ZnO-based catalysts impregnated with the metals Ag and Pd were synthesized for the photocatalytic degradation of two reactive dyes RB 19 and RB 21 in the textile effluents using an annular UV photoreactor. Among three synthesized catalysts, only the ZnO catalyst preserved the photodegradation of the dyes in the effluents. The density, mean particle diameter, surface area and porosity of the ZnO catalyst were 5550 kg/m(3), 1.19 x 10(-7) m, 16.830 m(2)/g, and 0.1, respectively. It was found that the ZnO nanoparticles photodegradated the RB 19 and RB 21 by 100 and 91% respectively over six hours..The photocatalytic degradation process was then modeled successfully based on the mass conservation law. The maximum photocatalytic degradation by the model can be obtained at the smaller catalyst size, lower initial dye concentration, and higher length per diameter ratio of the photocatalytic reactor. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.