Green and Sustainable Anaerobic Digestion of Waste Activated Sludge Through Integrated Primary and Secondary Pretreatment Balancing Solubilization, Biodegradability, and Energy Performance

Acid–thermal pretreatment can enhance anaerobic digestion (AD) of waste activated sludge (WAS), but its value depends on balancing methane recovery against additional energy demand. This study evaluated primary pretreatment (PP) of raw WAS and secondary pretreatment (SP) of anaerobically digested sludge in single- and two-stage AD configurations. PP increased methane production from 67 ± 1 to 104 ± 1 mL, corresponding to a 55% improvement, whereas digestates undergoing a second digestion stage without SP produced no detectable methane. After SP, methane production recovered to 55 ± 2 mL and 46 ± 4 mL from digestates derived from untreated and PP-treated sludge, respectively. Fluorescence excitation–emission matrix analysis and molecular-weight profiling showed differences in dissolved organic characteristics, supporting the interpretation that solubilization extent alone did not explain methane recovery. The combined PP + SP configuration achieved the highest methane yield (113 mL CH4/g COD) and COD removal (67%). However, all four configurations retained negative physical net energy balances under the stated calculation boundary. The combined configuration had the largest deficit (−492.8 MJ/t), owing to its additional pretreatment heating and temperature-maintenance requirements. Thus, increased methane recovery did not necessarily improve net energy performance. The scenario-based preliminary economic assessment indicated potential cost savings under the adopted disposal tariffs and costing assumptions, without establishing full-scale economic feasibility. These findings emphasize balancing methane recovery, organic quality, net energy performance and conditional economic benefits when selecting WAS pretreatment strategies.