There is a lack of literature reporting the measurement and prediction of biochemical methane potential (BMP) of vegetable crop residues (VCRs) and similarly, the kinetic assessment on the anaerobic digestion process of VCR is rarely investigated. In this paper, the BMP tests of five different vegetable (snap bean, capsicum, cucumber, eggplant, and tomato) crop residues were conducted at feed to inoculum ratio (F/I) of 2.0 under mesophilic (36 ± 1 °C) conditions. A series of single-variable and multiple-variable regression models were built based on organic components (hemicellulose, cellulose, lignin, total fat, total sugar, and crude protein) for BMP prediction. Three kinetic models, including the first-order kinetic model, the Chen and Hashimoto model, and the modified Gompertz model, were used to simulate the methane yield results of VCR and obtain valuable model parameters simultaneously. As a result, the BMPs and volatile solids (VS) degradation degree of different VCRs were respectively in the range of 94.2–146.8 mL g⁻¹ VS and 40.4–49.9%; the regression prediction models with variables lignin (R² = 0.704, p = 0.076), variables crude protein and lignin (R² = 0.976, p = 0.048), and variables total fat, hemicellulose, and lignin (R² = 0.999, p = 0.027) showed the best performance on BMP prediction among the single-factor, two-factor, and three-factor models, respectively. In addition, compared to the other two kinetic models, the modified Gompertz model could be excellently fitted (R² = 0.986–0.998) to the results of BMP experiment, verification deviations within 0.3%.

The effect of reducing particle size on physical properties, the methane yield and energy flow were investigated through the biochemical methane potential (BMP) experiment of aerobic-anaerobic digestion (AAD) of rice straw (RS). The whole straw was crushed through four sieves of different aperture sizes (1, 3, 5, and 7 mm) to obtain the actual and non-uniform particle size distribution (PSD). The results indicated that the actual particle sizes were normally or logarithmic normally distributed. Reducing particle size could significantly promote the aerobic hydrolysis and acidification process, increase the content of volatile fatty acids (VFAs) from 4408.78 to 6225.15 mg/L and the degradation of volatile solids (VS) from 40.56% to 50.49%. The results of path analysis suggested that particle size reduction played an important role in improving lignocellulosic degradability, which was the main factor affecting methane production with the comprehensive decision of 0.4616. The maximum methane production obtained at 1 mm sieve size was 176.47 mLCH₄g⁻¹ VS. The phyla of Firmicutes (61.5%), Proteobacteria (9.3%), Chloroflexi (8.3%), Bacteroidetes (4.1%), Cyanobacteria/Chloroplast (4.6%) were mainly responsible for VFAs production and lignocellulose degradation. However, the net negative energy balance was observed at the 1 mm sieve size due to the increased energy input. Therefore, the optimum sieve size for AAD was 3 mm.

A new method has been developed to improve the buffer capacity and methane production of the anaerobic digestion of Corn Stalk (CS), in which both an anaerobic co-digestion of CS with Straw Depolymerization Wastewater (SDW) and a mono-digestion of CS at different substrate concentrations (25.9, 36.2 and 45.3 mg/L) were investigated. Batch assays were conducted under thermophilic conditions for 70 days, which showed that an anaerobic co-digestion significantly increased the buffering capacity and methane production of the digestion process. The lag time for methane generation resulting from an anaerobic co-digestion of CS with SDW was 10 days, while the lag time for a mono-digestion of CS was 23 days. A maximum methane production of 214.81 mL/g-VS was obtained for the anaerobic co-digestion of CS with SDW when the substrate concentration was 36.2 g/L, which was around 13.54% higher than for mono-digestion of CS of 189.20 mL/g-VS. The removal rate for sulfate increased from 10.43% to 58.40% when the substrate concentration was increased from 25.9 to 45.3 mg/L for the anaerobic co-digestion of CS with SDW. Microbial communities were analyzed using 16S rDNA sequencing technology which showed that anaerobic co-digestion of CS and SDW promotes the growth of methanogens. The relative abundance of these methanogens (Euryarchaeota) for the anaerobic co-digestion of CS with SDW was increased significantly, being approximately 8.25% higher than that of a mono-digestion of CS, which was at a substrate concentration of 36.2 g/L. This means that the anaerobic co-digestion of CS and SDW is beneficial for improving buffer capacity and methane production from the digestion of CS, with higher organic matter and sulfate removal rates also being obtained.

To solve the difficulty of lignocellulose hydrolysis and the formation of crusted scum in anaerobic fermentation, various parts of corn stover, i.e., pith, rind and leaf, were subjected to a two-phase processing including aerobic hydrolysis (AH) and anaerobic fermentation. The results showed that AH significantly broke down the lignin structure of the various components of corn stover and increased the rate of lignin degradation. After 16 h of AH, the lignin degradation rates of the pith, rind and leaf were 4.20%, 3.91% and 4.90%, respectively, and the acetic acid produced accounted for more than 60% of the total amount of volatile fatty acids (VFAs) and ethanol. After hydrolyzing the pith and rind for 12 h and the leaf for 8 h, the maximum methane yields of fresh mass volatile solid (VS) were 323 mL g⁻¹, 251 mL g⁻¹ and 264 mL g⁻¹, respectively, which were increased by 35.02%, 30.05% and 8%, respectively, while the fermentation cycle of T90 (90% of the total gas production) was shortened by 4–5 days. After hydrolyzing the rind and leaf for 12 h and the pith for 16 h, the thicknesses of the scum layer were only 7.1%, 13.6% and 18%, respectively, of that of the untreated group, indicating that AH coupled with anaerobic fermentation can effectively degrade lignin, reduce the thickness of the scum layer and increase the methane yield.