Thermophilic anaerobic digestion of pulp and paper sludge: BMP and pilot-scale CSTR performance

This study evaluated the thermophilic anaerobic co-digestion of primary and secondary pulp and paper sludge through BMP assays and pilot-scale CSTR operation. Primary sludge exhibited a methane potential of 381 m³ CH₄/ton VS, 2.5 times higher than secondary sludge (149 m³ CH₄/ton VS). Co-digestion at a 3:1 TS ratio yielded 332 m³ CH₄/ton VSfed under thermophilic conditions - substantially higher than mesophilic operation. Pilot-scale thermophilic digestion achieved stable performance with 51.3% TCOD, 29.1% SS, and 52.5% VSS removal, alongside a methane yield of 333 m³ CH₄/ton VSfed. Transitioning from a two-phase to a single-phase configuration eliminated foaming and improved operational robustness. High CaCO₃-derived alkalinity ensured strong buffering, though it slightly reduced methane concentration. The system offered an energy recovery potential of 15,716 kWh/day and 2,834 tons CO₂-eq/year reduction. Although short-term stability was maintained, the COD:N:P ratio (∼400:10:1) suggests phosphorus may limit long-term performance.