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Draft PPS18: Renewable Energy
Annex 1: Energy from Waste (Biological Processes)

C1. This section offers guidance on systems using biological processes to extract energy from waste, in terms of their main characteristics, the basic technology and their environmental implications. This covers systems using the following as a fuel to generate heat and/or electricity: landfill gas; sewage gas; biogas from agricultural waste; digestible domestic or industrial waste. All these gases are products of an anaerobic digestion process, which is explained further below. Each process in this section begins by discussing anaerobic digestion in general, and subsequent to this, any differences relating to either sewage gas or landfill gas are described.

Anaerobic Digestion

C2. Anaerobic digestion (AD) is a method of waste treatment that produces a gas with high methane content from organic materials such as agricultural, household and industrial residues and sewage sludge (feedstocks). The methane can be used to produce heat, electricity, or a combination of the two. The process has the benefit of using waste substances that are otherwise difficult to dispose of in an environmentally acceptable manner. Energy from AD is also effectively carbon neutral in that the carbon it releases is approximately equal to the carbon absorbed from the atmosphere by the plants which constitute the origin of the organic waste. It can therefore reduce overall quantities of carbon dioxide released in the atmosphere when it is used to replace energy from fossil fuels. When used for heating, the process is simple, with the minimum pre-treatment of the gas required, and the use of simple, well-proven technology.
C3. Methane is a significant contributor to global warming (around 21 times more potent than carbon dioxide over a period of 100 years). AD with energy recovery offers an effective means of trapping this gas and converting it to carbon dioxide, which is less potent as a greenhouse gas, while producing a renewable source of energy. By-products of AD may be put to beneficial uses such as compost and liquid fertiliser. Such products can help reduce the demand for synthetic fertilisers and other soil conditioners that may be manufactured using less sustainable methods.
C4. The AD process has been used widely in the UK agricultural sector in the form of small on-farm digesters producing biogas to heat farmhouses and other farm buildings. An AD project is most likely to be part of an integrated farm waste management system in which the feedstocks and products all play a part. However there is potential for larger scale centralised anaerobic digesters (CADs) using feedstocks imported from a number of sources.

Sewage Gas

C5. Sewage sludge differs from farm waste in that it generally has a far higher inert content (usually >40% of the dry solid matter in sewage is ash). However, as it is only the organic matter that is digested, the gas produced from sewage is of a similar composition to that from farm waste, and the main difference in the digestion plant is one of scale: as sewage waste treatment is generally more centralised, sewage sludge digesters are usually much larger than farm waste digesters.
C6. Organic waste materials such as food, paper and garden wastes decompose in landfills to produce landfill gas (LFG), a mixture of methane, carbon dioxide and a wide range of minor components. Using LFG provides energy from a source which would otherwise be flared off or vented to the atmosphere and so wasted.
C7. The total waste produced in the UK is estimated to be about 434 million tonnes per year. Different types of waste vary immensely in their fuel values and characteristics. Municipal solid waste (MSW) and business waste are the largest potential sources of waste derived energy. However the composition and calorific value of these materials can vary markedly. The proportion sent to landfill will fall in the long term as a result of changes in waste management practices with, for example, increasing recycling. The EU Landfill Directive, implemented in Northern Ireland by the Landfill Regulations (Northern Ireland) 2003 , Opens link in a new browser window will also progressively ensure the diversion of organic material from landfill, reaching 75% of 1995 levels by 2010; 50% of 1995 levels by 2013 and 35% of levels by 2020. Nevertheless landfill is likely to remain a significant means of waste disposal for some time and the sites will remain biologically active for decades to come.
C8. The main difference between landfill gas systems and other forms of anaerobic digestion is that the landfill itself is effectively the digester, so there are no constructed tanks for this purpose. However, the generation plant used to extract the gas is broadly similar to that employed for other forms of anaerobic digestion.
This section contains the following sub-categories:
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