CYA and chlorine relation question.

[chem geek]

With 0 CYA: if you start your day at 0 FC and add CH to get to a FC of 5. Say every hour you lose 1 PPM to oxidizing whatever. After 5 hours, you reach 0 ppm and now things can multiply freely. (Question: assuming a constant addition of bacteria every hour, does the kill rate/usage rate stay constant at 5ppm down to 1ppm? Or does the oxidation rate change depending on the PPM level in this 0 CYA scenario)

We usually reserve "CH" to mean Calcium Hardness though clearly from your context you mean chlorine (perhaps using "Cl" for that is a better shorthand?). What you wrote isn't true because the FC level need not go to zero for pathogens or algae to grow. It just needs to get low enough to not kill faster than they can reproduce at which point they will grow faster and also deplete the chlorine faster. Now in practice, the FC level with no CYA that is needed to prevent green and black algae growth is only 0.06 ppm so calling this zero is not unreasonable though in practice it is not possible to maintain such a low chlorine level everywhere in the pool. As for the constant addition of bacteria every hour, that has more to do with properly maintaining the FC level so that you do not run out. So long as you maintain an FC level that kills faster than the pathogen can reproduce, it doesn't matter how quickly the bacteria are added to the pool. You just need a chlorine feed system that can maintain the FC level because the FC level would otherwise drop faster when more bacteria are added more quickly.

In other words, you need to distinguish between the instantaneous chlorine concentration vs. the rate at which that chlorine concentration is maintained by adding more chlorine. Those are two different things. It is the instantaneous chlorine concentration that determines the chlorine kill rate of pathogens. It doesn't matter how much you have in reserve (though with no CYA there is no reserve unless you count the hypochlorite ion as reserve).

So this analysis is why in commercial/public pools with higher bather loads one must have chlorine measurement systems (be it automatic electronic or regular manual testing) along with fast responsive chlorine dosing in order to maintain an appropriate FC level. For the situation you gave with no CYA, pathogens and algae are not a problem because in effect the pools are over-chlorinated. Their active chlorine level is far too high. This higher concentration of active chlorine is mostly just oxidizing swimsuits, skin, and hair faster and creating disinfection by-products faster. As you point out in your subsequent post, 5 ppm FC with no CYA would be horrible. Most commercial/public pools with no CYA (usually indoor pools) have around 1-2 ppm FC. There are plenty of commercial/public pools that use CYA but they are mostly outdoors. The pool industry doesn't understand CYA's significant moderating effect on chlorine so they end up over-chlorinating in all pools that have no CYA.

Europe has a much better understanding of this with the German DIN 19643 standard that doesn't use CYA but has a lower 0.3 to 0.6 ppm FC level and an even lower 0.2 to 0.5 ppm FC level when an ozonator is used.

With 50 CYA: you start at 4 and add CH to get to 9. Same amount of active (or, above saturation?) FC (5ppm) as the above scenario. So 5 hours later, your FC is at 4 and now the stuff can start growing, but not totally freely. The remaining 4ppm of CH will still get used up killing off things, but because the active concentration is lower than it was when FC was above 4ppm, it can't keep up with algae growth, and can only slow it a bit. (if this is true, then the "front line of soldiers" at 4ppm has fewer soldiers than the front line of soldiers at 9ppm. Right?)

You are incorrect about assuming the same amount of active chlorine as the previous scenario. It's night and day different. 5 ppm FC with no CYA has over 27 times higher active chlorine than 9 ppm FC with 50 ppm CYA. This is again why you need to completely divorce yourself from the idea that the FC level alone means anything other than the capacity or reserve of chlorine to not run out (i.e. a chlorine buffer). It has absolutely nothing to do with the active chlorine level as a number by itself. You have to additionally know the CYA level in order to know the active chlorine level (and also know the pH, though with CYA this isn't as important). Until you get that concept, you will be stuck thinking that FC means something.

For the soldier analogy, you are too focussed on the number in reserve (i.e. the FC). If you do not have enough front-line soldiers (active chlorine concentration) compared to your enemy, then they can kill your soldiers faster than you can kill them and they can overrun your lines. Furthermore, they can reproduce and grow in population. Sure, you may have lots and lots of soldiers in reserve, but what good do they do? The inadequate number of front-line soldiers are just a minor annoyance to the enemy who reproduces and increases in population faster than your front-line soldiers are able to kill them. You really need to get the idea of the reserve (FC) being that useful out of your head. It is ONLY important to have enough to get too low too quickly (i.e. before you next add chlorine), but they are not helpful in determining whether you will knock out the enemy.

In your 50 ppm CYA example, you are assuming that 4 ppm was too low but 4 ppm FC with 50 ppm CYA is enough to prevent green and black algae growth so it is not too low. That chlorine level still kills algae faster than it can grow. You are correct that at a constant CYA level having 4 ppm FC has fewer front-line soldiers than 9 ppm FC -- it's roughly 4/9th as much. Remember that the FC/CYA ratio is proportional to the active chlorine level so the concentration of front-line soldiers. At constant CYA, this means the number of front-line soldiers is proportional to the FC level, but again only for a constant CYA level. 3 ppm FC at 30 ppm CYA has the same number of front-line soldiers as 6 ppm FC at 60 ppm CYA or 10 ppm FC at 100 ppm CYA.

But then, adding the sun shielding effect, all the CH that was held in suspension with CYA was not degraded by the sun until it was released to active status, so the overall loss rate due to sun exposure is lower. plus, I think CG mentioned that the CYA might also have a sunscreen affect that prevents UV from penetrating deeper into the water, which would save the active/unbound FC from sun degradation as well, which is why you get better degradation rates at higher CYA in a not-totally-linear ratio.

The chlorine bound to CYA may also degrade in sunlight, but at a much slower rate than the chlorine that is unbound. In addition, CYA or possibly chlorine bound to CYA may block some UV from lower depths without degrading. Both of these factors together are why chlorine lasts so much longer in pools with CYA than without. The second effect is also why pools at higher CYA levels even with proportionally higher FC levels (so the same active chlorine level) lose less absolute FC per day.

 

[chem geek]

One additional concept that somewhat breaks the front-line vs. reserve soldier analogy is that the way chemical equilibrium works the number of front-line soldiers is a fairly constant fraction or proportion of the total number of soldiers. So if your reserve (FC) drops in half, then the number of front-line soldiers also drops in half. So while the FC number by itself is not meaningful except in roughly knowing the size of the reserve, it IS useful at a constant CYA level for knowing that the active chlorine level is changing. At a given CYA level, if the FC level drops in half, the active chlorine level drops in half.

Of course, knowing that the active chlorine level is changing and even by how much isn't very helpful information because it doesn't answer the question "did it drop too low?" To answer that you need to look at that FC/CYA ratio so you have to know both the FC and the CYA levels. You can consider the CYA level as a parameter that inversely determines the proportion of the total number of soldiers that will be front-line soldiers. If the CYA level is doubled, then at the same FC level (total number of soldiers) the number on the front-line is cut in half.

 

[morzh]

So, at a given CYA level, say 50ppm, is ALL my FC bound, no matter how much (5ppm or 30ppm)or is it that a part of it is bound and then the rest is not and so dissipated by Sun fast?
(Obviously the degradation rate is not constant and is the faster the more FC there is?)

 

[chem geek]

Yet another concept that breaks down is thinking that there is any sort of parity between the number of enemy front-line soldiers vs. your front-line soldiers. By thinking of parity, you think that having more pathogens or algae in the water requires a higher active chlorine level because you probably think the fighting is close to 1-on-1, but for the most part that isn't true until you get something like an algae bloom or clumped algae. The reality is that a 10,000 gallon pool at the FC/CYA levels we recommend has roughly 1x10²² active chlorine (hypochlorous acid) molecules. Just 1 milliliter has 2.6x10¹⁴ so 260 trillion active chlorine molecules.

So the number of active chorine molecules far outnumbers the number of bacteria, viruses, algae, etc. There are around 13 million fecal coloform bacteria in every gram of wet human feces. So unless this stays intact and devoid of water circulation within it, it will disperse in pool water and be surrounded by many more active chlorine molecules. These hypochlorous acid molecules look like water as shown below:

So what happens next is that these molecules are moving randomly getting jostled by water molecules and eventually run into bacteria or algae cell walls. Some of these react with nitrogenous organic molecules and especially with molecules containing sulfur and some penetrate deeper into the interior of the cell. There they can react with a large variety of nitrogenous and sulfur-containing organic molecules. Some of these reactions are relatively innocuous and do not affect cell function but others do including affecting the proper folding of proteins and in some cases causing a DNA mutation that prevents the cell from reproducing (see this paper for the latest and this paper for earlier discoveries).

The rate at which the hypochlorous acid is able to get inside the cell and do damage is proportional to its concentration in the water. So adding more bacteria to the water with more bathers does not require a higher concentration of chlorine in the water. The local concentration of chlorine around the bacteria is all that matters. Adding more bacteria to the water just puts such bacteria into other places in the pool where there is plenty of active chlorine to attack it. That is, the overwhelming quantity of active chlorine in the water means that you need not worry about its quantity but rather its concentration -- how much there is per unit volume of water. That is what determines how quickly a bacteria in that volume of water will get killed. Think of the large volume of water in the pool having trillions upon trillions of killing zones so that adding some bacteria to one zone doesn't affect the rate of kill of other bacteria getting into another zone. All that matters is that the concentration of active chlorine in each zone is high enough to kill the bacteria faster than it can reproduce. This is why circulation of the water is important -- to ensure that the concentration of chlorine is consistent everywhere in the pool.

Where the above explanation starts to break down is when the assumption of pathogens or algae being disperse in the water is no longer true. If the algae gets clumped or bacteria start to form biofilms, then it takes a higher concentration of active chlorine to be able to penetrate through a now much thicker barrier to kill cells on the inside of the clump. This is the primary reason the SLAM level is 10 times higher than the normal chlorine level. It is not only to have plenty of chlorine to not run out since it gets used up more quickly during an algae bloom, but also so that the concentration of active chlorine is high enough to more quickly penetrate into algae clumps to kill algae in the interior. Furthermore, it is why it is far easier to kill bacteria and algae before they grow to large clumped numbers so is why it is so important to maintain the proper FC/CYA ratio at all times.

 

[chem geek]

So, at a given CYA level, say 50ppm, is ALL my FC bound, no matter how much (5ppm or 30ppm)or is it that a part of it is bound and then the rest is not and so dissipated by Sun fast?
(Obviously the degradation rate is not constant and is the faster the more FC there is?)

Only part is bound (97%+) and part is unbound (<3%) and for a constant CYA level it depends on the FC level. Roughly speaking, the quantity (not proportion or fraction) that is unbound is the FC/CYA ratio. So at a 7.5% ratio the unbound chlorine is roughly 0.075 ppm (0.064 ppm is the more accurate number for 3.8 ppm FC with 50 ppm CYA). At a pH of 7.5, about half of this is active chlorine, hypochlorous acid, and the other half is hypochlorite ion. The rest is bound to CYA.

So while 3 ppm FC with 30 ppm CYA has the same unbound chlorine as 6 ppm with 60 ppm CYA or 10 ppm FC with 100 ppm CYA, obviously the amount of bound chlorine is higher at higher FC levels and furthermore the percentage of bound chlorine is higher. The following gives the relative amounts where I lump together all the different chlorine bound to CYA chemical species as "Cl-CYA" and this is at a pH of 7.5 and 77ºF water temperature. The missing part of FC minus Cl-CYA minus HOCl is hypochlorite ion which is roughly equal to HOCl at pH 7.5.

.. FC ... CYA . FC/CYA . HOCl . HOCl/FC . Cl-CYA . Cl-CYA/FC
. ppm . ppm .... % ...... ppm ..... % ......... ppm ........ %
=======================================
... 4 ..... 20 ...... 20 ..... 0.098 .... 2.5% ..... 3.802 ...... 95.1%
... 8 ..... 40 ...... 20 ..... 0.101 .... 1.3% ..... 7.796 ...... 97.5%
----------------------------------------------------------------------
... 3 ..... 30 ...... 10 ..... 0.042 .... 1.4% ..... 2.915 ...... 97.2%
... 6 ..... 60 ...... 10 ..... 0.043 .... 0.7% ..... 5.913 ...... 98.6%
... 9 ..... 90 ...... 10 ..... 0.043 .... 0.5% ..... 8.913 ...... 99.0%
----------------------------------------------------------------------
. 1.5 .... 30 ........ 5 ..... 0.020 .... 1.3% ..... 1.460 ...... 97.3%
... 3 ..... 60 ........ 5 ..... 0.020 .... 0.7% ..... 2.960 ...... 98.7%
... 5 ... 100 ........ 5 ..... 0.020 .... 0.4% ..... 4.960 ...... 99.2%

The active chlorine hypochlorous acid, HOCl, is broken down by the sun quickly (2 hour 10 minute half-life) and the roughly equal amount of hypochlorite ion (OCl^-) is broken down by the sun even faster (20 minute half-life) while the Cl-CYA amount is broken down more slowly in spite of there being much more of it. By the way, the first columns of the above table (through the HOCl column) was a table I proposed to be added to the CPO course, among other tables, but as of today the CPO course has not changed for any recommendation I have made (except getting rid of Gage-Bidwell, but I don't think that was because of my suggestion).

 

[BuckeyeChris]

I assumed most of what I said was wrong, so thank you for addressing them in turn, it seemed easier to state out the line of thinking than to put everything as a question. I think the few conclusions I pulled from your post, below, will serve as good bullet points.

In other words, you need to distinguish between the instantaneous chlorine concentration vs. the rate at which that chlorine concentration is maintained by adding more chlorine. Those are two different things. It is the instantaneous chlorine concentration that determines the chlorine kill rate of pathogens. It doesn't matter how much you have in reserve (though with no CYA there is no reserve unless you count the hypochlorite ion as reserve).

The reserves, which make up most of the FC level when CYA is in the pool, don't have an affect on kill rate, but having reserves does let you continue to fight the battle for a longer time, right? Even if those reserves are lesser in number (as in, fewer soldiers on the front line) it's better than having no front line. Delays the inevitable maybe..

You are correct that at a constant CYA level having 4 ppm FC has fewer front-line soldiers than 9 ppm FC -- it's roughly 4/9th as much. Remember that the FC/CYA ratio is proportional to the active chlorine level so the concentration of front-line soldiers. At constant CYA, this means the number of front-line soldiers is proportional to the FC level, but again only for a constant CYA level. 3 ppm FC at 30 ppm CYA has the same number of front-line soldiers as 6 ppm FC at 60 ppm CYA or 10 ppm FC at 100 ppm CYA.

the number of front-line soldiers is a fairly constant fraction or proportion of the total number of soldiers. So if your reserve (FC) drops in half, then the number of front-line soldiers also drops in half. So while the FC number by itself is not meaningful except in roughly knowing the size of the reserve, it IS useful at a constant CYA level for knowing that the active chlorine level is changing. At a given CYA level, if the FC level drops in half, the active chlorine level drops in half

So you could say that having a CYA buffer in the pool is like having an auto-sensing, automatic CL dispenser running 24/7 on your pool, but it keeps adding weaker and weaker bleach, so that by the end of the day it's close to not having enough active CL in the pool to stay above that breakpoint of outpacing growth...?

You mentioned that all you need to maintain in a 0 CYA pool is 0.3 ppm FC. Would another good summary be that, while a FC of 2ppm at 30 cya is sufficient, a FC of 2 at 60 CYA is not sufficient because a higher percentage of that 2ppm is bound to CYA, and your active level of FC is now less than that .3 ppm type threshold?

edit: crosspost with your chart above.

 

[BuckeyeChris]

.. FC ... CYA . FC/CYA . HOCl . HOCl/FC . Cl-CYA . Cl-CYA/FC
. ppm . ppm .... % ...... ppm ..... % ......... ppm ........ %
=======================================
... 4 ..... 20 ...... 20 ..... 0.098 .... 2.5% ..... 3.802 ...... 95.1%
... 8 ..... 40 ...... 20 ..... 0.101 .... 1.3% ..... 7.796 ...... 97.5%
----------------------------------------------------------------------
... 3 ..... 30 ...... 10 ..... 0.042 .... 1.4% ..... 2.915 ...... 97.2%
... 6 ..... 60 ...... 10 ..... 0.043 .... 0.7% ..... 5.913 ...... 98.6%
... 9 ..... 90 ...... 10 ..... 0.043 .... 0.5% ..... 8.913 ...... 99.0%
----------------------------------------------------------------------
. 1.5 .... 30 ........ 5 ..... 0.020 .... 1.3% ..... 1.460 ...... 97.3%
... 3 ..... 60 ........ 5 ..... 0.020 .... 0.7% ..... 2.960 ...... 98.7%
... 5 ... 100 ........ 5 ..... 0.020 .... 0.4% ..... 4.960 ...... 99.2%

so at 3/30, 97.2 % of your FC is bound to CYA, but at 9/90, 99% is bound to CYA. So even though that means there is less instantaneous active chlorine as a % of your total FC, there is still enough FC quantitatively to outpace growth.

You said above that you really only need .3 ppm in a 0 CYA pool to stay above the breakpoint. I'm trying to read that chart and find where it shows that for the given levels, I have more than .3 ppm available as active chlorine. Would that be the HOCl/FC column? If so, it seems that you do get closer to that breakpoint (0.3ppm) the higher your CYA/FC.

Thanks btw, you're patience is great.

 

[chem geek]

The reserves, which make up most of the FC level when CYA is in the pool, don't have an affect on kill rate, but having reserves does let you continue to fight the battle for a longer time, right? Even if those reserves are lesser in number (as in, fewer soldiers on the front line) it's better than having no front line. Delays the inevitable maybe..

Yes that is correct. CYA is a wonderful thing to have in moderation because it not only moderates chlorine's strength that is normally far too strong (unless you can maintain a very low FC level of 0.1 ppm or so and that is not practical), but it also acts as a chlorine buffer or reservoir releasing chlorine anywhere in the pool it is needed as the active chlorine level gets used up.

So you could say that having a CYA buffer in the pool is like having an auto-sensing, automatic CL dispenser running 24/7 on your pool, but it keeps adding weaker and weaker bleach, so that by the end of the day it's close to not having enough active CL in the pool to stay above that breakpoint of outpacing growth...?

Yes, that's basically correct if you were to let the FC level drop over the day. This is why you should add enough chlorine so that at the lowest point in the day your FC/CYA level isn't below the minimum needed to prevent algae growth.

You mentioned that all you need to maintain in a 0 CYA pool is 0.3 ppm FC. Would another good summary be that, while a FC of 2ppm at 30 cya is sufficient, a FC of 3 at 60 CYA is not because a higher percentage of that 2ppm is bound to CYA, and your active level of FC is now less than that .3 ppm type threshold?

The 0.3 ppm was the low end of the German DIN 19643 standard with no ozonator, but the amount needed to prevent algae growth (i.e. our minimum FC/CYA equivalent) is actually lower at around 0.06 ppm. Basically, even the Germans have their chlorine level still higher than is probably needed, but that's because their standard is similar to the EPA DIS/TSS-12 standard requiring 6-log reductions (99.9999% kill) in 30 seconds for some bacteria and 2 minutes for others. That is very likely to be overkill since pools in the U.S. with CYA in them haven't reported outbreaks at higher frequency than those without (with the possible exception of Crypto outbreaks though the difference is still pretty small due to their rarity). Fortunately bacteria are very easy to kill. Algae is much harder to kill by comparison but they don't need to be killed very quickly (i.e. there is no person-to-person transmission of algae and it does not cause disease).

2 ppm FC with 30 ppm CYA has around 0.027 ppm HOCl active chlorine which is the same as found in 0.06 ppm FC with no CYA while 3 ppm with 60 ppm CYA has around 0.020 ppm HOCl which is the same as found in 0.04 ppm FC with no CYA. So neither of these examples you gave are close to being similar to 0.3 ppm FC with no CYA, but your point that 2 ppm FC with 30 ppm CYA has a lower active chlorine level than 3 ppm FC with 60 ppm CYA is true and is easy to tell because 2/30 < 3/60. The 2 ppm FC with 30 ppm CYA would be below the 7.5% FC/CYA ratio needed to prevent algae growth or put another way is below the 0.030 ppm HOCl or 2.3 ppm FC with 30 ppm CYA. It's not much below though, but it is getting riskier especially if the pool is rich in algae nutrients.

- - - Updated - - -

so at 3/30, 97.2 % of your FC is bound to CYA, but at 9/90, 99% is bound to CYA. So even though that means there is less instantaneous active chlorine as a % of your total FC, there is still enough FC quantitatively to outpace growth.

You still aren't wording this correctly. It would be better to say that regardless of how much FC is bound to CYA, all that matters is the active chlorine hypochlorous acid (HOCl) concentration. The amount of instantaneous active chlorine as a % of total FC is not relevant. What matters is the concentration of active chlorine, period. I grouped entries in that table at similar HOCl concentrations so those all have the same killing rates.

You said above that you really only need .3 ppm in a 0 CYA pool to stay above the breakpoint. I'm trying to read that chart and find where it shows that for the given levels, I have more than .3 ppm available as active chlorine. Would that be the HOCl/FC column? If so, it seems that you do get closer to that breakpoint (0.3ppm) the higher your CYA/FC.

Forget the "0.3 ppm at 0 ppm CYA" since that is the German DIN 19643 standard low-end with no ozonator. That isn't what we are using and it's overkill, especially for residential pools. What we are using is the algae inhibition level which with some safety margin is equivalent to 0.06 ppm FC with no CYA.

 

[BuckeyeChris]

so at 3/30, 97.2 % of your FC is bound to CYA, but at 9/90, 99% is bound to CYA. So even though that means there is less instantaneous active chlorine as a % of your total FC, there is still enough FC quantitatively to outpace growth.

You still aren't wording this correctly. It would be better to say that regardless of how much FC is bound to CYA, all that matters is the active chlorine hypochlorous acid (HOCl) concentration. The amount of instantaneous active chlorine as a % of total FC is not relevant. What matters is the concentration of active chlorine, period. I grouped entries in that table at similar HOCl concentrations so those all have the same killing rates.

I think that's what I meant to get at, ungracefully, that the actual physical number, the ppm of HOCl, is what matters, not what % of your FC is unbound.

so looking at the chart, if at CYA of 30 has a FC of 1.5, that is only 0.020 HOCl, but a FC of 3 is 0.042. Recommended minimum is 2. So it seems that the HOCl target for outpacing growth is around that 0.027 number you referenced. 0.030 maybe?

 

[chem geek]

Yes, you have that correct. The Chlorine / CYA Chart in the Pool School is a rounded version of this more detailed chart where you can more readily see that the minimum FC level at each CYA level is that which results in 0.03 ppm HOCl and that roughly corresponds to an FC/CYA ratio of 7.5%. The actual HOCl concentration for a given FC and CYA level at any pH and other conditions is given in the Pool Equations spreadsheet, but that is not designed for novice users. I originally created it to calculate a more accurate Calcite Saturation Index (CSI) and then added the chlorine/CYA relationship and then some other items along the way.

 

[morzh]

So, in short:

Approximately:
HCLO (hypochloric acid) equals half of the FC/CYA decimal ratio in ppm.
HCLO(ppm)=FC/(2*CYA)

example

FC=5ppm
CYA=70ppm
HCLO=0.036ppm.

(compared to 0.06ppm so FC needs to go up to about 9ppm to meet the minimum).

Correct?

 

[chem geek]

The first part you write is correct, but you misinterpreted what the minimum is in the latter part. The minimum HOCl is 0.03 ppm. The 0.06 ppm was the FC level so roughly double. So the minimum FC/CYA ratio is actually 7.5% since the HOCl = FC/(2*CYA) is just an approximation and varies a little with the ratio level itself, but for the most part you've got this right. So your "example" is at a reasonable FC for that CYA level (i.e. it does NOT need to go up to 9 ppm).

 

[morzh]

Aha, OK, cause I could not find HOCL level and decided that 0.06 was it.

So 5ppm FC at at 70ppm CYA is a good minimum.