Citation: Kusch-Brandt, S.; Heaven, S.; Banks, C.J. Unlocking the Full Potential: New Frontiers in Anaerobic Digestion (AD) Processes. Processes 2023, 11, 1669. https:// doi.org/10.3390/pr11061669 Received: 21 May 2023 Accepted: 29 May 2023 Published: 31 May 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). processes Editorial Unlocking the Full Potential: New Frontiers in Anaerobic Digestion (AD) Processes Sigrid Kusch-Brandt 1,2, * , Sonia Heaven 1, * and Charles J. Banks 1 1 Water and Environmental Engineering Group, University of Southampton, Southampton SO16 7QF, UK; c.j.banks@soton.ac.uk 2 Faculty of Mathematics, Natural Sciences and Management, University of Applied Sciences Ulm, 89075 Ulm, Germany * Correspondence: mail@sigrid-kusch.eu (S.K.-B.); s.heaven@soton.ac.uk (S.H.) Anaerobic digestion (AD) is a bio-based solution designed to convert organic materials into renewable energy and other products, such as soil improver and organic fertiliser. AD is widely used in practice, with facilities at many thousands of sites worldwide: in Europe alone, more than 20,000 full-scale plants were in operation in 2022 [1]. The underlying biological processes are complex, and multiple options exist to steer the AD process towards optimised performance and a desired set of outputs in terms of energy and material flows. This puts AD in a prominent position with research agendas aiming for more sustainable resource management. Its ability to generate high-value products from organic wastes and residues is a key strength. The Special Issue on “New Frontiers in Anaerobic Digestion (AD) Processes” was initiated to explore recent developments and advanced concepts related to the valorisation of biomass via the application of AD. Fourteen submissions are included in the Special Issue, and each of these publications contributes towards unlocking the full potential of AD. Five thematic clusters to advance AD can be identified based on the included publications: • Understanding and monitoring the AD process; • Making substrates available and increasing the efficiency of the AD process; • Inspiring trust in non-academic stakeholders to adopt AD in practice; • Supporting decarbonisation of the energy system through hydrogen and biomethane; • Obtaining more value from a single unit of biomass. All included manuscripts contribute to more than one of these five clusters (Table 1). In this paper, some selected findings reported in the publications are highlighted. These are not intended to be exhaustive, but rather to provide some first insights into the rich body of new knowledge created by the authors of the Special Issue. For the purpose of maintaining the stability and performance of the AD process, in practice, only certain parameters can be monitored in real-time, while adequate methods are still lacking for many others. Yan et al. [2] reviewed the recent progress in applying soft sensor solutions. Some systems are available that use software-supported methods to determine the unmeasurable parameters based on measuring auxiliary variables online; but the need for more research remains high. Integration of deep learning elements into these software solutions is particularly promising. Liu et al. [3] focused on the residual biogas potential of digestate leaving the digester and the current time-consuming standard procedures to determine this indicator through experimental laboratory testing. Residual biogas potential is a key indicator of digestate stability, which in turn is an essential requirement for spreading digestate onto agricultural land. The authors showed that kinetic modelling, in particular when supported by machine learning, could be successfully applied to reduce the testing time for residual biogas potential. Processes 2023, 11, 1669. https://doi.org/10.3390/pr11061669 https://www.mdpi.com/journal/processes