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The aim of this study was to evaluate life cycle environmental impacts associated with the generation of electricity from biogas produced by the anaerobic digestion (AD) of agricultural products and waste. Five real plants in Italy were considered, using maize silage, slurry, and tomato waste as feedstocks and cogenerating electricity and heat; the latter is not utilized. The results suggest that maize silage and the operation of anaerobic digesters, including open storage of digestate, are the main contributors to the impacts of biogas electricity. The system that uses animal slurry is the best option, except for the marine and terrestrial ecotoxicity. The results also suggest that it is environmentally better to have smaller plants using slurry and waste rather than bigger installations, which require maize silage to operate efficiently. Electricity from biogas is environmentally more sustainable than grid electricity for seven out of 11 impacts considered. However, in comparison with natural gas, biogas electricity is worse for seven out of 11 impacts. It also has mostly higher impacts than other renewables, with a few exceptions, notably solar photovoltaics. Thus, for the AD systems and mesophilic operating conditions considered in this study, biogas electricity can help reduce greenhouse gas (GHG) emissions relative to a fossil-intensive electricity mix; however, some other impacts increase. If mitigation of climate change is the main aim, other renewables have a greater potential to reduce GHG emissions. If, in addition to this, other impacts are considered, then hydro, wind, and geothermal power are better alternatives to biogas electricity. However, utilization of heat would improve significantly its environmental sustainability, particularly global warming potential, summer smog, and the depletion of abiotic resources and the ozone layer. Further improvements can be achieved by banning open digestate storage to prevent methane emissions and regulating digestate spreading onto land to minimize emissions of ammonia and related environmental impacts.

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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 analyzed 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. We are pleased to enclose the revised version of our original manuscript of our paper entitled "Agricultural anaerobic digestion plants: What LCA studies pointed out and what can be done to make them more environmentally sustainable" which can hopefully be published in Applied Energy.Many thanks to the reviewers for their comments; they helped us to improve the manuscript.We hope that the manuscript in the revised form is appropriate for publication in Applied Energy.On behalf of all the Authors, as corresponding Author, yours sincerely, Cover Letter Title: Agricultural anaerobic digestion plants: What LCA studies pointed out and what can be done to make them more sustainable Dear Editor, we would like to thank you for the reviewers' comments. We are grateful to all the reviewers for devoting their time to review our manuscript. As you can see, we have taken most of them into consideration to modify the paper. We hope that the paper in this current revised version can be accepted for publication. Below, we enclose an explanation of how we have addressed the questions raised. Reviewer #1The manuscript is working on the review of the environmental impacts of biogas production. It includes good results, an interesting methodology, and related to the scope of this Journal. I suggest to accept it after some minor revisions. Thank you for your positive comments and for useful suggestions.The authors should write about the difference between their study and the following research: * Hijazi, O., Munro, S., Zerhusen, B. and Effenberger, M., 2016. Review of life cycle assessment for biogas production in Europe. Renewable and Sustainable Energy Reviews, 54, 1291-300. Done, we specified in the int...