Risks with high alkalinity?

[aquarium]

Okay,

That's what I thought, but was hoping maybe you knew something I didn't about getting CH lower.

High alk and CH usually come together in the fill water, which is my situation. About 220ppm alk, 430ppm CH.

I lower the alk each time I top up, but I know the CH is just getting worse and worse due to evaporation. So far not seeing a problem.

BTW, it's easy to use one of these to aerate each time during a top-off. I just screw it in for a day or so, then remove it:
EDIT - And add acid too. END EDIT

[chem geek]

High alk and CH usually come together in the fill water, which is my situation. About 220ppm alk, 430ppm CH.

Tom,

I am interested in what pH range you are operating with. It will be interesting to see what the relative CO2 outgassing rate is and the saturation index. More real-world data -- I love it!

Richard

[aquarium]

I try to keep it at 7.2-7.6 to maximize the effectiveness of the chlorine. Which, BTW, I can keep at 1.5-2ppm (EDIT - 30ppm CYA) and stay clear blue. If you'll notice in the pic above, this pool is cut into a hill and I think the deep end is cooled by the ground, so it stays close to 84 degrees F, which might be helping.

[chem geek]

I try to keep it at 7.2-7.6 to maximize the effectiveness of the chlorine. Which, BTW, I can keep at 1.5-2ppm (EDIT - 30ppm CYA) and stay clear blue. If you'll notice in the pic above, this pool is cut into a hill and I think the deep end is cooled by the ground, so it stays close to 84 degrees F, which might be helping.

So here are the "calculated" numbers for your pool, just so we can see if theory matches reality or not.

At pH 7.2, the CO2 outgas rate is 45.7 which is very high; at 1.5 FC your ppm HOCl is 0.023; at 2.0 FC it's 0.032

At pH 7.6, the CO2 outgas rate is 17.2 which is possibly near "stable"; at 1.5 FC your ppm HOCl is 0.019; at 2.0 FC it's 0.027

So this implies you are having a strong tendency of rising pH. The theory would say that if you started at 7.6, then you would see a much smaller pH drift upwards and would use less acid. In fact, somewhere in the range of 7.6 to 7.8 could be a place where you would be about stable with little or no perceptible drift.

As you can see from the ppm HOCl numbers, they don't change that much with pH (due to CYA acting as a buffer for HOCl). You are at the "low" end of Ben's table range so proabably what he considers to normally prevent algae though he has given me worst-case examples with algae that were at about 0.05 ppm HOCl.

Even if you let yourself drift to a pH of 8.0, your FC of 1.5-2.0 would mean a ppm HOCl of 0.017 to 0.024 which is lower, but not by much. You could just increase your FC to be between 2.0 and 2.5 (or 3.0) and you'd be in pretty much the same shape.

I think the fact that CYA acts as a chlorine buffer to offset the effects of pH is a useful piece of information from the theory that would allow you to operate at higher pH with proper disinfection and algae prevention. If you are willing to take a small chance to try it out, I think it would be worth it given your high alkalinity and CH situation.

Now, as for the saturation index, yours is 1.0 and since you have no evidence of scaling nor cloudiness, that shows that the index can get quite high (we still don't know how high) before scaling is an issue. By the way, do you know your level of TDS? A higher TDS lowers the saturation index. Just based on your alkalinity and CH your TDS has to be at least 860.

Richard

[jenpen400]

When I filled our AGP a year ago CH was 210 and ALK was 320. I was able to drop the ALK down to 110 but it is much harder to push down from a high number. Once I got the Alk to 220 it was easier. I know how you feel add acid, arate Alk goes down now add water to make up for evaporation and back up it goes. I found that just putting the fountain on low and keeping the ph around 7.4 or lower the alk just keeps creeping down without as much evaporation. Now my alkalinty is fine and my CH is 340. Oh well I guess thats another thread.

Hope this helps
jennifer

[chem geek]

When I filled our AGP a year ago CH was 210 and ALK was 320. I was able to drop the ALK down to 110 but it is much harder to push down from a high number. Once I got the Alk to 220 it was easier. I know how you feel add acid, arate Alk goes down now add water to make up for evaporation and back up it goes. I found that just putting the fountain on low and keeping the ph around 7.4 or lower the alk just keeps creeping down without as much evaporation. Now my alkalinty is fine and my CH is 340. Oh well I guess thats another thread.

Hope this helps
jennifer

Jennifer,

I just want to understand clearly. Are you now at an alkalinity of 110 or 220? Is the evaporation less because you've got the fountain on a lower setting? So now you have a balance between losing alkalinty through the aeration (and adding acid to restore the pH) and gaining alkalinity from make-up water for evaporation. Is that right? So your alkalinity is stable, but aren't you adding a lot of acid frequently to keep your pH down? I suppose that may be the tradeoff necessary.

Evap. --> Make-Up --> Alk. Up
  ^                       V
  |                   Fountain
Alk. Down                 V
pH Down <---- Acid <---  pH Up

Is that your cycle?

Richard

[Tredge]

Hi Rangeball!

My pool is Still at 340 Alk and I havent bothered to lower it.
Water is 'crystal' clear and no scaling problems.

My CA hardness is quite low...almost 0.
PH stabalizes at ~7.8 but I try to knock it down to 7.4 every few days.

My TDS is quite high due to the SWG. At around 3000 TDS currently.

Hope that helps, I've been happy with the pool and the alk hasnt bothered me a bit.

[chem geek]

My pool is Still at 340 Alk and I havent bothered to lower it.
Water is 'crystal' clear and no scaling problems.
My CA hardness is quite low...almost 0.
PH stabalizes at ~7.8 but I try to knock it down to 7.4 every few days.
My TDS is quite high due to the SWG. At around 3000 TDS currently.

For what it's worth, the pH stabilization at 7.8 is "predicted" by my spreadsheet and the CO2 graph except that on the graph I made it's off the chart (looks like I need to make a new graph for you high alkalinity folks ). As for your calcium hardness, you really don't want it to get to 0 if you have a plaster/grout pool. The problem with near 0 calcium isn't scaling, but corrosion (not of metal, but of plaster/grout). Is your pool vinyl? Is your CYA near 80 since you have an SWG? Mostly I'm asking just to get a sense of everyone's pool so I can see where the models start to break down.

Richard

[Rangeball]

Hey Tredge Glad things are working in your favor

Mostly I'm asking just to get a sense of everyone's pool so I can see where the models start to break down.

Richard, my pool fyi-

21,200 gal IG vinyl, 3' to 8' depth.
Last time I tested fill water- PH 7.2, ALK 320.
Chlorinated with .75 gal of 6% bleach nightly. Salt approx 1200 ppm, no SWG.

I ran some tests when I was home for lunch.

TC/FC- 3.0
PH- 7.6
ALK- 170
CYA- 20ish

I was gone the past 3 days, and right before I left the PH was 8+. I added some muratic acid, and this is where things stabilized when I returned.

I highly suspect that with anymore splashing about (another kid fest), my PH will again shoot rapidly up. I always panic when it does because my test kit's upper range is 8.2, and I don't want it to get above that level but won't be able to tell if it does, so I knock it back down. What conditions would it take for my pool to go much higher than 8-8.2?

[Tredge]

For what it's worth, the pH stabilization at 7.8 is "predicted" by my spreadsheet and the CO2 graph except that on the graph I made it's off the chart (looks like I need to make a new graph for you high alkalinity folks ). As for your calcium hardness, you really don't want it to get to 0 if you have a plaster/grout pool. The problem with near 0 calcium isn't scaling, but corrosion (not of metal, but of plaster/grout). Is your pool vinyl? Is your CYA near 80 since you have an SWG? Mostly I'm asking just to get a sense of everyone's pool so I can see where the models start to break down.

Richard

I have a heavy epoxy coating on the pool so the grout and plaster are protected.

I have my CYA at 40. I'm reluctant to follow instructions and bump it to 80