Draft PPS18: Renewable Energy
Annex 1 Energy from Waste (Biological Processes) Planning Issues: Anaerobic Digestion
Site selection, Transport and Traffic
C38. Many AD plants will be located close to the waste source. Small digesters on farms can sometimes be accommodated quite satisfactorily within the existing complex of farm buildings. Sewage sludge digesters are likely to be built in conjunction with new or existing wastewater treatment works, and will be less noticeable amongst the array of tanks and ponds performing other treatment functions than as a plant in isolation.
C39. Centralised AD facilities (CAD plants) handling large quantities of agricultural wastes, sewage sludge or MSW may be more economically viable for the plant operators, but have the potential to raise more complex siting issues. The most acceptable sites are likely to be beside existing industrial or wastewater treatment works or, in the case of digestion schemes using MSW, in close proximity to a landfill site or waste transfer station.
C40. Transport movements at on-farm digesters are not likely to add significantly to the impact of normal farm activities. By comparison, CAD plants will draw traffic to their central location as feedstock is delivered and products are distributed. The impact of these transport movements can be minimised by carefully considering fuel supply logistics, thereby reducing the distances travelled between the feedstocks, storage tanks, digester and local markets.
Feedstocks and Product Storage
C41. Planning permission may be given to a scheme specifying a certain feedstock and in these circumstances the feedstock will not be able to be changed without the further planning consent. The appropriate authorities should be consulted early in the process when considering waste handling issues and classifications.
C42. The storage of farm slurry is covered by the Control of Pollution (Silage, Slurry and Agricultural Fuel Oil) Regulations (Northern Ireland) 2003 and the Nitrates Action Programme Regulations (Northern Ireland) 2006 , which specify minimum standards relating to the design, construction and operation of any farm slurry storage system. Storage facilities will also be needed for the processed fibre. The market is seasonal, so storage could be needed for up to six months output. Liquors can be stored on the farm, or at a CAD plant. Once cooled, they can be stored in lagoons or large tanks. For CAD sites, liquid storage facilities will need bunding around storage silos.
C43. The AD of organic materials is, by its very nature, an odorous process. Predicted odour effects and proposed mitigating measures such as odour control systems should be examined. If a location is considered to be sensitive to odour nuisance, the Department will seek information from the developer to ensure that all possible sources of odour are accounted for in the proposals for odour control.
C44. Odour may arise from:
- waste input storage bays: this is especially important during the summer, when the breakdown of organic material can begin before it is even collected for disposal;
- sorting and mixing plant: here the waste is sorted or mixed with digestate prior to digestion;
- the digester: although this is sealed during use, this will release odours when opened to allow cleaning; and
- digestate draw-off and de-watering plant: digested material is significantly less odorous than raw organic material, but can still give off unpleasant smells.
It should however be noted that AD can bring benefits in terms of odour reduction. The digestion of slurry, for example, is significantly less odorous than the common practice of storing slurry in pits.
Emissions to Ground and Watercourses
C45. Serious farm pollution incidents can occur through the leakage or run-off of raw agricultural wastes. The AD of farm waste should reduce the likelihood and capacity of the material to pollute controlled waters. By following the Department of Agriculture and Rural Development Code of Good Agricultural Practice for the Prevention of Pollution of Water, Air and Soil, emissions to ground and watercourses should be minimised.
Emissions to Air
C46. The production and use of biogas through AD results in a number of emissions to air, including those from gas vents, engine exhausts and flare stacks. These emissions are generally minor and are unlikely to present any significant environmental problem, provided the equipment meets relevant design specifications and is properly serviced. The Department’s Environment and Heritage Service (EHS) will apply Integrated Pollution Control regulations to larger plant which will control emissions; this will apply to larger on-farm schemes as well as CAD plants.