Effect of Biochar on Nitrate Removal in a Pilot-Scale Denitrifying Bioreactor

Denitrifying bioreactors (DNBRs) harness the natural capacity of microorganisms to convert bioavailable nitrogen (N) into inert nitrogen gas (N) by providing a suitable anaerobic habitat and an organic carbon energy source. Woodchip systems are reported to remove 2 to 22 g N m d, but the potential to enhance denitrification with alternative substrates holds promise. The objective of this study was to determine the effect of adding biochar, an organic carbon pyrolysis product, to an in-field, pilot-scale woodchip DNBR. Two 25-m DNBRs, one with woodchips and the other with woodchips and a 10% by volume addition of biochar, were installed on the Delmarva Peninsula, Virginia. Performance was assessed using flood-and-drain batch experiments. An initial release of N was observed during the establishment of both DNBRs, reflecting a start-up phenomenon observed in previous studies. Nitrate (NO-N) removal rates observed during nine batch experiments 4 to 22 mo after installation were 0.25 to 6.06 g N m d. The presence of biochar, temperature, and influent NO-N concentration were found to have significant effects on NO-N removal rates using a linear mixed effects model. The model predicts that biochar increases the rate of N removal when influent concentrations are above approximately 5 to 10 mg L NO-N but that woodchip DNBRs outperform biochar-amended DNBRs when influent concentrations are lower, possibly reflecting the release of N temporarily stored in the biochar matrix. These results indicate that in high N-yielding systems the addition of biochar to standard woodchip DNBRs has the potential to significantly increase N removal.