A simple empirical model for hindered settling in activated sludge clarifier

The determination of activated sludge concentration at the bottom of secondary settler tanks is necessary for proper design and control of wastewater treatment plants. To model its time dependence and the effect of plant operation parameters, sludge thickening should be taken into account. Most of the models proposed in litterature require large computational efforts or the knowledge of parameters that require delicate measurements. This study gives a new equation for activated sludge thickening, X i = a*(t i + Δt) b , determined from 23 different experiments, with formulas to obtain all the model parameters (a, b, and Δt). They have been fitted using settling data obtained on biomass sampled on different activated sludge treatment plants (industrial brewery], municipal, and laboratory pilot plant). Model predictions are compared to experimental data obtained on a full-scale secondary clarifier under unsteady-state conditions.