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.