tds bogus or true???????

[chem geek]

Regarding:

"TDS: Check it out when the water doesn't behave."
By David Dickman
Reprinted from Pool & Spa Service Industry News
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This is an excellent presentation of confusing information. [EDIT] No offense to RavenNS -- I'm referring to the article reprint only [END-EDIT]. To lump all TDS together is ridiculous. A sea water pool which has 30,000 to 40,000 TDS operates just fine so obviously it's not just high TDS that is the issue. When the TDS is composed mostly of simple salts, especially sodium and chloride, then this is not a problem (it does change water balance slightly, due to ionic strength). Most of the chemicals you add to your pool add sodium or chloride either immediately or when they get used up or broken down (i.e. chlorine). Carbonates that get added to your pool (sodium bicarbonate or sodium carbonate) do not build up because they are always outgassing as carbon dioxide to the air.

On the other hand, organic material that gets into the pool and gets partly broken down by chlorine can sometimes stick around and not completely decompose. If the material is large enough, then it will get caught in the filter. If not, then it will remain dissolved. Now some of this organic material, if broken down into smaller molecules, can become food for bacteria and algae so that may explain why some higher TDS situations lead to what appears to be a reduction in sanitation or algae prevention (but is actually just that you don't have enough chlorine in the first place and were lucky that conditions for faster bacterial or algae growth weren't present). However, a conservative amount of chlorine (i.e. Ben's chart) should still work reliably in that environment. So while it may be true that a low TDS pool may be able to get away with a somewhat lower level of chlorine (below the minimums in Ben's chart, for example), I wouldn't say that a high TDS pool isn't manageable.

As for the smell of water that is high in TDS and has not been changed in a spa, that is most likely the result of incomplete oxidation of organics and resulting chloramines (if chlorine is used as a sanitizer or if used in conjunction with bromine). It would be interesting to find out if shocking a spa with high chlorine levels would take care of this problem.

In spite of what I just said, I think that regularly changing spa water makes a lot of sense since the quantity of organic material and bad stuff that gets put into the water is much, much larger per volume of water in a spa compared to a pool (even a commercial pool). If anyone has figured out how to maintain and shock such water so that they don't have to change it (or change it infrequently), then that would be useful information to know. As for pools, I believe they can go a lot longer without a water change, but in general you would end up doing some change of your pool water over time anyway through backwashing DE or sand filters. If you have a cartridge filter, then you probably have to manually drain and refill some pool water to refresh it periodically. In my own pool (which has a cartridge filter due to water restrictions in my area), I let each winter's rains effectively change about a third or so of the pool water (accounting for "continuous dilution").

Richard