What happens with sanitizer in an unused spa?

[Sjde]

When your hot tub isn’t used for a day or two ( or a week), what happens to the bromine level?

I know time and aeration bring it down, and an ozonater and a floater bring it up. So is there a net gain or loss?

 

[JoyfulNoise]

Sanitizing bromine gets reduced to bromide (Br-), basically a salt of bromine. When bromide is oxidized by a strong oxidizer (such as ozone), part of it is converted to sanitizing bromine, hypobromous acid, but another fraction of it is converted to bromate (BrO3-). Bromates are suspected carcinogens and persistent pollutants that build up in water and do not go away. So it is important to drain a bromine hot tub regularly to avoid bromate build up.

With chlorine oxidation, bromide is mostly converted to bromine and then the bromine reacts with something (oxidation or sanitation) and turns back into bromide. The cycle repeats as long as there is an oxidant that converts the bromide to bromine. This is why bromine tubs can start off with building up a bromide bank (adding sodium bromide to achieve a 30ppm concentration) and then just use regular bleach to create the necessary amount of bromine. No tablets necessary. However, bleach has to be added daily to account for the reduction of bromine into bromide so that means someone has to test and add daily. Tablets simply “automate” the process but leave behind undesirable chemicals (DMH).

 

[Sjde]

Thank you but my eyes were glazing over at that technical description! I just wanted to know what the reading tends to do over 24 hours of no use. I thought the purpose of bromine (vs chlorine) and a floater was so that it DIDN'T need daily additions.

 

[JoyfulNoise]

With a bromine floater and tablets, you should not have to add any oxidizer. The problem is, people often find it very difficult to “dial in” the right opening size on the floater and that typically leads to either too much or too little sanitizer.