The hydraulic efficiency of water process tanks can be significantly improved by performing simple modifications in the initial geometries of the tanks. This concept was applied in the main tanks of the water supply network of Athens, which are used for disinfection. Various modified geometries were formulated by "inserting" guiding walls in these tanks, a very common, simple and inexpensive modification. The hydraulic efficiency of the tanks was assessed with a CFD model, which calculated the flow fields and the flow through curves (FTCs) for the initial and the modified geometries. From the comparison of the results the following conclusions were drawn: (1) the flow for the initial geometries was characterized by high levels of short-circuiting, large re-circulation regions and high degrees of mixing, and (2) the use of guiding walls created flow fields in the tanks with significant volumes of "plug flow", reduced short-circuiting, smaller re-circulation regions and thus less mixing. The modified geometries showing the best hydraulic efficiency were proposed for construction. This procedure, which can be applied in the all types of water tanks, resulted in significant cost savings, because the modifications were examined with the CFD model, prior to their actual implementation in the real tanks. (c) 2007 Elsevier B.V. All rights reserved.