Optimization of methane production through co-digestion of pig manure with napier grass

Alkali-pretreated Napier grass (NG) was applied as a substrate to increase biogas yields in a mesophilic anaerobic co-digestion system with pig manure (PM). The goal was to identify optimal conditions for maximizing methane yield and improving the efficient use of these materials for sustainable energy. A laboratory-scale setup was used, applying Box-Behnken design and response surface methodology. Key variables included PM/NG ratio (1:2, 1.25:1, 2:1), organic loading rate (OLR) (0.5, 1, 1.5 g VS/L), and total solids content (TS) (1%, 3%, 5% was evaluated, with cumulative methane yield serving as the response variable. The optimal methane production from co-digesting PM and NG was found under conditions with a PM/NG ratio of 1.18, OLR of 0.62 g VS/L, and TS of 4.8%. Under these conditions, methane yield was predicted to be 331.59 mL/gVS, which closely approximated the experimentally observed value of 324.89 mL/gVS. This correspondence confirmed the validity of the optimization results. The kinetic study showed that the Modified Gompertz model accurately captured methane production dynamics, with a high R2. Additionally, significant quadratic effects for the three parameters and notable linear impacts of OLR and TS on biogas production were observed during the co-digestion process.