A methodology is developed for simulating the operation of flocculation tanks for potable water treatment in the presence of very low solids concentration. The main effort of the simulation is devoted to the detailed representation of the geometry of the system whereas the particle scale physicochemical processes are treated in a simplified way reducing the computational effort but retaining the parametric dependency of the process. As a case study, one of the flocculation tanks of the water refinery of the city of Thessaloniki (Greece) is examined. It is shown that for the case of low solids concentration in the feed stream, flocculation is very weak to lead to particles large enough that can settle in the subsequent sedimentation tank. In this case, flocculation need be enhanced by increasing the solids concentration in the flocculation tank by recycling sludge from the sedimentation tank. Furthermore, guidelines are provided on how the present computational tool can be used for the optimization of the design and operation of flocculation tanks treating potable water from relatively clean sources. (C) 2010 Elsevier B.V. All rights reserved.