Bioenergy in the era of circular economy: Anaerobic digestion technological solutions to produce biogas from lipid-rich wastes Vasileios Diamantis a, * , Alexandros Eftaxias a , Katerina Stamatelatou a , Constantinos Noutsopoulos b , Christos Vlachokostas c , Alexandros Aivasidis a a Department of Environmental Engineering, Democritus University of Thrace, Vas. Sofias 12, Xanthi, 67132, Greece b Sanitary Engineering Laboratory, Department of Water Resources and Environmental Engineering, School of Civil Engineering, National Technical University of Athens, 5 Iroon Polytechniou, Zografou, 15780, Athens, Greece c Laboratory of Heat Transfer and Environmental Engineering, Aristotle University of Thessaloniki, Box 483, Thessaloniki, 54124, Greece article info Article history: Received 11 August 2020 Received in revised form 22 November 2020 Accepted 8 December 2020 Available online 11 December 2020 Keywords: Anaerobic digestion Bioeconomy Circular economy model Fat oil grease Wastewater treatment abstract The circular economy model has gained increasing attention in policy agenda in modern times of climate change and decarbonisation. Undeniably, reintroducing waste in supply chains and cover energy needs for local communities and enterprises assists to rational management of waste, promotes resource ef- ficiency and properly applies the circular model. Biogas is a major vehicle for producing bioenergy in the frames of modern bioeconomy. In this short review, the fundamentals and the technology for biogas production from lipids and lipid-rich wastes are meticulously and critically discussed. Anaerobic di- gesters with external sludge separation and recycling are the mainstream option for the treatment of lipid-rich wastes. These systems retain both microorganisms and the lipids adsorbed onto the anaerobic biomass, thus improving the methane yield. Anaerobic membrane bioreactors can completely avoid the washout of microorganisms and enhance the retention of lipids, however membrane fouling often de- teriorates hydraulic performance. The review clearly depicts that data from the operation of full-scale facilities treating lipid-rich wastes are scarce in the literature. Effort is required by all stakeholders (practitioners, companies, scientists and politicians) to leverage the penetration of anaerobic technolo- gies, to exploit lipid-rich wastes in order to efficiently promote renewable bioenergy and support bioeconomy. © 2020 Elsevier Ltd. All rights reserved. 1. Introduction Earth is constantly pressed by human needs and global society is continuously transforming. Resources are both crucial and finite and their consumption should be optimized [1]. It is more than vital to rationalize their management towards promoting circularity, encountering climate change and sustainable development [2]. The circular economy model has gained increasing attention in policy agenda in modern times of climate change and decarbonisation. Undeniably, reintroducing waste in supply chains and cover energy needs of local communities and enterprises, assists to rational management of waste, promotes resource efficiency and properly applies the circular model. In this way bioeconomy is enhanced, renewable bioenergy is produced and bioproducts (e.g. energy, fertilizer) are valorised [3]. Circular economy can be combined with biotechnology to enhance local entrepreneurship and protect the environment from uncontrolled waste disposal [4,5]. This is also the case for lipids and lipid-rich wastes. For instance, there are important perspectives for the exploitation of approximately 1 million tons of waste cooking oils that are generated in European Union (EU), which are usually discharged through public sewerage systems, increasing the water treatment costs in wastewater treatment plants [6]. Large quanti- ties of lipid-rich wastes are also generated by numerous agro- industrial enterprises and more details can be found in Section 2 of this review. Biogas is the main vehicle for producing bioenergy from anaerobic digestion in the frame of modern bioeconomy [7]. In the EU, there is a gap between the centralized and industry level fa- cilities targeting lipid-rich wastes, and as a consequence these substrates are often disposed of, in an uncontrolled manner [8]. Biodiesel production is the prevailing process in the management of used cooking oil, however this is limited to high quality feed- stocks in terms of solids and free fatty acid content [9, 10]. It should * Corresponding author. E-mail address: bdiamant@env.duth.gr (V. Diamantis). Contents lists available at ScienceDirect Renewable Energy journal homepage: www.elsevier.com/locate/renene https://doi.org/10.1016/j.renene.2020.12.034 0960-1481/© 2020 Elsevier Ltd. All rights reserved. Renewable Energy 168 (2021) 438e447