Ozone is very effective at breaking down organics better than chlorine and at killing certain pathogens better than chlorine (cysts such as Giardia and Cryptosporidium and heartier bacteria such as Pseudomonas aeruginosa). The biggest problem with using an ozonator is that it does not provide a residual of disinfection in the general body of pool water so any pathogens growing on biofilms will not be affected and transmission of pathogens from person-to-person remains possible. This is why ozone systems are required by law to have an additional EPA-registered disinfectant such as chlorine (or bromine or biguanide). In the Claripure system, the residual chlorine is provided through a saltwater chlorine generation system (SWG). The problem is that ozone breaks down chlorine (ironically, the patents describe the reverse process -- more on that below). Their claims about any significant amounts of formation of chlorine dioxide are an exaggeration -- chlorate is the primary end product from ozone combining with chlorine. [EDIT] Chloride is also formed and is dominant with 77% while chlorate is the remaining 23% (see page 11 of the PDF, page 35 in the paper, in
this link or PDF page 3, page 104 in the paper, in
this link). Your pool is free of algae because you have a residual FC level and the ozonator kills any free-floating algae as well.
By not having any Cyanuric Acid (CYA) in your pool, the generated chlorine gets broken down by sunlight quickly so you wouldn't have the residual of chlorine in your pool that you should. However, the fact that you measure FC indicates that you may actually have a small amount of CYA since it doesn't take much to give chlorine a much extended life -- also, your SWG is set at 50% 10 hours a day so is on the high side. Adding a small amount of CYA in your pool to get the level to 30 ppm or even just 20 ppm (or even less, as JasonLion points out) should allow you to turn down your SWG to less than 50%, thus extending its life. That would be the main reason to add CYA in your pool. Another reason is that you actually have too much disinfecting chlorine in your pool at 4 ppm FC with very little (say less than 10 ppm) CYA so will cause faster degradation of swimsuits and corrosion (by a factor of 4 to 10 or more depending on actual CYA level). With your ozonator, you should be able to lower your SWG output and add CYA to run your pool at an FC target of 2 ppm with a CYA level of 20-30 ppm.
TECHNICAL SECTION
Most of the relevant patents may be found
here. The formation of chlorates and chlorites is not novel nor new and occurs when chlorine (hypochlorous acid) reacts with ozone. Putting an SWG after an ozonator will produce this effect and there is nothing novel about this. The proposed reactions that occur according to the patent are the following:
1) Cl
- +O
3 +H
+ --> HOCl + O
2
Chloride Ion + Ozone + Hydrogen Ion --> Hypochlorous Acid + Oxygen
2) HOCl + O
3 -->ClO
2- +H
+ +O
2 + e
-
Chlorine + Ozone --> Chlorite Ion + Hydrogen Ion + Oxygen + electron
3) O
3 + 2ClO
2- + 2H
+ --> 2ClO
2 +H
2O
Ozone + Chlorite Ion + Hydrogen Ion --> Chlorine Dioxide + Water
4) O
3 + 3ClO
2- + 2H
+ --> 3ClO
3 +O
2 +H
2O
Ozone + Chlorite Ion + Hydrogen Ion --> Chlorine Trioxide + Oxygen + Water
The second equation (2) from the patent apparently contains a typo since it has an extra "electron" in the reaction -- they probably took two half-reactions and forget to remove the electrons that canceled since the equation balances charge correctly if one removes the "e
-" or "electron" at the end. The third equation (3) from the patent also apparently contains a typo since it is missing an "O
2" or "Oxygen" on the right hand side. The fourth equation (4) from the patent is just plain wrong as it is not charge balanced (the left-hand-side has a net negative charge while the right-hand-side is uncharged). Obviously, the patent examiner was not very careful with this patent.
The rest of the reactions come from standard redox half-reactions so one can calculate their likelihood (thermodynamically, not reaction rate) as follows:
Cl
- + H
2O --> HOCl + H
+ + 2e
- ..... Eo = -1.482V
O
3 + 2H
+ + 2e
- --> O
2 + H
2O ..... Eo = +2.076V
------------------------------------------------------------------------------------------------
1) Cl
- + O
3 + H
+ --> HOCl + O
2 ..... Eo = +0.594V
Though this reaction is less likely at the normal pH in pools, in areas with high concentration of ozone, it is certainly reasonable and just states that some of the ozone does the same thing as electrolysis in terms of generating chlorine. However, other reactions are also possible as follows:
HOCl + H
2O --> HClO
2 + 2H
+ + 2e
- ..... Eo = -1.645V
O
3 + 2H
+ + 2e
- --> O
2 + H
2O ..... Eo = +2.076V
------------------------------------------------------------------------------------------------
2) HOCl + O
3 --> HClO
2 + O
2 ..... Eo = +0.431V
and HClO
2 --> H
+ + ClO
2- is also likely at the pH of pools.
2ClO2- --> 2ClO2(aq) + 2e- ..... Eo = -0.954V
O
3 + 2H
+ + 2e
- --> O
2 + H
2O ..... Eo = +2.076V
-----------------------------------------------------------------------------------
3) O
3 + 2ClO2- + 2H
+ --> 2ClO2(aq) + O
2 + H
2O ..... Eo = +1.122V
However, in addition to the above reactions, there is also the following very likely reaction that the patent oh so conveniently neglected to mention:
ClO
2- + 2OH
- --> ClO
3- + H
2O + 2e
- ..... Eo = -0.33V
O
3 + 2H
+ + 2e
- --> O
2 + H
2O ..... Eo = +2.076V
-----------------------------------------------------------------------------------
O
3 + ClO
2- --> ClO
3- + O
2 ..... Eo = +1.91V
Basically, the above reaction converting chlorite to chlorate is far more likely to occur at the pH of pools than the reaction (3) in the patent where chlorite is converted to chlorine dioxide and requires quite acidic conditions to occur. I'm not saying that there is no chlorine dioxide formed, but only that it is a small fraction of the end products and that the combination of ozone with chlorine mostly forms chlorate which is not an effective algaecide nor disinfectant. The writers of the patent were oh so clever to make the following bold statement:
The combination of ozonation and electrolytic chlorination produces at least one of chlorite, chlorine dioxide, and chlorate that react synergistically to kill bacteria, spores, viruses, algae, and other microorganisms, and to oxidize sulfides, cyanides, and other organic and inorganic substances.
The key phrase in the above are the words "at least one of" where in reality it is chlorate that is primarily formed and all of their talk about the superiority of chlorine dioxide while true completely misses the point that hardly any chlorine dioxide is, in fact, produced!
This is an example of a company using the patent system to generate an essentially invalid (obvious and full of errors) patent possibly for the purposes of touting "patented technology" to drum up more sales.
[EDIT] As already noted above,
this paper (which references
this paper describes how the reaction of ozone with hypochlorite ion produces 77% chloride ion and 23% chlorate ion. It does not appear that chlorine dioxide is produced. [END-EDIT]
Richard
