[curiouscat]

Does dissolved NaOH add to TDS (Total dissolved solids)? I suppose it should, since if I evaporate all the H2O out I'd be left with solid NaOH?

I'm wondering because it's often mentioned that adding lime to high-TDS sulfate-wastewater reduces TDS by an order of magnitude, because CaSO4 is very insoluble compared to Na2SO4.

Na2SO4 + Ca(OH)2    CaSO4 + 2 NaOH

How does that work? I've seen reports of an initial TDS of 35,000 ppm reduced to just 3500 ppm after lime addition. Only way this makes sense is if they don't include NaOH as TDS. Is that true?

[curiouscat]

This is the process I was reading about:

http://www.wateronline.com/doc/a-new-process-for-sulfate-removal-from-indust-0001

[Arkcon]

Well, the reference you have really clears it up for me.  I'd always thought CaO, aka, the lime pit, was the standard method of treating lab wastes.  Virtually all heavy metals in solution precipitate out as hydroxides, and you can safely release your lab water effluent, for certain values of the word, safe.  Removing sulfate is new, but I figured the excess NaOH was removed somehow, and your reference says it is, by CO2 addition to "re-carbonate" and reduce pH before effluent release, as sodium carbonate isn't as soluble in a sodium hydroxide solution.  But if you're sampling before all steps, then I have no idea.

*EDIT-spelling*