Aerobic granulation in a sequencing batch reactor

In a sequencing batch reactor (SBR) granules of aerobic heterotrophic microorganisms were cultured. The effect of different operational conditions on the formation of these aerobic granules were studied. The time allowed for settling was the main parameter to select for growth of bacteria in well settling granules. Both a short HRT and a relative high shear were found favorable for granulation. A substrate loading rate of 7.5 kg COD/(m3 day) was applied. This led to formation of granules with an average diameter of 3.3 mm and a biomass density of 11.9 gVSS/lgranule. Based on microscopic observations a hypothesis for the granulation process was formulated. The reactor was started up without any carrier material present. At the beginning filamentous fungal pellets dominated the reactor. These pellets functioned as an immobilization matrix in which bacteria could grow out to colonies. After a certain time the fungal pellets fell apart due to lysis in the inner part of the pellets, the bacterial colonies could now remain in the reactor because they were large enough to settle sufficiently fast. These colonies further grew out to granules. This paper shows that granule formation in aerobic reactors is feasible and can be exploited to increase the volumetric conversion capacity of such reactors.