When bromine is added to a spa it's in the form of bromide ions. It then activated with an oxidizer to form hypobromous acid. Hypobromous acid is the killing form of bromine. When hypobromous acid reacts with an organic contaminant it's reduced back to bromide ions. The bromide ions can then be reactivated back into hypobromous acid by the addition of an oxidizer. This is somewhat of a continuos cycle with bromine sanitized spas.
If 100 percent of the bromide ions became hypobromous acid and 100 percent of the hypobromous acid returned to bromide ions, you would never again have to add more bromide salt. But hypobromous acid can react with certain chemicals in the water that tie up the bromide ions and prevents them from becoming free bromide ions in the water. This happens when hypobromous acid produces bromate or bromoform for instance. There are many other combinations that tie up the bromine so it can not become a free bromide ions. For this reason more bromine salt or bromide ions need to periodically be added. The level of bromide ions should not go below 15 ppm.
The basis behind 2-part liquid bromine sanitizing systems is to add a salt of bromine (sodium bromide) to the water to get bromide ions and then oxidize the bromide ions with an oxidizer such as monopersulfate (MPS), hydrogen peroxide, percarbonate, ozone or any compound of chlorine to produce hypobromous acid. You need a minimum of 15 ppm of bromide ions for an oxidizer to work and to provide a "bank" of bromide ions for an oxidizer to react with.
Bromine tablets are typically 1-bromo-3-chloro-5,5-dimethylhydantoin. When added to water they hydrolyze to become hypobromous acid. With bromine tablets a separate oxidizer is not necessary to make hypobromous acid, it is already an ingredient in the tablets. When the hypobromous acid reacts with a contaminant and is reduced, it becomes a bromide ion. You then get a build up of bromide ions in the water. After a while, you could just start adding an oxidizer to reactivate the bromide ions to hypobromous acid, but most people don't, they just add more bromine tabs.
Currently is no way to test water to find out how much bromide ions are in it. This is because the same test that measures bromide ions also measures chloride ions and all water has chloride ions in it. Other than making an educated guess, there's no way to tell when the bromide ion level is too low. The 2-part bromine manufacturers know this and recommend that you add some bromide ions (liquid bromide salts) every few weeks or so.
Realistically, a bromine sanitized spa can not be switched over to chlorine, if there's still bromine in the water. All the chlorine added to the water is going to convert bromide ions into hypobromous acid. As long as there's 15 ppm or more of bromide ions in the water, all the chlorine added is going towards converting bromide ions into hypobromous acid, none of it will provide a chlorine residual. The spa will continue to be bromine sanitized until the bromide level gets below 15 ppm. But there is no test kit for measuring just bromide ions in the water and therefore no way to know when the bromide level is below 15 ppm. It could take a week. It could take 2 months.
There are typically two types of bromine systems, a 2-step system and a 3-step system. With a 2-step system sodium bromide, either granular or liquid, is added to the water. An oxidizer, such as chlorine or non-chlorine shock (MPS) is than added on a regular basis to oxidize the bromide into bromine. One of the more popular 2-step bromine systems is the Enhance/Activate Sanitizing System. It's chlorine free and easy to use, but does require some daily attention to maintain proper bromine levels in the water. A 3-step system is similar to the 2-step, but also uses bromine tablets in a floating feeder. Bromine tables consist of a combination of sodium bromide and an oxidizer, typically chlorine. The 3-step system requires less attention and maintains more constant levels of bromine in the water, but costs more than the 2-step system.
A 2-step system with an efficient ozonator might be able to achieve the constant bromine level without the use of, or by using less oxidizer (MPS or chlorine) since the ozone is constantly oxidizing the sodium bromide while it is on. However, the ozone may also deplete the bromide reserve more quickly, leading to the use of more sodium bromide, or a shorter time between necessary drain and refills. Also, ozone can cause bromates to form in your water. Bromates are a suspected carcinogen in drinking water.
To begin a bromine sanitized system on a freshly filled spa the first thing needed to be done is establish a bromide ion reserve of 30 ppm. This can be accomplished in a number of ways.
With either system the water will need to be shocked on a weekly basis. Shocking is done to completely destroy any organic material in the water and is accomplished by adding enough chlorine or non-chlorine shock (MPS) to raise the bromine level to above 10 ppm.
The above is a compilation of information, some of
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