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  Richard A. Falk January 17, 2011 at 8:29pm

  One additional piece of info that has been hinted at elsewhere but wasn't discussed in this thread is that any of the circulation-based methods for dealing with Crypto including improved filtration, ozone, and UV, all require multiple turnovers of the water to be effective.  The following is the cumulative amount of water that goes through the circulation system after the indicated number of turnovers:

   

  1 turnover --> 63.2%

  2 turnovers --> 86.5%

  3 turnovers --> 95.0%

  4 turnovers --> 98.2%

  5 turnovers --> 99.3%

  6 turnovers --> 99.75%

  7 turnovers --> 99.9%

   

  The above assumes perfect mixing which obviously doesn't happen so you can see that having a diarrhea incident can still infect many people even with the state-of-the-art systems available today.  There is a way to kill Crypto faster by adding something to the bulk water itself by using chlorine dioxide.  2 ppm for 8 hours would give a CT value of 960 that is high enough to provide a 99% kill of the most resistant form of Crypto as described in this paper.  Unfortunately, no manufacturer has obtained EPA approval for use of chlorine dioxide in swimming pools (or spas), not even as a shock basis after a diarrhea incident or as a weekly shock preventative measure (in case there was an undetected incident) against Crypto (so in both cases the pool could be shut down overnight to not expose anyone to chlorine dioxide).  This is in spite of the fact that chlorine dioxide is EPA approved for drinking water purification as in Katadyn Micropur MP1 Purification Tablets.  It makes little sense to me how the EPA can approve chlorine dioxide for DRINKING WATER, but requires millions of dollars to be spent to approve it even for limited no-exposure pool use.

   

  So no one in the swimming pool/spa industry is willing to spend the $2-3 million that is probably necessary to gain EPA approval.  I would think that the CDC and others might want to push fast-tracking this if used without exposure to swimmers, as I described above.  Using chlorine dioxide alone for sanitation has its own issues, most notably tending to break down by the UV in sunlight.  By the way, chlorine dioxide could easily be created on-site by adding an appropriate amount to stabilized chlorinated pool water since the water purification tablets are nothing more than a combination of Dichlor with sodium chlorite.  The chlorine with CYA is already in the water so only sodium chlorite needs to be added in sufficient quantity to minimize the production of chlorate.  After the 8 hour exposure is completed, one can add a reducing agent (chlorine neutralizer) to eliminate the chlorine dioxide.
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  Wes Burdine January 17, 2011 at 4:23pm

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  Richard A. Falk January 15, 2011 at 12:01pm

  P.S.

   

  This link to the CDC talks about using secondary disinfection to reduce disinfection by-products, especially for indoor pools.  Technically, it's a supplemental oxidation system that does this -- additional disinfection is not required.  The use of non-chlorine shock (MPS) or enzymes would also help reduce disinfection by-products.  Basically, anything that removes or oxidizes chemical precursors from the water before chlorine can react with them will help.

   

  Scott, do you have a specific link to the CDC site that talks about this recommendation or is it the link I just gave?  What I wrote in my first post regarding Crypto applies to all pools while the reduction of disinfection by-products, while important in all pools, is particularly needed for indoor pools since the air quality is worse (probably due to poorer air circulation and a lack of UV from sunlight).
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  Richard A. Falk January 15, 2011 at 10:18am

  It's important to note why the CDC is taking this stance.  Remember that they look primarily at overall health concerns (most notably transmitted diseases) and look at nationwide statistics.  Before regular sanitation of pools with good maintenance of a sanitizer residual, there were many incidents of transmitted disease including those coming from fecal bacteria, from some amoeba, from some viruses, as well as protozoan oocysts.  The overall numbers have dropped dramatically, primarily due to chlorination being used more consistently and while there are still too many commercial/public pools that do not consistently maintain sanitizer levels (around 10% of pools in recent surveys), the shift over recent years is to seeing larger numbers of outbreaks involving protozoan oocysts, most notably Cryptosporidium.

   

  The reason for this increase is that Crypto is very chlorine resistant, but part of the increase is due to reducing other forms of transmitted recreational water diseases so now Crypto stands out as the dominant one left in sanitized pools.  The chlorine resistance is primarily due to its having a relatively thick oocyst wall designed to protect the protozoa from harsh environmental conditions during its journey leaving the intestinal tract from a host out into the environment until it is picked up again (usually through drinking) by another host where the oocyst is removed and the protozoa released in the intestinal tract.  This is NOT "developed" chlorine resistance.  This is something that has existed long before chlorination was introduced.  See this link for a picture of the Crypto life cycle.

   

  The oocysts are small, usually around 4 to 6 microns.  So they can easily get through clean sand filters, but if there are good methods of flocculation/coagulation that are used, then these can help.  Slow sand filtration can capture more (though that requires a sticky mat of biological material to form on the sand surface and that has its own issues) and there is some evidence that some oocysts may be opened from passing through some sand filters in spite of the large pore size in such filters (and that's good as the protozoa themselves are killed by chlorine).  However, since so many oocysts are shed in a diarrhea event and it takes a relatively small number of oocysts to infect an individual, one tries to have a more thorough removal than possible through filtration (though every bit helps).  Ozone and UV systems have both been shown to be able to kill Crypto if such systems are sized properly.  It's probably also the case that strong oxidation systems, such as those using boron-doped diamond, are also effective though I haven't seen any literature on that.

   

  There are other reasons to have a supplemental system and ideally such a system would also provide supplemental oxidation of chloramines and organics as well because this will reduce the amount of chlorinated disinfection by-products.