Designing metallic iron based water filters: Light from methylene blue discoloration

Available water filtration systems containing metallic iron (Fe0 filters) are pragmatically designed. There is a lack of sound design criteria to exploit the full potential of Fe0 filters. A science-based design relies on valuable information on processes within a Fe0 filter, including chemical reactions, hydrodynamics and their relation to the performance of the filter. The aim of this study was to establish a simple method to evaluate the initial performance of Fe0 filters. The differential adsorptive affinity of methylene blue (MB) onto sand and iron oxide is exploited to characterize the evolution of a Fe0/sand system using the pure sand system as operational reference. Five systems were investigated for more than 70 days: pure sand, pure Fe0, Fe0/sand, Fe0/pumice and Fe0/sand/pumice. Individual systems were characterized by the extent of changes in pH value, iron breakthrough, MB breakthrough and hydraulic conductivity. Results showed that for MB discoloration (i) pure sand was the most efficient system, (ii) hybrid systems were more sustainable than the pure Fe0 system, and (iii) the pores of used pumice are poorly interconnected. Characterizing the initial reactivity of Fe0 filters using MB discoloration has introduced a powerful tool for the exploration of various aspects of filter design.