Granulation
There exists a variety of operational conditions, either aerobic or
anaerobic, that can result in the formation of dense granular
structures. In this study we investigated the characteristics of compact
anaerobic granular sludge producing VFA cultivated at varying SRT-values
(1-50 d). Other processes creating granules are the ‘Nereda®’ process
and the upflow anaerobic sludge blanket (UASB) reactor (Lettinga et al.,
1980; Pronk et al., 2015). In both these systems the influent is fed in
the bottom of the reactor and passes through a sludge bed which is not
(or only partly) mixed. The microorganisms encounter the highest
concentration of substrate at the bottom of the reactor, creating a
substrate concentration gradient along the height of the sludge bed.
Rapidly settling biomass therewith has a competitive advantage through
location in the lower section of the sludge bed. Low substrate
concentrations as found in CSTR type reactors negatively affect granule
formation (de Kreuk & van Loosdrecht, 2004). In this work the substrate
was added pulswise in a mixed reactor, creating a substrate
concentration gradient only in time and not in space and time as
described for the other processes. The substrate gradient is a
prerequisite for providing a competitive advantage for microorganisms
growing in a biofilm. Effective biomass granulation in this study was
achieved using the startup procedure as described in the material and
methods section. Enrichment of biomass growing in granules is a complex
process, in which the following operational steps were found to be
crucial to achieve effective granulation.
- First, to initiate granulation in a mixed reactor system full biomass
retention was aimed for, requiring a long settling time. In these
experiments an intial settling time of 3 h was implemented, achieving
full retention of biomass in the first 12 cycles of operation.
- Second, to achieve adequate selection of biomass growing in granules,
the settling time was step-wise reduced.
- Thirdly, the glucose was dosed pulse-wise creating a substrate
gradient over time
In this study at an SRT of 40-50 d a SVI60 of 11 ± 2
mL·gTSS-1 could be achieved, amounting to a very dense
sludge bed. At shorter SRT the SVI was higher and was around 34-44
mL·gTSS-1. The SVI for previously operated granular
systems varied though were in the same order of magnitude; 17-29
mL·gVSS-1 for granular acidogenic systems (J. Tamis et
al., 2015) and 12-45 mL·gTSS-1 have been reported for
aerobic granular sludge in lab and full-scale situation (De Kreuk,
Pronk, & Van Loosdrecht, 2005; Pronk et al., 2015). In upflow anaerobic
sludge bed (UASB) reactors the SVI can go as low as 7.8
mL·gVSS-1 (Grotenhuis, Kissel, Plugge, Stams, &
Zehnder, 1991). These results demonstrate well-settling granular sludge
and dense microbial structures can be obtained under varying process
conditions.