Constant carbon dosing of a pilot-scale denitrifying bioreactor to improve nitrate removal from agricultural tile drainage

Abstract

Denitrifying bioreactors are effective tools for removing nitrate from agricultural drainage water. However, as woodchips age with time, a shortage of labile carbon supply limits nitrate removal by microbial denitrification when high nitrate pulses occur in drainage waters. In this study, we investigated the potential of methanol dosing at a constant rate to increase nitrate removal rates in a 58 m³ pilot-scale bioreactor (25 m³ saturated volume) installed on a dairy farm for two consecutive drainage seasons. The drainage water in the bioreactor had a mean hydraulic retention time (HRT) of 12.7 days and 13.5 days in the 2020 and 2021 drainage seasons. With 14.4 L of methanol solution (8% v/v) added per day, the seasonal nitrate removal rates were 8.6 g N m¯³ d¯¹ in 2020 and 5.1 g N m¯³ d¯¹ in 2021 when the methanol dosing rate was halved. Both rates (2020 and 2021) were enhanced as compared to seasonal rates of 0.67–1.60 g N m¯³ d¯¹ in previous years (2017 and 2018) when the bioreactor was not dosed. When there were very large pulses of nitrate into the bioreactor, which exhausted added methanol, nitrate was observed at concentrations above limiting ranges (> 3 mg N L¯¹) at the outlet on several occasions in 2021. Cumulative nitrate load reductions of 1859 g N and 1620 g N occurred in 2020 and 2021, respectively, resulting in overall nitrate removal efficiencies of 85 and 73%, respectively. Methanol concentrations decreased by order of magnitude along the bioreactor length, from mean inlet concentrations of 327 mg CH₃OH-C L¯¹ in 2020 (higher dosing rate) to concentrations of <50 mg CH₃OH-C L¯¹ at the outlet. The mean methanol removal rates of 106 g CH₃OH-Cm¯³ d¯¹ in 2020 and 109 g CH₃OH-C m¯³ d¯¹ 1 in 2021. A 90% (2020) and 100% (2021) overall methanol removal efficiency was calculated. With methanol dosing, sulfate removal occurred in the bioreactor, with an average sulfate removal rate of 8.5 g SO₄²¯-S m¯³ d¯¹ in 2020 and 0.5 g SO₄²¯-S m¯³ d¯¹ in 2021. This work demonstrated that methanol additions to bioreactors could enhance denitrification rates even when nitrate was limiting without considerable losses of methanol being released to the receiving waters.