Life cycle assessment of manure biogas production: A review

Manure is a livestock residue that causes high environmental burdens in different categories. Its usage in biogas production is an environmentally beneficial way not only to reduce these impacts, but also to produce energy and biofertilizers. Although manure has low energy value and low biogas conversion efficiency, it is indicated for use in co-digestion with other substrates to produce biogas. The present work provides a systematic review of studies that have analyzed the production cycle of biogas from manure around the world, to detect and compare the critical points of the life cycle assessment (LCA) steps defined by the International Standardization Organization (ISO), as well as the biogas process steps. The critical points related to LCA structure in the analyzed articles involve the different boundaries considered, since the initial stage varies from one that considers only the input of raw material in the digestion process, to the one that considers the upstream steps to obtain the substrate. Despite this, the comparison of LCA structure in the analyzed articles is possible, since the ISO standards contain well-defined parameters and steps. The critical points in biogas production processes involve different variables in regional case studies, such as climate, raw material availability, transportation distance, source of inputs and energy, plant location and technological development level, hindering comparison between studies. In relation to the level of technologies, no details of industrial steps were observed in the LCA studies of manure biogas. In spite of the difficulty, comparison between stages is developed in this work to allow careful evaluation of critical points and environmental impacts, and consequently to help establish regional sustainability strategies for the production of biogas from manure. (C) 2019 Elsevier Ltd. All rights reserved.