The use of electrocoagulation for water treatment has drawn attention in the last few years due to several advantages it holds over traditional coagulation. However, gaps in the current understanding of the complex physicochemical processes performed during its operation have made the systematization of the design and the operation of this kind of systems quite difficult. This study intends to gather some knowledge about the factors that influence the coagulant formation when iron electrodes are used. Also, the electrochemical efficiency is verified at semi-pilot scale when the operation is performed out of the material passivation zone. This study shows that it is possible to determine the limits of the passivation zone with galvanometric tests at microelectrolysis scale for different conductivities, pH and anions in solution. These results make it possible to design a full-scale reactor with electrochemical efficiencies close to 100%.