Equilibrium aspects of the effects of shear and solids content on aggregate deflocculation

In this study the desorption of primary particles from activated sludge flocs in response to variations in total solids mass and turbulent shear intensity was studied. The kinetics of desorption could be described as a diffusion process. Equilibrium turbidity was found to increase more than proportional to the total solids content under the influence of turbulent shear (G) as well as without any shear. The increase in equilibrium turbidity in response to shear was close to linear in the lower shear domain, but levelled off for shear levels above approx. G=1000 s−1. Based on analogy to gas–liquid phase transitions a model for multilayer adsorption of primary particles was developed and successfully fitted to the data. The model is mathematically identical to the Langmuirian isotherm although conceptually different. From the model relative estimates of the interaction energy between primary particles can be obtained. The effects of kinetic energy were successfully treated by the vanʹt Hoff equation, when substituting T with G. This allows the determination of a characteristic parameter analogous to the change of enthalpy. The present model thus allowed for tentative calculation of ΔG- and ΔH-values of the desorption process.