(Organic Waste Biogas Plants)
Image: The surface of a well mixed high solids digester.
This list contains information on all the farm waste digesters in Northern Ireland.
In each case the project director was Dr. Les. Gornall
Operating information accurate as of December 1996
Index (In reverse chronological order)
Digester - 180 cubic meter Mode - Plug Flow (probably another reluctant CSTR in practice)
Designed feedstock - Broiler Poultry and Pig Manure
Feedstocks treated - Broiler Poultry and Cattle Manure (pig unit closed for non -digester related reasons)
Total Solids - 20%-25%
Operating Temperature - 35-40 C
Retention time - 5-9 days
History. Built to remove pressing pollution problems and provide an alternative farm income, the unit also includes large scale composting technology for downstream processing of the fibre into domestic organic peat substitutes. This is the largest digester fibre composting operation in Northern Ireland. The AJAX engine has proved to be extremely reliable.
Digester - 180 cubic meter Mode - Plug Flow (probably a 'reluctant' CSTR in practice) - two chamber design.
Designed feedstock - Broiler Poultry and Cattle Manure
Feedstocks treated - Broiler Poultry and Cattle Manure
Total Solids 20-22%
Mode - Plug flow Design
Operating Temperature 35-40 C
Retention time 10-15 days
History. Built to produce compost, the unit also includes sophisticated composting technology for downstream processing of the fibre. This unit was a test bed for development work on the processing of organic wastes as well as producing the finest golf course material seen to date from an anaerobic Digester. Sales of the compost and sports turf supplies are handled from the site under the trade name GrassRoots Tel. Contact Geoff Connell 01265 320972 Fax 01265 52342.
Digester - 180 cubic meter
Designed feedstock - pig slurry solids
Total Solids - 15% , rainwater screened out and irrigated
Feedstocks treated - pig slurry
Mode - Plug flow Design - Probably a CSTR in practice.
Operating Temperature 38-41 C
Retention time - 6 days
Built to assist with the disposal of pig effluent which was a farm limiting factor, the unit produced excess gas which was not used. Fibre from the plant was sold for further processing. The potential of the process and the site has been proved over the years and the owner won a NFFO (Non-Fossil Fuel Obligation) contract in 1996 to provide the grid with up to 250kwe. of power for 15 years. The site is being redesigned to accommodate a larger digester (incorporating the existing digester?) with full chp capacity - to be installed subject to contracts. Slurry is landspread using a large tanker.
Digester - 60 cubic meters
Designed feedstock - Dairy cattle manure and silage effluent
Feedstocks treated - Dairy cattle manure and silage effluent
Total Solids -15-20% (stackable plus some slurry)
Mode - CSTR design - operated in semi-batch load mode, ie. filled when the cattle sheds are cleaned.
Operating Temperature - 28-35 C
Retention time - variable 10-40 days
Outputs : Preserved Liquid fertiliser for grassland. Separated fibre to monastery compost production for some years.
This unit has reportedly removed the need for imported fertilisers and has significantly improved the farm economics. It was the first of the second generation digesters designed specifically for farm use and performs well under difficult loading conditions. Its main use is the production of stable liquid fertiliser spread using low soil compaction methods. The benefits have been removal of fertiliser from the farm budget. Better cash management, better cattle management, grazing a few days after slurry spreading, healthy grassland. This is one happy farm.
Digester - Kit form Farm Gas FGB 210 cubic meters.
Designed feedstock - beef cattle manure and silage effluent.
Total Solids - 10 - 12%
Feedstocks treated - beef cattle manure and silage effluent plus some poultry manure in early years.
Mode - Plug flow Design (actually proved to be a reluctant CSTR from lithium trace analysis)
Operating Temperature 28-35 C
Retention time - 20 days
Digester designed in 1984 and built from a kit of parts and assembled under the supervision of James Murcott of Farm Gas. The project was the subject of Les. Gornall's doctorate. Originally conceived as providing the gateway to large scale organic farming, integrating (as Cistercian Fr.Jim Conlon would put it) "more wholesome " farming of cattle using organic grass / clover/grain feed, with the production of organic oats for human consumption from the digested liquid fertiliser.
This unit won the 1986 Pollution Abatement Award from the Royal Society of Arts/Confederation of British Industry and the Department of the Environment.
In 1987 the system came equal second in a field of 1500 entries in the European Year of the Environment Awards in Brussels.
More importantly, the system has operated since 1985 on beef cattle slurry and the unit is still heating the monastery for 220 days a year when the cattle are housed.
1. (Interaction between anaerobic and aerobic processing of farm effluent - D.Phil, Dept. of Environmental Studies, University of Ulster,Coleraine -1990).
Les. Gornall preparing to boil an egg on a digester made from a plastic oil drum and tractor tyre inner tube for gas storage in 1978.
After some years of theoretical work there was only one way to see how high solids digestion worked and that was to DO IT. Operating at up to 70% dry matter (not a success!) the barrel digester above was able to boil an egg (just!) The straw bales are the insulation and the window provided heat in the morning. This photograph, taken by a Paddy Walsh (an ecologist with a real sense of humor) shows the earliest form of the digester - this is as crude as they come - but the lessons learned with this digester have stayed with the author for many years. Both the absolute temperature and temperature stability were important on this fragile little biogas maker. The feedstock had to be optimised for good gas production. Stirring (or technically speaking 'rolling') could dramatically improve the gas production (two eggs worth). Corrosion overtook even the simple valve arrangement on this unit - a small digester does not mean lower Hydrogen Sulphide! Something over a decade later, my ex-next door neighbour, a farmer, asked "did anything come of that little blue flame you made in the outhouse?"
The general decline of the agricultural economy in 1991/2 followed soon after by the BSE crisis mitigated against further installations in Northern Ireland, however there are signs of an upturn in the availability of finance to fund these large scale investments on the larger farms or in centralised anaerobic digester schemes. Furthermore, the BSE crisis has indicated the need to adopt more organic / natural farm practices. Experience of anaerobic digestion in Northern Ireland has clearly demonstrated that compared to the cost of another BSE type outbreak, (reportedly 3 to 5 thousand million pounds in the UK), the development of anaerobic digestion would be very, very cheap. The benefits of such a course of action would include better utilisation of fertiliser, soil reconstruction and separation of high phosphorous fibre to remove excess soil water phosphorus which is now causing problems in freshwater lakes. AD would produce significant environmental and economic improvements to agriculture in the long term and could provide a gateway for a truly green rural economy.
There are no other farm waste digesters in Northern Ireland - 100% success!
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