It sounds radical and way out there but it's easy to do and it works. You really can do this. Best of all, it uses muriatic acid which is cheap and something that is free — air.
In a typical pool, pH is maintained at a recommended level of minimum 7.2 and a maximum of 7.8 with the ideal range of 7.4 to 7.6 (per APSP and many health department guidelines). The total alkalinity is maintained at a minimum of 80 ppm to a maximum of 160 ppm, with an ideal range of 100 ppm to 140 ppm.
If the pH and alkalinity are both higher than recommended levels, muriatic acid (HCl 31.4 percent), dry acid (sodium bisulfate, NaHSO4), sulfuric acid (H2SO4) are added or carbon dioxide (CO2) is injected to lower them. However, pH is a logarithmic value and total alkalinity is linear, so they will not be lowered or raised at the same rate.
High and Low Problem
So a service tech, commercial pool operator or pool dealer is faced with lowering either pH or total alkalinity too much or not enough. If enough acid is added to adjust the pH to the recommended range, then the alkalinity will be too high. If enough acid is added to adjust the total alkalinity to the proper level, then the pH will be too low.
When the pH is a little low (about 7.1 or 7.2) and the total alkalinity is near recommended levels, adding soda ash will bring both up to proper levels. However, if pH is low and total alkalinity is OK, there has not been a practical way to raise only the pH.
You can't add a strong acid (muriatic) and then later add a strong base (soda ash) and expect the total alkalinity not to change. These two chemicals exactly cancel each other out, assuming you add amounts to end up at the same pH that you started with. This is the classic "yo-yo" effect, though in practice it's even worse because people often use soda ash to raise the pH, which actually increases the total alkalinity even more, so you end up with a higher total alkalinity than when you started! The only way to lower TA is to physically remove some of the carbonates in the pool, and by "carbonates" I mean the collection of chemical species that are in equilibrium — carbonate ion (CO3-2), bicarbonate ion (HCO3–), carbonic acid (H2CO3) and aqueous (dissolved) carbon dioxide.
In theory, one could intentionally form calcium carbonate by over-saturating the water — significantly raising the pH and then physically removing the precipitate. But doing that can cause scale to form on pool surfaces and in pipes and equipment. And it is not a precipitate that is easily removed. In addition, the high pH can cause metal staining if metals are in the water. If there were a chemical that strongly precipitated carbonate without forming scale, that would be great (similar to the lanthanum based products that precipitate phosphates for algae control), but there isn't any.
Remove the CO2
The best way of removing the carbonates in the water is to aerate the water to remove carbon dioxide (CO2) since the pool is over-saturated in the first place.
Lower Alkalinity and pH With Acid, Then Raise Only pH With Air
Lowering the pH significantly will increase the amount of carbonates that are in the form of carbon dioxide in the water and that will increase the rate of outgassing (carbon dioxide leaving thewater). Aeration will also increase this rate of outgassing. As the carbon dioxide outgases, the pH rises with no change in total alkalinity. You can then add acid to lower both pH and TA with the net effect of lowering only the TA.
In other words, add acid until the alkalinity reaches about 90 to 100 ppm. Then aerate until the pH rises to 7.4 to 7.6.
The main rule to keep in mind is that it takes 25.6 oz. of full-strength muriatic acid (31.45 percent hydrochloric acid) to lower the total alkalinity by 10 ppm in 10,000 gallons. The problem: If one adds enough acid to lower the total alkalinity all at once, the pH can get very low. Now, let's see how low it would actually go in some extreme cases.
For this example, assume there are no borates, the cyanuric acid is 30 ppm and the pH is starting out at 7.8 (if it's at 7.5 and is stable, then the total alkalinity probably doesn't need to be lowered, but usually high total-alkalinity pools are high in pH as well). If one lowers the total alkalinity from 200 ppm to 80 ppm, the pH would drop to 6.1. If one lowered the total alkalinity from 300 ppm to 80 ppm, the pH would drop to 5.8. So you can see that even extreme additions of acid to lower the total alkalinity, at least to 80 ppm, don't get the pH much below 6.0. While such a pH is not good in the long-term, having the pH at that level for a relatively short time — less than a day — is not a disaster. At such a low pH, carbon dioxide will be driven from the water fairly quickly since there is 220 to 440 times more carbon dioxide in the water as the equilibrium amount with the air above the water.
Add the Acid
The key to adding such large quantities of acid to the pool is to avoid overdosing in one place, as that could damage the plaster. Adding the acid slowly over a return flow with the pump running, and then brushing to ensure thorough mixing, will prevent the pH from getting too low in one spot, potentially damaging the plaster. (And obviously, one doesn't want a return fitting pointing up when adding the acid because you don't want it splashing back at you.)
Add the Air
Simply doing things like pointing the returns upwards and running the pump on high to create surface disturbance will aerate the water reasonably well. One could also add an aerator to a standard return.
If there are waterfalls, fountains, spillovers, etc., turning those on will also help. Even better: an air compressor with a head that has many tiny holes. It would produce lots of small bubbles and, when that end is placed in the deep end, the bubbles would spend as much time in the water as possible.
The idea is to increase the air-water surface area as much as possible, for as long as possible.
How Long Will It Take?
I don't have anything but relative qualitative estimates based on how far out of equilibrium the carbon dioxide in the water is with respect to the carbon dioxide in the air.
My rough guess is that if the full amount of acid was added and the pH lowered towards the 6.0 "limit," the outgassing would occur fast enough to have the pH rise to 6.5 or so within one to two hours, even with modest aeration from pointed-up returns. Getting from 6.5 to 7.0 could take longer, though still probably within the same day (likely about four to eight hours more). It's getting from 7.0 up to 7.5 that could take longer – more than a day or two, assuming that total alkalinity is near 80 ppm, unless there is significant aeration. Of course, that's what you want since that means the pH will become stable. The down side for a service tech is that raising the pH using this method could take many hours or even a couple of days.
Not for Everyone
Most pool owners would probably not attempt such a "drastic" or "scary" procedure, especially if they asked their pool store about it. (A question to which they would likely hear, "Are you crazy?")
Professional pool service techs, however, could easily use this method. Acid is inexpensive and air is free, plus, there's not much chance of overdoing the pH raising procedure. As the pH gets near 7.5 it takes more and more aeration to make a change. In fact, it is difficult to raise the pH much over 7.5 with aeration in a swimming pool.
It's certainly a heck of a lot better than the "acid column" or "slug" method, which are more likely to result in acid pools that can destroy the pool surface in the area where it is added, especially in a vinyl pool. In fact, the biggest risk to the procedure described above — dropping down to a pH of 6.0 — is to vinyl pools since they are far more sensitive to pH than plaster pools. However, having just one to two hours exposure to a 6.5 pH is not a serious problem in a vinyl pool.
If the total alkalinity drop is limited to increments of 40 ppm, then the pH would not get quite so low. In this case, adding 102 oz. per 10,000 gallons at one time would lower alkalinity by 40 ppm, and the pH would not drop below 6.5. Then one could add air before lowering by another 40 ppm. So lowering alkalinity by more than 80 ppm could be done over multiple days or visits.
Accelerating a Process
Just keep in mind that the above method simply accelerates a natural process that will occur more slowly over time anyway. The total amount of acid needed to lower alkalinity would be the same if some is added every week to lower the pH — over time, the total alkalinity would slowly drop. This procedure simply speeds up the process. You don't save on the amount of acid you need to add, but you save on the frequency of required acid addition. You basically get it over with in a shorter period of time. This assumes that the total alkalinity isn't increasing over time, but that can also happen with evaporation/refill since the fill water usually has some total alkalinity.
If you lower the total alkalinity of the pool water to about 90 to 100 ppm with muriatic acid, then the pH will be too low (usually less than 7.0 and maybe as low as 6.0). However, if you have the total alkalinity at 90 ppm and the pH is low, wait a couple of days and the pH will go back up to about 7.5 on its own without a change in total alkalinity. Carbon dioxide equilibrium will be reached.
Here is what happens:
1. Aeration causes CO2 to outgas.
So aqueous CO2 becomes gas CO2 and leaves the water: CO2 (aq)->CO2 (gas)
2. Carbonic acid (H2CO3) in the water then creates CO2 according to this reaction: H2CO3 ->CO2 + H2O
3. Bicarbonate (HCO3–) in the water creates carbonic acid (H2CO3) according to this: HCO3– + H+->H2CO3
Net result: H+ is consumed and pH goes up — alkalinity does not increase
— pH is a measure of H+ (hydrogen ion) concentration. The less H+, the higher the pH. We just want to make this natural CO2 out gassing and pH rising process go faster.
More Aeration and Turbulence = Faster pH Rise
Things to Do To Make CO2 Outgas Quicker
Whenever the pH is low and the alkalinity is OK, use air to raise only the pH.
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