Optimising Biogas Production from the Anaerobic Co-digestion of Primary Sewage Sludge with Intermediate Landfill Leachate using Response Surface Methodology

Anaerobic co-digestion has emerged as an effective approach for treating municipal and industrial wastewater, offering the added benefit of renewable energy generation through biogas production. This study investigated the optimisation of biogas yield via the co-digestion of sewage sludge and intermediate landfill leachate using Response Surface Methodology (RSM) based on a Box-Behnken Design (BBD). Three key parameters; co-substrate loading ratio (1:20–1:5 v/v), inoculum-to-substrate ratio (ISR; 1:2–1.5:1), and temperature (25-55 °C)—were examined for their influence on biogas yield (mL/gVS), chemical oxygen demand (COD) removal, and volatile solids (VS) reduction. Numerical optimisation identified the optimal conditions as a leachate (co-substrate loading) of 1:20, an ISR of 1.5:1, and a temperature of 42.21 °C, achieving a desirability of 90.10 %. Under these conditions, biogas yield reached 90.76 mL/gVS, with COD and VS removals of 33.62 % and 43.25 %. The developed RSM models showed strong predictive capability (0.9 < R² < 1) and were validated experimentally. Observed values closely matched predicted values, with discrepancies below 5 %, confirming the model's reliability. The model validation experimental runs resulted in an average biogas production of 87.90 mL/gVS, with corresponding chemical oxygen demand (COD) and volatile solids (VS) removal efficiencies of 35.16 % and 39.90 %. These findings underscore the potential of optimised co-digestion for enhanced biogas production and effective waste treatment.