Anaerobic co-digestion of cabbage waste and cattle manure: Effect of mixing ratio and hydraulic retention time

The aim of this study is to optimize the process parameters for the anaerobic co-digestion of cabbage waste and cattle manure, with the objective of maximizing methane production. Batch experiments were conducted using five different mixing ratios (1:0, 3:1, 1:1, 1:3, and 0:1), and continuous experiments were run at six different hydraulic retention times (HRTs) of 25, 20, 15, 10, 8, and 5 days. The results indicate that the co-digestion displayed a synergistic effect, as evidenced by a cooperative index greater than 1. Additionally, an appropriate cabbage waste to cattle manure ratio of 3:1 yielded a maximum methane production of 440 mL/g-VS. During the continuous study, the highest methane production rate of 1.55 L/(L·d) was achieved at 10-day HRT. Efficient methane yields in the range of 322–336 mL/g-VS were obtained at HRTs of 25, 20, and 15 days. Methane yield decreased when HRTs were shortened to 10 or 8 days due to the accumulation of volatile fatty acids and the loss of methanogens. At an HRT of 4 days, methane production nearly ceased. These results provide insights for the optimization of anaerobic co-digestion of cabbage waste and cattle manure, leading to higher methane production.