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You can write any chemical equation you like, but that does mean it actually occurs. The scientific paper peer-reviewed in a well respected journal that I linked to on the reactions of ozone with chlorine is a bit old (1984 with reference from 1983), but more recent papers do not differ in its conclusions. If you look at the EPA Alternate Disinfectants and Oxidants Guidance Manual you will see Chapter 4 on chlorine dioxide. You will see that all methods of creating chlorine dioxide involve sodium chlorite though there are also methods (referred to here) starting with chlorate (also see here where the chlorate with hydrogen peroxide is branded Purate® and mixed with sulfuric acid to produce chlorine dioxide; the EPA document lists the half-reaction of chlorate to chlorine dioxide). You wrote a reaction of hypochlorous acid with ozone to produce chlorite, but there is no evidence of this occurring to any significance. Besides, what is your point? Are you selling some sort of system where you are claiming to generate chlorine dioxide from an ozonator in a chlorinated pool? And if so, wouldn't the chlorine in the pool still cause all the disinfection by-product problems you mentioned? And what does this have to do with yellow/mustard algae that was the topic of this thread?
Richard A. Falk said:
As for yellow/mustard algae, it is about twice as resistant to chlorine compared to typical green algae. It tends to form more like dust or powder, prefers the shady side of the pool and likes to grow in light niches and under removable ladders because it's shady and chlorine circulation is typically poor.
It can be killed by using a higher shock level than the usual level for green algae. The normal green algae shock level is an FC that is around 40% of the CYA level, but for yellow/mustard this needs to be an FC that is around 60% of the CYA level. Though one can prevent its growth or return by maintaining a higher FC that is 15% of the CYA level, this can get costly or impractical so it's best to get rid of it completely and as Jeff noted one needs to put in pools and equipment into the pool during the shock process so that one does not reintroduce the algae later (also, get behind light niches and under removable ladders or wedding-cake steps).
Swimtrine is a copper-based algaecide so one has to be careful using this in plaster pools to prevent staining or having blonde hair turn greenish. The copper will not go away except by dilution. Chlorine alone can kill this algae, but if the CYA level is high it takes a rather high chlorine level so many people use algaecide alternatives including copper, sodium bromide (which turns the pool into a bromine pool), and ammonium chloride (to produce monochloramine). However, these are just stop-gap measures to get around a high CYA level which should really be addressed if one wants to be able to stop algae from growing using chlorine alone at a reasonable FC level.
As with most algae, prevention is far easier then curing an algae bloom.
As shown here and here at the EPA, chlorite itself is a regulated disinfection by-product. Chlorine dioxide is used to treat some drinking water, but it is not an EPA-approved disinfectant though I believe it should be considered. As for the equations you referenced, they do not reflect what we are talking about nor (for some of them) what really goes on. You show equations that imply that ozone and chlorine (hypochorous) combine to produce chlorite which then further reacts with ozone to produce chlorine dioxide. This is not what happens. Ozone and hypochlorous acid do react, but primarily to produce 77% chloride ion and 23% chlorate ion (not chlorite) as described in this scientific paper.
If chlorine dioxide were only present for nanonseconds as you describe, then it would be next to useless for disinfection. It takes around 8 hours of chlorine dioxide at 2 ppm to get a 2-log (99%) reduction in Cryptosporidium parvum for the worst case sources as described in this paper. Though this is vastly better than chlorine, it would still primarily be useful either as a shock treatment "after the fact" or as periodic prevention.
You can produce chlorine dioxide by starting with sodium chlorite and adding chlorine to it at a low active chlorine level such as found in pools with Cyanuric Acid (CYA) in the water. This is the principal of the water disinfection product I linked to in an earlier post where they use Dichlor with sodium chlorite.
Jeff Tarr said:
Chlorine adds THM's and Cancer causing elements to the water. Chlorite/Chlorate do not.
Chemical Formula.pdf
Andy A Preiss said:
Andy,
Unfortunately, chlorine dioxide is not an EPA-approved disinfectant for pools or spas, though I agree with you that it is used for some drinking water disinfection and is available in Katadyn Micropur MP1 Purification Tablets (technically, such tablets contain Dichlor and sodium chlorite which combine in water to produce chlorine dioxide). What do you do to protect the chlorine dioxide from breakdown from the UV in sunlight? That is its primary downside for use in outdoor pools. The other downside is the by-product of its production which can produce chlorate while chlorite is the primary product when chlorine dioxide reacts with natural organic matter. Chlorine dioxide does produce disinfection by-products, but they are probably more like the ones produced by ozone while the typical chlorinated by-products such as THMs, HAAs and chloramines are minimized. Personally, I think manufacturers and the EPA should take a serious look at using it, at a minimum as an overnight "shock" to eliminate Crytpo after a diarrhea incident or weekly as a preventative measure.
Chlorine! 12.5%, and lots of it. CYA levels will dictate how high, and leave the system running until the kill is over. Maintain the high chlorine level until you do not drop more than 1 ppm overnight.
Chlorine adds nothing more than water and a little salt to the pool, so you do not have to worry about all that other stuff that comes from Cal Hypo (calcium) or others that skew the water chemistry. Chlorine has an effective kill rate, and once the kill is done, you can get back to normal levels again without any other adjustments.
People seem to want a quick kill to algae, but it takes a little time. We have cleared very green pools in 2-3 days using just chlorine and letting the pump run. When we are done, the pool does not take much work to re-balance, and the algae is completely gone. Doesn't get much simpler than that!
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