The application of photocatalysis for water treatment and purification on an industrial scale can be accelerated by the development of both new photoreactor designs and mathematical models. A novel, pilot-plant, thin-film, slurry photocatalytic reactor for water treatment and purification is presented in this paper. The reactor is a "fountain" photocatalytic reactor, consisting of a flattened water bell irradiated from above. Such a reactor configuration is particularly suitable for large-scale solar applications of photocatalysis. A dimensionless mathematical model for the fountain photocatalytic reactor is presented in this paper. The model was developed using parameters that can be estimated easily from real systems and model solutions can be obtained with little computational effort. The model was validated with experimental results from the photocatalytic oxidation of salicylic acid in a pilot-scale reactor using titanium dioxide (TiO2) as the photocatalyst. Experiments were performed under a number of different conditions by varying substrate concentration, intensity of the incident radiation, catalyst loading, flow rate, recycle ratio and water fountain diameter. The model results were found to fit the experimental data well in all cases. The modeling approach can be extended to other thin-film slurry photocatalytic reactors.