How to Lower Alkalinity in a Pool
Today we'll be looking at a pool water treatment topic that a lot of pool owners have a tough time with. I'm talking about lowering alkalinity in a swimming pool.
First of all, What is pool water alkalinity?
Alkalinity is a measure of the capacity of water (or other liquids) to resist a change in pH. The alkalinity scale goes from 0 to over 150 and basically tells you how much acid needs to be added before you can get your pool pH down.
The number itself means the amount of calcium carbonate that has been dissolved in one kilogram of water (or 50 gallons).
When more calcium carbonate (lime or scale) is dissolved into the water, it takes more acid before the pH changes. Alkalinity, when higher than normal, causes some problems for swimmers and pool equipment.
Problems/effects of high alkalinity:
- Cloudy Water - Excess alkalinity may increase calcium and magnesium hardness, which in turn will cause the water to be cloudy.
- Pool surface staining - The high pH may also result in pool surface staining. Calcium and magnesium minerals from the water frequently stain pool surfaces if alkalinity is high.
- pH bounce - High total alkalinity (>200 ppm) can increase the frequency of pH bounce (high and low swings). In cases with extremely high alkalinity (>400 ppm), a pH swing of more than 0.4 units per day may be observed when either calcium or cyanuric acid drops. This causes an increase in demand for chlorine, so more chlorine is added by the pool owner...but this just causes even greater increases in pH!
- Chlorine loss - The high pH caused by excess alkalinity causes chlorine to be converted to forms that are less effective as a sanitizer. This causes the owner to use more chlorine, which increases the problem even more!
Common causes of high alkalinity:
- Surface water - Surface water (from rivers or lakes) may have higher alkalinity from decaying vegetation. In this case, you can lower calcium hardness and keep total alkalinity at pools' normal levels. In other words, you need to reduce calcium hardness first before you start lowering total alkalinity.
- Adding sodium bicarbonate - You can artificially increase your pool's total alkalinity by adding sodium bicarbonate baking soda). Be careful not to add too much, as this can raise pH and salt levels.
- Excess cyanuric acid - Pool stabilizer (aka conditioner) that has CYA in it also adds alkalinity! Make sure you don't have too much and if you do, lower the CYA level until total alkalinity is where it should be.
- Alkalinity causes an increase in the rate of chlorine loss due to evaporation, particularly when the pH is high. The higher your alkalinity goes, the more problems you're likely to experience. And if alkalinity gets too high (300 ppm or above), it can cause the pool owner a lot of headaches - especially with pH bounce and that nasty staining.
How to Lower Alkalinity in a Pool
1) Acid wash the pool - this is an effective way of getting your alkalinity down quickly if you're willing to dump a LOT of acid into the pool overnight. I don't recommend this method as it's going to be pretty tough on your pool surface and equipment, as well as possibly causing damage to your liner.
2) Balance the water to 100-150 ppm using calcium chloride or sodium bicarbonate (baking soda). This will take some time because there's no shortcut when it comes to balancing total alkalinity via calcium hardness. You have to either add a lot of minerals (calcium and sodium) or a lot of acids - both methods can be difficult depending upon how large your pool is and what type of equipment you have.
3) Make a blend such as this: 1. Add sodium bicarbonate to bring total alkalinity up to 150 ppm (the upper limit for balancing) and calcium hardness down to 100 ppm, then 2:1 the resulting water to plain water and retest. This gives you a nice "buffer" of mild alkalinity and should give you enough buffer without problems with pH swing - basically, it's almost like creating your own "super chlorinating" solution. Just be careful not to add so much sodium bicarbonate that you get other chemistry issues such as high PH or copper stains on the sides of your pool.
4) Use carbon dioxide injection at night time when pH is low along with a liquid acid such as muriatic acid to quickly reduce alkalinity. Inject CO2 into your return side using a bubbler. The bubbling will bring down the pH and the liquid acid will take care of the alkalinity. Just be careful not to dump too much liquid acid into your pool or you'll realize that it's "too much of a good thing"!
5) You can also use sodium bisulfate (liquid sulfuric acid) but you must lower pH slowly at night time because this stuff is very strong and may cause equipment damage if added directly to water before pH has been lowered enough. It does work, however...and works well for large pools with high alkalinity problems, but you have to be careful as you can damage your equipment if you add too much.
6) Or you can use the old standby, muriatic acid. It works well but is dangerous to pour directly into water so I recommend it only for very large pools or for commercial applications.
7) A final way of lowering alkalinity is by adding special resins that are meant to lower alkalinity - these are usually used when treating tarter problems on tile grout, etc., but have found limited applicability with regards to pool chemistry management. Once again, this is a "further down the road" solution that should be employed as a last resort - if at all.
8) You can also consider replacing your existing filter sand with softer silica sand which will remove some total alkalinity over time - but you'll need to add a lot of calcium chloride or sodium bicarbonate in order to replace what's been removed.
9) Last resort - empty the pool, acid wash it and refill. This is a last resort method for very high alkalinity pools where the only way to lower the alkalinity is to strip it all down and start over. It can be effective in some cases, but there are always risks involved with emptying your pool so again, this should be used as a "backstop" solution when other methods have failed or aren't working well enough for your circumstances.
Pool Alkalinity - Conclusion
So remember that total alkalinity is essentially just baking soda (sodium bicarbonate), which has many purposes including helping to maintain pH stability and acting as a buffer of sorts.
The problem with total alkalinity is that it can build up too high without you knowing it, resulting in sudden swings in water chemistry where the pH suddenly drops or goes up, possibly causing damage to your pool surface and equipment...not to mention your nose!
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