One-dimensional models for describing the secondary settler in activated sludge wastewater treatment are important with respect to process control and optimization. The most widely used one-dimensional model nowadays is the model presented by Takacs et al. (1991). In this paper, the model of Takacs et al. is thoroughly studied at the simulation level. Simulations have been performed to analyze the dynamic behaviour of the concentration profile and to examine the influence on the steady state concentration profile of (i) the loading characteristics (influent concentration and flow rate), and (ii) the number of layers considered in the settler. The simulations reveal a major shortcoming of the Takacs model, namely, its inconsistency with respect to the number of layers considered in the discretized equations. The identification problem resulting from this inconsistency is clearly illustrated. As an alternative, the (consistent) model of Hamilton et al. (1992) is proposed.