Aluminum hydroxide-coated pumice and biochar enhance biosand filter defluoridation and water treatment

Abstract

Chronic exposure to high fluoride levels in drinking water can cause serious health effects, including skeletal and dental fluorosis. Although conventional biosand filters (BSFs) can provide safe drinking water at the household scale, it has a low fluoride adsorption capacity. In this study, fluoride removal in BSF was enhanced by using aluminum hydroxide coated materials, such as pumice and biochar in place of sand. Bench-scale BSFs were set up with (a) uncoated sand as a control, (b) coated pumice, and (c) coated biochar. BSFs were charged with fluoride spiked surface water (≥ 5 mg/L) for more than a year. While fluoride breakthrough was observed in the sand column on day 49, fluoride concentrations remained below the World Health Organization limit (< 1.5 mg/L) for 313 and 418 days for coated biochar and coated pumice, respectively. Columns with coated media also effectively removed turbidity, fecal indicator bacteria, and organic matter and maintained acceptable filtration rates (> 0.25 m/h) without requiring frequent cleaning. Although pumice had a higher fluoride adsorption capacity (1.1 mg/g) than biochar (0.94 mg/g), filtered water pH in the pumice column was unacceptable (4.18 ± 0.25). However, a post-treatment process, using an oyster shell-filled column, restored pH to within drinking water standards without compromising water quality.