Recent studies on the Coffs Coast linked water quality issues to agricultural intensification.
Runoff from hothouses can contain high levels of nitrogen (N) from fertilisers often draining into
streams without any treatment. To manage nutrient pollution from hothouse drainage, we tested whether
a new woodchip bioreactor approach can attenuate the highly concentrated N effluent.
The experimental trial was designed and deployed as a collaboration between Southern Cross
University, North Coast Local Land Services and Coffs Harbour City Council. Here, we investigate the
effectiveness of buried, inline woodchip bioreactors in removing nitrate (NO3
--N) and the possibility of
pollution swapping by producing the greenhouse gas nitrous oxide (N2O).
Flow rates through the bioreactors across the 5 surveys were 65±29 L hr-1, and residence time
was 6.4±2.6 hrs, whilst water temperature was 17±1.0°C. Average dissolved NO3
--N removal and N2O
gas production were 6.0±5.2 g N m-3 hr-1 (0.9-12.3 g N m-3 hr-1), and 35.1±31.4 mg N m-3 hr-1 (14-83 mg
N m-3 hr-1), respectively. Average water depth was 8 cm, and only 21% of the bioreactor woodchips were
wet. A significant correlation between dissolved oxygen with NO3
--N and N2O-N indicated higher
removal occurred in hypoxic to anoxic conditions.
Overall, these inline pipe bioreactors achieved nitrate removal efficiencies (NRE) of 14.5±6.8%
(8.2%-25.0%) and N2O production equivalent to 0.7±0.6% (0.3-1.4 %) of nitrate removal. Our results
imply that these trial bioreactors are not significantly swapping NO3
--N removal with increased N2O
production to the atmosphere. Design improvements are required for better NO3
--N removal and a
review of on-farm fertigation is needed to minimize losses downstream. A suite of measures will be
required to achieve treatment levels that meet water quality guidelines.