Abstract The clarifier is a key element in the activated sludge treatment: it ensures both the separation of solids materials produced in the aeration tank and the thickening of the sludge. One of the main difficulties faced by operators of wastewater treatment plant (WWTP) is to anticipate the rising of the sludge blanket due to hydraulic or organic load increase (e.g. daily variations or rain events) whereas no suitable prediction model are available to investigate this problem. The one-dimensional clarifier model based on the Takács settling velocity function and on the flux theory for transport is among the most used because of its ability to predict effluent and underflow suspended solids concentrations. Nevertheless, results from literature highlight that the model parameter values depend on the sludge settling properties. Moreover, parameters values were unfortunately not set-up for activated sludge operated under extended aeration. Thus, the purpose of this paper is to study the validity of this model for sludge blanket height prediction at full-scale plant with changes in sludge properties. To reach this goal, experimental studies on a low loading rate municipal WWTP instrumented with on-line sensors were conducted. An experimental procedure was set-up to identify the model parameters with a combination of laboratory batch settling tests. The following main results were obtained for a given dynamic sludge volume index (DSVI): - The repeatability of the settling velocity determination in batch tests was around 10% in terms of dispersion, meaning that the column test is reliable. - The identification procedure was validated and the sludge blanket height behavior was predicted without any change in model parameters under simulation. - The results are discussed in order to point out the validity range of the studied model. Different tests are under process to establish the suitable operational correlation function between model parameters and DSVI for activated sludge operated under extended aeration. References Takács, I., G. G. Patry, et al, A dynamic model of the clarification-thickening process, Water Research 25(10), pp. 1263-1271, 1991.