In the present work, coagulation, electrocoagulation and sono-electrocoagulation were compared for the treatment of synthetic wastewater, which consists of a solution of Rhodamine 6G (used as a model of dye). The results are discussed in the light of the well-known coagulation mechanism of dyes. These results show that the primary coagulation mechanisms of coagulation and electrocoagulation are different because of the very different pH changes obtained during the treatment, being more relevant the enmeshment into the growing floc of the metal hydroxide precipitate in the electrocoagulation process and the charge neutralisation in the coagulation. Removals of 60% can be obtained for large doses of coagulants (or electric charge applied in the case of electrocoagulation) in both single technologies. Ultrasound irradiation allows the electrocoagulation process to enhance the efficiency up to 95% even if applying a lower electric charge. This enhancement is especially important during the treatment of wastewater with an alkaline initial pH because of the promotion of the floc enmeshment mechanisms. Electrochemically-based processes show an additional advantage: the pH is buffered during the treatment to a range 6-8, which allows the direct disposal of the treated waste without a further neutralisation process. (C) 2014 Elsevier B.V. All rights reserved.