Teagasc Grange Anaerobic Digestion Programme

Adriana Ferreira Maluf Braga Paul Crosson

Anaerobic digestion (AD) is a biological process conducted in a sealed vessel in the absence of oxygen. This process involves the fermentation of organic matter resulting in the production of biogas and digestate. There are four steps in the AD process: i) break-down of the organic material into soluble compounds (sugars, fats, proteins); ii) fermentation of the soluble compounds, generating organic acids; iii) further conversion of the acids into acetic acid; iv) conversion of acetic acid into biogas, a mixture predominantly composed of methane (CH₄) and carbon dioxide (CO₂). Although the steps are interdependent, they occur simultaneously in the digester, and a stable system provides an adequate environment for the growth and activity of all microbial groups.

In general terms, any organic material, residue, or even wastewater can serve as a feedstock for AD. However, the composition of the feedstock, e.g., dry matter (DM), the organic fraction of the DM, nitrogen (N) content, and other parameters, affects biogas production yield, i.e., the volume of biogas produced per unit mass or volume of feedstock. The operational conditions, such as the average time the feedstock remains in the digester and the amount of feedstock fed daily, can also affect the process. For example, fibrous feedstock requires more time for microbes to break it down, whereas sugar-rich wastewater can be rapidly fermented, and overfeeding can lead to imbalances and system failure. The simultaneous digestion of two or more feedstocks, i.e., co-digestion, is a strategy which can enhance process robustness, improve biogas yield, and reduce the risk of failure.

Ireland possesses great potential for biogas production from agricultural feedstocks. The Climate Action Plan 2023 (CAP23) has set a 25% reduction target for greenhouse gas (GHG) emissions in the agricultural sector and a significant reduction in the use of chemical N fertiliser. Providing animal slurry to AD plants can help the sector reduce CH₄ emissions from slurry management on livestock farms, while also providing additional income for farms. More broadly, the provision of grass feedstocks (mainly grass silage) can promote land use diversification and further increase farm incomes. Therefore, the co-digestion of animal slurry and grass silage has great potential as a feedstock for AD in the Irish context, provided it meets sustainability requirements.

The biogas produced in AD plants is mostly used to generate electricity and heat in combined heat and power (CHP) engines or for biomethane upgrading. These bioenergy options increase the proportion of renewables in the energy matrix, while promoting a more secure supply with reduced dependency on energy imports. In CHP systems, biogas is burned to drive a generator, which converts mechanical energy into electricity. While a standard generator releases waste heat into the atmosphere, a CHP system uses heat exchangers to capture thermal energy from the engine cooling water and exhaust gases. Combined heat and power is less expensive than biogas upgrading technologies; however, if the heat produced is not used (e.g. to dry grain, provide heat for farmhouse and livestock housing, provide hot water for cleaning and sterilising dairy equipment, etc.), it can result in up to 55% of the energy wasted. Biomethane is produced at AD plants by removing CO₂ and other ‘impurities’ from biogas and can replace natural gas for heating, transport and industrial uses. Increasing the production of biomethane is a key component of the National Biomethane Strategy, which set a target of delivering 5.7 terawatt-hour (TWh) (i.e. 10% of Irish gas demand) of indigenously produced biomethane from 2030.

In addition to biogas, digestate (i.e. the ‘slurry’ that remains after the digestion of the organic material) is a product of AD plants with great potential as an organic fertiliser. Digestate has a nutrient content similar to that of the feedstock used to feed the plant, conserving around 85% of the N in the mixed feedstock feeding, as the anaerobic microorganisms have low nutrient requirements (it is mainly the organic matter content which contributes to gaseous production). Furthermore, the ammonia proportion of the total N content is higher in the digestate compared to slurry and, consequently, more available to the plant. Technologies for further processing of digestate have been developed to increase nutrient recovery and separation. This involves the separation of nutrients into N, phosphorus (P) and potassium (K) fertilisers that can replace chemical fertilisers.

Teagasc Grange AD programme infrastructure

AD plant

The Grange AD plant, comprising a 1500 m³ digester and utilising cattle slurry and grass silage, underwent hot commissioning in 2024/25 including: i) digester inoculation with slurry as a source of microorganisms; ii) development of microbial community with the gradual increase of digester’s temperature; iii) gradual increase of the digester working volume, and iv) equipment test, maintenance and replacement, when required. Currently, the Grange AD plant producing 16 m³/day of biogas, which represents approximately 20% of its designed capacity (70 m³/day of biogas) to meet the digester’s heat requirements. The digester is fed daily with 10 m³ of slurry, equivalent to around 2,200 gallons, and 1.0 – 1.8 t fresh weight of grass silage. The AD lab has been used to monitor feedstock composition and digester stability, and to provide data for research. Production will increase further once the on-site biogas upgrading unit, which removes CO₂ to produce biomethane, is in place, enabling the biomethane to be transported and injected into the national gas grid.

Teagasc Grange AD laboratory

To properly monitor the digester at the Grange AD plant and support lab-scale experiments, a dedicated AD lab was set up in 2024. The AD lab can analyse total organic N and ammoniacal N, total organic matter, buffering capacity, total organic acids accumulation, and sulphide. Samples from the digester and the feedstock are analysed weekly for the key performance parameters. Monitoring is important to ensure adequate conditions for microorganisms and to understand how feedstock characteristics affect biogas production.

Changes in the feeding regime and feedstock composition and ratio can have a detrimental impact on an anaerobic digester, leading to microbial inhibition and changes in the community, resulting in a decline in biogas production that may take weeks or months to recover. Therefore, the optimisation of feedstock and other operational parameters are evaluated first through lab experiments. Using miniature digesters, Biomethane Potential (BMP) tests can be conducted in the AD lab to evaluate and compare the maximum biomethane that can be produced from co-digestion of feedstocks at different ratios. The effects of adding AD supplements and feedstock pretreatment can also be evaluated using BMP. The conditions are analysed with replicates, which is not possible with a single AD Plant. However, BMP tests have limitations, and after screening for conditions that positively impact biogas production, experiments are conducted in lab-scale digesters operated similarly to full-scale digesters to confirm the results and optimise operational conditions, e.g., feeding regime. Optimised conditions can be applied to the AD digester in Grange to validate the lab-scale results, provide additional information for economic assessments, and serve as a demonstration site of the benefits of optimised operation for stakeholders.

Teagasc Grange AD research programme

The AD research programme at Teagasc Grange combines desk-based analysis, laboratory methods, and pilot-scale AD system demonstrations. The AD research programme in Grange began with the work of Dr. Padraig O’Kiely, which was stimulated by an interest in assessing alternative uses of grasses. From that, the potential for AD in Ireland from agri-feedstocks such as grass silage and slurry became clear, and the Grange AD programme was established, building collaborations with national and international research groups. Alongside the AD research programme, a proposal was submitted to construct an AD plant in Grange for research and demonstration which is now operational. In recent years, interest in AD has grown in Ireland. Accordingly, researchers at Teagasc Grange have secured funding for a range of AD projects, which have required the establishment of a laboratory for AD digester monitoring and lab-scale experiments. In parallel with the research programme, the Grange AD Plant generates data that permits the evaluation of operational parameters, improve economic feasibility and sustainability, and serve as guidance for AD plants in Ireland. The programme now extends from (1) feedstock production, (2) optimising operational conditions of AD plants, (3) maximising nutrient recovery from digestate, (4) extraction/production of high-value products during the AD process, i.e., chemical compounds and proteins, and (5) desk-based analysis of the opportunities and challenges for an Irish AD sector at farm and sectoral level.

The following projects are currently underway:

Optimising the AD process:

Two Teagasc-funded PhD students are conducting projects at the Grange AD plant to optimise and better understand digester microbiology using agricultural feedstocks. The first project is a collaboration with the University of Galway to quantify fugitive emissions from the plant and digestate storage at Teagasc Grange, explore mitigation measures, and analyse the environmental sustainability of co-digestion of slurry and grass silage, with consideration of pretreatment and emissions mitigation. The other project, in collaboration with Trinity College Dublin, aims to characterise the microbial ecology of the Grange AD digester and investigate the activity of key functional species within the microbial community, thereby helping to optimise the system for more efficient operation. Both projects highlight the importance of the Grange AD plant in providing data and an environment for developing research with a direct impact on real-world problems that cannot be assessed in a laboratory.

BIOCHAR project:

The Teagasc AD programme is a partner in the BIOCHAR project, led by the University of Limerick and co-funded by the EU Just Transition Fund. This project focuses on producing biochar from indigenous biomass. Biochar is a solid product from thermochemical decomposition of biomass in absence or with limited oxygen. Depending on the raw material and the production conditions it might present characteristics that can reduce emissions in slurry storage tanks and enhance AD. The project will optimise production parameters for multiple applications and assess business opportunities for local stakeholders. Teagasc Grange will be responsible for evaluating the impact of adding biochar to improve the performance of the AD digesters through lab-scale experiments.

EXPAND project:

The AD programme is a partner in the EXPAND project, a Sustainable Energy Authority of Ireland (SEAI) and Department of Agriculture, Food and the Marine (DAFM) funded project led by University College Cork that examines how AD by-products (biogenic CO₂ and digestate) can be used within a broader bioeconomy context. Therefore, the EXPAND project aims to analyse the full potential of AD by-products, which involves research to identify feasible AD value chains and to evaluate the economic viability of establishing an AD by-product business ecosystem in Ireland.

GrassBÓ project:

Teagasc are leading the GrassBÓ project, also funded by SEAI-DAFM. The GrassBÓ project aims to develop decision-making tools to help stakeholders ensure a reliable and sustainable supply of materials, such as silage and slurry, to support the efficient development of the AD sector and achieve the biomethane target set for 2030. The project will investigate seasonal, regional, farm- and pasture-species dynamics and conduct feasibility analyses of farm-centric AD. It will also evaluate environmentally friendly ways to reuse digestate, a by-product of AD, on farms.

CliBeef project

The Teagasc Grange AD programme is a partner in the CliBeef project, supported by PEACEPLUS, a programme managed by the Special EU Programmes Body (SEUPB). The project is led by AFBI that focuses on reducing GHG emissions in Northern Ireland and Ireland’s beef industry. Twelve farms will be selected to evaluate the impact of improving breeding and grassland management, and efficient nutrient use and additives. The Teagasc Grange AD lab will conduct experiments to evaluate the impact of successful measures on the biomethane potential of slurry/manure from the animals in a scenario where the animal product is sent to an AD plant.

Conclusions

Anaerobic digestion is a crucial technology for achieving a circular bioeconomy in Ireland. By converting abundant agricultural feedstocks into renewable biomethane, AD can help to reduce gas imports while producing nutrient-rich biofertiliser, thereby reducing reliance on chemical fertiliser inputs. The Teagasc AD research programme’s core priority is to demonstrate how AD can support Ireland’s energy transition towards renewables and the decarbonisation of Irish farming by providing a model for biomethane production utilising sustainable agricultural feedstocks, nutrient management, and the valorisation of manures and digestate, and maximising circularity by incorporating digestate into feedstock production.

Compiled and edited by Mark McGee and Paul Crosson, Teagasc, Grange Animal & Grassland Research and Innovation Centre, and first published in BEEF2026 – Driving Sustainable Performance, additional reading from BEEF2026 is available here.