Pre-treatment of organic biomass in the context of stand-alone and integrated anaerobic digestion–pyrolysis for enhanced product recovery: A critical review of challenges and opportunities
Humera Sarwar, Arun K. Vuppaladadiyam, Adithya Venkatachalapati, Nimesha Ratnayake, Kamrun Nahar, Manoj Kumar Jena, Ramandeep Kaur, Hari B. Vuthaluru, Kalpit Shah
Abstract
Organic biomass is a widely available renewable resource with significant potential to reduce dependence on fossil-based energy and support large-scale sustainable biorefineries. Nonetheless, to enhance the suitability of organic wastes for conversion into valuable chemicals and fuels, identifying effective pre-treatment strategies that can reduce the inherent structural complexity and resistance to degradation is critical. This review critically examines various pre-treatment techniques, including mechanical, thermal, chemical, and biological methods, and their influence on biomass structure, inhibitor generation, and conversion performance in stand-alone and integrated anaerobic digestion (AD) and pyrolysis (Py) systems. The review explores how pre-treatment facilitates improved biodegradability in AD and alters thermal decomposition behaviour in Py, thereby enhancing the yield and quality of resulting products such as methane, bio-oil, and biochar. Furthermore, with growing interest in integrated AD–Py configurations to maximise resource recovery and process efficiency, this study discusses the role of pre-treatment in bridging these two technologies. In addition, the techno-economic and environmental implications of pre-treatment are also evaluated, highlighting trade-offs such as increased energy input or chemical usage versus improved energy recovery and product quality. The review identifies critical challenges, including inhibitor management, process scalability, and sustainability barriers. It also outlines future research directions to develop robust, low-impact, cost-effective pre-treatment strategies tailored for individual and integrated AD–Py pathways. This work aims to support the design of next-generation biorefineries capable of achieving high-efficiency, circular bioresource utilisation.