Value of Manure: Both Fertiliser and Biofuel in NI

NORTHERN IRELAND - Manures produced by housed farm livestock in Northern Ireland are a valuable resource both as a plant fertiliser and as a source of renewable energy.
calendar icon 26 November 2008
clock icon 5 minute read

According to the Agri-food and Biosciences Institute, housed farm livestock produce approximately 10 million cubic metres (2,200 million gallons) of undiluted manure each year in Northern Ireland. Almost 88% of this is cattle manure with pig and poultry manures accounting respectively for approximately 7% and 5% of the total, writes Dr Peter Frost.

Research work at AFBI Hillsborough is examining different ways of making use of this manure resource. Manure as a fertiliser

Manures contain plant nutrients that are a valuable fertiliser. Applying slurry at the wrong time wastes nutrients and money. The total annual requirements of grass and arable crops for fertiliser in Northern Ireland are much greater than that supplied by manures from housed livestock. When Northern Ireland is taken as a whole the major issue to address is redistribution of slurry rather than that of slurry surplus. This assumes that it is possible to distribute the manure produced from housed livestock in Northern Ireland over all grass and arable areas.

In Northern Ireland, most slurry is land spread by splash plate. This method can lead to much of the available N applied in the slurry being lost to the atmosphere as ammonia. Ammonia losses can be lowered by reducing the surface area of the spread slurry that is exposed to air. For grassland in Northern Ireland, slurry to air contact can be minimised by injection of slurry into shallow channels, band spreading and trailing-shoe application systems. Trailing-shoe application in which a metal shoe parts the grass and slurry is deposited in bands on the soil surface, with the minimum of herbage contamination, has been shown in work at AFBI Hillsborough to be effective. When compared with a splash plate, the trailing-shoe improved nitrogen efficiency and gave a window of opportunity for spreading slurry of up to three weeks after silage harvest.

The trailing-shoe has also been used during the last two years to apply slurry onto dairy grazing paddocks at AFBI, Hillsborough. Slurry was applied after the 1st, 3rd, 6th and 7th grazings in year 1 and after the 2nd and 5th grazing in year 2 to replace 150 kg per ha of fertiliser N in year 1 and 80kg per ha of fertiliser N in year 2. The impact of these slurry applications on animal and sward performance is currently being assessed.

Manure for renewable energy

Anaerobic digestion (AD) converts organic matter to biogas in the absence of oxygen. Biogas is an excellent form of renewable energy. Biogas can be burned through a gas boiler to produce heat or can be burned through a combined heat and power unit to produce electricity and heat. It is also possible to clean the biogas and use as a vehicle fuel. Many European countries are using biogas to generate electricity. AD can either be carried out centrally or at farm level.

In Denmark there are more than 20 centralised anaerobic digestion (CAD) plants. Typical agriculturally based 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.

Across Europe there are a large number of on-farm digesters in operation. In Germany, for example, there are almost 4,000 on-farm digesters operating. In the UK there are only about 30. In Northern Ireland, on-farm AD has the potential to provide additional income to farmers from the sale of renewable energy (mainly electricity) whilst at the same time preserve (or slightly improve) the fertiliser value of the slurry. At present, information available on the performance and economics of on-farm AD in Northern Ireland is at a very early stage.

In order to provide more detailed information, the Agri-Food and Biosciences Institute at Hillsborough has installed a 600 cubic meter on-farm anaerobic digester to handle the slurry from its 300 dairy cows. Biogas from this digester is currently producing heat through a gas boiler. Eventually the biogas will be used by a combined heat and power unit to produce about 26kW of electricity and about 43kW of heat. Data generated from research on this project will provide much needed information for the industry and will assist in informing and developing government policy.

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