Chlorine Dioxide

[sbn56]

OK, so I have forgotten almost everything I should have remembered from high school chemistry - but I ran across chlorine dioxide as an ingredient in some mouthwash stuff for my old dog recommended by the vet. Wiki says chlorine dioxide is used in municipal water supply disinfection - how does this differ (or not) from what we use in our own pools?

 

[chem geek]

Chlorine dioxide (ClO₂) has a lot of advantages over chlorine, most notably that it doesn't produce as many disinfection by-products and it's effective against the protozoan oocyst Cryptosporidium parvum that is highly chlorine resistant. Chlorine dioxide is actually a dissolved gas and is selectively reactive (with 1 electron transfer instead of 2) so is able to penetrate biofilms and oocyst shells. So you might wonder why it isn't used more. The reasons are that chlorine dioxide in concentrated form is dangerous and explosive and even when diluted in water it breaks down in sunlight and it's quite a bit more expensive. It can be generated on-site by mixing sodium chlorite with either a strong acid or with a hypochlorite source of chlorine. See the thread Is anyone using chlorine dioxide (ClO2) instead of chlorine?.

Unfortunately none of the manufacturers of chlorine dioxide products have paid for the safety and efficacy tests that would need to be done to get approval for use in pool/spa water. Though there are sodium chlorite plus Dichlor tablets used for drinking water purification such as when camping, the use of such products in pools would need to ensure that there were no long-term side effects (chemical buildup) from continued use. The EPA would likely be most concerned about chlorite, though in a chlorinated pool using chlorine dioxide the only buildup should be of chlorate. The dosing is tricky, but with so much focus by the CDC on Crypto it is being considered for remedial use after an outbreak, but again would need to get a green light from the EPA.

For use in outdoor pools exposed to sunlight, tests would need to be done to see if CYA's shielding effect is sufficient to protect it or whether it needs some other sort of UV shield such as titanium dioxide.