Optimization of Biogas Production from Co-digestion of Palm Oil Mill Residues with Zeolite Immobilization Using Response Surface Methodology

This study evaluated palm oil mill residues as feedstock and optimized biogas production via anaerobic co-digestion with cow dung, using zeolite as an immobilization medium. Response Surface Methodology (RSM) based on Central Composite Design in Design Expert v10.0 was used to assess the effects of hydraulic retention time (HRT), pH, pressure, and temperature. HRT was the most influential factor, followed by pH and pressure, while temperature had minimal impact. Optimal conditions—HRT of 17 days, pH 8.00, pressure 4 bar, and temperature 35 °C—produced a predicted biogas yield of 3410.75 mL, closely matched by a validated yield of 3412.71 mL with only 0.057% error. The model was robust (R² = 0.9995, insignificant lack of fit).Feedstock quality was confirmed by proximate and ultimate analyses (volatile solids 10.17%, volatile matter 17.53%, total solids 103.59 mg/L, C:N ratio 49.49). SEM revealed structural voids post-digestion, EDX showed reduced elemental content, FTIR indicated active biodegradation, and GC-MS showed 72.43% methane. This confirms the process's efficiency and the suitability of zeolite-enhanced co-digestion. RSM proved to be an effective modelling tool, supporting the potential for industrial-scale, sustainable biogas production.