Cya Vs Chlorine

[Pool-newb]

I have a new SWG and I want to understand why we even need any CYA at all. I do understand that without CYA the cell will be running much longer but I'm not certain why.

I also understand that CYA will bind to free chlorine in pool water and protect it from breaking down in sunlight. However, this binding will also prevent or at least reduce sanitizing action performed by the chlorine. Therefore our CL levels are highly dependent on the CYA level in the pool.

I also understand that in an indoor pool, zero CYA is needed, and chlorine at somewhere below 1 ppm would be plenty to achieve a sanitizing effect. This tells me that CYA is only necessary to protect from sunlight.

Understanding those two items, it seems to me that the chlorine only needs to be maintained about 1 ppm above the CYA level at which the rest is bound.

If in a test tube, 50 ppm of CYA resulted in a need of 6 ppm to achieve an arbitrary sanitizing effect, then ±5 ppm cl is bound to the CYA and the rest , ± 1 ppm is available to do the sanitizing.

Extrapolating then, at 100 ppm CYA we would have ±10 ppm cl bound to the CYA, and the rest, again about 1 ppm is free to perform the sanitizing work. These numbers seem to jibe roughy with the median values from the pool calculator.

My conclusion from all this would be that any measurable free chlorine available for sanitation would would be consumed by the sun very rapidly (< half hour half life), and while we would have a clearly measurable free cl level, none of it would be available in the water for sanitation because it would all be bound by the CYA.

Therefore it seem to me that the best CYA level would be zero, and while we would use much more chlorine, it would all be available for sanitation and would provide a much higher sanitation level.

Obviously this is not how things work in the real world so where did I mess up? Where does the sanitizing effect come from in a pool that has been in the direct sun long enough to burn off all the non-CYA-bound chlorine?

- Does CYA actually release some of the free chlorine to be available for sanitation as the sun burns it off?

- Does the CYA-bound chlorine actually have a much weaker sanitizing effect and therefore more is needed?

- Something else?

[chem geek]

My responses to your comments below in bold

I have a new SWG and I want to understand why we even need any CYA at all. I do understand that without CYA the cell will be running much longer but I'm not certain why.

In a pool exposed to direct sunlight, the UV will breakdown chlorine and this happens very quickly if there is no CYA in the water. In direct noontime sun, half of the FC is lost every 35 minutes. Even with CYA in the water, there is loss, but it is significantly slowed down. For an SWG pool, having the CYA be 70-80 ppm results in a fairly low loss of chlorine from sunlight. With a lower chlorine loss, one can turn down the SWG on-time so the SWG cell lasts longer since it's life is a function of on-time. The lower on-time also results in a slower rise in pH from aeration and chlorine outgassing.

I also understand that CYA will bind to free chlorine in pool water and protect it from breaking down in sunlight. However, this binding will also prevent or at least reduce sanitizing action performed by the chlorine. Therefore our CL levels are highly dependent on the CYA level in the pool.

The CYA protects chlorine from sunlight in two ways. One is by combining with most of the chlorine to form compounds that are resistant to breakdown (typically around 97% of the FC is chlorine bound to CYA). The other is by CYA (and the Cl-CYA combo) absorbing UV directly (without breaking down) thus shielding lower depths of water. The relationship between chlorine and CYA was definitively determined in the 1974 paper in this link. You are correct that sanitizing capability is significantly reduced when CYA is present since almost all of the sanitizing capability is from hypochlorous acid (some from hypochlorite ion and possibly a very small amount from Cl-CYA). However, it takes an incredibly low amount of chlorine to kill most heterotrophic bacteria and to inactivate most viruses (more on this below).

I also understand that in an indoor pool, zero CYA is needed, and chlorine at somewhere below 1 ppm would be plenty to achieve a sanitizing effect. This tells me that CYA is only necessary to protect from sunlight.

This is not true, though is the common conventional wisdom. If one does not use CYA in indoor pools, but chlorinates such pools with 1-2 ppm FC, then one is significantly over-chlorinating the water since the active chlorine concentration is way too high. There probably should be a small amount (say, 20 ppm) of CYA used in indoor pools with around 2 ppm FC (maybe 4 ppm FC for commercial/public pools) so that one has a low active chlorine level but still have enough buffer of chlorine to handle local bather loads (sweat/urine).

My wife experiences the consequences of not using CYA in indoor pools every winter season. She swims in an indoor community center pool with 1-2 ppm FC and no CYA over the winter and her swimsuits degrade (elasticity gets shot) after just one season of use and her skin gets flaky and hair frizzy. In our own outdoor pool with 3-4 ppm FC and 30 ppm CYA during the summer, the swimsuits have lasted for years with minimal degradation and her skin and hair aren't affected as much. The difference is that our pool has the equivalent of 0.1 ppm FC with no CYA so the indoor pool has 10-20 times as much active chlorine (hypochlorous acid).

The German DIN 19643 standard uses 0.3-0.6 ppm FC with no CYA precisely to have lower chlorine levels to prevent problems such as disinfection by-products (especially nitrogen trichloride). So the use of low active chlorine concentrations is well understood by many, including those in the water treatment industry. Why the 1974 info and subsequent disinfection studies are ignored by the industry is anybody's guess, but it may have serious ramifications since virtually all respiratory, ocular, asthma problems are associated with indoor pools and these pools generally don't have CYA in them. Though a lack of sunlight and poor air circulation are certainly contributing factors, the fact that the pools are significantly over-chlorinated due to a lack of CYA may be another factor as well and I show how this affects nitrogen trichloride production in this post.

Understanding those two items, it seems to me that the chlorine only needs to be maintained about 1 ppm above the CYA level at which the rest is bound.

1 ppm FC with no CYA is too high as noted above. CYA is a chlorine buffer so is useful to moderate chlorine's strength.

If in a test tube, 50 ppm of CYA resulted in a need of 6 ppm to achieve an arbitrary sanitizing effect, then ±5 ppm cl is bound to the CYA and the rest , ± 1 ppm is available to do the sanitizing.

Extrapolating then, at 100 ppm CYA we would have ±10 ppm cl bound to the CYA, and the rest, again about 1 ppm is free to perform the sanitizing work. These numbers seem to jibe roughy with the median values from the pool calculator.

Roughly speaking, the amount of disinfecting chlorine is proportional to the FC/CYA ratio. At a typical FC that is 10% of the CYA, this results in the equivalent of 0.1 ppm FC with no CYA. This is not only more than sufficient to kill bacteria quickly, but is also sufficient to prevent algae growth (the actual algae threshold even at high phosphate and other nutrient levels is somewhere below an FC that is 5% of the CYA level except yellow/mustard algae has a higher FC threshold requirement).

My conclusion from all this would be that any measurable free chlorine available for sanitation would would be consumed by the sun very rapidly (< half hour half life), and while we would have a clearly measurable free cl level, none of it would be available in the water for sanitation because it would all be bound by the CYA.

You are forgetting that the chlorine is bound to CYA in an equilibrium reaction and that the chlorine gets unbound and re-bound to the CYA quickly. So as the unbound chlorine gets broken down by sunlight, there is more chlorine released from the CYA to make up for it (technically, bound and unbound chlorine go back and forth relatively quickly). Half of the bound chlorine to CYA can convert to unbound form in 4 seconds from one species and 0.25 seconds from another species so for practical purposes the chlorine is released from its "buffer" attached to CYA rather quickly. In fact, this is what happens during the FC test itself since the unbound chlorine reacts with dye and more is freed from being bound to CYA all in the time of the test so that the test measures the sum of unbound and bound chlorine (i.e. hypochlorous acid, hypochlorite ion, and all chlorinated isocyanurates).

Therefore it seem to me that the best CYA level would be zero, and while we would use much more chlorine, it would all be available for sanitation and would provide a much higher sanitation level.

As noted above, 1+ ppm FC would be TOO much sanitation and would oxidize things too quickly not only be more damaging to skin, hair and swimsuits, but also generating more disinfection by-products. You could try and maintain 0.1 or 0.2 ppm FC with no CYA for a reasonable sanitation/oxidation level, but it is very hard to maintain such low FC levels throughout the bulk pool water with imperfect circulation and the presence of local bather load (urea and ammonia from sweat and urine, plus other skin oils, etc.). We have seen significant stainless steel corrosion in some SWG pools that had no CYA and had 3-5 ppm FC in them.

Obviously this is not how things work in the real world so where did I mess up? Where does the sanitizing effect come from in a pool that has been in the direct sun long enough to burn off all the non-CYA-bound chlorine?

You missed two points: 1) that it takes an incredibly low amount of chlorine to kill most bacteria and still a relatively low level to prevent algae growth and 2) chlorine is released from being bound to CYA quickly as this is a chemical equilibrium.

- Does CYA actually release some of the free chlorine to be available for sanitation as the sun burns it off?

Yes.

- Does the CYA-bound chlorine actually have a much weaker sanitizing effect and therefore more is needed?

Yes. This has been borne out by numerous scientific studies including the ones here, here, here, here and here. Of course, the pool industry says that this only happens in the lab and that in "real pools" this may not happen. Some studies have concluded that incorrectly such as the Pinellas County, Florida pool study I write about here where they never looked at hypochlorous acid concentration to see that it was at least as good a predictor as FC in terms of disinfection -- basically, bacteria are killed so quickly that you can't distinguish the effects of CYA only looking for higher HPC and related counts since you only need to kill bacteria faster than they reproduce in order to not have runaway growth. "Real pool" water has slower kill times due to other interfering substances in the water, but early studies used very low chlorine levels where the CYA buffering effect would be positive whereas in today's pools with a minimum of 1 ppm FC and usually 2 ppm FC or more, the effect of CYA reducing active chlorine concentration should be more noticeable (but no one has yet done a study in that situation -- using real pool water with sufficient FC and varying CYA and then adding bacteria and measuring kill times).

- Something else?

I think that about covers it.

[Pool-newb]

Ok, I think I get it.

There are two basic sanitation paths; one by the bound chlorine as it is released from the Cyanuric Acid as soon as the rest of the FC is consumed, and the other, perhaps weaker path is that of CYA combined with chlorine.

I didn't "forget" about the equilibrium reaction - I wasn't aware of it and that makes the use of CYA much more logical, thanks for the clarification (pun intended :-).

I also understand that values well under 1 ppm are effective sanitizers without the presence of CYA, but in the real world of pools, even the best test kits have a resolution of only 0.5 ppm (though I sometimes push that by doubling reagents or halving the sample with distilled water), and I'm just accustomed to refering to chlorine levels in terms of 1 or more ppm.

I googled the DIN19643 and while I didn't find the entire standard online I did find some excerpts and references to it (I am fluent in German). Apparently (according to the German Wiki article) they are planning to change it this year to add a section 6 which will allow "Ultrafiltration", using a membrane with 0.05µm porosity, which means not even viruses and bacteria get through. However, in the 30+ german pulic pools that are using this system they are still using chlorine. I guess you just can't get around the fact that not all water will get sucked into the filter and there can always be pockets of crud.

But I digress. None of the articles I read even mentioned CYA.

I have also spent many hours swimming in German pools (I lived there for 16 years) and neither my wife (who IS German) nor I noticed any huge difference in the swimsuit failure or hair frizzyness. Of course we didn't go every day but we were in many different pools, and they do have a huge selection of water temperatures and contents. One pool I lived near for several years was based on Bali. It had several pools including one with very warm water, and all of the pools had an intentionally very high salt content - from the taste I would say even as much as sea water. The most recent swimming adventure over there was around Christmas at this large complex near the Munich airport. They claim the longest indoor slide in Europe and have 14 different slides, and probably 12 different pools (including one with Dead Sea minerals) plus a huge sauna area. I wonder if they use any CYA because while we didn't notice ruined bathing suits ore frizzy hair, we were pretty tired afte a 15 hour day there.

[chem geek]

The FAS-DPD chlorine test using a 25 ml sample has a resolution of 0.2 ppm. The problem isn't in the testing of such low FC levels, but in maintaining them in spite of the onslaught of things in the pool to oxidize. In Germany, they take the approach of no CYA, low FC, and a combination of filtration AND adsoprtion with the latter being done by activated carbon which not only removes chloramines, but also removes chlorine itself. This means they must reinject more chlorine so the process isn't very efficient for a residential pool, but is OK for commercial/public pools where the chlorine demand is high anyway. Such a system would preclude use of CYA, but at least the FC level is only in the 0.3 to 0.6 range which is lower than most indoor pools in the U.S. (that don't have CYA).

The reason for the use of chlorine even in the German system is to have a fast-acting disinfectant in the bulk pool water. Filtration systems take too long since one turnover takes hours to complete and that only touches 63% of the water. It takes 4.6 turnovers to have 99% of the water get through the filtration system (under ideal mixing conditions).

If the water slide was indoors, then they probably didn't use CYA, but they also probably kept the FC as low as possible. Also, as you say, you aren't using these pools every day. My wife was literally swimming at least 4 times a week for 5 months in the indoor pool while in our pool over 7 months she swam at least 5 times a week. An infrequent exposure to 1-2 ppm FC with no CYA isn't going to do much -- it's the cumulative effect over an entire season of use.

Richard