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

Denitrifying woodchip bioreactors (WBRs) remove nitrate (NO3-) from agricultural drainage water at field-scale, but their efficacy at cold temperatures remains uncertain. This study shows how hydraulic residence time (HRT) controls NO3- removal and environmental side-effects of WBRs at low water temperature under pilot-scale conditions with controlled operation of nine WBRs (94 dm(3)). Hydraulic properties were assessed by a bromide tracer test, and NO3- 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 NO3- concentrations were increasingly reduced at higher HRTs. The relationship between HRT and the efficiency (%) of NO3- removal was linear (R-adj(2) = 0.94), while the relationship between HRT and NO3- reduction rates (NRR) was logistic (R-adj(2) = 0.88). Gaseous emissions of N2O were equally low at HRT5 of 10-30 h, but higher at 5 h (P< 0.05). Methane fluxes were small, but with consistent emissions at HRT5 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 HRT5 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. [GRAPHICS] .