Advances in carbonaceous promoters for anaerobic digestion processes – Functions and mechanisms of action

Various carbonaceous structures, such as graphite- and biochar-derived materials, have been recently developed to enhance biogas production by 1.3%–1130 % during anaerobic digestion (AD). There are several reviews discussing the promotion of direct interspecies electron transfer, but they lack a critical discussion of the various mechanisms that CMs can induce upon addition to AD systems. This paper specifically explores how the characterization of CMs (electrical conductivity, surface functional groups, specific surface area, surface charge, and hydrophobicity) affects the major mechanisms of action involved, i.e., interspecies electron transfer, buffering capacity, adsorption, and colonization, to fill the knowledge gap in this field. The feedstocks used and methods developed for synthesizing CMs and the applicability of various modification techniques to enhance the properties of CMs for such applications have been critically addressed. This review closes with a review of current challenges in the application of CMs and provides recommendations for future research. In conclusion, this review helps academic researchers and industrial professionals select suitable feedstocks and employ appropriate synthesis methods and modification strategies to prepare the most efficient CMs for AD enhancement.