A review of circularity and sustainability in anaerobic digestion processes.

The debate on the relationship between the long-established concept of sustainability, and the relatively novel one of circularity in constantly increasing biogas production remains. In this study, additional discussion elements to such an open debate are provided. With its role in the bioeconomy and ongoing ambiguity, a bibliographic review of anaerobic digestion is provided. In particular, this study aims to i) verify whether sustainability assessments and circularity measurements are performed in different ways in anaerobic digestion projects and ii) understand which indicators have been utilized for each pillar of sustainability. Initially, 152 scientific documents from the Scopus and Web of Science scholarly journal databases were selected. Specific eligibility criteria that were any type of measurement of circularity and/or assessment of sustainability, were used for screening. Fifty-eight articles met these criteria and were analyzed in depth. The results show that the terms circularity and sustainability are not always univocal concepts in the reviewed scientific contributions. Consequently, the relative criteria or measurements for their analysis are not the same. As a result, a different interpretation of the two concepts is suggested. Circularity should be considered as one of the ways to achieve the broadest objective of sustainability.