Nitrate Removal Performance of Denitrifying Woodchip Bioreactors in Tropical Climates

Fabio Manca,Louis Schipper,Peter Grace,Rhianna Robinson,Matthew Griffiths,Daniele De Rosa,David Rowlings,Christopher Algar,Carla Wegscheidl,Fiona George,Suzette Argent

doi:10.3390/w13243608

Fabio Manca, Louis Schipper + Show 9 more

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https://doi.org/10.3390/w13243608

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Abstract

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.

Highlights

In agricultural systems, nitrogen (N) applied in excess to plant requirements can be lost to the environment in the form of nitrate (NO3−) and can leach into shallow groundwater and enter surface waters through concentrated or diffuse discharges [1]
The wet tropics wall BR1 showed variable influent NO3− concentrations (Figure S6) that were positively correlated (r = 0.54, p < 0.001) with the groundwater level, with higher concentrations detected at shallow groundwater levels, similar to what was observed in other denitrification walls in Queensland [34]
The highest NRR performance was observed in the dry tropics bed BR5, due to a higher, more consistent influent NO3− concentration from the elevated NO3− in the groundwater used to irrigate the crop

Summary

Introduction

Nitrogen (N) applied in excess to plant requirements can be lost to the environment in the form of nitrate (NO3−) and can leach into shallow groundwater and enter surface waters through concentrated or diffuse discharges [1]. Common in the coastal floodplains adjacent to the GBR [5], have been identified as a main source of reactive N contributing to the deterioration of the water quality in the GBR [6]. The Australian and Queensland Governments have set targets for pollutant reductions and have invested in programs to improve agricultural management practices. The water quality targets will not be met at the current rate of management practice adoption and additional water quality improvement actions are needed [4]

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