Solid-State KOH Pretreatment of Corn Straw for Anaerobic Digestion: Methane Yield Enhancement, Potassium Flow Analysis, and Preliminary Economic Assessment

Pretreatment has been widely used to improve the anaerobic digestion (AD) efficiency of cellulosic substrates. However, intrinsic shortcomings of traditional pretreatment techniques limit their industrial applications. In this study, a solid-state alkaline pretreatment with KOH to improve the AD efficiency of corn straw was developed, which could largely decrease the chemical needed during pretreatment. In addition, it could be done during the feedstock storage without the need for liquid/solid separation before AD. Furthermore, the potassium added during pretreatment could be preserved in the AD digestate, which could benefit its utilization as a fertilizer. Results showed that the solid-state KOH pretreatment significantly improved the methane yield and AD efficiency that depended on KOH loading. Under 8% KOH load, the maximum methane yield was obtained, which was 31.9% higher than that of the original corn straw. Structural analysis showed that the intact structures (especially lignin) of corn straw were partly destroyed during pretreatment, which improved the AD performance of pretreated corn straw. The added K+ was well preserved in the digestate, more than 76% of which was distributed in the biogas slurry as water-soluble K+. The economy of solid-state KOH pretreatment was strongly associated with the KOH loads during pretreatment as well as with the utilization pattern of the biogas digestate. Only pretreatment with 1% KOH loading showed a positive net value of 0.16 USD/day without considering the value of the AD digestate.