ABSTRACT: Simultaneous saccharification and co-fermentation
(SSCF) of waste paper sludge to ethanol was investigated
using two recombinant xylose-fermenting microbes:
Zymomonas mobilis 8b and Saccharomyces cerevisiae
RWB222. S. cerevisiae RWB222 produced over 40 g/L ethanol
with a yield of 0.39 g ethanol/g carbohydrate on paper
sludge at 378C, while similar titers and yields were achieved
by Z. mobilis 8b at 308C. Both S. cerevisiae RWB222 and
Z. mobilis 8b exhibited decreasing cell viability at 378C when
producing over 40 g/L ethanol. A high ethanol concentration
can account for S. cerevisiae RWB222 viability loss, but
ethanol concentration was not the only factor influencing
Z. mobilis 8b viability loss at 378C. Over 3 g/L residual
glucose was observed at the end of paper sludge SSCF by
Z. mobilis 8b, and a statistical analysis revealed that a high
calcium concentration originating from paper sludge, a high
ethanol concentration, and a high temperature were the key
interactive factors resulting in glucose accumulation. The
highest ethanol yields were achieved by SSCF of paper sludge
with S. cerevisiae RWB222 at 378C and Z. mobilis 8b at 308C.
With good sugar consumption at 378C, S. cerevisiae
RWB222 was able to gain an improvement in the polysaccharide
to sugar yield compared to that at 308C, whereas
Z. mobilis 8b at 308C had a lower polysaccharide to sugar
yield, but a higher sugar to ethanol yield than S. cerevisiae.
Both organisms under optimal conditions achieved a 19%
higher overall conversion of paper sludge to ethanol than the
non-xylose utilizing S. cerevisiae D5A at its optimal process
temperature of 378C.