Microbial desalination cells (MDCs) are an emerging concept that uses bioelectric potential produced from organics via microbial metabolism to accomplish desalination. MDCs consist of three compartments, the anode, the cathode, and a salt compartment, which is between the anode and the cathode. This study reported a bench-scale laboratory experiment for evaluating the effectiveness of using MDC technology to remove hardness from several different hard water samples collected from across the United States, ranging in concentrations from 220 to 2080 mg/L as CaCO3. It was found that the MDC generally removed more than 90% of the hardness from the tested water samples driven by electron movement in the batch operation. Electricity production was highly related to the conductivity of the hard water samples. It was found that the MDC removed 89% of the arsenic, 97% of the copper, 99% of the mercury, and 95% of the nickel at the testing concentrations in a synthetic solution. These results provided a proof-of-concept that MDCs can be used to soften hard water that is driven by an electric current. Further development of the MDC softening process should optimize reaction configuration and reduce the volumetric ratio between wastewater and hard water. (C) 2012 Elsevier B.V. All rights reserved.