On maintaining FC...

hi all,

i've been trying to wrap my mind around a question about maintaining FC, and i just can't decide which side to take. let's say there are two pools:

pool A has 30 PPM CYA and needs about 3 PPM FC.

pool B has 60 PPM CYA and needs about 5 PPM FC.

here's the question: will pool B actually use more chlorine overall? in other words, if both pools are the same size and both are at their suggested PPM for FC, would they require the exact same amount of chlorine added to maintain those levels? i always kind of figured keeping a higher FC requires more bleach, and more expense. but when i put it this way, it seems like it would require about the same?

As usual, it depends.

It depends on many factors. If you lived in Louisiana, where aylad lives, the answer is easy--the CYA=30 would use more, because there isn't enough residual to prevent stuff from starting. aylad runs her CYA at around 70-80ppm and that keeps her chlorine usage fairly low, despite hot, steamy weather with VERY short winters.

Carl's got it right--if I kept my CYA at 30, it would use about double the amount of bleach that I use with CYA at 80, AND would require much more frequent additions. You need to look at how much sun, debris, and other chlorine-using factors are there for each pool to make that decision.

Janet

Roughly speaking, the chlorine amount needed to prevent algae growth is proportional to the Cyanuric Acid (CYA) level. That is, the FC/CYA ratio roughly determines the amount of "active" chlorine available. If you have twice as much CYA in the water, you need twice as much FC to have the same level of disinfection and algae prevention capability.

The breakdown of chlorine from sunlight is proportional to the amount of active chlorine. So a certain percentage breaks down so, in theory, at the same FC/CYA ratios the percentage of chlorine loss should be the same which means the absolute loss should be higher at higher FC and CYA levels.

However, Janet (aylad) and others have found that keeping a higher CYA level, especially around 70-80 ppm, has even the absolute FC loss be lower in spite of keeping a consistent FC/CYA ratio (i.e. having a higher absolute FC level). Mark (mas985) did an experiment in this thread that showed that at a CYA of 45 ppm, the chlorine loss per day was around 50% while at 80 ppm CYA the chlorine loss was only 15%. So even accounting for a higher absolute FC level, one would have a lower absolute FC loss at higher CYA levels.

For example, if one had the FC be around 7.5% of the CYA level which is close to Ben's "Min." column in the Best Guess CYA chart, then 3.4 ppm FC at 45 ppm CYA would lose half or around 1.7 ppm while 6 ppm FC at 80 ppm CYA would lose 15% or around 0.9 ppm -- around half the absolute FC loss per day.

I never figured out exactly why this effect is occurring. There is a protection effect (from UV in sunlight) from CYA for the lower depths of water and this effect is non-linear (exponential) with respect to distance (depth). So if the chlorine is depleted near the surface, then there can be a strong CYA "shielding" effect that is non-linear. In addition, there is some absolute chlorine loss independent of sunlight so that becomes a larger percentage of chlorine loss at lower FC and CYA levels. Say that there is a 0.7 ppm FC loss due to non-sunlight factors, then this would represent a 21% loss at 45 ppm CYA while it represents a 12% loss at 80 ppm CYA. This doesn't explain the entire effect, but combined with the CYA "shielding" effect, it might.

Richard

Richard:
I can't do the chemistry but let me pose an alternate hypothesis. CYA SLOWS the breakdown of FC. The higher the CYA, the slower the breakdown. Now, let's assume that chlorine at a fixed FC and a fixed CYA performs a certain amount of neutralization, and has a certain breakdown rate (whatever it is--we know in high UV with no CYA it can be as little as 15 minutes even with no significant contaminants). As CYA increases, both the amount of neutralization (of contaminanats) and the breakdown rate from everything decrease. But.....do they decrease at the same rate?

If, by increasing the CYA and there for the "chlorine residual" we are actually spreading the work load among more chlorine ions, so that far more ions are doing the job, but have now a lower breakdown rate than contaminant neutralization rate, wouldn't we see a reduction in chlorine demand?

Just a thought from a non chemist.

In theory, the rate of oxidation of contaminants and the breakdown of chlorine from sunlight are both proportional to the amount of "active" chlorine. With twice the CYA and no change in FC, there is about half the amount of "active" chlorine and as expected there would be a slowdown in chlorine usage.

The problem is that what we are comparing what happens when you raise BOTH FC and CYA to keep the FC/CYA ratio constant which is roughly what Ben's table does in order to provide the same amount of algae prevention. In theory, this has the rate of loss of "active" chlorine from sunlight and the oxidation of stuff be about the same as before if the only effect of CYA were lowering the active chlorine concentration. We know that CYA does more than that and shields lower depths as well, so that's the extra effect that depends only on the CYA level and is not diminished by higher FC.

In addition to "active" chlorine, the chlorine bound to CYA may also breakdown, though not as quickly. The amount of chlorine bound to CYA roughly doubles if both the FC and CYA are doubled. The amount of shielding is roughly proportional to the amount of CYA. So these two factors should roughly cancel each other out unless there was less chlorine at the surface.

Thanks for having me think it through a little more.

Richard

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the chlorine bound to CYA may also breakdown, though not as quickly.

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This may well be the key to the advantage Jan (aylad) has recorded year in and year out. If the bound-up chlorine ALSO breaks down, it's still doing work in addition to the un-bound chlorine. This MUST happen or why would there be any protection at all of the UNbound chlorine? Isn't it logical if the unbound chlorine isn't protected it would STILL break down as if there were no CYA at all to the residual level. Yet we know this doesn't happen. So there MUST be an effect from the bound chlorine.

So I think the next step is to evaluate the effectiveness of chlorine when the residual required base level is reached for increasingly greater CYA levels.

I don't understand chemistry at the scientific level, but I do follow logic and cause and effect. I leave to you, Richard, to evaluate and confirm or refute my hypothesis.

would it be fair to say that adding a solar cover to the mix significantly decreases chlorine breakdown due to sunlight? any numbers on how significantly?

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Originally Posted by docsobeck

would it be fair to say that adding a solar cover to the mix significantly decreases chlorine breakdown due to sunlight? any numbers on how significantly?

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Yes and no. Yes. The cover blocks UV and prevents evaporation loss of chlorine. No. I don't have metrics.

OH! I did not realize this! Off to add more CYA! LOL!

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Originally Posted by docsobeck

would it be fair to say that adding a solar cover to the mix significantly decreases chlorine breakdown due to sunlight? any numbers on how significantly?

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I can tell you that an opaque pool cover certainly eliminates the breakdown of chlorine from the UV in sunlight. My own pool is a case for that since it has only 30 ppm FC and with around 3 ppm FC average it loses around 0.8 ppm FC per day being open around 1-2 hours each day, though in practice even having the pool closed still has a loss of around 0.7 ppm FC or so per day. My daily loss appears to be mostly regular chlorine oxidation of stuff in the pool (including a slow oxidation of CYA by chlorine).

I don't know how much the "clear blue" solar covers block UV. If they did, but let visible and infrared light through, then that would be ideal as it would heat the pool without a loss of chlorine from sunlight.

Richard

Quote:

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Originally Posted by CarlD

This may well be the key to the advantage Jan (aylad) has recorded year in and year out. If the bound-up chlorine ALSO breaks down, it's still doing work in addition to the un-bound chlorine. This MUST happen or why would there be any protection at all of the UNbound chlorine? Isn't it logical if the unbound chlorine isn't protected it would STILL break down as if there were no CYA at all to the residual level. Yet we know this doesn't happen. So there MUST be an effect from the bound chlorine.

So I think the next step is to evaluate the effectiveness of chlorine when the residual required base level is reached for increasingly greater CYA levels.

I don't understand chemistry at the scientific level, but I do follow logic and cause and effect. I leave to you, Richard, to evaluate and confirm or refute my hypothesis.

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Cyanuric Acid (CYA) itself is able to absorb the UV from sunlight without breaking down. Absoprtion of photons does not necessarily mean destruction of a molecule. There is a number called the "quantum yield" that for CYA is near zero, but for hypochlorous acid and hypochlorite ion are near 1 meaning that virtually every UV photon absorbed by hypochlorous acid and hypochlorite ion results in its breakdown whereas for CYA such absorption just gets translated mostly to heat (i.e. the molecule gets "excited" and then relaxes from this state to mostly vibrational energy).

As for chlorine bound to CYA, the quantum yield must be close to zero or else we wouldn't see that much protection of chlorine from sunlight given that most of the chlorine is bound to CYA. I can't find any definitive information on whether and how much the chlorine bound to CYA breaks down, but it could be by some smaller amount.

As for the effectiveness of chlorine, there are many, many studies that show it is mostly related to the hypochlorous acid concentration and not to the chlorine bound to CYA which has little if any effectiveness (other than as a chlorine buffer or reserve). Also, any breakdown from UV photons getting absorbed by chlorine bound to CYA would have nothing to do with its effectiveness (i.e. rate of killing pathogens or oxidizing organics).

Basically, the bulk of the protection of chlorine is from most of it being bound to CYA and that molecule being much more stable when hit by UV. However, there appears to be a secondary protection that is more of a "shielding" effect and that can be done by CYA itself as well as by chlorine bound to CYA. The best way to determine this would be to measure chlorine loss in a thin layer of water (at various FC/CYA ratios) compared to water with some depth and see if there is a difference (accounting for the small absorption of UV by water itself).

Richard