Effects of Bioretention Cell Media Composition on the Removal of Nitrogen and Phosphorus
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Bioretention cells are engineered systems used in urban areas as stormwater treatment to remove unwanted nutrients from runoff. The objectives of the paper are to examine existing studies to determine effects of media composition, media depth, specific species of plants, and temperature on the removal/immobilization of nitrogen (N) and phosphorus (P) in bioretention cells. After reviewing studies performed to date, a number of conclusions can be reached regarding recommendations for bioretention cells to remove N and P. More times than not, N is removed through nitrification-denitrification within the media, and P is removed through sorption. Multiple studies investigating the appropriate media textures concluded that a sandy loam or sandy soil with the addition of at least 3% water treatment residuals (WTR) should be used in bioretention cells to help with removal of P. A specific organic matter (OM) recommendation could not be determined, but a positive correlation with NH4 removal supports the Virginia Department of Conservation and Recreation guidelines of 3-5% addition of OM. N removal was higher when a saturated zone plus organic matter was used versus an unsaturated cell zone without organic matter. The pH of the bioretention cell should be between 6 and 7. The suggested media depth should be 50 to 90 cm. Certain species perform better with respect to nutrient removal efficiencies, and it is recommended that Carex spp. be utilized when possible. Nitrogen removal is higher when both vegetation and a saturated zone are present in a bioretention cell. Native species are convenient for cell maintenance purposes, but not all native species help remove N and P more efficiently. Although temperature does have an effect on nitrification-denitrification and P sorption through slowing infiltrations rates because of frozen soil, the results show that overall performance of the bioretention cell is not negatively affected.