TDS Results

Paul__January 23, 2011

Help me understand...

I purchased a TDS measuring tool and did some measuring around the house with some contradictory results.

We live in Phoenix with hard water that as of the 2009 Water Quality Report states a Total Dissolved Solids (TDS) range of 518 to 676 ppm and a Total Hardness range of 166 to 305 ppm.

We have a United Standard Hydro-Quad HQ-1054 -1.5 water softener in place with the Fleck 5600 Econominder control valve. We believe the softening system is plumbed as a whole house softener.

The water at the sinks, showers, and faucets "feels" softened, but the TDS measurements indicate otherwise.

Here are the observations:

Water enter house at a TDS of 449 ppm and water after softening reads a TDS of 486 ppm ? ? ?

Water at the bathroom sinks read a TDS of 618 ppm and the shower at 655 ppm ? ? ?

Water at the bathroom toilet reservoir reads a TDS of 430 ppm ? ? ?

Water at the kitchen sink reads a TDS of 445 ppm ? ? ?

Running hot water only is a TDS 585 ppm ? ? ?

Water in the master suite measures TDS of 451 ppm at the sinks and a TDS of 473 ppm at the toilet ? ? ?

Measured with an HM Digital TDS-4TM with an accuracy of plus or minus 2%. Readings are corrected by the tool for temperature.

Shouldn't a functioning water softener reduce the TDS readings at all downstream points of measurement ?

How can the water "feel" softened and not be softened ?

By the way, TDS measurement on a R/O drinking water here shows 25 ppm, so calibration of the new TDS tester doesn't appear to be an issue.



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This phenomenon confuses lots of people...

Remember, your softener is softening the water by ion exchange and if you're using NaCl as a regenerant the exchange is at 2 sodium ions for each ion of hardness.

The harder the water being softened the more sodium is exchanged (@ 2 for 1).

See where we're going? Remove 100 ions of hardness and the softener adds 200 ions of sodium. That's how your TDS increases when softening water.

The lower the hardness of the service water the less noticeable the increase in TDS will be. The harder the service water the greater the increase in TDS will be but only someone who buys a tester and goes looking for confusion will ever see it.

If your softener is correct sized for the # of people, the water conditions, SFR, and it is set up to regenerate efficiently and the water feels soft then all is well.

Your TDS tester is the right tool to monitor the RO. Routine replacement of the pre and post filters is important, but when the TDS tester shows that the rejection rate falls below 90% it's time for a new membrane

    Bookmark   January 23, 2011 at 2:16PM
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Thanks for the response Justa...

Confusing is the right word.

I thought that the Ca and Mg ions in the incoming city water supply met the zeolite inside the water softener and attached to the zeolite itself.

Doesn't the briny regeneration simply reverse the attachment of Ca and Mg ions to the zeolite and flush the loosened Ca and Mg ions, now in the brine wash, to the city wastewater system as part of the brine waste ?

My understanding, limited as it is, was that the sodium or potassium chloride wash and rinse cycle cleansed the zeolite and only a small amount of that brine process would seep into the house system.

If most of what I am measuring downstream from the water softener is sodium or potassium, rather than Ca or Mg solids in the water, we are presented with quite a dietary dilemma. A liter of water from the tap would contribute around 445 milligrams of sodium to one's daily intake towards an RDA of 1,300 mg per day. Guess I'd better drink the R/O water at a TDS of 28 ppm, or 28 mg/L, after all.


    Bookmark   January 23, 2011 at 3:05PM
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If you want to learn the chemistry of ion exchange there are volumes you can read and years you can invest. Water softening is ion exchange... not ion wash or ion rinse.

Regarding the increased sodium in the softened water... we are talking ions exchanged not grams and your TDS meter is measuring PPM. Sodium exchanged into soft water is roughly 7.85 mg/l per grain of hardness exchanged. So 10 gpg * 7.85 = 78.5 mg/l. A slice of white bread has 150 mg of sodium. A glass of skim milk is 530

What you are measuring downstream of the softener is sodium or potassium along with whatever else is in the water when it enters your home that is not ion exchanged out by the softener.

Most people with hardness at your level and above do drink and cook and make ice cubes with RO water.

    Bookmark   January 23, 2011 at 3:40PM
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