- Methane - released to the atmosphere during normal storage and utilisation of farm slurries. Methane is 23 times more potent as a greenhouse gas than carbon dioxide (CO2). Anaerobic digestion (AD) collects methane and provides a source of renewable energy that is carbon neutral i.e. provides energy with no net increase in atmospheric CO2.
- Fertiliser - compared to undigested slurry, the nitrogen in digestate is more readily available as a plant nutrient.
- Smell - AD can lower the odour from farm slurries by up to 80%.
- Pollution - AD can lower the biological oxygen demand, (BOD - a measure of the polluting strength of a material) in the feedstock to less than 40% of that in the digestate. However, BOD of digestate is still extremely high relative to the discharge standards for wastewaters.
- Pathogens - pathogens in the feedstock, such as salmonella, are lowered by AD.
- Weed seeds - AD kills many weed seeds and hence there is less need for herbicides.
- Plant nutrients - management of plant nutrients is aided by mechanical separation of the digestate. Plant nutrients in the fibre fraction can be exported off farm as a soil conditioner, or further processed into granular organic fertiliser or combustible fuel.
- Handling - compared to raw slurry, digestate flows easier and requires less mixing before spreading.
- Grazing - cattle can reject grass spread with untreated slurry; they do not readily reject grass spread with digestate.
Biogas gives direct financial returns when used to generate electricity. Including the value for renewable obligation certificates (ROCs) further increases these returns. Use of a combined heat and power (CHP) unit to produce electricity and hot water is of further benefit, provided the heat produced can be utilised fully to heat the digester and for export. Biogas can also be used in modified gas boilers to produce hot water for use on site, or for export. In addition, biogas can be scrubbed of impurities and fed into a natural gas grid, or used as a fuel for cars, buses and trains.