The great majority of activated sludge bacteria exist incorporated in flocs. The increase in dispersed bacteria when exposed to increasing turbulent shear rates has been successfully modelled by a model assuming that the adhesion and erosion of cells may be considered in analogy to ordinary chemical phase transitions. By this adhesion-erosion model (AE-model), an “enthalpy” of cell adhesion can be estimated, and this value in turn determines the range of shear rates in which erosion of cells predominates. Application of the model has indicated that only a mass fraction less than ca. 6-17% may be released from activated sludge, even when exposed to a severe turbulent environment, i.e. only a small fraction of the flocs is dispersible by means of erosion by turbulence. The shear sensitivity and the dispersible floc fraction were found to depend on the floc composition. A net decrease in the floc EPS content during anaerobic sludge stabilisation causes a dramatic increase in the dispersed fraction, indicating the important role of EPS for the floc strength. It was found also that activated sludge cells do not reflocculate completely after exposure to high shear rates. This may be an indication that the cohesion energy of bacteria growing in colonies is greater than the energy of the more stochastic adhesion of dispersed cells to floc surfaces. It could also be another indicator of the importance of entanglement forces, which do not reform instantly, once broken. When the bond strength of cell attachment to sludge surfaces is altered by changes in the chemical environment, this may cause a change in the dispersible floc fraction as well as a change in the shear range of erosion. When the shear sensitivity constant kSS is adopted for sludge characterisation, an increased degree of dispersion under standard test conditions will result in increased shear sensitivity estimates. The shear sensitivity may be used for the estimation of en equivalent change in the Gibb's energy of cell adhesion (Δ(ΔGad/RT)).

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