Effectiveness of Denitrifying Bioreactors on Water Pollutant Reduction from Agricultural Areas

HighlightsDenitrifying woodchip bioreactors treat nitrate-N in a variety of applications and geographies.This review focuses on subsurface drainage bioreactors and bed-style designs (including in-ditch).Monitoring and reporting recommendations are provided to advance bioreactor science and engineering. Denitrifying bioreactors enhance the natural process of denitrification in a practical way to treat nitrate-nitrogen (N) in a variety of N-laden water matrices. The design and construction of bioreactors for treatment of subsurface drainage in the U.S. is guided by USDA-NRCS Conservation Practice Standard 605. This review consolidates the state of the science for denitrifying bioreactors using case studies from across the globe with an emphasis on full-size bioreactor nitrate-N removal and cost-effectiveness. The focus is on bed-style bioreactors (including in-ditch modifications), although there is mention of denitrifying walls, which broaden the applicability of bioreactor technology in some areas. Subsurface drainage denitrifying bioreactors have been assessed as removing 20% to 40% of annual nitrate-N loss in the Midwest, and an evaluation across the peer-reviewed literature published over the past three years showed that bioreactors around the world have been generally consistent with that (N load reduction median: 46%; mean ±SD: 40% ±26%; n = 15). Reported N removal rates were on the order of 5.1 g N m-3 d-1 (median; mean ±SD: 7.2 ±9.6 g N m-3 d-1; n = 27). Subsurface drainage bioreactor installation costs have ranged from less than $5,000 to$27,000, with estimated cost efficiencies ranging from less than $2.50 kg-1 N year-1 to roughly$20 kg-1 N year-1 (although they can be as high as $48 kg-1 N year-1). A suggested monitoring setup is described primarily for the context of conservation practitioners and watershed groups for assessing annual nitrate-N load removal performance of subsurface drainage denitrifying bioreactors. Recommended minimum reporting measures for assessing and comparing annual N removal performance include: bioreactor dimensions and installation date; fill media size, porosity, and type; nitrate-N concentrations and water temperatures; bioreactor flow treatment details; basic drainage system and bioreactor design characteristics; and N removal rate and efficiency

Denitrifying bioreactors - Expanding applications to stormwater

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