Pretreatment with composite microbe and NaOH to improve anaerobic performance of corn straw

Abstract: Due to the existence of lignin in crop straws, the anaerobic digestion by only crop straw would be demonstrated as low in conversion and utilization efficiency. Lots of studies reported that the pretreatment of crop straws would destroy the combination of cellulose, hemicelluloses and lignin, and enhance the anaerobic performance. To improve anaerobic performance, in this study, corn straws were pretreated by composite microbes and NaOH to investigate their effects on straw components, anaerobic digestion performance and production costs. Corn straws were pretreated by the addition of composite microbes and NaOH with 2%、4%、6%、8% (calculated by dry weight of corn straws), and retting for 3 days with a thermostat setting of (38±2℃). Then the samples of the treated and untreated corn straws were withdrawn to determined cellulose, hemicelluloses and lignose contents in dry matter. Then corn straws were inoculated with sludge (40% of sludge and 60% of treated corn straw in dry matter). Anaerobic digestion experiments were carried out in 30 days under temperature of 38±2℃, and the daily biogas yields were measured by the gas-collecting method of drainage water. A control with untreated corn straws was also done to compare the difference with treated ones. The experiments were set up in triplicate. The experiment results showed that corn straws' total contents of cellulose, hemicelluloses and lignose were decreased. Compared with the control, the pretreatment method by 8% NaOH, 2% NaOH and 8% NaOH demonstrated better degradation effects for cellulose, hemicelluloses and lignose, and the maximum degradation rate were 33.5%, 38.7% and 23.1%, respectively. The reduction of cellulose, hemicelluloses and lignose helped to improve the anaerobic digestion efficiencies. The anaerobic experiments revealed that the biogas yields and production efficiencies of corn straws after pretreatment were improved. Compared with the control, their biogas production for unit dry weight were elevated from 120.70 mL/g·TS to 334.76, 309.51, 333.57, 374.30 and 309.71 mL/g·TS, respectively. The NaOH pretreatment with 6% addition were demonstrated as giving the best result. The digestion times that were reached to 80% of the maximum gas production were 21, 24, 19, 18 and 26 d for pretreatment by composite microbes and NaOH with 2%、4%、6%、8%. However, the amount of biogas produced by the control in 30 days was only 32.25% of the maximum gas production. These results showed that the pretreatments increased the gas production rates. The pretreatment variational costs were calculated to analyze whether the methods were fit for popularization. The variational costs included the fees for both raw materials and pretreatment reagents. In this study, the prices of composite microbes, NaOH and corn straw were calculated as 4 yuan/kg, 25 yuan/kg and 150 yuan/t, respectively. The variational costs for addition of composite microbes and NaOH with 2%、4%、6%、8% are 0.72, 0.80, 0.98, 1.09 and 1.57 yuan/m3, respectively. Taking anaerobic digestion performances and pretreatment costs into consideration, the pretreatment method with composite microbe would be a more conducive way to popularize.