The carbon bio-reactor will be part of an experimental treatment wetland complex that also removes phosphorous and sediment.
Aquatic ecologist Dr Andy Hicks said a carbon bio-reactor could be as simple as a plastic-lined trench full of woodchips, which nutrient-laden water passed through, allowing nitrates to be converted to nitrogen gas. The cleaner water then enters the waterway.
''We will be experimenting with wood chips and oyster shells to determine how effective they are in removing nitrates and phosphorus from farm run-off,'' Dr Hicks said.
The trials will be carried during summer in partnership with NIWA and DairyNZ.
''We are going to finalise site selection and get to work this summer,'' Dr Hicks said.
Trials carried out in Finland had shown similar treatment wetlands extracted between 60% and 90% of sediment, between 45% and 90% of nitrates and between 50% and 90% of phosphorous from water during normal water flows.
A trial bio-reactor on a Waikato dairy farm achieved a nitrate load reduction of between 55% to 79% from the tile drain discharge.
''It is a method that has proven effective at removing nitrate from waterways.''
The woodchips, probably pine or eucalypt, remove nitrates from the water by a denitrification process. He said decaying vegetative matter contributed to anoxic (oxygen-less) conditions in the water, allowing dentrifying bacteria to thrive. They feed off carbon using nitrates as an energy source, producing nitrogen gas, which is released back into the atmosphere.
The bio-reactors can last for up to 20 years, and are temperature dependent (5degC plus).
Other parts of the treatment wetland will be covered by ponded water either by building a dam or digging a pond, and planted with native vegetation enhancing the aesthetic and recreational values of the area.
The areas of ponded water allow water to slow down and for sediment to fall out, as well as providing habitat and duck-shooting opportunities. He said another section would be filled with oyster shells, or calcium carbonate (lime), which should absorb phosphorus (P) from the water. The shells were readily available from Bluff.
''We want to see how long it takes for the shell to break down as we want something that will last for a while rather than continually replacing it.''
He said lime had a 64% reduction in a New Zealand laboratory trial but only a 18% reduction in a field trial.
The cleaner water would then flow to the waterway.
Woodchips and shells could also be added to conventionally constructed wetlands, duck ponds and other habitats with high aesthetic and recreational values. He was looking at using raupo, a wetland plant that acts as a filter and grows into attractive stands.
Once the trials have been done, Environment Southland will look at the cost and provide information to farmers.
''We are looking for a win-win situation that is cost-effective and practical.''
Some parts of the wetland will come to the fore very quickly, with P absorption taking place almost instantaneously and the denitrification bacteria building up over months.
Wood chip denitrification had been used in industrial waste before, but had not been tested in Southland farms, and the use of oyster shells would also be novel, he said.