The textile industries are known as one of the important source containing hazardous materials. Recently, different techniques (physical, chemical…) have been used for the wastewater treatment. Among of them, photocatalytic oxidation as an advanced oxidation process is a useful treatment for removal of the organic pollutants. According to the recent studies, it would be necessary to the choice of a photoreactor with high efficiency for wastewater treatment. Finding of the optimum parameters and flow regime inside the reactor are necessary to study the process efficiency. In addition, the hydrodynamics of a reactor can be characterized from the residence time distribution.