In water treatment processes, the hydrodynamic behavior in a flocculation unit is critical in order to achieve flocs with appropriate characteristics that allow their subsequent separation. In this contribution computational fluid dynamics (CFD) was used to investigate the flow characteristics of a Cox type hydraulic flocculator at the water purification plant in Viterbo, Colombia. In order to improve the original design, the effect of the windows configurations and the inlet volumetric flow was investigated. For the configurations performance assessment, the local velocity gradient and residence time distribution were calculated. The simulation results indicate that the window position have a considerable effect on the velocity gradient and retention time distribution, while the number of windows showed a negligible effect on the retention time. It was possible to decrease the local gradient mean error between $10$ and $20 \%$ by employing $2$ windows instead of $1$. The best hydraulic performance was obtained by locating the windows horizontally aligned and using $2$ windows between chambers. These results are encouraging to systematically improve the plant design thus performance.