Different Pathways to Integrate Anaerobic Digestion and Thermochemical Processes: Moving Toward the Circular Economy Concept

As one of the most environmentally friendly and cost-effective method, anaerobic digestion (AD) has been widely studied and developed as a conventional technology to degrade biodegradable materials and produce biogas simultaneously. Various substrate sources are used in this process such as organic fraction of municipal solid waste (MSW), waste activated sludge (WAS), animal manures, agro-industrial wastes, energy crops, micro- and macro-algae and etc. With the aim of process optimization, several publications have recently studied different configurations to integrate AD and thermochemical processes such as pyrolysis and gasification. These linking technologies seeks to optimize the use of products or by-products of thermochemical processes interchangeably. In this regard, this paper aims to review different potential pathways of feasible integration and coupling. Five hybrid pathways including biochar-amended anaerobic digestion, digestate-derived biochar and hydrochar, anaerobic digestion of aqueous phase liquid derived from pyrolysis and gasification of digestate were reviewed and their schematic diagram were presented. Despite several studies to combine AD with thermochemical valorization processes, further studies at the industrial scale are needed to prove the energy efficiency and economic viability of these coupling pathways.