Miscellaneous Short Discussions

Carl,

In this post you said that TA has a theoretical upper limit of 200 ppm. How is that so? There is a limit to calcium carbonate saturation, but at lower CH (or higher pH) one can have higher TA. In fact, if there is no calcium in the water at all, then there is no real limit to TA though obviously at very high TA (and a normal or low pH) the water will tend to outgas very rapidly. However, when one adds sodium bicarbonate (Alkalinity Up or Baking Soda) to water, it is very concentrated locally and yet carbon dioxide doesn't violently bubble out of the water and the pH isn't that high (unlike sodium carbonate or pH Up where the pH is very high).

So it seems to me that the "limit" is that of the solubility of sodium bicarbonate in water and that is 7.8g/100ml or 78g/l or 78,000mg/l which is 78,000 ppm of sodium bicarbonate. This is equivalent to about 46,000 ppm measured as CaCO3 alkalinity (the difference is from molecular weight and a factor of 2 since CO3 counts twice for alkalinity while HCO3 counts once).

I know that in practical terms, the calcium carbonate precipitation or cloudiness will be the real limit, but you said "theoretical limit" so I'm trying to figure out where this 200 ppm limit is coming from.

Richard

Carl,

In this post you say a couple of things I disagree with or perhaps am just unsure of or am misunderstanding.

1) Using a higher CYA of say 80 ppm may result in lower chlorine usage.

2) CYA should NEVER be used in an indoor pool.

Regarding the first point, I disagree because the benefit of CYA in protecting chlorine is a non-linear relationship of declining returns. The reason for this is that the chlorine bound to CYA (chorinated cyanurates) DOES degrade in sunlight. It just does so at a slower rate with a half-life of around 8.4 hours (though some sources say it's 6 hours). The half-life of unprotected (unbound) chlorine is 35 minutes (though some sources say that in a tube it's 11.6 minutes). These numbers are for direct noontime sunlight in summer (i.e. sun directly overhead).

Let's take a simplified example and assume that the chlorine bound to CYA is COMPLETELY protected from sunlight and lasts forever. The rough rule is that to get the same disinfecting chlorine concentration you need to keep the FC to CYA ratio constant so whereas at 30 ppm CYA you could have 3 ppm FC, at 90 ppm CYA you need 9 ppm FC. The amount of unbound chlorine in both cases is identical in this example so in this simplified example there is NO CHANGE in the amount of chlorine that is lost to sunlight. In other words, in this best-case you never use less chlorine with more CYA -- never (I'm not talking about SWG efficiency, but just chlorine lost to sunlight).

The reality, however, is that the chlorine bound to CYA IS lost to sunlight, albeit more slowly, and since the vast majority of the chlorine is in fact bound to the CYA it is this quantity that really determines what goes on. With three times the CYA and needing three times the FC you have roughly three times the amount of chlorine bound to CYA and three times the amount of chlorine lost to sunlight.

All of this is shown in this graph where you can see that the net overall effect from the two half-lives (of unbound and bound chlorine) is that at 30 ppm the half-life of 3 ppm FC chlorine is about 6 hours while at 90 ppm the half-life of 9 ppm FC chlorine is about 7.5 hours. So you only get a relatively small increase in the half-life of chlorine, but have three times as much to lose. The net result is that you lose FAR more chlorine at the higher CYA and FC levels. I show this net result in this graph where the 0.05 yellow line corresponds roughly to the mid-point of Ben's Min and Max columns. This clearly shows that the lowest total loss of chlorine is always at the lowest possible CYA level. In fact, with no CYA, the amount of chlorine required to produce 0.05 ppm of disinfecting chlorine is only 0.1 ppm FC. If there were somehow some way of putting 0.1 ppm FC into the pool everywhere and maintaining it, then that would lead to the lowest loss of chlorine since you would only be losing half, or 0.05 ppm FC, every 35 minutes. So over 8 hours, that's losing about 0.4 ppm FC total. Compare that to having 90 ppm CYA with 9 ppm FC where you lose half or 4.5 ppm FC in a little less than 8 hours. That's a HUGE difference!

Of course, there is no practical way to maintain 0.1 ppm FC everywhere in a pool at all times, even with an SWG blasting away. There can be localized demand for chlorine that overwhelms the ability of chlorine to diffuse from other areas of the pool fast enough. Also, you simply aren't going to be manually standing by your pool at all times adding chlorine to it continuously at a rate of around 0.05 ppm FC every half hour (0.1 ppm FC per hour).

This is what I mean when I say there is a tradeoff between using the lowest CYA possible to minimize chlorine breakdown from sunlight vs. the practical side of having enough chlorine buffer (capacity) in the pool to handle localized demand AND frequency of replenishment.

There is no question that Aylad may find that at a constant FC level that the chlorine lasts longer with more CYA, but there are two questions I would have. First, how much longer does the chlorine last, or put another way, what is the difference in loss? I doubt very much that it anywhere near a factor of 3 between 30 ppm and 90 ppm. I suspect that with, say, 5 ppm FC that at 30 ppm over 24 hours the end result is 2.5 ppm FC while with 90 ppm over 24 hours the end result is 3 ppm FC. The problem is that while at 30 ppm CYA one could keep 3 ppm FC; at 90 ppm CYA one needs to keep 9 ppm FC. In other words, to keep a minimum of 3 ppm FC and since half would get lost over a day, one has to start with 6 ppm FC (at 30 ppm CYA). At even double the CYA of 60 ppm, the half-life goes from 6 hours to 7 hours but the minimum FC level is around 6 ppm so now one needs to start with, perhaps, 11 ppm FC so one ends up with 6 ppm the next day. Part of what is happening is that Ben's chart is being interpreted at its extremes. The 30-50 ppm CYA says a Min of 3 ppm FC while the 60-90 ppm CYA says a Min of 5 ppm FC so it APPEARS that going from 30 ppm CYA at 3 ppm FC one can go to 80 or 90 ppm CYA at only 5 ppm FC and have the same disinfecting chlorine capability. At least from the chemistry, that is simply not the case. It's 30 ppm CYA at 3 ppm FC being equivalent to 90 ppm CYA at 9 ppm FC as seen in my chart (or accurately calculated in my spreadsheet).

So assuming half the FC is lost at 30 ppm CYA and somewhat less than half is lost at 80 ppm CYA, then the difference between Aylad going from 6 ppm FC to 3 ppm FC over a day to maintain the minimum of 3 ppm vs. going from perhaps 9 ppm FC to 5 ppm FC over that same day does seem to not be a huge difference, but even in this case it's a 3 ppm FC loss per day at 30 ppm CYA while a 4 ppm FC loss per day at 80 ppm CYA. Now if Aylad has some numbers for me that show these losses to be wrong, then we can see what is wrong about the model assumptions. But it would have to be a combination of two things: CYA would have to protect chlorine from sunlight much better than described (i.e. not 8.4 hours but far higher) AND the chlorine/CYA relationship would have to not be as linear as we think it is when CYA >> FC. I'm guessing that the issue here is more of a misinterpretation of Ben's chart due to its ranges.

One final point. In the above discussion I have only talked about the chlorine demand associated with breakdown from sunlight. In addition, there is a relatively fixed demand associated with typical algae and bather load (on average) and this is a constant to be added to the daily FC consumption. That doesn't change the end result of the analysis in making higher CYA better, but it does make the differences between different CYA levels smaller since there is an extra FC amount required. Note that I am NOT saying that 30 ppm CYA is the right number. This completely depends on the specifics of the total chlorine demand and the frequency of chlorine addition. I just doubt that 80 ppm CYA is the best level and this would be more apparent if the 9 ppm FC were the true target.

(CONTINUED BELOW REGARDING POINT 2)

Richard

(CONTINUATION FROM ABOVE)


As for point 2 (boy, I sure got long-winded about point 1!), there are reasons to use CYA in indoor pools that have nothing to do with the lack of sunlight. Remember that CYA not only protects chlorine from breakdown from sunlight, but it also significantly lowers the disinfecting chlorine concentration. Because one cannot maintain 0.1 ppm FC in a pool realistically, even indoor pools with no CYA tend to have at least 1 ppm and usually 2 ppm FC or even higher. That is a HUGE amount of disinfecting chlorine! Reaction rates are proportional to the concentration of disinfecting chlorine (most reactions are "first-order" so are proportional). The mid-point of Ben's chart is about 0.05 ppm of disinfecting chlorine while even 1 ppm FC with no CYA is about 0.5 ppm disinfecting chlorine so an indoor pool has AT LEAST 10 times the amount of disinfecting chlorine because no CYA is used. Swimsuits will degrade 10 times faster. Skin cells will get chafed 10 times faster. Disinfection by-products will get created 10 times faster. And yes, bacteria and algae will get killed 10 times faster, but so what? They were already killed more than fast enough at far lower disinfecting chlorine levels. All reports of asthma and respiratory problems in competitive swimmers and children have been in indoor pools and though better ventilation has helped a little, there is still a big problem there. Though some of the problem is due to a lack of sunlight to help breakdown combined chlorines including some disinfection by-products and though some of the problem is due to less air circulation, I believe a BIG part of the problem is the 10 times (or higher) faster production of disinfection by-products due to the 10 times (or higher) disinfecting chlorine levels. It is well known in the water treatment industry that high chlorine levels lead to faster production of DPBs and one of the ways this was historically handled was to try and minimize such chlorine levels.

So I am an advocate of using a small amount, say 10-20 ppm, of CYA in indoor pools. Since indoor pools have a very hard time dealing with combined chlorines and since having CYA in the water will slow down that breakpoint process (this happens in outdoor pools, but sunlight compensates for that by helping to break down CCs), then the use of a weekly non-chlorine shock (potassium monopersulfate) makes sense. That would also virtually eliminate the creation of DPBs as well by oxidizing organics before chlorine has a chance to combine with them. The downside is that there will be a buildup of sulfates over time due to the non-chlorine shock, but I don't see a way around that (so periodic regular dilution would be wise).

I know that my wife would be extremely happy if the indoor pool she went to used CYA since every winter (and it's been 4 years now) every single one of her swimsuits she uses in the indoor pool get degraded. They don't all fade because she uses "chlorine-resistant" swimsuits, but the rubber degrades or the fabric gets thin or other effects occur and sometimes there is even some visible fading. I finally checked their water and they did NOT have unusually high FC levels. They were below 3 ppm FC every time I checked (usually around 2-3 ppm FC), but had no CYA. The pH was normal and there was some CCs (it varied each time, but were about 0.5 - 1.5), but that is expected since I don't think they are using non-chlorine shock and the pool was always heavily used.

Richard

Watermom,

In this thread you mention how pouring bleach down the skimmer with the pump running is not a problem and from a practical point of view I would tend to agree, but do want to be more specific about the potential for harm.

If we assume a GPM rate of 40 when the pump is running, then 6% bleach is around 60,000 ppm FC and if it is poured slowly at a rate of about 5 seconds per cup, then that's 1.33 minutes per gallon (0.75 GPM) so this is a dilution ratio of 53 to 1 so an FC level hitting the pump, filter, etc. of about 1000 ppm FC (it's higher in the PVC line from the skimmer, but I'm assuming that all gets mixed together with the water from the other return lines; the GPM rate at the skimmer will be lower, but it will get diluted later so it nets out to the overall GPM rate of the system). This isn't all disinfecting chlorine since the pH will be high. I calculate that with normal pool conditions (pH 7.5, TA 100, CYA 30) that the pH will rise to about 9.3 and the disinfecting chlorine level will be 13.8 ppm. For comparison, the recommendation for the mid-point of Ben's Min/Max is 0.05 ppm and for shocking is around 0.3 ppm so the disinfecting chlorine level is indeed quite high and is similar to having 27 ppm FC with no CYA.

The time for exposure is probably only around 30 seconds to a minute depending on how much is being added, but this could happen every day.

I am bringing this up not only for the issue of how to pour chlorine into the pool, but to address concerns with using a liquid form of chlorine in vinyl pools. Some pool builders swear that using a sodium hypochlorite will damage vinyl pools and some say to use lithium hypochlorite instead. Though more expensive, it dissolves quickly and can be broadcast around the pool so is not so concentrated in one place. I'm guessing that pouring chlorine quickly in one place may have the chlorine settle briefly near the bottom (it's denser than water so may settle until it mixes). Clearly this can happen with Cal-Hypo which is why it should get mixed in a bucket before adding to a vinyl pool. I was hoping that pouring sodium hypochlorite slowly in front of a return would be the best approach, especially in the deep end where the water flow is strong from return to skimmer and return to floor drain(s). It's almost impossible to calculate what happens, but I can imagine that slowly pouring in front of a return such that very little chlorine avoids hitting the flow would cause the chlorine to rapidly disperse into a larger volume of water and therefore get diluted rather quickly and therefore be safer.

No conclusions here -- just putting out some info.

As for Cyanuric Acid dissolving in the skimmer, that's a whole other matter since it dissolves so slowly. In this case, it may take many hours to dissolve so if it took as long as one turnover of water, then it essentially would not cause any higher concentration of acid or CYA at all. Of course, the pump must be running continuously or else there can be a localized buildup of acidity.

A Trichlor tablet in the skimmer may also be safe for the same reason, so long as the pump is continually running. I don't remember how long it takes for a tablet to dissolve, but I do think it's hours.

Richard