Nitrate removal and environmental side-effects controlled by hydraulic residence time in woodchip bioreactors treating cold agricultural drainage water

Abstract

ABSTRACT Denitrifying woodchip bioreactors (WBRs) remove nitrate () from agricultural drainage water at field-scale, but their efficacy at cold temperatures remains uncertain. This study shows how hydraulic residence time (HRT) controls removal and environmental side-effects of WBRs at low water temperature under pilot-scale conditions with controlled operation of nine WBRs (94 dm3). Hydraulic properties were assessed by a bromide tracer test, and removal, emissions of nitrous oxide (N2O) and methane (CH4), and losses of dissolved organic carbon (DOC) were measured at HRTs of 5–30 h. Inlet concentrations were increasingly reduced at higher HRTs. The relationship between HRT and the efficiency (%) of removal was linear ( = 0.94), while the relationship between HRT and reduction rates (NRR) was logistic ( = 0.88). Gaseous emissions of N2O were equally low at HRTs of 10–30 h, but higher at 5 h (P < 0.05). Methane fluxes were small, but with consistent emissions at HRTs of 20–30 h and uptake at 5–15 h. HRT had limited effect on effluent DOC concentrations, but strong effect on mass losses that were five-fold higher (320 mg L−1) at the HRT of 5 h than at 30 h. In summary, at cold temperatures HRTs of ≤ 20 h resulted in suboptimal NRR, accelerating DOC losses, and increased risk of N2O losses at least below a threshold HRT of 5–10 h. HRTs of 20–30 h gave maximal NRR, smallest losses of DOC and N2O, but an increased risk of CH4 emissions.