To obtain a high methane yield during the anaerobic digestion of a straw and animal manure mixture, an integrated process of thermo-alkali-bi-enzyme hydrolysis-anaerobic digestion was proposed. A mixture of corn straw and cattle manure was selected as the experimental object. A higher dissolution efficiency of cellulose, hemicellulose, and protein in the thermo-alkali pretreatment, dosages and hydrolysis times of cellulase and protease in the bi-enzyme hydrolysis, and the methane yield and biogas production cycle in the anaerobic digestion with mixed slurry and hydrolysates were investigated respectively. The results showed that the dissolution efficiency (%TS) of cellulose (24.84%), hemicellulose (12.24%), and protein (8.92%) reached their highest levels at 0.5% NaOH and 80℃ (compared with the control group). The bi-enzyme hydrolysis process and conditions were as follows:cellulase hydrolysis was 80 U·g-1 and 18 h, and protease hydrolysis was 20 U·g-1 and 4 h. The hydrolysis efficiency of cellulose and protein reached 74.08% and 74.01%, respectively. The sugars in the hydrolysate were increased by 12-32 times. During anaerobic digestion, the maximum yield of methane from hydrolysate after thermo-alkali-bi-enzyme hydrolysis was 750 mL·h-1, and the gas production cycle was 50 h. Compared with the mixture after the thermo-alkali pretreatment, the methane production efficiency of the mixed hydrolysate after thermo-alkali-bi-enzyme hydrolysis was increased by 14 times, and the gas production cycle was noticeably shortened by 17 d. The results indicated that the thermo-alkali and enzyme hydrolysis pretreatment could effectively accelerate the hydrolysis rate in the anaerobic digestion with the mixture. The results of this study provide a new reference for developing efficient technology of high-value energy utilization of agricultural waste.
Keywords: anaerobic digestion; enzyme hydrolysis; methane yield; mixture; straw and animal manure; thermo-alkali pretreatment.