Electrocoagulation (EC) technology is based on rapid in situ dissolution of sacrificial anode to generate precipitates and flocs capable of removing heavy metal ions. The lack of mechanism-based approach to reactor optimization has limited its implementation. The continuous EC process for treating heavy metal ions (Ni2+) was studied. Effects of some experimental parameters on the heavy metal ions removal were investigated. The back mixing was investigated through the concentration distribution at the direction of streamline. In continuous EC process characterized by in situ generation of absorbents, the degree of back mixing is dependent on both fluid dynamics and mass transfer. The degree of back mixing increases with the increment of current density or residence time. A new universal parameter named as molar ratio of M/Al (M is the heavy metal ions) was introduced to quantify the utilization rate of electro-generated Al ions (or Fe ion) in EC process. The M/Al ratio is found to decrease with the increment of current density and residence time. Further, the relationship between the ratio and traditional parameters (removal efficiency and the consumption of energy and electrode) was discussed in this article. The optimization and reactor design could be based on the optimization of M/Al ratio. (C) 2015 Elsevier B.V. All rights reserved.