Submitted to: Ecological Engineering
Publication Type: Review Article
Publication Acceptance Date: 4/2/2010
Publication Date: 9/17/2010
Citation: Schipper, L.A., Robertson, W., Gold, A.J., Jaynes, D.B., Cameron, S.C. 2010. Denitrifying Bioreactors – An Approach for Reducing Nitrate Loads to Receiving Waters. Ecological Engineering. 36:1532-1543. Interpretive Summary:
Technical Abstract: Low-cost and simple technologies are needed to reduce watershed export of excess nitrogen to sensitive aquatic ecosystems. Denitrifying bioreactors are an approach where solid carbon substrates are added into the flow path of contaminated water. These carbon substrates (often fragmented wood-products) act as an energy source to support denitrification; the conversion of nitrate to nitrogen gases. Here, we summarize the different designs of denitrifying bioreactors, their hydrological connections, effectiveness, and factors that limit their performance. The main denitrifying bioreactors are: denitrification walls (intercepting groundwater), denitrifying beds (intercepting point source discharges) and denitrifying layers (intercepting soil leachate). Both denitrifcation walls and beds have proven successful in appropriate field settings with nitrate removal rates generally ranging from 0.01 to 3.6 g N m-3 d-1 for walls and 2 to 22 g N m-3 d-1 for beds, with the lower rates often associated with nitrate-limitations. Nitrate removal is also limited by the rate of carbon supply from degrading substrate and removal is operationally zero-order with respect to nitrate concentration primarily because the inputs of nitrate into studied bioreactors have been generally high. In bioreactors where nitrate is not fully depleted, removal rates generally increases with increasing temperature. Nitrate removal has been supported for up to 15 years without further maintenance or carbon supplementation because wood chips degrade sufficiently slowly under anaerobic conditions. There have been few field-based comparisons of alternative carbon substrates to increase nitrate removal rates but laboratory trials suggest that some alternatives could support greater rates of nitrate removal (e.g., corn cobs and wheat straw). Denitrifying bioreactors may have a number of adverse effects, such as production of nitrous oxide and leaching of dissolved organic matter (usually only for the first few months after construction). The relatively small amount of field data suggests that these problems can be adequately managed or minimized. An initial cost/benefit analysis demonstrates that denitrifying bioreactors are as cost effective and complementary to other agricultural management practices aimed at decreasing nitrogen loads to surface waters. We conclude with recommendations for further research to enhance performance of denitrifying bioreactors.