Build-up on Salt Cell

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Mandy

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Silver Supporter
Jul 5, 2017
43
Dubbo Australia
I have been following TFP method since July 2017 and my pool water has never looked better or been as easy to maintain. However, I have an issue with the build-up of a white substance on my salt cell.

This cell was installed in October 2015. Here is a photo of the build-up as at 24/04/17

I have diligently kept my chemical levels all within TFP range for my size/type of pool and am wondering why this build-up continued on my cell. These photos were taken 26/03/18

My calcium hardness level during this period has been in the range of 225ppm – 300ppm and generally sits at 290ppm. A pool shop tested CH at 205ppm in October 2017 and 189ppm in January 2018 (my test at this time was 290ppm).

My CSI during this period has been in the range of -0.20 and -0.89 and my pH between 7.3 and 7.8

My fill water tested at 110ppm in October 2017 (my test) and I have not added any calcium chloride since May 2017.

Due to an on-going metal staining issue I was adding 1 cup of sequestrant on a fortnightly basis following an initial dose commencing May 2017. My phosphate level has climbed from 186ppb in May 2017 to over 5093ppb when last tested in January 2018 (pool shop tests).

To address the metal issue we replaced the gal fittings in our taps in January 2018 (I'm sorry, I don't know how to link to that post) and in March installed a water filter for our pool top-up water.

I have not added any further sequestrant since the water filter installation and to date have not had a return of metal staining.

Could the build-up on the cell be from the phosphates or is my CH level too high? I cleaned the cell in March 2018 and to date there are just a few tiny specks that have reappeared.

As it is winter in Australia I have not been testing as regularly as I normally do so my last full test results were done 01/06/18 and were as follows:

FC 12.5 (had not turned cell down low enough to account for the lower FC demand)
CC 0.5 (there was a dead mouse in the skimmer basket)
pH 7.8 (not reliable due to FC being above 10 but did add 150mls of acid to reduce by 0.3)
TA 70
CH 290
CYA 80
Salt 4200 ​
Pool Temp 10.2°C
CSI -0.53

Any advice would be greatly appreciated, thank you.
 
That's an amazing summary with complete information! Nothing jumps out at me, so I've put up the bat signal to some experts who know heaps about such things.

One question I have is whether or not dry acid was used in the past, but even if it was, that scale doesn't look like it could be related to use of dry acid anyway.

It might be worthwhile turning the SWG off for the winter and using liquid chlorine if/when needed. It wouldn't be much for the next couple of months while the water is cold, and if the scale dissolves somewhat, it would be a clue.

At any rate, I'll leave it to the chemistry experts!
 
Scale typically forms as one of three compound - calcium carbonate, calcium phosphate or calcium sulfate. Of the three, only calcium carbonate will dissolve easily in a diluted strong acid like muriatic acid. When you say you cleaned the cell, how did you clean it?

Also, what output setting do you normally run your cell at, 50%, 100%? Do you know if your power supply does periodic current reversal to help reduce scale build up? If you don’t know, does scale build up all on one side of the cell or does it build up evenly?
 
Hi Matt

I cleaned the cell using a solution of acid and water, I can't remember the ratio but I did research it here and followed whatever I found. I remember the scale was very hard to remove and hardly fizzed at all. I typically run my cell for 5-6 hours at 75% during summer (more when bather load and conditions dictate) and yes it does the periodic current reversal as part of it's self cleaning process. The scale was definitely built up more on one side than the other. There is no evidence of any scale in the pool itself.
 
It seems like Mandy is doing really well on TFPC water chemistry. A short period of dry acid wouldn't be enough to cause this I don't think, so we can probably rule out sulphates as an issue.

To the weird chemistry experts... would it be worth removing the 5 ppm phosphates? (to rule out calcium phosphate scale)
Note: not weird experts, just weird chemistry, haha

And by the way, thanks for becoming a supporter, Mandy!
 
Phosphate scale tends to be hard and will not be etched by acid. Sulfate scale is mushy but also resistant to acid. Sulfate scale is easier to develop than phosphate scale at SWG pH levels (the pH inside the cell is higher than the bulk pool volume).

My guess is you’re seeing some kind of combination scale (phosphates and sulfates). How much dry acid was used??
 
Hi Mandy...I’m late to the party here and Matt has you covered but I have similar myster scale issues in my heater ;)
In my swg, it might collect but doesn’t actually stick.

In my case, I’ve tested the scale and indeed it has phosphate in its composition. I would suggest in this case that you try specifically some lanatham chloride po4 remover (don’t use alum as it will add to your sulphates and you’ve already used dry acid). Seakleer is the brand I’ve had the most success with.

If after treatment, your cell remains clear, then that may have been in part the culprit.
 
Hi Swampwoman

Thank you for your advice. I haven't been able to find that particular brand of phosphate remover in Australia. My pool shop sells a product which is in granular form and is advertised as an ultra pure phosphate remover. It's active constituent is 980g/Kg Rare Earth Salts. I rang the company and the chemist confirmed it is a lanthanum chloride salt. He advised doing 2 treatments 3 days apart to avoid over-pressurising my filter. One 250gm packet should remove 1850ppb in 50,000 litres so in my case that would equate to roughly 3500ppb reduction per treatment. The instructions suggest the use of a clarifier 2 hours following treatment, would you recommend that? Regarding sulfates, prior to TFP I used Potassium Monopersulfate whenever I needed to remove CC's. Could this type of sulfate be part of the issue?
 

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Lanthanum chloride salt is fine to use. The high quality bottled stuff we get here is simply the lanthanum chloride mixed with water and the pH is adjusted to make it acidic. The lower quality stuff usually has a lower concentration of LaCl3 in it and clarifier.

You have a sand filter so you’re going to need to use a clarifier to help capture the lanthanum phosphate particles. Get a high quality polymeric clarifier and follow the dosing instructions.

Yes, all sulfates are bad. If you were using MPS previously then what you’re seeing is calcium sulfate scale. I can attest to sulfate scale as I have seen it in my own pool. The only fix is to do a partial drain of the pool and hope it’s still not too high. Sulfates are not easily tested for and you typically need to send a sample out to a water quality testing lab (line the kind that you would use if you were on well water) to get an accurate sulfate level. Around here, that would cost about $100 or so and so it’s simply just cheaper and easier to dump the pool water and start fresh rather than waste a lot of time and money figuring out the precise level.
 
Ok thanks Matt. As luck would have it I have been manually vacuuming to waste today (only getting 1/4 of pool surface clean before stopping and waiting for the hose to catch up, so 4 times in total) therefore have replaced quite a bit of water today. From previous experience with phosphate remover I'm going to be doing a good deal more vacuuming to waste in the coming week. Hopefully solve both issues in one hit. Interesting about the MPS being an issue, could it's use also have the potential to cause sulfate scale on a different type of filter? And have I just been lucky that the scale appears to be limited to my cell and has not affected the pool surface? I don't want to think about my pipework............! But I do have to ask, could they also be affected with any buildup of scale?
 
Hi Mandy. Given the new info re MPS, maybe just a partial drain would be easier and accomplish the same thing in terms of dilution...Eg swapping 50% of your water will reduce the sulphate and po4 at the same time.

Just wanna save you on the phosphate removal hassle if there’s a chance it’s not even an issue ;) My levels were 5x higher when I last did a removal.
 
Mandy said:
Ok thanks Matt. As luck would have it I have been manually vacuuming to waste today (only getting 1/4 of pool surface clean before stopping and waiting for the hose to catch up, so 4 times in total) therefore have replaced quite a bit of water today. From previous experience with phosphate remover I'm going to be doing a good deal more vacuuming to waste in the coming week. Hopefully solve both issues in one hit. Interesting about the MPS being an issue, could it's use also have the potential to cause sulfate scale on a different type of filter? And have I just been lucky that the scale appears to be limited to my cell and has not affected the pool surface? I don't want to think about my pipework............! But I do have to ask, could they also be affected with any buildup of scale?
Click to expand...

Scaling is pH andvtemperature dependent. At normal pool water pH, sulfate scale is unlikely to happen. However, inside the SWG cell, the pH is quite a bit higher (above 10) and so scaling can happen quite easily. Same is true inside a heat exchanger except it’s the higher temperature water that drives scaling. Your pipes, filter and pool surfaces are fine.
 
Mandy said:
Ok thanks Matt. As luck would have it I have been manually vacuuming to waste today (only getting 1/4 of pool surface clean before stopping and waiting for the hose to catch up, so 4 times in total) therefore have replaced quite a bit of water today. From previous experience with phosphate remover I'm going to be doing a good deal more vacuuming to waste in the coming week. Hopefully solve both issues in one hit. Interesting about the MPS being an issue, could it's use also have the potential to cause sulfate scale on a different type of filter? And have I just been lucky that the scale appears to be limited to my cell and has not affected the pool surface? I don't want to think about my pipework............! But I do have to ask, could they also be affected with any buildup of scale?
Click to expand...

Scaling is pH and temperature dependent. At normal pool water pH, sulfate scale is unlikely to happen. However, inside the SWG cell, the pH is quite a bit higher (above 10) and so scaling can happen quite easily. Same is true inside a heat exchanger except it’s the higher temperature water that drives scaling. Your pipes, filter and pool surfaces are fine.

[EDIT]

Upon further discussion with other Experts, calcium sulfate scaling is not very pH dependent but is temperature dependent. It is conceivable that one could get a small bit of calcium carbonate scale that acts as a seed for the growth of calcium sulfate scale if the amounts of calcium and sulfate ions are at a high enough concentration. Given the complex physical and chemical reactions happening inside the cell, a mixed scale of precipitates is always possible and they would be less affected by acid than pure carbonate scale.

I believe Swampwoman’s advice is best here - try some partial draining to reduce the overall levels of dissolved minerals and then rebalance. Very likely a good fraction of water replacement will solve a lot of these scaling issues.

[END-EDIT]
 
You've probably considered this, but draining has some risks that should be avoided. Fibreglass pools are backfilled as they're filled with water, so draining can cause bulging or worse. Draining a foot or so, and then refilling is low risk, but it takes more water to achieve the same dilution with serial dilutions.

Another option is to float the fill water on top, and draw the heavier treated water from the bottom. If you have a main drain, it's easier, but can also be done with a vacuum hose, or by siphoning. This method eliminates draining risks entirely. If you'd like to go this route, I can provide more description if needed. You could achieve almost a complete water exchange for around $60/$65 if you're paying the same as we pay.
 
I have done the layered water replacement method by putting my submersible pump in the deep end and running hose fill water into the skimmer at the shallow end (no pumps running). It does work but takes a lot of baby sitting to make sure the outflow is balanced with the inflow or else you could overflow the pool. In my case, my submersible pump would run for an hour or so before tripping the GFCI outlet. So I wasn’t able to replace more than about 1200 gallons of water that way. The decrease in CH corresponded to the almost exact dilution one would expect from removing 1200 gallons of water so the density difference between the pool and fill water is sufficient to get a good exchange without mixing. I will be doing more partial draining myself soon but I can attest to the principle mentioned above.

SWG’s make managing the water a bit more complicated because of these scaling issues but I think the complexity is definitely outweighed by the total ease of chlorine production one gets. As I’ve said many times, if I could not have an SWG then I would not want to own a pool.
 
Thank you for your advice and input. As I don't have a main drain I think the best way forward would be to go ahead with the phosphate removal process, that way I will be able to replace water via vacuuming to waste rather than just dumping water thereby lowering my phosphate and sulfate levels at the same time.

Needsajet: Will placing the hose into the top of the skimmer above the vacuum plate be able to keep the new water on top of the existing water whilst I drain via the vacuum hose? Your offer of advice on this is appreciated

Matt: Regarding calcium sulfate scaling being temperature dependent - my cell is in the sun for part of the day so I made a cover to place over it for that reason. Could these be the cause of the high temperature you are referring to?
 
No. The temperature variation would have to be quite low (near freezing) or extremely high (over 100C) to see any solubility differences. Your unit being in the sun is not the culprit.
 

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