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).
3.3. Effect of enzyme loading on SSF of de-ashed paper mill sludge
To examine the effect of enzyme loading on ABE production, SSF
of PS7 was performed (3.8% solid loading) with different enzyme
loading (5, 10, and 15 FPU/g-glucan) (Fig. 3a–c). Under 5 FPU, the
glucose and xylose were released in a small amount (1 g/L) in the
solution from 24 to 96 h (Fig. 3a). At 120 h, the glucose surged to
5.2 g/L. However, the low glucose concentration did not appear
to inhibit the butanol production. Butanol increased from 0.3 g/L
at 24 h to 5.3 g/L at 120 h. At the end of fermentation, the total solvent
reached 7.4 g/L, the solvent yield was 0.20 g/g sugars and the
productivity was 0.062 g/L/h.
When the enzyme dosage was increased to 10 FPU, about 2.3
and 0.40 g/L of glucose and xylose were released in the solution
at 24 h. After that, both glucose and xylose concentration were
kept less than 2 g/L until 120 h. The butyric acid first increased
to 4.7 g/L at 24 h, and decreased gradually to 2.2 g/L at 120 h.
The acetic acid increased to 4.5 g/L and was kept at the same level
until 120 h. The butanol concentration increased linearly and
reached 6.8 g/L at 120 h. The total solvent concentration reached
10.6 g/L, the solvent yield was 0.29 g/g sugars and the productivity
was 0.088 g/L/h. As comparing to the SSF process at 5 FPU, both
solvent yield and the productivity increased by 45%. This indicated
enzymatic hydrolysis was a rate-limiting step in SSF process. It
should be noted that xylose was also consumed in the SSF process,
this agreed well with previous reports on C. acetobutylicum being
able to ferment xylose to butanol as well