White Crystalline Precipitate

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T

TreeFiter

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In The Industry
Jul 2, 2012
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Saugerties, NY
Hello,

I'm a pool tech in New York, and I've been seeing a white crystalline precipitate in certain pools. It seems to be a new thing that neither I or anyone else working at my company have seen in the past. It seems to happen in new pools. They are all pools with liners, and salt systems. Commonly it is seen laying on the bottom near the return jets. Initially I suspected it could be a result of too much salt, but salt levels are usually around 3000ppm, and even after vacuuming it out on waste, it shows up week after week. It looks almost like granular conditioner/stabilizer laying on the bottom. One of our techs said that he collected a sample and that it was very hard.

I'm wondering if it could have something to do with the water we use to fill new pools. The water comes from an underground lake situated in limestone. We've been using this water supply for years, and never had the problem until about 2 years ago. Has anyone seen anything like this before. Any ideas?

Water chemistry is typically well balanced, as we regularly maintain the chemistry.
 
Would calcium form as neat granules that settle out? My experience has been that calcium usually grows as a crusty scale on surfaces. When I first saw this, I thought it might have been saw dust from cutting the PVC pipes when the plumbing was installed.
 
TreeFiter said:
Would calcium form as neat granules that settle out? My experience has been that calcium usually grows as a crusty scale on surfaces. When I first saw this, I thought it might have been saw dust from cutting the PVC pipes when the plumbing was installed.
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It's probably forming on the SWCG cell and flaking off.
 
No they aren't well formed crystals, but they are better formed than scale. They look like most granular chemicals do before they are added to the pool (conditioner/stabilizer, granular chlorine, or Calcium Hardness). The first time I saw them, I thought they were pvc cuttings from when the pool was plumbed, and after that I considered granular chemicals being blown back into the pool like they would if there was a bad multiport. These were all new pools, and no sand was passing into the pool, so that rules out a bad multiport.

I think the idea that it could be scale forming within the salt cell and flaking off makes quite a bit of sense.
 
Can you post chemicals levels for the pools and the fill water, including the highs and lows for each?

Can you post all chemicals you have added to the pools?

Are you adding phosphate remover?

The more detailed information you can provide, the better.

The fill water is a likely source of the problem. Limestone is calcium carbonate, which would typically result in high calcium and TA (Total Alkalinity) levels.
 

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I don't have the chemistry history readily available. For the most part these are well balanced pools. It is possible that the pH is on the high side in a few of them, but probably right around 7.8 I know some of the guys I work with tend to keep pH a bit low, so 7.8 is usually looked at as high and brought down a bit. Typically our salt pools will see granular shock, ph increaser, Total Alkalinity, Conditioner/Stabilizer and ph minus. If hardness is low we will add Calcium Hardness. If there is Iron in the water we use Conquest. Most of our chemicals are Nu Clo brand. We only add phosphate remover if we notice a significant drop in Chlorine levels, such that the salt system can't keep up.
 
For the most part these are well balanced pools.
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The actual chemistry numbers can be very informative. When trying to determine the cause of the type of problem you are describing, the numbers, including temperature, are critical.

Typically our salt pools will see granular shock, ph increaser, Total Alkalinity, Conditioner/Stabilizer and ph minus.
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Salt pools should almost never need pH increaser. How is the pH increaser being added? If it is being added in the skimmer, then it can definitely cause problems. Even if it isn't added in the skimmer, it can cause problems. I prefer to avoid sodium carbonate entirely whenever possible.

pH minus can contribute to sulfates, which might build up to high enough levels to contribute to (calcium sulfate) scaling in the cell. Muriatic acid would be a better choice.

Are the pools heated? If yes, are the heaters large gas heaters before the salt cell? Increased temperature can contribute to scaling. Scaling might be happening in the heater and breaking loose.

The problem might be related to calcium phosphate if phosphate levels are high enough. Some tap water contains phosphates. Phosphate can also come from sequestrants based on HEDP, or other phosphonate based chemicals. Phosphate treatment can lead to lanthanum phosphate precipitation.

Overall, there are many possible explanations. Without very good detail as to levels, and chemicals added, it can be very difficult to determine a cause.

Can you post pictures of the White Crystalline Precipitate?

http://aquamagazine.com/content/post/Th ... osits.aspx
 
I'm not sure I understand why salt pools shouldn't need pH increaser. I've heard this from other people as well, but I haven't seen it to be true. Rain water is acidic, sweat, urine, and most of the other stuff that ends up in pools is acidic. Salt is neutral. So where does the high pH come from?

We do use sodium carbonate to raise ph, and we do typically add it through the skimmer. However we do this on approximately 600 pools on a regular basis, and we are only seeing a small handful of pools with this issue.

Most of the pools are heated, but often the heaters don't run much once the warm part of the season hits. However they are typically configured as you said with the gas heater before the salt cell in the return line. I think that a good case has been made for scale developing either in the heater or the salt cell and flaking off in the return stream.

If I can get some pictures I will, but I seldom see these pools. I've been to them a few times to see it for myself, but its usually other techs dealing with them regularly. I have asked one of the techs that regularly services one of these pools to try to get a sample of the material for me.
 
The salt generation process causes the pH to rise. It can be annoying to deal with it rises so consistently and rapidly in most cases.
 
JohnT said:
The salt generation process causes the pH to rise. It can be annoying to deal with it rises so consistently and rapidly in most cases.
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Can anyone elaborate on this a bit for me? What in the process causes the rise in pH? How high will it go if let go? I see some of my pools like to stay right around 7.8, but I seldom see them any higher.
 
Mostly, the salt process is (theoretically) overall net neutral as far as pH is concerned. The cell converts water into hydrogen gas and hydroxide, and it converts chloride ions into chlorine gas.

The hydroxide raises pH, and the chlorine gas lowers pH as it dissolves into the water. There is a net pH rise because the hydroxide ions slightly outnumber the hydrogen ions created when the chlorine gas dissolves. As the chlorine is used up, there are more hydrogen ions generated, which mostly offsets the hydroxide.

In practice, most salt pools find that the pH rises over time. I don't think that I have ever seen any that require pH increaser.

In addition to the pH rise from the salt process, there is pH rise from carbon dioxide offgassing. This is due to the carbonate alkalinity.

If you are adding pH increaser to salt pools, there is something else going on. Perhaps, your service people are overusing pH reducer. In any case, adding sodium carbonate through the skimmer is a bad idea, especially if the cell is on.

Also, as chem geek mentioned, there is no need to raise the calcium level in a vinyl liner pool.

What are your target chemical levels?
 
I had similar white crystals at the returns to my pool. It was forming in the SWG and when the polarity reverses it is then flaked off into the pool water. I have high CH levels and when I lowered my TA and kept a very close watch on pH, it went away. At TA below 75 and pH never allowed to get above 7.8 you will never see it again.
 
When I say I'm adding pH increaser to salt pools, I don't mean on a weekly basis. Typically we need to bring the pH up in the early spring when we open the pools, and during the rainy part of the spring. Once summer hits, we seldom increase pH. But I personally have not witnessed a true increase in pH in the pools I take care of. Some of the other guys I work with say they have, and are constantly adding pH minus.

As far as things like offgassing, would the amount of use these pools see effect that drastically. I think the majority of my pools are second homes that only get used occasionally. I'm wondering if there would be a difference betweeen pools that are used regularly and pools that aren't.
 
Carbon dioxide offgassing is affected by the pH, TA, water temperature and amount of agitation the water gets. High use pools, or pools with other sources of aeration, such as fountain(s), infinity edge(s), etc. will experience more offgassing than a pool with minimum aeration, all else being equal.

If the pH is constantly too high, then the TA is most likely too high. 60 ppm seems to be a good number, assuming a CYA of about 60 ppm. If the pH is too low, and the TA is too low, then adding sodium bicarbonate and aerating is usually a better choice than sodium carbonate.

CYA should be kept at about 60 to 70 ppm.

Borates are an option that can help reduce pH rise.
 

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