Need some basic advice -TA, pH, CH, FC all things chemical

[SoDel]

Can I just interject some actual experience into this thought-experiment … the vast majority of the time spent SLAM’ing a pool is in the clearing phase and really has nothing to do with sanitation or oxidation. When you add chlorine to a level of 40% of CYA, the algae is pretty much dead within a few hours and any bacteria that may have been in the water is completely wiped out. After the first 24 hours, the next 5 days is spent filtering the water. So even if you wanted to do some kind of super-shocking, it's not really doing anything useful. TFP suggests maintaining the SLAM level because, again, it's a good balance between keeping the water sanitary and the loss of chlorine. If one tried to maintain a level much higher than 40% of CYA, it would be nearly impossible to pass the exit criteria of less than 1ppm overnight chlorine loss simply because the thermal loss rate of chlorine would exceed that.

This makes much more sense. I get practicality.

 

[mgtfp]

Just on a side note: To get an idea about toxicity of chlorinated isocyanurates in low concentrations, have a look at the ingredients list of water purification tablets used for hiking:

https://www.katadyn.com/downloads/katadyn/safety-data-sheets/micropur-forte-mf-1t_ghs_en.pdf

 

[SoDel]

Just for completeness: Technically you also have to consider OCl^-, which also shows in the FC-test. OCl^- is not an efficient sanitizer, but still a powerful oxidizer. There is a chemical equilibrium between HOCl and OCl^-. At very low pH it's nearly all HOCl, at very high pH it's nearly all OCl^-. Somewhere between pH 7.4 and 7.5 it's 50/50. And with CYA, the vast majority of chlorine is bound to CYA, but still tests as FC.

That’s a good point and routinely ignored. I don’t know much about the interaction between CYA and OCL^-. My assumption (uh oh) is it chlorinates cyanurates same or similar to HOCL so we don’t jump in that additional rabbit hole? Is that not right? How does the OCL fit into this and if it’s significant, then pH takes on a new dimension in this discussion (woo hoo lol).

 

[mgtfp]

When you add chlorine to a level of 40% of CYA, the algae is pretty much dead within a few hours and any bacteria that may have been in the water is completely wiped out. After the first 24 hours, the next 5 days is spent filtering the water.

That's a very good point. Standard SLAM-FC can be safely maintained for however long it takes filtering, and allows for an easy, one-fits-all SLAM process.

 

[mgtfp]

That’s a good point and routinely ignored. I don’t know much about the interaction between CYA and OCL^-. My assumption (uh oh) is it chlorinates cyanurates same or similar to HOCL so we don’t jump in that additional rabbit hole? Is that not right? How does the OCL fit into this and if it’s significant, then pH takes on a new dimension in this discussion (woo hoo lol).

You can't separate the chemistry. There are many equilibriums between all the involved species. If you want to know the details, search for the O'Brien paper on the forum.

Edit: One of the advantages of having CYA is that the pH-effect becomes less dominant. TFP levels ensure that at the higher recommended pH-end is enough HOCl, and levels are safe over the whole relevant pH-range. Details are in the stickies in the Deep End, no need to reiterate them here.

 

[SoDel]

Just on a side note: To get an idea about toxicity of chlorinated isocyanurates in low concentrations, have a look at the ingredients list of water purification tablets used for hiking:

https://www.katadyn.com/downloads/katadyn/safety-data-sheets/micropur-forte-mf-1t_ghs_en.pdf

So if I understand what you’re saying correctly, in terms of bather safety, we need to be concerned about not only FC but also FC bound to CYA as chlorinated isocyanurates. This is not something I’ve seen before — at what levels in a typical pool might we be concerned? It would seem easy enough to measure pool chemistry to arrive at the level. My guess is given the state of CYA regulation (or state of confusion), there are no standards related to this.

 

[mgtfp]

No, I wanted to say that we don't need to be concerned. Dichlor is the active ingredient in water purification tablets.

 

[Oly]

Great discussion.
I've always loved swimming in the Deep End.

 

[SoDel]

You can't separate the chemistry. There are many equilibriums between all the involved species. If you want to know the details, search for the O'Brien paper on the forum.

Edit: One of the advantages of having CYA is that the pH-effect becomes less dominant. TFP levels ensure that at the higher recommended pH-end is enough HOCl, and levels are safe over the whole relevant pH-range. Details are in the stickies in the Deep End, no need to reiterate them here.

I scanned through the O’Brien paper and ai understand the difficulty as you point out in something as dynamic as a pool. But what I also got out of it is that at given CYA / FC levels there is a calculable FC equivalent to water without CYA. This seems to tell me the HOCL vs OCL ratio is not relevant (assuming that pH and CYA and FC levels are not crazy).

 

[mgtfp]

But what I also got out of it is that at given CYA / FC levels there is a calculable FC equivalent to water without CYA. This seems to tell me the HOCL vs OCL ratio is not relevant (assuming that pH and CYA and FC levels are not crazy).

Pretty much. This calculation is what's behind the FC/CYA chart.

And yes, within TFP's recommended ranges, there is no practically relevant influence of pH on HOCl-levels.

EDIT:

I only added the comment regarding OCl^- in my earlier post, because I wanted to clarify that FC = HOCl without CYA is not quite correct. Correct is:

FC = HOCl + OCl^- + chlorinated isocyanurates (all measured in ppm Cl₂).

Without CYA, the last term disappears.
And at pH of about 7.4-7.5, you would get HOCl=0.5ppm at FC=1ppm.

 

[mgtfp]

See this post in the Pool Water Chemistry thread for the pH dependencies as calculated based on O'Brien:

Pool Water Chemistry

This thread presents my findings so far on pool water chemistry including the following: More Accurate Calcite Saturation Index (CSI) to replace Langelier Saturation Index (LSI) Calculation of ppm HOCl (disinfecting chlorine) at various levels of Total Free Chlorine (FC) and Cyanuric Acid (CYA)...

www.troublefreepool.com

 

[mgtfp]

One last link, which shows nicely the pH-dependence with and w/o CYA, and also shows some background on why HOCl (or FC/CYA as a measurable substitute that is proportional to HOCl) is the relevant parameter and not FC:

Richard Falk’s presentation on Risk of illness in stabilized chlorine pools - PWTAG - The Pool Water Treatment Advisory Group

www.pwtag.org

 

[tim5055]

One last link, which shows nicely the pH-dependence with and w/o CYA, and also shows some background on why HOCl (or FC/CYA as a measurable substitute that is proportional to HOCl) is the relevant parameter and not FC:

Richard Falk’s presentation on Risk of illness in stabilized chlorine pools - PWTAG - The Pool Water Treatment Advisory Group

www.pwtag.org

For those unaware, Richard Falk is @chem geek

He was (is) a member Centers for Disease Control and Prevention Council for the Model Aquatic Health Code Ad-Hoc Committee on CYA.

The committee issued a report saying FC should be a ratio of CYA, but I'll be darned if I can find it now. Here is an article you may want ot read.

Cyanuric Acid Debate - PoolPro

There was a standing-room-only panel discussion at the International Pool | Spa | Patio Expo this past November on cyanuric acid in pool water care. Panelists

poolpromag.com

 

[mgtfp]

The committee issued a report saying FC should be a ratio of CYA, but I'll be darned if I can find it now.

You are probably referring to this paper:

Assessing the Impact of Cyanuric Acid on Bather’s Risk of Gastrointestinal Illness at Swimming Pools

Current regulatory codes for swimming pool disinfection separately regulate free chlorine (FC) and cyanuric acid (CYA). It is well-known that CYA affects disinfection rates by reversibly binding to FC in aqueous solutions. However, limits for these regulated parameters have neither...

www.mdpi.com

Above Powerpoint presentation is basically an easier to digest summary of this paper.

 

[SoDel]

This is all starting to make more sense as a mix of the science with practicality. So, at least so far in this thought experiment, what I take away (at least for my own pool):

1. Maintain CYA at 30 to 50 ppm (at least to this point I’m rejecting the idea that salt pools somehow need more CYA until somebody eventually figures out exactly why and and back it with both scientific and empirical evidence).

2. assuming 30 CYA, then FC between 2 and 10 shall be considered safe to swim in;

3. if the pool goes green, raise FC to around 15 and start charting FC loss per time interval. Maintain a minimum FC of around 10 and add enough LC at each top up (up to around 20 max to keep things reasonable) so that it won’t fall below 10 between each top up (based on the experienced FC loss and making a reasonable guess as to what it will be in that next so many hour interval, anticipating the rate of loss will slow as the nasties burn off but also keeping UV in mind).

(this presumes that all other water chemistry parameters achieve saturation index range goal, which I’m really careful about because pool plaster is the one and only thing I can’t fix if it needs replacement).

I need to ask for some field experience for those who have done the ”SLAM” for a green pool — what would be your experience for FC loss per time interval as you went through the process and what were your observations of the green to gray to clear as you went through the process?

 

[Bperry]

I’m rejecting the idea that salt pools somehow need more CYA until somebody eventually figures out exactly why and and back it with both scientific and empirical evidence).

I’m not sure who came up with that idea, but it wasn’t in this site. Salt water chlorine pools don’t “require” more CYA. It’s just that from experience, it’s been observed that the FC levels can be somewhat lower when using a SWCG and not have an algae outbreak. Assuming that’s the case, you can then choose to run a higher ratio of CYA to protect the chlorine from sunlight degradation. I’m somewhat experimenting with that now since my CYA is in the low end of SWCG use.

 

[IceShadow]

I believe the current running theory on being able to run FC levels lower with a SWG is that the constant addition of chlorine means you don't run the risk of dipping too far below minimums at any given time. Adding LC is done once daily at most during normal operation (SLAM aside), and so the dosing for that should be higher to ensure you don't run too low before the next addition can start.

 

[kimkats]

I wanted to step in to say THANK YOU to everyone in this thread! @SoDel for asking the in-depth questions to @mgtfp @JoyfulNoise @IceShadow @Bperry @tim5055 for answering in such a way anyone reading this could follow and understand what was being said! We have some great ROCK STARS on TFP!

Kim

 

[SoDel]

This blog article came across my desk yesterday and though it looks to be short on references, the concept is that vinyl contains small amounts of calcium carbonate as a filler. We know what a calcium starved water balance will do with that. Should calcium recommendations related to vinyl liner pools be reexamined?

What say the experts lol?

Understanding Vinyl Liner Pools

Swimming pool vinyl liners are more affordable, faster to install, and should be easier to maintain chemistry. But aggressive water can still damage them.

blog.orendatech.com

 

[PowerPigg!!]

The referred study appears to be dated 2019. Things seem to have changed since according to this 2021 post.

Link to 2021 comments on similar question

Footnote 4 seems very recently updated but makes no reference to calcium content. Did the author maybe deduce content the certification tests?

The author seems to be saying that corrosive low-calcium water will otherwise actively leaching into vinyl anyway looking for what little calcium there may be. I may be missing something because this doesn't really make sense to me. How would water "know" that calcium not exposed to the water by direct contact is somewhere in the liner to be found, if vinyl is not supposed to be permeable in that way?

The author then discusses a mechanism by which this still corrosive water continues to attack vinyl after all the calcium had been leached out of the liner. Wouldn't we expect water that is otherwise balanced to no longer affect the vinyl at this point? And how then would a higher calcium content prevent this exactly?