Waste produced by forestry is being used in an ingeniously circular system to mitigate the runoff from dairy farms.
DairyNZ and the Institute of Environmental Science and Research (ESR) won the research and science category at the 2024 Primary Industries NZ Summit Awards.
The win recognised the work DairyNZ senior scientist Dr Lee Burbery has done in partnership with ESR over the past 10 years. Together, they researched and developed woodchip denitrifying bioreactors as an edge-of-field practice for mitigating nitrogen loads to New Zealand waterways.
“I was inspired by the forestry conversions that were happening in Canterbury around 2014,” Lee says.
“I noticed an opportunity where we were disposing of a significant amount of wood in Canterbury and intensively farming on porous soils that were susceptible to nitrate leaching.
“By adopting a circular economy approach, we could use some of the deforestation waste to create a water treatment facility for nitrate filtration.”
The bioreactors are pits filled with woodchips that work like a water filter. The carbon in the wood helps bacteria turn harmful nitrate in the water into harmless nitrogen gas during their natural respiration process.
Lee has a PhD from the school of environmental science at Lancaster University in the UK. He applied his hydrological knowledge to tackle the challenge of setting up woodchip filters in that environment.
“Water follows the path of least resistance, so the woodchip bioreactor had to be as permeable as the gravel found in Canterbury,” Lee explains.
“We mixed gravel with the woodchips to strengthen the bioreactor and extend its performance. This involved testing various woodchip and gravel combinations.”
He knew that in Canterbury, and much of the South and some of the North Island, there are deposits of very coarse gravel material. It was crucial to design something that would be fit for purpose in these environments, which hadn’t been done before.
Working with ESR, Lee oversaw the construction of two trial woodchip denitrification sites. One was a wall designed to treat shallow groundwater nitrate at a site near Kaiapoi and the other was a bed to treat farm drainage on a dairy farm near Geraldine.
DairyNZ and ESR worked together to monitor the woodchip bioreactor near Geraldine for two years, conducting some innovative experiments during the process. The concept has been successful, but Lee notes there is a caveat.
“We’ve demonstrated that woodchip bioreactors are highly effective at removing nitrate. However, they are a niche solution with limited applicability due to hydrological challenges, construction costs and compliance issues.”
He explains that there is currently no design standard in NZ so if a farmer was interested in installing their own they would need to do a bit of homework to piece it together.
“What we do on land – to reduce nitrogen inputs and better manage losses of nitrogen from the soil – is where we get the biggest return for reducing our nitrate footprint.”
Spending most of his 25-year science career working on New Zealand groundwater resource management issues, Lee has enjoyed learning about dairy farm systems and engaging with farming communities since joining DairyNZ in 2021.
This story first appeared in Inside Dairy, the official publication of DairyNZ.
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