Microwave assisted anaerobic digestion of waste-activated sludge: Integrating multi-scale characterisation and energy assessment for enhanced biogas recovery

Anaerobic digestion (AD) is a promising technology for energy recovery from waste-activated sludge (WAS), yet its efficiency remains constrained by poor biodegradability and limited substrate accessibility. Pretreatment processes can enhance sludge disintegration and improve methane production, but their energy efficiency and large-scale feasibility remain uncertain. In this study, the impacts of microwave pretreatment (MP) and conventional pretreatment (CP) on sludge characteristics and AD performance were systematically investigated using integrated experimental and analytical approaches. Both pretreatments significantly enhanced methane yields, with moderate microwave intensity (MP600) achieving the highest improvement (69.1 %) compared to CP (57.9 %) and high-intensity microwave treatment (MP1000, 62.8 %). Multi-scale characterisation using fluorescence excitation-emission matrix (EEM) spectroscopy with parallel factor analysis (PARAFAC) and self-organising maps (SOM), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and multi-omics analyses revealed that microwave pretreatment effectively disrupted sludge microstructure, promoted solubilisation of organic compounds, and induced beneficial microbial community shifts that facilitated hydrolysis and methanogenesis. A full-scale energy assessment further confirmed the process feasibility, demonstrating that all pretreatments yielded positive net energy gains. MP600 achieved the highest net balance (13.97 kJ/gVS), surpassing CP (13.22 kJ/gVS) and MP1000 (13.30 kJ/gVS). These results highlight that low-power microwave pretreatment offers the most favourable trade-off between enhanced methane recovery and energy efficiency, representing a technically and economically viable strategy for sustainable sludge management and bioenergy generation.