Valorization of pulp and paper industry sludge to produce methane using different F/M ratios and its kinetics and...

The pulp and paper industry (PPI) produces various lignocellulosic wastes, including paper sludge, which is rich in cellulose and suitable for biofuel production. Anaerobic digestion (AD) has emerged as a promising treatment technology due to its environmental and economic benefits. Biochemical methane production (BMP) assays were performed at different food-to-microorganisms (F/M) ratios: 1.0, 1.5, 2.0, 2.5, and a control (Inoculum). Among the four different F/M ratios, F/M 1.5 achieved the highest methane yield of 272 mL CH₄/g VS, followed by 2.0 > 1.0 > 2.5 with methane yield of 250 > 247 > 227 mL CH₄/g VS over a period of 42 days. The effect of the F/M ratio on specific methanogenic activity (SMA) was evaluated to support the interpretation of BMP assay results. Further effects of substrate biodegradability and methane production rate were also evaluated in a kinetic study using two simplified models, among them the modified Gompertz model provided the best fit (R² = 0.997) to the experimental data. Statistical analysis using one-way ANOVA revealed that the F/M ratio significantly affected methane yield (p < 0.05). Fourier Transform Infrared Spectroscopy (FTIR) analysis confirmed the presence of active organic functional groups, which highlights the lignocellulosic degradation pattern before and after digestion. A phytotoxicity assay using Vigna radiata L. (mung bean) revealed a concentration-dependent decline in seed germination, shoot and root length, and biomass after digestion relative to the control. This study uniquely integrates F/M ratio optimization, biodegradability assessment, kinetic modeling, and statistical analysis to provide a complete understanding of methane production from PPI sludge. It also links process performance with environmental safety through phytotoxicity evaluation, offering a comprehensive waste-to-energy framework.