Defluoridation of reject water from a reverse osmosis unit and synthetic water using adsorption

Abstract

Many parts of the world have excess fluoride in drinking water. At some locations, nitrate is also in excess. Hence, reverse osmosis (RO) units have been installed at several villages in India. Reverse osmosis is a good technique, but it has the disadvantage of discarding a considerable amount of the inlet water as a reject stream. This is an unsustainable way of using water. Two adsorbents, namely, a hybrid anion exchange resin embedded with zirconium oxide nanoparticles (HAIX-Zr), and activated alumina (AA) were used in column experiments. For water containing only F−, HAIX-Zr had a better capacity than AA. The same trend was observed with synthetic water samples containing other ions in addition to F-. However, for RO reject, the converse was true, and the capacities of AA and HAIX-Zr decreased significantly. For AA, the presence of a small concentration of HCO3− increased the uptake of F- by 100% compared to water containing only F-. For HAIX-Zr, the adsorption capacity decreased as the concentration of co-ions increased. The cost of treated water varied from ▪ 0.1–1.5/L (US $ 0.002–0.03/L) for AA and ▪ 0.2–11.5/L (US $ 0.004–0.23/L) for HAIX-Zr.