Reduce Total Alkalinity Levels

[chem geek]

The only way to lower the CYA level is through water dilution, including partial drain/refill. To prevent the CYA from rising, you can use 6% unscented bleach or 10% or 12.5% chlorinating liquid. If your CH is low, you can also use Cal-Hypo at least for a time. When using these sources of chlorine you have to add the pool every day or two unless you have a pool cover in which case you can usually add the chlorine twice a week. This is what I do in my own 16,000 gallon pool shown here and here and it costs only $15 per month including the small amount of acid I add every month or two. I don't need to use any algaecides, phosphate removers, metal ions, clarifiers, flocculants, or weekly shocking. The same is true for most of the tens of thousdands of pool owners on multiple pool forums including this one, The Pool Forum and Trouble Free Pool.

You can also get automated dosing systems (The Liquidator or a peristaltic pump) but if you don't want to buy chlorine regularly then a saltwater chlorine generator (SWG) will generate chlorine on-site from the salt in the pool. With most SWG pools, the main issues are rising pH and adding a lot of acid to compensate, but these things can be mitigated as described in Water Balance for SWGs. If you have soft stone coping or hardscape, then you should seal it since salt splash-out can damage soft stone.

[Guest]

Larry one quick question my total alk is slightly elevated you said aeration would help reduce so I have pointed my return up so it actually shoots out and creates a splashish bubbly area would this be adequate to create the necessary aeration

[chem geek]

It isn't aeration that reduces TA, but the addition of acid that reduces TA. The aeration is so that you can raise the pH without changing the TA so that you can add more acid. If you only added acid to lower the TA, then the pH would get too low. It is the combination of acid addition with aeration that results in TA getting lower without lowering the pH overall.

Yes, your aeration from returns pointed upwards can work well and is what I've done in the past in my own pool, turning the pump on high as well (if you have a variable speed pump).

[Billjohn]

Larry,
For thirty years I have had a 20,000 gallon indoor pool filled with well water. During that time I have tried diligently to lower the 230 ppm TA to the recomended 130 for painted pools. The chemical people have always given me the same "voo-doo" advice that you refer to as the slug method. Even though it never worked, I just kept doing the same thing expecting a different outcome because they all said the same thing. Finally I just gave up and accepted the scaling. Recently, I decided to install a geo-thermal pool heater but knew that it would not tolerate the scaling. I came across your post concerning the use of aeration and acid. I had an old 1/2 hp shallow well pump which I rigged with a hose nozzel and dip tube to suck water from the pool and blast it back into the surface at the location of a return. With a little experimentation I found that I could run the aerator constantly, add 2 quarts of acid every 2 hours, and drop the TA 10 ppm every 2 hours with the PH always testing at 7.2 at the end of each 2 hour period.
You would not make a very good chemical salesman, but I can't tell you how much I appreciate your sharing this information with me.

[chem geek]

Isn't it ironic how the "chemical people" don't really understand chemistry? The principles are basic first-year college chemistry yet the pool/spa industry is driven a lot by hearsay and baloney, some of it to drive more profits while some of it is just someone making a mistake and everyone else repeating it like lemmings.

Ben Powell at The PoolForum in this post described the acid/aeration procedure while the slug or acid column method was discredited in this JSPSI report. Note that it isn't that the slug method doesn't work -- the amount that the TA drops is solely a function of the amount of acid added and not how it is added -- it's that it can be dangerous and can result in a very low pH (if you try raising the pH with a base, then the TA rises as well defeating the purpose of adding the acid). Adding 25-1/2 fluid ounces of full-strength Muriatic Acid (31.45% Hydrochloric Acid) in 10,000 gallons lowers the Total Alkalinity (TA) by 10 ppm and it doesn't matter how you add that acid in terms of that lowering of TA (obviously it matters in terms of safety to pool surfaces so the acid should be added slowly over a return flow in the deep end with the pump running). The trick is how to lower the TA substantially without lowering the pH too much since that can harm pool surfaces and equipment. The answer is to add the acid in steps and aerate the water since that raises the pH with no change in TA -- it forces carbon dioxide out of the pool. Pools are intentionally over-carbonated in order to provide a pH buffer and to saturate the water with calcium carbonate to protect plaster surfaces.

As for the ideal aeration, have the bubbles be as small as possible and put the pipe outlet in the deep end so the bubbles have maximum time exposure in the pool. A pipe or tube with very small holes would work well. It sounds like you've got a great system since 10 ppm TA drop per hour is very good. Disturbing the water surface also works well. You want to maximize the surface area between the water and air any way you can.

By the way, 130 ppm TA for painted pools is also a bunch of bunk. There is no single absolute TA number that is the "right" number. What is required for plaster pools is that the saturation index be near zero and that is a combination of pH, TA, CYA, CH and Temperature. You can calculate the index using The Pool Calculator. If you find that your pH tends to rise too much over time, then the TA level is probably too high. When using hypochlorite sources of chlorine, one should usually have the TA be at 80 ppm or lower. The TA should only be higher if you are using acidic sources of chlorine such as Trichlor (or even Dichlor since it is net acidic when accounting for chlorine consumption/usage which is an acidic process). You need to watch your Cyanuric Acid (CYA) level when using stabilized chlorine sources. Read the Pool School for more info.

Richard

[somaholiday03]

Awesome method, works like a charm in my 12,000 G indoor. I also have a 200,000 G indoor that needs some TA lowered (150 to 100). I use a 2' section of PVC with holes drilled connected to a compressor for aeration. My question is...is that big enough for my large pool...and this is going to take me a couple nights, correct?

[T3]

The more you aerate, the more the carbon dioxide will off-gas. It depends on how much air you inject and how vigorously the water is agitated. Aeration is not necessary for the process to work, it just speeds it up. I wouldn't be too concerned about aerating unless you had some reason to speed up the process. Just drop the pH a little and wait for it to go back up.

What are all of your numbers for pH, TA and calcium?

[somaholiday03]

The more you aerate, the more the carbon dioxide will off-gas. It depends on how much air you inject and how vigorously the water is agitated. Aeration is not necessary for the process to work, it just speeds it up. I wouldn't be too concerned about aerating unless you had some reason to speed up the process. Just drop the pH a little and wait for it to go back up.

What are all of your numbers for pH, TA and calcium?

It's a commercial pool so everything is time critical and mainly done at night.

My levels are
FC 2.0
pH 7.4
TA 150
CA 260

[T3]

Assuming that you cyanuric acid is zero and the temp is about 86 F, then your CSI is only +0.17. That's fine. Why do you want to lower the TA? What does your pH tend to do? Rise, fall or stay at 7.4?

What type of chlorine do you use?

[somaholiday]

Assuming that you cyanuric acid is zero and the temp is about 86 F, then your CSI is only +0.17. That's fine. Why do you want to lower the TA? What does your pH tend to do? Rise, fall or stay at 7.4?

What type of chlorine do you use?

CYA is zero and temp is 82.

Just wanted to keep it around the target level of 100 but if it's unnecessary then so be it. pH is always steady at 7.4 and I use sodium hypo along with MPS.

[chem geek]

Though it's unusual to be using a hypochlorite source of chlorine and have the TA be at 150 ppm and have the pH be stable, if that's the way it is for this pool, then leave it as is. As noted above, your saturation index is OK so nothing to fret about.

I suspect that you must be using enough MPS that this is keeping the pH down. MPS is acidic, though not as strong as using Trichlor, for example. In terms of the net effect on pH and TA, one pound of non-chlorine shock (43% MPS) is equivalent to 6 fluid ounces of full-strength Muriatic Acid (31.45% Hydrochloric Acid).

Unless you have a lot of evaporation and refill, the use of MPS should have the TA slowly drop over time.

[somaholiday]

Cool. Always wondered why my TA stays somewhat under control even while using Co2 for PH control. Regular use of MPS.

You guys are awesome, thanks a lot.

[chem geek]

If you were to lower the TA level, you should find that you will need far less carbon dioxide to keep the pH down. You had said your pH was stable, but really it isn't because you are adding carbon dioxide to keep it down. If you want to continue to use MPS, which is acidic, then just lower the TA until you find that you no longer need to add carbon dioxide and instead have stable pH just from the chlorine and MPS.

Adding carbon dioxide does not change the TA level. Yes, it adds carbonates to the water, but it also lowers the pH and these two effects exactly cancel each other out. Essentially, adding carbon dioxide is like adding carbonic acid (which is carbon dioxide plus water) and the increase in bicarbonate (and some carbonate) that increases TA equals the increase in hydrogen ion that decreases TA (and decreases pH).

There is a net rise in TA over time when using hypochlorite sources of chlorine along with carbon dioxide for pH control, but that is mostly due to your chlorinating liquid since that has some excess lye in it. There may also be some increase from evaporation and refill with water which has TA (this also increases CH over time as well, unless you've got water dilution from splash-out, backwashing or rain overflow).

[somaholiday]

Awesome. I was under the wrong assumption for c02 and I'm a little ashamed I didnt put 2 and 2 together. As for a stable pH, I have a relatively high bather load (swim teams, aerobics, parties, etc.) which would make a stable pH without automated c02 control impossible, right? I mean within 10 minutes of our largest swim team getting in that c02 is kicking on. I hope I'm not overlooking something else, but I'm all for making my job easier and my budget better looking.

I don't mean to get too too personal, but some of this knowledge and advice is very impressive. Do we have industry professionals here? Chemists? Or just years of experience? This advice is second to none.

[chem geek]

No need for any embarrassment. Understanding how CO2 works isn't obvious and many in the industry get this wrong as well. People who don't know chemistry well just think "carbon dioxide adds carbonates to the pool therefore it must increase TA" and don't consider the pH effect nor the difference in adding sodium bicarbonate (a carbonate salt) which increases TA vs. effectively adding carbonic acid which does not (while a weak acid such as carbonic acid does not decrease TA, a strong acid such as Muriatic Acid does).

Anyway, what happens with the bather load is that the splashing and kicking aerates the water and that drives carbon dioxide out of the water and into the air faster and this causes the pH to rise with no change in TA (essentially carbonic acid is removed from the water). The rate at which this occurs depends roughly on the square of the TA level (from experiments made by Wojtowicz) so a lower TA level reduces the rate of pH rise from outgassing. Yes, a lower TA provides less pH buffering, but the outgassing effect outweights the buffering effect. If the TA were at equilibrium with the air, which is very low at around 10 ppm (if there is no CYA in the water and at a pH of 7.5), then there would be no pH rise at all since there would be no carbon dioxide outgassing. However, one needs some TA to provide at least some pH buffering and to provide a source of carbonate ions which along with calcium ions protect plaster surfaces.

Basically, with your higher TA, you are just see-sawing between faster carbon dioxide outgassing during bather load that you are compensating by adding carbon dioxide back into the pool. The higher the TA, the faster this outgassing occurs requiring even more CO2 and likewise the lower the TA the slower this occurs requiring less CO2.

In a residential pool, one can use 50 ppm Borates to provide an alternative source of pH buffering thereby letting one have a much lower TA level (and to provide other benefits such as inhibiting algae growth), though with plaster pools one must have a higher CH to compensate. And as I noted earlier, the rate of outgassing is higher at lower pH so one can set a higher pH target to minimize this effect. This chart shows how much the water is out-of-equilibrium with the air in terms of the amount of dissolved carbon dioxide in the water compared to its equilibrium amount. This chart is roughly linear with respect to TA, but the rate of outgassing seems to be related to the square of the TA -- Wojtowicz postulates that this is due to facilitated ion transport.

So you can try to see what happens if you target a lower TA of 70 or 80 ppm and also target a pH of 7.6 or 7.7. Then see if this significantly reduces the amount of CO2 that you need to inject to maintain the pH. If it does and you want to keep the lower TA level, then use The Pool Calculator to calculate the saturation index and add calcium chloride (or use Cal-Hypo for a while) to increase the Calcium Hardness (CH) to get the index closer to zero to protect plaster surfaces.

As for my background, I do not work in the pool/spa industry nor in the chemical industry. I am just a pool homeowner who developed a strong interest in pool water chemistry (I majored in physics & chemistry in college). If you want to learn a lot more that isn't taught in the industry, then look at Certified Pool Operator (CPO) training -- What is not taught.

Richard