Quote Originally Posted by RavenNS
( why is it difficult to believe that when there is tons of crap in your tub, or high TDS, that your sanitizer might get 'busy" with all that & that it won't be as effective? is it not plausibile?)

( also note that if you add fragrance for every tub-ride, you'll hit the high TDS mark very fast as well. & I suspect that the fragrance binding/bonding materials interfer somewhat with the sanitizers, although I have no proof of that. lol)
Normally dissolved substances, specifically charged ions, in water only interfere with chemical reactions due to what is known as "ionic strength". With sufficiently high TDS, this does in fact start to shift equilibrium towards charged ionic species since their "activity" is reduced relative to their actual concentration. My spreadsheet actually calculates all of this. However, the effect is very small in terms of its effect on chlorine concentration (though not on LSI or water balance) and essentially can be ignored for the ranges of TDS we are dealing with. It would be very important in sea water, but even salt SWCG pools do not have enough TDS to be of concern.

As an example, in a pool without CYA and with 2 ppm chlorine, 7.5 pH, 100 TA, 300 CH, 510 TDS (the minimum with these other parameters), and a temperature of 80F, this results in 0.968 HOCl (now correctly interpreted in ppm Cl2 units as all chlorine is measured due to historical convention). If I increase the TDS to 3500, then the HOCl drops to 0.909 which isn't a huge change.

Interestingly, if I repeat the above with CYA of 30 ppm and use 3 ppm chlorine, then I get at 525 TDS (again, the minimum which is higher due to the CYA), 0.042 for HOCl. If I increase the TDS to 3500, then the HOCl drops to 0.041 which is hardly a change at all.

The reason that changes in TDS have virtually no effect on HOCl in the presence of CYA is that the CYA acts like a chlorine (specifially an HOCl) buffer and resists changes in HOCl level due to outside factors. Specifically, the dominant equation where CYA buffers HOCl has charged ions of equal strength (charge) on both sides of the equation so changes in ionic strength due to TDS mostly cancel each other out. Without CYA, it is the HOCl/OCl- relationship that is affected and here there is charge on one side of the equation and no charge on the other.

Now in spite of what I just said it is of course possible that some forms of organics that get into the water may behave somewhat like CYA and tie up the chlorine and reduce its effectiveness. However, any such substance would have to also report its chlorine in the standard chlorine test for FC and would have to not be destroyed (oxidized) by the chlorine. Chlorine bound to CYA gets reported as FC since it is released from CYA in a matter of seconds. So yes, it is possible for some other organic compounds to behave in a similar fashion, but most are simply 1) combined with chlorine and stay that way as "combined chlorine" until fully oxidized (and are measured as the CC part of TC, but are not in FC) or 2) are not affected by chlorine at all. Obviously CYA is an "organic substance" that inhibits chlorine so certainly it is possible for other substances to behave this way as well.

So again, it is not the high TDS in and of itself that is a problem, but rather the fact that a high TDS may indicate older water that has more organics in it (in addition to salt) and some of these organics may interfere with chlorine's effectiveness. My hunch, however, is that for most standard organics that get into the pool or spa, that few if any inhibit chlorine's effectiveness (assuming you do not allow combined chlorine to build up since obviously that is chlorine that is not effective).

Richard