Moderate thermal-chemical-enzymatic pretreatment for waste activated sludge: Methane yield enhancement and life cycle...

Anaerobic digestion (AD) is a promising pathway for waste activated sludge (WAS) valorization, but its efficiency is often constrained by slow hydrolysis. Although pretreatment can enhance hydrolysis, intensified conditions frequently increase energy demand and environmental burdens. In this study, a combined thermal-chemical-enzymatic pretreatment strategy was systematically evaluated using an orthogonal design, together with methane production kinetics, life cycle assessment (LCA), and scenario-based techno-economic assessment (TEA). The protease-based pretreatment at 55 °C and pH 10.0 (PHB) achieved the highest methane yield (204.65 mL/g VS) and solubilization efficiency, but also resulted in the highest global warming potential and energy demand. In contrast, the cellulase-based strategy at 45 °C and pH 10.0 (CMB) achieved 92% of the maximum methane yield (189.06 mL/g VS) with lower environmental impacts and reduced sensitivity to electricity fluctuations. The TEA further showed that electricity recovery alone could not offset pretreatment costs, whereas sludge-disposal credit, combined heat and power (CHP) heat recovery, and low-cost crude enzyme substitution substantially improved the economic performance of moderate pretreatment strategies. Overall, this study highlights the trade-off among methane recovery, environmental burden, and economic constraints, and provides an integrated framework for optimizing sludge pretreatment under practical conditions.