, resulting in supply of carbonate alkalinity, Ca2+ ions and also (indirectly) Mg2+ ions, using the “calcite dissolution–ion exchange” process introduced in previous works. Thereby, the dosage of the strong acid to the feed water accomplishes two goals, making the approach cost effective. The paper addresses experimental (at both laboratory and full scale) and theoretical aspects of the proposed process, as well as assessment of engineering and economic feasibility.

► Dosage of 155 mg/l HCl to BWRO feed was shown to eliminate antiscalant requirement.
► HCl dosage also enabled reusing CO2 within CaCO3 dissolution post treatment process.
► 155 mg HCl/l into feed water enabled dissolving 260 mg CaCO3/l in post treatment step.
► The attained high [Ca2+] can be exchanged with [Mg2+] for improved water quality.
► The OPEX of the proposed method (acidification and Mg2+ supply) was 2.29 $cent/m3.