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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:
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.
AD is a multi-faceted process and has a number of different benefits:
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.
An AD plant can provide multiple economic benefits, such as:
The economic viability of an AD plant is very dependent on the type and quantity of feedstock and the utilisation of the by-products, biogas, bio-fertiliser and, to a lesser extent, heat. The optimisation of the AD process is key to the productivity – both biological and financial – of any AD plant.
There are three potential types of feedstocks that can go into an AD system either singly or, in most cases preferably, combined:
These include animal wastes such as cattle and pig slurry, manures, feed waste and bedding.
Purpose grown crops cultivated for their high calorific value. The key crops used in the UK are maize silage, grass silage and whole crop cereals.
Industrial, commercial and domestic.
The feedstock enters the digester where bacteria feed on the organic matter to produce solids, liquids and biogas. Feedstock has varying biogas yields:
|Feedstock||Biogas Yield (m3 per tonne)||Equivalent Value (£ per tonne)|
|cattle slurry||15 - 25||4.00 - 7.00|
|pig slurry||15 - 25||4.00 - 7.00|
|poultry manure||30 - 100||8.00 - 30.00|
|maize silage||200 - 220||54.00 - 60.00|
|grass silage||160 - 200||43.00 - 55.00|
|whole crop wheat||170 - 190||50.00 - 60.00|
|fodder beet||420 - 500||110.00 - 125.00|
|crude glycerine||580 - 1000||155.00 - 270.00|
Slurry is an excellent feedstock that helps promote a healthy AD system, however, it is not the best producer of biogas as the animal has already digested the readily available material. We would therefore recommend that, even on large slurry-based systems, co-digestion is considered with fresh undigested feedstocks to increase biogas production. This may be the addition of wet energy crops, such as grass silage, whole crop cereals, or maize. Glycerine is also a very valuable as a co-digestion product within a biogas plant which is currently widely available from bio-diesel producers.
You may also consider bringing in food waste to digest for which a gate fee could be demanded. Food waste requires additional permitting and processing (pasteurisation) which can increase capital and operating costs, but these costs can be offset by high gate fees.
To find out the energy potential of your available feedstock please complete our enquiry form giving us as much information as possible. We will get back to you to discuss your options.
Digestate is a nutrient-rich substance produced by the AD process consisting of left over indigestible material and dead micro-organisms. The volume of digestate is around 90-95% of feedstock inputs.
Using digestate in place of synthetic fertilisers saves energy, cuts consumption of fossil fuels and reduces farm expenditure and management costs.
Typical values for digestate nutrients are:
However, the nutrients are considerably more available to plants than in raw slurry. This is an often underestimated benefit of an AD plant. Significant improvements in both yield and crop quality are possible.
Biogas produced from the AD process can be utilised in power and heat generation through a Combined Heat and Power (CHP) unit, or, after upgrading and purifying, can be injected to gas grid or used as biomethane transport fuel.
The biogas upgrading process is often expensive, and an undeveloped UK infrastructure can make it difficult to realise benefits. The most accessible and cost-effective benefits to many AD operators is therefore through power and heat generation. Electricity and heat can be utilised by operators leading to significant reductions in energy costs. Electricity can also be exported to grid.
A range of government financial incentives are in place to assist uptake of renewable AD energy (see below).
The Feed in Tariff is a government index linked incentive offering guaranteed payments for each kWhr of electricity generated.
In addition any electricity exported to the grid receives a further payment from the energy supplier, currently set at a minimum 4.5p/kWhr. Exporters can negotiate higher payments with energy companies, with up to 6p/kWhr achievable.
The RHI provides a fixed income (per kWh) to generators of renewable heat, and producers of renewable biogas and biomethane AD facilities completed after 15 July 2009 are eligible for the RHI. The lifetime of the tariff is 20 years. The current RHI for AD is as follows (from April 2012)*:
*As of 20th Sept 2012 The Department of Energy and Climate Change (DECC) have announced proposals to remove the current 200kW limit. See here
Update Feb 2013: Further consultation is underway which may lead to different conclusions from the Sept announcement. Watch this space.
A Renewables Obligation Certificate (ROC) is a green certificate issued to an accredited generator for eligible renewable electricity generated within the United Kingdom and supplied to customers within the United Kingdom by a licensed electricity supplier.
An anaerobic digester will receive 2 ROCs/ MWh until April 2015. This makes ROCS a viable option for larger scale AD plants, particularly after FiT degression in 2014.
Unlike FiTs the value of ROCs is not fixed and may reduce as well as increase - an indication of values can be found here.