That’s the belief of those involved in research into an effective design for denitrifying bioreactors suitable for treating subsurface drainage waters under New Zealand conditions.

Hamilton based Roland Stenger, Principal Scientist in the Environmental Group of Lincoln Agritech Ltd, Aldrin Rivas, Catchment Hydrologist, also with Lincoln Agritech Ltd and Greg Barkle, Principal Environmental Engineer of Aqualinc Research Ltd have pooled their knowledge and talents to design and build a pilot-scale denitrifying bioreactor on a dairy farm near Tatuanui.

So far, the results are promising.

Early monitoring results show a reduction of nitrate-nitrogen levels from seven-to-eight milligrams per litre of drain water to almost undetectable levels after the water has been filtered through the wood chips.

The research is funded by Ministry for Business, Innovation and Employment via the Institute of Environmental Science and Research, with additional support provided by Waikato Regional Council.

 Cooperative farmer

The research also benefits greatly from a very cooperative farmer, who is not only tolerating the activities on his land, but even lent a helping hand during the bioreactor construction.

“Denitrifying bioreactors are being used in the US and Europe, but overseas designs can’t just be uplifted and applied in New Zealand, because of the difference in drainage systems, land use and climate,” says Roland.

However, the technology is reasonably simple and installation cost is low, so if proven to work reliably in this country, the method should appeal to New Zealand farmers.

The reactors work by passing water from a subsurface field drain through a pit filled with woodchips, naturally colonised by bacteria, which feed on the nitrate in the water, turning it into harmless nitrogen gas and so reducing the amount of nitrogen leaching into waterways.

Aldrin says the design of the bioreactors had to take into account the fact that in New Zealand, subsurface drains are typically installed at a depth of between 0.6 to 0.8m, compared to 1.5-1.8m common in those parts of the US where most bioreactors have been trialled.

“This meant designing a bioreactor below the level of the drain and making sure water could flow through it without the use of costly pumping systems.”

 Hardworking bacteria

The hardest workers in the project are the bacteria themselves, which live in the woodchips. “We don’t have to introduce them,” says Aldrin. “They are everywhere. We just provide an environment they like.”

The aim of the project, says Greg, is to ensure the system not only works effectively, but is also an affordable option for farmers.

Artificial drainage is used on approximately 40 per cent of New Zealand’s dairying land and is essential to enable many pastures to be used year round.

However, says Roland, subsurface drains can also provide a pathway for fast and non-attenuated nutrient transfers, able to treat water before it enters surface drains, streams or rivers meaning it will be of value to farmers and the environment.

The Tatuanui pilot-scale bioreactor has a 60 cubic metre capacity and is five metres by nine meters at the base of the pit, with a one-to-one slope rising to the surface where it measures approximately 12 metres by eight.

Filled with wood chips, there is an inlet control structure at one end, and an outlet control at the other. The Tatuanui reactor drains an area of 0.65 hectare.

“For research purposes, the pilot reactor is equipped with a lot more monitoring equipment than would be required for a routine farm reactor,” says Aldrin.

That’s because to prove it works reliably, the researchers need to extract detailed data, including weather, rainfall, inlet and outlet water chemistry and flow rates.

 Two cyclones

The early stages of construction were more than a little hampered by two cyclones in April, but the reactor was finally operational by July 2017 and will continue to be monitored for at least two years.

Greg says the woodchips, which are readily available, should last 15 or more years before they need replacing.

“The biggest costs for farmers installing a bioreactor will be digging the pit, which many can probably do with their own equipment, plus the cost of the liner, and inlet and outlet control structures,” he says.

While the Tataunui reactor is fenced off from stock, Roland says there’s no reason the grassed area above a reactor, in a normal farming situation, couldn’t be grazed.

“Bioreactors aren’t a silver bullet to mitigate nitrogen leaching, but they may be one useful new tool in the toolbox and could appeal to farmers who are coming under pressure to reduce their farm’s impact on water quality.”

At this point in the project Roland, Aldrin and Greg aren’t prepared to say it is right for New Zealand conditions until they have gathered more solid information. One concern that people have raised, says Roland, is that bioreactors may have some undesirable side effects.

“One of them is that incomplete denitrification may result in emissions of the greenhouse gas nitrous oxide (N₂O).

“We will monitor if, or how much, nitrous oxide gets produced and how a bioreactor can be optimised to minimise any emissions.

“If a bioreactor becomes more strongly reduced than what is required for denitrification, mobilisation of phosphorus and trace metals and hydrogen sulphide production (‘rotten egg smell’)

may occur.

“Our research will therefore determine if and under what conditions these processes occur and, if required, develop strategies to reduce these unwanted effects.”

Denitrifying bacteria are estimated to make up ten to 15 per cent of bacteria found in soil, water and sediments and there are more than 126 different species which can convert nitrate to a harmless gas.