Monochloramine, Chlorine, CYA and pH

[smallpooldad]

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

[waterbear]

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

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.

[smallpooldad]

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

[waterbear]

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.

[smallpooldad]

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

[chem geek]

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

[smallpooldad]

Chem Geek,

I understand, thank you.

Will do next time I have to help someone.

Aloha