Denitrifying bioreactors typically comprise a buried bed of woodchips that receive a portion of tile drainage flows from an adjoining field. The woodchips provide a carbon source, which combined with the reducing (oxygen limiting) conditions in the saturated subsurface environment, encourage naturally occurring bacteria to reduce nitrate to di-nitrogen gas in a stepwise process (denitrification). A number of recently published studies on this practice are available; Schipper et al. (2010) provided a good overview. Field performance studies (e.g., Christianson 2012) have shown a range of efficiencies. In general, hydraulic retention times between 8 and 18 hrs are needed to reduce nitrate concentrations by half. Assuming a 12 hour hydraulic retention time, Moorman et al. (2015) found that nitrate loads in tile drainage can be reduced by 20-30% by bioreactors of 1 m (3.3 ft) depth and occupying <0.3% of the drainage area. We locate bioreactors sized at 0.5% of the field drainage area to account for construction disturbance and the possibility to allow retention times >12 hrs in the actual design, which may be needed where increased drainage volumes occur under cold conditions that slow microbial processes. Hydraulic retention time requirements in bioreactors are a subject of ongoing research (Moorman et al., 2015).

Christianson, L., A. Bhandari, M. Helmers, K. Kult, T. Stuphin, and R. Wolf. 2012. Performance evaluation of four field-scale agricultural drainage denitrification bioreactors in Iowa. Trans. Am. Soc. Agric. Biol. Eng. 55: 2163–2174.

Moorman, T.B., M.D. Tomer, D.R. Smith, and D.B. Jaynes. 2015. Evaluating the potential role of denitrifying bioreactors in reducing watershed-scale nitrate loads: A case study comparing three Midwestern (USA) watersheds. Ecological Engineering. 75: 441-448.

Schipper, L.A., W.D. Robertson, A.J. Gold, D.B. Jaynes, and S.C. Cameron. 2010. Denitrifying bioreactors – an approach for reducing nitrate loads to receiving waters. Ecol. Eng. 35(11): 1532–1543.