CO2 as a pH reducer - Problems with TA

[Retep]

I came across a public indoor Pool ,which is using CO2 in order to bring down the pH.
Sodium Hypochlorite is used as a disinfectant.
Now : It seems the TA creeps up by using CO2 and more and more CO2 is used the higher
the TA gets. . .
Doesn't really make sense to me that CO2 brings up TA - I suspect it might be the Sodium Hypochlorite
which adds to the Alkalinity build up.
Apparently TA levels should be kept at 80 - 90 ppm in order for the CO2 to do it's job ,
the pool in question got now a reading of 120 ppm. pH is kept at 7.35
In order to get down the TA one needs to add Hydrochloric Acid or Sodium Bisulfate.
Naturally this will lower the pH as well, which should not be a big deal as long it doesn't go below
6.80. Eventually it will creep up anyways, since Sodium Hypochlorite is used.
The pool manager wants to add Sodium Bisulfate instead of Muriatic Acid. . . I remember reading
somewhere that Sodium Bisulfate ( commonly refered as "Dry Acid") will bring down the pH , but not as much
the Alkalinity . . (?)
Any suggestions ?
Thanks in advance

 

[JasonLion]

CO2 does indeed tend to cause TA to rise. My understanding is that some of the TA increase comes from the excess lye in Sodium Hypochlorite. There isn't much, but over time it will slowly drive the TA up. TA will also go up any time water is added to the pool to replace evaporation. Even if the TA level of the fill water is low, it won't be zero.

Sodium Bisulfate will have the same effect on both PH and TA as Muriatic Acid, it just ends up costing more and adding sulfates (which aren't a big deal unless you use a lot over a very long period).

 

[chem geek]

Using CO₂ for pH control when the primary source of rising pH is CO₂ outgassing is insane. By having a lower Total Alkalinity (TA) level and targeting a somewhat higher pH, one can reduce this outgassing rate. If necessary, one can have a higher Calcium Hardness (CH) to maintain the same saturation index. As Jason points out, the rise in TA over time is from basic sources such as the excess lye in bleach or chlorinating liquid and TA in the fill water or from plaster curing. The best way to compensate for these is to use Muriatic Acid.

Technically, the increase in TA at the same pH level comes from the combination of a basic source such as excess lye plus adding CO₂. It is the exact opposite of reducing TA by a combination of an acidic source such as Muriatic Acid plus aeration. Adding CO₂ or having it outgas (with or without aeration) doesn't change the TA, but since it changes the pH the subsequent addition of acid or base is what ends up changing the TA when you get back to your target pH.

 

[Retep]

Thanks Jason and Chem Geek for your input.

I also think the easiest and least expensive method would have been to simply use a liquid based pH reducer (e.g. muriatic acid 35 %).

As we all know, there are always some political reasons and justifications involved when a municipality decides
to use a certain water treatment . Many times it is the company which builds the pool who suggests
a certain method ( in this case it was the pool builder who suggested CO2 - his justification were "safety issues" with acid. . . ).
Now the municipality is stuck with paying the bill for excessive CO2 consumption. . .The company which built the
pool has long since walked away after getting paid. As usual they now blame anything and everybody else for the high CO2 consumption.

Perhaps it might be ok to use CO2 when your filling water from the City comes in at a low TA ( like 40 - 50 ppm) , but this municipality gets already
80 - 90 ppm straight out from the tap.

 

[JamesW]

If they are going to use CO₂, then they should still follow better practices, as chem geek has indicated.

First, they should use a higher target pH. I would suggest 7.8. Second, they should target a lower TA (perhaps around 60 ppm, depending on the CH, CYA and Temp.). The best TA depends on the Calcium Hardness, Cyanuric acid level and water temperature. They should maintain a neutral CSI to avoid dissolving the plaster, or getting scale.

Even with using a higher pH and a lower TA, some carbon dioxide will off-gas and cause the pH to rise. If you are careful to distinguish if the pH rise is from basic sources or from carbon dioxide off gassing and use correct pH-lowering chemical at the right time, you should get good results and lower your costs dramatically.

If the pH is high and the TA is above the target TA, then use muriatic acid. If the pH is high and the TA is at the target TA, then use the CO₂.

If you only use CO₂, then the TA will continue to climb indefinitely. And, the higher the TA goes, the more CO₂ it takes to keep the pH under control. Therefore, you will eventually need to use muriatic acid, or you will end up needing dangerous amounts of CO₂ just to try to keep up.

 

[Retep]

First, they should use a higher target pH. I would suggest 7.8.

IMHO a pH of 7.8 is way too high.
7.3 is good - if it's up to me I'd bring it down even lower ( 7- 7.2) but the municipality which owns the pool is afraid to to so, due to misinformation ( partly again by the pool builder).

By running the pH at 7.8 the Hypochlourous Acid is probably only 1/2 as active as with a pH of 7.2.
Therefore you'd need more Sodium Hypochlorite ( as it the case with this particular pool) , which raises your pH , which in turn will require
more quantities of muriatic acid .

Pool in question is an indoor pool, does not use CYA , Volume is 450.000 gal , Calcium hardness = 230 ppm , TA = (presently) at 120 ppm .
Turbidity (measured after Filters) = 0.06 NTU ( = Crystal Clear) , FAC = 1.36 ppm ( = way too high) ; TC = 1.51 ppm , Redox = 829 mV , pH = 7.35

I am planning of going down to 0.80 ppm of FAC - still will give me most likely around 750 mV, which will be sufficient to have good disinfection.

 

[JamesW]

A higher pH does change the percentage of hypochlorous acid vs. hypochlorite. However, you can just increase the FC to compensate. This will give you a greater reserve and better performance. It will not increase the total amount of chlorine needed. (See edit below)

A lower pH increases the percentage of carbon dioxide vs. bicarbonate, which will increase pH rise due to loss of carbon dioxide.

If you change to a pH of 7.7 and a TA of about 60 to 70, your water would be balanced and would require much less CO₂.

[edit]If the pool were exposed to UV, such as from being outdoors exposed to the sun, or from a UV system, and no cyanuric acid was used, then the higher percentage of hypochlorite at a higher pH would result in higher chlorine usage.

If you could provide some more complete information, that would be helpful:

What is your turnover time?

How often do you test the chlorine?

What are you using to measure the chlorine and pH?

What is the water temperature?

How much carbon dioxide do you use every day?

What is the daily bather load?

How much chlorine is used per day?[end edit]

 

[JasonLion]

I am planning of going down to 0.80 ppm of FAC - still will give me most likely around 750 mV, which will be sufficient to have good disinfection.

FC levels that low don't work at all well in a public pool. One person peeing and your chlorine is all gone across a significant volume of water.

 

[Retep]

James - There are actually 2 Pools in this building.

I described the bigger one ( 450.000 gal) , but the "Kiddie" Pool ( 65.000 gal) has more or less the same readings than the big pool.

Turnover time on the big pool = once every 3 hours ( 8 times in 24 hours )
small pool = once every 1.5 hours ( 16 times in 24 hours)

Temp. big pool = kept at around 78 - 80
small pool = around 83

Amperometric automatic control (Siemens) which displays FAC, pH and ORP. Additionally I take my test kits ( Electronic Photospectrometers ) along to compare readings on automatic control , which - more or less - give me the same readings.

Chemical Consumptions on Sodium Hypochlorite and CO2 :
Obviously the small pool will consume ( in proportion to the big pool ) more chemicals, since there are quite a few kids and the elderly who frequent this pool.
If the pool builder / engineering firm would have done a good job there would be 2 separate containers in the filtration room for the Sodium Hypochlorite , which is being fed by the small dosage pumps on demand into the pool(s).
Now there is only one big container ( 660 gal) and there is no way of knowing how much is consumed for the big pool and how much for the small pool.
I calculated consumption for both pools to be at 7.66 gal/day ( average ) of Sodium Hypochlorite.
Assuming the concentration is around 10-12% - which would translate to about 7.6 pounds of 100% available chlorine content ( if chlorine gas were used instead of Sodium Hypochlorite.)
Average bather load is around 500 people per day , which means that each person "consumes" around 7 grams .

Carbon Dioxide consumption around 50 pounds / day , which I find excessive.

Jason - there should not be any problem in running the pool at 0.80 ppm of FAC. My company takes care of other municipal pools where I am running lower than that. Microbiological tests as well as tests on THM's and TOC are done once / month at those pools . We got less than 15 micrograms/ liter of THM , therefore even surpassing German standards (DIN 19643), which are set at max. 20 micrograms/l of TTHM.

 

[JamesW]

Assuming each person is in the pool for about 45 minutes, then the chlorine usage seems about right.

The carbon dioxide usage is excessive due to the high TA and the low pH. I think that you could lower the CO₂ usage to 5 pounds, or less, if you lower the TA to about 60 ppm and raise the pH to about 7.7 or 7.8. If the pH is high and the TA is above the target TA, then use muriatic acid. If the pH is high and the TA is at the target TA, then use the CO₂.

The chlorine usage should remain the same (unless you have a UV system). You can increase the target FC to maintain the desired hypochlorous acid level.

 

[Retep]

Assuming each person is in the pool for about 45 minutes, then the chlorine usage seems about right.

The carbon dioxide usage is excessive due to the high TA and the low pH. I think that you could lower the CO₂ usage to 5 pounds, or less, if you lower the TA to about 60 ppm and raise the pH to about 7.7 or 7.8. If the pH is high and the TA is above the target TA, then use muriatic acid. If the pH is high and the TA is at the target TA, then use the CO₂.

The chlorine usage should remain the same (unless you have a UV system). You can increase the target FC to maintain the desired hypochlorous acid level.

The majority of chlorine consumption per person happens right after the swimmer goes into the pool. If they stay 45 min it is only slightly higher than if they stay just 3 min. in the pool. (unless it is a competitive swimmer who sweats a lot and does heavy physical exercise in the pool ).

As I mentioned before - raising the pH to 7.7 or 7.8 is not an option. Max. pH I would be willing to go up is 7.4
Raising the FC from the currently 1.3 ppm is also not an option. Those 1.2 - 1.3 ppm are already too high.
The higher the Cl2 the more byproducts of chlorination.

Concerning chlorine usage with UV irradiation : Every pool which uses UV irradiation ( high - or medium pressure lamps) will use MORE Chlorine and not less. Additionally it it will raise THM's in the pool water .
UV is great for drinking water applications, since the water will be chlorinated after it passes through the UV chamber. A pool is a closed circuit , therefore chlorinated water will always pass through the UV chamber , which will raise THM levels.

 

[JamesW]

Why not try increasing the pH in stages. Go to 7.4 for a while to see if you're still getting good results and how much CO₂ you're using. Then go to 7.5 etc.

If you don't want to raise the pH, you can still lower the TA. With a low pH, you're going to use a lot of carbon dioxide unless you lower the TA very low, which creates its own problems.

You might have some carbon dioxide loss due to poor dissolution of the carbon dioxide in the water. How much CO₂ does not dissolve and you can see as bubbles in the return water?

Also, if you raise the pH, you can raise the FC to get the same hypochlorous level as before, which will give you the same effective chlorine level.

 

[chem geek]

The majority of chlorine consumption per person happens right after the swimmer goes into the pool. If they stay 45 min it is only slightly higher than if they stay just 3 min. in the pool. (unless it is a competitive swimmer who sweats a lot and does heavy physical exercise in the pool ).

If people do not shower or rinse before going into the pool, then there is a larger initial chlorine demand, but it is not true that the subsequent demand is so negligible as to have a 3 minute soak equal a 45 minute soak or that this is the same as 1-1/2 hours. As just one example, see Figure 5 in this paper showing that an arm put into tap water with added chlorine immediately started to react with chlorine and the chlorine loss continued for over 2 hours. This isn't just sweat, but rather chlorine reacting with organic compounds in skin.

In residential spas, we have data showing that chlorine consumption is very much related to person-hours, but in that case there is clearly far more sweating going on. Roughly speaking, for such spas the chlorine demand is around 9 grams per person-hour while for pools the more typical demand is 4 grams per person-hour except for competitive swimmers that are roughly double this demand (i.e. more similar to that of spa use).

 

[Retep]

The majority of chlorine consumption per person happens right after the swimmer goes into the pool. If they stay 45 min it is only slightly higher than if they stay just 3 min. in the pool. (unless it is a competitive swimmer who sweats a lot and does heavy physical exercise in the pool ).

If people do not shower or rinse before going into the pool, then there is a larger initial chlorine demand, but it is not true that the subsequent demand is so negligible as to have a 3 minute soak equal a 45 minute soak or that this is the same as 1-1/2 hours. As just one example, see Figure 5 in this paper showing that an arm put into tap water with added chlorine immediately started to react with chlorine and the chlorine loss continued for over 2 hours. This isn't just sweat, but rather chlorine reacting with organic compounds in skin.

In residential spas, we have data showing that chlorine consumption is very much related to person-hours, but in that case there is clearly far more sweating going on. Roughly speaking, for such spas the chlorine demand is around 9 grams per person-hour while for pools the more typical demand is 4 grams per person-hour except for competitive swimmers that are roughly double this demand (i.e. more similar to that of spa use).

I do remember coming across a study ( which I can't find anymore) which showed into more details the consumption rate in regards to the time being spent in the pool. Possible it was a bit longer than those 3 min. I stated.

I find the chlorine demand you stated ( 4 grams / person) very low, unless you got those numbers from a German swimming pool where usually sophisticated filtration as well as good coagulation/ flocculation is being used.
Maximum FC levels in Germany is 0.60 ppm.

Reality is that in public indoor pools almost nobody takes a shower before going in the pool.
A typical indoor pool in North America which runs at 2 -3 ppm, pH 7.5 (or higher) , uses about 20 grams/person. We have seen pools where the
consumption rate was 40 grams/person.
Having come across the Erdinger study you linked already a while ago I found it interesting to read it again, since it deals with DBP's and THM's.
Still puzzles me why there are no regulations on THM in public pools in the USA or Canada.

 

[JamesW]

Here is some information regarding adjusting the FC level for various pH levels to maintain the same level of hypochlorous acid.

pKa hypochlorous acid = 7.53

pH.........% HOCl.........% OCl^-

7.0............77..............23
7.1............73..............27
7.2............68..............32
7.3............63..............37
7.4............57..............43
7.5............52..............48
7.6............46..............54
7.7............40............ .60
7.8............35..............65
7.9............30..............70
8.0............25..............75

If you were using a pH of 7.3 and an FC of 1.0, then the hypochlorous acid level would be 0.63 ppm. If you wanted to increase the pH to 7.6 with no loss of chlorine effectiveness, then you would raise the FC to 1.37 ppm, which would also give you a hypochlorous acid level of 0.63 ppm (1.37 x 46 % = 0.63).

If you were to raise the pH to 7.7, then the FC could be raised to 1.58 ppm to maintain the same 0.63 ppm level of hypochlorous acid.

If you raised the pH to 7.6, lowered the TA to 80 and increased the calcium to 260, then you would have a balanced pool and you would lower the carbon dioxide usage by probably at least 75 % (about 12.5 pounds, or less).

At a pH of 7.35, your hypochlorous acid level is 60 % of your FC. Since you were planning to go down to an FC of 0.80 ppm, then your hypochlorous acid level would be 0.48 ppm. To maintain the same 0.48 ppm hypochlorous acid level at a pH of 7.6, you would keep the FC at 1.05 ppm.

Do you have a salt or TDS reading?

______________________________________________

More information

CO₂ + H₂O <> H⁺ + HCO₃^-

Carbon dioxide + water <> Hydrogen ion + bicarbonate ion.

As you can see, the carbon dioxide does add bicarbonate ions, which increase the TA. However, the hydrogen ion is the acid, which lowers the pH, and also counts against the TA. Therefore, you get a net 0 change in TA.

If a basic source adds hydroxide ions to the water, then the hydroxide ions combine with the hydrogen ions to form water, and you're left with the bicarbonate, and a higher TA.

CO₂ + H₂O + OH^- <> H₂O + HCO₃^-

Carbon dioxide + water + hydroxide ions <> water + bicarbonate.

 

[chem geek]

I find the chlorine demand you stated ( 4 grams / person) very low, unless you got those numbers from a German swimming pool where usually sophisticated filtration as well as good coagulation/ flocculation is being used.
Maximum FC levels in Germany is 0.60 ppm.

Reality is that in public indoor pools almost nobody takes a shower before going in the pool.
A typical indoor pool in North America which runs at 2 -3 ppm, pH 7.5 (or higher) , uses about 20 grams/person. We have seen pools where the
consumption rate was 40 grams/person.

Are you serious? 20 grams/person and sometimes 40 grams/person? A high bather-load pool with one bather per 1000 gallons if swimming for one hour and the pool open for 8 hours (so 8 person-hours per 1000 gallons) would be 20*8 = 160 grams chlorine in 1000 gallons (3785 liters) or 42 mg/L (ppm) of chlorine per day. Are you sure you have your numbers right? Perhaps you are talking about the weight of some specific type of chlorine. If it were 12.5% chlorinating liquid, for example, then 20 grams would be 0.125*20 = 2.5 grams per person or 1000*20*8*0.125/3785 = 5.3 mg/L (ppm) which sounds more accurate though low (more expected would be 4 grams per person or sometimes somewhat more, perhaps 6 or so, maybe 8 if people are dirty).

If you are really seeing 20 grams chlorine (Cl₂, not product such as chlorinating liquid) per person, then you probably have biofilms growing in your sand filters or other sources of extreme chlorine demand.

 

[JamesW]

Retep, also note that a certain percent of the bicarbonate + dissolved carbon dioxide (carbonic acid) total will be carbon dioxide and the rest will be bicarbonate.

pKa carbonic acid = 6.35

pH.........% CO₂.........% HCO₃^-

7.0............18..............82
7.1............15..............85
7.2............12..............88
7.3............10..............90
7.35...........09.............91
7.4............08..............92
7.5............06.6...........93.4
7.6............05.3...........94.7
7.7............04.3...........95.7
7.8............03.4...........96.6
7.9............02.70.........97.3
8.0............02.2...........97.8

There is about 2.55 times as much carbon dioxide at a pH of 7.35 and a TA of 120 vs. a pH of 7.6 and a TA of 80. Moreover, there seems to be a non-linear effect where the loss of carbon dioxide increases faster than the increase in the amount of carbon dioxide.

I would estimate that the carbon dioxide loss rate would be somewhere between 2.55 and 3.8 times as much at a pH of 7.35 and a TA of 120 vs. a pH of 7.6 and a TA of 80. That means that you could cut your CO₂ usage by 60 to 75 %.

[edit]Actually, since you measure TA and not TA + CO₂, the difference would be (9/91)/(5.3/94.7) x 1.5 = 2.65. Therefore, I would estimate that the loss rate difference would be 2.65 to 4 times as much (still about 60 to 75 % savings). Moreover, that assumes that there is no loss of carbon dioxide due to not dissolving. If you factor in a certain inefficiency, then that increase the difference even more.[end edit]

If you increased the pH to 7.7 and lowered the TA to 60 or 70 ppm, then you would save even more.

 

[Retep]

Are you serious? 20 grams/person and sometimes 40 grams/person?

If you are really seeing 20 grams chlorine (Cl₂, not product such as chlorinating liquid) per person, then you probably have biofilms growing in your sand filters or other sources of extreme chlorine demand.

Yeps - very serious.
You just made me go back into my basement to look for those hand written notes from a public pool about 20 years ago. . . lol
Bather load as well as Sodium Hypochlorite consumption was daily documented by the technical staff from this particular indoor swimming pool.
Back then I did calculate the last 60 days and averaged it on a daily basis :

Bather load : 550 people/day
Sodium Hypochlorite consumption : 31.70 gal / day
Strenght was supposed to be 12 % , but let's assume it was 10 % :

31.70 gal = 120 litres x 10% = 12 litres ( or approx. 12.000 grams ) : by 550 bathers = 21.80 grams / person

FAC at the time was at 2 ppm , TC = 3 ppm, pH = 7.6. Pool volume = 265.000 gal

Couldn't even breath in this pool - air quality was just terrible. Deep end had to be closed on a constant basis, since the water was always cloudy.
I did not have a Turbidity meter at the time, but it was very very cloudy.
Funny thing was that the management from this municipality didn't think there was a problem, since their Langelier Index and the Saturation Index was spot on ! lol
Swimmers did not really complain either, because for them it was just normal to swim in this chemical loaded soup, since they never saw a proper pool in their lives.

Yes - probably there was biofilm not only in the sand filters but also in the whole system ( pipes, heat exchanger, inlet jets and everywhere water passed through).

And - Yes again - I also saw indoor pools which used twice the amount per person than this pool did.

 

[Retep]

There is about 2.55 times as much carbon dioxide at a pH of 7.35 and a TA of 120 vs. a pH of 7.6 and a TA of 80. Moreover, there seems to be a non-linear effect where the loss of carbon dioxide increases faster than the increase in the amount of carbon dioxide.

I would estimate that the carbon dioxide loss rate would be somewhere between 2.55 and 3.8 times as much at a pH of 7.35 and a TA of 120 vs. a pH of 7.6 and a TA of 80. That means that you could cut your CO₂ usage by 60 to 75 %.

If you increased the pH to 7.7 and lowered the TA to 60 or 70 ppm, then you would save even more.

Thanks Jason - I knew there was a non-linear effect.
Still - I think that by keeping a constant pH of 7.30 and a TA of 70 the consumption of CO2 should not be more as if I would keep the pH at 7.5 and the TA at 70. Naturally one would need a bit more quantities of CO2 at the beginning to bring it down to the desired 7.3 , but once this is done it should stabilise providing the TA is also kept at 70. Can anyone confirm that or am I totally off with my assumption ?
Tomorrow I will be back at this pool and set the automatic control to 7.4 and tell the
maintenance guys to bring down the TA to around 60 - 70. Told them at the beginning of the week to purchase Muriatic Acid 20 Baume - hopefully they got it in by now.
Thanks you guys for all your input

 

[JamesW]

Still - I think that by keeping a constant pH of 7.30 and a TA of 70 the consumption of CO2 should not be more as if I would keep the pH at 7.5 and the TA at 70. Naturally one would need a bit more quantities of CO2 at the beginning to bring it down to the desired 7.3 , but once this is done it should stabilise providing the TA is also kept at 70. Can anyone confirm that or am I totally off with my assumption ?

A pH of 7.3 will require about 1.57 times as much carbon dioxide as at a pH of 7.5 for the same TA. I think that you're missing that there is more dissolved carbon dioxide in the water at a lower pH . And, if there is more dissolved carbon dioxide, then there will be more loss because it's out of equilibrium with the ambient air.

Also, you're going to have to lower the TA in stages according to the Lowering TA procedure. You're going to need to turn off the carbon dioxide feeder until the TA is brought down by the muriatic acid.

pool-school/lowering%20total%20alkalinity

You will also need to consider the saturation index as you lower the TA. You could increase the calcium hardness, but that can lead to excessively high levels over time depending on dilution rates and the calcium hardness of the fill water.