The reactor design for tannery wastewater treatment using computational fluid dynamics (CFD) techniques was carried out. The effluent contains different types of pollutants such as organic matter, salts, heavy metals, and dyes difficult to degrade. Due to the complexity of the effluent, a treatment based on an electrocoagulation process is proposed. In most of the references is mentioned that the characterization and design of this type of process is carried out empirically or semi-empirically, so in this work the influence of hydrodynamics, mass transfer of aluminum ions and secondary current distribution was studied. An electrochemical rotating cylinder electrode (RCE) reactor was used, with 6 working electrodes and an aluminum RCE as counter electrode. Besides, a decanter was incorporated in the reactor design. Current distribution and fluid velocity profiles as well as particle tracking animations were obtained. The most suitable geometry was designed from the theoretical results of the sedimentation process and aluminum ions generation. Finally, some operating parameters such as velocity inflow and velocity of the RCE were evaluated.