240 min. The reason for such a fact can be explained by the
association of positively charged molecules released during pretreatment
and negative sites of microorganisms or polysaccharides
(Fernández et al., 2012).
3.2. Biogas production
The effect of pretreated temperature and time on cumulative
biogas production of water hyacinth is shown in Fig. 1. Untreated
biomass started to produce biogas until the 8th day, but a couple
of days were enough for the pretreated water hyacinth to convert.
This indicates that the pretreatment of ionic liquid can accelerate
biogas production and reduce lag phase. The water hyacinth pretreated
at 100 C for 120 min, 120 C for 60 min, and 120 C for
120 min showed higher biogas yield (100, 104, 170 L kg1 VS) than
untreated water hyacinth (86 L kg1 VS). However, the acids such
as formic acid and levulinic acid, which are toxic for anaerobic
microorganisms (Chen et al., 2008), were produced from the pretreatment
and acetogen at the initial digestion and then the pH value
of the systems was quickly dropped within the first 3 days. On
the other hand, the formation of aggregates may make the digestion
difficult. Therefore, the samples pretreated at 140 C for
120 min and at 120 C for 240 min showed less production of biogas
(32 and 30 L kg1 VS), and only lasted for about 8 days,
although higher pretreated temperature or longer pretreated time
could cause more loss of lignin.