Quantification of the bond energy of bacteria attached to activated sludgefloc surfaces

The great majority of activated sludge bacteria exist incorporated in flocs . The increase in dispersed bacteria when exposed to increasing turbulent s hear rates has been successfully modelled by a model assuming that the adhe sion 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 mean s of erosion by turbulence. The shear sensitivity and the dispersible floc fraction were found to depend on the floc composition. A net decrease in th e flee EPS content during anaerobic sludge stabilisation causes a dramatic increase in the dispersed fraction, indicating the important role of EPS fo r the Roc strength. It was found also that activated sludge cells do not re flocculate completely after exposure to high shear rates. This may be an in dication that the cohesion energy of bacteria growing in colonies is greate r than the energy of the more stochastic adhesion of dispersed cells to fle e surfaces. It could also be another indicator of the importance of entangl ement forces, which do not reform instantly. once broken. When the bond str ength of cell attachment to sludge surfaces is altered by changes in the ch emical environment, this may cause a change in the dispersible flee fractio n as well as a change in the shear range of erosion. When the shear sensiti vity constant k(SS) is adopted for sludge characterisation, an increased de gree of dispersion under standard test conditions will result in increased shear sensitivity estimates. The shear sensitivity may be used for the esti mation of an equivalent change in the Gibb's energy of cell adhesion (Delta (DeltaG(ad)/RT)).