A batch electrocoagulator was developed for optimal removal of reactive blue 21 from wastewater. The data from the batch reactor were then used to develop a continuous-flow reactor with similar performance for reduction of chemical oxygen demand (COD) and color removal. In the 1.8-L batch system, COD removal of 90% and color removal of 95% were achieved using a current density of 30 A/m(2), electrode area of 0.10 m(2), and electrocoagulation time of 7 min. The continuous-flow electrocoagulator consisted of two hollow iron tubes put together in a concentric configuration with a working volume of 1.5 L and an effective electrode area of 0.15 m(2). By keeping the current density and the applied electric charge the same in both the batch and the continuous-flow systems, the COD and color removals and energy consumption of both systems were the same within 10%. However, the color and COD removals by the batch reactor were higher than those of the continuous-flow electrocoagulator because the distribution of electroactive species in the batch reactor was more effective than that in the tube-in-tube electrocoagulator. (C) 2015 Elsevier B.V. All rights reserved.