Water Quality/TDS Readings


I have a home RO system and change the 3 filters annually as recommended by the vendor. The vendor also suggests changing the membrane filter every 3 or so years. Before changing my membrane filter (which is now over 3 years old) I ran a TDS test which shows a rejection rate of 97.60% of TDS. The vendor recommends changing this at any reading below 85%. I am assuming my reading shows my membrane is still acceptable. My question on TDS is, why do I get a higher reading on my soft water (1001.8 PPM) than I get on my hard water (484.6 PPM)? My readings for PPM after the soft water RO filtration are .024 which of course relate to the 97.60% rejection rate. I ran the TDS PPM test several times on all three. (Soft Water, Hard Water & Soft RO Water) Your advice is sure appreciated.

ANSWER: Hi Rolf,
I guess my first question would be why are you running soft water through your RO unit?  It would make much more sense to run the tap (or hard) water directly to the RO unit.  The reason is that the soft water has a higher TDS than the tap water and makes the RO unit work harder and reject more water.  The reason that soft water has a higher TDS than the hard water is pretty complicated - because TDS is actually a measure of electrical conductivity of the water and soft water is more conductive. Here is a good article which may help:




I totally disagree with changing the RO membrane when the rate drops below the 85% level.  I change mine at 93%. But that is a personal preference.  I have found that the RO membranes tend to get fouled, especially when treating well water, after a couple of years.

Hope that helps - if you still have questions don't hesitate to let me know.  
Take care,

Environmental Engineer

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I pasted the following note from the Montana.edu site you referenced.

"Note: Water softeners do not reduce TDS. They remove magnesium and calcium ions, which cause hard water, but these ions are replaced with an equal number of sodium or potassium ions. This leaves overall TDS unchanged."

According to this note, there should have been very little variance in TDS from the soft water sample to the hard water sample. As you can see in my earlier comments, the TDS/PPM for the soft water was more than double the hard water sample. Your comment?

My reason for using the soft water to feed the RO system is again a recommendation of the vendor for the RO system. Here is a quote from my installation instructions from Pure Earth Technologies, Inc. in Marietta, GA. "Installing a water softener (available from PureEarth) ahead of the R/O system on water that is 10 grains per gallon or harder will prolong its life".

After reading this again, my only thought is perhaps the softener they are talking about here is something different than my standard home water softener I have been using for years. Comments?

Unless you have come to a different conclusion, I believe I will change the feed to my RO from my soft water to my hard water. I also have a whole house sediment filter which I change every 30 days on my hard water from the well. In addition, I believe I will also change my membrane filter as it is over 5 years old and has done a satisfactory job of giving us good drinking water to date.  

Appreciate your help and expertise.

ANSWER: Hi Rolf,
I may have assumed a few things that I shouldn't have and this could explain what you are seeing. Lets go back a little.  What is the analysis of your water? (If you have a total analysis that would be great).  You have well water - is that correct?  What is the model of your water softener - I know you said it is Pure Earth but does it have a model number?  Do you add salt to this unit?  If so is it sodium chloride or potassium chloride?  How much do you add per week? What kind of TDS meter are you using? (Brand, model, etc.)  Does it read ppm or mMho?

Brand and model of your RO unit?  Do you use it only for drinking water?

If you could get me this information I will see if I can figure out the discrepancy.  Don't change anything until I get back with you.


---------- FOLLOW-UP ----------

RO-50 Info
RO-50 Info  

TDS Meter
TDS Meter  
Attached are two files for your information.
Answers to your questions:
1. I have not had a total analysis of my water. Only the TDS test.
2. Our water comes from a 40' well of many years old. Water from the well is generally pretty good tasting but does have some sediments which I filter through a whole house filter before entering the house. I change this filter religiously every 30 days. The sediment is not iron but vegetative in composition.
3. We don't have a PureEarth water softener. It is a Whirlpool, Model  WHES 30. Installed on 8/2006. Whirlpool Toll Free #.
4. Yes...I add salt. Yes it is sodium Chloride(CAS#7647-14-5). I go through less than 200 lbs. per year or 16-17 lbs. per month.
5. Yes...The RO water is generally used for drinking, ice, coffee, cooking, etc. There is only 2 adults so not a lot of use.
6. The TDS Meter reads PPM and is made by Liquatec. Model PM-3000. 1-2999PPM. Home office is in Fullerton, CA (www.liquatec.com) See also info attached.
I keep pretty good records. If there is anything else you need, let me know.

Hi Rolf,
I don't think I can be much more help without a water analysis.  Frequently the county health departments will run tests on individual homeowner wells for free.  You may want to check with them.  Without knowing how much Calcium, Magnesium and Sodium you have in your well water plus other ions such as sulfate, chloride, etc. it's really hard to make any recommendations.  The fact that your TDS exit the softener is higher than it is coming in may indicate you are bleeding some excess salt through the system or even hardness. Since you only run a TDS test you don't really know how well your softener is working. However, it would be hard to determine this without an analysis.  Just make sure of 2 things:

1. That you are using the correct amount of salt for each regeneration
2. Your rinse cycle is sufficient to get all the excess salt out of the system.

You can calculate how much water your softener will treat if  you know your incoming water hardness and the amount and type of resin in your softener.

Most of the time I recommend using a hardness test to determine when a softener needs to be regenerated.  Once you start bleeding hardness thru the unit it's time to regenerate.  Otherwise how do you determine the length of time between regens?  You can get a test kit from  Amazon which will perform all the tests you would need for $15-100.  

I would encourage you to do some testing so that you know more about your system and how efficiently it is working.  Let me know if you need some help with this - I will be glad to help  you.  Good luck,


Go Colts!!.

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Steve Young


I will answer questions on residential water treatment, i.e. use of softeners, filters, reverse osmosis systems, disinfection, iron and/or sediment removal and other issues facing the homeowner. I have worked extensively with well water systems.


I have worked in the water treatment area for many years as a consultant and as a professional environmental engineer in the private industry sector. I have designed reverse osmosis, deionization, and water softening systems. I have also done work in water quality and stream and river remediation as it relates to the Clean Water Act.

I am currently serving as an Expert for Home Improvement, Travel (New Mexico) and Travel (North Carolina);

BS Environmental Eng.

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