Denitrifying bioreactor technology to reduce nitrate discharges from artificial drainage - a novel tool to enable viable farming within limits?

Abstract

Aims - Artificial drainage is essential for viable use of poorly drained soils, which account for approximately 40% of dairying land in New Zealand. However, subsurface and surface drains can also provide a pathway for fast and unattenuated nutrient transfers to our streams and rivers. A denitrifying bioreactor, fundamentally a pit filled with carbon source such as woodchips, is a recently developed technology for treating drainage water at the edge of the field (Schipper et al. 2010). Naturally occurring microorganisms utilise carbon in woodchips to transform nitrate in the drainage water into gaseous forms of nitrogen (largely N2) through the denitrification process. The technology has been widely adopted in cropped lands in the USA (Christianson et al. 2012). However, a different bioreactor design is necessary in New Zealand due to the shallower subsurface drainage systems in our flat pastoral lowland areas. While bioreactors have been found to effectively remove nitrate in the drainage water, possible pollution swapping (particularly N2O emissions) and other unwanted side effects (including high concentration of dissolved organic matter in the outflow) also need careful consideration (Schipper et al. 2010; Weigelhofer and Hein 2015). Thus the main objective of this research is to assess the applicability and performance of denitrifying bioreactor technology in reducing nitrate loads from subsurface drains in New Zealand pastoral lands. We aim to identify the factors affecting the performance as well as potentially occurring detrimental side effects of denitrifying bioreactor technology to optimise the cost and efficiency of future installations in New Zealand. Method - We designed and constructed a pilot-scale denitrifying bioreactor at a farm in the Hauraki Plains where high nitrate concentrations (>10 mg nitrate-N L-1) were found in the drainage water (Figure 1). The bioreactor has an effective volume of approximately 60 m3 filled with locally sourced untreated pine (Pinus radiata) woodchips. We route the drainage water from a lateral subsurface drain into the bioreactor through an inlet control structure and the flow rate through the bioreactor is controlled by the difference between the heights of weirs in the inlet and outlet control structures (Figure 2). The inlet control structure allows excess drainage water during high flow events to by-pass the bioreactor. We continuously monitor flow through the bioreactor and any by-pass flow, electrical conductivity at the inlet and outlet, temperature at the inlet, outlet and within the bioreactor, and rainfall at the site. Inlet and outlet waters are proportionally sampled for nitrogen and carbon species to assess the effectiveness of the bioreactor in attenuating nitrate and for a range of other analytes to investigate the possible occurrence of negative side effects. Results - We will present our approach to the design of the bioreactor for typical New Zealand subsurface drainage systems in comparison with the approach applied in other countries, such as the USA. Monitoring data from the first season of the bioreactor’s operation will also be presented to show the performance of the bioreactor in reducing nitrate in the subsurface drainage water and to assess any potentially occurring negative side effects.