Anaerobic Digestion (AD) is a natural biological process where organic material is broken down by bacteria. It is similar to composting but differs in that it takes place in the absence of oxygen. The process occurs naturally in swamps and bogs and also takes place in landfill sites.
AD is a 4-stage process: hydrolysis, acidification, acetogenesis & methanogenesis. These are long words which describe why cows have four stomach compartments..
An Anaerobic Digester is a device for optimizing and controlling the digestion process to produce:
- Biogas (made up of around 60% methane, 40% CO2 and small amounts of hydrogen, hydrogen sulphide, nitrogen, and water vapour).
- Biofertiliser (digestate).
AD has been applied for nearly a century in the water industry as the favoured method for the stabilisation of sewage sludge, with biogas production viewed as a bonus. There are thousands of digesters in rural China, India and Nepal where village-scale plants are important for the recycling of nutrients in farm manure and producing biogas for cooking and lighting.
The technology has been widely adopted across Europe. In Denmark there are a number of farm co-operative AD plants which produce electricity and district heating for local villages. In Sweden biogas plants have been built to produce vehicle fuel. In Germany and Austria, encouraged by favourable government renewable energy policies, there are several thousand on-farm digesters treating mixtures of manure, energy crops and restaurant waste, with the biogas used to produce electricity.
How It Benefits You
AD stabilises organic wastes and prevents unwanted pollution. Uncontrolled methane (CH4) emissions are avoided (CH4 is 22 times more powerful than CO2 as a greenhouse gas), foul odours are all but eliminated, pathogenic organics are killed (complete eradication is achieved by the inclusion of pasteurisation) and weed seeds are destroyed.
Organic wastes, including animal manures, contain valuable nutrients, but significant proportions are locked up in unavailable forms. The AD process retains all the nutrients and, importantly, converts them into available forms enabling a farm to plan its nutrient management and reduce its use of fossil fuel dependant mineral fertilisers.
AD produces renewable energy in the form of biogas. Biogas can be used either in a conventional boiler, or as the fuel for a combined heat & power (CHP) unit. Alternatively it can be cleaned of impurities to use as a vehicle fuel or injected to gas grid.
Typical Finance Plan
Typical Site Plan
Here is a typical site plan of an Anaerobic Digester that runs Grain Barns and Chicken Broiler Sheds.
As you’ll produce both heat and electricity with an Anaerobic Digester you’d be eligible for both the RHI and Feed-In Tariffs offered from the government. For more information on these visit our incentives page.
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