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Two years ago I installed a very large ozone system on a 72,000 gallon residential pool. The unit produces 27g/hr ozone in a sidestream configuration off the filter pump, with a 120 gallon contact tank. I also installed ORP/pH control with peristaltic pumps for liquid chlorine & muriatic diluted 4/1. The chemical automation system operates off from a separate pump & filter that feeds a waterfall in the middle of the pool. The chlorine system turns on a couple of hours before the ozone every day, to ensure a light chlorine residual (.02-.05ppm) for algae control. 


When I installed the system, the pool had 150 ppm of CYA, but we decided to keep the water as is and not worry about it, as we weren't relying on chlorine that much anyway.


Very early on, I recieved concerned calls from the service tech that the pool was losing about 40 ppm of alkalinity a week. I figure this is due to aeration, both from the ozone system and the waterfall, and the fact that we were not adding much in the way of additional chemicals to buffer the water. Running the pool at a higher pH has helped with this issue.


But a year later, we measured the CYA, and it was zero. The only method I know of for removing CYA is to drain the pool. Is there a possible explanation in a chemical reaction, or do we need a leak detection service pronto? (No wet spots in the ground or other observable phenomena are immediately apparant).

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There are two different things going on here.  First is the loss of TA over time.  Aeration does not change the TA level -- it only increases the pH.  When carbon dioxide outgasses, it is equivalent to removing carbonic acid from the water and that does not change the TA though raises the pH.  The reason is that the carbonate removal lowers the TA but the hydrogen removal (since carbonic acid is hydrogen and carbonate, H2CO3) raises the TA and these exactly cancel each other out.  The loss of TA is due to acid addition -- the muriatic acid.  You mentioned ORP/pH control so it is the acid addition from the pH control that caused the TA to drop over time.  Every gallon of full-strength Muriatic Acid (31.45% Hydrochloric Acid) in 72,000 gallons lowers the TA by 7 ppm.  In fact, the amount of acid added over time should have dropped as the TA dropped.  Remember that TA (specifically, the carbonate alkalinity) is a SOURCE of rising pH, not just a pH buffer.  You should not try to maintain a higher TA.  You might find much better pH stability at a TA of 60 ppm or even lower, for example, with a pH target of 7.7 instead of 7.5, for example.  If this is a plaster pool, you can raise the CH level to make the saturation index closer to 0.


Now it is true that the aeration has the pH rise more thereby requiring more acid so in that sense you can associate the aeration with the drop in TA, but it is preventable by not trying to maintain as high a TA nor pH target -- the result being a much lower use of acid.  If one wants an additional pH buffer that doesn't cause the pH to rise, one can use 50 ppm Borates (though check local regs to see if that is allowed).  By the way, as you noticed, running at a higher pH target helps, but so would running at a significantly lower TA target.  This table shows how over-carbonated the water is at various TA and pH levels.  With significant aeration, you would need to be well into the green area with over-carbonation level probably not more than 4, though you just have to experiment to see.  The outgassing rate of carbon dioxide varies as the square of the TA and that is not reflected in the table so lowering the TA has a greater effect than indicated in the table.


One more factor to consider is that the waterfall may also increase evaporation so evaporation with refill will add whatever is in the fill water to the pool, including TA and CH.  So that may partially counter the TA drop from the acid addition.  If the CH level rose over time and there is CH in the fill water, then that would be why.


As for the CYA, are you sure the FC was 0.02 to 0.05 or did you mean 0.2 to 0.5 ppm?  With a high CYA level, that low chlorine level will be next to useless for algae control, if that was your intention.  It is also very hard to maintain a low FC level with ORP when CYA is present because the active chlorine level will be extremely low with the ORP mV being in the region where most ORP sensors don't do very well.  So I wouldn't be surprised if there were long periods of time with no chlorine in the pool -- did you do regular manual testing to ensure that the FC was indeed not zero?  If the chlorine level went to zero, then bacteria can convert CYA into ammonia creating a huge chlorine demand.  The chemical automation system might have tried to correct that every now and then oxidizing the resulting ammonia.  Technical details about this are in this post and my personal experience with this is in this post.


If this is an outdoor pool, then you need CYA in the water to protect chlorine loss, but you can't have 0.2 to 0.5 ppm FC with CYA in the water.  If you want to prevent algae growth using chlorine alone, then you need the FC to be around 7.5% of the CYA level and this will have the same active chlorine level as 0.08 ppm FC with no CYA so is very low.  If the customer wants an even lower active chlorine level or just can't stomach the concept of a higher FC level, then you can use supplemental algaecide (e.g. weekly PolyQuat) or a phosphate remover for algae control (at extra cost, of course).

Richard, I understand what is going on with the TA part of the equation. It is essentially what you explain in the post about lowering TA. When we decided to live at a lower TA and adjust pH according to calcium & temoerature, we stopped pumping acid & TA stabilized.


As far as how much chlorine is in the pool at a given time, I really don't know for sure. DPD & OTO test kits do not differentiate between ozone, chlorine or bromine, neither does ORP. This pool delivers 700-880 (depending on temperature of the water) ORP with ozone alone without a bather load.


If the CYA were conveted to ammonia by bacteria, would that explain its disappearance? Since there is an alkalinity component to CYA, is it possible that we destroyed it in the process of lowering the alkalinity? The CYA loss is what concerns me, because if we can't explain its disappearance chemically, then the pool leaks badly enough to completely exchange the water in the the pool in a year.





If there was a pool leak, then you should see a drop in CH or in salt (TDS) levels unless it is made up for by evaporation/refill.  You can always do a leak bucket test where you place a marked bucket of pool water on a step so that the bucket is in the water enough so that the water in the bucket is at the same temperature as the pool water and then see if the drop in water level in the bucket is the same as that in the pool.  If not, then you've got a leak (see Leak Detection).


Absolute ORP measurements are next to meaningless and cannot be used to know if the pool is disinfected.  As described in this post, different ORP sensors from different manufacturers measuring the same water at the same time showed more than 100 mV differences in 23% of the 130 pools.  You really need to use a FAS-DPD test kit in an area of the pool far away from the returns.  Yes, it cannot distinguish between ozone, but if there is ozone in the bulk pool water enough to significantly affect the chlorine test, then you've got a health hazard as ozone will outgas (do you smell any ozone above the pool water?).  Normally, ozonators are either woefully undersized in residential pools or have separate contact/degassing tanks/sections that ensure that little ozone is left going into the bulk pool water.  Where is this ORP sensor located?  I would hope that it is in the circulation path before the ozonator (and chlorine/acid injection point).


Be aware that ozone will break down chlorine to chloride (77%) and chlorate (23%) (see this paper) so an ozonator powerful enough to actually do something will usually result in a higher chlorine demand.  This is most readily seen in residential spas where it typically doubles the chlorine demand if the spa is not used, but cuts it in half when the spa is used every day or two.  This is because ozone can oxidize the bather waste so the chlorine doesn't have to.  This is why ozone makes sense in high bather load situations such as frequently used residential spas or in commercial/public spas and pools, but doesn't make much sense in a residential pool since the bather load is generally low.  The ozonator just ends up increasing the chlorine usage, though ozonators in many residential pools are woefully undersized so they end up not doing much of anything.


Yes, CYA has an alkalinity component so if CYA were to drop then the measured TA would drop as well.  If you were to lose 100 ppm CYA, then at a pH near 7.5 that's a drop of around 31 ppm TA.  However, adding acid to the pool will not break down CYA.  I've tried to see if ozone will break down CYA and can't find anything on that (simizine is broken down, but that's not CYA; this paper describes break down with zinc oxide solutions to promote hydroxyl radical formation, but otherwise describes CYA as recalcitrant).  Experience in residential spas would indicate that ozone doesn't break down CYA or does so very slowly.  There is a drop of around 5 ppm CYA per month, but is explained by chlorine oxidation of CYA which is known to occur and at hotter spa temperatures would roughly have that rate.


Anyway, for bacteria to convert CYA to ammonia, there must not be any chlorine in the water since even small amounts of chlorine will kill bacteria faster than they can grow.  What amount of chlorine was fed to this pool per day?  That might give a clue as to the actual chlorine level in the pool and whether it was close enough to zero for bacteria to be able to grow.  This assumes that ozone didn't make it to the bulk pool water in high enough concentration to kill bacteria.  Also, though the chlorine tests would not distinguish ozone from chlorine, what sort of FC measurements were you getting with the DPD test?  I would doubt that bacteria would grow if you measured FC, even if it was from ozone.


Admittedly I don't know all that much about ozone and cyanuric acid reactions, but I do know that Truoz's Peroxolyte will deplete cyanuric acid. That being said, and being that Peroxolyte is partially an oxygen-based reaction, maybe ozone being an unstable O3 molecule may also end up having a similiar effect on cyanuric. I doubt it, but maybe a possibility. The more probable scenario is water loss, maybe combined with inaccurate testing of the cyanuric at some point. Maybe Richard can clarify this more.


That is quite the range of ORP... (700 - 880 mV with just ozone). This doesn't really make a whole lot of sense considering  that the ORP bypass and controller bypass's are separate. If you think about it, (even on a low use residential pool), having that much ozone get back through the main body of the pool to the sensors to get 880 mV is perplexing to say the least, particularly on older water. Even on shock situations with chlorine, this level is sometimes diffucult to reach... yet you say this level of ORP is without chlorine. Are you sure the chlorine test isn't being bleached out, and that you actually have high levels of chlorine? Next time this happens, maybe try a dilution test on the chlorine to see if this is the case. You could also use Hi-Range chlorine papers.


I operate several pools this way, starting about 8 years ago, and the numbers are pretty consistent from pool to pool. I take great care to address all the issues that Richard spoke of above, including Contact tank, care with off gas, using a destruct unit if needed. You don't smell ozone in these pools until you get way above 700 mv, and that only happens when the water is too cold to swim in. On this particular pool, the water is not old, nor is it high TDS, it just started out with high CYA because the pool guy used a lot of dry chlorine in addition to adding CYA by hand. Some old school pool guys out here love high CYA readings.


I appreciate your input about Truox, I think I might be losing the CYA through oxidation somehow. I haven't had any indicators that there has been bacteria or ammonia in the water. There are indicators that the pool may be losing some water, but not a complete exchange in less than a year.


BTW, I have tried to locate a way to buy Truox products in So Cal, wholesale distribution has never heard of it, and I have sent two e-mails that remain unanswered for weeks. I saw an article that you posted about their boilout process & wanted to try it out.

We've seen some strange cases of "disappearing CYA" also, with no explanation!  We have also noticed large swings depending on water temperature.  The standard CYA test seems to do real well in water about 75 degrees, but is not as accurate on colder or warmer water.


Leaks aside, there must be some phenomenon that exists that may not be real common, yet has an effect on this.  Perhaps it just doesn't get tested enough to be chronicled, as no one seems to check CYA as often as they check chlorine, for example!


I'll be interested to see what you learn and why you've lost all the conditioner in this pool!



Wow, older topic but one I like. To prevent me going on forever I will just try to show my basic idea for Ozone on this pool.

Without knowing a ton about this pool, or any really, I like to use about a 6 hour turnover to size. There are many methods and formulas people use so things can always come out differently. This pool then breaks down to about 200gpm circ rate. Great slipstreams will be about 30% down to 20% so roughly 50gpm. I would be looking at about a generator here. For heavy commercial it could be larger and for light residential it could even be smaller. I am a huge proponent of 4 minute contact time as that is what really make Ozone work well without the detrimental effects that seem to pop up.

For a 50gpm system I would be looking to something like a 250 gallon tank because they are not 100% efficient. A 200 gallon tank might see 2-3 minutes if lucky. Large generators and small contact tanks tend to waste Ozone out the offgas and destruct and put too much Ozone back into the pool. Tank will seem big, but anyone who can build a 72K pool in their yard can find the space and money for it I am sure.


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