Effects of liquid digestate pretreatment on biogas production for anaerobic digestion of wheat straw

Abstract

In this study, the effects of liquid digestate pretreatment of wheat straw on its biogas production efficiency and anaerobic digestion were investigated using a laboratory-scale biochemical methane potential test. Scanning electron microscope, X-ray diffraction analysis and Fourier transform infrared spectroscopy were used to analyze the changes in main compositions and physico-chemical structure of wheat straw after different pretreatment durations (3 days, 5 days, and 7 days). The results evidenced that liquid digestate pretreatment can dissolve the lignocellulosic structure, which positively promoted anaerobic biogas production. Liquid digestate pretreatment achieved a satisfactory anaerobic digestion efficiency with wheat straw as the substrate. Furthermore, the production of biogas and methane were also found to increase after pretreatment. Thus, it was concluded that a duration of 5-day liquid digestate pretreatment was optimal as it yielded 39.76% more biogas production, and 49.58% more biomethane yield, in 35.71% shorter digestion time (T80) as compared to raw wheat straw. The modified Gompertz model was able to explain the kinetic behavior during anaerobic digestion of wheat straw. Based on the aforementioned experimental observations, a comparison between effects of equivalent NH4+-N concentration of ammonia solution and liquid digestate pretreatment was conducted, which indicated that ammonolysis played crucial role in the enhancement of biogas production process by pretreated liquid digestate.