Chlorine/CYA Chart

[Freelancer]

They do not teach the chlorine / CYA relationship in the NSPF CPO nor the APSP TECH courses nor any other courses that I am aware of. I'm trying to get that changed, starting with the NSPF CPO course and I've written about that in this thread where that's just one of several topics missing or incorrect in the course (or at least the CPO Handbook since I have that, but have not taken the course). I've given my feedback to NSPF (including extensive comments and new tables and new or changed text) and we'll see if anything changes.

Richard,
I recently attended a pool seminar where the recommended FC level was 1-2 PPM in a pool with 50 - 70 PPM of CYA with 70 being the ideal. I asked the lecturer where is the chlorine to sanitise the pool? He said the 1 - 2 PPM of FC, when I stated that this chlorine was going to be bound to the CYA and that 1 - 2 PPM of FC was not suffiecient for a CYA level of 70 PPM and surely he needed more FC to ensure adequate sanitisation he stated that any higher FC would be detrimental to the swimmers. I also mentioned that I run all my pools at between 7% - 10% of the CYA level. He stated that this was far too high and the swimmers would get pool itch and some burning, when I mentioned that I run quite a number of large communal pools at these levels and have done so for some time without a single complaint and infact I get a lot of comments of how nice the water is to swim in, I got the impression that he did not believe me.

He then quizzed me about my qualifications. I replied that I have all the qualifications that I need, real world experience, happy customers and fantastic looking pools.

I do not think that chlorine / CYA relationship is taught here in Europe either.

 

[chem geek]

Please show your instructor the 1974 O'Brien scientific paper and have him read the first part of the INTRODUCTION and the SUMMARY at the end of the paper. Then have him look at the CPO post I wrote including the file of my comments that has charts that may be useful to him and note all of the links to scientific references in my post that he should also look at.

He is simply misinformed because manufacturers have intentionally withheld materially important information that consumers and pool operators need to know to make informed purchase and operational decisions. It's not his fault, but if he continues to teach inaccurate information after you have given him the truth, then he will be responsible for it from this point forward.

 

[Freelancer]

I will certainly show him the paper and your rather excellent CPO post which I found to be most informative. After he has read them I will let you know his thoughts on them. Thanks once again for all your great work.

 

[Brakel]

Interesting reading, thanks.

Some thoughts, particularly for constant algae battlers.
Algae requires light and nutrients (P04 and or N03), blocking all light is not possible but can help while treating, while ever organics are added you will always have nutrients, and algae will grow even with nutrient levels lower than testable with common kits. Hence FC is requires or some other inhibitor. However we can reduce these requirements by reducing nutrients, the challenge is to do it cost effectively.
Some ideas, has any one looked into the use of zeolites?, do people test there top up water for nutrients, what about anaerobic filtration for Nitrates, regeneratable ion exchange resins for Phosphates.
I'm thinking more for those that suffer regular algae blooms, bacterial (heterotrophic) blooms can also be reduced by reducing organics, autotrophice bacteria is a different in that it can use no organic energy, so some level of FC, ozone, etc is going to be required.

 

[chem geek]

As the tens of thousands of pool homeowners on this forum and on The PoolForum can attest, you can prevent algae growth using chlorine alone if it is maintained at an appropriate level relative to the CYA level. This works for almost all pools with a failure rate of perhaps 1 in 5000. Once you understand the FC/CYA relationship, you no longer are a constant algae battler.

As for nutrients, the only one that is somewhat easily lowered is the phosphate level, but only for inorganic phosphate (known as orthophosphate). There is no easy way to lower nitrates or organic phosphates. Anaerobic filtration might be practical for an aquarium, but not for the large volumes of a pool. The same is true for ion exchange resins -- OK for smaller quantities of water, but not for pool volumes. One can use Reverse Osmosis (RO) to remove virtually everything in a pool and this is one way to reduce CH as well as CYA, for example, but it's not something to be done frequently as it isn't cheap.

Zeolite does not remove algae nutrients. A fine filtration filter, such as DE, can help remove organic precursors, but that doesn't necessarily lower algae nutrients depending on what is getting into the pool.

Bacteria are even less of a problem than algae in properly maintained pools since they are much easier to kill. So maintaining proper FC/CYA ratios prevents algae growth and also kills bacteria quickly. I have had over 3000 ppb phosphates in my pool as have others and yet algae growth is prevented through maintaining a proper FC relative to the CYA level.

You seem to be looking for a solution to a problem that does not exist when one properly maintains their pool understanding the FC/CYA relationship. Now high bather-load pools such as many commercial and public pools could use supplemental oxidation in order to oxidize organic precursors before chlorine gets to them since that would reduce disinfection by-products and also lower chlorine usage. However, in outdoor residential pools, the bather loads are so low and the sunlight and air circulation are good enough that disinfection by-products are much, much lower. Also, most chlorine loss in residential pools is due to sunlight whereas in commercial/public pools it is due to bather load.

 

[Brakel]

You seem to be looking for a solution to a problem that does not exist when one properly maintains their pool understanding the FC/CYA relationship

Agree, I was assuming from my short time and reading here that some people might be having problems, this may not be the case or a case of poor FC/CYA management.

 

[Brakel]

As for nutrients, the only one that is somewhat easily lowered is the phosphate level, but only for inorganic phosphate (known as orthophosphate). There is no easy way to lower nitrates or organic phosphates. Anaerobic filtration might be practical for an aquarium, but not for the large volumes of a pool. The same is true for ion exchange resins -- OK for smaller quantities of water, but not for pool volumes. One can use Reverse Osmosis (RO) to remove virtually everything in a pool and this is one way to reduce CH as well as CYA, for example, but it's not something to be done frequently as it isn't cheap.

Zeolite does not remove algae nutrients. A fine filtration filter, such as DE, can help remove organic precursors, but that doesn't necessarily lower algae nutrients depending on what is getting into the pool.

Simply for the purpose of the discussion, not to suggest its a better method.
If a pool was high in Organic Phosphates, it could be fixed over the closed season. Grow algae and harvest, this would reduce nitrates also.
A set up similar to a sand filter with flow or media that has anaerobic states would maintain nitrates,
Of cause filling the pool via RO is ideal if water wastage and costs are not an issue, prevention is better than a cure.

Zeolites can take up Ammonia, therefor there is less nitrite>nitrate. It would also have low oxygen states with in the rock so would provide anaerobic conditions to aid in reduction of Nitrate to Nitrogen via bacteria.

There are many other methods to reduce algae, these are just ideas that I have not seen mentioned so I put them out for food for thought for the more experienced pool enthusiest, I'm happy to start my pool husbandry learning curve of with traditional pooven methods.

 

[chem geek]

Zeolites can take up Ammonia, therefor there is less nitrite>nitrate. It would also have low oxygen states with in the rock so would provide anaerobic conditions to aid in reduction of Nitrate to Nitrogen via bacteria.

This is simply not true in a pool with chlorine in it. Chlorine combines with ammonia VERY quickly to form monochloramine, in a matter of seconds if there is no CYA to a matter of a minute or two when CYA is present, such that ammonia will never reach the zeolite filter. The zeolite will not absorb nor breakdown monochloramine and the release of ammonia from monochloramine is so slow that the monochloramine will get fully oxidized by chlorine (mostly complete in a matter of hours) long before enough ammonia is released from monochloramine to get absorbed into the zeolite filter. This whole marketing campaign for zeolites saying "no ammonia, no chloramines" is just bunk -- not that it isn't true, because without ammonia one can not have chloramines, but that it is terribly misleading because the ammonia doesn't survive long enough to make it to the zeolite filter.

As for bacteria converting nitrate to nitrogen, again this won't happen in a chlorinated pool because the chlorine will kill bacteria -- that's the whole point of chlorinating the pool.

 

[twilson]

I scanned all the replies to this post on the Chlorine/CYA chart and didn't see an answer to the following question. In other parts of the site, and in the instructions for my test kit, it says that a good rule of thumb for shocking ("superchlorinating") pools is to raise FC to 10 times CC. Yet, this chart suggests a shock level that, while dependent on CYA, is independent of CC (unless I'm reading it wrong). Can you explain?

 

[JasonLion]

The whole raise FC by 10 times CC story is a myth, one of many that float around the pool industry. It is loosely based on the fact that it takes about nine times as much chlorine as ammonia to break down ammonia in the water. CC isn't ammonia, and it doesn't take anywhere near nine times the chlorine to break CC down. Even if it was ammonia, there is no need to add the chlorine all at once. Adding chlorine in batches works just as well and is safer for the pool. A shock level based on the CYA level is about maintaining an FC level that is high enough to be effective and low enough to be as practical as possible and safe for the pool.

 

[chem geek]

See this post for technical details. The industry got this one wrong, not just a little bit, but way, way, way off. A classic example of not paying attention to differing units of measurement, in this case between ammonia expressed as ppm N vs. chlorine expressed as ppm Cl₂. On top of that, monochloramine already has one of the needed chlorine attached to it. And as Jason wrote, oxidation of CC is continuous and doesn't get stuck -- so long as there is measurable FC the oxidation will continue. The higher the active chlorine (roughly proportional to the FC/CYA ratio), the faster such oxidation occurs, hence the shock level we set that usually gives a decent RATE of such oxidation (and algae kill/clearing rate).

 

[houstonTex]

Re:

There are several factors that enter into the rate at which algae grows. Algae requires sunlight, food (mostly phosphates but also some other things), low chlorine levels, etc. Any one of those factors can constrain the rate of growth. Poor water circulation enters in because algae can sometimes consume all of the chlorine in one local area allowing the algae enough time to form a biofilm in that area and thus resist higher chlorine levels in the future. Good circulation keeps the water mixed up enough to help prevent the biofilm from forming. Under ideal growth conditions algae can grow very quickly, turning a pool green in two or three days. But conditions are rarely ideal.

Curious as to why - (given their need for direct sunlight) - do mustard algae grow in the shade?

 

[chem geek]

Re: Re:

Curious as to why - (given their need for direct sunlight) - do mustard algae grow in the shade?

Algae are like single-celled plants in that nearly all use photosynthesis so require light (they are sometimes classified in their own kingdom, but I won't get into that here). Like land plants, some grow better in more sun and some grow better in more shade. Each species developed in specific conditions of sunlight and is optimized for that level of sun. If there is no light at all, the algae will die, but if there is more than it has evolved to expect to be "optimum" then it can slow down in growth or even die if there is more damage from the UV in sunlight that isn't getting repaired. For land plants, too much sun can dry them out, but obviously that's not a problem for algae that are in the water itself.

 

[randyb123]

I created the chart in a pdf so it is easy to print, cut, and laminate.View attachment CYA_Chlorine.pdf

 

[chem geek]

I created the chart in a pdf so it is easy to print, cut, and laminate.View attachment 31581

Welcome to TFP!

Most people use the Chlorine / CYA Chart in the Pool School, but thanks for making the PDF for those who want the more detailed chart for non-SWCG pools.

 

[Catanzaro]

I have been reading this particular post during the evening and have one simple question (Please see below). I fully understand the CYA/Chlorine Chart. Outside high CYA levels being a problem during the SLAM process, is it not the daily chlorine consumption that matters.


If my CYA is 40, I have a minimum of 3 and Target of 5
If my CYA is 50, I have a minimum of 4 and Target of 6

All the way up to a CYA of 60, there is a 2 ppm difference. Once we get between 70-90 CYA, the difference is 3 ppm. At 100 CYA levels the difference is 5 ppm.

Assuming my daily consumption is 2 ppm, does it really matter whether I am at 30 or at 60 (or anywhere in between). Or am I over thinking this relationship. Does chlorine burn off faster at higher CYA levels? Or will chlorine burn off at the same rate, whether it be 30 or 60? Does chlorine have a higher stabilization rate @ any levels?

 

[Smykowski]

At higher FC levels, you lose a higher ppm per day. However, the rate of loss stays about the same, which is usually 30-50%.

I wouldn't worry too much about the "target" number. It's a suggestion. The much, much more important number is the minimum. Every pool is different (size, bather load, temperature, sun exposure, etc.), so it's impossible to predict what any particular pool will lose in a day.

The goal is figure out what your pool will lose in a day, and plan so that your lowest FC level at any point of the day never goes below the minimum for your CYA level. Your personal "target" will lilely be different from thw chart. Your "minimum" is hard and fast, and use that as the basis for all of your planning.

 

[chem geek]

See the chart in this post for a rough idea of how higher CYA levels even with proportionately higher FC levels results in lower chlorine loss from sunlight. So the loss isn't always 2 ppm -- that loss is dependent on the CYA level even for the same FC/CYA ratio. We aren't sure exactly why this occurs, whether it's some sort of CYA shielding effect of UV or something else, but the effect is real and seen in both pools and in bucket experiments that were done.

In theory, if chlorine losses were only related to unbound chlorine, then the losses should be the same for the same FC/CYA ratio regardless of CYA level, but that's not what is actually seen.

 

[Catanzaro]

At higher FC levels, you lose a higher ppm per day. However, the rate of loss stays about the same, which is usually 30-50%.

If this is the case then it may make sense to keep the lowest CYA possible in your pool. The savings on liquid chlorine may also be negligible during the season making it a mood point. It may be better to stay at the 50 CYA level and then if during the course of the year the level fall near the bottom range, then stabilizer can be added. I will understand more about my pool next season. I keep a daily log of additions and only perform the extended tests 1X per week. I like being safe so I always shoot for slightly higher minimums and target FC based on my CYA. I do not wish to deal with an ALGAE problem unless it is in my cards.

 

[chem geek]

The % loss rate does NOT stay the same at all CYA levels. For a given CYA level, the loss is a percentage so higher FC levels have higher absolute FC loss, but at higher CYA levels the percentage FC loss is lower. That's what the chart I linked to roughly showed and is why SWG pools are recommended to be at 80 ppm so that the SWG on-time percentage can be turned down, in spite of the proportionately higher FC levels (i.e. same FC/CYA ratio).

This post from Mark describes the bucket test he did that confirmed that at a CYA level of 45 ppm he saw 50% FC loss per day while at 80 ppm it was only a 15% FC loss. The chart I showed you uses numbers gleaned from averages from pools as well as Mark's experiment so aren't identical to his experiment.

Now you may have less sunlight on your pool so that a lower CYA level is acceptable to you in terms of the total amount of chlorine you are using, but if you want to use less absolute amounts of chlorine, then a higher CYA level with the same FC/CYA ratio will do that for you. The risk is that at a higher CYA level IF you ever get algae by having the FC drop too low then it takes more absolute amounts of chlorine to fight it since the shock FC level is higher at a higher CYA level. This is why we normally recommend around 50 ppm CYA for manually dosed pools. Nevertheless, some people go higher, up to 80 ppm, in very hot sunny areas and they just remain diligent about not letting the FC go below the minimum for their CYA level.