The effect of calcium on arsenate removal by electrocoagulation with aluminum electrode was investigated in this study. The addition of calcium salt dramatically improved the removal efficiency of As(V). The residual arsenate decreased from more than 0.5 mM to less than 0.05 mM after the addition of stoichiometrically equivalent amounts of calcium salt (gamma = 1). The arsenate cannot be completely removed if the is under 1. If the gamma is over 1, the calcium concentration rose and lead to the increase of hardness. The calcium ions can neutralize the negative surface charges of the precipitate and increase the binding energy of As-O. The addition of calcium also prevented the formation of deposit layer on anode surface which caused an increase of applied potential and decrease the concentration of dissolve aluminum. (C) 2014 Elsevier B.V. All rights reserved.