Why do so many people prefer bromine over chlorine for spa and hot tub water disinfection?

While it is true that bromine, in the form of hypobromous acid, is a strong oxidant and disinfectant, it is not as strong as chlorine.

Bromine is a less effective sanitizer that chlorine on a ppm basis. For example, 0.3 ppm free available chlorine provides 99.99 percent inactivation of both S. faecalis and P. aeruginosa after two minutes at 25°C and pH 7.5. By comparison, 5 ppm electro-generated bromine provides 92.8 and 85.5 percent inactivation under the same conditions.

Bromine cannot be effectively stabilized with cyanuric acid to protect against ultraviolet degradation from the sun as can chlorine. Although bromine does react with cyanuric acid to form bromoisocynanurates, it requires very high concentrations of cyanuric acid to observe a significant effect, so it is effectively not stabilized, according to chemist John Wojtowitcz.

Not only is bromine less effective and cannot be stabilized – it is also more expensive.

It seems possible that the preference for bromine for spa applications is based as much on misinformation as anything else.

For example, bromine is frequently touted as being “more stable than chlorine at higher temperatures.”

It is also frequently said that “bromine works faster than chlorine at higher temperatures.”

Neither of these statements is true. In fact, both chlorine and bromine work faster at higher temperatures. But chlorine actually works faster than bromine to oxidize bather wastes at higher temperatures. And that’s actually a good thing, but it also means that chlorine gets depleted faster. This may be the reason for the perceived “stability” difference.

What is true is that without any stabilizer in the water, chlorine is more volatile than bromine and therefore outgases faster than bromine. But adding cyanuric acid makes any difference in “stability” negligible.

Another difference between bromine and chlorine is the sanitation efficacy of combined bromine compared to combined chlorine. As is the case with chlorine, when bromine reacts with water contaminants, combined bromine is

Toxicity of halo-disinfection byproducts (Chloro, Bromo, Iodo) formed. Unlike chlorine, however, combined bromines are also pretty good disinfectants. According to Wojtowicz, combined bromine is about half as effective as free bromine.

That said, according to scientific studies, combined bromines, otherwise known as disinfection by-products (DBPs), are worse than combined chlorines from a health standpoint.

Studies published by genetics specialists at the University of Illinois have demonstrated that the genotoxicity (damages DNA) of bromine is about 10 times higher than that of chlorine. Meanwhile, the cytotoxicity (kills cells) of bromine is more than 100 times higher than that of chlorine. This was illustrated in graphic from a presentation given by Dr. Michael Plewa and Dr. Elizabeth Wagner, entitled “Mammalian and Human Cell Toxicology of Emerging Drinking Water Disinfection Byproducts.” It can be seen that the chlorodisinfection byproducts (chloro-DBPs) are significantly less toxic that their bromine analogues, and this may be an important consideration for the health conscious, as some of these byproducts can be absorbed through the skin or are volatile and can enter through the lungs.

So, if the choice is between chlorine and bromine and a person is concerned about health effects, then chlorine would be the better choice.

Perhaps one big selling point of bromine is that after it is used up in the various chemical reactions it performs, it can be regenerated with an oxidizing agent. When either chlorine or bromine react with the contaminants in a pool, chloride or bromide ions are formed. But bromide ions are easily oxidized back to hypobromous acid with the simple addition of chlorine, potassium monopersulfate, or ozone. For chloride ions to regenerate back to hypochlorous acid, a salt water chlorine generator is needed.

At the end of the day, the main reason to use bromine in spa applications has to do with convenience. Just as using trichlor tabs for swimming pools simplifies water maintenance, it is easy to use bromine tabs to maintain a sanitation residual.

Trichlor tabs dissolve slowly in swimming pool water, but rather quickly in hot water, and trichlor is also very acidic, which can be extremely corrosive to the spa’s surfaces and equipment.

But bromine tabs dissolve slowly, and after bromine has performed its oxidation duties, can be very simply and automatically regenerated.

For those considering switching to bromine for spa use, there are two different ways that it can be administered to the water.

One way is to add bromine tablets in floating feeder or some other feeder system. The tablets contain a mixed bromine sanitation compound, bromo-chloro-dimethyl-hydantoin (BCDMH) that provides a continuous bromine residual for extended times.

The bromine should be maintained to between 2-4 ppm, and it’s a good idea to shock the water after each bather use. Users should be aware that this tablet dissolves faster at elevated temperatures, which can be a problem for spas. Therefore, use a spa bromine feeder to avoid overbromination.

Another way to administer bromine is to add sodium bromide on a weekly basis. Alone, this compound does nothing at all. When sanitation and oxidation is required, an oxidizer is added, which oxidizes the bromide ion to hypobromous acid.

Some people use chlorine as the oxidizer. Others, preferring a non-chlorine spa, use potassium monopersulfate, potassium peroxymonopersulfate, or ozone as the oxidizer.

The shock is added only after bathers have used the spa. After bathers exit the spa, they administer the required dose and put the cover in place. Those endorsing this method enjoy the fact that bathers are not exposed to any sanitizer while in the spa.

To use this method, an initial sodium salt solution is added to the water after the spa has been filled. The filtration system is used to mix the salt. Two ounces of a non-chlorine shock oxidizer is used for every 250 gallons of water.

After this point, the non-chlorine shock oxidizer is added after bather use. Once a week, one ounce of sodium bromide salt solution is added for every 250 gallons of spa water.