Monochloramine, Chlorine, CYA and pH

[chem geek]

This post is a response to this post where the response is technical so is going into The China Shop.

Monochloramine is a weaker oxidizer and sanitizer than hypochlorous acid AT THE SAME CONCENTRATION. For example, 3-log inactivation of Giardia by chlorine with no CYA in the water at a pH of 7.5 is a CT value in the 40-55 range depending somewhat on concentration. 4-log inactivation of viruses by chlorine with no CYA near a pH of 7.5 is a CT value of around 2. By comparison, with monochloramine 3-log inactivation of Giardia has a CT value of 750 while 4-log inactivation of viruses has a CT value of 497. (A higher CT value means it takes a higher concentration to have the same effect in a fixed amount of time. "CT" stands for the product of concentration times time, with concentration usually in ppm and time in seconds.)

So upon first appearance, it looks like a factor of 15 difference for Giardia inactivation and a factor of 250 difference for virus inactivation. Similar differences are seen for bacterial kill times as well, on the order of 30-50. HOWEVER, in water with Cyanuric Acic (CYA), the actual disinfecting and strongly oxidizing form of chlorine (i.e. hypochlorous acid) concentration is very low. At the normally recommended levels of an FC that is about 10% of the CYA level, the equivalent FC with no CYA is around 0.1 ppm. So having monochloramine at around 1-3 ppm is 10-30 times higher in concentration. In water with a lot of CYA in it, the effect is more dramatic. With 3 ppm FC and 100 ppm CYA, the equivalent FC with no CYA is around 0.03 ppm so monochloramine at around 1-3 ppm is 30-100 times higher in concentration.

The net of the above is that in pools with lots of CYA, the use of monochloramine can be more effective than the use of chlorine. Also, algae are different than bacteria, protozoan cysts (e.g. Giardia), and viruses. Algae can use ammonia as a source of nitrogen so monochloramine, which looks somewhat like ammonia, can be taken in more readily by algae. The difference in CT values for algae kill are probably much smaller between hypochlorous acid and monochloramine, but I cannot find definitive sources that give such numbers (CT values are usually not quoted for algae kill since algae is not a pathogen, but rather an annoyance). Of course hypochlorous acid looks like water which is what makes it so effective, but the point is that monochloramine can be effective at killing algae.

Everything that you were quoting about chlorine being more effective than monochloramine is true only when there is no CYA in the water. With CYA, the entire argument shifts since it effectively reduces chlorine concentrations by orders of magnitude.

As for the effects of pH on chlorine effectiveness, the true story is shown in this post where you can see (especially from the log graphs) that the effect of pH on hypochlorous acid concentration is not nearly as strong in the presence of CYA. This is because CYA acts as a hypochlorous acid buffer resisting changes in its concentration from changes in pH. In fact, whereas going from a pH of 7.5 to 8.0 results in a roughly 50% drop in hypochlorous acid concentration when there is no CYA, there is only a roughly 15% drop in concentration when CYA is present at typical concentrations. Of course, the ABSOLUTE amount of hypochlorous acid when CYA is present is far lower so the comparison isn't completely fair, but the point is that the traditional industry graph is not only wrong when CYA is present, but misleading in its conclusions with regard to the effect of pH on chlorine effectiveness.

As for nitrates in the water or other effects from ammonia or monochloramine, first of all there won't be growing bacteria in the water since you are sanitizing it. Second of all, the amounts we are talking about are very small. Third, you are not drinking the water and it does not get readily absorbed through the skin in significant quantities. Fourth, you generally don't go swimming while shocking a pool to get rid of algae -- at least not until the algae is dead and the water is, perhaps, cloudy.

Richard