Biogas represents a key component of a balanced renewable energy system and plants provide useful waste management solutions for biological waste. However, recent studies carried out by Life-Cycle consultancy Footprints4Food using an analysis method developed by Bangor University show that under some circumstances crop-fed anaerobic digestion plants can generate electricity with a higher environmental burden than grid electricity.

Additional environmental benefits may arise from the use of other outputs produced by biogas plants, such as using digestate to replace synthetic fertilisers in crop production. However, depending on the design of the plant and digestate management practices, there is a risk of increased ammonia emissions.

While many biogas plants reduce greenhouse gas (GHG) emissions and resource depletion through avoided waste management and replacing grid electricity generation, some may also increase eutrophication and acidification burdens, resulting in a decrease in water and air quality. This is mainly due to ammonia emissions from the digestate storage and subsequent application of the digestate to land.

Ian Finlayson of Footprints4Food explains, "It all depends on the types of feedstock digested, what would have happened to those feedstocks otherwise, and how the biogas plant is designed and managed. Our recent analyses have thrown up a few surprises."

Understanding the parameters that are explored during life cycle assessment also means that plant developers and managers can optimise the management of their plant and reduce the overall environmental impact, not just the GHG balance of the resulting biogas.

David Styles of Bangor University adds, "Adjusting the mix of feedstocks digested in AD units can have a significant impact on their environmental balance. Sound business and operational management of AD requires robust scientific evidence on opportunities and risk factors.

"Up to now Life Cycle Assessment has primarily focused on the GHG balance of the gas or electricity produced. However, tighter environmental legislation, such as the revision of the Emissions Ceiling Directive 2001/81/EC could lead to further control measures on major sources of ammonia, including digestate storage and land spreading. It is vital that biogas plants are designed and managed to cope with these potential requirements."

Footprints4Food – an 'environmental footprinting service' linked to Bangor University – is now offering its 'LCAD Eco Balance' tool to biogas operators and developers, providing them with detailed reporting and highlighting which options operators can apply to improve the eco-balance of their anaerobic digestion system. More information, including an example LCAD Eco Balance Report is available from the company's website: www.footprints4farming.co.uk.