Optimizing Biofiltration Media for the Capture of Phosphate and the Support of Vegetation Growth

Abstract

AbstractBiofiltration is a stormwater management practice designed to treat runoff for harmful contaminants. A critical component of these systems is the granular media. In this study, I investigated six different types of base media (10% leaf compost, 20% leaf compost, 10% food compost, 20% food compost, sphagnum peat, reed sedge peat) and four different amendments (spent lime, biochar, iron and sphagnum peat) in various combinations with sand to test their capacity in mitigating phosphate release and supporting the growth of Switchgrass (Panicum virgatum) as an indicator of potential to support vegetation. The study consisted of an outdoor mesocosm experiment over three years, with 34 events total where 30 seeded mesocosms containing various media mixes received water from the Mississippi River (near downtown Minneapolis, MN, USA) that was spiked with phosphorus to simulate stormwater inputs. Soluble reactive phosphorus and nitrate concentration, pH, Switchgrass height over time, and Switchgrass biomass at senescence was measured each year. In general, mixes containing food compost, leaf compost, biochar with compost and spent lime with compost leached phosphorus whereas mixes containing peat, iron with compost, and sand adsorbed phosphorus. The mixes that leached phosphorus supported the most plant growth. Spent lime mixes and biochar mixes had the highest effluent nitrate concentrations indicating effects on N mineralization or nitrification. The iron and leaf compost layered media mix performed the best of all the mixes tested in terms of mitigating the release of phosphate and having the potential to support vegetation. Future research is necessary to determine if it can continue preventing phosphorus leaching and support vegetative growth with a higher percentage of compost and different plant species. Keywords: phosphate, nitrate, biofiltration, stormwater treatment, switchgrass (Panicum virgatum), compost, peat, biochar, water treatment residuals, iron