Ethanol production from waste biomass: enzymatic hydrolysis and fermentation of sulphite pulp mill primary clarifier sludge
Creators: name: Moritz, John William
Publisher: name: University of British Columbia
Year: 2009

description: Waste fibre (cellulose) in the form of primary clarifier sludge (PCS) from pulp mills is an abundant and renewable resource which is currently used merely as a dewatering agent for activated sludge, and as a fuel source in hog fuel boilers. One company which follows this practice of disposal is Tembec Inc., a low yield sulphite pulping operation located in Temiscaming, Quebec. Tembec produces 40 tonnes/day of PCS on a dry basis, primarily as a by-product of sulphite pulping, but also from chemithermomechanical pulp (CTMP) production. The operation also produces azeotropic (97%) ethanol from the hexose fraction of its spent sulphite liquor (SSL) in a fermentation process that is currently underutilized. While the market for pulp is historically volatile, ethanol has always been a highly-valued product with a stable market and selling price. Bioconversion of PCS to ethanol through enzymatic hydrolysis and fermentation is a well understood process which could be implemented at a modest cost by using existing equipment and personnel. Therefore a unique opportunity exists at Tembec to increase ethanol productivity, and at the same time reduce the amount of PCS of which it must dispose. In this thesis, the suitability of PCS as a substrate for ethanol production through a variety of technologies has been explored. Efforts to characterize the amenability of PCS to enzymatic hydrolysis have shown that initial (1 hour) hydrolysis rates in acetate buffer as high as 7.9 g-L'^h"1 are possible at an initial enzyme loading of 10 filter paper units (FPU)/g PCS, using cellulases derived from Trichoderma reesei. The hydrolysis rate was found to be proportional to enzyme loading. A 66.4% conversion of PCS to reducing sugars was possible, coinciding with a 56% increase in residual solids dewaterability. Wood sugars derived from PCS were readily fermented to ethanol by Saccharomyces cerevisiae yeast, with yields as high as 66% of the theoretical obtained. To reduce the deleterious effects of end product inhibition, simultaneous hydrolysis and fermentation (SSF) experiments were carried out; In batch trials, sugar accumulations (in amounts proportional to the enzyme loading) were observed over the first 10 hours of SSF. After this time, yeast populations multiplied and the sugar was converted to ethanol. In SSF reactions using 100% SSL (20% total solids) as the reaction matrix ethanol production could be increased by 25% over a 24 hour period (the residence time in the fermenters at Tembec). This result could possibly be improved upon if the SSL were prefermented prior to the addition of PCS and enzymes, hence simulating the conditions of the industrial process and reducing the effect of hexose inhibition of cellulase activity. It should be stressed however that pre-fermented SSL was found to significantly inhibit hydrolysis and fermentation; hence, a kinetic penalty is associated with fortifying SSL with sugars via PCS hydrolysis in situ. A novel system of simultaneous saccharification and extractive fermentation (SSEF) was developed using PCS as a substrate. It was found that ethanol production (relative to aqueous volume) from Tembec PCS could be increased by 50% through simultaneous in-situ liquid extraction of ethanol by oleyl alcohol. The benefit of ethanol extraction increased in proportion to the concentration of ethanol produced. A model for SSF of PCS in Tembec's existing continuous fermentation was presented, and selected kinetic and yield parameters for use in with the model were obtained. The model remains in the early stages of development.