I don't understand the need to use CyA

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K

Knetsel

Member
Jan 1, 2022
24
France
Pool Size
14500
Surface
Vinyl
Chlorine
Liquid Chlorine
Hello,

There must certainly be a few (many ?) places here where this is discussed in depth.
Could you please show me the way to the most relevant ones ?
Thank you in advance.

Nota : pool in France, pH and chlorine controlled via peristaltic pumps, pH and ORP probes, sulfuric acid (PVC liner) and bleach
 
The primary reason to use CYA is to reduce the harsh effects of hypochlorous acid (HOCl) which is formed when chlorine is dissolved in water. A well managed pool only needs about 50-100ppb of HOCl to control pathogens and algae. That’s 0.1 to 0.2ppm FC. It’s very difficult to control such a low level of FC without a chemical buffering species that holds most of the chlorine in reserve (bound to a CYA molecule) especially in an outdoor pool where the UV extinction rate can be very high (half life of unstabilized chlorine is less than 30mins).

Can one not use CYA and deploy a positive feedback system to measure and dose liquid chlorine to keep up with demand? Sure. But the equipment is expensive and finicky and prone to failure. You will almost always be in a situation where the hypochlorous acid levels are over-dosed relative to what is needed which will make the water harsh to eyes, skin, and bathing suits. You will also be at risk of developing much more potent combined chlorine compounds and THM’s because the formation reactions are much more favorable.

In short, you are dancing on the head of pin as the saying goes and much more likely to lose control of the sanitation which will result in either and algae bloom or, worse, biofilm formation in the plumbing that will then lead to bacterial reservoirs of fecal coliforms and other nasty pathogenic bacteria.

Having a reserve buffer of chlorine at all times makes the water much more comfortable to swim in and gives enough margin of safety to effectively manage a pool without a lot of burdensome equipment.

Your pool, your choice.
 
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Knetsel said:
Hello,

There must certainly be a few (many ?) places here where this is discussed in depth.
Could you please show me the way to the most relevant ones ?
Thank you in advance.

Nota : pool in France, pH and chlorine controlled via peristaltic pumps, pH and ORP probes, sulfuric acid (PVC liner) and bleach
Click to expand...
If you’re having problems getting a predictable chlorine level you may want to consider getting rid of the ORP probe.
I tried using one for a while and it didn’t work for me. I know I’m not the only one.
 
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Hello,
Thank you for your answers which helped me understand the situation a bit better.
These answers, however, raise a few other questions to me :
1 - you s
 

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Hello

Thank you for your answers which helped me understand the subject a bit better.
These answers, however, raise a few other questions to me :

1 - You say that 0.05 ppm of HClO is enough to control pathogen and algae an overdosing of chlorine would make the water harsh to eyes, skin, and bathing suits.
The minimum and maximum values for HOCl in drinking water that I found in the technical literature range from 0,1 ppm to 5 ppm ! (CDC, EPA, WHO); up to now I admitted that these values could be used for swimming pool as well without damage to bathers and there suits as this is what we drink and use for washing. Is there another reliable source for these data ?

2 - It's very difficult to control such a low level of FC without a chemical buffering species that holds most of the chlorine in reserve and a positive feedback system to measure and dose liquid chlorine to keep up with demand is expensive and finicky and prone to failure.
Even with an SWG or a dosing pump, with or without feedback ? My dosing pump with ORP feedback has been running very fine for 10 years with a target value of 700 mV adjusted from time to time depending on measurement results from my photometer.
May be CyA could be used for instance at around 10 ppm in a pool where one would drop 1 to 2 g/m3 of équiv. Cl2 of bleach every morning. Even then, I have always been convinced that a simple peristaltic pump with flow control and timer would do as well if not better. Something I might test to confirm my idea (or not..).

3 - Cyanuric acid protects chlorine from degradation by UV light especially in outdoor pool where the UV extinction rate can be very high (half life of unstabilized chlorine is less than 30mins).
As chem geek states : The presence of cyanuric acid causes chlorine to become bound to it and held in reserve. Because of this, there is much less active chlorine (hypochlorous acid and hypochlorite) present in the water. Since there is less active chlorine, there is less chlorine to react with UV and, ultimately, lose.
Reading John A. Wojtowicz (JSPSI - V. 2, N. 1, pp. 34–41), I understood that it is mainly OCl - that is dissociated by UV and that the quantity of dissociated OCl- is proportional to its concentration in the water (everything else being constant : pH; temperature, TDS, CyA, ..). We should than have an exponential decrease of OCl- when there is no CyA and simultaneously observe an exponential decrease of FC as thetherelationship between both fits quite well with something like [FC ~ a * [OCl-] ^ b. Moreover, I feel that an exponential decrease of FC should very likely be observed as well when there is a constant CyA level etc. To be checked...
 
Knetsel said:
Hello

Thank you for your answers which helped me understand the subject a bit better.
These answers, however, raise a few other questions to me :

1 - You say that 0.05 ppm of HClO is enough to control pathogen and algae an overdosing of chlorine would make the water harsh to eyes, skin, and bathing suits.
The minimum and maximum values for HOCl in drinking water that I found in the technical literature range from 0,1 ppm to 5 ppm ! (CDC, EPA, WHO); up to now I admitted that these values could be used for swimming pool as well without damage to bathers and there suits as this is what we drink and use for washing. Is there another reliable source for these data ?
Click to expand...

Hi Knetsel and welcome TFP’s forums,

I am under the impression that it is free chlorine, FC, that is regulated by the various authorities. I am unaware of any test that directly tests hypochlorous acid, HOCl.

Knetsel said:
2 - It's very difficult to control such a low level of FC without a chemical buffering species that holds most of the chlorine in reserve and a positive feedback system to measure and dose liquid chlorine to keep up with demand is expensive and finicky and prone to failure.
Even with an SWG or a dosing pump, with or without feedback ? My dosing pump with ORP feedback has been running very fine for 10 years with a target value of 700 mV adjusted from time to time depending on measurement results from my photometer.
May be CyA could be used for instance at around 10 ppm in a pool where one would drop 1 to 2 g/m3 of équiv. Cl2 of bleach every morning. Even then, I have always been convinced that a simple peristaltic pump with flow control and timer would do as well if not better. Something I might test to confirm my idea (or not..).
Click to expand...

ORP is not in itself a sanitation product. I have used ORP control and am not convinced that it is a viable option for the average pool owner that struggles to take a water sample to the pool shop once a month. Without CyA pH becomes a critical factor that needs to be controlled with a specific range. ORP and pH control is beyond the average pool owner.

All chemical species in a pool contribute to the ORP value. Changes to any one species will change the ORP control set point. I believe ORP is better suited to commercial applications.

I‘ve owned and used two photometers over the years and don‘t trust the results they provide.

10ppm of CyA is not possible without very expensive lab grade equipment. At 10ppm you loose the benefit of protection from UV degradation that starts at and above ~25ppm.

Knetsel said:
3 - Cyanuric acid protects chlorine from degradation by UV light especially in outdoor pool where the UV extinction rate can be very high (half life of unstabilized chlorine is less than 30mins).
As chem geek states : The presence of cyanuric acid causes chlorine to become bound to it and held in reserve. Because of this, there is much less active chlorine (hypochlorous acid and hypochlorite) present in the water. Since there is less active chlorine, there is less chlorine to react with UV and, ultimately, lose.
Reading John A. Wojtowicz (JSPSI - V. 2, N. 1, pp. 34–41), I understood that it is mainly OCl - that is dissociated by UV and that the quantity of dissociated OCl- is proportional to its concentration in the water (everything else being constant : pH; temperature, TDS, CyA, ..). We should than have an exponential decrease of OCl- when there is no CyA and simultaneously observe an exponential decrease of FC as thetherelationship between both fits quite well with something like [FC ~ a * [OCl-] ^ b. Moreover, I feel that an exponential decrease of FC should very likely be observed as well when there is a constant CyA level etc. To be checked...
Click to expand...

Not sure where you’re going with this. I don’t believe that FC loss via OCl- loss to UV is exponential. All species are in constant equilibrium and are constantly shifting in concentration as any one undergoes a change in concentration.
 
Knetsel said:
Hello,

There must certainly be a few (many ?) places here where this is discussed in depth.
Could you please show me the way to the most relevant ones ?
Thank you in advance.

Nota : pool in France, pH and chlorine controlled via peristaltic pumps, pH and ORP probes, sulfuric acid (PVC liner) and bleach
Click to expand...

In very basic terms it seams illogical to me not to adopt or embrace such a simple method of reducing FC loss to UV when we are spending hard earned money to buy or produce chlorine. In terms of of toxicity CyA is similar to magnesium. And in terms of simplicity for the average pool owner, CyA at 50 - 80ppm is much easier than complicated and expensive process control that ultimately requires regular water testing to confirm correct operation anyway. The real question is not why would you, but rather, why wouldn’t you use stabiliser or CyA.

Edit: Be careful with that sulfuric acid it’s more dangerous to handle than hydrochloric. Constant use of sulfuric will cause an increase in sulphate ions which is better to avoid. Hydrochloric adds nothing that’s not already in pool water.
 
Last edited:
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To put into more simple terms.
It would be difficult to maintain that minimum of chlorine in the pool unless you are constantly measuring the level in the water and adjusting the feed.
The Sun’s UV burns off chlorine very quickly, within a couple of hours. It would take a lot of chlorine to maintain it.
CYA keeps the UV from burning it off so quickly reducing how much it takes to maintain level required.
CYA also acts as a buffer holding a quantity and as chlorine is ‘used up’, it releases some to maintain the level of free chlorine, as buffers naturally do.
So, overall, with the proper amount of CYA, your chlorine usage will reduce, less cost to maintain.

I’ve seen requirements by Pennsylvania and other states as guides for level of chemistry to maintain in public pools. These would be good guides for any pool, same with the guides here in this forum. There is likely a similar guide in France or the EU for pools if one does some searching.
 
AUSpool said:
In very basic terms it seams illogical to me not to adopt or embrace such a simple method of reducing FC loss to UV when we are spending hard earned money to buy or produce chlorine. In terms of of toxicity CyA is similar to magnesium. And in terms of simplicity for the average pool owner, CyA at 50 - 80ppm is much easier than complicated and expensive process control that ultimately requires regular water testing to confirm correct operation anyway. The real question is not why would you, but rather, why wouldn’t you use stabiliser or CyA.

Edit: Be careful with that sulfuric acid it’s more dangerous to handle than hydrochloric. Constant use of sulfuric will cause an increase in sulphate ions which is better to avoid. Hydrochloric adds nothing that’s not already in pool water.
Click to expand...
Before TFP I used to read a lot on the Orenda site.
They would talk about how the contact time is a factor when dealing with CYA. That being the time taken to eradicate certain contaminants. How there were CYA limits placed on commercial pools, about 15 ppm I think, to keep the contact time within specifications.
My question is, if the correct Cya/FC ratio is used then the correct amount of HOCL is in the pool so why would the CYA affect contact time?
 
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Contact time is for commercial operators, commonly associated with fecal or biological accidents. It is the time required to kill a specific pathogen at a given hypochlorous acid concentration. Commercial pools need to limit CyA so they can raise their FC and hypochlorous acid high enough to kill their target pathogens within the shortest possible contact time in order to keep the pool open. Cryptosporidium oocysts from the link above are considered the hardest nasties to kill for commercial operators, domestic pool owners don't need to go to this extreme. If the FC/CyA ratio is maintained at or above 7.5% there will always be enough sanitiser with a healthy surplus to the kill the pathogens most likely to be found in a domestic pool. With a ratio at or above 7.5% the CyA concentration will not affect contact times, in effect the appropriate contact time or kill rate has been factored in when establishing the the 7.5% ratio.
 
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Unless you know someone has been in your pool with crypto or any of the other critters, it’s not likely a problem to the typical home pool. Otherwise, it would’ve been in the news everyday.
Because the public pool keepers don’t know who’s in their pool, when when they find yuckey things floating, everybody is chased away, the pool is shut down for a day or two, and they put everything but the kitchen sink in to kill the yuckeys.
Just keep the numbers you see here, and algae and all the other critters will be kept at bay.
 
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