Fermentation of duckweed
Fig. 4 shows the biogas and biohydrogen production during
fermentation of acid-pretreated duckweed biomass at
different combinations of temperature and initial pH. At
ambient temperature, the biogas production at an initial pH of
5.5 reached 166 mL g1 after 13 days, which is much higher
than those at other initial pHs (Fig. 4a). Similarly, the initial pH
of 5.5 also resulted in the highest biohydrogen production
(81 mL g1 after 13 days, Fig. 4b). This pH is much lower than
the working pH of the anaerobic digester (pH ¼ 7.5) where the
inoculum was collected, however that seed was acclimated
for methane generation on a different substrate. The change
of pH range where microbes perform efficiently might be the
result of heat treatment of the inoculum which inhibited
methanogens, and changed the microbial composition of the
inoculum, thus significantly affecting its response to environmental
pH. The lag periods for both biogas and biohydrogen
productions varied between zero and five days
depending on the initial pH. The hydrogen concentrations in
the biogas were similar for initial pHs of 5.5e8.5 (46e55%), but
remained below 15% throughout the fermentation at the
initial pH of 4.0. This indicates that an initial pH of 4.0 was
detrimental to hydrogen-producingmicroorganisms. At 35 C,
the initial pH of 5.5 remained the best condition among all
initial pH levels (Fig. 4c and d). The lag periods for biogas and
biohydrogen productions disappeared at 35 C. After 7 days,
the biogas and biohydrogen production leveled off at
179 mL g1 and 75 mL g1, respectively, which are comparable
with those achieved at ambient temperature. Although biohydrogen
production started earlier and the production rate
was higher at 35 C, the final volume of biohydrogen produced
was not increased. This indicates that the final hydrogen yield
is probably determined by the pretreatment method rather
b
Time (