Accounting for CYA / ratio

[JohnA902]

So I have been searching for topics on what to do if a kid poops in the pool. ( hypothetical, thank goodness this hasn't happened). I found a bunch of good stuff, one link led me to the CDCs site which suggests "raising free chlorine to 2ppm for 30 min"

That is confusing.

Clearly they aren't accounting for cya. So how would one scale up their 2 ppm for a certain cya level. I'm familiar with the chlorine / cya chart so I know my own recommended levels but I was curious how this 2 would compare. Is 2 the CDCs shock level ignoring cya?

I'm having trouble expressing my question. I guess I'm looking for a ratio? Or what assumption is the CDC making about cya? Then I could do the following ratio:

cdc's cya / 2 ppm = my 40 cya / x ppm

maybe the answer is right in that chlorine / cya chart but I'm just not connecting it.

Thoughts?

john

 

[ned8377]

If some kid had diarrhea in my pool I would just dump a gallon of 12.5% bleach in. Then check FC and CC the next morning.

 

[Richard320]

I think what you want is in this graph. Note that the scales are logarithmic.

There's more to confound and amaze you in Pool Water Chemistry

 

[OTPirate]

My thought is that you need some sanitizer in there immediately. I have seen a "code brown" literally, that is what they call it at public pools and no one tests the CYA. I would raise my FC to SLAM level and then check in the morning. It seems like it would almost be like having algae in your pool, maybe worse. Thoughts?

 

[JohnA902]

Hmmm. That's a tough graph to extrapolate from. I guess I still want to know how to use their "2ppm" in a real world example of having 50 ppm cya.

The idea of "raising to 2ppm" is meaningless when I'm already at 5.


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[JohnA902]

For the poop idea, I guess I would just raise to shock level (which is like 16 or so), let it circulate and then let the other kids back in after 30 min. Anyway, I just wanted a plan in case this happened.


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[tim5055]

For the poop idea, I guess I would just raise to shock level (which is like 16 or so), let it circulate and then let the other kids back in after 30 min. Anyway, I just wanted a plan in case this happened.


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Good plan, that was along the lines of what I was going to suggest.

 

[911medic]

Looking at the CDC pdf document on the topic (found here: https://www.cdc.gov/healthywater/pdf/swimming/pools/fecal-incident-response-recommendations.pdf), they delineate two responses, depending on if the stool is formed (not as dangerous) or diarrhea (much higher risk). For "formed stool incidents," disinfection times are based on killing Giardia cysts, while for "diarrheal incidents," times are based on killing Cryptosporidium (they explain why in the document).

The 2ppm recommendation is for formed stool, and only if the existing chlorine concentration is less than 2 ppm. Additionally, you must "ensure pH 7.5 or less and a temperature of 77°F (25°C) or higher."

The formed stool guidelines go on to say "State or local regulators may require higher free chlorine levels in the presence of chlorine stabilizers, which are known to slow disinfection."

So, not great guidance on what to do about your CYA level there.

However, in the diarrhea guidelines, they discuss stabilizer effects on disinfection further:

"Raise the free chlorine concentration to 20 ppm and maintain pH 7.5 or less and a temperature at 77°F (25°C) or higher. The free chlorine and pH should remain at these levels for at least 12.75 hours to achieve the CT inactivation value of 15,300.
Crypto CT inactivation values are based on killing 99.9% of Crypto. This level of Crypto inactivation cannot be reached in the presence of 50 ppm chlorine stabilizer, even after 24 hours at 40 ppm free chlorine, pH 6.5, and a temperature of 77°F (25°C). Extrapolation of these data suggest it would take approximately 30 hours to kill 99.9% of Crypto in the presence of 50 ppm or less cyanuric acid, 40 ppm free chlorine, pH 6.5, and a temperature of 77°F (25°C) or higher."

To me, all this means the following:

If it's formed stool in the pool, scoop out as much as possible immediately. Formed stool has less pathogenic bacteria than diarrhea, and it's fairly well contained. It sounds like your standard daily chlorine levels (for your CYA level) will kill the bacteria pretty quickly, but raising to shock level almost certainly will disinfect completely.

If it's diarrhea, you're in for more of a battle. Higher chance of bad bacteria, more dispersed, etc. Scoop/bucket out what you can, and crank up the chlorine to high levels (beyond shock level for your CYA) for days. Fun.

My two cents.

 

[JoyfulNoise]

I'll only add this point to what has been posted - you have to differentiate between a residential pool which is normally confined to one single family versus a public pool. All State, local & CDC recommendations and codes are going to apply to commercial/public pools which, in the event of a fecal incident, would more than likely shut down for the rest of the day. In some areas, the remediation & reporting requirements for a fecal exposure would require not only FC/pH data but also fecal coliform testing as well.

In a residential pool setting, many of these guidelines make no practical sense because it would be a rare circumstance to find a family that has active cryptosporidium or giardia lamblia. Those are the two major, chlorine-resistant disease vectors that you have to worry about in pools with a fecal exposure. So, at then end of the day, if you have a fecal exposure in your pool, your best bet is to simply get everyone out of the pool and raise the FC up to shock level for 24 hours and then let it drift down naturally. Everyone can still swim at shock levels if they chose to.

TFP advocates a method of pool care that puts the active chlorine level (which is proportional to the FC/CYA ratio) at a value which will destroy bacterial and viral pathogens very quickly. In a pool with no CYA and 2ppm FC (very harsh chlorine levels), the hypochlorous acid (HOCl) concentration would be close to 1ppm at a pH of 7.5. Normal bacteria and viruses are killed at HOCl concentrations of 0.01ppm, i.e. 100 times lower level than what is advocated in the CDC document but designed to be overkill to handle an exposure. With the TFPC Method of pool care (5-7.5% FC/CYA ratio), the HOCl concentration is ~0.05ppm or so. That is 5X what is needed to inactivate most pathogens. At shock level, the FC/CYA ratio is 40% so you would neutralize pathogens even more quickly.

 

[JohnA902]

Thanks everyone. Good replies. Lots of info.


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