Particles of natural volcanic ashes as photocatalyst configured in a packed-bed photocatalytic reactor were studied for the photodegradation of wastewater pollutants under solar light. The photocatalytic system was equipped with a cylindrical parabolic sunlight concentrator, and the photocatalytic treatment has been developed with a continuous flow of wastewater under sunlight irradiation. The influence of incident radiation, amount of photocatalyst, and several configurations in the photoreactor hydrodynamics were studied. Moreover, results obtained were compared with those obtained from the photolysis and adsorption studies. Good photocatalytic activities have been observed, and this allows concluding that heterogeneous photocatalytic system in a packed bed is an effective method for wastewater pollutants removal. In a sunny day, more than 90% of the contaminant can be removed after 5 h by the continuous-flow photocatalytic treatment of the wastewater in the packed-bed reactor under sunlight. The possibility of combining the renewable energy (solar energy) and the photocatalytic technology, by means of a sustainable photocatalytic material, offers a powerful alternative in the wastewater treatment sector.