Ethanol production from the mixture of hemicellulose prehydrolysate and paper sludge

Much of the hemicellulose fraction of pulp mill feedstock is released into black liquor during the pulping process, and it is combusted to recover chemicals and energy in the form of steam and electricity. It is technically feasible to recover this fraction of carbohydrates and convert it into value-added products. In this study, a portion of the hemicellulose in pulp feed was hydrolyzed to soluble sugars by hot-water treatment. The sugars (mixtures of pentose, hexose, and their oligomers) were then converted to ethanol by simultaneous saccharification and fermentation (SSF) employing pectinase and the ethanologenic microorganism, Saccharomyces cerevisiae. The prehydrolysate produced from wood also contained toxins, primarily lignin and sugar degradation products, which strongly inhibited the microbial and the enzymatic reactions. De-toxification of the prehydrolysates was achieved by over-liming (addition of excess CaO). The total sugar concentration in the prehydrolysate obtained from softwood was below 4 wt%, which is roughly equivalent to 2 wt% ethanol, far below the acceptable level for downstream processing. In our previous study (Kang et al. 2010), a certain amount of water is added to attain fluidity required for SSF operation. In this study, prehydrolysate, in place of water, was added into the bioreactor along with the sludge. The proposed scheme has proven that total sugar concentration as well as product concentration in the bioreactor can be significantly increased above that of the sludge-alone operation.