If you look at the "Chlorine/CYA Relationship" section in the first post in the thread Certified Pool Operator (CPO) training -- What is not taught, you will find numerous references to the effects of CYA on the kill times of bacteria, viruses, protozoan oocysts, and algae as well as oxidation rates and even ORP levels. The algae paper is out-of-whack (for reasons I won't get into here), but the others are all in line with theoretical predictions based on the equilibrium chemistry.
The CDC is well aware of this, but most bacteria are VERY easy to kill so the slower kill times aren't really a problem. As shown in the chart in this post, you can see that even with CYA in the water and an FC that is roughly 10% of the CYA level, the common pathogenic fecal bacteria are 99.9% killed (3-log reduction) in under 1 minute. Even Legionella pneumophila which is more chlorine resistant is 99.9% killed in one hour, but more importantly that implies a 50% kill in under 7 minutes which is faster than the 15-60 minutes it takes for bacteria to reproduce in ideal conditions. As for viruses, they do not reproduce so the longer kill times are also reasonable. Of course, an FC of 1 ppm with a CYA of 100 ppm would have kill times around 10 times slower, yet such FC and CYA ranges are allowed even in most outdoor commercial/public pools in the U.S. In practice, high bather-load pools have higher monochloramine levels and that somewhat increases disinfection rates (and helps with algae prevention as well). At the other extreme, most indoor commercial/public pools have no CYA at all yet have FC of at least 1-2 ppm if not more so have at least 10-20 times the active chlorine concentration.
So the primary issue with slower kill times would be for person-to-person transmission of disease. However, having higher chlorine levels to get to faster kill times has the side effect of more disinfection by-products and faster oxidation of swimsuits, skin, hair, corrosion of equipment, etc. The main problem is that ALL of the pool and spa regulations today including the more recent APSP-11 guidelines give separate broad FC and CYA ranges rather than using the FC/CYA ratio as a proxy for active chlorine level. I tried my best to change that, but I was late to the APSP-11 public comment review period and was not on the committee anyway so had little influence. Quite frankly, allowing 1 ppm FC with 100 ppm CYA for the outdoor pools while recommending against CYA for indoor pools which have 1-2 ppm FC or higher is absolutely ridiculous with at least a factor of 1000 difference in active chlorine levels! In spite of such absurdity, trying to get the pool/spa industry to follow the basic chemistry (validated by many scientific peer-reviewed studies) has been a frustrating and so far completely fruitless task.
The CDC is well aware of this, but most bacteria are VERY easy to kill so the slower kill times aren't really a problem. As shown in the chart in this post, you can see that even with CYA in the water and an FC that is roughly 10% of the CYA level, the common pathogenic fecal bacteria are 99.9% killed (3-log reduction) in under 1 minute. Even Legionella pneumophila which is more chlorine resistant is 99.9% killed in one hour, but more importantly that implies a 50% kill in under 7 minutes which is faster than the 15-60 minutes it takes for bacteria to reproduce in ideal conditions. As for viruses, they do not reproduce so the longer kill times are also reasonable. Of course, an FC of 1 ppm with a CYA of 100 ppm would have kill times around 10 times slower, yet such FC and CYA ranges are allowed even in most outdoor commercial/public pools in the U.S. In practice, high bather-load pools have higher monochloramine levels and that somewhat increases disinfection rates (and helps with algae prevention as well). At the other extreme, most indoor commercial/public pools have no CYA at all yet have FC of at least 1-2 ppm if not more so have at least 10-20 times the active chlorine concentration.
So the primary issue with slower kill times would be for person-to-person transmission of disease. However, having higher chlorine levels to get to faster kill times has the side effect of more disinfection by-products and faster oxidation of swimsuits, skin, hair, corrosion of equipment, etc. The main problem is that ALL of the pool and spa regulations today including the more recent APSP-11 guidelines give separate broad FC and CYA ranges rather than using the FC/CYA ratio as a proxy for active chlorine level. I tried my best to change that, but I was late to the APSP-11 public comment review period and was not on the committee anyway so had little influence. Quite frankly, allowing 1 ppm FC with 100 ppm CYA for the outdoor pools while recommending against CYA for indoor pools which have 1-2 ppm FC or higher is absolutely ridiculous with at least a factor of 1000 difference in active chlorine levels! In spite of such absurdity, trying to get the pool/spa industry to follow the basic chemistry (validated by many scientific peer-reviewed studies) has been a frustrating and so far completely fruitless task.
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