By diverting slurry solids from farms to anaerobic digestion (AD) plants, we can generate green energy, ease nutrient pressures, and close the loop on nutrient cycles. However, understanding the key drivers and barriers to this feedstock model is critical for unlocking its full potential.
Anaerobic Digestion (AD) in Northern Ireland
AD is the breakdown of organic materials by microorganisms through a series of biological reactions, resulting in the release of biogas. Biogas is made up of methane and carbon dioxide, with a typical methane content of 50-70%, depending on the feedstock used. Common uses of biogas include combustion in a Combined Heat and Power (CHP) unit to generate electricity and heat. Biogas can also be upgraded to biomethane to replace fossilised gas. Currently in Northern Ireland (NI), there are around 80 AD plants, predominantly producing renewable electricity through a CHP engine, with an average export per AD of 500kW.
Of the material fed into an AD plant (feedstock), only a fraction (roughly 10-20%) is converted into biogas. The remainder exits the plant as digestate, an unavoidable output of the process. Digestate is a sludge-like material, somewhat mirroring the nutrient profile of the feedstock. Most plants in NI are agriculturally fed, meaning the feedstock is from agricultural sources such as energy crops and manure material, classified in waste management terms as non-waste and waste material respectively.
AD and Slurries/Manures
Recent work conducted through BH Estates has uncovered that approximately 182,600 tonnes of slurry or manure are fed into NI AD plants annually, from a 32 AD sample group. By weight, the majority of this material is raw cattle slurry (63%) and chicken litter (27%), with slurry solids representing only 2.8% of the feedstock (Frederick, 2025 (unpublished)).
Slurry solids is a term used to describe a specific fraction of slurry, often produced through separation. Screw press separation is one technology that produces slurry solids by partitioning slurry into two main fractions (solid and liquid), based on particle size. The liquid fraction often exhibits a dry matter lower than raw slurry, while slurry solids, retained by the separation process, are a more solid material (25-30% dry matter).
Benefits of Redirecting Slurry Solids to AD
Through BH Estate’s Farm2Export project, part-funded by DAERA, research has shown the value associated with utilising slurry solids as an AD feedstock.
For the farm: slurry separation followed by export of slurry solids, lowers the volume of slurry to store, freeing up storage space. A proportion of the slurry methane is also exported, reducing farm methane emissions and therefore total GHG emissions. In relation to nutrient loading, the export of slurry solids produces an export stream for agricultural nutrients, which reduces nutrient loading on the farm. In NI, with an intense livestock-dominated farming system, identifying new export pathways for nutrients is beneficial in the context of water quality protection.
The creation of the liquid fraction (which remains on the farm) is easier mixed and applied, showing evidence of less slurry stripes (fibre accumulation) after application. The remaining liquid fraction is also more suited to Low Emission Slurry Spreading equipment and can exhibit a higher infiltration rate (compared to raw slurry), reducing ammonia losses from application.
For the AD: slurry solids represent an energy rich biomass, and economic value can be placed on displacing grass silage with slurry solids as a feedstock – which also enables the rerouting of energy crops elsewhere. From the point of view of energy generation, the use of slurry solids for AD unlocks a new energy source that is currently relatively untapped. This is important in the context of future renewable energy targets.
Main Challenges and Enablers for Redirecting Slurry Solids to AD
Challenge 1: The Operational Protocol for ammonia emissions in NI. The planning framework for deploying nutrient recovery (separation) technology is a current barrier to utilising slurry solids at AD, due to the presence of reactive nitrogen in the biomass. Designed to protect sensitive (designated) habitats from ammonia emissions, the Revised Operational Protocol sets out a methodology for assessing ammonia impacts, however multiple stakeholders have identified that the approach limits the deployment of manure processing technology. The barrier stems partially from the high background levels of ammonia in NI, combined with the extensive spatial distribution of designated sites. This decreases permitted ammonia fluxes on a large proportion of land area, making the introduction of separation technology complex.
Limiting the introduction of nutrient recovery technology significantly restricts nutrient supply chain development. Like water travelling through a hose, if flow is blocked, this will lead to damage or the requirement for the flow to stop completely. Effective flow of nutrients from AD will enable increased flow of nutrients into AD i.e., slurry solids. It is recommended that the unintentional planning barriers to manure processing are reviewed, and that technology capable of reducing the impact of agriculture on air and water quality is permitted under planning rules.
Challenge 2: The creation of nutrient supply chains. As slurry solids largely contain more nutrients than energy crops, a feedstock displacement can increase the nutrient levels in digestate; an undesirable consequence if digestate continues to be land spread and operators need to produce balanced nutrient management plans. Creating a supply chain of nutrients onwards from AD is therefore critical to fully realise the benefit potential of utilising slurry solids. This is the main aim of the Farm2Export project, to move nutrients from farms, through AD, into a standardised biofertiliser for the efficient and sustainable redistribution of agricultural nutrients.
Challenge 3: The uncertainty in NI around government support for AD. There is a strong view that dealing with wastes needs government support, which currently is lacking. This makes investment and commitment to new sustainable feedstock models difficult. Without a clear NIROC replacement, which currently subsidies electricity production, and/or the lack of clear direction for biomethane, it is challenging to efficiently plan for the future. To overcome this barrier, support frameworks are required to reduce uncertainty and allow industry to properly invest in more sustainable feedstock streams. It is recommended that support frameworks consider or are even subject to the sustainability of feedstock used, to position and credit AD as a waste management process and increase the digestion of slurry solids moving into the future.
Given the significant benefits of utilising slurry solids at AD for NI, including reduced agricultural nutrient loading, lower methane and ammonia emissions, increased renewable energy generation, and enhanced nutrient supply for biofertiliser production for nutrient recycling, it is essential that the current challenges to implementing this feedstock model are addressed and overcome.
Dr Áine Anderson - Project Manager at BH Estates and Technical Lead on the Farm2Export Project. Her expertise lies in sustainable slurry management, and the developing synergies between nutrient surpluses in agriculture and renewable energy production through AD. She co-authored the 2022 QUB/CASE biomethane report and contributed to the Phosphorus Stock and Flows in the Northern Ireland Food system report within the REPHOKUS Project. Dr Anderson holds a PhD focused on modelling and mapping nutrient solutions for Northern Ireland Agriculture and was awarded the Climate Change Thought Leadership Award (PhD category) (2022).
The research including in this report is part funded by DAERA. The views expressed in this publication are those of the author and not necessarily those of DAERA.