Optimization of process condition for methane yield from co-digestion of both thermochemically treated pig manure and Napier grass using response surface methodology

13 Pretreating pig manure (PM) and Pak Chong 1 Napier grass (NG) before co-digestion is 14 essential for enhancing methane production. This study aims to optimize methane output under 15 various experimental conditions using a semi-continuous laboratory-scale reactor and Response 16 Surface Methodology. Different PM/NG ratios (1:2 to 2:1), organic loading rates (OLR) between 17 0.41 and 1.22 g VS/L, and dilution volumes (750 to 950 mL) were investigated, with cumulative 18 methane production (CMP) as the primary response variable. The Box-Behnken design was 19 employed, and data were analyzed using a quadratic model and analysis of variance to assess the 20 significance of factors and their interactions. Over a 60-day period, the optimal parameters 21 identified were a PM/NG ratio of 1.08, an OLR of 1.04 g VS/L, and a dilution volume of 903.54 22 mL, leading to a predicted CMP of 22,112 mL. The analysis indicated significant linear effects of 23