Assessment of corn stover pretreated with shock and alkali using methane-arrested anaerobic digestion.

Corn stover, an underutilized agricultural residue, is a promising lignocellulosic feedstock for producing biofuels. To fully utilize it, pretreatment is needed. Typically, pretreatments are rapidly assessed using extracellular enzymes that release sugars from cellulose and hemicellulose. In contrast, this study uses methane-arrested anaerobic digestion (MAAD) to assess pretreatments. Although time consuming, MAAD is a more accurate assessment technique when lignocellulose is employed in the carboxylate platform, a promising approach that utilizes nearly all biomass components. Using recommended pretreatment conditions identified from a previous study, three corn stover pretreatments were compared using MAAD: (1) shock-only, (2) NaOH-only, and (3) shock + NaOH. Air-dried sewage sludge was used as nutrient source. At 100 g/L initial substrate concentration, compared to untreated corn stover, shock-only decreased conversion (amount of biomass digested) by 14%, NaOH-only increased conversion by 82%, and shock + NaOH increased conversion by 104%. Using batch MAAD data, the continuum particle distribution model simulated four-stage countercurrent fermentation. At an industrial non-acid volatile solids (NAVS) concentration of 300 g/Lliq , for both NaOH-only and shock + NaOH, the model predicts total carboxylic acid concentration of about 58 g/L and conversion of about 0.85 g NAVSdigested /g NAVSfed at liquid retention time of 35 days and volatile solid loading rate of 4 g/(Lliq ⋅day). At this long solid residence time, shock is not necessary; however, with short solid residence times, shock acts synergistically to aid NaOH pretreatment. Shock treatment offers a way to reduce pretreatment costs without sacrificing pretreatment efficacy.

Assessment of shock pretreatment and alkali pretreatment on corn stover using enzymatic hydrolysis.

This study investigates digestibility enhancements of lignocellulose from shock pretreatment, alkaline pretreatment, and combination. Shock pretreatment subjects aqueous slurries of lignocellulose to shock waves, which disrupts its structure rendering it more susceptible to hydrolysis. Alkaline pretreatment submerges the biomass in aqueous alkali (NaOH, Ca(OH)2 ), which removes lignin and acetyl groups. As indicators of digestibility, cellulase (CTec3) and hemicellulase (HTec3) were used to saccharify the pretreated corn stover and the resulting filtrate which contains about 10% of the sugars. Shock is most effective when it precedes alkaline pretreatment, presumably because it opens the biomass structure and enhances diffusion of pretreatment chemicals. Lignocellulose digestibility from calcium hydroxide treatment improves significantly with oxygen addition. In contrast, sodium hydroxide is a more potent alkali, and thereby eliminates the need for oxygen to enhance pretreatment. At low hydroxide loadings (<4 g OH- /100 g dry biomass), both NaOH and Ca(OH)2 provide similar increases in digestibility; however, at high hydroxide loadings, NaOH is superior. For animal feed, Ca(OH)2 treatment is recommended, because residual calcium ions are valuable nutrients. In contrast, for methane-arrested anaerobic digestion, NaOH treatment is preferred because NaHCO3 is a stronger buffer. At 50°C, shock pretreatment improves sugar yields at all NaOH loadings. The effect of shock is most pronounced when the no-shock control employed the same soaking-and-drying procedure as the shock treatment. The recommended conditions are shock treatment (5.52 bar [abs] initial H2 /O2 pressure) followed by 50°C alkaline treatment with NaOH loading of 4 g OH- /100 g dry biomass for 1 h.