In Northern Ireland, housed livestock produce 9.7m t/y of manure - 88% cattle; 7% pigs; and 5% poultry (Frost, 2005). This manure has an annual potential to produce approximately 73 MW_electric and 60 MW_heat through anaerobic digestion (Frost, 2005). Furthermore, AD plants can include mechanical separation of fibrous solids from the digestate that, after further processing, can give a value added product, such as compost or pellets (fertiliser or combustible fuel). The most appropriate way for Northern Ireland to realise this energy potential is through centralised anaerobic digestion (CAD) schemes (Frost, 2005). In addition to energy production, there is considerable potential for CAD to assist in centrally managing the distribution of plant nutrients in manures, together with minimising biosecurity risks (pathogen kill) (Frost, 2005). Whilst CAD has potentially a major role in Northern Ireland and offers the most appropriate and immediate way forward, there is also significant potential for on-farm AD. This potential is as yet undefined, as are the economics and the most appropriate specification for an on-farm digester.

Across Europe there are a large number of on-farm digesters in operation, though there are only about 30 within the UK. In Germany, for example, over 2,500 on-farm digesters are currently in operation. Frost and Beck (1991) in a report for the Department of Agriculture, Northern Ireland (DANI) concluded that, at 1991 oil prices and capital costs for digesters, on-farm AD in Northern Ireland was not a viable economic proposition. Current oil prices are considerably greater than those in 1991 and as a result on-farm AD may be financially viable. Within Germany, for example, the current economics of on-farm AD are favourable. This is as a result of the Renewable Energy Sources Act (EEG) 2000 and 2004 that guarantees (for 20 years) a premium price for electricity generated from solar energy, hydropower, wind power, geothermal power and biomass.

Typical agriculturally based centralised AD (CAD) plants use farm products (livestock manures and crops) as the main feedstocks, as well as other organic material from, for example, food processing. Co-digestion can provide an additional source of income through gate fees and can improve the yield of biogas per unit of feedstock input. CAD plants can be thermophilic or mesophilic. Compared to typical on-farm plants, CAD plants are larger (0.1-1.0 MW_electricity), give economies of scale and offer better market opportunities for heat (for local industry and/or district heating) and fibre production. CAD schemes can involve a number of farms within a radius of about 10 km from the plant. All agriculturally based CAD schemes distribute digestate back to agricultural land, normally that of the supplying farms. Raw slurry and digestate are rich in plant nutrients (nitrogen, phosphorus and potassium). Digestate must be applied to agricultural land in accordance with crop requirements for plant nutrients. Nutrient management is a major issue for consideration when determining the feasibility of any AD scheme. CAD schemes have major potential to assist in managing and redistributing plant nutrients in slurry. When redistributing digestate to farms it is very important to ensure biosecurity. All CAD schemes should include sterilisation of material prior to redistribution.