salt water vs. ozone purification

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jdjeff

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Jul 15, 2009
21
Houston, Texas
Topic Split off from this Thread.

First I want to say that this board is very interesting and helpful, and I don't mean to start off my stay here by disagreeing, but.... :lol:

It is true that sunlight can cause the production of ozone, but in such miniscule amounts that it has no real world effect in a pool. It is also true that most ozone generators for residential pools do next to nothing. Particularly here in Houston where the humidity is high, many appear to do little more than produce nitric acid, and you won't see any reduction in the need for chemicals. In fact, by relying on these systems for their "magic" effect on chemical use, you often end up with a worse situation than with no ozone at all.

That being said, ozone systems are being used successfully in residential and commercial pools (as well as in water and waste treatment) around the world as well as in the US as primary sanitizers with all of the good effects advertised, but these are well engineered, well laid out systems that tend to be expensive. My own set up uses an oxygen concentrator, drier, 120 gallon holding tank and an in floor system to push ozonated water through the floor heads 24 hours a day (I use a dedicated variable speed pump at low speed for most of the day and kick it up to a higher speed an hour before people tend to use the pool and at an even higher speed during pool use....and highest when the whole neighborhood is over). Of course, slow speed means even more time in the contact tank so more complete oxidation, etc.... I also maintain a residual amount of cholorine of .2ppm which given the size and output of my ozone system may be unnecessary but this is Houston with all of its nasty attendant pool problems, so for now I'm most comfortable going this way. It also means I have chlorine in the water should the power go out. I believe that because of my ozone system, TDS remains very low, PH is stable and ORP is high and I have incredibly clean, sparkling water. Haven't experienced algae problems and no amount of bather load appears to get my chemistry out of whack. I'm not enough of a chemist to say that ozone does or does not destroy chlorine, but in my system the primary ozone action is in the holding tank, and chlorine is introduced way downstream just before the water enters the pool. I do know that ozone eats up chloramines produced by chlorine's interaction with organics, but the ozone itself also oxidizes the organics very well itself, leaving very little if anything for the chlorine to turn nasty with.

I think it is easy to dismiss ozone because here in the US on the residential side, there has been so much snake oil sold that the indifferent to bad reputation is mostly well deserved. And not everyone will want to invest enough to make it work right. As many of you suggest, with constant attention and a little bit of smarts (and good pool education!) you can get some if not many of the same results at perhaps a lower price (although my chemical costs are waaaay low put up against a very small increase in my electric bill), so I guess it is a matter of preference. Just didn't want to see ozone dismissed entirely as a very good option. Instead, I will save my incredulity for those fish pond/algae pools. Yuck. :mrgreen:

SWG certainly have their advantages in terms of ease of use, but salt in the pool (and chlorine itself) has its drawbacks as well. Careful research is required to figure out what fits your situation!
 
jdjeff said:
Ozone . . . as primary sanitizers with all of the good effects advertised
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{EDIT}No matter how well designed an ozone system you have, ozone will not remain active in the bulk of the pool water, and a pool without sanitizer in the bulk of the water is simply not safe.{/EDIT} Using a chlorine residual of 0.2, or even 0.5, is not sufficient. Chlorine can get used up very quickly in the presence of swimmers or any kind of organic contamination, resulting in substantial pockets of water without sanitizer, which is simply not safe.

Ozone has some real value if used correctly in a spa or public pool, where very high bather loads result in lots of stuff to get oxidized. But ozone is a waste of time in most residential outdoor pools, where chlorine levels that are sufficient to be safe have no problem keeping up with the oxidization load. I suppose that if you routinely have very high bather loads, ten or more swimmers for hour each day, similar to a public pool, there might be some point to ozone in a residential setting, but that is quite unusual. Also, ozone can be problematic in indoor pools/spas, where air quality problems make it a bad choice.
 
Respectfully (and chemically!) it is not correct to state that ozone is not a sanitizer. It both oxidizes and sanitizes and does so more effectively than chlorine in all but one respect....its residual. The half life of ozone in water is relatively short, which is why a contact vessel (or surge pool in a commercial set up) is often used to ensure sufficient contact with the water to do the bulk of its work, but within its effective life, it more quickly and completely oxidizes and sanitizes than chlorine (or bromine, etc...). That being said, the half life of ozone is about 20 ish minutes (give or take depending on temperature, etc...), so it is not correct that it has no residual in the pool. So the real issues are more about the concentration of ozone within the pool water as it circulates, and the amount of supplemental sanitizer needed to ensure constant sanitation. So again, a pool properly designed for ozone as the primary santizer needs to ensure that the ozone concentration within the tank (not the contact vessel, but rather the holding tank) and also in the pool is sufficient for the load (I would also note that ozone in excess of what can be held in solution is off-gassed and destroyed in my system before it ever gets to the pool, so air quality is not an issue). 24 hour a day circulation, frequent turnovers plus a better mixing in of the ozone and chlorine as a result of constantly being fed through the wall and floor returns at high speed seems to do the trick and .2-.5 ppm of chlorine has proven sufficient to keep the water in very good shape (I've had the water commercially tested...I can't go entirely on faith of course). Remember, the contact time for chlorine required to kill microbes/bacteria/viruses is relatively high, and chlorine, while obviously a very effective oxidizer, is also slower than ozone, which is why you need a higher residual amount in the water to ensure complete sanitation and oxidation, and chlorine both binds and transforms (affecting your water chemistry significantly--but in a way that is easily handled as you point out), while ozone goes poof when it has done its work. Again, this ain't for everyone, and slapping a $500 ozonator on a 25,000 gallon pool is probably a complete waste of time and money. It ain't for everyone, but done correctly, it works very well.
 
Sorry, you are correct, ozone is a sanitizer. I got mixed up with UV.

In a typical pool, water remains in the pool, without passing through the pump area for an average time in excess of 24 hours. Even with a oversized, extremely aggressive, circulation system, which will draw dramatically more electricity, it is difficult to get the water to pass through the pump area more often than once every 4 hours on the average. Even ignoring the fact that some portion of the water will actually pass through the pump much less often than that, four hours does not compare well to the 20 minute lifetime of ozone in the bulk pool water.

You can't talk about the kill times of chlorine relative to ozone without considering the concentration of each in the pool water. Higher concentrations have faster kill times. In the pump area, it is completely possible to raise ozone levels high enough to achieve faster kill times than chlorine normally has. But in the bulk pool water, ozone concentrations will be far far lower and ozone kill times will be dramatically longer, far longer than chlorine kill times even using specially designed circulation systems that maximize the ozone levels.

You can not claim that you are maintaining ozone in the bulk pool water without air quality becoming an issue. Using special equipment, it is possible to remove all of the ozone from the water before it enters the pool, eliminating air quality issues. However, if you do that, you remove all of the protection you are claiming to achieve by having ozone in the bulk pool water.
 
No worries! Your edit makes the statement more generally correct, but is only true if the amount of ozone that enters the pool is not sufficient on a continuous basis, combined with the residual effect of a minimal amount of some halide, to provide germ busting action. Consider that in a well designed ozone pool, the load of the chlorine is significantly reduced (ie: it has less to do as it hangs around until something extra passes by that hasn't already been handled by the oxidation and santization affect of the ozone in the tank and in the pool). In my pool, free chlorine remains high (as does ORP), so something is working right (by all appearances). Ozone also causes stuff (including de-activated organics and inorganics such as iron etc...) to flocculate and as a result get filtered out much better, which also increases the sanitation of a pool (remember that the EPA view of sanitation in a pool is a combination of filtration and chemical means to eliminate the varoius baddies). Public water supplies and bottled water producers use ozone because it is a better sanitizer (kills crypto and other chlorine resistant micro-organisms lightening quick...pun intended. SWG and chlorine in general produce all sorts of bad by-products in swimming pools even when they work as they are supposed to. Ozone does not (again assuming proper design and installation...you shouldn't have bubbles of ozone coming out of your returns). The only point I'm trying to make is that done right, both work. My own personal issue is the production of by-products as a consequence of salt systems and chlorine in general that I'm trying to reduce or eliminate entirely, while maintaining the safety of bathers. My system does it, while I'm sure others don't. But that is true for chlorine/SWG pools as well. Each has to be applied consistent with its advantages and limitations. Ozone for residential pools can't simply be dismissed.

To your other point, water going through the venturi would only want to take so much ozone, and it's this excess over the amount solubilized that is off-gassed from the holding tank. Otherwise, it just tends to stay in solution until it comes into contact with something in the water (which is almost constant), when it becomes oxygen again. It is another reason to use a holding tank. It ensures the proper holding time, allows for excess to be vented, and sends the rest that remains in solution and available to sanitize/oxidize into the pool. Many studies have shown that ozone off-gassing at the pool surface is not an issue (frankly, even with the bad systems that put bubbles into the pool). There are much worse things in any case that are produced in a chlorine pool, even one that is well maintained. I take your point on the "bulk of the water issue", but I would invite you to test (I have) the chlorine levels at the top (where chlorinated water is being returned...gently) and bottom of your pool. There can be wide variations throughout. High speed jets in an in-floor system can help avoid dead zones in either a chlorine or ozone pool.

One other point. You are correct on the turnover issue, but 25% of my filter output is sidestreamed by the the ozonater and into the holding tank before heading out to the pool, but 20 mins. vs. 4 hours isn't the right way to look at it. The entire pool is being stripped by the ozone every 4-6 hours in my pool, but water with residual ozone is constantly being introduced into the pool. While it will only remain in the pool for 20 mins, it is constantly being replenished.

Cheers!
 
The biggest problem with ozone is that is it NOT residual, in fact, there are set levels of ozone that are permitted in commercial pools and if they go over the pool is shut down. Ideally, residual ozone should be below .03 -.1 ppm, depending on local regulations .1 ppm is the highest level acceptable to OSHA) while an active level of ozone for disinfection is tenfold that, ususlly .4- 1 ppm (and according to OSHA, 10 parts per million is considered Immediately Dangerous to Life, requiring urgent evacuation of the area.) This is why ozone is supposed to be applied in a reaction vessel such as you have in your pool (but the vast majority of pools and spas do not). Ozone buildup is toxic to the lungs so this is particularly important for indoor pools where ozone can do the most good, yet air handling is often inadequate.
Bottom line, ozone still requires an adequate level of residual sanitizer in the water if you want maintain properly sanitized water. The only way an ozone only system will maintain properly sanitized water is by having a bather load of 0. Each time someone enters the pool they will add sweat, urine, and feces no matter how clean they think they are and with no residual sanitize pathogens can and will grow quickly, much faster than the water can go through the ozone system.

Also, in the original post that this thread was split from
salt-water-vs-ozone-purification-t14616.html
the OP was asking which was better, SWG or ozone AND copper ionizer. In that case it's the first hands down! both ozone AND copper need to be used with a fast acting residual sanitizer such as chlorine to maintain propoerly santized water.
 
If the point is whether a properly designed ozone system can work as a supplemental OFF-LINE (i.e. not residual in the bulk pool water) oxidizer and sanitizer, then the answer is yes. However, this still requires a bulk pool water sanitizer and chlorine is still great for that. As for the lower chlorine level, that is true only in high bather load pools where the chlorine level was intentionally higher to not "run out" due to oxidizing local bather load, so with ozone helping out there this can result in a lower chlorine usage and potentially somewhat lower chlorine levels. However, if the bather load is not high, then the chlorine level is determined not only by oxidation or even sanitation, but by algae prevention (unless some supplemental bulk water algae preventative is used such as algaecide or borates or, with side effects, copper or phosphate remover).

By the way, it is incorrect for you to say that your entire pool is getting exposed to the high ozone level every 4-6 hours. One turnover of water only has around 63% of the water pass through the circulation/filtration system (and that assumes that all of it is getting to the ozone tank, not 25%). It takes 4.6 turnovers of water for 99% of the water to get through the circulation/filtration system. The reason is that some of the water passes through more than once during a single turnover while other water doesn't go through at all. Yes, one turnover means one pool VOLUME of water passes through, but it does NOT mean that 100% of the actual water molecules, bacteria, viruses, algae, organics, etc. passes through in a single turnover.

The purpose of the sanitizer in the bulk pool water (even when an ozonator is used off-line to supplement oxidation of bather waste) is primarily to prevent:

  1. runaway bacterial growth[/*:m:lyk6hwmk]
  2. person-to-person transmission of disease[/*:m:lyk6hwmk]
  3. algae growth[/*:m:lyk6hwmk]

Item #1 requires a very low level of sanitizer (for most pathogens, roughly an FC/CYA ratio of 1%) while #2 requires more depending on what level of risk one wants to have (for 99% kill of most pathogens in 1-2 minutes it's roughly an FC/CYA ratio of 5-10%) and #3 requires far more than #1 (for green algae, roughly an FC/CYA ratio of 5% or more). Note that the ozone doesn't really help with any of these three items if there is bacterial or algae growth on pool surfaces as opposed to free-floating (or attached to particles that can get filtered) or if the people excreting mucous or fecal matter aren't doing so directly into the skimmer.

In a low or even medium bather load residential pool, one may be less concerned with person-to-person transmission unless one has guests using the pool. Yes, you still don't want to drink your own fecal matter, but the risk for one sick person infecting many is far lower. So one may relax requirement #2 which means the chlorine level is mostly determined by #3, algae prevention. So the only way one can really lower the chlorine level even in a residential pool is to use a supplemental algaecide or equivalent. In that case, one just needs #1 which is a rather low chlorine level of perhaps an FC/CYA ratio of 1% or so. In practice, you don't want to run out of chlorine locally so you'd have at least 1 ppm FC at any rate so 1 ppm FC with 100 ppm CYA would be about the lowest one would go. This isn't normal, but if someone had to minimize their chlorine exposure to a bare minimum, then this is what they could do.

So the bottom line with ozone is that in high bather load pools it can reduce total chlorine usage/consumpation, but it really doesn't change the need for having residual FC in the bulk pool water. So ozone is really a supplemental oxidizer with the side benefit of strong sanitation off-line so useful for more chlorine-resistant pathogens (perhaps Crypto, though UV or microfiltration also work). In fact, UV is very similar to ozone in terms of supplemental oxidation and additional off-line sanitation.

Reduction of chloramines is a whole other topic and there are other ways of handling that including a lower production of at least some chloramines (nitrogen trichloride) in the first place by proper moderation of active chlorine level as I describe in this post. Since your primary concern seems to be with disinfection by-products (DBPs) from chlorine, you should really understand that the biggest problems with this in terms of respiratory and ocular problems are with indoor pools likely not using CYA. An outdoor pool exposed to sunlight and with CYA in the water has a far lower rate of DBP production, at least for nitrogen trichloride. It is possible that ozone would help remove other non-volatile precursors that are slower to react with chlorine so would otherwise build up in pool water so would be of benefit to remove, but it's too early to tell if this problem really exists in a typical low-to-medium bather-load residential pool. I can tell you that in most outdoor residential pools (including my own), the Combined Chlorine (CC) level almost always measures <= 0.2 ppm which is the lowest measurable in our FAS-DPD tests.

In your residential pool, you may be solving a problem that doesn't exist.

Richard
 
I think there is also a lot of confusion about ozone and chlorine demand. Ozone will not really lower the needed chorine levels to maintain properly sanitized water but can lower the amount of chlorine needed to maintain that level. However, the reverse is often true and total chlorine consumption goes up after ozone is applied to a pool.This is more a factor of how the ozone is applied than anything else. Often to bring the price down to an acceptable level for residential use a few too many corners are cut, IMHO.
 
I agree that too many corners are cut on residential systems typically, and the systems that really work are very expensive and probably not worth it...although I did it anyway for a number of reasons. You folks are way more expert than I, but my experience (and those of over 20 friends and colleagues in Europe and Australia...both commercial and residential) suggests that you can have a sufficiently high concentration of ozone (measured at the return into the pool and also at the return to the filter) that still does not create ozone at the surface in excess (and generally well below) the limits for ozone in the air that can provide sufficient sanitization within the pool. Again, I should only speak for my experience directly, but with very low (but generally steady) levels of CC in my pool supplementing my ozone system (which was sized and designed for my application), I don't have algae and my water tests have come back very well. I can't argue with the math on turnover, but my system was sized to take that into account. Short of that I just cry "Magic" and also cry "Uncle". Thanks for a very interesting discussion!
 
I just want to stress again a point that waterbear made in passing. Ozone simply can't be acting as a sanitizer in the bulk pool water and have the pool be safe to swim in. If there is enough ozone in the pool to be acting as a sanitizer, then the water is not safe to swim in and the air around the pool is not safe to breathe. This, of course, doesn't happen because the reaction chamber lowers the ozone level before the ozone enters the pool. At that point there is no longer enough ozone in the pool to act effectively as a sanitizer. In a properly designed ozone system, the ozone level in the pool will be exceedingly low, and ineffective as either a sanitizer or oxidizer in the bulk pool water.

The water coming out of the reaction chamber may indeed be totally germ free, sanitary, water (if everything is working correctly). But then it sits in the pool for anywhere from 4 to 96+ hours (depending on circulation rate). If extremely low FC levels are used, below about 1.0 in practice, during the time the water is away from the pump area, it can lose it's residual chlorine, become infected with pathogens, and no longer be safe, regardless of how pure it was when it came out of the pump.

There is a way to work around this, but it involves extreme engineering and extremely high electrical usage. The water could in theory flow through the pump area every 10 or 15 minutes. But achieving that would require probably a dozen large circulation pumps/filters/ozone systems just for a small residential pool, along with special plumbing and circulation design to minimize the amount of water that does not pass through the pumps each turnover. The pools used for the olympics in China claimed to use something along these lines, replacing almost 100% of the water between each event, though they used UV instead of ozone because of air quality issues. At least that is what their press releases appeared to be saying. Of course, building such a pool is not practical unless your national pride is on the line or someone wealthy wants to show off.
 

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Rather than extending this out any further, we will just have to agree to disagree. You folks are obviously very knowledgeable, and most of your points are excellent and well considered. There are only a couple on which we disagree that really drive the debate that we can save for another day.

Cheers! :cheers: :cheers:
 
jdjeff said:
I also maintain a residual amount of cholorine of .2ppm
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If you didn't mistype that and literally meant 0.2 ppm FC as your chlorine residual then I can guarantee you that your pool is not sanitary as it is virtually impossible to maintain this low a chlorine level consistently throughout the pool, especially when there is any bather load. The sweat/urine will consume such low levels of chlorine locally very readily and then you've got zero chlorine for some time. What kind of system do you have that is monitoring this chlorine level at multiple points in the pool and adding more chlorine where it is needed almost immediately?

It would be one thing if you said you were maintaining a low 1-2 ppm FC even with a higher CYA level for a low effective chlorine concentration, but having 0.2 ppm FC is almost like having nothing at all, especially if you have CYA in the water. Do you understand how CYA lowers chlorine effectiveness by orders-of-magnitude? See this post for links to numerous studies showing the effect of CYA on active chlorine (hypochlorous acid) levels and the significant effect on kill times.

jdjeff said:
I don't have algae and my water tests have come back very well.
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A lack of algae growth is not indicative of sanitary water. One can have water low in phosphates or other algae nutrients or just be lucky to not have algae, but still have bacterial growth. For viruses and protozoa, the nutrients are irrelevant anyway since they don't grow, but just get transmitted from person-to-person.

Did your water tests include extensive and numerous samples looking at bacterial plate counts in the pool under conditions of bather load?

Even Germany in Europe with their DIN 19643 standard still uses chlorine, though at lower levels of 0.3 to 0.6 ppm FC, but with no Cyanuric Acid (CYA) in the water and they are continually removing the chlorine (and chloramines via activated carbon filtration) and then reinjecting chlorine. This is mostly done for indoor pools not exposed to sunlight in a way to minimize disinfection by-products, but as far as I can tell, they never considered using CYA in pool instead (but then they couldn't use their chloramine removal method in that case since it would remove all of the chlorine so be more costly to inject high FC levels on each pass).

On another forum, for spas, there were several reports of hot tub users getting hot tub itch caused by Pseudomonas aeruginosa bacteria in spite of having working ozonators. The common factor in these incidents was low or no chlorine. Now you can say that these ozonators were not sufficiently strong enough, but they did keep the spas clean and clear even with no chlorine (like your no algae situation) yet did nothing to prevent bacteria forming biofilms on surfaces of the spa that then break off with water movement getting into sore spots in the skin.

Yes, we agree-to-disagree -- I only just noticed the ".2" number so felt I needed to respond to that.

Richard
 

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