CYA for Salt Chlorinator Pools II

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Originally Posted by tphaggerty

Richard & Poolsean,
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.

In thinking about this though, you did say that it might take a couple of days to "shock" all of the water in the pool, perhaps this is happening and is "good enough" to keep the CC reading at or close to zero. I am wondering why or whether this information is available in Austrailia where they have been using these systems for a lot longer than here (at least for non-commercial pools).

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A properly maintained outdoor pool using manually dosed chlorine will also report a CC of 0 all the time. If the chlorine level gets too low, then [EDIT]when you later add some more chlorine[END-EDIT] you can build up chloramines, but if it is constantly maintained, then this is much less likely [EDIT]since breakpoint occurs "on the fly" when sufficient chlorine is present[END-EDIT]. Also, outdoor pools exposed to sunlight may have UV breakdown the chloramines [EDIT]and possibly chlorinated organics[END-EDIT]. If you think about it, if you were to manually drip liquid chlorine (remember this is 60,000 ppm chlorine concentration) into your pool near an output jet and did so more or less continuously whenever the pump was running, you are essesntially doing exactly the same thing that the SWG system is doing [EDIT]including exposing some volume of water to very high chlorine levels[END-EDIT]. This implies that for a manually dosed pool it *may* be better to add chlorine in small doses more frequently rather than wait to do it in larger doses every few days [EDIT](and maybe this slower dosing helps keep the CC at 0 by oxidizing CC albeit a small amount of pool water volume at a time).[END-EDIT]

[EDIT]From what I have read, chlorinated organics (aka Disinfection ByProducts or DBPs) are more likely to form at higher chlorine concentrations (which CYA keeps down low) and in particular they form even more when there are chloramines also present so an incomplete breakpoint is the worst situation to be in (i.e. to not have enough chlorine to meet bather loads). The "bad" chlorinated organics, known as THMs, form more at higher pH while at lower pH you get more non-volatile chlorinted organics that are not considered harmful to health, but would still show up in CC if they persisted.[END-EDIT]

To really test your theory, an analysis of indoor pools with and without SWG would be the place to start. Indoor pools have a much harder time avoiding CC since there is no sunlight to break them down nor is there good air circulation to sweep away the products of breakpoint chlorination (though I've never really been convinced that this is an issue since nitrogen and oxygen gasses are already pretty dominant in air so I doubt they get that much higher in concentration above an indoor pool's surface -- perhaps it's other gasses, such as carbon dioxide, that need to be swept away???).

Richard

Evan (waterbear) also makes good points. If we really want to get to the bottom of this, we would need ALL of the relevant pool water chemistry information that all of you in these forums know is what is asked of you when you want people to help you with your water problems. Of particular relevance for this SWG algae discussion would be minimum FC level, CYA, and pH. The TDS is already roughly known and not a big factor nor is the CH or alkalinity (except that very low alkalinity can lead to large pH swings you might not be aware of) -- I'm not talking about water balance to protect plaster, but rather those components that affect disinfecting chlorine concentration.

Anyway, I don't want to bash SWG systems in any way. Even though I don't have one, I think they are great and the most important service they provide is that they automatically maintain FC levels. And then there's that silky feel of salt water...oooooooo, you've got me thinking about getting an SWG myself!

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Originally Posted by chem geek

Anyway, I don't want to bash SWG systems in any way. Even though I don't have one, I think they are great and the most important service they provide is that they automatically maintain FC levels. And then there's that silky feel of salt water...oooooooo, you've got me thinking about getting an SWG myself!

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IF you do you won't reget it! Also, you might be able to clear up some of the issues raised here!;)

Seriously, IMHO, SWGs are the way to go! Pool maintenance becomes much easier (I only have to worry about keeping a jug or two of acid on hand. Every time I buy bleach for the pool 'just in case' it gets used in the laundry since I don't need it for the pool! I keep an extra bag or two of salt on hand for the water softener and If I need to bump up the salt in the pool a bit I use some of that unless I decide to spluge on bag of Morten or Diamond Crystal pool salt...learned my lesson about off brand pool salt with I got iron stains from one!)

There are a few things that I would be interested in understanding better. I have pretty much forgotten everything learned in Chem 101 & 102:mad:.

From what I understand about SWGs, after the chlorine gas is generated, it dissolves very quickly into water and creates HOCL and HCL.

Based upon the amps in the cell, how much chlorine gas is generated and does this match the advertised production of the cell (see below)?

Does some of the HOCL and HCL revert back to salt before exiting the cell, which might explain the higher levels in the cell?

How long does it take for some of the HOCL to break down to OCL- & H+?

How long does it take CYA to bind to the chlorine?


Next, I found this derivation on a university web site:

In an electrolysis of sodium chloride solution experiment a current of 7 A was passed for 1 minute

o Electrode equations:
(-) cathode 2H+ + 2e- ==> H2 and (+) anode 2Cl- -2e- ==> Cl2
o (a) Calculate the volume of chlorine gas produced.
Q = I x t, so Q = 7 x 1 x 60 = 420 C
420 C = 420 / 96500 = 0.00435 mol electrons
this will produce 0.00435 / 2 = 0.002176 mol Cl2 (two electrons/molecule)
vol = mol x molar volume = 0.002176 x 24000 = 52.23 cm3 of Cl2
o (b) What volume of hydrogen would be formed?
52.23 cm3 of H2 because two electrons transferred per molecule, same as chlorine.


First, is this formulation correct?

If so, a cell running for 24 hours would produce 4418658 cm3, 4419 liters or 14 kg (31 lbs). This is 21x the advertised production rate so if true, a majority of chlorine produced quickly reverts back to salt before returning to the pool.

Anyway, I have measured the water coming out of the return and, as Sean pointed out, it was only 2-3 ppm higher than the pool water. So my premise is that there may be some HOCL in very high concentrations in the cell but does not survive very long.

Sean knows his cells better than I, but I'll take a stab at explaining things from a general chemistry perspective and I'll do so in-line in bold in your quoted post below.

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Originally Posted by mas985

From what I understand about SWGs, after the chlorine gas is generated, it dissolves very quickly into water and creates HOCL and HCL.
True.

Based upon the amps in the cell, how much chlorine gas is generated and does this match the advertised production of the cell (see below)?
I do not know, but suspect the production is close to the generation except for the "side-reactions" that can occur and inefficiency from the buildup of products (Cl2 and HOCl).

Does some of the HOCL and HCL revert back to salt before exiting the cell, which might explain the higher levels in the cell?
There is always an equilibrium that goes on, though the generation is very fast and things are out-of-equilibrium at various points in the system. First chlorine gas is produced. Then this gets converted to HOCl. Then this gets bound up with CYA. So there *is* a peak of concentration of HOCl in some region of the water, but this exact same thing also happens (minus the chlorine gas) when you add liquid chlorine or bleach to your pool.

How long does it take for some of the HOCL to break down to OCL- & H+?
This is an extraordinarily fast reaction, but I don't have the rate constant at hand (in general, protonation reactions with acids are extraodinarily fast).

How long does it take CYA to bind to the chlorine?
Now in this case I have the actual rate constant and the answer depends on the concentration of the different CYA species in the water. The reaction of CYAs taking up HOCl in pool water is as follows at 80 ppm CYA:

... rate ......... [CYA-]
7.4x10^4 * 4.64x10(-4) * [HOCl] = 34 * [HOCl]
... rate ......... [CYA(2-)]
2.2x10^7 * 7.98x10^(-8) * [HOCl] = 1.8 * [HOCl]

Though the reaction rate is dependent on the HOCl concentration, one can calculate the half-life of HOCl which is how long it takes for half of the HOCl to get converted as follows (I'll only use the faster of the two reactions which is the CYA- one):

Half-life of HOCl = -ln(0.5)/34 = 0.02 seconds

It is this rather slow reaction (in chemistry 0.02 seconds seems like an eternity) that is why such high CYA levels are required to make the salt cell more efficient. If CYA reacted faster, then lower levels would "take up" the HOCl fast enough to keep the cell operating efficiently. In fact, if you calculate how long water spends inside the salt cell at normal flow rates, you get a time that is only somewhat longer than the 0.02 seconds which is why the 80 ppm CYA concentration is needed (for today's sizes of cells and chlorine production rates). With a CYA of 20 ppm, the time rises to around 0.1 seconds which is longer than the time water spends inside the salt cell. This means that Cl2 and HOCl build up more inside the cell and that degrades the efficiency of the cell.


Next, I found this derivation on a university web site:

In an electrolysis of sodium chloride solution experiment a current of 7 A was passed for 1 minute

o Electrode equations:
(-) cathode 2H+ + 2e- ==> H2 and (+) anode 2Cl- -2e- ==> Cl2
o (a) Calculate the volume of chlorine gas produced.
Q = I x t, so Q = 7 x 1 x 60 = 420 C
420 C = 420 / 96500 = 0.00435 mol electrons
this will produce 0.00435 / 2 = 0.002176 mol Cl2 (two electrons/molecule)
vol = mol x molar volume = 0.002176 x 24000 = 52.23 cm3 of Cl2
o (b) What volume of hydrogen would be formed?
52.23 cm3 of H2 because two electrons transferred per molecule, same as chlorine.


First, is this formulation correct?
Yes, this computation is correct (some constants were rounded so the final answer isn't as accurate as implied, but its essentially correct). The 0.002176 moles of chlorine is equivalent to (0.002176 moles/minute)*(2*35.4532 g/mole)/(453.59237 grams/pound)*60(min/hr)*24(hours/day) = 0.49 (pounds/24hours)

If so, a cell running for 24 hours would produce 4418658 cm3, 4419 liters or 14 kg (31 lbs). This is 21x the advertised production rate so if true, a majority of chlorine produced quickly reverts back to salt before returning to the pool.


52.23 * 60 * 24 = 75211 cm3, not 4418658 that you calculated. It looks like you multiplied by an extra factor of 60 thinking that the above example was per second when it was done for one minute.

The electrolysis current was 7 Amps. One of the manufacturers I looked up said their cell current was 4.5 to 7.8 amps, presumably adjustable as a "power level". They claim they can produce up to 1.45 pounds per day of chlorine. Remember that we calculated 0.49 pounds per day of Cl2 above. I don't know how to reconcile that, but the cell certainly isn't producing more chlorine than they claim.


Anyway, I have measured the water coming out of the return and, as Sean pointed out, it was only 2-3 ppm higher than the pool water. So my premise is that there may be some HOCL in very high concentrations in the cell but does not survive very long.
There is most certainly some HOCl in very high concentrations in the cell and most of it gets bound up by CYA, but that is also what happens when you add liquid chlorine to the pool.

If instead there were huge amounts of HOCl produced over larger areas of the cell and if this did in fact break down back into chloride ion before getting into the pool, then the "super-chlorination" effect would be real. However, over the short lifetime of the water from the cell and into your pool, the water can tolerate incredibly high concentrations of HOCl without breaking down. Remember that 12.5% chlorine (which is around 11% available chlorine) is over 100,000 ppm in concentration and yet takes months to breakdown (the half-life under normal conditions is over 200 days) naturally (at normal temperature and *not* exposed to sunlight).

Now if you had a special kind of salt cell that had TWO reactions in it where it generated HOCl at one end of the cell and then intentionally destroyed HOCl at the other end of the cell, then this would produce a middle area with high levels of HOCl that could super-chlorinate, but it is my understanding that this is not how the salt cells operate. Instead, they operate more like you described with the electrode equations.

[EDIT]
The fact is that the computations based on current (amps) shows that the salt cell simply does not produce enough chlorine to cause the average chlorine level of the entire cell to be higher than what you are measuring at the outlet which is just a few ppm higher UNLESS some water in the cell is moving much more slowly (more on that below). We have a conflict between the claimed 1.45 pounds/24hour maximum rate and the 0.5 pounds/24hour rate calculated from the amperage, but let's just see what we get using some reasonable numbers. One user in an earlier post estimated 90 GPM flow rates and I'd say that's about the same as my pool as well so let's use that.
0.5(pounds/24hour) * 453.952337(g/pound) * 1000(mg/g) / 24(hours/24hour) / 60(minutes/hour) = 158(mg/minute)
90(gallons/minute) * 3.785(liters/gallon) = 341(liters/minute)
158(mg/minute)/341(liters/minute) = 0.46 ppm
So the increase in chlorine at the outlet would only be 0.46 ppm at maximum. Since people measure more than this, something is wrong. If the 1.45 pounds/24hr is correct and the amperage is wrong, then you would get around 1.5 ppm incrementally higher output which sounds closer to being what people are measuring.

So how is it that someone can measure the ppm inside a cell and find it to be so high, seemingly over a reasonably large volume of the cell? Ignoring problems with how you perform such a measurement, if there are parts of the cell with slowly moving water then this water can indeed have very high chlorine concentration, but that means that most water moving through the cell bypasses this high chlorine area so even though it seems like a good-sized volume of the cell has high chlorine (which is true), it is not true that this "superchlorination" is getting applied to large volumes of pool water.

Sean mentioned how slowing down the water made it possible to give such water enough time to be exposed to the superchlorination so all of that makes sense, but it also means that the amount of total pool water exposed to this superchlorination is very small. So my point about taking many turnovers to get much of the water through this superchlorination is still valid. And this is essentially the same situation that happens when you add liquid chlorine (or bleach) to the pool except that an SWG dones this much more slowly over an extended period of time and therefore is more efficient and better. I just don't like the claim that implies a superchlorination of your entire pool water over a short period of time -- say, a day or two -- and leaving out the fact that bugs or algae stuck to walls aren't super-chlorinated at all.
[END-EDIT]

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I've onlhy been running the SWG for a few weeks now and have observed
that the pool liner is "squeaky clean" ... the exact opposite of slimy.

I have been raising the CYA since discovering it all went away during the winter.
It is now in the 40-50 range
Keep the ph in 7.2 to 7.5 range.
CC always at or near 0
FC 3 to 5... still playing with settings to get a balance between run time and power.

We have an unprotected hill top site, so lots of debris. Every morning about 30 to 50 crickets, equal numbers of some other beetle like bug, and 15 or so young frogs ... mostly dead by morning.

The water is astonishingly clear, despite the debris.

Reading all this I'm inclined to keep moving the CYA upward.

Sean,

I updated the end of my post
See Above
to reflect what is consistent with your point of slower moving water in the cell getting longer contact time in a superchlorinated environment. We haven't heard from you so I hope I didn't offend. The theories about extra chlorine being generated and then breaking down just don't add up, but the theory of slower moving water at higher chlorine concentration does make sense. It's just that this doesn't actually superchlorinate all of the pool water quickly -- it would take weeks. You can either slow down the flow in a good part of the cell and have this part be exceptionally superchlorinated or you can have the water flow evenly through the entire cell at small chlorine boost, or a combination of the two, but you can't get the full benefits of both (superchlorination and full pool volume) at the same time.

Richard

Richard,
I don't mean to offend you but it is obvious that you have never worked with or lived with a pool that has a SWG. Chlorine levels will change very slowly with changes in pump run time and cell output...sometimes it can take days for it to stabilize. While you are right that only the water passing through the plates is superchlorinated, in actual practice this seems to be enough to keep the CC at 0 with FC levels that are way below the best guess chart. Also, since algae will first start in the water column before it attaches to the sides and walls of the pool, once again, in actual practice, this seems to be sufficient to prevent algae blooms in normal operation. I view this in the same light as UV sanitizers and ozone generators. Any sanitation only takes place in the reaction chamber and not in the pool proper. The residual chlorine in the water takes care of the rest in all threee of these cases. Perhaps SWGs need to be looked at in the light of this as having 2 separate functions: To generate a residual FC level for the pool and to superchlorinate a small quantity of water in the reaction cell during cell operation to constantly destroy free floating algae and oxidize any nitrogenous compounds in much the same way an ozonator would. As far as superchlorinating all the water, the boost or superchlorinate functions on most units are, IMHO, not really good for this since it can take up to 24 hours to reach the high chlorine levels necessary. I feel it is much more effective to add sodium hypochlorite in the recommeded dosage all at once than to let it gradually climb up. If I understand breakpoint chlorination properly then many undsirable forms of combined chloramines would form at high chlorine levels that are below breakpoint and make breakpoint even more difficult to achieve thus the gradual rise in FC levels by running the celll 24/7 at 100% are not the optimum way to achieve it and may acutally hinder it.
I have only emperical evidence from my own experimentation with my pool but I can tell you this. If I maintain the cya in the 60-80 range and my FC at least 3 ppm and my pH 7.6 or below I do not have any CC nor does the pool ever get cloudy. I have never had an algae breakout since the SWG went online. I have experimented with the above factors and have found that
1. When my CYA is below 60 ppm I have to turn the output up to maintain a 3 ppm FC level.
2. When my pH is above 7.6 I start to show some CC on testing. When it reaches 8.0 my FC level starts to drop.
3. When my FC level is below 3 ppm I have experienced CC and cloudiness of the water.

It is a given that theory often does not hold up in field conditions because of other factors that are not taken into account.

Your thoughts on these points would be appreciated.

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Originally Posted by waterbear

Richard,
I don't mean to offend you
Perhaps SWGs need to be looked at in the light of this as having 2 separate functions: To generate a residual FC level for the pool and to superchlorinate a small quantity of water in the reaction cell during cell operation to constantly destroy free floating algae and oxidize any nitrogenous compounds in much the same way an ozonator would.
It is a given that theory often does not hold up in field condtions because of other factors that are not taken into account.
Your thoughts on these points would be appreciated.

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Absolutely no offense taken at all. You are correct that I have zero experience with SWG systems, I agree completely with your points, and I'm the one who has to be careful not to offend. My educated guess as to what is going on is not inconsistent with the real-world results and yes, it's the theory that must match the real world, not the other way around.:) I was just dumbfounded with the initial claim that the salt cell has superchlorinated levels of chlorine AND that this was getting applied to all of the pool water (in a reasonable time).

There is one big difference between UV and ozone sanitizers vs. SWG as far as I understand them and that is that the former do in fact "do their work" on the entire volume of water that flows through their chambers so that after a few hours with one turnover of pool water, the majority (I forget the number and how to calculate it, but I think it's around 70%) of the water in the pool has been sterilized and oxidized at least once. This does not appear to be the case with the SWG and instead some fraction of the water goes through amounts of superchlorination. I could be dead-wrong about how the UV and ozone systems work so if anyone knows if they "slow down" some part of the water in their systems in order to more effectively disinfect and oxidize, please let us know.

Since putting in liquid chlorine (bleach) also superchlorinates a part of the pool water when it is introduced into the pool, then the incremental benefit of the SWG probably derives from its continual dosing which is more optimal. I wonder if people without SWG were to manually dose their pools more frequently (with smaller amounts of chlorine each time) and did so over pool jets at both ends of the pool if they, too, could operate at lower levels of chlorine safely. My guess is that they could, but I have no idea what the required FC level would be between "once a day" well-distributed manual dosing vs. the continual dosing of the SWG.

The 3 ppm FC level that you found to be required for your pool is still very safe from a disinfection point of view since 70 ppm CYA, 7.6 pH and 3 ppm FC gives 0.017 ppm HOCl which is above the 0.011 "minimum" that appears to be needed for disinfection (and that's in non-SWG pools and spas).

Again, I think SWG is great and just because I "bash" a specific claim doesn't mean I think the technology as a whole has no net benefit. On the contrary, I regret not having one installed when we put in our pool (it was not mentioned as an option by our pool contractor and I knew even less about pools then than I do now).

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Originally Posted by chem geek

Absolutely no offense taken at all. You are correct that I have zero experience with SWG systems, I agree completely with your points, and I'm the one who has to be careful not to offend. My educated guess as to what is going on is not inconsistent with the real-world results and yes, it's the theory that must match the real world, not the other way around.:) I was just dumbfounded with the initial claim that the salt cell has superchlorinated levels of chlorine AND that this was getting applied to all of the pool water (in a reasonable time).
You are right that it is only being applied to a fraction of the pool water but it is an ongoing process and I would believe the effect would become cumulative after a while...in much the same way UV sterilization would only kill the bacteria in the reaction tube but with each pass there would be less and less bacteria in the whole of the water. I know that UV light has to be in close proximity to the water for sterilization to occur from my experience with it in aquarium use and not all the water flowing through the tube receives enought intensity to completely kill the pathogens present. Ozone, also only works on the water in the reaction chamber. In fact, no residual ozone is supposed to be in the water when it enters the pool. Once again the benifit seems to be gained from an incremental effect. Plus ozone acutally depletes chlorine levels somewhat so a negative factor is introduced.
There is one big difference between UV and ozone sanitizers vs. SWG as far as I understand them and that is that the former do in fact "do their work" on the entire volume of water that flows through their chambers so that after a few hours with one turnover of pool water, the majority (I forget the number and how to calculate it, but I think it's around 70%) of the water in the pool has been sterilized and oxidized at least once. This does not appear to be the case with the SWG and instead some fraction of the water goes through amounts of superchlorination. I could be dead-wrong about how the UV and ozone systems work so if anyone knows if they "slow down" some part of the water in their systems in order to more effectively disinfect and oxidize, please let us know.

Since putting in liquid chlorine (bleach) also superchlorinates a part of the pool water when it is introduced into the pool, then the incremental benefit of the SWG probably derives from its continual dosing which is more optimal. I wonder if people without SWG were to manually dose their pools more frequently (with smaller amounts of chlorine each time) and did so over pool jets at both ends of the pool if they, too, could operate at lower levels of chlorine safely.
It would seem that systems that use ORP controllers and peristaltic pumps would achieve this but I don't believe that optimin ORP mv levels are reached with lower chlorine concentrations. And it is a well known fact that CYA disturbs ORP readings. Whether these actually have an effect on the actual sanitation I do not know. I would be intersting to compare pools with SWGs and peristaltic pumps at comparable FC levels and CYA levels and see if, in actual use, similar results would be obtained.
My guess is that they could, but I have no idea what the required FC level would be between "once a day" well-distributed manual dosing vs. the continual dosing of the SWG.

The 3 ppm FC level that you found to be required for your pool is still very safe from a disinfection point of view since 70 ppm CYA, 7.6 pH and 3 ppm FC gives 0.017 ppm HOCl which is above the 0.011 "minimum" that appears to be needed for disinfection (and that's in non-SWG pools and spas).

Again, I think SWG is great and just because I "bash" a specific claim doesn't mean I think the technology as a whole has no net benefit. On the contrary, I regret not having one installed when we put in our pool (it was not mentioned as an option by our pool contractor and I knew even less about pools then than I do now).

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A most interesting discourse!

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

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

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?

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Originally Posted by 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?

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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). 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

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.

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Originally Posted by 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.

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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

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

Quote:

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Originally Posted by 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

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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

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.

Quote:

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Originally Posted by chem geek

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

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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/showpos...91&postcount=7
http://www.poolforum.com/pf2/showpos...3&postcount=13
http://www.poolforum.com/pf2/showpos...1&postcount=17

I'd like to bring this thread back to its original topic so I'm putting my response in a new Saturation Index thread where I hope to hear from you and Ben (and others).

Richard

Quote:

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Originally Posted by tphaggerty

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.

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

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.

[Edit]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.
[/Edit]

Sentient

My comments in bold.

Quote:

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Originally Posted by Sentient

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.

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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.

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

Quote:

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Originally Posted by 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

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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!

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

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

I posted some questions/answers here:

http://www.poolforum.com/pf2/showpos...77&postcount=3