A disinfection device for a trapped-liquid comprising of a drain trap for a liquid flow having an inner surface to trap a volume of the trapped-liquid and a helical coil capable of generating metal ions to eliminate biological cells attached to said drain trap. An electric valve is connected to a tap water and the helical coil and a power supply provides a current and a voltage to the helical coil for a period of ionization and a sensor system installed in the drain trap to sense flow of a liquid through the drain trap. A timer system to determine the length of a stagnant-time that the trapped-liquid is stagnant in the drain trap; and to determine the length of an ionization-time for the period of ionization.