- Mar 5, 2020
- 3,221
- Pool Size
- 66000
- Surface
- Plaster
- Chlorine
- Salt Water Generator
- SWG Type
- Astral Viron V35
Melbourne is currently dealing with an increased number of Cryptosporidium outbreaks in public pools, requiring them to close for "Hyper-Chlorination":
www.theage.com.au
What does ""Hyper-Chlorination" in the Victorian regulations mean?
The recommended incident response in Victoria is described here (Appendix 6):
The steps are:
No CYA used (CYA not allowed for public indoor pools in Victoria):
1) Adjust pH to 7.5 or lower
2) "Hyper-Chlorinate" to achieve an FC contact time (CT) of 15300 ppm x min, e.g. 13 hours @ FC 20ppm or 26 hours @ FC 10ppm
CYA used (up to 100ppm allowed in public outdoor pools in Victoria):
1) Reduce CYA to 15ppm or lower
2) Adjust pH to 7.5 or lower
3) "Hyper-Chlorinate" to achieve an FC contact time (CT) of 31500 ppm x min, e.g. 28 hours @ FC 20ppm
The FC-CT of 15300 is a commonly found value for crypto for a kill target of 99.9%, so far so good. They added a factor of 2 when CYA up to 15ppm is used. Let's see if that makes sense.
As we all know, it is actually the HOCl-CT that matters, and not the FC-CT. Assuming that the FC-CT of 15300 ppm x min was derived at pH 7.5 without CYA where 50% of the FC is HOCl, then this would be equivalent to an HOCl-CT of 7650 ppm x min, which is our benchmark to compare with the CYA-scenario.
At CYA 15 ppm, the regulation tells us to use the 31500 FC-CT value.
Let's assume first that we want to "hyper-chlorinate" at FC 10 ppm, which would yield (calculated with chem geek's sheet) 0.75ppm of HOCl (or 7.5% of the FC). This would be roughly the HOCl concentration at TFP's mustard algae SLAM level. This means that the FC-CT of 31500 yields an HOCl-CT of only about 2350. This will actually only kill about 88% of the crypto population, rather than 99.9% (2348 / ln(1-0.88) x ln(1.0.999) = 7656).
When "Hyper-chlorinating"with CYA 15ppm at FC 20ppm (which is the example they give in above guidelines) we'd have 3.5ppm of HOCl (or 17.5% of the FC). This is nearly 5 times of our mustard algae SLAM level - note that at these extreme FC/CYA ratios, the HOCl concentration is no longer proportional to the FC/CYA ratio). This gives us an HOCl-CT of about 5500 ppm x min, killing about 99.3% of the crypto population (5500 / ln(1-0.993) x ln(1.0.999) = 7657), which is getting closer to the intended 99.9%.
I'd say their simplified approach of just doubling the FC-CT time to account for CYA seems to do the trick. The challenge would be to maintain FC at 20ppm for 28 hours in an outdoor pool with only 15ppm of CYA.
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What does ""Hyper-Chlorination" in the Victorian regulations mean?
The recommended incident response in Victoria is described here (Appendix 6):
The steps are:
No CYA used (CYA not allowed for public indoor pools in Victoria):
1) Adjust pH to 7.5 or lower
2) "Hyper-Chlorinate" to achieve an FC contact time (CT) of 15300 ppm x min, e.g. 13 hours @ FC 20ppm or 26 hours @ FC 10ppm
CYA used (up to 100ppm allowed in public outdoor pools in Victoria):
1) Reduce CYA to 15ppm or lower
2) Adjust pH to 7.5 or lower
3) "Hyper-Chlorinate" to achieve an FC contact time (CT) of 31500 ppm x min, e.g. 28 hours @ FC 20ppm
The FC-CT of 15300 is a commonly found value for crypto for a kill target of 99.9%, so far so good. They added a factor of 2 when CYA up to 15ppm is used. Let's see if that makes sense.
As we all know, it is actually the HOCl-CT that matters, and not the FC-CT. Assuming that the FC-CT of 15300 ppm x min was derived at pH 7.5 without CYA where 50% of the FC is HOCl, then this would be equivalent to an HOCl-CT of 7650 ppm x min, which is our benchmark to compare with the CYA-scenario.
At CYA 15 ppm, the regulation tells us to use the 31500 FC-CT value.
Let's assume first that we want to "hyper-chlorinate" at FC 10 ppm, which would yield (calculated with chem geek's sheet) 0.75ppm of HOCl (or 7.5% of the FC). This would be roughly the HOCl concentration at TFP's mustard algae SLAM level. This means that the FC-CT of 31500 yields an HOCl-CT of only about 2350. This will actually only kill about 88% of the crypto population, rather than 99.9% (2348 / ln(1-0.88) x ln(1.0.999) = 7656).
When "Hyper-chlorinating"with CYA 15ppm at FC 20ppm (which is the example they give in above guidelines) we'd have 3.5ppm of HOCl (or 17.5% of the FC). This is nearly 5 times of our mustard algae SLAM level - note that at these extreme FC/CYA ratios, the HOCl concentration is no longer proportional to the FC/CYA ratio). This gives us an HOCl-CT of about 5500 ppm x min, killing about 99.3% of the crypto population (5500 / ln(1-0.993) x ln(1.0.999) = 7657), which is getting closer to the intended 99.9%.
I'd say their simplified approach of just doubling the FC-CT time to account for CYA seems to do the trick. The challenge would be to maintain FC at 20ppm for 28 hours in an outdoor pool with only 15ppm of CYA.
I remember once being at the common pool when we lived in an apartment complex and this fat hairy old guy (not me) walks into the shallow end, submerged himself and then puts a finger to the side of his nose and blows out a wad of snot into the water. Then proceeds to clear the other nostril of mucous … like the pool was his personal bathtub … people are generally gross and do gross and nasty things. I don’t need to be swimming in their filth 
