Operating diagnostics on a flocculator-settling tank using Fluent CFD software

Recently, Computational fluid dynamics (CFD) software has become easy to use, fast and user-friendly. This new generation software offers an inexpensive means of testing and optimizing hydraulic operation of both existing constructions and those under design. In the present case, 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 would overflow, impairing the quality of the treated water. The phenomenon was easily linked to large-scale external temperature swings, sometimes rising as high as 50 degrees C in the shade during the daytime. However, observations made in situ offered no explanation for the full range of mechanisms triggered by these temperature variations. The CFD simulations enabled to understand the precise mechanisms in operation when the phenomenon is observed and to have a good base from which to search for a solution to the problem.