Zero Alkalinity Acid Wash questions/prep-work

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S

shaawnzee

0
Jul 12, 2016
5
Katy/TX
First off, thank you to all on this forum for the wealth of knowledge and education y'all have given me.

I have only owned a pool for 8 months. The prior owners did not maintain the water chemistry and the aggregate surface is frosty and there is some scaling on the waterline tile. I have always had trouble keeping the pH from creeping up, despite dropping the pH/alkalinity and letting it come up naturally. It seems like in the summer I could add a gallon of acid a week.

Would the scale buildup be causing this and would the acid wash fix this once the chemistry is brought back in line or is it normal for a SWG pool's pH to continually rise?

It's getting that time of year in Houston that I need to get the water back inline for swimming season (mainly bring my CYA and borate back up after being diluted in the off season, so my below numbers will be low).

I want to do the no drain acid wash anyway to dissolve all of the scale and I have some questions for those more experienced than I.

Everything I have read, the amount of acid and target pH ranges a bit (4-12 gallons and 3.5-6, respectively). Pool Math says I need 17 gallons to get to a pH of 4, which seems excessive. What is the correct target pH for an aggregate pool and is there a more realistic acid volume to get there?

Do I need to protect the metal fixtures around my lights while the pH is down? If so, how best to do this?

I have also read sometimes two rounds is needed to achieve the desired results. Why is that? Couldn't you just add more acid to bring the pH back down to target since the acid gets spent as it dissolves the scale? That way it saves the money on chemicals to bring the TA back up just to knock it down again.

What's the best way to clean the tile at the waterline, since much of the scale will be out of the water? Garden sprayer with straight acid and brush? I also have some mineral structures on my waterfall that would make a cave jealous that I think need the straight stuff.

Any other advise/information you could throw my way would be very appreciated (aside bring never-ending brushing, kill pumps due to heater w/o a bypass, and a sub pump...I see those recommendations a lot).

Thank you all in advance,
Shawn


22000 gallons
SWG
Aggregate plaster
pH 7.8
TA 70
CH 300
CYA 30
Borate 40
Salt 3300
 
Welcome to TFP!

Here is the best info on a no drain acid bath, The Zero Alkalinity Acid Treatment

As for your scale, it is fairly easy to prevent. Keep your CSI slightly negative, between -0.3 and 0, to prevent scale. Many folks find that TA around 50 and pH at 7.8 world's best to prevent pH rise. When pH gets to 8.0 lower it back to 7.6. 50 ppm of borates also helps slow pH rise.

More here about lowering TA, Pool School - Lower Total Alkalinity

More here about scale, Pool School - Calcium Scaling
 
Thank you for the input. I have read that article several times but it doesn't address whether I need to somehow protect the metal housings on my lights or advise on best practices to tackle stuff that's above the water line. Are you able to help with these aspects?
 
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If you are going to do a no drain acid wash, then you should pull the lights from their niches and put them up on the deck along with the metal retaining ring. I would only do the process as long as you have a way of bypassing the pool heater (if you have one? please update your signature with equipment) because the metal heater core can be damaged by low pH. You'll also need to pull out or by-pass your SWG as low pH is detrimental to the SWG electrolytic cell plates.

PoolMath is not going to be good at estimating large pH changes. If you're going to do this, I would say you need a well-calibrated pH meter to properly know where the water is at; the phenol red drops with your test kit will not help once the pH is below 7.0. You're going to need 3.25 gallons of MA to simply drop the TA from 70ppm to 0. Then you'll need some additional acid to drop the pH down to about 4.0. Once the TA and pH are reduced, you need to brush pool and keep a close eye on the surface to ensure that you're not causing excessive etching of the plaster. Once the scale is reduced to your satisfaction, then you need to immediately neutralize the acid and bring the pH up using baking soda. It will take roughly 25lbs of baking soda to change your TA from 0 to 70ppm. You're going to want to have more than that on hand though as the process will take a bit more baking soda to neutralize the acid before it will begin to raise pH. You'll want to add the baking soda in small batches, trying to achieve an incremental increase in TA (say 20ppm with each addition) and brush it around a lot to help with mixing. If you raise the pH and TA too quickly, the scale can return.

The no-drain acid wash is NOT a simple process and over-doing it can screw up your plaster. People think it as an easy method to fix scale but it is dangerous to your plaster surface and can cause equipment damage if it's not executed properly. I would consider other, less radical alternatives before going this route as it is too easy for a novice pool owner to cause irreparable damage.
 
JoyfulNoise said:
If you are going to do a no drain acid wash, then you should pull the lights from their niches and put them up on the deck along with the metal retaining ring. I would only do the process as long as you have a way of bypassing the pool heater (if you have one? please update your signature with equipment) because the metal heater core can be damaged by low pH. You'll also need to pull out or by-pass your SWG as low pH is detrimental to the SWG electrolytic cell plates.

PoolMath is not going to be good at estimating large pH changes. If you're going to do this, I would say you need a well-calibrated pH meter to properly know where the water is at; the phenol red drops with your test kit will not help once the pH is below 7.0. You're going to need 3.25 gallons of MA to simply drop the TA from 70ppm to 0. Then you'll need some additional acid to drop the pH down to about 4.0. Once the TA and pH are reduced, you need to brush pool and keep a close eye on the surface to ensure that you're not causing excessive etching of the plaster. Once the scale is reduced to your satisfaction, then you need to immediately neutralize the acid and bring the pH up using baking soda. It will take roughly 25lbs of baking soda to change your TA from 0 to 70ppm. You're going to want to have more than that on hand though as the process will take a bit more baking soda to neutralize the acid before it will begin to raise pH. You'll want to add the baking soda in small batches, trying to achieve an incremental increase in TA (say 20ppm with each addition) and brush it around a lot to help with mixing. If you raise the pH and TA too quickly, the scale can return.

The no-drain acid wash is NOT a simple process and over-doing it can screw up your plaster. People think it as an easy method to fix scale but it is dangerous to your plaster surface and can cause equipment damage if it's not executed properly. I would consider other, less radical alternatives before going this route as it is too easy for a novice pool owner to cause irreparable damage.
Click to expand...


Thank you JoyfuNoise.
I appreciate the words of caution. My equipment will be off as I do not have a bypass and i will me using a submersible pump to help with circulation.

No worries on the pH. I have a very expensive pH meter and the calibration standards to go with it...benefits of being a chemist. :D

Regarding bringing the pH up slowly...
Once the pH is increased, there isn't much you can do to keep the scale from reforming (time independent). I thought you wanted to do it quickly to make all the scale precipitate out as fine particles to be caught by the filter versus slowly and created a supersaturation scenario (high scaling index) and giving the scale crystals a chance to nucleate on the plaster surface and grow larger resulting in the original scaling issue? This is at least my experience with scale and water treatment working in the oil patch.
Or is the slow addition more so you do not overshoot your target?

Is it better to use baking soda or soda ash and finish with baking soda?


Thank you for the continued help.
 
shaawnzee said:
Thank you JoyfuNoise.
I appreciate the words of caution. My equipment will be off as I do not have a bypass and i will me using a submersible pump to help with circulation.

No worries on the pH. I have a very expensive pH meter and the calibration standards to go with it...benefits of being a chemist. :D

Regarding bringing the pH up slowly...
Once the pH is increased, there isn't much you can do to keep the scale from reforming (time independent). I thought you wanted to do it quickly to make all the scale precipitate out as fine particles to be caught by the filter versus slowly and created a supersaturation scenario (high scaling index) and giving the scale crystals a chance to nucleate on the plaster surface and grow larger resulting in the original scaling issue? This is at least my experience with scale and water treatment working in the oil patch.
Or is the slow addition more so you do not overshoot your target?

Is it better to use baking soda or soda ash and finish with baking soda?


Thank you for the continued help.
Click to expand...

You ABSOLUTELY DO NOT want to use soda ash! You use baking soda because, as you bring the alkalinity up, you want the carbonate alkalinity to remain as bicarbonate ion (HCO3-) and not as carbonate (CO3^2-). Calcium bicarbonate will only exist as an aqueous solution whereas calcium carbonate will precipitate. The solubility of calcium bicarbonate is 166 g/L while the solubility of calcium carbonate is 0.013 g/L. In a plaster pool you will never be able to ensure that scale will occur in bulk solution versus the plaster sidewalls - in fact, you are highly more likely to develop scale on the sidewalls as the number of nucleation sites is infinitely larger than in bulk pool water. The energy barrier for spontaneous nucleation of most crystalline materials from solution when a substrate is present is much lower. That is why, if you want to make rock candy, you always use a string or stick that has granular sugar on it before dipping it into the saturated sugar solution.

You can absolutely control scale - my CH is nearly 1000ppm and I have no problem managing my pool water balance to avoid scale. You simply need to keep your CSI in the negative range (pool water is undersaturated with calcium carbonate) to keep scale from forming. You can also use a scale control product (chelating agent) to make it harder for calcium ions to react with carbonate and form scale. Another approach would be to do the no-drain acid wash and then dump half the pool water after neutralizing some of the acid and refill. That way, you get rid of some of CH causing the scale.
 
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Thank you JoyfulNoise and pooldv, I'll use bicarb then. I thought I read somewhere that you might overshoot the TA before the pH is up to where it needs to be. I wasn't sure I could trust Pool Math with such a large pH swing as it's disclaimer states. I already purchased boric acid for a later treatment, so I wasn't considering borax an option for aid in pH elevation.

Good point regarding surface area for nucleation.


I'll get my signature filled out soon.
 
At pH values lower than 6.5, it is impossible for a lot of bicarbonate to exist for any substantial period of time as the equilibrium chemistry will force the dissolved inorganic carbon to be in the form of aqueous CO2. If your TA reaches normal levels before the pH is above 7, then simply wait or run some water features or brush the pool vigorously - the outgassing of CO2 will cause the pH to rise.
 

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Perhaps you should read this article to see what can happen when someone doesn't get the chemistry right in a no-drain acid wash -

Acid soak/bath questions

It's a warning to all those that would try this process that you need to be extremely careful...
 
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