Using large amounts of acid with infinity edge pool

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petros4060

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Feb 13, 2011
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I have a 98000 litre infinity edge pool with about a two metre drop to the trough below the infinity edge.
I have an automatic chlorine and acid dispenser that maintains pH and OCP.
The chlorine and acid dispenser do a good job of maintaining pH around 7.5 and chlorine level around 3.
I run alkalinity around 70-90, CH around 250-300 and cyanuric acid around 40.
The problem with the setup is that the pool is using large amounts of HCl acid. About 20 litres every two weeks. If I allow the alkalinity levels to drop to around 40 this reduces markedly. I have hypothesised that the pH correction that is happening at higher alkalinity is due to the large water fall over the infinity edge alkalinising the water.
The high use of acid is a problem and I would value advice regarding possible solutions. Can I continue to run the alkalinity at 40 without damaging my pool. It is a tiled inground pool. Is there another way to reduce acid consumption.

Peter :?
 
would it be cheaper to switch to a Dry Acid day tank? (Similar to a Cal Hypo Chlorine Day Tank where you top up with water and add the dry chemical to dissolve, with usually some type of agitator or manual stirrer inside) I never thought about that before, as on deck level pools I have never had to buy the chemicals for it, but for a residential owner this will be an expensive overlay to maintain an overflow system.

When your TA was at 40 did you not experience a considerable amount of pH bounce? Any I have worked on have bounced all over the place with TA as low as that.

Another possibility is to use Chlorine Gas as it's Acidic, but can be expensive.

Quick queery though - does your infinity pool have a balance tank? Nothing whatsoever to do with the problem, just total curiosity!!

Regards
Stuart
 
Thanks for the reply Stuart. Sorry it has taken so long for me to reply - life is too busy at the moment and the pool is hogging my time at present.

My pool does have a balance tank about one metre wide and 60mm deep. There is a fall of about 1.5 metres from the infinity edge to the surface of the water in the balance tank. There is considerable turbulence when the falling water hits the balance tank. The pool is about 15 metres long and 5 metres wide. It has an infloor cleaning system. I use hypochlorite chlorine and hydrochloric acid diluted 1 in 5 which is fed through the chemigem controller.
The chemigem controller I use is really convenient when it is working and automated - I don't have any knowledge of dry acid day tanks but sounds like potentially more work.

Interestingly I found an entry from chem geek on France Forum with a chemigem question similar to mine. I am not sure if it is the same person as chem geek on this forum.

The reply was:

"Assuming you are using a hypochlorite source of chlorine (and it sounds like you are via an automated dosing system), you do NOT want your Total Alkalinity (TA) to be very high -- probably no higher than 80 ppm. Basically what is happening is that you are adding baking soda which makes the TA rise and to some extent the pH as well, but the higher TA causes faster carbon dioxide outgassing that has the pH rise. This is triggering your pH controller to added acid which lowers both the pH and the TA. You then add more baking soda and are in a vicious cycle.

When using a hypochlorite source of chlorine, you generally want the TA to drop as needed until the pH becomes more stable with less of a rise, though I wouldn't go below 50 ppm in any event. If your TA does end up on the low side, you can increase the Calcium Hardness (CH) to compensate for the calcite saturation index or you can increase your pH target a little on your controller (but probably not above 7.8 at the most).

This chart shows how far out of equilibrium pool water is in terms of being over-carbonated with respect to air at various TA and pH levels. I'll bet you find that you use a lot less acid at a lower TA level.

As for hydrochloric acid vs. sulfuric acid, they are identical in their acidic effects and differ only in what is leftover -- chloride with the former, sulfate with the latter."


If this is correct it answers my question. It means when I allowed TA to drop to around 50 I used much less acid and the pH seemed to be stable. I see to compensate you should increase calcium hardness which I had already increased to around 300.

Unfortunately the help staff at chemigem didn't seem to have this knowledge. They advised me to increase alkalinity to over 110 to reduce pH bounce and turn off the acid feed for 48 hours after adding the bicarbonate. After 24 hours my pH was 8.0 and I didn't feel comfortable to allow it to get any higher. They said the large infinity edge drop to the balance tank would make no difference. The infinity edge would make a significant difference if it was increasing the carbon outgassing referred to by chem geek.

I wondered if chem geek reads this if he could confirm the advice I have read on the other forum and my theory about the infinity edge :-D .

regards

Peter
 
Yes, I'm the same chem geek and yes, if you drop your TA further (which it will do on its own from the acid additions), then you should see a drop in the rate of pH rise and especially in the amount of acid that needs to be added.

I would not use dry acid (sodium bisulfate) instead of HCl because higher sulfate levels can damage concrete/plaster (see this thread and this thread).

If you do end up with a lower TA and if that does help, then you can use a higher pH target (say 7.8) and a higher Calcium Hardness (CH) level to have the saturation index still be near zero to protect plaster surfaces.

Obviously, the staff at chemigen don't know what they are talking about. This is not uncommon in the pool industry where the idea that TA is a SOURCE of rising pH due to carbon dioxide outgassing seems to be known by relatively few. TA is mostly a measure of bicarbonate and how over-carbonated the pool water is with respect to the equilibrium amount from the carbon dioxide in air. In almost all pools there is ALWAYS outgassing, but in most pools it's fairly slow. When there is lots of aeration, the outgassing is faster as it is when the pH is lower or the TA higher as shown in this table. Perhaps after you've demonstrated in your own pool that lower TA works better with less acid usage, you can then explain this to the folks at chemigen so they don't give such inaccurate advice to others.
 
Thank you all for the replies and thank you chem geek for confirming your previously posted advice and giving further advise.
At last after 2 years I have a clear description of the problem and possible solution. I had been putting 20 litres of hydrochloride acid into the pool every two weeks which meant diluting to 100 litres to achieve the 1:4 dilution recommended. I was maintaining pH at 7.5 ant trying to keep TA above 70. The bicarbonate costs were considerable as well. I noticed when TA dropped in winter, when i was testing the water less, to around 40-50 the acid consumption dropped hugely.
I will increase pH to 7.8 and maintain alkalinity at a lower level. Calcium hardness is already pretty high.
My last area of confusion is the relationship between ORP and pH. There are a few articles written on this and I find them a bit difficult to follow. My current understanding is that ORP measures oxidation potential of the chlorine which is the HOCL component. I also understood that despite HOCL reducing with increased pH the ORP continues to represent an accurate measure of the sanitising capacity of the chlorine in the pool until pH reaches about 8.0. Would anyone mind confirming or correcting my understanding. My apologies if I should have started a new thread on this one. I am new to forum use.

Regards

Peter
 
Sorry to interject, but I do not follow chem geek's recommendation. If you start maintaining a high pH, don't you have to LOWER the CH to maintain the same CSI? Although lowering the TA also lowers the CSI, so maybe that is the piece of that sentence I was missing.
 
By lowering the TA in the 60-80 range that is recommended for SWG users, the CSI goes negative really fast and by keeping the pH in the 7.6-7.8 range along with a somewhat higher CH value, that will offset the low TA and bring the CSI up some.
 

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The devil is in the details so it really depends how low he puts the TA and how high he moves the pH. The real answer comes from the saturation index using whatever numbers for TA and pH one finds are "stable" and satisfactory. Then one just adjusts the CH accordingly. If he goes from pH 7.5 TA 80 to pH 7.8 TA 50, then the saturation index goes up by only 0.04 so not very much change. If he went from pH 7.5 TA 90 to pH 7.7 TA 50, then the saturation index goes down by 0.12 so perhaps the CH should be raised from 300 to 400 in that case (for example).

As for ORP, forget about it except for process control. That is, you separately measure your Free Chlorine (FC) with a proper FAS-DPD chlorine test kit and do that when the pH is where you want it. Note the ORP and set the ORP setpoint to that value to maintain the FC. Forget the absolute ORP value itself -- it is close to useless except in pointing out extreme problems.

When the pH is higher, the ORP drops not only because HOCl drops but also because of a direct effect of pH (hydrogen ions) that are part of the half-reaction potential being measured. However, with CYA in the water, the drop in HOCl isn't that much. When one goes from 7.5 to 8.0, then the HOCl drops in half if there is no CYA, but with usual CYA levels the HOCl drops by only around 15%. This is shown graphically in this post. ORP is a proxy for HOCl, but is not a direct measurement of it and other things affect ORP that don't affect disinfection rates. Also, see this post for how ORP differs a lot from different manufacturer's sensors measuring the same type of water (and this comes from their own tabled data!).
 
Thanks again chem geek

I thought from my reading that ORP was an indicator of HOCL but better to rely on FC levels.

Thank you for the links to the posts. I scanned them and will come back to them later. You have such a great way of condensing complex information into understandable bytes.
In my simple understanding as long as there is adequate cyanuric acid levels - and I guess you're recommending 50 or over but less than 100 - and adequate FC levels - I'm thinking of using the fc/cya chart - the pH is not that critical. Thus I can raise the pH to accommodate the change in alkalinity and maintain a correct saturation index as long as it say stays below 7.9. The CH also may need to be adjusted.

The ORP is useful as it can be a reasonable predictor of FC once a correlation has been measured on my system between the FC I want and the ORP at that level. Thus ORP can be used to maintain the FC level. It is not as the chemigem tech advised me that I shouldn't worry about the FC and cya relationship as long as the ORP is 650.

I think I understand. Thanks again for your time and advice and all the incredible work you' ve published on the forum.

Stuart I will try and post a picture but there is another tale to be told, to go with the picture, about construction problems with infinity edge walls.

Peter
 
Peter,

You've got it! Just follow the FC/CYA chart. I wouldn't have the CYA get to 100 ppm -- 80 ppm would be the highest but given your ORP sensor you probably don't want to go above 50 ppm. Many ORP sensors seem to have a problem with higher CYA levels. At first I thought that was because the FC wasn't raised proportionately so the sensors were measuring lower ORP values that were more noisy (see the second graph in this post), but some have reported that even at the same FC/CYA ratios (so roughly same HOCl levels), the higher CYA levels did something to have the ORP become erratic (perhaps some chemical "fouling" of the sensors). Chemigem may have some recommendation for maximum CYA level.

As for pH, though the higher pH will slow down carbon dioxide outgassing so use less acid, the risk of getting too high in pH is mostly that of metal staining. So if you have metals (iron, copper, manganese) in your water, then you may not want to go as high in pH. Hopefully, though, you don't have metals and can target 7.7 or 7.8 readily and find a nice sweet spot for better pH stability (along with the lower TA level). You can also just experiment and see the difference in acid usage at different pH targets.

After you get things settled and have an FC that is a minimum of 7.5% of the CYA level and have set your pH target, let us know what ORP gets reported. If I assume 4 ppm FC with 50 ppm CYA and a pH of 7.8, then that is an "official" (based on tables or field measurements) ORP of around 655 mV for Chemtrol, 627 mV for Oakton, 542 mV for Aquarius and 418 mV for Sensorex. So you can see how ridiculous it is to look at an absolute ORP level to mean anything, at least for some of the sensors.
 
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