Under the EU Landfill Directive (1999/31/EC) the UK is obliged to meet tough targets for reducing the amount of biodegradable waste which we send to landfill sites. Biodegradable waste means such things as food waste, garden waste, paper/card etc, basically more or less anything produced from an animal or plant. Based on 1995 figures, we must reduce to 50% by 2013 and to 35% by 2020.
The reason biodegradable waste is such a problem is that, as it degrades, it gives of both carbon dioxide and methane, both “greenhouse gases”. In addition it can pollute both soils and groundwater.
What is Anaerobic Digestion?
Anaerobic digestion is a natural process which biodegradable waste will eventually go through. It involves microorganisms breaking down the waste and producing methane gas and carbon dioxide as a bi-product. This bi-product is known as biogas and can be used just like natural gas for either space heating or to fuel a generator to produce electricity.
An anaerobic digester is a device which accelerates the digestion process and so the production of biogas. It is fed with biodegradable waste, heated to the appropriate temperature and left to do its work.
Why Use Anaerobic Digestion?
If left to degrade naturally in landfill, organic waste will still produce methane which will then be released into the atmosphere. Methane is about 21 times more effective than carbon dioxide as a greenhouse gas making it a major contributor to global warming.
When burned as fuel, the methane releases carbon dioxide into the atmosphere. Obviously this is greenhouse gas too, but a much less effective one. Also, the use of biogas will generally be in substitution for the use of fossil fuels which would have produced their own carbon dioxide if burned.
What Happens to the Waste After Digestion?
The amount of waste left over after anaerobic digestion is roughly equal to the amount that went in, however it comes out free of offensive odours and is used as a very effective fertilizer, both solid and liquid.
Feed In Tariff & Renewable Heat Incentive
Under the Feed In Tariff scheme, electricity produced by burning biogas from anaerobic digestion is eligible for a payment 11.5 pence per KWh of electricity produced. The plan is for the renewable heat incentive to include a payment for biogas exported to the gas grid.
How Much Does an Anaerobic Digester Cost?
This depends greatly on the size and type of the plant and the type of waste to be used and it would not be appropriate to suggest a figure here. Projects tend to be bespoke rather “off the shelf” as with, say, a biomass boiler or solar panels.
How Much Energy Can an Anaerobic Digester Produce?
This depends on the type of system and the type of waste. Estimates range from 20 cubic metres – 800 cubic metres per tonne of waste (a metric tonne is around 2,204 lbs). One tonne of cow manure produces 36 cubic metres of biogas whereas the same amount of kitchen waste produces 140 cubic metres.
One cubic metre of pure methane biogas can produce 10 kWh of energy. In practice the gas is usually around 60% methane and 40% carbon dioxide, so producing 6 kWh per tonne of waste. To give some perspective, the average UK home uses around 21,500 kWh of energy for heat each year.
Could I Install an Anaerobic Digester at Home?
Although possible, a domestic anaerobic digester is not really practical at the moment. First of all, it would not be large enough to reach the optimal temperature of 35°C for producing biogas and second, the average home does not produce enough waste.
In the UK we might produce around one tonne of biodegradable waste per household per year, which at most could produce around 140 cubic metres of biogas. At a rate of 6 kWh per cubic metre this equates to 840 kWh of heat energy, which is only about 4% of the total energy a typical home will use in a year.
It is possible that a community project, where the biogas (and the fertilizer) was sold, could be viable, subject to planning permissions and licences (biogas is combustible so there are obvious health and safety issues).