Denitrification during the sedimentation process in secondary settlers in waste-water treatment plants has been reported to be still a significant problem. The modelling of such a process with the conservation of mass leads to a nonlinear convection-diffusion-reaction partial differential equation, which needs non-standard numerical methods to obtain reliable simulations. The purpose of this study is to provide a first extension of the Burger-Diehl settler model, which models sedimentation without reactions, to include biological reactions. This is done with a reduced biokinetic model that contains only the relevant particulate and soluble components. Furthermore, the development is limited to batch settling. The final model describes the last settling stage of a sequencing batch reactor with denitrification. The main result of the paper is a numerical scheme that is tested for simulations with several initial conditions. (C) 2016 Elsevier Ltd. All rights reserved.