The objective of the present study is to optimize the treatment of the cardboard industry wastewater generated in the process of machine washing. This type of effluent is usually treated by traditional physicochemical processes such as coagulation/flocculation and sedimentation. These processes give a limited purifying efficiency, particularly for the COD reduction. In this work, the treatment by coagulation-electroflotation process was adopted. In batch mode treatment, current density, pH and coagulant concentration are the operating parameters to optimize. The methodology of experimental research, with an orthogonal central composite plan was adopted. Good agreement between theoretical analysis and experimental results was obtained. Continuous mode was also studied in order to optimize the residence time. A physicochemical characterization including COD, BOD and suspended solids charge was done before and after the treatment in order to improve the efficiency of this process. (C) 2007 Elsevier B.V. All rights reserved.