In the US, the EPA has approved the use of chlorine, bromine, or biguanide as a spa sanitizer. One can also use potassium monopersulfate (MPS) and silver ions but only if the water temperature is consistently maintained above 98F otherwise the bacterial kill rate falls too low. MPS/silver sanitized hot tubs tend to get cloudy more quickly and need more frequent water changes.
The silver cartridges adds the silver ions (silver nitrate embedded in a slowly dissolving matrix) and the MPS acts mainly as the oxidizer for bather waste. Silver ions are known to be anti-viral and anti-bacterial when in solution and at higher temperatures (104F). The MPS enhances the ability of the silver ions to kill pathogens and somewhat stabilizes the silver ion in solution (silver will readily form insoluble oxides and chlorides when in aqueous solution). The down side to these systems is that there is no way to know what the silver concentration is nor is it easy to figure out how much MPS is in the water, so reliable testing and accurate dosing are out the door and you have to rely on trial and error. Not the best way to manage a body of water.
