Assessment of physical techniques to regenerate active slag filters removing phosphorus from wastewater

Active slag filters are an emerging technology for removing phosphorus (P) from wastewater. Currently, the lifespan of these filters is limited by their available reactive sites. An increasing number of researchers suggest that drying active filters can renew their P removal capacity, although there has only been one research paper that has achieved regeneration. Hence, this study investigated techniques to regenerate the P removal efficiency of exhausted melter slag filter media which had successfully treated effluent for 5 years in the field before becoming inactive. Several techniques were performed to expose fresh surface sites on the slag, including: (1) drying; (2) agitation; and (3) crushing of the slag granules. Crushing resulted in the best regeneration of P removal efficiency by the slag, however, after just 2 months the renewed P removal efficiency of the crushed slag fell back to a similar level to that exhibited by exhausted slag. Furthermore, none of the other physical methods caused long-term rejuvenation of the exhausted slag to remove P from effluent. Scanning electron microscopy revealed that none of the regeneration techniques exposed substantial new phosphorus adsorption sites on the slag granules. These findings therefore challenge the validity of the existing dogma that active slag filters can be effectively regenerated by simple physical treatments, indicating further research is required to optimise active filter performance.