Physical/Mechanical Pretreatment Strategies for Sewage Sludge: A Comprehensive Review on Unlocking Biogas Yields Through Process Optimization

Sustainable sewage sludge management has become a major global concern due to increasing sludge generation, rising wastewater treatment demands, strict disposal regulations, and the growing need for energy and resource recovery. Anaerobic digestion (AD) is widely applied for sludge stabilization and biogas production; however, its efficiency is limited by the complex structure of waste activated sludge, in which biodegradable organics are protected within extracellular polymeric substances, microbial cells, and compact flocs. Pretreatment technologies have been widely studied to enhance sludge solubilization and methane production. However, the relationships between process optimization, methane yield, biodegradability, and energy performance remain insufficiently understood. In this review, physical/mechanical and thermal pretreatment strategies, including ultrasonication, microwave irradiation, high-pressure homogenization, electro-kinetic disintegration, and thermal pretreatment, are critically evaluated to determine their potential for unlocking biogas yields through process optimization in applications ranging from laboratory to full scale. Reported studies indicate that pretreatment can improve organic matter solubilization and enhance methane or biogas production, depending on sludge characteristics, solids concentration, energy input, temperature, pressure, exposure time, and digester configuration. Nevertheless, excessive pretreatment severity can decouple solubilization from methane recovery by generating refractory soluble compounds, increasing energy consumption, impairing dewaterability, or destabilizing anaerobic conversion. The review concludes with a comparative assessment of these pretreatment technologies, providing insights into their relative productivity, optimization limits, and implementation potential. This study proposes a process-oriented framework for more efficient AD operation and stable methane generation, supporting sustainable biogas recovery within circular wastewater treatment systems.