| Summary: | 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.
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