Removal of sulfate and chloride ions from reverse osmosis concentrate using a two-stage ultra-high lime with aluminum process

Abstract

Reverse osmosis (RO) concentrate generated from RO desalination is necessary to be managed due to its discharge toxicity and mineral scaling. In this work, the removal of SO42− and Cl− from RO concentrate and further improvement of water quality were systematically investigated using the two-stage ultra-high lime with aluminum (UHLA) process and carbonation post-process, respectively. Firstly, batch results of the one-stage UHLA process proved the competitive inhibition effect of coexisting SO42− on Cl− removal. Thus, a two-stage UHLA process was designed to preferentially precipitate SO42− as ettringite (Ca6Al2(SO4)3(OH)12) in stage I and then dislodge partial Cl− as Friedel's salt (Ca4Al2Cl2(OH)12) in stage II. Results showed that almost all of SO42− and 77.8 % of Cl− could be removed at the optimal conditions. And the XRD and SEM analysis results showed formation of ettringite and Friedel's salt with good crystal morphology in stage I and stage II, respectively. This technology was further applied in the real RO concentrate treatment and achieved almost complete SO42− removal and high Cl− removal of 70.5 %, which confirmed the feasibility and effectiveness of the two-stage UHLA process. Finally, the carbonation post-process was applied to precipitate remaining Ca2+ as well as lower the pH and conductivity, so as to improve the treated water quality. This two-stage UHLA process is promising to be applied to saline wastewater containing high concentrations of SO42− and Cl−.