Removal of scale-forming ions (e.g., Ca2+, Mg2+) from seawater using a bioelecrochemical system (BES) was investigated in a fed-batch and a continuous mode. In the fed-batch mode, Ca2+ removal with different anolyte buffers at different external resistances (10 Omega and 11 k Omega) was evaluated. As a result, Ca2+ was effectively removed with PBS (phosphate buffer solution) and PIPES (piperazine-N,N'-bis(2-ethanesulfonic acid) buffer solution. However, the removal of Ca2+ with PBS was not solely due to current generated from the anode but also from precipitation induced by combining Ca2+ with phosphate ions transferred from the anode chamber while the removal of Ca2+ with PIPES entirely relies on a cathodic reaction. Continuous removal of Ca2+ and Mg2+ with PIPES was more effective than the fed-batch mode with removal efficiencies of > 97 +/- 2% (Ca2+ removal) and 98 +/- 1% (Mg2+ removal). The XRD spectra revealed that the precipitates were mainly composed of brucite (Mg(OH)(2)), calcite, and aragonite (CaCO3). These results indicate that BES could be a promising way to remove scale-forming ions that can promote inorganic fouling in membrane-based processes.