Using Liquid Chlorine and Raising Pool Salinity

Michael1

Well-known member
May 3, 2015
193
Moorpark, CA
Since I am on a septic system, I have to drain my pool into a natural gulch. Therefore, I don't want to be discharging illegal salt water, which would ruin the flora in the area.

Right now I am using pucks to chlorinate, but am running into high CYA levels. If I switch to liquid chlorine (sodium hypochlorite), how much is that going to raise my salinity? I'm sure there must be a formula for this, but I can't find one.

Michael
 

smackdab

Well-known member
Mar 17, 2015
157
Georgia
For 10,000 gallons of water:
151 fl oz of 8.25% Bleach will raise the FC level 10ppm and the Salt level 16ppm.
15 oz weight of Trichlor will raise the FC level 10ppm and the Salt level 8.4ppm.
So it looks like you'll add about twice as much salt per equal amount of FC.

All forms of chlorine add salt. Some more than others. I believe your thinking of a salt water pool with a SWG that has to have a high amount of salt before it will work. Typically in the 3500 ppm range. It would take you a while to get to 3500 ppm with a non swg pool.

Typical pools loose 2-4 ppm of FC per day. Take the extremel loss of 4ppm FC per day and multiply that by 7 to get 28ppm of FC loss per week. That would be 259 ppm of salt per week added. 10 weeks would be 2590 ppm of salt. 2ppm loss FC per day would be half that at 1295ppm of salt in 10 weeks.
 

Michael1

Well-known member
May 3, 2015
193
Moorpark, CA
For 10,000 gallons of water:
151 fl oz of 8.25% Bleach will raise the FC level 10ppm and the Salt level 16ppm.
15 oz weight of Trichlor will raise the FC level 10ppm and the Salt level 8.4ppm.
So it looks like you'll add about twice as much salt per equal amount of FC.

All forms of chlorine add salt. Some more than others. I believe your thinking of a salt water pool with a SWG that has to have a high amount of salt before it will work. Typically in the 3500 ppm range. It would take you a while to get to 3500 ppm with a non swg pool.

Typical pools loose 2-4 ppm of FC per day. Take the extremel loss of 4ppm FC per day and multiply that by 7 to get 28ppm of FC loss per week. That would be 259 ppm of salt per week added. 10 weeks would be 2590 ppm of salt. 2ppm loss FC per day would be half that at 1295ppm of salt in 10 weeks.

Great information here! Much appreciated. I had no idea trichlor added salt, too.

So even at 1295 ppm salt in 10 weeks, in just over half a year, your pool would have as much salt as a salt water pool? It doesn't seem like it takes long to build up salt.

I've read that most people drain and refill their pools every seven years. At the 1295 ppm 10 week salt rate, the salt level would be 47,000 ppm. That saltier than the ocean! I must be missing something.

Michael
 

duraleigh

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Apr 1, 2007
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Sebring, Florida
I must be missing something.
Splash out and rainwater. Don't have any idea where Moorpark is (please enter your state) but most of the eastern US gets enough rainwater to replace more than 1/2 of their pool
 

chem geek

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LifeTime Supporter
Mar 28, 2007
12,083
San Rafael, CA USA
Michael,

You aren't missing anything. The calculation by smackdab was wrong by a factor of 10 he forgot (for division since the rule had "for every 10 ppm" in it) and had other errors as well. While it is true that for every 10 ppm FC added by chlorinating liquid or bleach that it increases the sodium chloride salt level by 17 ppm (after accounting for chlorine usage/consumption), if one uses 2 ppm FC per day, then after 10 weeks this would be 2*7*10*17/10 = 238 ppm (not 1295).

Your calculation over 7 years assumed that a pool was open all year long. That is generally only the case (at 2 ppm FC per day) in places like Florida. Even so, over 7 years the salt level buildup with no water dilution (rain overflow, backwashing, splash-out, etc.) would be (using a more accurate 1.65 ppm salt per 1 ppm FC) 7*365*2*1.65 = 8432 ppm. Florida gets a lot of rain dilution all year long. They get summer rains and have humid months so not that much evaporation -- with 62" of rainfall per year that is 5.2 depth of water so more than average pool depth.
 

epro05

Bronze Supporter
Jun 5, 2014
281
Keller, Texas
I always plug zero into PoolMath for Salt, because I don't have a SWG, but if I have actually have built up some level of salt from the bleach, the CSI could be significantly wrong. So it sounds like non-SWG pool owners should add Salt Strips to their test kit, so that PoolMath CSI calculation will be correct.
 

smackdab

Well-known member
Mar 17, 2015
157
Georgia
The calculation by smackdab was wrong by a factor of 10 he forgot (for division since the rule had "for every 10 ppm" in it) and had other errors as well. While it is true that for every 10 ppm FC added by chlorinating liquid or bleach that it increases the sodium chloride salt level by 17 ppm (after accounting for chlorine usage/consumption), if one uses 2 ppm FC per day, then after 10 weeks this would be 2*7*10*17/10 = 238 ppm (not 1295).
I thought my figure sounded awful high. Thanks for the correction.
 

chem geek

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Mar 28, 2007
12,083
San Rafael, CA USA
I always plug zero into PoolMath for Salt, because I don't have a SWG, but if I have actually have built up some level of salt from the bleach, the CSI could be significantly wrong. So it sounds like non-SWG pool owners should add Salt Strips to their test kit, so that PoolMath CSI calculation will be correct.
The salt or TDS level doesn't affect the CSI very much, though if you go from < 500 ppm to 3000 ppm it does lower the CSI by about 0.2 units. Though that doesn't sound like a lot, CSI is a logarithmic scale where -0.3 is a factor of 2 lower in calcium (or carbonate) concentration than full saturation. Going from no calcium in the water (infinitely negative CSI) to a level at -0.3 CSI (assuming the pH is the same) could have the plaster last twice as long but going to full saturation could protect it for decades. I'm starting to see some pools getting reported (on another forum) with -0.6 having degraded plaster after around 5 years, but it depends on the quality of plaster, the pH, and other factors. onBalance plans to do some experiments to get some controlled experimental evidence. Note that the Recommended Levels for SWCG on the low side have a CSI of -0.7 (with Bleach it's around -0.3) and I've brought up before how such levels should probably be higher.
 

epro05

Bronze Supporter
Jun 5, 2014
281
Keller, Texas
Thanks chem geek. Does a CSI of zero equal full saturation. If not, what CSI level would indicate full saturation. (I obviously don't understand CSI very well).
 

chem geek

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Mar 28, 2007
12,083
San Rafael, CA USA
Yes, a CSI of 0 means full saturation. It's the level of calcium and carbonate where there will be no net change in the amount of solid calcium carbonate (a component of pool plaster). That is, there will be no net dissolving of calcium carbonate from pool plaster nor will there be any net scaling or precipitation of calcium carbonate from the water. There is still, nevertheless, an exchange of calcium and carbonate to/from the plaster wall (i.e. a calcium or carbonate is lost in one place but added in another) so over enough time it could still get rough but that could be a very long time (decades).

pH is likely a much more important factor in determining the rate of plaster degradation but having the CSI be 0 even at lower pH would prevent any net loss of plaster. So the worst combination is to have a negative CSI along with a low pH since the negative CSI means there can be a net loss of calcium carbonate from plaster and the lower pH means that this process can be accelerated.