CYA for Salt Chlorinator Pools II

[Poolsean]

Richard, no offense taken in any of the responses here. What's interesting is that scientific calculations "should" prove to be consistent with lab conditions, and I'm sure it does. However, real world applications show differently, mainly due to (IMHO) external parameters that are too diverse to pinpoint.

My unscientific mind says that what I see in real life, is what I can report on. Just like Ben's best guess charts are based on his many years of actual experience and research and have proven highly effective. The longevity and membership here, proves that point. Who would have ever thought to grab the gallon of bleach and box of 20 mule team from the laundry room to take care of a pool?!
Ben's wife must have absolutely killed him the first time she saw him sneaking off with the bottle of Chlorox!

You chemistry GEEKS, sorry Chemgeek for borrowing on your name, absolutely are amazing with your knowledge. I have a hard enough time converting Celsius to Fahrenheit! I cheat and just double ( I THINK it's suppose to be times 9/5, rather than times 2) the Celsius and add 32.
I have to admit though, I am learning quite a bit from this string.

Thanks y'all

[chem geek]

Well, we got moved to The China Shop (thank you Ben, if you're the one who did it) and this is appropriate for where this discussion has moved. It's not really about advice on problems or answering questions on CYA for SWG systems anymore since most everyone is in agreement that with current salt cells the CYA should be kept at the manufacturer's recommended higher level (70-80) to improve efficiency of the cell. And I think it's pretty clear from the real-world experience that SWG pools generally have no CC and fewer algae problems (with a few exceptions) on average than manually dosed pools. The issue is why this is the case and it's not clear to me that it's the chemical process in the salt cell that is the source of these benefits or if it's the constant and automatic maintenance of chlorine levels or the continual dosing and at least partial superchlorination or what.

I absolutely agree with you and appreciate the real-world experience and feedback and also know that it won't always match a scientific model. The main reason I like to have a scientific model or at least a partial understanding of the real-world isn't just a curiosity, but also to be practical to be able to predict and potentially to be more accurate. However, if a good model that reasonably fits the real-world data and predicts accurately is not found, then I'm fine with using the "gut-feel" tables based on experience. After all, the chemical and biological model for how asprin works was only recently discovered, yet asprin was (and still is) a very valuable drug for over a hundred years (for the synthetic -- willow bark was used long before that).

There's also another more personal reason that I got "plugged in" to the superchlorination debate in SWGs and that is that I had just come back from the off-site library storage facility in Richmond where I looked up the original O'Brien article on "Equilibria in Aqueous Solutions of Chlorinated Isocyanurate" that was presented at the "Chemistry of Water Supply, Treatment and Distribution" symposium in 1973 and published in 1974. In his paper, he explicitly states the problem of using too much CYA in pools because of how much it ties up the chlorine leaving very little disinfecting chlorine left. He recommended CYA concentrations on the order of 25 ppm. His paper is the de facto industry standard for the equilibrium constants that control the chlorine CYA relationship. So I was upset at how an entire industry of CYA manufacturers would (probably intentionally) ignore this data and promote Tri-Chlor (and Di-Chlor) without limitation until rather recently. It's kind of like the tobacco industry ignoring their own results on the health hazards of smoking. Well, OK, a greater likelihood of developing algae in a pool isn't the same as the risk of getting lung cancer, but you know what I mean. I was upset, nevertheless, and let this emotion spill a little into the superchlorination discussion.

Richard

[aquarium]

So, the entire residential pool chemical industry has organized itself for intentional failure since 1974? Why haven't there been huge class action lawsuits against the obvious deep pockets, like, oh say, DuPont (http://www.lowes.com/lowes/lkn?action=productDetail&productId=232441-23132-10037&lpage=none)?

[chem geek]

So, the entire residential pool chemical industry has organized itself for intentional failure since 1974? Why haven't there been huge class action lawsuits against the obvious deep pockets, like, oh say, DuPont (http://www.lowes.com/lowes/lkn?action=productDetail&productId=232441-23132-10037&lpage=none)?
I was partly being facetious when I said the CYA manufacturers "intentionally" ignored these results. I have no evidence that they saw these results back when it was discovered though I do know that at least one company (whom I've had contact with) knew about it since 2004 (see pages 15-17 of EPA Document (http://www.epa.gov/chemrtk/sdditriz/c14660rr.pdf)). I do not want to cast dispersions on these companies. The guy I communicated with was fairly forthcoming and we did not discuss the historical promotion of CYA products -- I was mostly just trying to solidify the technical data through alternative sources.

Nevertheless, it doesn't take a whole lot of imagination to see that companies that make a product whose excessive use can lead to problems would not be the first ones to say "limit your use of our product", especially since this mostly just leads to people buying highly profitable specialty products (algaecide, non-chlorine shock, etc.) to fix the resultant pool problems. Technically, you could just keep ramping up your (FC) chlorine levels to higher and higher amounts to still reamin "safe", though you would be losing an awful lot of chlorine doing so (since even the chlorine tied up in CYA degrades in sunlight, albeit more slowly with a half-life in direct intense sunlight of around 8 hours).

I don't want to start a crusade on this issue, especially since it seems that the manufacturers are now starting to disseminate the advice of keeping CYA levels in check.

Richard

[aquarium]

I understand Richard,

It just seems very odd to me. I'm an architect, and contrary to public perception that we're all about making things look nice, our first and foremost task is ensuring public safety. As is that of every other professional that works in constructing/managing the built environment. It just seems too odd that this particular part of the built environment is somehow exempt from that goal.

[chem geek]

I understand Richard,

It just seems very odd to me. I'm an architect, and contrary to public perception that we're all about making things look nice, our first and foremost task is ensuring public safety. As is that of every other professional that works in constructing/managing the built environment. It just seems too odd that this particular part of the built environment is somehow exempt from that goal.
The manufacturers recommend keeping the FC at 3.0 or higher and that this is OK to do for CYA up to 70 and they say you shouldn't go above 100. So from a disinfection point of view at normal pH (7.5), even at 100 ppm CYA the HOCl is 0.012 which is a minimum for disinfection that roughly corresponds to an ORP near 650 mV which is the same level that commercial pools use as a minimum. So the issue is not one of health safety, at least for easy-to-moderate-to-kill bugs (i.e. up to E.coli), but of having sparkling clean pools without algae. Now some in the industry claim that this 3 ppm rate will also prevent algae, but Ben's table and real-world results dispute this (except, perhaps, for SWG pools where the jury is still out on the exact chlorine level needed to prevent algae in such pools).

The study I looked at done by one manufacturer, however, used statistical averages to prove their point saying that the average chlorine level in a group of pools (with varying amounts of CYA) without algae was 3 while in a group of pools with algae the average was below 3. That is, of course, not the right way to look at data. It should be the maximum chlorine level (actually HOCl level) in the pools with algae that is a starting point for analysis. The minimum chlorine level in the pools without algae is also useful and these will likely overlap due to "lucky" pools without enough chlorine but that haven't yet developed algae. Of course, algae growth depends on lots of other factors including nutrients in the pool, sun exposure, rate of introduction of algae into the pool, etc.

So I do not believe that the manufacturers are trying to skirt public safety. My analogy with the tobacco companies was again facetious and way too extreme. A better analogy might be drug companies who promote their solutions rather than non-patentable alternatives that sometimes work as well or better. Also, it could just be that the manufacturers just aren't being as careful with their studies, not intentionally, but just based on who did the study -- these aren't scientifically peer-reviewed articles, but industry conference presentations (the O'Brien paper, on the other hand, was done scientifically with university professors and may have had peer-review).

Richard

[aquarium]

Thanks Richard,

I guess I have a 'lucky pool' then. :D

Using just bleach and muriatic acid I run it at 1.5-2ppm chlorine as measured with a dumb little OTO test kit, with 30ppm CYA at about 7.2-7.4pH, adjusted via aeration to about 80ppm alk.

Clear blue water.

I mean -really- clear blue water. :p

TW

[chem geek]

Thanks Richard,

I guess I have a 'lucky pool' then. :D

Using just bleach and muriatic acid I run it at 1.5-2ppm chlorine as measured with a dumb little OTO test kit, with 30ppm CYA at about 7.2-7.4pH, adjusted via aeration to about 80ppm alk.

Clear blue water.

I mean -really- clear blue water. :p

TW
An FC of 1.5-2, CYA of 30, pH of 7.3 gives an HOCl of 0.022-0.030 which is probably equivalent to the Min. FC in Ben's chart (not if you look it up directly with his "ranges", but if you convert Ben's chart into HOCl levels, then his Min. FC column is approximately 0.02 or 0.025 though he has it rise up at the lowest CYA levels, probably just to ensure a minimum FC as a reserve). So your pool isn't just lucky, it's also within the relam of the theory we're trying to develop.

On the other hand, Ben has said he has seen pools with algae that were maintained with levels that, after I run through the calculations, are closer to 0.05 ppm HOCl which seems to be the worst-case scenario. So the real answer for algae prevention is likely to be somewhere in that 0.02-0.05 range. Of course, the "real world" creeps in with factors like uneven pool circulation that can cause localized conditions to be worse than the minimum. This is why Ben's chart tries to be conservative -- to account for such conditions.

By the way, what is your CH level and pool water temperature? The saturation index I calculate at 80ºF and 300 CH (with 7.3 pH) would be -0.28 (using my newer index, though the old index is -0.30 so about the same in this case). This isn't horrible and may be a very mild corrosive condition (if you have a plaster pool), but if you want to be in balance, then CH would need to be 700 (with my new index; 600 with the old). Of course once you commit to this high a CH, you have to keep your pH and alkalinity at your current low levels unless you drain your pool water to dilute the calcium.

I try to keep my pool's alkalinity closer to 100-115 with a CH of 260 and a pH of 7.4-7.5 and a water temperature of 88ºF and CYA of 15-20 (hard to tell for sure given the tube stops at 30). My primary motivation for this balance is that the higher alkalinity makes the need for pH adjustment much less frequent, though in practice I hardly ever have to adjust it anyway. I keep an FC level of 2-3 which is safe and conservative since I can drop to 1 FC and still have about 0.020 ppm HOCl similar to your pool. I only lose about 0.5 ppm FC or less per day due to an electric opaque pool cover. I, too, have never had algae and the water is crystal clear blue.

By the way, do you ever get CC > 0 and have to shock your pool?

Richard

[aquarium]

The calcium hardness is about 430ppm.

Arm's length down into the deep end with the pumps running at 3:45PM (full sun in central Texas) the water temp is 84 degrees F.

Plaster pool, about 15,000 gallons, 6 foot deep end.

TW

EDIT: I don't bother with much beyond the OTO test kit, so I don't know CC and I've never shocked since doing BBB starting in June. Just lucky again I guess.

[waterbear]

By the way, what is your CH level and pool water temperature? The saturation index I calculate at 80ºF and 300 CH (with 7.3 pH) would be -0.28 (using my newer index, though the old index is -0.30 so about the same in this case). This isn't horrible and may be a very mild corrosive condition (if you have a plaster pool), but if you want to be in balance, then CH would need to be 700 (with my new index; 600 with the old). Of course once you commit to this high a CH, you have to keep your pH and alkalinity at your current low levels unless you drain your pool water to dilute the calcium.


Richard
Langelier Saturation Index is, IMHO, a bogus measurement for pools. It was designed for closed systems and a pool is an open system. See these posts (2 of mine and one from Ben):
http://www.poolforum.com/pf2/showpost.php?p=5291&postcount=7
http://www.poolforum.com/pf2/showpost.php?p=2823&postcount=13
http://www.poolforum.com/pf2/showpost.php?p=3891&postcount=17

[chem geek]

I'd like to bring this thread back to its original topic so I'm putting my response in a new Saturation Index (http://www.poolforum.com/pf2/showthread.php?t=4523) thread where I hope to hear from you and Ben (and others).

Richard

[Sentient]

The ONE thing that hasn't been addressed in your analysis, Richard, is the fact (using the term loosely) that almost everyone using a SWG with fairly well balanced water reports CC at 0 all the time. This is not true (I believe) of those that use bleach to maintain normal FC, at some point they will get a buildup of CC and have to shock (which is another interesting thread you are involved in!). This would seem to support the idea that the SWG is "super-shocking" in the cell and it must be some larger percentage of the water that passes through the cell than you are calculating.

I hope this doesn't duplicate something said farther below, where I haven't read yet.
Sure enough, it was beat to death. Sorry to waste the bandwidth

I have a SWG and have seen CC chlorine levels above zero. It has been after a swim party or other event that put a good load on the pool. My solution was simply to add some bleach, and let things circulate over night.

This does not deny that some form of superchlorination may be occurring in the cell, but it also shows that water not having gone through the cell recently is still at risk.

I think I will try and up my CYA based on what I read here. I was keeping it lower because I didn't understand how it would help. Now that I can grasp a reason why it is a good thing I will follow through.

In reading all of this I had a couple of thoughts.

First, my aquarite turbo cell has what looks like plastic mesh around the plates. This should spit out lots of vortices to both slow the water down and mix it a lot near the plates, suddenly that makes sense. High turbulence should move the chlorine away from the plates more quickly. Clever mechanical tricks are always a good thing.

Second, someone with a two speed pump should be able to test whether slower flow is more or less efficient somehow. This would not be an easy test since fairly accurate estimates of flow rate would be needed, but worth considering.


Sentient

[chem geek]

My comments in bold.



I have a SWG and have seen CC chlorine levels above zero. It has been after a swim party or other event that put a good load on the pool. My solution was simply to add some bleach, and let things circulate over night.

This does not deny that some form of superchlorination may be occurring in the cell, but it also shows that water not having gone through the cell recently is still at risk.

I would guess as much though with sufficiently high levels of chlorine (independent of whether there was an SWG) the CC should get reduced between a somewhat slow breakpoint and a somewhat faster exposure to sunlight. At least that's the current thinking (subject to change, of course). Ironically, the typically higher CYA levels and somewhat lower chlorine levels found in most SWG pools means that achieving breakpoint takes even longer outside of the salt cell (that is, it takes longer to get breakpoint from the chlorine in the pool because the HOCl levels are relatively low). Fortunately, the salt cell itself has high chlorine levels so eventually after many turnovers most of the water has gone through areas of high chlorination to achieve breakpoint. Again, my speculative theory.

First, my aquarite turbo cell has what looks like plastic mesh around the plates. This should spit out lots of vortices to both slow the water down and mix it a lot near the plates, suddenly that makes sense. High turbulence should move the chlorine away from the plates more quickly. Clever mechanical tricks are always a good thing.

Yup, creating all sorts of vortices and somewhat turbulent flow (what I would call "controlled" turbulence since normal uncontrolled turbulence causes all sorts of problems) helps sweep away the generated chlorine away from the plates. Sounds like a good design in your system.

Second, someone with a two speed pump should be able to test whether slower flow is more or less efficient somehow. This would not be an easy test since fairly accurate estimates of flow rate would be needed, but worth considering.

I would suspect that the SWG manufacturers designed their systems for typical flow rates and that slowing down the flow would result in lower efficiency. Anyway, I think we've all pretty much concluded that SWG systems have a lot going for them regardless of the specifics of the (electro)chemistry that is involved.

[Poolsean]

Slower flow provides better efficiency. Think of it as longer contact time of the combined chlorines and the "shocking effect" of the salt chlorine generator cell. This is evident in the Pool Pilot's manifold assembly, the Hayward TurboCell body, the various other 2"inlet and larger body designs.
The larger body allows the water flow to actually decrease, thus giving you the slower flow - Higher Efficiency.
The problem is that at low flow, you need to make sure that there is still sufficient flow to activate the flow switch, or evacuate all the air with Gas Trap systems.

Hope this helps.

[chem geek]

I think we misunderstood each other.

When I said that slowing the flow would lower the efficiency, I meant slowing it down from the pump side with the already slowed down flow that is in the cell itself. In other words, the cell has already slowed things down for higher shocking effect and optimum efficiency with higher CYA to bind the chlorine, but if you were to slow down the pump a lot, then wouldn't there be a buildup of chlorine at the plates because there wouldn't be enough flow to sweep it away?

In other words, I figured that the salt cell had already designed into it the proper amount of slowdown for high efficiency and having a flow that is much higher (wouldn't build up as high a localized chlorine level to shock) or much lower (would build up too high a chlorine level near the plates to be efficient) would be less optimal.

Richard

[Sentient]

I think we misunderstood each other.

When I said that slowing the flow would lower the efficiency, I meant slowing it down from the pump side with the already slowed down flow that is in the cell itself. In other words, the cell has already slowed things down for higher shocking effect and optimum efficiency with higher CYA to bind the chlorine, but if you were to slow down the pump a lot, then wouldn't there be a buildup of chlorine at the plates because there wouldn't be enough flow to sweep it away?

In other words, I figured that the salt cell had already designed into it the proper amount of slowdown for high efficiency and having a flow that is much higher (wouldn't build up as high a localized chlorine level to shock) or much lower (would build up too high a chlorine level near the plates to be efficient) would be less optimal.

Richard

So in a two speed system, one could theoretically expect higher "cell shock" at low speed, or more overall chlorine getting into the pool for a given run time and duty cycle at high speed. I expect the difference is probably not very noticable by the average user. They will still just set the duty cycle control maintain the desired chlorine level as they always have.

In any event it is all very intersting!

[chem geek]

Well, I believe I have figured out why the salt cell seems to do a much better job of superchlorinating a portion of the pool water -- both in disinfecting (killing bugs) and in oxidizing (getting rid of combined chlorine) than simple addition of chlorinating liquid or bleach. When I described earlier how adding chlorinating liquid or bleach was similar to what was done in a salt cell in that there are high levels of chlorine as the concentrated chlorine (125,000 ppm for 12.5% chlorinating liquid) gets diluted in the pool water, I didn't account for the large differences in pH between the two methods. The pH of chlorinating liquid or bleach is very high and though this gets reduced as it is diluted, it is still somewhat high until the dilution is quite broad. This means that most of the chlorine is in the form of the hypochlorite ion (OCl-) which is a much less powerful oxidizer and disinfectant than hypochlorous acid (HOCl).

If I run through some dilution calculations (using standard pool values of pH 7.5, TA 100), I find that the disinfecting chlorine level even with no CYA present only reaches a peak at a dilution of 195:1 and a disinfecting chlorine (HOCl) level of 14.2 ppm. With 30 ppm CYA, the peak is reached at a dilution of 112:1 and a disinfecting chlorine level of 7.1 ppm. At 80 ppm CYA, the peak is reached at a dilution of 75:1 and a disinfecting chlorine level of 3.5 ppm.

In other words, the strong alkalinity of chlorinating liquid or bleach severely cuts down its disinfection and oxidation ability. Of course, the hypochlorite ion (OCl-) is very high (my earlier posts' calculations of dilution would apply mostly to this hypochlorite ion concentration), but is at least 30 times less effective an oxidizer and disinfectant (and possibly worse than that).

Contrast this with what goes on in an SWG cell. At the plate where the chlorine gas is generated, the chlorine gas combines with water to form hypochlorous acid and hydrochloric acid so is a very strongly acidic process. This means that almost all of the chlorine is in the form of the powerful oxidizer and disinfectant hypochlorous acid (HOCl). It is only in the area in the middle between the two plates where the alkalinity from the other plate combines with the acidity to have net alkalinity similar to chlorinating liquid. So, near the acid environment of the chlorine generating plate, water will indeed be exposed to very high levels of disinfection and oxidation -- i.e. will be superchlorinated. Of course, this will be a small subset of the total water flow through the cell, but that is still much better than what happens when simply adding chlorinating liquid or bleach to a pool.

So to simulate the salt cell through adding chlorinating liquid or bleach, one would have to add acid to lower the pH of the pool, then add the chlorine, then add base to raise the pH back up. This is obviously impractical and would swing the pH too much and have other side effects. (Also remember that you never want to add concentrated acid to concentrated chlorine as that produces large amounts of chlorine gas which is quite poisonous).

So that explains why the SWG pools seem to require less FC than non-SWG pools, at least for the pool water that gets circulated through the cell. There are still some users' pools that seem to need higher levels of FC (say, 5-6 instead of 3) to prevent mustard/yellow algae, but that may be due to biofilms stuck to pool surfaces. Regular (once a week) brushing might help to force algae and other microorganisms from pool surfaces into the general body of pool water that will eventually go through the superchlorinated portion of the salt cell. However, if growth rates of algae are high, then even regular brushing might not keep up with such growth rates so the higher chlorine levels in the body of pool water would be the only way to keep the algae at bay.

Richard

[Strawfoot]

Novice here, so please have a little patience with my question. I have a new salt water pool, approx 27,500 gallons, in-ground, with Goldline/Aqualogic equipment. The problem I am having is my Free Chlorine level has been practically zero. PH and all other levels are fairly normal.

After intial shocking using the chlorinator, I ran the CYA at about 30ppm. I used bleach a couple times to get the FC level up, but now have started to rely on the chlorinator.

After hearing that salt water pools require higher CYA, a couple days back I added stabilizer (using tied up socks in the skimmer to desolve the stuff). Today the CYA read 70 ppm (where I absolutely cannot see the black dot.) But my FC level is still pretty much zero.

Do I need to wait a couple more days for the CYA level to take effect, or is there something else I need to do. Chlorinator level of the spa is 10% and pool is at 50%. I'm thinking I may need to shock again. Weather here has been cold, with some rain.

Any suggestions or advice will be appreciated.


Mike

[mas985]

I posted some questions/answers here:

http://www.poolforum.com/pf2/showpost.php?p=41977&postcount=3