Energy and cost effective pretreatment of seawater is essential for any desalination plant to convert seawater into clean water successfully. This study shows a way to pretreat seawater without excessive input of electrical energy or high pressure; it uses a bioelectrochemical cell with two chambers, divided by an anion exchange membrane (AEM). When bacteria attached on the anode produce current using organic matter as a source of fuel, hydroxyl ions can be generated in the cathode chamber, which can increase the alkalinity of the catholyte (seawater); scale-forming ions such as Ca2+ and Mg2+ in seawater can be removed by forming precipitates in the cathode chamber. Our results show that 84% of Ca2+ and 37% of Mg2+ in seawater can be effectively removed while treating wastewater in the anode chamber. These results indicate a new method to pretreat seawater without external energy input since the cell accomplishes selective removal of divalent cations using an electrochemical cathode reaction. This study provides evidence that this new method can be a viable option for membrane based seawater pretreatment and desalination processes to significantly mitigate scale problems of membranes. (C) 2014 Elsevier B.V. All rights reserved.