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.
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.