This paper summarizes the results of a work aimed at the design of pilot and industrial size photocatalytic reactors for wastewaters treatment. The work is based on the findings of previous lab-scale experiments and model development. The physical reactor model was used to simulate the performance of the photocatalytic system in various configurations and different operating conditions. The cost of the water treatment was determined by considering both the energy consumption and the periodic renewal of commercial UV lamps. The results of this work show that the optimal conditions for a photocatalytic purification unit remarkably depend on the kinetics of the pollutants degradation. In the case of fast degradation reactions low-power UV sources and low absorption catalysts are preferable. On the contrary the use of high-power lamps and dense catalysts can reduce sensibly the treatment cost when the degradation processes is slow. (C) 2004 Elsevier B.V. All rights reserved.