Water sanitization device, system and method

A water sanitisation system comprising:

• an electrolysis cell arranged in a water circulation system of a body of water in a swimming pool or spa, the body of water containing (a) sodium chlorite and/or sodium chlorate and (b) sodium chloride, the electrolysis cell operable to convert sodium chlorite and/or sodium chlorate to chlorine dioxide, and sodium chloride to chlorine,
• a control unit in communication with the electrolysis cell;
• and
• a sensor configured to detect a level of chlorine dioxide and a level of free chlorine present in the body of water,
• wherein the control unit is configured to stop or slow the electrolysis cell when the sensor determines that the level of chlorine dioxide present in the body of water has exceeded a first predetermined threshold,
• wherein the first predetermined threshold is between 0.1 ppm and 1.5 ppm chlorine dioxide in a body of water, and
• wherein the control unit is configured to stop or slow the electrolysis cell when the sensor determines that the level of free chlorine present in the body of water has exceeded a second predetermined threshold,
• wherein the second predetermined threshold is between 1 ppm and 10 ppm free chlorine in a body of water, and
• wherein the electrolysis cell is controlled by the control unit to result in:
• high chlorine dioxide production if the sensor determines that the level of chlorine dioxide is less than 0.1 ppm;
• low chlorine dioxide production if the sensor determines that the level of chlorine dioxide is between 0.1 and 1.5 ppm;
• and
• no chlorine dioxide production if the sensor determines that the level of chlorine dioxide is above 1.5 ppm.