The Role Of Microbial Selection Pressure In Granulation

The feasibility of of the technique of aerobic granulation based on a continuous-flowcomplete mixing bioreactor depends on the aerobic granulation mechanism.Previously, the studies of the aerobic granulation mechanisms were all based on thesequencing batch reactor, which is unable to suppressing interference of operatingcharacteristic for granulation formation. This study explored the dynamic granulationprocess and microbial community succession in a continuous-flow complete mixingbioreactor by using fluorescent tracer technique, polymerase chain reaction anddenaturing gradient gel electrophoresis (PCR-DGGE) technique, and multiplexfluorescence in situ hybridization(FISH) technique, and analysis the function ofmicrobial selection pressure for granulation formation in SBR, for ensure theinfluence of it and lay the function of further study.The results showed the granulation process is a dynamic process withgranulation aggregation, detachment, crush and re-granulation. During the initial stageself-aggregation of the bioflocs was the dominant granulation mode (100%), theattachment of free bioflocs onto the newly formed granules was the dominantgranulation mode on day12~20, which accounts for81%of the three granulationmodes, the dominant mode of granulation was self-aggregation again (possessed61%of the three modes) on day21~33, the detached flocculent biomass,new cells andcrushed aged granules re-granulation mode became the final dominant mode(possessed52%of the three modes) when the granulation rate was steady. It wasnoted that the terminal way of the granulation was an aggregation of crushedgranulation and new biomasses randomly. The results of PCR-DGGE profile andUPGMA dendrogram further showed the microbial communities of seeding, theflocculent sludge and the granular sludge were highly similar, over0.86, even more than0.98between the flocculent sludge and the granular sludge. Microbial selectionpressure was confirmed to be not a prerequisite for aerobic granulation from both ofthe dynamic granulation steps and molecular biology aspects. Moreover, the results ofFISH indicated that the filamentous fungus and bacteria were equally distributed inmatur granulation. It revealed that although the fungus were not eliminated from thesystem as no microbial selection pressure, but the fungus could be the inner backbonestrengthening the structure of aerobic granules.The filamentous bulking would nothappen if the operating conditions were suitable.The study predict that the cultivation of aerobic granulation under thecontinuous-flow has a good development space. It is of great significance todevelopment of granulation technology.