Determining the optimum Fe0 ratio for sustainable granular Fe0/sand water filters

• Fe0/additives mixtures are widely used for aqueous contaminant removal.
• Reliable methods for designing Fe0 filtration systems are still lacking.
• Additives have been mostly used to account for construction width requirements.
• Various Fe0/sand ratios were tested for dye discoloration in dynamic systems.
• Results revealed the 25% Fe0 system as the most efficient and sustainable.

Packed beds of metallic iron (Fe0) and sand are tested for their efficiency at discoloring 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 ratio 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 filter 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 nonsite-specific system design.