Balancing Hydraulic Control and Phosphorus Removal in Bioretention Media Amended with Drinking Water Treatment Residuals

Green stormwater infrastructure such as bioretention can reduce stormwater runoff volumes and trap sediments and pollutants. However, bioretention soil media can have limited capacity to retain phosphorus (P) or even be a P source, necessitating addition of P-sorbing materials. We investigated the potential trade-off between P removal by drinking water treatment residuals (DWTRs) and hydraulic conductivity to inform the design of bioretention media. Batch isotherm and flow-through column experiments showed that P removal varied greatly among three DWTRs and across methodologies, which has implications for design requirements. We also conducted a large column experiment to determine the hydraulic and P removal effects of amending bioretention media with solid and mixed layers of DWTRs. When DWTRs were applied to bioretention media, their impact on hydraulic conductivity and P removal depended on the layering strategy. Although DWTR addition in solid and mixed layer designs improved P removal, the solid layer restricted water flow and exhibited incomplete P removal, while the mixed layer had no effect on flow and removed nearly 100% of P inputs. We recommend that DWTRs be mixed with sand in bioretention media to simultaneously achieve stormwater drainage and P reduction goals.