Thermophilic Solid-State Anaerobic Digestion of Alkaline-Pretreated Corn Stover

Abstract

Solid-state anaerobic digestion (SS-AD) of corn stover (CS) was conducted at mesophilic (37 °C) and thermophilic (50 °C) conditions with/without solid-state NaOH pretreatment. Fourier transform infrared (FTIR), X-ray diffraction (XRD), and scanning electron microscopy (SEM) were employed to help with the understanding of the changes of the physicochemical structure of pretreated CS. Additionally, thermophilic SS-AD of pretreated CS and chicken manure (CM) was investigated. First-order, Cone, and modified Gompertz models were used to evaluate the methane production. Results showed that solid-state NaOH pretreatment could partly dissolve lignin and hemicelluloses and significantly increase the internal surface area of CS, which could make it more readily biodegradable. At a substrate/inoculum (S/I) ratio of 3, mesophilic SS-AD of CS failed because of the accumulation of organic acids. However, under thermophilic SS-AD conditions, biogas and methane yields of pretreated CS were found to be 386.3 and 194.8 mL g–1 of VSadded, respectively, which were 29.4 and 40.1% higher than those of untreated CS. Thermophilic solid-state co-digestion of pretreated CS with CM showed no beneficial effects for enhancing the biogas and methane yields, because of the accumulation of volatile fatty acids and ammonia. Methane production could well be explained by the Cone and modified Gompertz models compared to the first-order model, because higher R2 values were obtained. On the basis of the results of the Cone model, the first-order rate constant (k) of thermophilic SS-AD of CS and CM decreased from 0.132 to 0.039 day–1 with the increasing portion of CM. Through the modified Gompertz model, a higher lag phase time (λ) and lower maximal methane production rate (μm) were found, with the content of CM increasing under thermophilic solid-state co-digestion conditions. These results collectively suggested that thermophilic SS-AD of pretreated CS could be a promising way to produce biogas in the future.