Cyanuric acid is a chemical that has created a battleground of differing opinions over the years.
The following is from the Queensland Health Water quality guidelines for public aquatic facilities December 2019.
Interestingly, in the recommended Free Chlorine levels in the table above, we need a minimum FC level of 1ppm when there is no cyanuric acid in the pool and 2ppm when there is cyanuric acid in the pool. Why?
You will also find in swimming pool documentation that if a pool contains too much cyanuric acid, then it becomes almost impossible to treat Cryptosporidium. In the Queensland Health Guidelines, you need to remove water from a pool and replace it with fresh ($) when the cyanuric acid is too high until you reach a level of 15 ppm or less before you can start to kill off the cryptosporidium.
It should be mentioned here that cryptosporidium is dangerous for young children. The severe diarrhea attack that the bug causes will dehydrate anyone, and this is a big problem for small bodies, which has resulted in death in the past. As a pool manager you need to have a management process ready to go, a button to push, should you be confronted with the problem. This is covered in the Queensland Heath Guidelines.
The CDC in the USA has mandated the 15 ppm is the maximum level of cyanuric acid allowed in a pool.
Several years ago, while operating a pool service business we made the decision to drop our cyanuric acid level to between 10 and 20 ppm, even though there was good money to be made selling cyanuric acid (called ‘Stabilizer’ or ‘Sunscreen”). We were previously aiming at 70 ppm. The pools improved out of sight.
Further evidence that high cyanuric acid levels cause problems for a pool became clear to us when dealing with green pools. We learnt that the first thing to do when treating a green pool was to test the cyanuric acid level. If the cyanuric acid level was too high then it was cheaper and quicker to empty, refill and rebalance the pool.
Apparently, the half-life of chlorine is 45 minutes. The following shows the effect the sun has on a pool where the cyanuric acid level was 3ppm. The pool is about 300,000 liters and not heavily used. Chlorination is done using a 400g Saltwater Chlorinator. The trend curve with dosing information was downloaded from an Aquarius Ultima Chemical Controller during a period of changing weather conditions. The Solar Exposure data was downloaded from Bureau of Meteorology web site.
Pretty amazing isn't it. It would be hard to argue against the relationship between the sun and chlorine consumption.
For domestic pools where the filtration only runs for 8 hours or less per day cyanuric acid is an essential.
Pools that have 24-hour dosing (and that’s pretty much every other type of pool) need to be, I think looked at differently.
Cyanuric Acid locks up Free Chlorine. When you add it to a pool that is managed using a dosing system that runs off an ORP level, the ORP will initially drop while the stabilizer sucks the chlorine out of the pool. The system will keep dosing until it reaches the set point. Your DPD test result will show a higher level of chlorine than it did previously at the same ORP. This is very important when establishing the ORP-Free Chlorine Relationship for a pool.
If your dosing system can maintain the set point such that the ORP drop is less than 20mV, then is the cyanuric acid doing anything? This will more likely occur when dealing with a liquid chlorine pool with a decent size chemical pool. At what level does the stabilizer release chlorine back into the pool?
On the other hand, if the pool is dosed by using a Saltwater Chlorinator the ORP will usually drop somewhat lower since a chlorinator can only dose at a set rate. This link shows some math on this. Maybe the cyanuric acid is of some use when faced with this type of dosing.
When you are responsible for the operation of a pool you need to know that the active chlorine level in the pool is at an acceptable level. Since chlorine demand will vary throughout the pool, I believe that a small amount of cyanuric acid will assist,
Studies have shown that for every 20ppm CNA you will need an increase in FC of 1 ppm to maintain the same level of sanitation. Another method is to use the CAN to FC ratio. This has been worked out to be 7.5%. In other words, the minimum FC level required in a pool is 7.5% of the CNA level.
I currently believe that if you think there is a need to use cyanuric acid then keep it below 10ppm.
I will continue to look for a more complete answer to this question.
For further information I recommend the following site, which I was very relieved to find as it confirmed what I was seeing as well as being the source of the above formula.
Richard Falk - the Chem Geek
