Draft PPS18: Renewable Energy
Annex 1 Energy from Waste (Biological Processes) Technology: Fuel Sources
C12. The main types of feedstock employed in AD are:
- Sewage sludge. This is the sediment that is removed from foul sewage during the course of treatment by a process of settlement. AD of sewage sludge currently takes place at many sewage treatment works in the UK, and some schemes already include energy recovery. The raising of sewage treatment standards, together with tighter controls on the disposal of sludge, has lead to greatly increased arisings, particularly in coastal areas where sludge dumping at sea ceased to be an option in 1998. Water Service are placing a priority on finding alternative methods of safe disposal. Energy recovery will potentially become more economically attractive where AD is the chosen waste treatment measure.
- Farm slurry. The intensive rearing of livestock, particularly cattle and pigs, produces large quantities of slurry – manure in liquid form – which is not only odorous but which can also present pollution problems if it is not carefully disposed of. Silage effluent can cause similar problems. Farmers can face stiff penalties for causing these substances to pollute watercourses.
- Municipal solid waste (MSW).Municipal refuse contains large quantities of food, garden waste, paper and packaging with a high organic content, and is therefore suitable for energy extraction via AD. With the introduction of the Renewables Obligation, the market for CAD plant, utilising 100% food processing waste for example, is large and is likely to grow still further.
C13. Digestion reduces the volume of the waste and also has the benefits of reducing odour and removing harmful pathogens, which is a particular advantage in the case of farm slurry and sewage sludge.
C14. Feedstocks for AD inevitably contain plant or animal pathogens (such as Salmonella) and parasites (such as Cryptosporidium) to different degrees in different materials. Precautions are therefore needed in AD projects, especially CAD projects which involve transporting residues from various sources to a central point, which could lead to cross-contamination unless appropriate preventative measures are taken. Mesophilic AD will reduce pathogens and bacteria, but will not eliminate them from waste. Thermophilic digestion will further reduce the levels, but cannot guarantee total removal.
C15. After any necessary pre-treatment, the waste is fed into a digester tank. The contents are then mixed thoroughly, either mechanically or by pumping gas through suitably located tubes inside the tank. Digesters are usually operated at temperatures of 35°C or 55°C. The rate at which the digestate breaks down through microbial action increases with temperature. At the same time, the survival rate of pathogens such as Salmonella reduces significantly.
C16. After the AD process has taken place, the gas generated is collected in a storage tank, with any excess gas being flared off. The contents of the digester will be a mixture of solids and liquids (digestate solids and digestate liquor), which might be suitable for beneficial use as fertiliser or soil conditioner (subject to legislation), or will otherwise require disposal.
C17. In ‘sequential batch’ digesters, the tank is loaded with the feedstocks (farm slurry etc), AD proceeds and the residues (i.e. the digestates) are then removed to make way for a new load. This method is often used in small-scale digestion schemes, such as those on individual farms. Larger scale digesters often employ a ‘continuous feed’ system in which the incoming feedstock is fed into the tank while an equivalent volume of processed waste is drawn off. The transport implications of peak movements need to be borne in mind for sequential batch digesters.