Aerobic granulation in a sequencing batch reactor

In a sequencing batch reactor (SBR) granules of aerobic heterotrophic micro organisms 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 se ttling granules. Both a short HRT and a relative high shear were found favo rable for granulation. A substrate loading rate of 7.5 kg COD/(m(3) day) wa s applied. This Ted to formation of granules with an average diameter of 3. 3 mm and a biomass density of 11.9 gVSS/l(granule). Based on microscopic ob servations a hypothesis for the granulation process was formulated. The rea ctor was started up without any carrier material present. At the beginning filamentous fungal pallets dominated the reactor. These pellets functioned as an immobilization matrix in which bacteria could grow out to colonies. A fter a certain rime the fungal pellets fell apart due to lysis in the inner parr of the pellets, the bacterial colonies could now remain in the reacto r because they were large enough to settle sufficiently Fast. These colonie s further grew out to granules. This paper shows that granule formation in aerobic reactors is feasible and can be exploited to increase the volumetri c conversion capacity of such reactors. (C) 1999 Elsevier Science Ltd. All rights reserved.