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Thread: Basic chemistry on SWG systems

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    Basic chemistry on SWG systems

    Hi, I'm a noob here at TFP with a basic question that the Web doesn't seem to answer.

    I have an in-ground plaster pool/spa/waterfall with SWG, built 8 years ago. I keep up with the chemistry myself, and it's been very low maintenance (I run it year round). Recently bought a "real" chemical test kit (Taylor) and I'm enjoying the fine tuning and learning lots on this site.

    My question: If the salt merely provides chlorine from a different source, and chlorine is consumed by organics and sunlight (hence CYA is needed), then why don't I have to constantly add salt to my pool to replace the "consumed" chlorine? I do add a little during the season, but so little that I don't keep track (maybe just to replace backwash and splashout?). However, I DO have to add muriatic acid REGULARLY. I'm always told, "new plaster pools will require a lot of acid," but it's 8 years old now. And many pool sites note that SWG pools "mysteriously" will have pH creep up and require routine addition of acid - but don't say why. I found only ONE site on the internet that claims the ACID is actually what's being consumed - i.e., instead of adding chlorine manually (solution or tabs), with a SWG you're adding acid manually. This site even had some chemical equations purporting to show how this works (as the salt's chlorine is used, sodium hydroxide is formed, and the HCl busts that up to give you NaCl and water again, or something like that). It made sense to me, but I'm just surprised there's so little talk about this, given the debates and misconceptions about SWG versus other methods. I just have a hard time believing in a "perpetual motion machine." Any thoughts are appreciated!

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    Mod Squad tim5055's Avatar
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    Re: Basic chemistry on SWG systems

    Welcome to TFP!!

    It is a continuous closed loop. The SWCG turns the salt into chlorine via a chemical reaction. The "consumption" of chlorine is a different chemical reaction that turns teh chlorine back into - salt.

    You have to add a little during the swim season due to splash out and the water that is carried out of the pool in swimsuits and such.

    The pH creep is more from areation, water falls, people splashing and the like.
    TFP Moderator 39 X 18 23,000(ish) freeform gunite; built 2007ish; Pentair Triton II TR100 600lb Sand filter; 2 HP Pentair pump with 2.2 HP AO Smith single speed motor; 2 skimmers, 1 main drain, 4 returns w/waterfall, Stenner 45MHP2 3GPD running@ 60% - 15 gal Tank; heated by the sun CYA 200+ when I started - 50 now. Dolphin Supreme M5 Pool Cleaner. Hot Springs SX Spa, 285 gallon

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    Mod Squad pooldv's Avatar
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    Re: Basic chemistry on SWG systems

    Yes, it is closed loop, with no splash out or overflow you would never have to add salt again.

    There is a recent discussion about it here: Salt not affected by sunlight?
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    Richard320's Avatar
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    Re: Basic chemistry on SWG systems

    Welcome!

    What happens is that NaCl + H20 react in the cell to make NaOCl + H2 That's Sodium Hypochlorite - bleach - and hydrogen gas. The gas dissipates when it hits the surface. And as the hypochlorite oxidizes things, it reverts back to NaCl. I imagine if you use the search box here for one of Chemgeek's threads, you'll find a more complete and accurate answer.
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    Re: Basic chemistry on SWG systems

    I suggest you read the Pool School article Water Balance for SWGs. By having a lower TA level and higher pH target and higher CYA level you should significantly lower your acid usage and using 50 ppm Borates the rate of pH rise can be further slowed.

    The chemistry you saw regarding pH rise left out the very important steps of what happens when chlorine is used/consumed since this is an acidic process. Technical details are in this post.
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    Re: Basic chemistry on SWG systems

    Thanks to all for the quick and detailed responses. Clearly I have a lot to learn. The "closed loop" idea just seemed to good to be true, though like I said I've been happy that my pool stays pretty clean and clear. Actually, right now I am using liquid chlorine to sanitize, because I have a leak in the pool return pipes, and the in -line salt cell is disabled while I run the pool water through the spa jets/waterfall only... but that's another story...

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