Slurry processing offers a practical route to better water quality and nutrient use

Date published: 01 March 2026

AFBI has been leading research that shows that separating slurry into solid and liquid fractions can help reduce phosphorus, improve nutrient use, protect water quality and even create new income streams.

Background

Northern Ireland’s agricultural sector, particularly its intensive livestock farming, plays a vital role in the region’s economy. However, this success comes with a significant environmental challenge: a persistent surplus of phosphorus (P). Put simply, more phosphorus is coming onto farms in feed and fertiliser than is leaving in crops, meat or milk. This surplus ends up accumulating in soils and, and in some cases, entering local waterways. 

Phosphorus is not only an environmental issue – it is also a valuable, finite global resource. As such, capturing and managing surplus phosphorus will support improvements in water quality in Northern Ireland and could create additional opportunities.

One promising solution is slurry processing, which offers the opportunity to manage a portion of the surplus P on farms by exporting phosphorus-rich slurry solids to areas which need phosphorus in the UK, Ireland or beyond – enabling farmers to reduce local nutrient loading without cutting productivity.

Sustainable Utilisation of Livestock Slurry

AFBI has been leading research into slurry processing technologies that enable effective phosphorus partitioning into solid fractions. By concentrating phosphorus into the solid fraction, AFBI’s work demonstrates opportunities to reduce the amount of surplus phosphorus being applied to the local land base as these solids can then be exported as a raw material or further processed into horticultural products, fertilisers or biochar for regions where phosphorus is needed, thereby reducing NI’s surplus and mitigating environmental risks. 

Mechanical separation systems, such as screw presses, typically remove around 15–25% of the phosphorus, although performance depends on factors such as, screen size, clamp pressure, and flow rate, as well as the dry matter content of the slurry, amongst other management factors. Incorporating chemical coagulants or flocculants, currently being investigated through CASE-DfE funded research, can also improve phosphorus recovery. More advanced systems, including dissolved air flotation and centrifugation combined with polymer dosing, have achieved phosphorus removal rates of 80–90% aiming to maximise phosphorus capture while maintaining economic and practical feasibility for farms.

The benefits go beyond nutrient management. AFBI’s research is also considering on-farm separation, storage, digestate management, nutrient mass balances, and life cycle assessments to quantify environmental benefits, including in the impacts on greenhouse gas and ammonia emissions and overall nutrient management. 

Across 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 ultimately export.

Early results from AFBI’s work investigating the effectiveness of centrifugation have found that 71% of phosphorus can be removed from digestate into solids with improvements possible with adjustments to dosing as well as bowl and auger speeds. The goal is to deliver solutions that are environmentally sustainable and economically viable at scale.

Building on this foundation of knowledge, 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 includes nutrient recovery technologies, ammonia stabilisation, and the development of value-added products such as fertilisers and biochar, while engaging directly with farmers to ensure practical implementation. 

Furthermore, producing fertiliser and horticultural products from separated digestate creates new economic opportunities. These products, rich in phosphorus, nitrogen, and potassium, can be pelletised or granulated for ease of transport and sale, serving both domestic and international markets.

On-farm separation can be carried out using fixed or mobile equipment, including screw presses and centrifuges. The solid fraction, which contains a higher concentration of phosphorus, can then be transported off farm.

The remaining liquid fraction retains valuable nitrogen and potassium and can be used more efficiently on farm. Covered storage and the use of Low-Emission Slurry Spreading Equipment (LESSE) are essential to maximise nitrogen use efficiency and minimise ammonia losses when managing this liquid fraction.

Farmers involved in trials have reported that separated slurry is easier to handle and apply, with improved soil infiltration when spread using LESSE. There is also scope to further reduce ammonia emissions through pH adjustment or ammonia stripping at central processing facilities, producing nitrogen-based fertiliser products in the process.

Environment & Renewable Energy

There are several environmental benefits to slurry processing. By reducing the amount of phosphorus applied to land, the risk of runoff to rivers and lakes is lowered, helping to protect freshwater ecosystems. Furthermore, the integration of anaerobic digestion into the slurry processing chain allows for the production of biomethane as a renewable energy source. This not only displaces fossil fuels but also contributes to reducing the carbon footprint of Northern Ireland’s society.

Going Forward

While the potential is significant, several challenges remain. Investment is needed in processing infrastructure, along with robust systems for slurry processing, collection and transport. Clear frameworks will also be required to support product development and nutrient export beyond Northern Ireland.

Equally important is stakeholder and public buy-in, as slurry processing represents a shift in how nutrients are managed across the farming system.

Off-farm slurry processing will not solve the phosphorus challenge on its own, but it offers a powerful additional tool. By turning surplus nutrients into valuable products, farmers can reduce environmental risk while strengthening the long-term resilience of their businesses.

With continued research, farmer engagement and policy support, slurry processing could play a key role in ensuring Northern Ireland’s land continues to support both food production and nature for generations to come.