Reducing Stormwater Nitrogen with Denitrifying Bioreactors: Florida Case Study

Abstract

Growth of suburban areas has led to significant nonpoint source nitrate (NO3−) pollution. Wet detention ponds reduce peak flows but are not designed to reduce NO3− loading downstream. Denitrifying bioreactors have shown high rates of NO3− removal in runoff and effluent from agricultural fields. Bioreactors modified for wet detention ponds may improve NO3− treatment in suburban settings. This study retrofitted two wet detention ponds with submerged denitrifying bioreactors and monitored water quality for 1 year. Samples taken after installation showed significant and persistent NO3− reductions compared with preinstallation measurements. This was coupled with decreased chlorophyll-a levels, suggesting a concurrent reduction of algae growth. High levels of dissolved organic carbon and reduced sulfate corroborate denitrification as the likely removal pathway. Estimates of the NO3− removal rate were much higher than observed in agricultural bioreactors. This may be due to increased pondwide denitrification supported by the release of dissolved carbon from the bioreactors into the surrounding pond. Further installations, broader sampling regimes, and longer monitoring are necessary to confirm the viability of bioreactor retrofits in wet detention ponds.