Technical Report

Utilisation of digestate from biogas plants as biofertiliser

June 2010

Greenhouse gas emissions, nitrate and phosphate enrichment of inland and coastal waters and deteriorating air quality are major issues throughout the world. While there is a growing global demand for more efficient heat, power, transport and food production, all of these processes are still largely reliant on fossil fuels. Within agriculture the manufacture and use of inorganic fertiliser is recognised to give rise to particular issues, arising both from fossil fuel use and from nitrous oxide emissions. Many governments are now giving a high priority to lowering pollution from all of these factors.

This brochure describes how Anaerobic Digestion of manures and other organic residues can be used to produce both renewable energy and organic fertiliser – as biogas and digestate. Renewed worldwide interest in Anaerobic Digestion is being driven by different factors in different parts of the world. In the EU, for example, current issues such as global warming, demand for renewable energy, landfill tax on organic waste, demand for organic fertiliser, high fossil fuel prices, pollution of the environment and legislation relating to the treatment and disposal of organic wastes are all important factors influencing increasing levels of investment in Anaerobic Digestion. The information in this brochure is intended to inform prospective biogas/digestate producers as well as policy makers and regulators. The reference section lists valuable sources of further information; these should be helpful to anyone wishing to explore the subject of Anaerobic Digestion in more detail.

Anaerobic Digestion (AD) in a biogas plant is a well proven process in which organic matter breaks down naturally in the absence of oxygen to produce two valuable products – biogas and digestate. Biogas is an extremely useful source of renewable energy, whilst digestate is a highly valuable biofertiliser (Lukehurst 2010). AD can also offer a range of other benefits.

• Lowers fossil fuel use
• Lowers mineral fertilisers use
• Lowers GHG emissions from open manure stores
• Provides a highly efficient method for resource recycling
• Closes the production cycle

Over the last 50 years, increasing use of inorganic fertilisers throughout the world has been central to increased farm production. However, the volatility of world oil prices has had a major effect on the use of oilbased fertilisers. For example, high oil prices are leading both to increased costs to farmers and also to lower fertiliser consumption. The digestate is a very useful organic1 fertiliser that can be used to offset the financial as well as the environmental costs associated with the use of mineral fertiliser.