An Insight into the Observed Discrepancies in the Effect of Biochar on Removing Nitrate in the Denitrifying Bioreactors

High nitrogen concentrations in agricultural drainage waters are in part responsible for formation of dead zones in the oceans. To reduce nitrate concentrations, denitrifying bioreactors are increasingly being used in agricultural tile effluent. During high flow events the efficiency to remove nitrate decreases in these reactors. This study examines whether biochar can increase the nitrate ( removal in short HRTs by 2 mg L-1 d-1, it decreased the removal of nitrate in longer HRTs by 20%. Better performance of the biochar amended bioreactors in short HRTs was attributed its ability to lower the dissolved oxygen in the bioreactors more efficiently. With the elimination of dissolved oxygen from the influent, no significant difference was observed between the biochar amended bioreactors and the non-amended members. However, in long retention times, the biochar amended bioreactors performed poorly, possibly due to biochar not being bioavailable to denitrifiers. Therefore, its addition to the bioreactors decreased the share of the organic matter bacteria would otherwise obtain.