Bromine for Ozone Pool

[rbroders]

I have an ozone pool which has been running very well for about 5 months now. I have two very large (22gm/hr) units and do not use supplemental sanitizer (yet). The pool is covered almost all the time and has very low phosphate levels (~300ppb). I have not had any algae problems and the water is very clear. The only problem I have had is that the Ph is quite high (7.8-8.0) and I have difficulty lowering it (even with 1.5qts MA/day). The TA is ~80.

My ozone system has a flash mixer, a serpentine contact pipe, an air-water separator and an ozone destruct unit. Despite the large amount of Ozone produced, there is no outgassing in the pool (even small concentrations of gaseous O3 are easily detectable by smell). Dissolved O3 in pool temperature water has a half-life of ~10minutes (http://www.ozoneapplications.com/info/ozone_properties.htm), thus ozonated water is able to spread throughout the pool and kill any bugs that get in. I have measured a positive DO3 residual in the pool, though there is none in the infinity edge overflow tank. As an added measure, the pool cleaner is fed from the ozone loop and it travels throughout the pool.

The system runs 11hrs/day (in two 5.5hr shifts) @45gpm (I installed a flowmeter) on a separate loop from the heater (hot water reduces both O3 solubility and half-life) turning over the pool water once per day.

Is my pool safe? I have tested for bacteria a few times and haven't found any. People take baths together in water with no "chlorine" residual for hours at a time. Hmmm. My solution is most definitely not economical, the units were very expensive, and they consume a fair amount of electricity. It is worth it to me though, because I really, really, really hate chlorine. Sorry guys, I know you love miss Cl, but she's just not for me. When I get out of the pool, I just towel off and get dressed. No post swim shower required.

Well unfortunately my experiment is coming to an end. One of my Ozone units had a pump seal failure, and they were installed such that both have to be shut down to stop the leak. The new pump won't be ready for three weeks, so I need to have a sanitizer until it is repaired (and replumbed so I don't have this problem again).

My current plan is to add Sodium Bromide and MPS, but I'm not entirely sure how to do it. From what I have read normally, you add lots of NaBr (30ppm?), and then control the amount of MPS to adjust the Bromine levels. However, once my Ozone system comes back on line, the O3 will activate all of the NaBr and I'll have too much Bromine.

Also, is it true, that If I add NaBr, I'll have Bromine in my pool forever? Maybe I just bite the bullet and use Chlorine because I know the O3 will oxidize it out of existance once repairs are complete.

Thanks in advance -- Bob

 

[JasonLion]

Ozone alone is not considered safe. If you can't smell any ozone then there isn't enough ozone to keep the bulk pool water sanitized. Water coming out of the pump will be sanitized if the ozone system works correctly, but the residual amounts required to keep the bulk pool water sanitized are very high, because of the rapid break down rate. You would certainly smell ozone, and have respiratory problems, if you were at those levels. The whole point of the serpentine contact pipe is to maintain contact in the plumbing for long enough that the ozone levels have dropped before the water reaches the pool.

Bromine and ozone tend to combine to form bromate, which is a suspected human carcinogen. Bromine is also broken down by sunlight fairly quickly, which isn't a big deal if you keep the pool covered, but can become somewhat expensive if you leave the pool exposed to sunlight. Bromine/bromide is very difficult to remove from the pool, not literally forever but close enough.

 

[chem geek]

As for your higher pH, the aeration from the air injected by the ozone generator may be increasing carbon dioxide outgassing in which case lowering the TA level may help (try 60 ppm to see if the pool is able to get to a lower pH). However, normally if a pool remains covered the pH is more stable. If the pool is relatively new or replastered in the last few months or even year, then the pH rise could be from plaster curing.

"Safe" is a relative term. Having low phosphate levels (which helps slow down algae growth) along with some amount of residual ozone is safer than having nothing at all in the pool, but not as safe as having an EPA-registered disinfectant such as chlorine. Note that if you are measuring dissolved ozone levels, you need to do so away from the returns at the far point of worst circulation. As for bacteria, that needs to be measured during bather load as the point of a bulk sanitizer is to kill pathogens quickly as they are being shed from the body. It's possible that you have enough ozone in the pool to prevent uncontrolled bacteria growth or it may be that your test kit is invalid (you can check it against a saliva cheek swab or against fecal matter to see if it is able to register either).

 

[rbroders]

Jason, its important to understand that there are two types of Ozone at work. Gaseous ozone and Dissolved ozone. Both are excellent sanitizers. Gaseous ozone is toxic, but dissolved is not. The air-water separator and ozone destruct unit remove the gaseous ozone from the water before it is returned to the pool. The serpentine contact pipe is designed to allow the gaseous ozone time to sanitize and dissolve properly (ozone does not dissolve instantly in water, of course). The contact pipe is not so long that the dissolved O3 decays into O2 according to its half-life. Thus allowing the sanitizer to build up a small residual in the pool. An important consideration is to not have the ozonator in front of the heater. Water temperature increases will reduce the ozone solubility and lead to ozone outgassing. BTW, the solubility of ozone in spa temperature water is basically zero so all the sanitization comes from contact with gaseous ozone.

It is not my intent to change your well-formed opinion on alternative sanitizers, or your faith in "EPA-registered disenfectants". Though it is my understanding that Germany allows Ozone only pool sanitization as long as the ozone is injected in sufficient quantities (can't find the reference now though).

I'm really hoping for advice and dosing suggestions for my current "failed ozone system" problems. Yes, Bromine is a suspected carcinogen and is banned in food products. It is likely safe in pool/spa water though. I like Bromine because its not chlorine (I dislike Cl), and also because it works harmoniously with ozone and will supplement my system nicely when it comes back on-line. However, it will be around forever (?) which worries me. If I use Chlorine for my temporary problem, it will get oxidized away once my ozone system comes back and I'll have a nice pristine pool again. Which do you prefer, and if NaBr, how much?

Thanks -- Bob

 

[JasonLion]

Yes, Bromine is a suspected carcinogen and is banned in food products. It is likely safe in pool/spa water though.

Bromine is considered safe in pool water. But bromate, which pretty much only forms when you have both bromine and ozone, is not considered safe in pool water.

Even ignoring the bromate issue, I would not call bromine and ozone harmonious. Once you add bromine you will have a bromine pool, even when the ozone system starts working again. Any ozone that is added will interact with the bromide and turn it back into bromine, breaking down the ozone in the process. This reaction proceeds rather quickly, keeping the ozone levels extremely low. Your only sanitizer will be bromine, even after the ozone system is repaired, unless you replace 100% of your water or wait ten to twenty years.

If you are happy with your pool as it was when the ozone was working, you should use chlorine while the ozone system is off line. When the ozone comes back on the chlorine will start breaking down, and you will have an ozone only pool again in just a couple of hours.

 

[rbroders]

Sorry, I meant to say Bromate. I refer you to this thread http://www.troublefreepool.com/will-bromine-used-with-ozone-create-carcinogens-t24905.html regarding its carcinogenic properties in a pool/spa. Chem Geek seemed convinced of its safety. Still, you don't want to mess around with potential cancer if you can avoid it.

Yes, the ozone will activate the Bromide and turn it into Bromine. The Bromine runs around to the little cracks and crannies that the DO3 can't reach, kills the bugs and reverts to Bromide. Then the ozone re-activates the Bromine and the cycle continues harmoniously - no? You suggest the Bromide will absorb all of the ozone and I will essentially have an "ozone activated" Bromine pool. This seems contingent on the ratio of Bromide to Ozone in the pool doesn't it? Once all the Bromide is Bromine the Ozone will be free to be Ozone won't it? My plan was to add only a little NaBr to my pool with its HUGE Ozone system such that I would have an Ozone pool with a small Bromine residual. Is this not possible? I don't want a huge "Bromine Bank" a small portion of which is activated/controlled by MPS or other oxidizer. I want just enough Bromine to create a small residual, but I'm beginning to think it can't be done.

Perhaps nobody does it because while you can add more NaBr, it is impossible to remove NaBr if you accidently add too much. Strangely, my pool builder added Bromine (chlorine activated, I think) when the pool was first starting (before my Ozone units were operational). Today (5mos later), I cannot detect any Chlorine or Bromine with my standard test kit. If there was Bromine, where did it go? Maybe I'll add just a little NaBr to the pool. 10oz in 30,000gal and MPS shock to see if I get a measurable residual. Trial and error should work, but makes me a little nervous. Maybe I'll just use Chlorine. Hmm..

--Bob

 

[chem geek]

Jason, its important to understand that there are two types of Ozone at work. Gaseous ozone and Dissolved ozone. Both are excellent sanitizers. Gaseous ozone is toxic, but dissolved is not. The air-water separator and ozone destruct unit remove the gaseous ozone from the water before it is returned to the pool. The serpentine contact pipe is designed to allow the gaseous ozone time to sanitize and dissolve properly (ozone does not dissolve instantly in water, of course). The contact pipe is not so long that the dissolved O3 decays into O2 according to its half-life. Thus allowing the sanitizer to build up a small residual in the pool. An important consideration is to not have the ozonator in front of the heater. Water temperature increases will reduce the ozone solubility and lead to ozone outgassing. BTW, the solubility of ozone in spa temperature water is basically zero so all the sanitization comes from contact with gaseous ozone.

You are forgetting that there is an equilibrium between dissolved ozone in the water and gaseous ozone in the air. It is incorrect to assume that once a gas dissolves in water that it will not outgas. After all, carbon dioxide is dissolved in pool water yet it outgasses into the air because it is higher than its equilibrium concentration. I think you are mixing up gaseous vs. dissolved (aka aqueous) based on the rates of outgassing. If you've got a gas not yet dissolved in the pool, then the bubbles can get to the surface and outgas faster, but this does not mean that dissolved/aqueous ozone will stay in the water -- it won't.

So outgassing of ozone is very real and is precisely why even dissolved/aqueous ozone is not allowed in bulk pool water except at very low concentrations. Honest ozone manufacturers will tell you that they intentionally design their systems to have very little residual ozone entering the pool water. As described here:

“It is possible to generate too much ozone for a given application, although due to economic reasons, it is more likely that ozone is misapplied. Generally speaking, if the ozone system allows residual ozone to enter the main body of water, be it a pool or a spa, exposure of the bathers to ozone gas is possible, “ he explained.

Following this line of reasoning, if ozone gas exposure is possible, it is also possible for a person to be exposed to a level that exceeds regulatory limits (OSHA 0.1ppmv TWA 8-hours; 0.3ppmv STEL).

Therefore, engineering controls are used to prevent ozone from entering the occupied space. First, ozone is applied in a manner which will cause it to be consumed prior to its being able to enter the main water body. In the case of a spa, this means injecting the ozone in the circulating filter line and allowing it to do its job before the water re-enters the spa. “The relatively low ozone levels applied, the high water temperatures, and the ozone demand all contribute to giving ozone a very short life,” Mouw noted.

Secondly, devices are available to destroy excess ozone, in either the dissolved phase or gas phase. In the case of swimming pools, in which relatively high ozone levels are applied to the filter flow and water temperatures are much lower, ozone is consumed during the time it is detained in a contact tank following the ozone injection point, with any residual ozone destroyed by passing through a GAC filter or UV unit prior to reentering the pool.

You are also assuming that it is only gaseous ozone that sanitizes in water, but by definition that is not true. If the ozone is not dissolved in water, it will not sanitize that which is in water. To be a gas not dissolved in water means, by definition, that the gas is in a gaseous state such as in a bubble and there is therefore no contact with pathogens since there aren't any in the bubble. It is dissolved ozone that is a molecule in the water itself that comes into contact with pathogens to kill them. The killing power of ozone in the contact pipe comes from the ozone that is getting dissolved in the water, not from that which is still in a separate gas bubble. The purpose of the contact pipe is to dissolve the ozone, not to get rid of it.

Is my pool safe? I have tested for bacteria a few times and haven't found any. People take baths together in water with no "chlorine" residual for hours at a time. Hmmm. My solution is most definitely not economical, the units were very expensive, and they consume a fair amount of electricity. It is worth it to me though, because I really, really, really hate chlorine. Sorry guys, I know you love miss Cl, but she's just not for me. When I get out of the pool, I just towel off and get dressed. No post swim shower required.

Bath water is completely replaced after each use and it's usually chlorinated (with either chlorine or monochloramine) if it comes from a municipal water supply. A small bath is not a commercial/public pool or spa where one person can infect dozens if not hundreds and where the water isn't changed every hour or two. As described in this post, Stacy contacted Legionairre's Disease from a hotel hot tub that was improperly sanitized. In this post, Martin got hot tub lung using Aqua Finesse. In this post one person nearly died from only sanitizing their spa with ozone (to be fair, spa ozone systems are notoriously weak).

If you want to minimize chlorine exposure, then there are ways of doing that by using supplemental methods to prevent algae growth and have supplemental oxidation (the latter you have by using ozone). You could minimize your FC/CYA ratio to something that provides only sanitation so would minimize any chlorine side-effects such as smell on skin.

Though it is my understanding that Germany allows Ozone only pool sanitization as long as the ozone is injected in sufficient quantities (can't find the reference now though).

The German DIN 19643 standard allows ozone to be used, but NOT by itself. If no ozone is used, then chlorine levels must be between 0.3 and 0.6 ppm FC (with no CYA) while if ozone is used, then chlorine levels must be between 0.2 and 0.5 ppm. In fact, when ozone is used, great pains are taken to strip all ozone from the water before reintroduction of the water into the pool and even chlorine gets removed (in addition to the ozone) via activated carbon such that additional chlorine must be added downstream.

Sorry, I meant to say Bromate. I refer you to this thread http://www.troublefreepool.com/will-bromine-used-with-ozone-create-carcinogens-t24905.html regarding its carcinogenic properties in a pool/spa. Chem Geek seemed convinced of its safety. Still, you don't want to mess around with potential cancer if you can avoid it.

I was only commenting that the bromates, which are a problem when ingested, are not volatile. You should know that the brominated disinfection by-products tend to be much more carcinogenic than the chlorinated ones at the same concentration. In a bromine pool, you will get more brominated trihalomethanes (1 in 1 million cancer levels for bromoform at 4.0 µg/L, dibromochloromethane at 0.4 µg/L, bromodichloromethane at 0.6 µg/L) in addition to chloroform, while in a chlorine pool you mostly just get chloroform (with 1 in 1 million cancer level of 0.04 µg/m³ in air and lowest effect limits with margin of safety for ingestion at 0.01 mg/kg/day which for 2 liters of water per day and 50 kg is 0.25 mg/L (250 µg/L) or about 60 times less toxic than bromoform). While the cancer risk is low, it is nevertheless higher with bromine pools than with chlorine pools and this is consistent with scientific papers (some of which I write about here).

 

[rbroders]

Okay, fair enough, if there is a higher concentration of ozone dissolved in the water than there is in the surrounding air, it will outgas. That is how we got it to dissolve in the first place, right? Putting bubbles with a high concentration of ozone in the water. However, I believe surface outgassing occurs slowly due to the small air/water contact area (compared to the bubbles used in the ozone injector/mixer). My pool seems to indicate this, as I have measured DO3 near the surface, but never smelled O3. Fortunately we can smell O3 at levels far below toxicity limits. I have also never smelled O3 under my cover (which is not sealed, but does limit airflow quite a bit). Obviously this could be a bigger problem for spas which have a tight sealing cover.

You are also assuming that it is only gaseous ozone that sanitizes in water, but by definition that is not true

Actually I said that both gaseous and dissolved O3 are excellent sanitizers. Perhaps I was wrong about gaseous O3, but I thought bacteria and other organics that came in contact with an ozone bubble would be destroyed. Otherwise how do hot tub ozonators work at all? The typical UV system produces very low ozone concentrations which have a solubility in hot water of <1ppm.

Anyway, I guess I'll throw away the NaBr and get some chlorine for the duration. I'm still curious though, it is possible to add a little NaBr and control Bromine levels with NaBr addition instead of putting in a lot of NaBr and controlling Bromine levels with MPS shock?

--Bob

 

[JasonLion]

When you add NaBr you are essentially adding bromide, which does not sanitize by it's self. You need something to convert it into bromine. Chlorine, ozone, and MPS will all convert bromide into bromine. Of those three, chlorine is the most common choice. MPS costs more and ozone has the issues noted above. NaBr is also available mixed with chlorine, which both raises the bromide level and converts some (or all) of the bromide into bromine. Some spa treatment systems use this mixture both to add bromide and to convert it into bromine, but that system is inefficient. NaBr costs more than chlorine, so you really only want to add as much as you need to add, and not keep adding more just to get the chlorine it is packaged with.

 

[chem geek]

I believe surface outgassing occurs slowly due to the small air/water contact area (compared to the bubbles used in the ozone injector/mixer). My pool seems to indicate this, as I have measured DO3 near the surface, but never smelled O3. Fortunately we can smell O3 at levels far below toxicity limits. I have also never smelled O3 under my cover (which is not sealed, but does limit airflow quite a bit).

You are right that the outgassing of ozone is slower in the bulk water due to lower surface area to volume ratio. The ozone odor threshold varies a lot anywhere from 0.0005 to 0.5 ppm (remember these are gas concentrations, not water) as denoted here and here. Remember the OSHA limit is 0.1 ppmV over 8 hours or short-term exposure of 0.3 ppmV. Basically, the odor threshold for some people is near the OSHA limits, though still below immediate danger to life and health which is 5 ppm. I wouldn't count on your nose as a guide for safety from ozone, at least not for long-term exposure.

Actually I said that both gaseous and dissolved O3 are excellent sanitizers. Perhaps I was wrong about gaseous O3, but I thought bacteria and other organics that came in contact with an ozone bubble would be destroyed. Otherwise how do hot tub ozonators work at all? The typical UV system produces very low ozone concentrations which have a solubility in hot water of <1ppm.

There isn't very much absolute quantity of ozone produced in spas, but enough given relatively small spa sizes to be able to oxidize some bather waste as well as reacting with chlorine. In a typical residential spa, the chlorine usage for bather load is cut in half with most ozonators, but the chlorine demand with no bather load is roughly doubled. The ozone is completely dissolved and you generally don't smell it unless it builds up under the cover and you can barely measure it. This chart shows that at 40ºC (104ºF) ozone solubility is low, but depends on the gas concentration (so when it gets in the bulk spa water, it will outgas, though dependent on aeration rate, surface area, etc.). In practice, ozonators are automatically turned off when one is using the spa (for some period of time after any controls are touched) and are turned on automatically after spa use which is also when the water is usually cooler than the maximum as well. If this were not done, then the aeration jets would drive much of the generated ozone from the water into the air when the spa was in use which would not be good.

 

[rbroders]

Well, my nose correlates well with my safety monitor: EcoSensors C-30ZX, which is installed in my pool equipment room. When the ozone destruct units are not functioning, lots of O3 can build up near the generator.

I still find it difficult to believe that UV based spa ozonators cause any O3 to dissolve. I used the exact same solubility graph. Noting that according to this, the highest concentration from a 185-nm UV lamp is only 0.2% by weight. At this point we are so far off the graph extrapolations are unreliable, but suggest a solubility of only 0.2ppm. I suppose that is just barely sufficient to sanitize the water directly in contact with the ozonator. Once the water begins to mix with non-ozonated water the concentration will drop below the useful threshold.

I suppose the fact that it is changing the chlorine demand is proof that something is happening.

BTW I finally found a NaBr dosing guideline for Ozone pools of 20g per 1000L every 6mos. here. Not sure I trust it, but at least it is something.

Bit the bullet and added Bleach to my non-ozonated pool. Still no sign of algae except for a little bit of yellow/black slime on my pool cleaner hose (wiped it off with paper towels).

--Bob

 

[chem geek]

I suppose the fact that it is changing the chlorine demand is proof that something is happening.

Yup, though I can't say that these systems were UV -- most were likely to have been corona discharge (CD) units. At any rate, there doesn't have to be very much ozone generated and dissolved to be effective. It basically gets generated, injected, dissolved, and consumed all very quickly whether that be against bather waste or against chlorine. Remember that the drop is over 24 hours and the ozonator is on most of the time as is the circulation pump. The chlorine loss with no bather load is around 1-2 ppm out of 2-4 ppm FC. The "half" of bather load usually works out to up to 3-4 ppm FC equivalent (for one person-hour which is the high end of what I've seen resulting in half chlorine savings). Over 24 hours, that's only 0.17 ppm FC per hour equivalent at the most. In 350 gallons, that's around 60 mg chlorine per hour or around 40 mg ozone per hour (assuming ozone units use the molecular weight of ozone which is 47.998 g/mole). CD Ozone units for spas are typically at 35 mg/hour for spas up to 1000 gallons (see here) while other units are 250 mg/h or higher so this is fairly consistent with what I've been seeing. There are a few spas that have ozonators that are too powerful so cannot hold chlorine well while some others seem to have stopped working after a time. It's pretty easy to tell based on the chlorine demand with and without bather load compared to what is expected.

 

[rbroders]

These ambient air CD units also produce lots of Nitric Acid (highly corrosive), Nitric Oxide (poison gas) other Nitrous byproducts, and even Nitrates (Algae Food). Maybe the Nitrate production explains the increased chlorine demand with no bather load.

Ambient air CD units also suffer reduced performance when humidity increases (above -60 dewpoint). Seems to me a body of hot water increases the local humidity significantly.

Oxygen concentrators don't scale down well, so hot tub sized ozone generators seem doomed to me. Get an ambient CD unit with nitrogen and humidity problems, or get a UV unit with nearly useless output concentration...

--Bob

 

[chem geek]

These ambient air CD units also produce lots of Nitric Acid (highly corrosive), Nitric Oxide (poison gas) other Nitrous byproducts, and even Nitrates (Algae Food). Maybe the Nitrate production explains the increased chlorine demand with no bather load.

Nitrates are fully oxidized so won't consume chlorine (nitrite, on the other hand, will). As for ozone reacting with chlorine, this is well known and definitively described in this paper. There are two reactions of ozone with chlorine as follows:

O₃ + OCl^- ---> [O₂ + Cl-O-O^-] ---> 2O₂ + Cl^- ..... (77%)
2O₃ + OCl^- ---> 2O₂ + ClO₃^- ..... (23%)

The first reaction has the oxygen in the chlorine get reduced so in effect the ozone oxidizes the oxygen in hypochlorite so this is thermodynamically favorable. In the second reaction, the ozone oxidizes the chlorine in hypochorite to produce chlorate ion.

In a spa with a low hypochorous acid concentration due to CYA in the water, the half-life of ozone is roughly in the 20-60 minute range. The half-life with 1 ppm monochloramine is also around an hour. This latter reaction is as follows:

NH₂Cl + 3O₃ ---> 2H⁺ + NO₃^- + Cl^- + 3O₂

Because the generation rate of ozone is so low and the half-life is close to an hour, the steady-state ozone level surviving in the spa (remember that no one is in the spa during this generation) is roughly in the 0.1 ppm range IF there were no outgassing at all. So spas often do have some ozone in the bulk water, but when you touch the controls to turn up the spa heat and possibly turn on jets to aerate, the ozone generation stops, more ozone is outgassed, and the rest gets used up with chlorine. So by the time you get in to soak, the ozone levels are far lower. At least that's the theory of how these residential spas are supposed to work.

I don't think the CD ozone systems produce that much nitric acid since the pH doesn't seem to drop or rise any more slowly in the ozone hot tubs. In fact, if anything their pH tends to rise somewhat more quickly due to the increased aeration from the bubbles, though this is largely controlled by keeping the TA lower. I don't know if these systems for spas have a desiccant air dryer as reducing moisture helps reduce formation of nitric oxide (as does oxygen concentration, but I doubt these relatively inexpensive systems have oxygen concentrators).

 

[rbroders]

Regarding Nitrates increasing chlorine consumption, I was assuming the algae would eat the nitrates and then the chlorine would be consumed dealing with the ensuing bloom

In a spa with a low hypochorous acid concentration due to CYA in the water, the half-life of ozone is roughly in the 20-60 minute range. The half-life with 1 ppm monochloramine is also around an hour.

Where do you get this? My research suggests the half-life is only a few minutes (5?) at 40C. If the presence of chlorine (hypochlorous acid or monochloramine) *increases* the half-life of DO3 it would make a huge change in my understanding of how ozone hot tubs function.

My math agrees with your 0.1ppm if all of the ozone dissolves (and none outgasses), the unit is producing full rated output (250mg/hr), and the half-life is a full hour. If the half-life is actually only a tenth of an hour and the unit is only making half advertised amount and only a third of that actually dissolves (typical rates with good injector mixer) you end up with a useless .0017ppm.

--Bob

 

[chem geek]

The ozone oxidation of chlorine and of monochloramine produces nitrates and the chlorine oxidation of ammonia produces nitrates. With a reasonable FC/CYA ratio in the water at nearly all times, algae won't grow -- it's also unusual in a spa due to the temperature extremes and general lack of sunlight if it's a covered spa.

The source you have for half-life has the following footnote: "These values are based on thermal decomposition only. No wall effects, humidity, organic loading, or other catalytic effects are considered". The link I gave has a rate formula and graph for the reaction of ozone and chlorine (the paper is not free and must be purchased). However, the paper's data was at 20ºC so you are right that the rate at 40ºF will be faster. They didn't give an activation energy so I don't know exactly how much faster, but if one uses the rough rule-of-thumb of a doubling in rate for every 10ºC then that's 4 times faster so 5-15 minutes. Since the thermal decomposition seems to be on the order of 5 minutes, the net half-life between the two is probably just 2-3 minutes or so.

Since the generation rate is so slow, that would mean the steady-state in the spa would be more like 0.17/(60/2.5) = 0.007 ppm which is very low. You call this "useless", but the point of ozone in these spas isn't for bulk water disinfection, but for supplemental off-line oxidation. A bulk water sanitizer is still expected to be used. Nevertheless, modern spas turn off the ozonator when controls are used (i.e. when the spa is seen to be in use) as a safety measure which tells you how they try to avoid ozone outgassing at all costs.

 

[rbroders]

Well, you only get 7ppb residual under ideal conditions (100% mass transfer, perfect ozone production, etc.). Realistically I think only a few parts per billion. Hard to imagine that little even causes "supplemental off-line oxidation", but perhaps smaller localized concentrations have useful effects.

It is rather interesting that the ozone manufacturers are so paranoid about gaseous ozone from such an impotent unit. Lawyers rule, I guess. Supposedly there has never been a workmans comp claim due to ozone exposure. Anecdotally, I read a study regarding ozone exposure to mice. When they were exposed to copious quantities of ozone from an ambient air CD machine, they lost all their hair and got rashes on their skin. When exposed to the same dose of ozone from an oxygen CD machine, mice showed no ill effects. In fact when the sick mice were moved to the oxygen ozone machine, their skin cleared up and their hair grew back.

--Bob

 

[chem geek]

Well, you only get 7ppb residual under ideal conditions (100% mass transfer, perfect ozone production, etc.). Realistically I think only a few parts per billion. Hard to imagine that little even causes "supplemental off-line oxidation", but perhaps smaller localized concentrations have useful effects.

I didn't make my point clear. By "supplemental off-line oxidation" I mean at the ozonator. I completely agree with you that the amount of ozone residual left in the spa is negligible. So yes, it's "useless" in the main body of the spa which is why a bulk sanitizer (e.g. chlorine) is needed. I just meant that even these weak ozonators are not useless for spas since they do oxidize bather waste and cut down chlorine demand in regularly used spas.

That's interesting about the mice, implying that it is the by-products that were a problem. Nevertheless, the OSHA limits came from ozone exposure in more controlled conditions.

 

[Pb2Au]

Bromide ions (Br-) in water can be converted into HOBr (hypobromous acid – the killing form of bromine) by adding an oxidizer to the water. The most common oxidizer for this purpose in the pool and spa industry is potassium peroxymonosulfate (Oxone®, monopersulfate or MPS) but any form of chlorine or ozone or an electrolytic device like a chlorine generator will also make HOBr from bromide ions. The HOBr can then undergo sunlight-induced photolysis and decompose into bromate (BrO3- ) or bromide ions. Bromates have been considered carcinogenic (cancer-causing) since 1991 by the U.S EPA and are only allowed in drinking water up to 10 ppb (parts per billion). Once formed in water, bromates can not be oxidized or removed. There is a chemical that prevents bromate formation but it must always be present in the water.

Formation of Bromate during the Ozonation of Water Containing Bromide Ions
In the bromide-ozone process sodium bromide (NaBr) is added to the water to get the bromide ions ( ) and then the bromide ions are converted into HOBr (hypobromous acid) with the ozone. Here are the reactions as described by Rip G. Rice, Ph.D. in the article “Chemistries of Ozone for Municipal Pool and Spa Water Treatment,” that appeared in Journal of the Swimming Pool and Spa Industry, Volume 1, Number 1, 1995:
Br- + O3 --> O2 + (OBr-)

bromide ion plus ozone yields oxygen plus hypobromite ion

Then the hypobromite ion forms hypobromous acid:
(OBr-) + H2O --> HOBr + (OH-)

hypobromite ion plus water yields hypobromous acid plus hydroxide ion

An equilibrium is set up between hypobromous acid and hypobromite ion based on the pH of the water. The percentage of each is determined by the water’s pH:
HOBr + H2O <--> (OBr-) + (H3O+)

hypobromous acid – hypobromite ion equilibrium reaction

When hypobromous acid disinfects microorganisms or organic or inorganic pollutants, the bromine atom is reduced to bromide ion which then becomes available to be recycled or reoxidized by ozone to hypobromite ion again. The oxidation of hypobromite ion by ozone produces bromide ion 77 percent of the time and bromate ion 23 percent of the time (Haag and Hoigné 1983, 1984).

Or to put it another way, this study reveals that 23 percent of the time bromide ions are being made into cancer-causing bromate ions when bromide and ozone are in the water.

During normal pool and spa water conditions, bromate ion can be formed at concentrations ranging from 0 to 150 ppm during the oxidation and disinfection of water containing bromide ions with ozone. One theory suggests that dissolved ozone in water reacts with bromide ions to produce hypobromite ion (OBr-), which is further oxidized to bromate ion.

Formation of Bromate during the Process of Electrolytically Generating Bromine

Bromine can be generated electrolytically by having bromide ions (Br-) in the water and then placing an electrode or a series of plates in the flow of water. A small amount of DC electricity is then applied to the electrodes or plates and elemental bromine (Br) is produced at the anode and some sodium hydroxide and hydrogen gas is produced at the cathode. The elemental bromine produced quickly dissolves in the water and forms hypobromous acid (HOBr) – the killing form of bromine.

The equation showing this conversion is as follows:

NaBr + H2O --> Br2 + NaOH + H2

Salt + Water + electricity yields bromine + sodium hydroxide + hydrogen gas

The bromine then forms hypobromous acid (HOBr) according to this equation:

2 + H2O --> HOBr + HBr

Then the HOBr dissociates into hypobromite ion ( ) according to this equilibrium equation:

HOBr ↔ + H+

The dissociation of HOBr is dependent on the pH of the water. The lower the pH the more HOBr and the higher the pH, the more . This is an equilibrium reaction and the percentage will always total 100. At a pH of 7.0, 98.0 percent HOBr is available while 2.0 percent is ; at a pH of 7.5, there is 94.0 percent HOBr and 6.0 percent ; and at a pH of 8.0 there is 83.0 percent HOBr and 17.0 percent .

During the process of electrolytically producing chlorine or bromine other oxidants are produced. In the case of chlorine production from electrolysis, other oxidants such as ozone, nascent oxygen, hydrogen peroxide, chlorine dioxide, free radicals and some super oxides may also be produced. These have been well documented in the literature. Some companies manufacturing this type of equipment enhance the production of these other oxidants and then claim that they are making more and stronger oxidants than just chlorine.

Similar oxidants are produced from electrolytically producing bromine. There is no doubt that some ozone, hydrogen peroxide, nascent oxygen, free radicals and super oxides are produced.

Most pool and spa waters contain some level of bromide. Both ozone and hydroxyl radicals, a decomposition product of ozone, contribute to the formation of bromate. As shown in the above reactions, a direct reaction with ozone is responsible for the oxidation of bromide (Br-) to hypobromite (OBr-), and the oxidation of bromite (BrO2-) to bromate (BrO3-). The intermediate steps may be carried out through indirect reactions with hydroxyl radicals (OH•) and other secondary oxidants.

Formation of Bromate from a Disproportionation Reaction

There is another mechanism for bromate formation that we have not discussed. This is what is called a Disproportionation Reaction.

In aqueous solution, at a pH greater than 8.8, bromates are formed according to the following equation:

2 HOBr + OBr- --> BrO3- + 2HBr

This would of course happen at or near the electrolytic cell as sodium hydroxide (NaOH) is produced which will locally raise the pH to higher than 8.8 for a short time until it is mixed with surrounding water. But it would be enough time to produce some bromate.

A similar disproportionation reaction takes place with elemental bromine (Br2) and sodium hydroxide (NaOH).
The sodium bromate then dissociates into sodium and bromate.

Both of these mechanisms may be happening in, at or near the bromine generator cells.

Sunlight-Induced Bromate Formation

In addition to the bromates formed by bromide-ozone process and by electrolytic generation, bromates can also be formed when hypobromous acid (HOBr) and hypobromite ion (OBr-) undergo UV (Ultraviolet) a degradation or photolysis reaction that forms bromates (D.C. Macalady, J.H. Carpenter and C.A. Moore Science, 25 March 1977, Volume 195, pp. 1335-1337).

Sunlight causes up to 50 percent conversion to bromate which is persistent in recreational water.

The implications of this are that any hypobromous acid or hypobromite ions produced by any means will create bromates.

Any bromide ions converted into HOBr or by any means will produce bromates.
Formation of Bromate during addition of Algaecides, Clarifiers or Other Specialty Chemicals Containing Bromide Ions

There are some specialty chemicals in the pool and spa industry that have as their active ingredient sodium bromide (NaBr). The most common specialty products containing sodium bromide are algaecides. They work by using chlorine already in the water or added in a mega-dose to convert the bromide ions from the sodium bromide into hypobromous acid (HOBr – the killing form of bromine). The HOBr then kills the algae.

The clarifiers and problem preventer chemicals containing sodium bromide all produce HOBr as well but the HOBr kills bacteria or algae which “clarifies” the water.

Once hypobromous acid (HOBr) and the subsequent (OBr-) are formed, they are subject to the sunlight-induced bromate formation mentioned above.


Bromates are Cancer Causing

Bromate has been classified by the International Agency for Research on Cancer (IARC) as having sufficient evidence of carcinogenicity in laboratory animals. In 1993, the World Health Organization (WHO) set a provisional value of 25 µg/L in their drinking water guidelines. The USEPA has proposed a maximum contaminant level (MCL) of 10 µg/L for bromate.
The FDA recently identified the compound bromate as a potential carcinogen when present at levels frequently found in ozonated spring waters (almost all spring waters are ozonated prior to bottling). The limit established by the FDA is very low (< 10ppb) and went into effect in 2002.
A Solution to the Problem?
In March of 1999 a patent was granted for methods of generating residual disinfectants during the ozonation of water. It is U.S. Patent 5,888,428, Howarth, et. al.
The background and summary of the invention section of the patent describes “adding a small amount of a hypobromite ion scavenger possessing imide or amide functionality that is capable of intercepting the hypobromite ion and forming biocidal derivatives thereof that are stable to ozone attack.” The hypobromite ion scavenger is dimethyl hydantoin or DMH. It is part of the compound that makes up BCDMH (bromine tablets or 1-bromo-3-chloro-5,5-dimethylhydantoin).
According to the patent, 10 ppm residual of DMH will prevent the formation of bromates from hypobromous acid and hypobromite ions. Waters using bromine tablets and ozone are protected against bromate formation because the tablets have DMH in them.
This may be the reason that bromates have been found in most recreational waters but not in waters that have been using bromine tabs (BCDMH). The DMH is preventing bromate formation.

DMH Concerns
At present we have not much information on DMH and what levels might be needed. We do not know what the maximum safe level of DMH might be. We do not know how fast DMH will degrade. There currently is not a quick, inexpensive field test for DMH. As long as you have 10 ppm of DMH in the water you will not have bromates forming from the bromide-ozone process or from OBr- decomposition. However, you have no way of knowing what the DMH level is in the water or when it might go below 10 ppm and allow bromates to form. Test kit and test strip manufacturers should soon have some new test methods for determining this.
Bromide Concerns
We currently do not have a method for determining what level of bromide ions might be in the water. Many products containing bromide do not list the bromide as an ingredient on the label. You do not know how much bromide might be in the product and how much it might add to the pool. Even if the bromide-based products did not produce bromates, there is a concern that a pool or spa owner could add a product and switch their pool or spa to a bromine system.
Bromates will become a much larger issue as time goes by and industry and consumer awareness heightens. Progressive companies will alert consumers of the potential risks now or restrict product use so that bromates are not produced while others will adopt a “wait until there is a regulation” attitude.

 

[chem geek]

The implications of this are that any hypobromous acid or hypobromite ions produced by any means will create bromates.

That isn't true because the concentration of hypobromous acid and hypobromite ions in pools and spas is very low and the pH is well below 8.8 so the disproportionation reaction doesn't occur enough in the bulk pool/spa water to create a significant amount of bromate. The other sources from sunlight and from electrolysis and especially from ozone produce the bromate that would be of concern, but creation of bromine by adding an oxidizer to bromide ions or by dissolving bromine tablets shouldn't create significant bromates.

[EDIT] According to this paper (which is also visible as section 5 in this link) about 50% of the bromine is converted to bromate after 2 hours of exposure to sunlight with much of the rest converted to bromide. The conditions used were with seawater (which is around 67 mg/L Br2) and at a pH of 8 with around 4.5 ppm chlorine used in the experiment. Lower pH causes the decomposition and disproportionation reactions to occur more quickly, but without sunlight and at diluted concentrations in pools such reactions are slow. This link used de-ionized and raw water and found similar results with sunlight-induced bromate formation of 6.6% to 32% of bromide ion. [END-EDIT]

As for DMH, it needs to be looked at the same way as CYA and chlorine -- it's an equilibrium reaction so it does not eliminate the formation of bromate, but rather lowers its rate of production.

So using a 2-step bromine method where one creates a bromide bank and adds an oxidizer to create bromine should be OK (as far as bromates are concerned) unless the water is exposed to sunlight. So for most spas this is probably OK. Using a 3-step bromine system that uses bromine tablets will add DMH so over time (as DMH builds up) reduces the amount of bromate formation. Since many spas have ozonators, the use of bromine tabs rather than the 2-step system might make more sense. As for pools which are often exposed to sunlight, the bromine tabs will eventually cause the same sorts of problems of low active bromine levels that we find in chlorine pools with CYA.