FLUENT CFD software was used to form an understanding of a periodic dysfunction at a drinking water production plant at a site in North Africa. The structure involved is a circular, 27-metre diameter, flocculator-settling tank, supplied with surface water taken from the nearby river. The following dysfunction was observed on the structure in question. In summer, during the hottest time of the day, a cloud of flocs would sometimes rise to the surface, appearing in the centre of the settling zone. The cloud then progressed slowly to the outer zone of the tank, where a proportion of the total suspended solids (TSS) would overflow, impairing the quality of the treated water. The CFD simulations enabled us to understand the precise mechanisms in operation when the phenomenon is observed.