Feasibility of anaerobic co-digestion for biogas production from recycled paper industry sludge: optimization of mixing ratios and application in two-stage CSTR system design

Anaerobic co-digestion (AnCoD) presents a promising route for valorizing sludge generated from recycled paper processing. This study explored the co-digestion of primary sludge (PS) and secondary sludge (SS) at various mixing ratios to enhance methane generation and system stability. Batch biochemical methane potential (BMP) assays revealed that the 1:3 PS:SS ratio produced the highest methane yield (918.66 mL CH₄/g VS_fed) with a notably short lag phase of 1.59 days. Kinetic assessment using both modified Gompertz and logistic models indicated that the former offered superior fitting accuracy (R² > 0.986), effectively describing methane production dynamics. A two-stage continuous stirred-tank reactor (CSTR) system operated under this optimal ratio showed distinct functional separation: the acidogenic stage facilitated hydrolysis and volatile fatty acid (VFA) degradation, while the methanogenic stage supported biogas generation with stable pH and low VFA/alkalinity ratios. Microbial analysis confirmed a clear differentiation between fermentative and methanogenic communities, with evidence suggesting enhanced electron transfer pathways. These findings underscore the potential of AnCoD for efficient sludge stabilization and bioenergy recovery in the pulp and paper sector.