Denitrifying bioreactors have proven effective at reducing nitrate loads from agricultural tile drainage. However, flows associated with storm events can cause conditions that may decrease the effectiveness of the bioreactors for nitrate reduction by decreasing hydraulic retention time. Stormflow may also shift flow paths, alter chemistry, and cause sloughing of biofilm and microbes in the bioreactor. As storms can contribute significantly to annual loads of excess nitrate, the ability for management practices to address stormflow is crucial.

In this research, field and lab bioreactors were observed during stormflows to compare how performance, as measured by removal rate and removal efficiency, was impacted by varied inflow hydrographs. The field study showed that removal rate significantly increased during peak flows but removal efficiency decreased both during and after storms events. The lab study confirmed this trend and found removal rate was most closely associated with internal flow patterns. When bioreactors exhibited predominantly distributed flow rather than preferential flow, event-averaged removal rate and efficiency were both significantly higher. Both studies of novel applications of bioreactors in stormwater infrastructure demonstrated effectiveness beyond agricultural fields with removal rates higher than agricultural bioreactors. The submerged bioreactors reduced nitrate in wet detention ponds below recommended levels within one month of installation. This also reduced chlorophyll-a levels. The ditch bioreactor was able to significantly reduce nitrate loads even during stormflows despite the small size. During peak flows, instantaneous removal rate was orders of magnitude higher than previously reported. This work confirms that denitrifying bioreactors are an effective management strategy for reducing nitrate load in stormwater though peak flow rates can cause disruption of high denitrification. Wider application of bioreactors will reduce excess nitrate pollution reaching receiving water bodies and improve water quality.