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Thread: Scientific justification of CYA/FC recommended levels

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    Scientific justification of CYA/FC recommended levels

    Is there a rough scientific explanation for the minimum (7.5% CYA) and maximum (11.5% CYA) FC levels recommended here or are they based on experience?

    I mean, I understand that HOCl is approximately 0.3*(FC/CYA) but how do we know how much HOCl is considered sufficient?

    I apologize if that has been covered before but I could not find anything in my little search....

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    Re: Scientific justification of CYA/FC recommended levels

    I know there are scholarly articles and peer reviewed papers detailing the relationship. I'm sure an expert will chime in soon with the location.

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    Re: Scientific justification of CYA/FC recommended levels

    The HOCl number is based primarily on experience, but then backed up with some papers. Ben Powell and Richard Faulk (chem geek) put a lot of work in to zeroing in on the most accurate numbers based on both math and real world experience.

    Here is chem geek's comprehensive post on this specific subject: Pool Water Chemistry

    One of our resident chemical experts can chime in with more specific answers, but I think that post is the best place to start.
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    Re: Scientific justification of CYA/FC recommended levels

    here is another link in addition to what Donaldson posted.
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    Re: Scientific justification of CYA/FC recommended levels

    i recommend you search out and read chem geek's links. i'm sure some of his posts have addressed an issue like this before. in my humble opinion, he is the resident expert in all things chemical.

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    Re: Scientific justification of CYA/FC recommended levels

    The basis of the FC/CYA relationship is very simple and covered in detail in many of the threads linked to in the previous posts. Basically, you need enough hypochlorous acid (HOCl) concentration in the water to satisfy the CT kill rates for various waterborne pathogens (although algae isn't really considered a "pathogen" but a nuisance). Your kill rates have to be faster than the ability of the colony forming unit (CFU) or individual microbe to reproduce itself. Most CT kill rates are reported as the number of Log reductions in CFUs or pathogen count per unit of time. So if a specific concentration of hypochlorous acid has a 6-log reduction in say 2 mins of exposure, then that means that 99.9999% of the pathogens were destroyed. Again, your rate of inactivation has to be greater than the pathogen's intrinsic rate of reproduction (typically measured as a time to double their numbers) in order to keep the water clean.

    Ben Powell ("PoolDoc" at the The Pool Forum) used his long time experience with commercial pool management and service to figure out the rough amounts of FC needed for specific CYA levels to maintain a "clean" pool. Richard Falk ("chem geek" here and on many other forums) used that data as a starting point and scoured CDC, EPA and other sources of biological science papers on pathogen kill rates for chlorine as well as calculated the chemical equilibrium reactions between chlorine, water and cyanuric acid. This led to a determination of the threshold disinfection level of ~ 0.01ppm HOCl for bacteria and ~0.05ppm for algae. Once you know that level, you can then back-calculate the amount of CYA and FC needed to achieve it. A ratio of 7.5% FC/CYA gives you roughly the correct amount of HOCl needed to maintain clean water. However, given the daily loss of FC to sunlight and oxidation of organics, one should target 11.5% FC/CYA to be safe. The other ratios of FC/CYA are what are considered shock levels of chlorine.
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