Nutrient run-off solutions found

ROB TIPA
Last updated 13:29 15/04/2014
farm effluent pond
BOTTOM TO TOP: A farm effluent pond that has been covered to collect methane gas for use as energy on the property.

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In the on-going battle for the dairy industry to control nutrient runoff into the country's waterways, National Institute of Water and Atmospheric Research (Niwa) scientists are excited about a range of techniques they are developing for farmers.

They hope to save money by capturing and recycling nutrients and harnessing the free energy generated by farm effluent.

Dr John Quinn, leader of a team of freshwater ecologists working with experts in environmental pollution at Niwa in Hamilton, said the group was investigating a combination of measures and good land management practices to capture and recycle nitrogen and phosphorus close to their source before they contaminated waterways.

"Big complex problems like diffuse farm pollution will likely be solved by working on a number of fronts in a linked-up way - no silver bullets but a quiver of armaments to win the battle," he said.

"When you're trying to deal with nutrient runoff issues, it's important to frame it in the context of the catchment," he said, and all catchments were different. "You need to start at the bottom of the catchment and look up."

Farmers needed to focus on the key nutrient they were dealing with because the pathways nitrogen and phosphorus took were different and the methods required to contain them often also differed.

Scientists in the AgResearch-led Clean Water Productive Land programme are testing the 80/20 rule that 80 per cent of the contaminants come from 20 per cent of the land area, which means focusing on the hot spots creating most of the contaminants and dealing with them as close as possible to the source.

"Not all options will work on all farms or in all catchments," Quinn said. "You need to put things together in the right combination in the right place."

Because a lot of natural wetlands had been drained for agriculture, he said it was important to preserve those that were left.

"The type of engineering options we are talking about are quite expensive to put in place, so if you've got natural wetlands in place it's important to look after them for a start."

Niwa scientists are currently undertaking three projects targeting key pollution issues for the dairy industry.

Dr Rupert Craggs is exploring the potential for farmers to use dairy farm effluent as a resource, generating electricity or heat from the methane gas produced, growing algae to assimilate nutrients which could be fed to stock as a protein supplement or to generate energy and recycling the water itself.

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In the last 15 years dairy farmers had moved towards direct irrigation of effluent on to land to reduce the nutrient loading on rivers, he said. Many farmers were using weeping walls and mechanical separators to remove solids or using effluent ponds to allow the solids to settle, producing methane.

Increasing intensification of farming had major implications for New Zealand's greenhouse gas emissions, but a positive benefit for farmers was the potential to cover effluent ponds and recover the methane gas as a potential energy source for use on farm.

Craggs said farmers were interested in this technology because it was viable today to collect the effluent off a feed pad carrying 400 cows and the economies of scale improved with larger herds.

"It doesn't work for smaller farms but with the intensification of dairying in New Zealand it is becoming more and more promising."

Growing algae on effluent ponds also presented several opportunities to assimilate nutrients for use as a possible soil conditioner, turning it into silage for use as a protein supplement for livestock, or to recycle it through a digester to generate energy.

Scientific trials had shown these options worked in the New Zealand climate and scientists were looking for funding to advance these ideas, he said.

Environmental chemist Dr Chris Hickey is investigating the use of nutrient absorbent filters, such as aluminised zeolite, to intercept, capture and store contaminants in tile drains or constructed wetlands so they could be removed from effluent and reapplied to soils.

He said iron-based products used in sewerage systems and aluminised zeolite were particularly effective in absorbing and binding nutrients like phosphorus and ammonia.

Currently his research is focused on the longevity and sensitivity of some nutrient absorbent materials, the practicalities of designing the best systems for their use and any unexpected consequences.

Hickey is working with ecologist Dr Chris Tanner to link P-sorbing filters with wood-chip filters to achieve both nitrogen and phosphorus removal. The wood-chip filters provide ideal conditions for conversion of nitrate into nitrogen gas which is released back into the atmosphere.

A large scale trial funded by Fonterra and Dairy New Zealand is demonstrating this technology in the ecologically sensitive Waituna catchment in coastal Southland.

Tanner's research also includes scientific trials of constructed wetlands on working farms in Northland, Waikato and Southland with farmer participation and dairy industry funding.

He said monitoring of those wetland trials, over three and five years though wet and dry years, had provided the basis for preparation of practical guidelines for farmers.

Research suggested wetlands needed to occupy between one and five per cent of the catchment area they were treating, with a marked reduction in nitrate levels in wetlands occupying between 2-3 per cent of the farm area.

"The more you have, the better they work," Tanner said.

Constructed wetlands could be recreated in low-lying, lower value areas of the farm using wetland plants like reeds, rushes and sedges to supply organic carbon and provide the right environment to convert nitrates in the water into nitrogen gas.

While Niwa scientists believe individual farmers can achieve great things on their own, ultimately they said it was the combined efforts of farmers working together in wider catchments that could make a significant difference.

Niwa is planning a series of in-depth articles on these technologies and their applications over the next few months.

- Manawatu Standard

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