Mechanism of ball milling pretreatment to improve the anaerobic digestion performance and energy conversion efficiency of corn straw

Abstract

In this study, the effects of ball-milling (BM) pretreatment on the anaerobic digestion (AD) performance and energy conversion efficiency of corn straw (CS) were explored. The AD testing was conducted by varying BM times (0–120 min) and TS (4–10 %). The results showed that BM pretreatment increased microbial and enzyme accessibility to the CS and facilitated the hydrolytic acidification process of AD. The peak concentration of volatile fatty acids (VFAs) (5325.69 mg/L) and chemical oxygen demand (COD) (14524 mg/L) were increased, resulting in more substrates available for methanogenic bacteria. The lag phase time was shortened (37.41–56.12 %) and the cumulative methane production was increased (45.63–104.11 %). Accelerated rate of total ammonia nitrogen (TAN) release resulted in increased buffering capacity of the digestion system. Reduced the initial density of the digestive slurry (6.91–9.58 %) and altered the water distribution (maximum increase in free water content to 70.4 %), which facilitated the agitation of the AD process. Excessive BM times (>60 min) and TS (>8%) are not recommended. The BM60-6 presented the highest volatile solids (VS) removal (78.79 %), cellulose degradation (92.07 %) and biochemical methane potential (BMP) (309.69 mL/g VS). The BM pretreatment also improved the rheological characteristics of digestive slurry and reduced the stirring power consumption of the AD process. The BM60-6 showed the largest net energy benefit (28.2101 KJ/g VS) and net residual value (0.5000), and the high energy input of the pretreatment was balanced. This discovery breaks the conventional wisdom that mechanical pretreatment is meaningless because of higher power consumption.