This paper presents the mathematical modelling and design of a flat plate photoreactor with a fixed film photocatalyst to degrade phenol. Phenol was used as target pollutant as it is one of the most studied contaminants in photocatalysis and it is present in many of the wastewater streams containing pesticides, drugs, and dyes which cannot be treated with conventional wastewater technologies. To model the photoreactor, first a Zero Reflectance Model assuming no energy absorption or scattering by the fluid phase was used to compute the rate of photon absorption. Interest was also placed on determining the effect of the photocatalytic film thickness on both the internal diffusion and the rate of photon absorption. Afterward, the material balance of the pollutant was computed considering convection, diffusion, and a Langmuir Hinshelwood type of kinetics to describe the degradation of phenol. As the reaction just takes place at the bottom of the reactor, external mass transfer limited the overall phenol degradation. Therefore, several reactor heights were investigated to diminish external mass transfer limitations and determine the optimal photoreactor dimensions. For instance, for a throughput of 50 liters per day and assuming 10 hours of sunlight, to reach 95% phenol conversion the photoreactor height should be below 0.45cm, which corresponds to a photoreactor length of approximately 25 meters. In contrast, when the reactor height was diminished to 0.1cm, the required reactor length to reach 95% conversion decreased to 10 meters.