Potential method for CH reduction

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G

Gimpy

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LifeTime Supporter
May 27, 2007
40
Spring Hill, Florida
My CH level has gone from 300 ppm to 500 ppm in a year, with only temporary and insignificant reductions as a result of rainwater additions. I attribute the rise of the CH to fill water concentrated by evaporation. Realizing that it won’t be long before the escalating CH will reach a tipping point and manifest itself with the appearance of other problems, I’ve been seeking a method to lower the CH without a partial drain and refill – a Magic Bullet, if you will. I don’t want to drain and refill because its expensive, inconvenient, and disruptive of the other components of pool water balance.

The pool is a 20,000 in-ground plaster pool. The current water test results are:
FC = 2.8
CC = 0.0
pH = 7.5
Alk = 70
CH = 500
CYA = 80
NaCl = 3,200
Borate = 50
Temp = 86.4°F

The (municipal) fill water test results are:
FC = 0.4
CC =0.0
pH = 7.8
Alk = 150
CH = 170

Here’s a potential method I found, and I’m soliciting the analysis and comments of folks more knowledgeable in the chemistry department than I am. (Are you reading, Chemgeek?) As you can see, this involves the use of trisodium phosphate, and I’m kinda sorta reluctant to add a phosphate to the pool water.

http://www.deh.enr.state.nc.us/ehs/chem.htm
“To lower calcium hardness anhydrous trisodium phosphate may be used. One pound trisodium phosphate per 10,000 gallons will lower calcium hardness 11 ppm. Use in small increments or clouding may occur. Another method of lowering calcium hardness is to simply drain off part of the pool water and dilute the remaining water with fresh make up water.”

http://www.watermaid.com/help/alltopics ... .htm#Scale
“Another method is to precipitate the calcium as Tricalcium phosphate by the addition of trisodium phosphate. 1 lb. of anhydrous trisodium phosphate will precipitate 1 lb. of calcium carbonate. This is readily removed from the bottom of the pool by vacuuming.”
 
TSP in the pool is not particularly a good thing. While phosphates are seldom a problem, massive doses in a marginally sanitized pool may lead to a significant algae bloom.

Other than that, it sounds interesting, but I don't know what other affect(s), other than clouding, TSP may have on your pool water.
 
Take a look at this thread which talks about two products or systems that reduce Calcium Hardness. The Cal-Sok is an anionic (negatively charged) organic polymer so essentially goes into the filter and attracts calcium to it which then needs to be cleaned or backwashed. The Calsaway system doesn't say how it works.

The use of TriSodium Phosphate (TSP) is, as you point out, not a good idea as it adds phosphates to the pool, so let's see how much would be left over after it precipitates with calcium. Though some amount isn't a problem, high amounts aren't good. The solubility product for calcium phosphate, Ca3(PO4)2 is 1x10^-26 so that means:

[Ca2+]3 x [PO43-]2 = 1x10^-26

A Calcium Hardness of 300 ppm is about 3x10-3 moles/liter concentration. Solving for the phosphate concentration from the above results in about 6x10-10 moles/liter. Phosphates have an equilibrium with different species derived from phosphoric acid and at a pH of 7.5 this implies a concentration of HPO42- of 9x10-5 and a concentration of H2PO4- of 4.6x10-5 and a concentration of H3PO4 of 2x10-10. So the total phosphate concentration is 1.4x10-4 which is 13 ppm or 13,000 ppb (assuming this is based on the molecular weight of phosphate, PO43- which is 94.97 g/mole and not on phosphorous alone which is 30.97 g/mole).

So it seems that this technique of using phosphates is not practical except under very basic/alkaline conditions which would increase the concentration of phosphate ion. It should be pretty obvious that phosphates are found in pools today with the presence of calcium and have been measured to at least 5000 ppb so the above analysis is probably correct. Yes, TSP will reduce the calcium in the pool, but then you'll have leftover phosphate (up to 13,000 ppb). Of course, you could then remove that with expensive phosphate removers (which are lanthanum chloride). To minimize this problem, one should use the TSP at very high pH to maximize precipitation. At a pH of 8.0, the above analysis results in about 3 ppm or 3,000 ppb which is more manageable. At a pH of 8.5, the above analysis results in about 900 ppb while at a pH of 9.0, the above analysis results in about 300 ppb.

Richard
 
That doesn't sound very practical, and is precisely what I was afraid of - high phosphates at the end.

I'm wondering how a five gallon bucket test would turn out. Are the phosphate test strips sufficientl reliable for the purpose?

My quick and dirty SWAG hints that I would end up with about 35+ lbs. of precipitate (from the 20,000 gallons), which is a whole lot of filter cartridge rinsing....
 
You could use a bucket test, but remember that the quantities are very small so you have to carefully measure. I think the test strips would be reasonable to measure the leftover phosphate, but they don't measure very high levels so you'll have to use a dilution method in that case (using filtered or distilled water that has virtually no phosphates).

You can even repeat the experiment at different pH levels. If you use Lye (caustic soda; sodium hydroxide) to raise the pH, then later adding Muriatic Acid to restore the pH essentially just has you increase the salt level as a side effect to minimizing the resulting phosphate level.

And yes, you are talking about a LOT of precipitate, but would generally fall to the bottom of the pool (if the pump is turned off) and would then need to be vacuumed to waste and/or scooped up. You could even use a floc to force more particles to the bottom.
 
You know, if my analysis was correct, then I've hit upon another way to remove phosphates without having to use a phosphate remover. If raising the pH to 9.0 truly can precipitate out the phosphates down to 300 ppb (possibly with the help of a floculant) IF one has 300 ppm CH, then that's an interesting (though generally unnecessary) technique.

The presence of phosphates would also be another source of cloudiness when the pH rises. So if the saturation index doesn't seem to indicate that cloudiness would occur at higher pH -- say, the TA is low, but the CH is normal to high -- then the cloudiness could be due to phosphates and the precipitation of calcium phosphate. Very interesting.

It would take 26 ounces weight of Lye (Caustic Soda; Sodium Hydroxide) to raise the pH to 9.0 in a standard pool (TA 100, CYA 30, starting pH 7.5) and 62 fluid ounces of Muriatic Acid to restore the pH back to 7.5, all in 10,000 gallons. The cost of Lye is about $1.50 per pound while the cost of Muriatic Acid is about $5 per gallon. So the cost of the Lye and acid to lower phosphates in normal CH pools is around $5 per 10,000 gallons for any amount of phosphates.

32 ounces of SeaKlear Phosphate Remover removes 1000 ppb of orthophosphate from 20,000 gallons of water, so that's 16 ounces to remove 1000 ppb from 10,000 gallons. 32 ounces is around $29 (or $23 if bought as $92 for one gallon).

So for pools that have normal CH levels (i.e. plaster pools), then simply adjusting pH may be a less expensive way of reducing phosphate levels. Though normally we don't care about high phosphate levels, those few pools with 5000+ ppb could simply use the pH swing technique along with calcium to precipitate calcium phosphate. At least that's the theory.

One BIG problem, though, is that the high pH will also tend to precipitate calcium carbonate, first as cloudiness and then as scale. So to have the above method work more reliably, one would need to lower the TA level to extremely low levels (say, 20 ppm with 30 ppm CYA -- so carbonate alkalinity only 10 ppm).

OK, so this was a stupid idea, but I didn't realize it until getting this far.

Richard
 
The use of TSP to reduce calcium hardness is very 'old school' and the practice has been pretty much abandoned since it does lead to major algae problems. The amount of phosphates introduced in this way is extremely high. Also many places have banned TSP because of phosphate pollution and only a TSP substitude is sold ( usually sodium carbonate or sodium sesquicarbonate) which is pretty much usless for this purpose. I know that TSP was banned in Florida many years ago.
 
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