The control of a drinking-water treatment plant aims to produce the correct quantity of water, with a constant quality. Achieving constant water quality is not an obvious task, since the online water-quality measurements and possible control actions are limited. Applying model-based control improves disturbance rejection and online process optimisation. For the softening process step, the integral control scheme is shown with multiple controllers for different time scales and process detail. The dosing control is elaborated and verified using simulation experiments. The control is implemented and tested in the pilot plant of Weesperkarspel (Amsterdam). It shows that in the case of accurate state estimation, quick changes in setpoint can be tracked.