Aerobic granulation is considered as a promising alternative to conventional activated sludge in biological wastewater treatment. Instead of forming flocs the bacterial communities aggregate to dense granular biofilms. In the present study, granules were cultivated in bubble column reactors operated in sequencing batch mode. The operating cycle consisted of 4 phases: i) an anaerobic feeding phase, ii) a long aeration phase where the sludge was mixed, iii) a short settling phase and finally iv) a withdrawal phase where the treated water was taken out of the reactor. Feeding with synthetic wastewater was done from the bottom of the reactor in plug-flow mode without a mixing of the settled sludge. We made two observations which let us conclude that all the granules were not equally provided with substrate: 1) We measured the carbon consumption rate and calculated that only the lower 15-25% of the sludge bed got carbon in each cycle. If all granules would have had the same settling properties, this heterogeneity would have been cancelled over time. But 2) we observed a clear gradient in granule size within the sludge bed. Larger granules settled faster than smaller ones. Sludge from the bottom of the reactor was composed up to 80% by granules with diameter larger than 3.5 mm, whereas over the whole reactor the threshold diameter for 80% of the sludge volume was only at 2.4 mm. The heterogeneity in carbon accessibility is expected to influence the composition of bacterial communities and the wastewater treatment performances. For example, dephosphatation and denitrification are directly depending on the presence of bacteria containing internal carbon reserve in form of polyhydroxyalkanoates. We are presently investigating possible differences between “feast” and “famine” granules.


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