Kinetic properties of the enrichment
The \(q_{S}^{\max}\)of the culture decreased at increasing SRT. At an
SRT of 1-2 d a \(q_{S}^{\max}\) of 0.7
gCOD·gVSS-1·h-1 was achieved, which
reduced to a value of 0.2
gCOD·gVSS-1·h-1 at an SRT of 40-50
d. Rombouts et al., (2019) found a \(q_{S}^{\max}\) of 4.3
gCOD·gVSS-1·h-1 in a pulse-fed
glucose SBR operated at a pH of 8.0 and a SRT of 8 h (Rombouts et al.,
2019). This \(q_{S}^{\max}\) value is an order of magnitude higher than
the values obtained in this study. Possibly the pH, SRT or a combination
of both played a crucial role. In Rombouts et al. (2019) the influent
contained 4 g·L-1 glucose however, the operating pH
was 8.0 and product inhibition was not likely to occur at only 4
g·L-1 glucose, possibly enabling this high
qSmax. Another reason for the lower \(q_{S}^{\max}\)values found for granular sludge is the production of EPS in granular
sludge. When in both cases the VSS concentration gets classified as
catalytic biomass the biomass content gets overestimated in a granular
system as more parts of the VSS is EPS and not catalytic biomass.
Furthermore, part of the biomass could be inactive but retained in the
system as VSS incorporated in a granule resulting in lower observed qS
values. Additionally, there is the possibility that microorganism over
the depth of the biofilm are not doing all processes simultaneously.
These reasons would result in the underestimation of
qSmax in a granular system compared to a suspended
cell system. The qSmax value obtained in this work at
an SRT of 1-2 d was lower compared to the values found previously in
comparable conditions (0.7 versus 1.6
gCOD·gVSS-1·h-1(J. Tamis et al.,
2015). Potentially, the different history of the enrichments obtained in
both studies resulted in a different local optimum, characterized by
different types of dominant microorganisms and corresponding kinetic
properties, i.e. in this study Bifidobacterium scardovii was the
dominant microorganism versus Clostridium pasteurianum in Tamis
et al. (2015).
Despite the relatively low observed \(q_{S}^{\max}\) a volumetric
glucose consumption rate of 1100
gCOD·L-1·d-1 was found in the 40-50
d SRT system. This is orders of magnitude larger than continuous stirred
tank reactors (CSTR) operated using similar conditions as in this study.
A consumption rate of 12
gCOD·L-1·d-1 can be estimated when
the HRT is 12 h and the influent concentration is 6
gCOD·L-1. The overall volumetric capacity in the
granular sludge process is orders of magnitude higher compared to a
chemostat due to effective biomass retention, and despite the lower\(q_{s}^{\max}\) values achieved.