It is interesting to note that the composition of biogas produced
during anaerobic fermentation was affected significantly by sludge
type (Fig. 5). Almost no hydrogen was observed in PFS-F and PFS-TF
during anaerobic fermentation. Conversely, a relatively high
hydrogen yield was observed in both PGS-F and PGS-T-F. PGS produced
more n-butyric acid and less propionic acid in comparison
with PFS during anaerobic fermentation (Figs. 3 and S2). This
was reported to benefit the increase in hydrogen production
(Wang et al., 2013). Furthermore, some hydrogen-producing bacteria
such as Clostridium and Enterobacteriaceae_unclassified were
detected in PGS-I (see Section 3.5), which also facilitated hydrogen
production in the early stage of PGS fermentation. In addition, the
hydrogen yield of PGS-T-F was higher than that of PGS-F, indicating
that the yield of hydrogen could be improved greatly by lowtemperature
thermal pretreatment. Previous studies also showed
that thermal pretreatment can select spore-forming hydrogenproducing
bacteria and increase the amount of soluble substrate,
facilitating the production of hydrogen (Li and Fang, 2007; Guo
et al., 2010). The methane yield showed an opposite trend to the