The Agri-Food and Bioscience Institute (AFBI) has been leading research that shows separating slurry into solid and liquid fractions can help reduce phosphorus, improve nutrient use, protect water quality and even create new income streams.

According to AFBI, Northern Ireland's agriculture sector plays a vital role in the region's economy, however its success comes with a major significant environmental challenge - persistent surplus of phosphorus (P).

More P is coming onto farms in food and fertiliser than is leaving in crops, milk, or meat, with the surplus P ending up accumulating in the soil and in some cases, waterways, the institute said.

The AFBI said slurry processing is "a promising solution" that offers the opportunity to manage a portion of the surplus P on farms by exporting phosphorus-rich slurry solids to areas which need P in the UK, Ireland, or beyond.

This would farmers to reduce local nutrient loading without cutting productivity.

Utilising livestock slurry sustainably

The organisation has been spearheading research in slurry processing technologies that allow the effective phosphorus partitioning into solid fractions.

This work demonstrates the opportunity to reduce the amount of surplus P being applied to the local land base, as these solids can then be exported as a raw material.

It can also be further processed into horticultural products, fertilisers, or biochar for regions where phosphorus is needed.

This will help to reduce, Northern Ireland's surplus and mitigate environmental risks, AFBI said.

Mechanical separation systems, such as screw presses, can typically remove around 15–25% of the P, though performance depends on several factors, such as screen size, clamp pressure, and flow rate, as well as the dry matter content of the slurry, amongst others.

AFBI is also currently investigating incorporating chemical coagulants or flocculants to improve P recovery.

Advanced systems, including dissolved air flotation and centrifugation combined with polymer dosing, have achieved P removal rates of 80–90%, aiming to maximise phosphorus capture while maintaining economic and practical feasibility for farms.

There are other benefits to the system that just nutrient management, according to the AFBI, which is researching on-farm separation, storage, digestate management, nutrient mass balances, and life cycle assessments to quantify environmental benefits.

Anaerobic digestion

In Northern Ireland, several anaerobic digestion (AD) plants are already accepting separated slurry solids.

These facilities generate biogas for heat and electricity while recovering nutrients for onward digestate processing, reuse, and export.

AFBI has said early results from its work investigating the effectiveness of centrifugation have found that 71% of phosphorus can be removed from digestate into solids.

With the knowledge gathered from this research, the Department of Agriculture, Environment and Rural Affairs (DAERA) has initiated the Sustainable Utilisation of Livestock Slurry (SULS) project, which is piloting the full slurry-to-product pipeline.

This pipeline includes nutrient recovery technologies, ammonia stabilisation, and the development of value-added products such as fertilisers and biochar, all while engaging directly with farmers to ensure practical implementation, AFBI said.