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 United States is guided by the United States Department of Agriculture Natural Resources Conservation Service 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 between 20 to 40% of annual nitrate-N loss in the Midwest, and an evaluation across peer-reviewed literature published over the past three years showed bioreactors around the world have been generally consistent with that (N load reduction median: 46%; mean ± st dev: 40±26%; n = 15). Reported N removal rates were on the order of 5.1 g N/m3-d (median; mean ± st dev: 7.2±9.6 g N/m3-d; 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 N-y to roughly $20/kg N-y (although they can be as high as $48/kg N-y). A suggested monitoring set-up 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.
- Non-point source pollution
- Subsurface drainage