A high-voltage capacitive electrostatic device is immersed in the water line feeding equipment in dental offices to remove existing biofilms and to prevent their recurrence. The device is operated continuously in a totally capacitive mode at very high voltages, most preferably greater than 30,000 volts DC. The biofilm present in the system is altered by the electrostatic field so generated and the change is found to cause the separation of existing biofilm from the surface to which it adheres in the water lines and to prevent the formation of new biofilm bacterial colonies, thereby materially improving the quality of the water delivered at the various use points of the dental unit.