The separation of solid particles from the liquid phase is the final operation in an activated sludge wastewater treatment plant. This paper introduces a one-dimensional, knowledge-based dynamic model incorporating both mass and momentum balances, designed for use in the development of a closed-loop controller. The objective of such a future controller would be to regulate both energy consumption and the quality of water discharged from the clarifier. This model is simulated using an explicit Euler time discretization, along with a spatial discretization based on the finite volume method. The fluxes are approximated using the Rusanov scheme, a method particularly suited for handling nonlinear hyperbolic systems that exhibit discontinuities or shock waves. Simulation results are compared with experimental data, obtained from measuring the sludge blanket (the upper interface of the solid particle zone) during a transient-state experiment in an urban wastewater treatment plant. Additionally, two predictive scenarios are provided to demonstrate the potential of this model as a decision support tool in wastewater treatment processes.