The increasing amount of organic solid waste produced worldwide highlights the need for effective and durable treatment methods. Unlike traditional liquid anaerobic digestion, High Solid Anaerobic Digestion (HS-AD), which typically operates at total solids (TS) contents above 15%, has become a promising technology because it requires less reactor volume, less heating energy, and generates less leachate. However, HS-AD often faces challenges such as mass transfer limitations, ammonia inhibition, and process instability. Co-digestion-the simultaneous digestion of two or more substrates-has proven successful in overcoming these issues by balancing nutrients, neutralising inhibitory chemicals, and regulating the carbon-to-nitrogen (C/N) ratio. This paper thoroughly examines recent advances in High Solid Anaerobic Co-Digestion (HS-AcoD). It critically analyses the fundamental mechanisms, the synergistic effects of different substrate combinations, and the influence of operational parameters on process performance. Particular attention is given to reactor configurations, microbial community dynamics, and methods to mitigate inhibition. The study also discusses technological and financial challenges that hinder the large-scale implementation of emerging techniques, such as bioaugmentation and the addition of conductive materials. The review concludes with perspectives on future research directions aimed at enhancing the efficiency and resilience of HS-AcoD systems for waste valorisation and bioenergy production.
High solid anaerobic digestion, co-digestion, synergistic effects, bioenergy, microbial community, process stability
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