Phosphorus Removal by Two Urban Stormwater Sand Filters Enhanced with Alum-Based Drinking Water Treatment Residuals

Stormwater runoff from urban areas threatens water quality and ecosystems around the world. For freshwater ecosystems, phosphorus (P) is often a primary concern, as excess P loading can cause eutrophication, symptoms of which include harmful algal blooms and oxygen depletion. Sand filters are stormwater management practices that trap particles and thereby reduce downstream sediment and P loads. However, sand typically has low P sorption capacity. To target both particulate and dissolved P species in stormwater, a P-sorbing material amendment can be added to sand filter media to increase P sorption capacity. This research centered on a field study of two urban stormwater sand filters enhanced with alum-based drinking water treatment residuals (DWTRs) (3%–5% of the sand layer by volume), a byproduct of drinking water treatment, to improve P removal in sand filter media under field conditions in Chittenden County, VT. The composition of influent stormwater was markedly different between sites, dominated by dissolved P at the residential site and mostly particulate P at the more industrial/commercial site. Due to this difference in influent water quality, >99% of the total P removed at the residential sand filter was in the form of dissolved P, while only 4% of the total P load removed at the industrial/commercial site was dissolved P. Because removal of dissolved P by sand filters tends to be negligible, the dissolved P load reductions observed at both sites are likely attributable to the DWTRs. Overall, the two systems reduced total P loads by 65%–78% during monitored events. This field study indicates that adding DWTRs to sand filter media is an effective way to couple both physical and chemical P removal mechanisms and thereby enhance water quality improvement performance. We provide guidance on future use of DWTRs in stormwater infrastructure based on our findings.