SSF of alkali-pretreated switchgrass was performed with a solid
loading of 5% and enzyme loading of 15 FPU/g-glucan (Fig. 2b). In
the first 24 h, glucose (10 g/L) and xylose (1.0 g/L) was released,
small amount of acetic and butyric acids (2 g/L) was produced.
No solvents were produced until 48 h, but the solvents production
quickly stopped at 72 h. The final total solvents was very low (3 g/
L). More than 9 g/L of glucose was present in the solution from 24
to 120 h. This indicated microbes could not actively consume sugars
in the SSF of Alkali-pretreated switchgrass, and enzyme hydrolysis
was not a rate-limiting step in SSF. It was most likely that
residual lignin (10.4%) and other degradation compounds in
alkali-pretreated switchgrass inhibited the microbial fermentation
step. Similar results have been reported on converting dilute acid
pretreated corn stover to butanol in a SSF process (Qureshi et al.,
2014), they found the pretreated corn stover still contained toxic
chemicals that inhibited cell growth and fermentation activity. A
further detoxification of pretreated solid was required to improve
the ABE yield.
In contrast to alkali-pretreated switchgrass, SSF of de-ashed
paper mill sludge (7.4% solid loading and 10 FPU) showed very
good ABE solvents yield (Fig. 2c). Specifically in the first 24 h, glucose
and xylose reached 10 and 6 g/L, respectively. Both acetic and
butyric acids reached 4.0 g/L, respectively. After 24 h, the glucose
concentration continued to increase and reached at 16.1 g/L at
48 h, then decreased gradually to 5.0 g/L at 120 h as SSF progressed.
The xylose concentration was kept at 6 g/L from 24 to
120 h. Butanol was steadily produced from 24 to 120 h and
reached 9.7 g/L at the end of fermentation. The total ABE solvent
concentration was 17.1 g/L, the solvent yield was 0.24 (g/g sugars)
and the productivity was 0.20 g/L/h. As comparing to the SSF of
Avicel, the total acids concentration (5.7 g/L) was significantly
higher than that in SSF of de-ashed sludge. Most likely, it is due to the buffering effect of ash in paper mill sludge. The culture tends
to produce more acids to maintain the medium pH near its optimum
for solvent production. Most of the solvent-producing cultures
used for ABE production has shown the tendency to
produce high level of acids when it comes to the fermentation of
the hydrolysates of lignocellulosic substrate (Qureshi et al.,
2010b; Sun and Liu, 2012).