Effects of Freezing-Thawing Pretreatment on Anaerobic Digestion of Wheat Straw and its Kinetics Analysis

Abstract In this study, freezing-thawing (FT) pretreatment of different freezing time and freezing temperatures was investigated to find the effect on anaerobic digestion of wheat straw (WS). The freezing temperature gradient is -10, -20, -40 and -80℃, and the freezing time gradient is 12 h, 24 h, 48 h and 96 h. Total methane production exhibited a mere distance among all samples. Morphology change sculptured by SEM showed this method broken the structure of WS leaving fragments and pores in varying degrees. Three kinetic models were performed on WS to represent the behavior of experimental data. Kinetic model parameters of total methane production and lag phase time showed that logistic function model had the best fit, followed by modified Gompertz model, yet transfer function model lost efficacy in this experiment. Logistic function model was then used to reveal the influence on lag phase caused by freezing time and freezing temperature, the results implied that FT pretreatment can shorten the lag phase time of anaerobic digestion (AD), providing a 21.39% improvement under the optimal conditions of -20℃ 96 h. The analysis of response surface regression shows that the freezing temperature has more effect on the lag phase time of anaerobic digestion than freezing time. Warmer freezing temperature of -20℃ do better than -80℃ on lag time, which can be achieved in most cold regions, so this treatment can occur naturally in such area without additional energy input.In this study, freezing-thawing (FT) pretreatment of different freezing time and freezing temperatures was investigated to find the effect on anaerobic digestion of wheat straw (WS). The freezing temperature gradient is -10, -20, -40 and -80℃, and the freezing time gradient is 12 h, 24 h, 48 h and 96 h. Total methane production exhibited a mere distance among all samples. Morphology change sculptured by SEM showed this method broken the structure of WS leaving fragments and pores in varying degrees. Three kinetic models were performed on WS to represent the behavior of experimental data. Kinetic model parameters of total methane production and lag phase time showed that logistic function model had the best fit, followed by modified Gompertz model, yet transfer function model lost efficacy in this experiment. Logistic function model was then used to reveal the influence on lag phase caused by freezing time and freezing temperature, the results implied that FT pretreatment can shorten the lag phase time of anaerobic digestion (AD), providing a 21.39% improvement under the optimal conditions of -20℃ 96 h. The analysis of response surface regression shows that the freezing temperature has more effect on the lag phase time of anaerobic digestion than freezing time. Warmer freezing temperature of -20℃ do better than -80℃ on lag time, which can be achieved in most cold regions, so this treatment can occur naturally in such area without additional energy input.