Nitrate Removal Performance of Denitrifying Woodchip Bioreactors in Tropical Climates

In Australia, declining water quality in the Great Barrier Reef (GBR) is a threat to its marine ecosystems and nitrate (NO₃⁻) from sugar cane-dominated agricultural areas in the coastal catchments of North Queensland is a key pollutant of concern. Woodchip bioreactors have been identified as a potential low-cost remediation technology to reduce the NO₃⁻ runoff from sugar cane farms. This study aimed to trial different designs of bioreactors (denitrification walls and beds) to quantify their NO₃⁻ removal performance in the distinct tropical climates and hydrological regimes that characterize sugarcane farms in North Queensland. One denitrification wall and two denitrification beds were installed to treat groundwater and subsurface tile-drainage water in wet tropics catchments, where sugar cane farming relies only on rainfall for crop growth. Two denitrification beds were installed in the dry tropics to assess their performance in treating irrigation tailwater from sugarcane. All trialled bioreactors were effective at removing NO₃⁻, with the beds exhibiting a higher NO₃⁻ removal rate (NRR, from 2.5 to 7.1 g N m⁻³ d⁻¹) compared to the wall (0.15 g N m⁻³ d⁻¹). The NRR depended on the influent NO₃⁻ concentration, as low influent concentrations triggered NO₃⁻ limitation. The highest NRR was observed in a bed installed in the dry tropics, with relatively high and consistent NO₃⁻ influent concentrations due to the use of groundwater, with elevated NO₃⁻, for irrigation. This study demonstrates that bioreactors can be a useful edge-of-field technology for reducing NO₃⁻ in runoff to the GBR, when sited and designed to maximise NO₃⁻ removal performance. View Full-Text