Ozone Systems - Further Reading

Ozone Systems in Pools

TFP does not use or endorse the use of ozone systems. Ozone is not a water sanitizer and ozone systems have little to no value in a residential, outdoor pool.

Ozone manufacturers push Ozone systems by saying you need to maintain a minimal amount of chlorine in the water. Here at TFP we group them together as an "alternative" system that is potentially unsafe to the swimmers in the water.

Additionally, ozone systems are impossible to test to see if they are working or not.

Ozone provides no residual effectiveness. It only cleans what comes in direct contact with it. We would much rather maintain a higher Free Chlorine level and know that pool water is safe since it's the water itself that carries the sanitizing effects along with it.

Ozone can act as both a sanitizer (kills pathogens) and an oxidizer (breaks chemical bonds and breakdown complex organic compounds into simpler ones).

Ozone has a very low Henry's Law constant and so ozone does not dissolve in water readily. Just like how an SWG produces both chlorine gas and hydrogen gas, the chlorine gas readily dissolves into the water while the hydrogen gas remains undissolved and bubbles out of the return. Ozone will also remain largely undissolved and simply bubble out of the returns.

Ozone only works on the water that it is in intimate contact with. As ozone travels along the return pipes, it can react with pathogens and oxidize organics but as soon as it enters the pool volume it will bubble to the surface and escape the pool water. This is mostly why ozone is ineffective - it does not create any residual concentration in the water.

Pools have very poor hydraulic efficiency and so even when you run multiple turnovers, the pool equipment does not "See" all of the water. Even if you assumed "perfect" mixing of the pool water, you would see the following dilutions:

• 1 turnover: 63.2%
• 2 turnovers: 86.5%
• 3 turnovers: 95.0%
• 4 turnovers: 98.2%
• 5 turnovers: 99.3%
• 6 turnovers: 99.75%
• 7 turnovers: 99.91%
• 8 turnovers: 99.966%
• 9 turnovers: 99.988%
• 10 turnovers: 99.995%

You never get to 100% and again, the above assumes perfect near instantaneous mixing in the bulk pool water.

Thus, in a theoretical pool with perfect mixing (so basically, no pool ever in existence), it takes 7 turnovers of the water to get the pool equipment to "see" 99.9% of the water. Therefore, any sanitizer that doesn't establish a residual concentration (ie, UV, Ozone) will be very ineffective at sanitizing.

Because there is no residual concentration of ozone, you can not measure it. There is no simple way to measure the output gas stream of an ozonator and simply applying power to an ozone cell does not guarantee it is actually making ozone gas.

Ozone gas can be easily created using two different methods - corona discharge (CD) or medium-pressure mercury arc lamp (UV light). Corona discharge is a lot like the sparks generated in an old electric motor where you can smell the ozone. A corona discharge is a gas plasma that forms on a non-reactive surface (glass typically, you can think of it like a neon sign) with oxygen gas forming a small fraction of ozone when you strike a plasma with it. However, most CD ozone generators simply use air to strike a plasma and air is mostly nitrogen gas (78-80%). A further complication arises when using atmospheric air as the plasma gas because most air has high relative humidity and water vapor is known to "poison" (or reduce) the generation of ozone within a plasma. This is why most commercial scale generators (the kinds found in water treatment plants) use either pure bottled oxygen or a mixture of an inert gas (like argon) and oxygen with a desiccating pre-filter to remove water vapor.

In short, ozone is a bad choice for residential pool sanitation and, at the end of the day, all pools MUST use chlorine as ozone is not approved as a primary disinfectant. So if you have to use chlorine anyway, why bother adding the complexity of an ozone generator to begin with??

Ozone Systems in Spas

The biggest plus with a functioning ozonator (not all work properly or put out enough ozone to work well) is that it will oxidize many contaminants in the water, but it's not fast as it takes time to get water circulated through the ozonator. Though spas circulate water faster than pools, it still takes 4.6 turnovers of the water to get 99% of the water through the ozonator and that's assuming no dead spots. You still need a residual sanitizer if you're going to prevent bacterial growth in the bulk pool water.

The biggest minus with an ozonator is that it mostly injects air into the water and that aerates the water. If the ozonator is always on (and many are), this leads to a rise in pH if you use a hypochlorite source of chlorine. So many spa users use Dichlor which is acidic (when accounting for chlorine usage) and this helps maintain the pH, but has the CYA rise.

An ozonator probably makes more sense in a bromine spa than a chlorine spa as it can reactivate the bromide to bromine (though can create bromates -- so don't drink the water) and most bromine systems are net acidic so the pH will be more stable with the ozonator. As was pointed out, it is technically unnecessary if one maintains a residual sanitizer (chlorine, bromine, PHMB/biguanide/Baqua).

Just keep in mind that the sanitizer usage in a spa is MUCH higher than in a pool due to the lower water volume (higher bather load) and higher temperature that causes one to produce more sweat and causes chlorine to outgas more (especially with an ozonator).

If you use your hot tub a lot and keep it covered when not in use then an Ozonator can help, however if you only soak in it on the weekends, etc. then an Ozonator will actually consume more chlorine than the tub would use to oxidize bather waste if you did not have one. Also unlike Ozonators for pools, ones built for hot tubs are relatively cheap.

Does Ozone Increase pH or TA?

Bubbles from an ozonator are air plus added ozone, so it contains mostly nitrogen gas and oxygen gas with some ozone gas (some of the oxygen has been converted to ozone via corona discharge or UV depending on the system). The amount of CO2 will be that in air and will be less than that in pool water so there will be some degree of mixing and aeration as with any air bubbles going through the water. The ozone will mostly dissolve into the water as an aqueous (dissolved) gas.

The pH will rise more than it would if you had no ozonator, but it's not because of the ozone but rather because of the air bubble that not only pulls some carbon dioxide from the water, but more importantly physically disturbs the surface of the water where the bubbles escape. Such disturbance increases the rate of carbon dioxide outgassing.

The same principles hold for saltwater chlorine generators and their hydrogen gas bubbles except they have no carbon dioxide in them initially so have a somewhat greater effect on aeration.

High-end commercial ozonators have oxygen concentrators feeding nearly 100% oxygen gas to the ozonator to produce higher levels of ozone (6%+) and in this case the rest of the gas is oxygen.