Pathogen kill times with and without cyanuric acid

[09659]

I'm looking for information that lists the kill times of various pathogens with and without cyanuric acid.

For example,

Cryptosporidium
Giardia
Shigella
Escherichia coli
Norovirus
Adenovirus
Hepatitis A
Pseudomonas aeruginosa
Legionella pneumophila
Other bacteria, viruses, and protozoa.

I want to use this information for customers that are resistant to the idea of diluting pool water to reduce high cyanuric acid levels. There seems to be the perception that "If it's clear, it must be good."

Thanks

 

[JasonLion]

"with and without cyanuric acid" doesn't really mean anything. Adding CYA changes the effective chlorine level, and the kill times change in direct correlation with the effective chlorine level. So are you assuming without CYA compared to with CYA but with FC adjusted to correct the effective chlorine level to what it was before (which is what we recommend doing), or what?

 

[09659]

For example, a fixed free chlorine level of 1 ppm, with no cyanuric acid, 30 ppm cyanuric acid, and 300 ppm cyanuric acid.

Thanks

 

[chem geek]

This post has a column of the time it takes in minutes for a 3-log (99.9%) reduction of pathogens with the equivalent of 0.1 ppm FC with no CYA which is roughly equivalent to an FC that is around 10% of the CYA level. You can scale the times proportionately to the FC/CYA ratio since that ratio is proportional to the active chlorine level. So if someone has 3 ppm FC with 300 ppm CYA, then the kill times are roughly 10 times longer than shown in the table (where the table is equivalent to 3 ppm FC with 30 ppm CYA, for example).

However, it's not really fair to try and scare people with the pathogen kill times when their CYA is high since the risk for disease is very low in residential pools even at high CYA levels. The person-to-person transmission of disease risk is increased and is why commercial/public pools have tighter limits, but the risk in residential pools is low. Even with a small amount of chlorine and a high CYA level most bacteria are killed faster than they can reproduce. It's algae that would tend to show up. If in the pools you are talking about there is no algae or cloudiness, then the algae nutrient level (phosphates and/or nitrates) may be very low or there may be use of an algaecide.

When we recommend against using a copper/silver ion combination, for example, it's not so much because of the slower kill times as shown in the comparison table as it is for the risk of metal staining though both points are valid. Copper-only is more of a problem because it does not address fecal bacteria, but the risks for transmitting viruses or protozoan oocysts in a residential pool are very low. Again, a commercial/public pool where one sick person can infect dozens or hundreds of others is quite a different matter. And of course metal ions alone do not oxidize bather waste so you would eventually be soaking in your own sweat and urine.

 

[09659]

With proper sanitization, phosphates and/or nitrates are irrelevant, correct? Because if there's no algae, it can't feed on phosphates and/or nitrates.

It seems I should mention algae prevention as the main reason for dilution.

 

[chem geek]

You are correct that with proper chlorine levels the phosphates and/or nitrate levels are irrelevant. Algae prevention is the main reason for lowering the CYA level. For higher bather loads, then faster chlorine oxidation of bather waste is another reason (so that's important in residential spas since they are typically higher in bather load than pools). You can certainly say truthfully that pathogen kill rates are slower with the higher CYA -- just don't make it sound like it's high risk. Chlorine effectiveness (active chlorine level) is reduced at higher CYA levels unless the FC is proportionately raised -- that's the bottom line.