NI study suggests slurry separation hubs could offer solution to nutrient surplus

A study examining the potential of slurry separation technology has suggested the establishment of processing hubs in Northern Ireland could help the industry make better use of agricultural by-products.

It could be one of a series of new ‘climate-smart’ technologies used to further improve the environmental sustainability of the industry.

The paper, which is based on research carried out at the Agri-Food and Biosciences Institute (AFBI) site in Hillsborough, was published in the latest edition of the scientific journal Agronomy.

The project examined the economic and practical viability of both screw press and decanting centrifuge separation technology which could be used in the dairy, beef, pig and anaerobic digestion sectors.

Northern Ireland’s high stocking density has meant the region produces 9,000t more phosphates than it requires, meaning that even if all nutrients were applied where they were needed for crops and grass production, there would still be a significant surplus in the region.

As a result, Northern Ireland, along with parts of Belgium, Denmark, the Netherlands, Scotland, and more recently, Wales, have been designated as nitrate vulnerable zones (NVZs).

The report lead by AFBI researcher Gary Lyons, states: “Both separation technologies could be integrated into agricultural manure and digestate management systems in order to provide a more sustainable approach to managing agricultural phosphorus loss and its associated impact on water quality.

“Screw press separation could reduce phosphorous loss to water bodies by 34%, and depending on separation efficiency, decanting centrifugation could achieve a reduction of between 30% and 93%.”

How slurry separation would work for NI

The separators act to reduce the liquid content of slurry, making it lighter and less voluminous, meaning it is easier to transport and less likely to leach into water bodies.

The squeezed off liquid is still a valuable biofertiliser. Technologies to further treat the liquid such as membrane filtration, N-stripping, and struvite crystallisation for example, could reduce its nutrient content, while evapo-concentration could be used to reduce volume and concentrate nutrients.

However, there are some constraints with the technology. The study showed separation worked most efficiently when carried out within 10 days of excretion as the Dry Matter (DM) content of slurry reduces the longer it is stored.

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Decanting centrifuge technology was the more expensive of the two systems, however, the technology allowed for a much greater volume of slurry to be processed meaning that its cost could be reduced through scale of efficiency.

The study found plant uptake of N delivered in liquid form was better. However, field application of digestate and separated digestate liquids resulted in similar grass yields and N uptake in comparison to plots treated with commercially available N fertilisers.

While limited research has been carried out into the quality and yield of temperate grassland, some evidence suggests that there is potential for separated liquid manure to increase yield when comapred to untreated slurry, due to higher bio-available levels of N.

Acidification was also found to play an important role. Losses from acidified separated liquid were reduced by 92% compared to separated liquid which had not been acidified.

The study also suggested there could be the possibility for further processing, including pelletising manure to create renewable fertilisers, soil conditioners or biofuel.