High Performance Phosphorus Removal Through use of Bioretention Soil Mix Amended with Phosphorus-Adsorbing Media

An increasing focus of water managers is the impairment of water bodies due to nutrient loads transported in stormwater runoff. Regional authorities for watersheds in the U.S. and Canada, including Chesapeake Bay and Puget Sound, have targeted phosphorus as a primary pollutant of concern. The widespread adoption of bioretention as a low-impact stormwater treatment practice has produced some well-documented water quality benefits, however, an increasing number of monitoring studies have detected substantial leaching of phosphorus from compost-containing bioretention installations.A study was undertaken by researchers at Fleming College in Ontario, Canada to assess the impact on phosphorus removal from artificial stormwater using bioretention soil amended with varying quantities of a commercially available adsorptive media, Imbrium® Systems Sorbtive® Media. Five bioretention cells were constructed, comprising a control of quartz sand and peat with no adsorptive media and four cells with the additive blended in at 3%, 5%, 10%, and 17% volume basis, respectively. Batches of artificial stormwater were spiked with potassium dihydrogen phosphate (KH2PO4) at concentrations of 0.2, 0.4, 0.6, and 0.8 mg/L (P-basis). Starting with the artificial stormwater containing the lowest concentration of phosphorus, each bioretention cell was subjected to a series of daily simulated storm events of controlled water volume for five consecutive days. This five-day simulated storm series was undertaken five times for each phosphate concentration. The total volume of spiked artificial stormwater applied to each cell was representative of two years of regional cumulative urban runoff for a drainage area five times the size of a bioretention cell.Influent and effluent samples were collected for each cell and analyzed for total phosphorus and total dissolved phosphorus. Study results demonstrated a very substantial improvement in phosphorus removal with the amended bioretention soil compared to the control. Over the course of the study, the control cell's total phosphorus removal efficiency decreased from 60% to 25%, while each of the amended cells maintained removal efficiency of up to 99% and at least 84% for the duration of the study. These results suggest that the adsorptive media utilized in this study, even when blended into the soil mix at only 3 - 5% volume basis, would be highly effective for improving phosphorus removal in bioretention installations.