Mathematical simulation and laboratory experiments were conducted to investigate the controlling factor for aerobic sludge granulation. A model was used to describe the biomass dynamics during the granulation process. The simulation results indicate that the selective discharge of small and loose sludge flocs is the key controlling factor for granulation. In the experimental studies, tests were conducted with four batch column reactors (BCR) that were seeded with both activated sludge flocs and mature granules. Three different sludge discharge methods were tested, including unselective discharge of mixed sludge, selective discharge of small and slow-settling flocs, and selective discharge of settled dense sludge. The results show that mixed sludge discharge and discharge of dense sludge resulted in disappearance of granules from the reactors. Only selective discharge of small and slow-settling sludge flocs led to complete granulation. Small and loose sludge flocs were found to have a clear advantage over large and dense granules in substrate uptake. It can be concluded that selective discharge of loose flocs removes these competitors in suspended-growth mode from the reactors and makes the substrate more available for uptake and utilization by the biomass in attached-growth form, leading to granulation.

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