Co2 for PH control

[mkohr]

I have been using the BBB method for several years now and always seam to have high PH. Everyone in this form says liquid chlorine doesn't drive up PH but i seams too in my pool.
I have a 25,000 gallon plaster swimming pool and i use a Stenner 3GPD pump with a 7.5 gallon tank.
I inject my liquid chlorine at night at a rate of about 3/4 gallon per day. I usually check my PH when i get home from work which is about 12 hours after the chlorine was dispensed and always have high PH... dark red on the test kit cant even tell a number... off the chart!.
I try and maintain around 1PPM and 0-.5 PPM used chlorine. It is typically in that range when i get home form work around 5:30 pm.
So i dump some acid in and it comes down as expected, only to start the cycle all over again. So decided to try the co2 thing, i used a 5lb tank from a beer tap and injected into the return water. My method was lets say less than scientific. and not sure how much CO2 went in. I basically let in on for a few burst of 10-15 seconds each.
The co2 brought down ph as expected but it seams to keep it down longer (next day was still good)
So this brings me to several questions...
why is my PH always high?
Does acid not work as well as co2 for longevity?
Is there a economical co2 injection system for residential pools?
Should i use an acid injection system.
Which is better co2 or acid?
How do you determine how much co2 to dispence to bring down PH to desired level?

Water chemistry is...
FC 1.0
PH7.4
Alk 110
Calcium Hardness 260
CYA 40

This is the chlorine i have been using

 

[kellyfair]

Your FC is scary low for your CYA. Should be in 5-7 range.

Do you have any kind of fountains or water features going? That will raise your pH. So will lots of action in the pool splashing etc. Have you tried to lower your alkalinity by repeatedly dosing with muriatic acid down to 7.2 pH and then aerating back up and then lowering again? Your alkalinity might be a bit high to keep a stable pH, and if you lower it to about 80 or 90, you might find that your pH is more stable.

 

[mkohr]

FC5-7 really I was always told 1-2ppm

 

[tim5055]

FC5-7 really I was always told 1-2ppm

Not by anyone around here. We follow this chart - [FC/CYA][/FC/CYA]

You may want to let that TA come down some and you may see less pH rise.

Here are our recommendation for a plaster pool chlorinated with bleach


FC 3-7 (See FC/CYA chart)
pH 7.2-7.8
TA 70-90+
CH 250-350
CYA 30-50

 

[JamesW]

1 to 2 with zero CYA might work.

With CYA, you need to maintain the fc at about 7.5% to 15% of the CYA.

If your pH keeps rising, you're keeping the TA too high. Try a TA of 60 and a pH of 7.8.

CO2 carbonates the water like a soda. But, the CO2 just offgasses into the air like a soda going flat. It's an endless and expensive process that has no real benefit.

 

[mknauss]

Put "co2 injection system for pools" in to Google. They make them. Be aware it adds alkalinity. You must use muriatic acid to reduce your alkalinity.

Take care.

 

[mkohr]

Not by anyone around here. We follow this chart - [FC/CYA][/FC/CYA]

You may want to let that TA come down some and you may see less pH rise.

Here are our recommendation for a plaster pool chlorinated with bleach


FC 3-7 (See FC/CYA chart)
pH 7.2-7.8
TA 70-90+
CH 250-350
CYA 30-50

I would guess the FC higher in the am I typically test when I get home which is 12 hours after the chlorine was Injected and typically is around 1-1.5 ppm FC and used is around .5-1.0 worst case at that time. I have had the same rate of injection for years and have always been chasing ph. No fountains or aeration. I know myTA is a bit high at the moment.

 

[JamesW]

Adding or losing carbon dioxide only affects the pH. The TA does not change.

However, if the cause of the pH rise is not from carbon dioxide loss, introduced carbon dioxide can convert hydroxide alkalinity into carbonate alkalinity.

 

[mknauss]

The website for a CO2 injection system for Pools states:



CO2 increases the alkalinity of water and is therefore not recommended for applications where high alkalinity is a problem. Total Alkalinity (TA) shoul be maintained between 80 and 125 ppm.

High alkalinity increases acid demand, makes it difficult to lower the pH and may result in the formation of bicarbonate scale,

High alkalinity may be reduced with muriatic acid HCl or dry acid.

 

[JamesW]

Adding CO2 converts hydroxide alkalinity into bicarbonate alkalinity.

CO2 + OH --> HCO3

So, carbon dioxide is not responsible for increasing the total alkalinity.

For the total alkalinity to increase, something alkaline needs to be added.

If a sodium hypochlorite solution has excess lye (hydroxide), it will raise pH and TA. It's the hydroxide that raises the total alkalinity.

Adding CO2 to lower the pH converts hydroxide alkalinity into bicarbonate alkalinity.

To lower pH rise caused by something other than CO2 loss, hydrochloric acid should be used.

H + OH --> H2O

When CO2 is lost, pH rises with no change in TA.

HCO3 + H --> H2O + CO2

So, the injection of CO2 is the reverse of aeration. You're just forcing CO2 into the water instead of forcing it out.

CO2 + H2O --> HCO3 + H

So, bicarbonate is created, which increases the alkalinity but a hydrogen ion is also created, which lowers the pH and alkalinity for a net change in the total alkalinity of zero.

If pH rise is only due to carbon dioxide loss, reinjecting the carbon dioxide brings you back to where you were before the carbon dioxide was lost.

If the carbon dioxide is injected at the same rate as it's lost, the pH and TA remain stable.

The problem is that CO2 is expensive and you have to keep reinjecting it to replace what offgasses into the air.

 

[mkohr]

How do I figure out how much co2 to use?

 

[JamesW]

3.67 lbs CO2 is equal to adding 1 gallon of acid as far as lowering the pH.

Edit: It depends on the final target pH. For a target pH of 7.8, you need 3.8 lbs CO2. For a target pH of 7.5, you need 3.9 lbs CO2. For a target pH of 7.0, you need 4.49 lbs CO2 to match the pH drop of one gallon of 31.45% muriatic acid.

For example, with a TA of 110, you need 26 oz acid to lower the pH from 7.8 to 7.5. That's equivalent to 0.79 lb or 12.68 oz (weight) CO2.

Also, that's at 100% of the carbon dioxide getting dissolved into the water.

If the injection process is not good, you might have low efficiency in transfer, which will cause you to use more.