Optimization of the Process Conditions for Methane Yield from Co-Digestion of Mixed Vegetable Residues and Pig Manure Using Response Surface Methodology

To determine the optimal co-digestion conditions for four mixed vegetable crop residues (including cucumber, tomato, eggplant, and pepper, mixed in equal parts by mass) with pig manure, we conducted a study using response surface methodology (RSM). The interaction effects of parameters, such as manure-to-mixed vegetable residue ratios (M/S), initial pH, and organic load (OL) with respect to cumulative methane yield, were investigated. The results showed that the influence of each factor on the CMY is in the following order: initial pH > M/S > OL. The highest cumulative methane yield was calculated to be 380.50 mL/g VS at an initial pH of 7.3, OL of 18.8 g VS/L, and M/S of 3.9:1. Comparison and verification experiments showed that under optimized conditions, the co-digestion process showed increased methane yield and had practical application value. The microbial analysis showed that the relative abundances of bacterial taxa, such as Clostridium_sensu_stricto_1, Fastidiosipila, and Terrisporobacter, were all highest in the co-digestion samples under optimized process conditions (PV). Different types of methanogenic archaea taxa in PV samples were richer than in other samples, which showed higher relative abundances of Methanogenium, Methanobrevibacter, Methanoplanus, Methanospirillum, and Methanobrevibacter. Thus, the co-digestion system of a mixture of vegetable residues and pig manure can enrich different types of methanogenic archaea taxa, which leads to increased digestion performance and may strengthen process stability. Importantly, pig manure and mixed vegetable residues can be included in anaerobic digestion applications through co-digestion, thus enabling the valorization of these substantial residues and engineering for applications.