Bioretention systems have many benefits including adding aesthetic value to properties, improving biodiversity, reduced flooding, and removing pollutants from stormwater runoff. However, when it comes to treating dissolved nitrogen species, a nutrient of concern in aquatic environments, conventionally designed bioretention do not consistently perform well. Conventional bioretention designs support nitrification which transforms nitrogen from ammonia to nitrate. An effective strategy to more consistently remove nitrogen from the effluent is by allowing for the biogeochemical cycle of nitrogen to complete its course of denitrification.  In this study, two bioretention designs, a conventional and a modified design, were compared side-by-side in the field in Tampa, FL. The modified system contains a supplementary layer to support denitrification. The media and design were first studied and tested in the laboratory. The optimal media was selected for further study of a bioretention in the field. The additional layer is called the internal water storage zone (IWSZ), a fully saturated region supplied with eucalyptus wood chips as a carbon source to promote denitrification. Dissolved organic carbon becomes bioavailable for denitrifying bacteria to transform nitrate coming from the sand layer to nitrogen gas and exit the system.  The results from two years of simulated storm events at three different hydraulic loading rates demonstrate that the biological and chemical processes that occur within the modified bioretention system have successfully removed over 75% of TN while the conventional system removed approximately 40%. These field results call for additional research to better understand how modified bioretention systems can be designed to optimize nitrogen removal with a changing climate. A more in-depth understanding of the different mechanism at play, such as the chemistry, microbiology, and hydrology, that impact the removal of nitrogen species, is necessary to properly design bioretention systems to consistently deliver a more reliable water supply into the environment.

From the laboratory to the field: Study of a modified denitrifying bioretention system. Innovative green stormwater infrastructure for nitrogen removal

2019 Fall National Meeting

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