If you don’t want to use chlorine in your spa then you can use sodium bromide salt to make bromine instead of chlorine. Sodium Bromide is significantly more expensive than sodium chloride and you can get it in liquid or powder form. You will need about 3000 PPM so start with about 2 pounds per 100 gallons.
If you use NACL – sodium chloride with a chlorine generator you will get chlorine as the sanitizer.
If you use NABR – sodium bromide with a chlorine generator you will get bromine as the sanitizer.
There are a few bromine generators for hot tubs and spas out there. Do not use them with regular salt (NACL), they can only make bromine.
For a 400 gallon spa, using regular pool salt will cost a couple dollars at the most. To use sodium bromide the cost will be over $100. If you replace your water 2 times per year the cost to use sodium bromide becomes significant.