Heavy historical use of phosphonic metal sequestrant, soft water and and swg cell?

[Swampwoman]

Good morning. ChemGeek got me thinking about my planned spring conversion to SWG with this comment on another thread which I didn't want to hijack:

A standard water softener that is a cationic exchange resin will substitute calcium and magnesium with sodium so reduces water hardness, but does not change the chloride level and chloride is what gets measured in most salt tests. So yes SODIUM is increased with the water softener, but not chloride.

If you also had an anionic exchange resin, then THAT could substitute carbonate or bicarbonate for chloride and would be used to reduce high TA in fill water.

Recently, my pool water was tested by machine, not strips, at pool builder's store - tested 3500 salinity. They also tested my tap water to confirm 450 from tap/soft water source, from which I fill the pool.

I have a dual tank softener; don't think its anionic.

Based on chrmgeek's comment, I took it to mean that my salinity reading is not necessarily related to the softened water. My salt buildup could be strictly from chlorine. Historically, I don't use a tonne of MA, but in past have used a to be of Jack's pink, then Metal Magic for previous iron load, which is now diluted to about .5 ppm or just under.

Likewise, the highest my TA ever runs is 90-ish, though pool is happier at 70. In 4 years I've never personally added TA, but on purchase original pool techs had.

I never tested for salt before and there was no evidence of a prior swg but I can reliably predict that the water was not likely lowered for winter etc (the same techs close it who built it and they don't lower the water due to water table for which they'd plumbed a sump.)

So apart from backwash not a ton of dilution. This is the first year that the water "tasted like a tear" and only since switching to the dual softener...the single unit would tap out on refills, meaning more well iron went in

Here are my questions:

1. Can high use of diphosphonic acid (eg Metal Magic) and its by-product of spent phosphate (to the order of about 25,000 ppb in attempted dilution) increase salinity...and more importantly, if I switch to swg, will it possibly create phosphate scale on the salt cell, reducing life span?

If so, is this one of the rare times a major phosphate floc is in order, and if so, what is the best, commercial-level/high volume remover to use with minimal contraindications?

2. Is it possible that sodium from soft water is skewing my salinity readings across the board? Or that the sodium is combining with my chlorine? Do I need to look into my softener setup further? Or just be happy that when I switch, I don't have to add salt?

3. Going forward, when I make the switch, assuming I stll have .5ish iron and .3 copper, will I have problems using Metal Magic to a maintenance level? Would I be better off switching to jack's magenta for swg?

4. Any other forseeable issues with the combo of metals, soft water additions, and SWG apart from manage salt build up if it gets too high?

5. My liner is about 15 yrs old and showing signs of wear...I don't plan to change it for two more seasons if possible, but when I do change it, I'm likely to have startup water trucked in to mitigate well/softener demand. Should I wait until the liner/water change to switch to SWG?

Thanks in advance for any insight

 

[JoyfulNoise]

Here's my general impression - given the "funkiness" of your water and its history I think I would advise you to wait on the SWG and do everything at once (liner, new water and new SWG). I know that's a bigger nut to squirrel-away, but I have hesitations saying that your water is A-OK for a new SWG.

Let me answer your questions (if I can and certainly not definitive) -

1. Can high use of diphosphonic acid (eg Metal Magic) and its by-product of spent phosphate (to the order of about 25,000 ppb in attempted dilution) increase salinity...and more importantly, if I switch to swg, will it possibly create phosphate scale on the salt cell, reducing life span?

If so, is this one of the rare times a major phosphate floc is in order, and if so, what is the best, commercial-level/high volume remover to use with minimal contraindications?


I believe chem geek once calculated the rate of calcium phosphate formation inside an SWG and found that you'd need very high inorganic phosphate levels to even begin to consider it a problem. See HERE and HERE for details.

However, with that said, your phosphate levels are quite high....heck, you should water your garden with your pool water, the plants would love it You could try a phosphate remover like Orenda PR10000 in this thread. The Orenda stuff is the most concentrated on the market but it does require an additional clarifier to be used. The Leslie's stuff (PhosFree) already contains a clarifier but the lanthanum chloride concentration is much lower.

Also, the HEDP should have little bearing on your waters salinity.

2. Is it possible that sodium from soft water is skewing my salinity readings across the board? Or that the sodium is combining with my chlorine? Do I need to look into my softener setup further? Or just be happy that when I switch, I don't have to add salt?

OK, so there's two things here that are confusing. When folks here talk about measuring their salinity, they are really talking about measuring their total chloride (Cl-) level since that is what is important for an SWG. It really doesn't matter what the cation is (Na, Ca, K, Mg, etc). Chloride is measured using the argentometric method of titration (silver nitrate reacts with sodium chloride to precipitate silver chloride in the presence of a chromate indicator dye which changes color from yellow to salmon/brick red). All the argentometric titration measures is TOTAL chloride (Cl-) concentration, it doesn't measure the concentration of the counter ion. There is no specific and simple colorimetric method for measuring sodium. The only way I know of to accurately measure it would be with a flame spectroscopy method where one looks at the intensity of specific emission spectra for Na. You could potentially measure total hardness (Ca and Mg) and then subtract that number from a total chloride measurement to get a rough estimate of the amount of sodium and potassium in the water, but that's about it. There are no electrical meter methods of measuring sodium directly.

I think your softener setup is right for your well water. If you get a new liner with a fresh fill and an SWG, you really don't want metals around. Sodium is not anything to worry about unless you plan on drinking your pool water


3. Going forward, when I make the switch, assuming I stll have .5ish iron and .3 copper, will I have problems using Metal Magic to a maintenance level? Would I be better off switching to jack's magenta for swg?


Because of your metal laced fill water, you should keep using whatever metal sequestrant regimen works for you. If you start with a fresh fill of trucked in water, you'll be able to manage your metals better.


4. Any other forseeable issues with the combo of metals, soft water additions, and SWG apart from manage salt build up if it gets too high?

None that I can see.


5. My liner is about 15 yrs old and showing signs of wear...I don't plan to change it for two more seasons if possible, but when I do change it, I'm likely to have startup water trucked in to mitigate well/softener demand. Should I wait until the liner/water change to switch to SWG?

As I said in my intro thoughts, I think doing it all at once is the better approach. Why potentially foul up a new SWG with suspect pool water.

 

[chem geek]

Recently, my pool water was tested by machine, not strips, at pool builder's store - tested 3500 salinity. They also tested my tap water to confirm 450 from tap/soft water source, from which I fill the pool.

I have a dual tank softener; don't think its anionic.

Based on chrmgeek's comment, I took it to mean that my salinity reading is not necessarily related to the softened water. My salt buildup could be strictly from chlorine. Historically, I don't use a tonne of MA, but in past have used a to be of Jack's pink, then Metal Magic for previous iron load, which is now diluted to about .5 ppm or just under.

I don't know how they are testing for salinity. They might be using conductivity and then assuming a certain combination of ions. Or they could be doing the same salt test that we use for pools which is a chloride test. The fact that the result is reported as ppm sodium chloride does NOT mean that this is what was measured. In our pool salt test, all that was measured is the chloride level because the test uses silver nitrate to precipitate silver chloride where extra silver beyond that then reacts with an indicator chemical to turn it red.

Jack's Magic The Pink Stuff and ProTeam Metal Magic are both HEDP which over time with breakdown from chlorine will result in phosphates. These will NOT show up in chloride-based salt tests but they WILL show up in conductivity tests that might be used to report salinity but the amount added isn't likely to be very much (as I discuss below).

1. Can high use of diphosphonic acid (eg Metal Magic) and its by-product of spent phosphate (to the order of about 25,000 ppb in attempted dilution) increase salinity...and more importantly, if I switch to swg, will it possibly create phosphate scale on the salt cell, reducing life span?

If so, is this one of the rare times a major phosphate floc is in order, and if so, what is the best, commercial-level/high volume remover to use with minimal contraindications?

If you actually measured 25,000 ppb phosphate then this is only 25 ppm and while this WILL increase conductivity so affect salinity measurements, it's not that much of an increase. At pool pH, most phosphate will be in the form of HPO₄^2- and H₂PO₄^-. So with its double charge maybe you'd see a conductivity increase that implies around 50 ppm salinity so not that much.

As Matt pointed out, you need rather high calcium AND phosphate levels to get significant precipitation, though in an SWCG cell the pH is higher so the phosphate level is higher as a result. This post gives the best formula for predicting calcium phosphate scaling. If I assume 25,000 ppb phosphate and solve for calcium hardness I get the following and I'll use 30ºC (86ºF) tempertaure and 7.5 and 8.5 pH:

pH_c = (11.755 - log(ppm CaH) - log(ppm PO₄) - 2log(ºC)) / 0.65
CaH = 10^[11.755 - log(ppm PO₄) - 2log(ºC) - (0.65 * pH)]
CaH = 10^[11.755 - log(25) - 2log(30) - (0.65 * 7.5)] = 337 ppm
CaH = 10^[11.755 - log(25) - 2log(30) - (0.65 * 8.5)] = 75 ppm

So you can see that at the elevated pH in the SWCG cell that calcium phosphate formation can occur at rather low CH levels. So unless your CH is very low, you'll probably need to lower your phosphate levels in your pool. The least expensive way to do that (other than water replacement) is to use Orenda PR-10,000 where removing 25,000 ppb phosphate in 22,000 gallons will take somewhat more than 1.4 gallons. Of course, you don't need to remove all the phosphate but even removing half would take a lot of product. As shown in this link the product is $139 per gallon. If you add this very slowly to the skimmer (maybe even in split doses) you might avoid the cloudiness and not need to use a clarifier. Obviously do not backwash your filter for a while (say, a week) after adding the phosphate remover.

This is why if one is going to use a phosphate remover for whatever reason, then they should use a non-phosphate based metal sequestrant (but not EDTA since that reacts with chlorine too quickly).

2. Is it possible that sodium from soft water is skewing my salinity readings across the board? Or that the sodium is combining with my chlorine? Do I need to look into my softener setup further? Or just be happy that when I switch, I don't have to add salt?

Your softener being a cationic exchange resin is simply substituting calcium and magnesium for sodium and should not affect the chloride salt test. As for whether it affects the salinity test, that depends on how that test is being done. If it's measuring conductivity, then I don't think it will change that much. If it's measuring sodium specifically, then yes certainly the water softener will increase sodium levels.

As far as your SWCG is concerned, it only really cares about the chloride level since that is what is used to generate chlorine. It also needs conductivity, but with charge balance of the chloride it doesn't care whether the positive charge balance is from sodium, calcium, magnesium, potassium, etc.

3. Going forward, when I make the switch, assuming I stll have .5ish iron and .3 copper, will I have problems using Metal Magic to a maintenance level? Would I be better off switching to jack's magenta for swg?

Yes, Jack's Magic The Magenta Stuff uses an acrylic acid copolymer which is a negatively charged polymer that attracts positively charged metal ions. You want to be careful not to use a standard clarifier when using The Magenta Stuff because standard clarifiers are positively charged polymers and the result will be a precipitated cloudy mess, though it will filter out.

4. Any other forseeable issues with the combo of metals, soft water additions, and SWG apart from manage salt build up if it gets too high?

No, not other than what I already mentioned above.

5. My liner is about 15 yrs old and showing signs of wear...I don't plan to change it for two more seasons if possible, but when I do change it, I'm likely to have startup water trucked in to mitigate well/softener demand. Should I wait until the liner/water change to switch to SWG?

Up to you, but that would certainly save you the cost of the phosphate remover.

 

[Swampwoman]

Thank you both for weighing in -- it's a bit of a heady topic for me because the math is just a bit outside my power of recall I think I need to call the PB and confirm what kind of test that was, though the strips did corroborate.

Chemgek, the 25000 ppb was based on a 1:5 dilution in a Hasa aquarium kit since it had higher capability than my Taylor phosphate kit. But I don necessarily trust my powers of precision dilution

While my phosphate levels are comparable to SmallPoolDad's in his thread, my calcium is much lower (eg 60). I can't quite estimate what that does to my threshold (re me and math) but even if it gves me way more head room (eg i will never let ph get that high...i ride 7.4 now to control metal) my hunch is still that I'm best off with either a fresh start for swg OR to do a serious commercial grade phosphate floc...the latter of which I've thought would make sense next year -- I didn't test phosphates this year and since my "diet" of sequestrant reduced dramatically and my metal levels aso dropped there's a good chance my phosphate level has as well.

Pool was closed today, ( )so I'll measure the phosphate level come spring.

Thanks also for tip about clarifier and Jack's magenta -- haven't and don't plan to use clarifirs...water is always crystal, and I'm optimistic that if I get my variables sorted ahead of time the switch to swg will not change that

(Or I will be in big trouble with hubby, who as you may recall, has been wary of salt when I've lobbied for it over the last four years What changed this year was he suffered a distal tendon rupture (complete) and surgery this summer, and discovered we're getting old. Hi ho.)

 

[chem geek]

When I used the 8.5 pH example, that wasn't for your pool pH but rather the pH at the SWCG hydrogen gas generation plate. Without borates, it could be that high of higher there and therefore you seem to be on the edge of getting calcium phosphate scaling in the SWCG cell. Probably best to retest phosphates next year and re-evaluate. If you can wait until you change your liner so change your water, then that's obviously safest and a fresh start.