In this paper, the role of the diffusion coefficient in one-dimensional convection - diffusion models for secondary settling tanks is addressed. A simulation analysis is performed using a representative convection - diffusion model to demonstrate the large sensitivity of the model solutions with respect to the value of the diffusion coefficient. In addition, it is illustrated that the numerical behaviour of the model is strongly influenced by the value of the diffusion coefficient. As the results of the simulation study clearly demonstrate the crucial role of the diffusion coefficient in convection - diffusion models, accurate calibration of the diffusion coefficient is of utmost importance when using such models. Therefore, a new closure for the evaluation of a spatially dependent diffusion coefficient is introduced and analysed.