4. Discussion
Paper mill sludge was utilized directly as feedstock for butanol
production via simultaneous saccharification and fermentation.
Effects of solid loading and enzyme loading on SSF of PS7 were
compared (Table 2). Under a solid loading of 6.3–7.4% and enzyme
loading of 10–15 FPU/g-glucan, a total of 16.4–18.0 g/L of ABE solvent
was produced, which is comparable to that of the control test
(pure cellulose). In comparing ABE production from different substrates
in SSF processes, the final ABE solvent concentrations from
paper sludge were 5–34% higher than that from dilute acid pretreated
wheat straw (Table 3). The ABE yields from paper sludge
were similar to that from dilute acid pretreated wheat straw. In
comparing ABE production in SSF and SHF processes, the ABE
yields in SHF process appeared relatively higher than those in
SSF process, but the ABE final concentrations were similar. It
should be noted that different enzyme loading and different
Clostridium strains will affect the final ABE concentration and
ABE yields was well. Two major advantages of paper mill sludge
as feedstock for butanol production are that no pretreatment is
needed and no pH control is required as ABE fermentation from
sugars. Yang et al. previously found that supplementation of calcium
carbonate in ABE fermentation from glucose significantly
improved the solvent yield by stabilizing the pH of fermentation
broth (Yang et al., 2013). Remarkably, significant amount of CaCO3
already exists in paper mill sludge and can be used as a buffering