Combined sodium hydroxide and ammonium hydroxide pretreatment of post-biogas digestion dairy manure fiber for cost effective cellulosic bioethanol production

Abstract

BACKGROUND:The current higher manufacturing cost of biofuels production from lignocellulosics hinders the commercial process development. Although many approaches for reducing the manufacturing cost of cellulosic biofuels may be considered, the use of less expensive feedstocks may represent the largest impact. In the present study, we investigated the use of a low cost feedstock: post-biogas digestion dairy manure fiber. We used an innovative pretreatment procedure that combines dilute sodium hydroxide with supplementary aqueous ammonia, with the goal of releasing fermentable sugar for ethanol fermentation.RESULTS:Post-biogas digestion manure fiber were found to contain 41.1% total carbohydrates, 29.4% lignin, 13.7% ash, and 11.7% extractives on dry basis. Chemical treatment were applied using varying amounts of NaOH and NH3 (2-10% loadings of each alkali on dry solids) at mild conditions of 100[degree sign]C for 5 min, which led to a reduction in lignin of 16-40%. Increasing treatment severity conditions to 121[degree sign]C for 60 min improved delignification to 17-67%, but also solubilized significant amounts of the carbohydrates. A modified severity parameter model was used to determine the delignification efficiency of manure fiber during alkaline pretreatment. The linear model well predicted the experimental values of fiber delignification for all pretreatment methods (R2 > 0.94). Enzymatic digestion of the treated fibers attained 15-50% saccharification for the low severity treatment, whereas the high severity treatment achieved up to 2-fold higher saccharification. Pretreatment with NaOH alone at a variety of concentrations and temperatures provide low delignification levels of only 5 - 21% and low saccharification yields of 3 - 8%, whereas pretreatment with the combination of NaOH and NH3 improved delignification levels and saccharification yields 2-3.5 higher than pretreatment with NH3 alone. Additionally, the combined NaOH and NH3 pretreatment led to noticeable changes in fiber morphology as determined by SEM and CrI measurements.CONCLUSIONS:We show that combined alkaline treatment by NaOH and NH3 improves the delignification and enzymatic digestibility of anaerobically digested manure fibers. Although pretreatment leads to acceptable saccharification for this low-cost feedstock, the high chemical consumption costs of the process likely will require recovery and reuse of the treatment chemicals, prior to this process being economically feasibility.