Why is the TDS on my RO system so high?

Why is the TDS on my RO system so high?
By Mark Timmons
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Why is the TDS on my RO system so high?

Dear Sir,

With your 40 years experience in drinking water, I'm hoping you can help.  At this point I'm worried and confused about the new drinking water system we just had installed in our home by a local company that we will be renting from.

For the past 17 years we have been renting a Reverse Osmosis system (4 Stage) from a  company that does supply or install ro/alkaline filter equipment.  Our equipment is under the kitchen sink.  I thought RO water was healthier for myself and my family.  I recently read that RO water is Acidic water because it takes all the minerals out.  I was told by a friend that adding an Alkaline filter to our RO system would make the water more alkaline which is healthier for the body. This past Wednesday, I had a 6-Stage Reverse Osmosis/Alkaline Ionized Water filter system installed by a different company.  I read it would be healthier because the alkaline filter adds Minerals back into the water which makes the water more Alkaline. Drinking a more alkaline water supposedly makes the body less acidic.

The RO water I had before always tested between 10-20 (company serviced every year or sooner if needed).  I have a HM digital TDS meter tester and would check it myself now and then.  

The new company is family operated.  The wife did a lot of research before adding this alkaline ionized filter to their RO systems.  She claims it's one of the best filters out there.  Her older son takes care of the water system for the military so I thought this was a great testimonial.

Before they installed our new system, we tested my water from my "old" RO water system, it tested 12, both on my tester and his.  After our new RO/Alkaline Ionized filter water system was installed it tested at 49.  He said it's higher because of the minerals added back into the water.  He also told me to drain the tank tonight and in the morning re-test the water, it should be much lower probably in the 20's.  In the morning it was 72.  This made me worried about drinking this water.  I called the owner and she was surprised that it was that high & said she would call the manufacturer and get back to me. I tested the water again this morning, and it's even Higher, 96.  As I said at the beginning of this long email, I'm worried about this water and don't know if we should be even drinking it.

When I saw your site and with all your experience in this field I thought you could tell me what's happening & if we should be drinking this water.  We also buy bottled water to take out with us. We buy "DaSani" water.  I tested the DaSani, it was 24.  DaSani claims they add a very small amount of minerals back into their RO water for taste.  I thought 24 is a pretty good #  for bottled water.  Which makes me more confused as to why my new system is testing out so high.

I hope you can help.  I would be more happy to pay you a fee for your unbiased opinion on this water situation, please let me know.  It would be worth it for someone with your experience.  


I.S. - Westlake Village, California

The Water Doctor responded:

First of all, the debate about Alkaline verses Acidic water is sad, because most people do  not understand the difference, and there are lots of unscrupulous companies competing for the business.  For the record, I do not think a few points on the pH scale make a difference because you drink the water and  the water penetrates the cell wall, it becomes the pH of the body anyway.

You have evidently purchased a reverse osmosis system that adds minerals back to the water (usually calcium carbonate) which is why your TDS is higher than before.  Your RO is taking the minerals out and the re-mineralization filter is adding some back.  It is functioning perfectly in all likelihood.  No matter what the label says, you do not have a Alkaline Ionized filter.   You have a RO system where additional minerals are added which adds TDS (Total Dissolved Solids) back to the water.

Some manufacturers say that they have ionized alkaline water, but unless the ORP (Oxidation Reduction Potential) is lowered and the pH (potential of Hydrogen) is raised and alkalinity added, you cannot change the alkalinity.  The water may be alkaline in nature (pH above 7.0) but it does not add alkalinity and there is no benefit to that.

The people you bought the RO from are probably innocent of any deception, simply uneducated about these things, but just adding minerals to raise the pH (and TDS) really does nothing.  To have alkaline water, you would have to use an alkaline water ionizer (an electrical device that uses electrolysis), but that may not be wise either, simply because you can't use RO water to make alkaline water.  You have to leave the contaminants, and then you concentrate all the impurities removed by reverse osmosis.  A good ionizer can cost up to $7,000.00!  Silly, in mymineral-water-sparkling opinion when you can simply do one of two things:

  1. Use 1/2 teaspoon of baking soda in 12 ounces of water once a day; or
  2. Use Gerolsteiner Mineral Water once a day.
I am not a medical doctor (well I am a water doctor) and this is not medical advice, but Alkaline Ionizers are not the solution.  The two above methods can produce true alkaline water, but you are not getting it from your reverse osmosis system. You may want to read, a previous blog post I have written on Alkaline vs. Acid Water.


November 20, 2013
April 19, 2014 at 10:18 AM
I just bought a pro pur. I tds it and it reads 142. I emailed manufactureer and she said that's normal, that it would be higher than tap water because of the minerals put back in. can that possibly be true? if so how would know if a filter's working in that case? so confused.
Mark Timmons
April 19, 2014 at 10:12 PM
What is your TDS before the filter?
June 23, 2017 at 10:45 AM
my ro tds before 10-20 ppm. but now increased to above 70-100ppm.why?
Mark Timmons
June 23, 2017 at 10:56 AM
What kind of RO do you have ? How old is is? What is your incoming TDS? I need a lot more info in order to answer your question.
Cecilia Ghilardotti
July 13, 2017 at 6:42 PM
Hi, I just got an RO system that adds back minerals as well and the TDS meter reads 96 from it. The tap water reads 372. Is this a normal level to have for an RO system? The RO is from Aquasana, I just installed it so it is pretty much new. Additional info: the guy that installed it, who apparently has 25 years installing RO's, said he thought there was something wrong because the water from the RO faucet came full force as soon as the RO was installed. He said that water first drips until tank fills up, he thought the membrane was not working. He measured TDS then and both tap and RO were 230. When called Aquasana, the tech said that since the installation guy hooked the refrigerator's icemaker to the RO that probably caused the water from the icemaker to flow back and come out of the faucet.. In any case, they recommended purging the system for 24 hs and then try again. After doing that, the water intensity from the faucet lowered to the normal level when the tank emptied. But now testing the TDS, 96 seems too high to me for an RO system?
Mark Timmons
July 14, 2017 at 8:23 AM
Yes, it is too high. You might as well not have an RO. Really, adding back minerals is useless (unless it is for taste) because even if you drank a bathtub full of highly mineralized water a day, you would not get 5% of the minerals your body needs. Water is not a significant source of minerals.
July 14, 2017 at 8:23 AM
I read somewhere that RO water which has low TDS (total dissolved solids) is not good to drink long term due to the fact the water has a demand. Is it true that if the water lacks minerals, it will begin to feed off the calcium in your bones deeming them brittle over time? Hence, RO purifies the water and then being reintroduced with minerals to feed the “demand” in the water therefore not feeding off of us while also making them less acidic? I have been also reading about PFAS and PFOS and trying to figure out the best way to cleanse my family’s drinking water in the right way. Your feedback is greatly appreciated.
Mark Timmons
March 8, 2020 at 10:42 AM
No, that is not true. #1: I believe that RO removes the widest spectrum of contaminants and why on earth would anyone not want to remove as many contaminants as possible; #2: There are things that can be done after the RO process to enhance the water quality. We are currently developing a new Process and Products, called Livation Hydration Technology which requires a different way of looking at and treating water. We hope to release it very soon.
Mark Timmons
March 15, 2020 at 1:11 PM
Once the water penetrates the cell wall, it becomes the pH of your body, so that is not true. We are coming out with a new 100% American-Made RO that is also available with Livation Hydration Technology... a new technology that includes true alkaline water, higher pH, lower ORP and molecular hydrogen, Watch for it. PFAS and PFOA are best removed with carbon filtration and reverse osmosis for drinking. We are seeing more and more people opt for whole-house RO Systems. This is what we use on PFAS: https://www.uswatersystems.com/pioneer-lead-cyst-pfoa-pfos-removal-system.html
October 9, 2017 at 9:37 PM
I have a spectrapure ro/so my ppm read 10-16. I noticed it's radon n 240-250 my water is well water and it reads 260's. I replaced the carbon and micron filter but I ran out of di resin for now. My ppm has not went back down it's still 240-250. Could this be because of the resin is old?
Mark Timmons
October 16, 2017 at 12:25 PM
Most likely, but we need more info... specifically a Lab Water Test.
John Carlson
October 31, 2017 at 4:22 PM
My water before the r/o system is a little over 600ppm. After the filter it's 60 so about 1/10 of the initial water. I have read this is normal to reduce by 90% but need a professionals opinion. Thanks in advance
Mark Timmons
November 5, 2017 at 9:49 AM
That is acceptable!
March 16, 2018 at 2:37 PM
No need to remineralize. WE got mineral from our food.
Mark Timmons
March 18, 2018 at 10:37 PM
Exactly! That is correct.
April 17, 2018 at 9:28 PM
Hi - We have well water and have an RO system installed under our sink. I just bought a Zerowater water pitcher and it has a TDS tester with it. I tested our water from the tap and it was 133 and then tested from the RO tap and it was 633. I took another sample and that one was 690. Then I tested the water after it went through the Zerowater filter and it was 0. We also get bottled water and it tested at 002. I don't understand how the RO water can be that high. Thank you.
Mark Timmons
April 28, 2018 at 1:49 PM
Zerowater uses DI resin, so of course, it will be ZERO, but on your water, it might only last a few gallons. What is the TDS of your water before it is treated?
April 28, 2018 at 11:34 AM
I have a great question....i test my uri e its 5.5 rough avg...i drink high alkiline water, it goes up. Why? If i add banking soda it goes up into the 7's...why is this? if your stomach acid neutralizes alkilinty in water then my ph shouldnt change, right?
Mark Timmons
April 28, 2018 at 1:40 PM
Hank, There may be some typos - I am not sure. Can you re-frame your question?
May 2, 2018 at 11:41 PM
Lol....4 sure. I may have meant that for a different site.... This question/issue doesn't have much to do with this article. What i was trying to say is....my urine ph usually tests at 5.5 I drink 20 oz of water with a half tsp of baking soda, the ph shoots to 7 or higher Its obviously not good to have urine ph of 5.5. I was trying to ask if im drinking water with a good amount of alkalinity my ph should go up right? the body will balance/buffer mineralized water better than ro water with baking soda, right? If i add the soda to ro water and drink it, does it end up pulling minerals out of my body? Reason i ask is i can go upto a tbsp of soda to 20oz of water, do that for a few days, stop for a few days, then check ph and it goes down past the original 5.5...its gone to 4.5 a couple of times... I understand you're not a dr and cant give medical advice. Have you heard of this happening to others? My thought is the ro doesn't gain minerals from the baking soda, its pulling minerals out of my body at the same time its giving me a false ph reading because of the high ph from the baking soda water...if that doesn't m ake sense im going to get checked for dyslexia....lmao!
Mark Timmons
May 5, 2018 at 10:02 PM
I am not a Doctor, but water is not a significant source of minerals. I do know that when water penetrates your cell wall, it becomes the pH of your body. If you drank a bathtub of water a day, you might get 5% of the minerals your body needs, so it seems silly to re-mineralize when you could just take a supplement to get better results. I think you are looking for something where there is nothing there.
May 2, 2018 at 11:52 PM
Can you re mineralize ro water with baking soda? My guess is no...so i would have to buy a calcium/mineral supplement with the sole specific use to remineralize the ro water?
Mark Timmons
May 5, 2018 at 9:58 PM
The baking soda just adds alkalinity and adjusts the ORP. Why do you want to re-mineralze?
Adelina M Lopez
June 1, 2018 at 12:28 AM
So Mr Water Doctor if I take 1/2 teaspoon baking soda in 12oz of water once a day or use Gerolsteiner Mineral Water once a day should be enough??
Mark Timmons
June 2, 2018 at 10:41 AM
I am not a medical doctor, but I know people who do that with success. I am not advising you for or against it. You should talk to a health professional.
Christopher Caton
October 11, 2018 at 3:32 PM
I have an generic 3 stage undersink RO. sediment - membrane - post carbon with permeate pump. i have traced all lines and i am satisfied all are connected properly. MY Tap water (originally from well) is 502ppm. RO is 330. I've never been able to get it below 125. I just changed both filters and membranes, drained system multiple times to flush. Before i changed the membrane, the TDS was matching the tap water suggesting a damaged membrane (pass through). Is the post carbon the culprit for keeping the TDS so high? any other suggestions.
Mark Timmons
October 12, 2018 at 11:32 AM
Chris, It sounds like a design or manufacturing flaw. Post carbon should not have any effect, but if it's only a three stage, I would question the pre-filtration if you have a post carbon filter. Can you send a photo of it?
October 22, 2018 at 4:53 PM
Christopher, did you run at least 5 gal out of the ro side after you installed it? I've seen similar issues when people did not run enough water through the entire system before testing the TDS.
October 25, 2018 at 5:54 PM
Hello Mark, I have RO system installed to use to produce cosmetics. TDS reading after installation was 0 ppm but after 2 weeks rise to 86 ppm. It is still ok to use this water to production?
Mark Timmons
October 27, 2018 at 5:46 PM
Barbara, I would think not and you need to determine why the huge rise in TDS? Is it lack of pre-treatment that fouled the membrane? I need more information.
Amy B.
December 21, 2018 at 9:52 PM
This is a wonderfully helpful post and comments. I've just installed a countertop New Wave Enviro 10 Stage Water Filter System and tested with a TDS stick. It's testing in the 100s. I tested water from a Zero Water pitcher and it was 0. Is the 10 stage filtered water safe to drink? Thank you!
Mark Timmons
December 22, 2018 at 12:12 PM
I can't say whether it's safe or not, but contaminanted water can read ZERO. TDS is not a measure of purity - it is a measure of the minerals in the water. Zero Water is ZERO TDS because it uses Deionization to remove all the minerals. Zero does not mean it is good to drink or even safe to drink. It just means that it has ZERO minerals.
June 9, 2019 at 10:32 PM
Hi Mark, I just bought the TDS tester. I've tested the Tap water and the reading is 55 ppm, while RO water is 43 ppm. Is it normal? Can I use the TDS calculation to prove the quality of the RO water (by rejection rate) if the reading as mentioned?
Mark Timmons
June 17, 2019 at 9:10 PM
That is not even close to good. Call our technical support department to troubleshoot!
Roy StJohn
December 22, 2019 at 11:58 AM
Just replaced 1 sed 1 carb 1 10gpd membrane in old 3-stage RO setup, fully sanitizing and rinsing each housing. Newly calibrated TDS meter, cross-checked with new ZERO water filter output. Tap ppm ranges 550-600. New R.O. filters ppm 550-600 following 25+ gallons of flushing and having double and triple checked for tight membrane insertion. I am GOBSMACKED at these results! Any thoughts on what has gone wrong here would be GREATLY appreciated by my entire family and our pets!
Mark Timmons
December 22, 2019 at 5:53 PM
Something is obviously very wrong. If you bought the filters from us, please call our Tech Support Department at 800-608-8792 on M-F from 8 AM to 8 PM EDT.
January 16, 2020 at 10:23 AM
TDS of my tap water is 97 and after filter, TDS is only 7. I am using kent RO What is problem ??
Mark Timmons
January 24, 2020 at 3:29 PM
I would say nothing is wrong. Try emptying your tank on a regular basis and realize that a TDS Meter is within 5% = or -. You are getting over 90% rejection.
February 2, 2020 at 10:46 PM
hi, i recently purchased a water filter that us 4-stage filter (pp sediment, granular carbon, Block Carbon & UF Membrane). i took a sample for TDS reading. i noticed it is higher than my tap water. tap water : 175 ppm, after filteration : 350 ppm. how is this possible? does the filters add more mineral to my water?
Mark Timmons
March 15, 2020 at 10:58 AM
It sounds like something si defective or not hooked up properly. Can't you contact the manufacturer?
Phyllis Freeman
February 20, 2020 at 3:29 PM
We just had a new puromax ro 5 stage installed. The sink water is 440 or so and the ro tests sometimes at 109 and once at 77. The company says it is fine. The plumber is willing to return the unit and purchase another one. We are very confused if our brand new ro is functioning. we have a brand new whole house water softener and brand new acid neutralizer as well. Can you comment? Can we drink this water?
Mark Timmons
March 15, 2020 at 12:01 PM
Most RO systems are made of Chinese parts. They may be assembled in the USA, but all of the parts, including the filters, housings, tanks, and faucets are from the Far East. In the next week, US Water Systems will release it's 100% All-American RO whereby everything is made in the USA. I'd wait for that... but I am biased!
March 26, 2020 at 4:38 PM
We just changed our whole reverse osmosis by buying new tank and new filters and membrane (supposedly made in usa).The filters/membrane we bought now are the same as the ones used previously. The problem we have now is that the tds measure really high,way higher than the tap water we have ,which usually measures between 159 and 162.Our reverse osmosis's tds is now over 200.We had readings from 800+ to 150+ and now to over 200.The water taste really good though,like fresh.I grew up on the farm in eastern Europe and we only drank fresh well water,so I know what fresh taste like. What could make the tds numbers so high?Our reverse osmosis is a 5 stage as I don't see the point of adding all those expensive,extra filters that don't make any sense.We do have mineral drops that we could add to our water,but I concentrate on healthy food to get them from first.
Mark Timmons
March 27, 2020 at 3:02 PM
There are several things that can cause this. I will put up a blog about this in the next couple of days.