Denitrifying Bioreactors for Nitrate Removal: A Meta-Analysis of Controlling Factors.

Carbon substrate bioreactors that denitrify nitrate are an edge-of-field technique receiving extensive study, however, there are many questions about controlling factors on their performance. Bioreactors are typically beds or walls filled with wood chips, with the former intercepting tile drainage lines and potentially receiving large loads of nitrate. A meta-analysis (using the MetaWin software program) was conducted using published literature to examine the effects of various controlling factors on nitrate removal rates (g m^-3 d^-1), including: temperature, nitrate concentration, retention time, age, and bed or wall design. Data were coded into categories to allow for a detailed examination of each of these factors on nitrate removal rates. Mean effect sizes and bias-corrected 95% confidence intervals were calculated for each category using a bootstrapping procedure. As expected, both higher temperatures and greater nitrate concentrations led to greater nitrate removal rates. However, other factors such as bed age (in years) and retention time also were factors in nitrate removal rates. Results demonstrate the importance of all studies reporting nitrate removal rates using a common metric (g m^-3 d^-1), as well as supporting information on temperature, bed age, and retention times. These results show the conservation community both how well this nitrate reduction technique can work, along with the many limitations observed.