SWG Operation - Lessons Learned

[chem geek]

Carl,

Salt does not need to be regularly added to the water unless it is getting diluted through splash-out or backwashing. The consumption of chlorine in the water produces chloride ion so essentially the chloride ion is REGENERATED when the chlorine gets used up -- the chlorine/chloride does not (usually) evaporate. This happens regardless of whether the chlorine gets used up through oxidation (including disinfection) or through breakpoint with ammonia/urea or through breakdown from sunlight.

The only way that chloride will leave the system on its own (i.e. without sodium) is if there is outgassing of chlorine gas or hypochlorous acid gas. Both can outgas from a pool with hypochlorous acid being about 100 times faster to outgas (it's equilibrium quantity as a gas is much higher than for atomic chlorine gas). However, such chlorine outgassing is very slow and minimal in a pool. By far, most chlorine in a pool gets used up through the breakdown by sunlight and therefore gets regenerated back into chloride ion.

So the bottom line is that though it is true that theoretically you can build up sodium ion by having to add salt to make up for the lowering of chloride ion through the outgassing of chlorine (hypochlorous acid), the amount is exceptionally small. Anyone who has an SWG pool with a cartridge filter can probably tell you that they need to add very little salt to maintain the salt level (which is actually measuring chloride ion though the units are in ppm sodium chloride).

You can see the equations for chlorine consumption (and generation in a salt cell) at this link where you can see that the net reaction of chlorine generation AND consumption in an SWG system is:

4H2O + 2Cl- --> 2HOCl + 2OH- + 2H2(g)
2HOCl --> O2(g) + 2H+ + 2Cl-
2H+ + 2OH- --> 2H2O
------------------------------------------
2H2O --> 2H2(g) + O2(g)

6H2O + 3Cl- --> 3HOCl + 3OH- + 3H2(g)
2NH3 + 3HOCl --> N2(g) + 3H+ + 3Cl- + 3H2O
3H+ + 3OH- --> 3H2O
------------------------------------------------------
3H2O + 2NH3 --> N2(g) + 3H2(g)

So the net reaction of chlorine generation AND consumption in an SWG system is that you simply produce hydrogen and oxygen gasses if the chlorine breaks down from sunlight (which is what most of it does) or you produce nitrogen and hydrogen gasses if the chlorine burns up (so to speak) ammonia. Other reactions are possible, but these are the common ones. Chlorine can combine with organics, but generally sunlight breaks these down and liberates the chloride ion (carbon dioxide may also be produced in this process).

Richard

[CarlD]

OOPS! Wrong "Carl"!!!!