Fig. 4 shows the number of sequences of different genera
obtained during operation at HRTs of 24 and 72 h (operational
taxonomic unit showing >97% 16S rRNA sequence similarity
to known taxa were taken into account in the microbial
analysis). The microbial characterization demonstrates that
the phylum Firmicutes represent more than 99% of the OTUs
found in the sample of the reactor. The family Veillonellaceae
represents 99% of the OTUs of the Firmicutes analyzed.
Members of this family are all obligate anaerobes and occur in
habitats, such as rivers, lakes and the intestines of vertebrates.
It has been reported that this family could contribute
to greater hydrogen production at low OLRs using easy-todegrade
substrates (as glucose) and lactate [22]. The phylogenetic
analysis of the representative OTUs (with a relative
abundance greater than 0.1%) is shown in Fig. 5.