The charging and discharging processes of conducting polymers, e.g. polypyrrole (PPy), are accompanied by the exchange of ions. This ion exchanger ability was used to develop an electrochemically switchable ion exchanger for softening drinking water. Ion exchanger electrodes based on modified PPy nanoparticles were prepared and their applicability was tested. A special cell arrangement containing two ion exchanger electrodes was developed to provide continuous operation of the softening and regeneration process. The test solutions were analysed by conductivity sensors. The ion exchanger cell was tested in laboratory loops and scaled up to meet requirements for practical application. (C) 2005 Elsevier Ltd. All rights reserved.