BBB pools and sanitary conditions - CDC comment

[chem geek]

What about someone with Giardia?

Giardia is slowly inactivated, but it is inactivated even in normally chlorinated water. With the FC at around 10% of the CYA level, a 3-log reduction (99.9%) kill takes around 232 minutes so nearly 4 hours. Basically, half are inactivated every 23 minutes. So in a chlorinated pool, this probably wouldn't prevent person-to-person transmission of this pathogen if there were an incident and you were nearby and drank some pool water, but a full-fledged ongoing multi-day outbreak would very likely be prevented.

In a commercial/public pool, personnel are supposed to be on the lookout for formed stools or diarrhea incidents and to close the pool when they occur. Of course, this isn't always possible to see and is why one gets outbreaks. In practice, if you look at the Cryptosporidiosis Surveillance compared to the Giardiasis Surveillance and you look at Surveillance for Waterborne Disease, you will find the following:

Cryptosporidium has become the leading cause of reported treated recreational water–associated outbreaks of gastroenteritis.
:
During 1999–2008, Giardia was identified as a causal agent of eight (3.5%) of 228 reported recreational water-associated gastroenteritis outbreaks.

From the graphs in Figure 4 in the Surveillance for Waterborne Disease report, it shows that in treated recreational water in 2007-2008, 82.9% of the gastrointestinal illness outbreaks were due to Crypto alone while 4.3% were Giardia and 1.4% were both. Table 7 shows there were 60 Crypto outbreaks with 12,492 cases while there were 3 Giardia outbreaks with 19 cases and 1 combination outbreak of Crypto and Giardia with 19 cases.

This paper shows that 6.2% of pools without outbreaks had Giardia while 1.2% had Crypto giving further evidence that Crypto is more likely to cause an outbreak when it is present. This may be partly due to its greater infectivity, but is also due to its greater chlorine resistance. If an incident remains undetected, it may take weeks before a correlation of later gastrointestinal illness is associated with swimming in a particular pool. That's why you see so many more Crypto outbreaks and that they affect so many more people.

 

[msgtdan]

They had another item on the local news this morning suggesting users test the water themselves with strips provided by the health dept. They said as long as there was 1 ppm FC it was okay to swim. I've thought about sending the TV station a link to here.

 

[ptbrunet]

I finally got time to look at Richard's post covering kill times. Some questions:
1) HSV apparently takes 30 hours to kill but an earlier posting said STDs weren't an issue - so why isn't it an issue?
2) Cyrpto takes 106 days to kill. How is that dealt with? It seems you'd need to test for it daily and then if detected drain the pool and use something more effective than chlorine to clean the surfaces before refilling.

 

[chem geek]

Most STDs require sex for transmission and are not transmitted casually. As for HSV which is usually transmitted by direct skin-to-skin contact (HSV-1 for cold sores, HSV-2 for genital herpes). I calculated kill times from this paper, but even distilled water with no chlorine had HSV have a 2-log reduction (99% kill) in 6 hours (the virus does not do well in water) and a 3-log reduction (99.9% kill) in 18 hours. Tap water with 4 ppm FC (and no CYA) had a >2-log reduction (>99% kill) in 30 minutes and I extrapolated from that, but was very conservative. In practice, the 3-log kill for 0.1 ppm FC equivalent (an FC that is 10% of the CYA level) would not be 30 hours but would be between 30 minutes and 18 hours -- perhaps around 4-8 hours.

However, I can find no other sources indicating actual transmission of herpes via chlorinated swimming pool water.

As for Crypto, it is introduced from the diarrhea of infected persons. If a diarrheal incident is detected, the pool is shut down and disinfected (super-chlorinated or other means) and usually closed until the next day. It is when such an incident is not detected that one can have an outbreak that lasts for weeks until the source is determined and the pool shut down and disinfected. There are supplemental systems such as UV or ozone that will kill Crypto via circulation taking 7 turnovers for a 3-log reduction (99.9% kill). One can also use techniques of coagulation/flocculation with filtration or microfiltration. There are no simple on-site detection kits for Crypto and public pools do not have it checked at labs. The CDC is proposing in their Model Aquatic Health Code (MAHC) for high-risk venues to install ozone or UV systems to help reduce the duration of Crypto outbreaks.

 

[ptbrunet]

Thanks Richard, What is the recommended resolution for residential pools? From what you said it sounds like super-chlorination for 24 hours. Is super-chlorination equivalent to raising to shock level as shown in the CYA/Chlorine chart, i.e. to ~50% CYA?

 

[chem geek]

For residential pools it's simple. If someone is sick with a gastrointestinal illness, don't let them swim. It's virtually unheard of to get Crypto from a residential pool. Most people don't let sick people get into their pool. If you do end up with a loose diarrhea incident and you suspect Crypto, then super-chlorination when you've got CYA in the water can be impractical -- you'd have to raise the FC to be at least 10 ppm higher than the CYA level and keep it there for 25 hours (the CDC recommendation is 10 ppm FC for 25 hours, but that's with no CYA in the water) -- you need to lower your pH substantially first to around 7.0 before super-chlorinating.

I think it would be easier to just get some sodium chlorite and add it to the pool when there is around 4-5 ppm FC in the pool with CYA (amount doesn't matter, but have some in the pool) and dose to get 2 ppm chlorine dioxide and add this when the sun goes down so the pool is disinfected overnight. This is roughly 2 ounces weight of sodium chlorite anhydrous per 10,000 gallons (about 3.2 ounces weight sodium chlorite trihydrate per 10,000 gallons). Then the next day you can use the pool either as is or you can dechlorinate and then add chlorine back again (not really necessary since the chlorine dioxide is OK to swim in and it will get broken down by sunlight during the day).

With regard to HSV, I think that in practice the dilution in the larger body of water does a lot to prevent transmission since normally direct contact is needed. Even if the 3-log reduction was over 8 hours, that's still an inactivation of half the virus every 50 minutes and it may be faster than that. There's a difference between theoretical survival from lab experiments vs. actual infectivity. Otherwise, we would see some infectivity from "toilet seats" in frequently used bathrooms, but we don't.

 

[msgtdan]

What effect to pool surfaces can be expected at super-chlorination levels, and does using the sodium chlorite change that?

 

[chem geek]

The superchlorination is equivalent to 10 ppm FC with no CYA so is roughy 25 times higher active chlorine concentration than regular shock level or roughly 15 time higher than yellow/mustard algae shock level. It's only for 25 hours so is unlikely to seriously damage equipment or plaster, but it most certainly is very strong and for sure you do not want to get in the pool at that level. You would also likely smell the chlorine itself, not just chloramines. Since the active chlorine is around 130 times higher than the minimum FC level, figure that the exposure for one day is roughly equivalent to somewhat more than 4 months of regular chlorine exposure. So while not great, it's not going to immediately corrode metal or bleach vinyl. It's more like accelerated wear.

Chlorine dioxide will be far less harsh. It's not as strong an oxidizer and is more selective an oxidizer, but happens to be more effective against Crypto (as well as biofilms) because it is a dissolved gas so penetrates the protozoan oocyst shell more readily (and does not react with that shell as much as chlorine) to inactivate the protozoa inside.

 

[msgtdan]

exposure for one day is roughly equivalent to somewhat more than 4 months of regular chlorine exposure. So while not great, it's not going to immediately corrode metal or bleach vinyl. It's more like accelerated wear.

Chlorine dioxide will be far less harsh.

Great thanks, that's exactly what I was looking for. So the treatment won't create larger equipment costs. Hopefully never have to do it but it's good info to keep filed away just in case.

I've found sodium chlorite powder 80% online and a few ounces of it isn't very expensive, but sodium chlorite trihydrate flakes seems more expensive and I couldn't find sodium chlorite anhydrous. The powder was ~$20 for 4 ounces, if it is suitable and has a good shelf life I might get that to have on hand if there is ever an incident.

 

[chem geek]

That sodium chlorite 80% is anhydrous (essentially) and you just factor in the fact that it's only 80% pure (the impurities are mostly sodium chloride salt with some sodium hydroxide, sodium carbonate, sodium sulfate and sodium chlorate all of which are OK). You can get one pound of 80% in flake form for $45 though is more than what you need.

I seriously doubt you will ever need to use this, but it is the easiest way to inactivate Crypto if there is a possible incident. The principle of adding sodium chlorite to stabilized chlorine to produce chlorine dioxide is used in the Katadyn Micropur MP1 Purification Tablets.

 

[ptbrunet]

It's sounding like about 4oz 80% is the right amount for a 15K pool. I can get 8 oz powder for $25 here. Would the full 8 oz be too much?

 

[chem geek]

No it wouldn't be too much. Excess sodium chlorite is OK and will result in the chlorine getting used up and you having chlorine dioxide with leftover sodium chlorite. When you later add chlorine to the pool the next morning (remember that you have to add this at night since chlorine dioxide breaks down in sunlight), it will use up some chlorine to produce more chlorine dioxide and that's OK. It will drop in the sunlight so just be aware that you may see an FC drop rapidly at first until the chlorine dioxide gets used up. After that, the chlorine should hold as well as it normally does.

 

[msgtdan]

What is the shelf life and best storage practice? My youngest is 4 with cousins who are 2 so we're planning to use the pool for years to come. And it could happen that the first incidence of diarrhea happens in the pool. If sodium chlorite 80% has a good shelf life it might be worth it to have on hand just in case. If it doesn't keep very well then I'll order as needed, either way the pool shouldn't have to be closed for to long.

 

[chem geek]

According to A2Z Health Products, it has a multi-year shelf-life, but I really don't know. There's actually a website selling only sodium chlorite so this stuff should be pretty easy to get. Keep in mind that what you would be doing is not an EPA-approved disinfecting method for swimming pools.

 

[msgtdan]

I'll probably wait until I need it if that ever happens.