Bartier Disinfection Index

[smallpooldad]

Ben and Michael,

Thank you both, I think I will stick with your tables. Only one thing it did seem that there was less irritation to the eyes when the Cl dropped to Bartier's recommended level or is this purely a function of not enough CL?

Aloha

[chem geek]

I am the user that Ben mentioned who created the spreadsheet that accurately calculates the chlorine species in water including the chlorinated cyanurates.

With regards to the Bartier Disinfection Index, the pH Factor table is reasonably accurate and simply describes the HOCL to OCl- ratio that is independent of CYA (the RATIO is independent of CYA, but the absolute levels are certainly not).

However, simply subtracting the CYA level from the disinfection index is NOT an accurate way of predicting disinfection (assuming disinfection is directly related to the concentration HOCl as appears to be the case). There are two primary problems with the Bartier formula. First is that with no CYA it sets the minimum chlorine levels too high as required for disinfection (see below), though this level and even higher IS required for a reservoir of chlorine. For outdoor pools exposed to sunlight, you'd have to continually add chlorine as half of it could be gone in a half-hour. Second, their formula woefully underestimates the effect of CYA on reducing HOCl concentration (and therefore on reducing disinfection ability).

I won't put in the whole CYA/FC/HOCl table into this post (I'll create a new China Shop forum topic instead) but results from a commercial spas study and HOCl vs. ORP correlation data from a reliable ORP sensor manufacturer indicate that a 0.011 ppm HOCl concentration is the minimum required for disinfection and roughly corresponds to 650 mV ORP (at pH 7.5). Of course, prevention of algae may require more (this is part of what I want to find out from forum users). The "Ideal" Bartier Disinfection Index value of 91 implies the following levels of chlorine at different CYA levels and a pH of 7.5 (and I also show the calculated ppm HOCl):

CYA ..... ppm FC ..... ppm HOCl
.. 0 ....... 1.9 ............ 0.916
. 10 ....... 2.1 ............ 0.109
. 20 ....... 2.3 ............ 0.052
. 30 ....... 2.5 ............ 0.036
. 50 ....... 2.9 ............ 0.024
. 70 ....... 3.4 ............ 0.020
100 ....... 4.0 ............ 0.016

As you can see, a small amount of CYA will store (in chlorinated cyanurates) most of the total chlorine. The Bartier "VERY LOW" level of 50 with a CYA of 100 results in a ppm HOCl of around 0.011 so everything in their table does provide for minimum disinfection (which is good), but their "equivalency" of chlorine levels at varying of CYA is wrong. Also, I suspect that 0.03 or 0.05 ppm HOCl may be needed to prevent algae and if this is the case, the chlorine levels they propose at high CYA would be too low.

Ben's table is a better guide to chlorine requirements vs. CYA since it more closely tracks a minimum level of 0.02 ppm HOCl though the mid-points of the ranges are closer to 0.05 ppm HOCl. It's nice to know that real-world experience has a basis in chemical theory (or vice versa). As Ben mentioned elsewhere (on another forum), the shock values are not as consistent, but we really need more real-world experience to know what minimums are truly required to zap algae at various CYA levels.

Richard (aka "chem geek")

[chem geek]

With regard to HOCl vs. OCl- as a sanitizer, there appears to be valid science to back the notion of HOCl being the more powerful sanitizer for certain kinds of bugs. This is because most pathogens have cell walls that are negatively charged as follows:

1) Gram-Positive bacterial cells (so named due to a positive result from a stain test developed by Hans Christian Gram) have teichoic acids (phosphate groups joined by glycerol or ribitol) that give a negative charge to the outer cell wall.
2) Gram-Negative bacterial cells have lipopolysaccharides that give a negative charge to the outer cell well.
3) Many, but not all, viruses have a net negative electrical charge since their viral envelopes often come from host cells (which have a negative charge on their cell wall). Viruses enter cells at specific binding sites that overcome the electrostatic repulsion.

Since HOCl is neutral, it is more readily able to enter cells than OCl-. Polar molecules and ions generaly only enter cells through specific channels that only allow ions of certain sizes and shapes generally blocking OCl-. On the other hand, algae have different types of cell walls like cellulose that does not appear to be charged. So while HOCl may be a more effective sanitizer for bacteria and viruses, it may not be more effective for algae, but I cannot confirm that.

(See WHO water sanitation document)

Once inside a cell, the mechanism of sanitation appears to come down to two different mechanisms though this appears to be somewhat speculative:

1) Chlorine substitution where the HOCl substitutes its chlorine with hydrogen from an organic compound (a protein, typically an enzyme). This substituion inteferes with the enzyme's function (folding of protein, intereference of active site, etc.).
2) Chlorine oxidation of organics (typically from production of free radical oxygen O• from HOCl or OCl-). This can destroy proteins (including enzymes) as well as DNA (typically during cell division).

The dependence of Oxidation-Reduction Potential (ORP) on pH that follows the concentration of HOCl plus my analysis of the commerical spas study (Oregon) data using HOCl lead me to believe that HOCl is indeed the effective sanitizer against bacteria. I couldn't find scientifically valid information on algae to know whether HOCl or OCl- is more effective, but in any event I would doubt that OCl- is more effective than HOCl even for algae (though it might be about equally effective -- I just don't know).