Removing calcium and magnesium?

[aquarium]

Is there a relatively safe way to precipitate calcium and magnesium out of the water? My fill water is very high in both and I think it's causing the girl's hair to get more and more 'crinkly' as time goes by and evaporation causes the pool water to become more and more concentrated.

[chem geek]

If you add Sodium Carbonate (typical pH Up or Soda Ash or washing soda) you can raise the TA and pH which will add carbonate to the water and (along with the higher pH) will precipitate out calcium carbonate. With enough carbonate you can even precipitate magnesium carbonate though it is about 10 times more soluble.

Adding just 1 ounce (weight) of Sodium Carbonate to 100 gallons of water will make the equilibrium of calcium carbonate out of balance by a factor of 45 which should precipitate over 90% of the calcium. You might also precipitate half of the magnesium. This assumes you get the pH over 9 (which will happen unless your water has over 100 TA to start with).

Of course, you'll then be left with water with high TA, but you know how to handle that. Using Ben's procedure of adding acid and aerating, you can get rid of the excess TA. Adding 1 ounce of Muriatic Acid will bring you back down to 7.5 pH, but your TA will be higher by about 35.

Then you'll be left with salty water (Sodium from the Sodium Carbonate and Chloride from the Muriatic Acid). Just with adding what I described above, your TDS in this make-up water goes up by 100. It will go up even more with the acid/aeration process, but I can't tell how much since it depends on how efficiently you aerate.

Well, you asked! Sorry I don't know of a cleaner method.

Richard

[dannyboy]

I use a water softener to remove calcium. It also removes magnesium and
iron to a lesser extent.
I recirculate my water thru it when necessary to maintain 150CH
My fill water is over 500CH and over 300 TA.
What I know for sure is that it works. It raises my salt level and I think my
TA goes down a bit and PH up a bit. I was not to specific about before and
after measurements.
I have not seen much for discussion on the use of water softeners on this
forum.
I hope the CHEM GEEK will follow on and give us the break down of what
the water softener removes and adds to the pool and its possible effects.

[chem geek]

A water softener is just an ion exchange material (typically in the form of beads) so it will accept certain ions, calcium and magnesium which form the basis of "hard" water, and release sodium and/or potassium ions in their place. If this is all that it does, then it should lower CH, but everything else should stay the same (pH, TA, etc.). TDS (in ppm) would actually go down if Calcium were traded with Sodium (which has a lower molecular weight and is singly charged).

This is a good idea that I didn't think of. I just don't know how economical it would be for volumes of water in a pool. When a water softener cartridge gets filled up with magnesium and calcium, you replace [EDIT] or regenerate [END-EDIT] the cartridge, so they are typically rated in terms of how many gallons of water at a certain hardness they can filter. It's the best idea proposed so far, however, for reducing calcium and magnesium and I'm really glad dannyboy thought of it. Clearly for fill water, it is very economical -- I'm just not sure how much it would cost to filter your entire pool if you've already got high CH in it.

Richard

[aquarium]

I think the typical whole-house water softener is rated in the 500 gallons per day range. At that rate it would take two weeks to fill a 15,000 gallon pool. Doable, but not practical.

[catnip]

aquarium, as I understand water softener operation (I've had two over the years), you should be able to operate the softener continuously until you've reach the resin bed capacity which would be around 1800 gallons for a CH of 340 ppm in a 36000 grain capacity softener. Then you regenerate the resin in the softener which takes around two hours and proceed with operation. The only caveat is that you need to make sure the salt used to regenerate the resin is totally dissolved which could take some time depending upon the type of salt you use. You can get automatic regenerators that will regenerate based on the water usage vs. timer based regeneration or manual regeneration.

So, given those capabilities, I think you could reasonably fill a 18000 gallon pool in 3 to 5 days...or faster with a high flow rate softener.

[catnip]

chem geek, would you recommend your method for reducing calcium in an entire pool vs. just fill water?

[chem geek]

Chuck,

If you are referring to my proposed method of adding Sodium Carbonate (Soda Ash) to raise TA and pH to ridiculously high levels, I would not do that in an entire pool. If you did that in a pool with gunite/plaster, you probably wouldn't just precipitate calcium carbonate, but would form scale on the plaster making the surface rough (both unpleasant and more likely to retain algae). I was proposing that this method be used in a large tub where you put your fill water and "process it" before putting it into your pool. So you would precipitate the calcium carbonate, then remove the solid (or drain the water to another tub), then lower the pH back down and aerate to lower the TA and then finally adjust the pH and put the water into the pool (sounds like a pain in the butt to me).

The water softener approach proposed by dannyboy is much more sane, but is again only practical on smaller volumes of water, so for fill water (though what you said about taking 3-5 days doesn't sound so bad, but man that's a lot of regeneration needed).

If you've already got an entire pool with high CH and magnesium and have fill water high in CH and magnesium, then there don't seem to be great choices available. One could try using either of the above two techniques on the fill water over time and replace (drain and refill) the pool water with this processed fill water.

The easiest thing to do, of course, it to just live with the higher calcium and magnesium content. To avoid scale, one would use lower than normal TA and possibly lower pH levels (say, 7.2-7.3). Having a lower TA level is actually not a bad thing since it will reduce the outgassing rate of carbon dioxide and thereby lower the rate of rise in pH and acid demand. If one wants more buffering capability, then one can add borates (about 50 ppm should do). Of course, one would still have "hard" water in terms of calcium and magnesium so the original problem of crinkly hair would probably not be solved.

Richard

[dannyboy]

The short story is I did not have a choice. My PH always 7.2-7.8 PS233
With 300 TA and 500 CH my brand new pools sand filter after 8 # dichlor
from the pool builder turned into a brick. I had to chisel it out with a hammer.
This sites removing ALK solved 1/2 of my problem. However my good mornig
precipitate covering the bottom of my liner was only 2/3 eliminated.
A 30000 grain water softener and a shallow well pump total $600.
I cycled the water from the pool thru the system continuously. 35K gallons
9 days 36 manual reginerations later. I have got TA=110 CH=150
It used 5 bags of salt.
Some where about CH=300 all the scale and precipitate disolved and went
away.
My CH slowly goes up with fill water and I repeated the water softener
for 4 days this year. However my TA does not go up. I was curious about
that untill I read the CHEM GEEKS relative out gas rates.
Overall the water softener works for me.
My first salt reading 3 weeks ago from a talor kit was 1200. With 1 year old
water this had to come from bleach and the water softener.
I just installed 2 aquarite SWG that are working flawlessly.

[mbar]

Thanks for posting this method. It is great to have someone share what works for them, it is how we all learn That is what I love about this forum, you can learn so much from the experiences of others!