Nitrate removal efficiency and secondary effects of a woodchip bioreactor for the treatment of agricultural drainage

Artificial drainage has been instrumental in the viable use of poorly drained soils for agriculture. However, artificial drains can also provide a pathway for fast and unattenuated nutrient transfers to streams and rivers. To remove nitrate from drainage water, bioreactors have recently been widely adopted as an edge-of-field mitigation measure, particularly in the USA. Bioreactors are fundamentally a lined pit filled with woodchips as a source of carbon, which microorganisms use to transform nitrate through the process of denitrification into gaseous forms of nitrogen, mostly N₂. However, there is a lack of information on the performance of these bioreactors under the very flashy agricultural drainage flow conditions typical for New Zealand. Moreover, to avoid pollution-swapping, any possibly occurring negative side effects need to be investigated. A pilot-scale woodchip bioreactor was constructed on a dairy farm on the Hauraki Plains in Waikato and was monitored for one and half drainage seasons (part of 2017, 2018). The nitrate removal efficiency of the bioreactor, calculated from the difference in nitrate load between the bioreactor inflow and the outflow, was 99% and 48% in 2017 and 2018, respectively. The difference in removal efficiencies can be attributed to the much longer residence times and greater organic carbon (OC) availability in the bioreactor in 2017. While the long residence times in 2017 resulted in nearly complete denitrification with reduced concentrations of the greenhouse gas nitrous oxide in the bioreactor outflow, it also led to very strongly reduced conditions with production of methane (another greenhouse gas) and hydrogen sulphide (“rotten egg smell”). The shorter residence times occurring in 2018 following the modification of the bioreactor inlet manifold rectified this strongly reduced condition; however the nitrate removal efficiency concomitantly decreased. Elevated discharges of OC and dissolved reactive phosphorus (DRP) were evident during the first start-up phase of the bioreactor in 2017. In 2018 significant removal (89%) of DRP was measured over the drainage season, with no initial elevated DRP discharge. Ongoing investigations aim to optimise installation costs and treatment efficiency, while minimising any potential side effects. Specifically, options to improve the poor treatment during high flows will be investigated in the 2019 drainage season (e.g. by adding readily available OC source such as methanol)