Evaluation and optimization of bioretention design for nitrogen and phosphorus removal

Laboratory and field investigations were conducted into the performance of modified bioretention system designs to reduce nutrient loads from stormwater runoff. Bioretention design characteristics of particular interest were filter media composition and structural configuration. A filter media admixture of raw aluminum-based water treatment residuals (WTR) in bioretention soil mix effectively adsorbed orthophosphate in laboratory tests (90-99% median removal efficiencies). WTR dewatered to 33% solids demonstrated consistently higher removal efficiencies (\u3e99%). A bioretention system constructed in Durham, NH in 2011 includes a bioretention soil mix with raw WTR admixture (9% solids) and a structural design modeled after the UNHSC subsurface gravel wetland with an internal storage reservoir to promote denitrification under anaerobic conditions. Bioavailable nutrients, orthophosphate and nitrate, were generally reduced in this system, with median removal efficiencies of 20% and 60%, respectively. This modified system achieved median removal efficiencies of 55% and 36% for total phosphorus and total nitrogen, respectively