Removal of fluoride in membrane-based water and wastewater treatment technologies: Performance review

The presence of excess fluoride in aqueous media above local environmental standards (e.g., the U.S. Environmental Protection Agency (EPA) standard of 4 mg/L) affects the health of aquatic life. Excess fluoride in drinking water above the maximum contaminant level (e.g., the World Health Organization (WHO) standard of 1.5 mg/L) also affects the skeletal and nervous systems of humans. Fluoride removal from aqueous solutions is difficult using conventional electrochemical, precipitation, and adsorption methods owing to its ionic size and reactivity. Thus, new technologies have been introduced to reduce the fluoride concentration in industrial wastewater effluents and various drinking water sources. Membrane technology is one of the newer technologies found to be very effective in significantly reducing fluoride to desired standards levels; however, it has received less attention than other technologies because it is perceived as a costly process. This study critically reviewed the performance of various membrane process and compared it with effluent and zero liquid discharge (ZLD) standards. The performance review has been conducted with the consideration of the theoretical background, rejection mechanisms, technical viability, and parameters affecting flux and rejection performance. This review includes membrane systems investigated for the defluoridation process but operated under pressure (i.e., reverse osmosis [RO] and nanofiltration [NF]), temperature gradients (i.e., membrane distillation [MD]), electrical potential gradients (i.e., electrodialysis [ED] and Donnan dialysis [DD]), and concentration differences (i.e., forward osmosis [FO]). Moreover, the study also addressed the advantages, limitations, & applicable conditions of each membrane based defluoridation process.