Determining the optimum Fe-o ratio for sustainable granular Fe-o/sand water filters

International audiencePacked beds of metallic iron (Fe0) and sand are tested for their efficiency at discolouring an aqueous methylene blue (MB) solution (2.0 mg L-1) in gravity driven systems for up to 95 days. The aim was to determine the optimal Fe0/sand ration for sustainable filters. Six different Fe0/sand volumetric ratios were explored: 0/100, 20/80, 30/70, 40/60, 60/40 and 100/0. The columns were characterized by (i) the time-dependant extent of MB discoloration and (ii) the evolution of the hydraulic conductivity (permeability). Results clearly showed increased permeability loss with increasing Fe0 ratio. The Fe0/sand ratio dependent extent of MB discoloration was not monotone. These observations corroborated the working hypothesis that properly designing a Fe0/sand filters is finding a compromise between: (i) increased sustainability by lowering Fe0 ratios and (ii) decreased efficiency by lowering Fe0 ratios. This work provided the first experimental evidence for an optimal Fe0/sand volumetric ratio of 25/75. This result will accelerate efforts for non site-specific system design