Agricultural anaerobic digestion plants : What LCA studies pointed out and what can be done to make them more environmentally sustainable

Anaerobic Digestion (AD) has been recognized as a viable solution to produce renewable energy and to reduce global warming especially when secondary feedstock and/or wastes are used. Several LCA studies analysed the environmental performances of biogas production systems. The results of this review highlight that the goal, scope, life cycle impact assessment (LCIA) methodology, feedstocks and geographical regions covered by the studies vary widely. Most studies are based in Europe, several in China and few in South and North America and in Africa. To better highlight how the choices on the feeding mix, the digestate storage, the surplus heat valorisation as well as the plant size can affect the environmental performances of agricultural AD plants four plants have been analysed in this study. The results suggest that the energy crops production and the operation of anaerobic digesters, including digestate emission from open tanks, are the main contributors to the impacts from biogas electricity. This entails that it is environmentally better to have smaller plants using slurry and waste rather than bigger plants fed with energy crops. Recovering heat waste as well as covering of digestate tank would improve significantly the environmental sustainability of biogas electricity, and particularly the global warming category.