Enhanced Nutrient Management of Stormwater through a Field Demonstration of Nitrogen Removal in a Modified Bioretention System

Excess nutrients in stormwater runoff may lead to eutrophic areas, negatively impacting ecosystems, social, and economic well-being. A solution to remove nutrients such as nitrogen from stormwater may be the use of bioretention, a low impact development (LID) technology. However, conventional bioretention systems are not designed specifically to remove or recover nitrogen. Nitrogen removal in a bioretention system can be improved by modifying the conventional system to include an internal water storage zone (IWSZ) that contains an electron donor (e.g., organic carbon source from wood chips) to enhance denitrification. This study provides evidence that modified bioretention systems are a promising LID technology for reducing nitrogen loads to downstream waters. The study demonstrates the nitrogen removal performance of field-scale conventional and modified bioretention systems in the subtropical climate of Tampa, FL. Both systems are similar in design except the modified system has an upturned elbow and is 0.3 m deeper to accommodate the IWSZ. Initial results show greater total nitrogen mass removal in the modified (73%) compared to the conventional (62%) system. This is likely due to the enhanced denitrifying conditions and increased hydraulic detention time associated with the IWSZ. Furthermore, nitrogen removal in the modified system was observed to be greater during the initial period of a storm event compared to the final period. The results also demonstrate that careful attention to design considerations can reduce nitrogen removal performance variability, which has been observed in prior modified bioretention studies.