Effects of mixing ratios on anaerobic co-digestion of swine manure and rice straw: methane production and kinetics

The effects of seven mixing ratios (1:0, 3:1, 2:1, 1:1, 1:2, 1:3, and 0:1, swine manure (SM)/rice straw (RS), based on total solid) on anaerobic co-digestion (co-AD) of SM and RS mixtures were investigated in batch tests. The results showed that the highest methane yield (188.79 mL/g VS added ) was obtained at the ratio of 1:1, which was higher than that of the mono-digestion of RS or SM by 178.77% and 18.94%, respectively. For the remained six ratios, the methane yield increased with the increase of SM proportion. The highest organic matter degradation rates were also observed at the ratio of 1:1. The methane content of biogas and the pH, total alkalinity, and total ammonia nitrogen of the digestate all decreased with the increase of RS proportion and decrease of SM proportion. Additionally, the kinetic results of each anaerobic digestion (AD) step suggested that the co-AD of SM and RS at the ratio of 1:1 significantly improved the reaction rates of the four AD steps. Meanwhile, the methane production time was prolonged and the lag time of methanogenesis was shortened at the ratio of 1:1.