Photocatalytic oxidation (PCO) over titanium dioxide (TiO2) is a "green" sustainable process for the treatment and purification of water and wastewater. However, the application of PCO for wastewater treatment on an industrial scale is currently hindered by a lack of simple mathematical models that can be readily applied to reactor design. Current models are either too simplistic or too rigorous to be useful in photocatalytic reactor design, scale-up, and optimization. In this paper a simple mathematical model is presented for slurry, annular, photocatalytic reactors that still retains the essential elements of a rigorous approach while providing simple solutions. The model extends the applicability of the thin-film model of photocatalytic reactors previously presented to include the case of geometrically thick photoreactors (i.e., those reactors in which the thickness of the annular zone is significant as compared to the outer radius of the reactor). The model uses a novel six-flux absorption-scattering model to represent the radiation field in the reaction space, which assumes that scattered photons follow the route of the six directions of the Cartesian coordinates. The model was successfully validated with experimental results from the photocatalytic oxidation of the pesticide isoproturon in an experimental reactor. The mathematical model presented may be used as a tool for the design, scale-up, and optimization of annular photocatalytic reactors for water treatment and purification.