Microbiological tips from combined UASB and MBR system
In order to characterize the population of Archaea FISH analysis
were performed. As can be seen in Fig. 4a, the population of Methanosaeta
spp. dominated the anaerobic biomass, however, no positive
signal of Methanosarcina spp. was found. Methanosarcina and
Methanosaeta are acetotrophic methanogens and compete with
each other for acetate. Dworkin et al. [25] reported that Methanosaeta
usually out-compete Methanosarcina in environments in
which turnover is slow and acetate concentrations are low. This
could happen in the case of the combined UASB and MBR system
studied in this work, since the organic load applied to the UASB
stage was low in comparison to typical anaerobic digesters. Moreover,
the system was operated at ambient temperatures (17–
25 C), which also narrows the pool of methanogens able to acclimate
and grow under these conditions, since the optimum temperature
for the majority of methanogenic bacteria is within the
mesophilic range.
On the other hand, in the subsequent MBR stage abundant methanotrophs
type I were found (Fig. 4b). These bacteria use methane
as a carbon and energy source [26]. Taking into account the abundance
of these bacteria and gaps in COD conversion to methane
discussed in Section 3.5, it can be assumed that a large portion of
dissolved methane was oxidized by methanotrophs.
3.4. Biogas