Denitrifying Bioreactors: A Synthesis of Removal Rates, Controls and Utility.

Denitrifying bioreactors are designed to intercept water containing elevated nitrate concentrations and convert this nitrate to nitrogen gas reducing loads on receiving waters. There are many potential designs but typically these systems contain a slowly degrading carbon material (e.g., wood chips, sawdust or cobs) that support the growth and activity of denitrifying bacteria. These systems have been installed in a variety of hydraulic settings including interception of tile drains, groundwater, streams and wastewaters. Rates of nitrate removal can vary between bioreactors and through time depending on temperatures, nitrate concentration, flow rates and the carbon sources used. Across studies, nitrate removal rates averaged 3.4 g m^-3 bioreactor d^-1. In general, removal rates increase with increasing temperature (roughly doubling for 10°C increase) when nitrate concentrations were non-limiting. When nitrate concentrations are high, removal rates are not concentration-dependent, concentrations at which removal becomes nitrate limited have not been well quantified. A range of carbon sources have been shown to support different rates of nitrate removal and it is expected that as removal rate increases the longevity of the bioreactor is likely to decline. Therefore construction needs to take into account both nitrate removal rate and expected frequency of carbon renewal.  Bioreactors made using wood chips have not yet been observed to fail and have remained functional in excess of 14 years. In comparison to other N control strategies in agricultural systems, bioreactors are cost competitive ranging $2-15 (USD) per kg N removed depending on the degree of land owner involvement in construction. There are potential adverse effects that may require mitigation strategies to be developed; such as, an initial release of dissolved organic carbon, greenhouse gas emissions (particularly N₂O) and formation of methyl mercury. Overall, denitrifying bioreactors are simple in principle, are being installed in a variety of locations and, as such, are an additional tool for land managers to address nitrate loads into receiving waters.