Monochloramine, Chlorine, CYA and pH

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

Chem Geek and Waterbear,

Thank you for your replies.

I think I understand, and am glad I asked the question and challenged Waterbear's assumption, as I now know why this is the case. As stated I sat on the fence on this one and was open to having my mind changed, the explanation you give seems clearer and more concise here than in the other posts covering Monochloramine. I am therfore convinced that what Waterbear was trying to explain makes sense. What I am not sure of is how to best achieve results using this method.

The whole point of the previous log was to get rid of algae so which of the following methods would be preferable, and less expensive, assuming a pool which has a CyA of 40. and a pH of 7.5?

1. Get the chlorine to the recommended shock level based on Best Guess CYA chart not adding any acid to lower it to 7.2.

2. Lower the pH to 7.2, then shock based on Best Guess CYA chart .

3. Lower the pH to 7.2, then shock at FC 35-40.

3. Leave the pH at 7.5 then shock based on Best Guess CYA chart.

4. Raise the pH to 7.8 then shock to a based on Best Guess CYA chart.

5. Raise the pH to 7.8 then shock at FC 35-40.

6. Something entirely different.

7. It really does not matter which of the above methods one takes.

Looking forward to your reply(ies).

Aloha

Quote:

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Originally Posted by smallpooldad

Chem Geek,

Thank you for your reply.

I think I understand.

The whole point of the previous log was to get rid of algae so which of the three following methods would be preferable assuming a pool which has a CyA of 40. and a pH of 7.5?

1. Get the chlorine to the recommended shock level based on Best Guess CYA chart not adding any acid to lower it to 7.2.

2. Lower the pH to 7.2, then shock based on Best Guess CYA chart .

3. Raise the pH to 7.5 then shock based on Best Guess CYA chart.

4. Raise the pH to 7.5 then shock to a higher level say FC 35 or 40

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For #3 and #4 why are you raising the pH to 7.5 when it is already at 7.5 according to the first paragraph of your post.

As to what is best the first thing to consider is what kind of algae. Green is easiest to kill, black algae and the bacterial pink slime and white water mold the most difficult. All must be taken into consideration.

Also, the pool surface needs to be taken into consideration. A vinyl pool can be bleached by too high a chlorine level (as can dark colored plaster and fiberglass but not as readily) so 'nuking' a pool is not always a viable option.

Bottom line is this. As long as the pH is within aobut 7.2 -8.0 and the pool is stabilized normally and not overstabilized then the FC needs to be raised high enough and kept there long enough to kill the algae. The best guess chart works for green algae and is on the border for mustard. Black, pink, and white require even higher chlorine levels. If you are going to use an adjuct such as sodium bromide or an ammonia compound then the pH is ususally raised to 7.0-8.0 before using the product and shocking to activate it. These are more useful at high stabilizer levels than at lower levels but can be useful at normal stabilizer levels with the more resistant algae types.

Waterbear,

Thank you for your near instaneous time reply. Your right I did mention the same method twice, I realized that just after I posted and was in the process of editing it out but your reply came in so quickly that I had no chance to change it. But I think you mean #1 and #3 (part 2), number sequence is wrong, in #4 I'm raising it to 7.8, but that is my fault for not editing it quick enough.

Anyhow I think I got it. So if one had strong algae it might be better to raise it to 7.8 or 8.00, is that correct? Do you have any pictures white water mold, or an internet link to an example?

Thank you for all your help.

Aloha

No matter what method you choose it will take a lot of chlorine to kill mustard algae. If you do not have a vinyl pool I would not worry about the pH but rather just 'nuke the pool' until the algae is gone.
If you have a pool surface that might be affected by high chlorine levels then I would use either an ammonia based or bromine based 'chlorine enhancer' to take the CYA out of the loop. If CYA levels are high I would definately use bromine or ammonia based products. The algae will die faster but then you have to keep shocking to burn off the ammonia or bromine.

Waterbear,

Once again thank you for this great and clear reply. I know you think I am a pain in the "okole" as we say in Hawaii. But look at how much you have taught me in just a few weeks.

My dad always said never trust a person who always agrees with you and always challenge their assumption. True life experiences have born this out for me . The only times I have gotten into a lot of hot water was when I dealt with someone who agreed with me totally. Well my dad would have loved you, he might have punched you in the nose a few times due to a little harsh delivery, he was a champion boxer when he was young; but he would have learnt to respect you. I have learnt to trust what you say.

Thank you

If you are going to be shocking with chlorine as opposed to adding ammonia to form monochloramine or adding bromide to from bromine (similar arguments for its effectiveness since it does not combine with CYA), then I would lower the pH to around 7.2 IF you are shocking at very high levels. This is generally the case when fighting mustard/yellow algae since such shocking is usually done with an FC that is 60% of the CYA level. Though the pH dependence of chlorine when CYA is present is less, it's not zero, so heavy shock levels would be better if the pH is lowered first.

As a concrete example using a TA of 80 for reference (and assuming no Borates), adding chlorine to raise the FC by 10 ppm would go from a pH of 7.5 to 8.15 with a hypochlorous acid concentration of 0.17 ppm. Starting with a pH of 7.2 you would get to 7.53 and 0.21 ppm so 24% higher in chlorine. However, if you raised the FC by 20 ppm, then starting with a pH of 7.5 goes to 8.56 and 0.43 ppm vs. starting with a pH of 7.2 going to 8.1 and 0.64 ppm which is 49% higher. A 30 ppm FC increase would be 7.5 to 8.72 and 0.69 ppm vs. 7.2 to 8.4 and 1.08 ppm which is 57% higher (and a starting pH of 7.0 would go to 7.95 and 1.83 ppm which is 165% higher than starting with a pH of 7.5). So I generally tell people to lower the pH first if they are going to be shocking with 20 ppm or more incremental FC. Don't forget that the pH will drop back down to where you started as the chlorine gets used up (assuming not much pH rise from outgassing).

The above heavy shock levels are for pools that can more readily handle it such as plaster pools. I would be more conservative using lower chlorine levels with vinyl pools to be on the safe side. And as waterbear mentioned, different types of algae require some different methods. Black algae is most effectively killed by frequent brushing to remove the slime layer "head" so that lower layers get exposed to the shock level of chlorine.

Richard

Chem Geek,

I understand, thank you.

Will do next time I have to help someone.

Aloha