Fate of Lignocellulosic Biomass during Pretreatment and Anaerobic Digestion of Lignin-rich Cattle Manure

Cattle manure (CM), a lignocellulosic biomass, is an energy-rich feedstock which is poorly biodegradable during anaerobic digestion (AD). In this study, milled CM was primarily subjected to chemical (pH 4-12), thermal (50-90°C), and thermochemical batch pretreatment tests at contact times (CTs) of 6-24 hr, to determine the optimum conditions for testing in fed-batch AD systems. Lignin, cellulose, and hemicellulose were determined according to standard NREL protocols. Thermochemical pretreatment at 90°C and pH 12 achieved the highest methane yield, biodegradability, and maximum biomass-specific methane production rate (MSMPR) of 180 mL CH₄/gCOD; 51%; 28.3 mL CH₄/gVSS-d, respectively, surpassing chemical (149; 43%; 20.9) and thermal (162; 46%; 15.3), with the greatest lignin, cellulose, and hemicellulose (LCH) removals (lignin 14%, cellulose 41%, hemicellulose 51%). The correlation between solubilization and maximum specific methane production rate (MSMPR) showed an overall increasing trend; however, at high solubilization (~235 mg SCOD/gVSS), no further improvement was observed, likely due to the formation of inhibitory or poorly biodegradable compounds under severe pretreatment conditions. Solubilization for all pretreatments correlated well with a modified severity index (MSI). Methane yield was calibrated/validated using a Box–Behnken design, which showed significant temperature dependence. Fed-batch reactors with chemical or thermal pretreatment were stable but showed no improvement over batches, indicating that long-term acclimatization was not advantageous.