RO TDS Observation and Question

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Reader90

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Hello All,

Been awhile since I have been on the forum -- lots going on.... Anyway, here is my setup: I have a "5 stage" RO under kitchen sync system. It has 4 Omnipure filters and a typical RO 50 gpd membrane.

Stage 1 sediment, Q5605
Stage 2 carbon, Q5621
Stage 3 carbon, Q5621
Stage 4 membrane (I have used various supplier membranes, typically Dow is my 1st choice)
Stage 5 GAC final filter, K2533KK

Typical set up with a 5 gallon holding tank, with a tee into refrigerator for ice/cold water. City water PSI is ~46 to 50 (have have a pressure gauge in-line), city water RDS is from 440 to 480, so I will say average 460. No permeate pump. System does have a auto shutoff valve (ASV) and I do have a green capillary drain flow restrictor installed on RO capsule drain line.

Observation and Question:

I recently changed all component filters/RO (2 weeks ago), a task I do once a year for all filters and I change the RO membrane once every 2 years. I know I could do the membrane less frequently, but this is my routine. After running a few tank fulls of RO water, after a 3 or 4 days, I am seeing TDS readings of 45 to 55 consistently (turn on RO water output, let run for 10 seconds, then take RO water sample in clean glass. Typically I would see 15 to 25.

While the TDS is much lower and perfectly okay to use, I feel I should be getting lower TDS measurements. Any troubleshooting ideas? Think I have a bad or sub-par RO membrane? Should I be changing the capillary drain flow restrictor?

Thanks in advance for any comments!!
 

Reader90

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Hello All,

Been awhile since I have been on the forum -- lots going on.... Anyway, here is my setup: I have a "5 stage" RO under kitchen sync system. It has 4 Omnipure filters and a typical RO 50 gpd membrane.

Stage 1 sediment, Q5605
Stage 2 carbon, Q5621
Stage 3 carbon, Q5621
Stage 4 membrane (I have used various supplier membranes, typically Dow is my 1st choice)
Stage 5 GAC final filter, K2533KK

Typical set up with a 5 gallon holding tank, with a tee into refrigerator for ice/cold water. City water PSI is ~46 to 50 (have have a pressure gauge in-line), city water RDS is from 440 to 480, so I will say average 460. No permeate pump. System does have a auto shutoff valve (ASV) and I do have a green capillary drain flow restrictor installed on RO capsule drain line.

Observation and Question:

I recently changed all component filters/RO (2 weeks ago), a task I do once a year for all filters and I change the RO membrane once every 2 years. I know I could do the membrane less frequently, but this is my routine. After running a few tank fulls of RO water, after a 3 or 4 days, I am seeing TDS readings of 45 to 55 consistently (turn on RO water output, let run for 10 seconds, then take RO water sample in clean glass. Typically I would see 15 to 25.

While the TDS is much lower and perfectly okay to use, I feel I should be getting lower TDS measurements. Any troubleshooting ideas? Think I have a bad or sub-par RO membrane? Should I be changing the capillary drain flow restrictor?

Thanks in advance for any comments!!
** bump ** any feedback or comments from anyone?? Thx!
 

Bannerman

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After running a few tank fulls of RO water,
Did you confirm the storage tank was actually empty when RO water stopped being discharged from the faucet?

If the storage tank has lost air, there maybe insufficient air remaining in the tank to push out all of the water. Too little air can lead to higher TDS since the unit will be forced to start production when even a small quantity of water is removed from the tank.

Each time production is started, the RO water first produced will be higher in TDS than later while the unit has been producing continuously for some time. The larger volume of lower TDS water will dilute the smaller volume of higher TDS water, but if your unit is repeatedly commencing production each time a small quantity of water is drawn, then the quantity of higher TDS water will cause the average TDS to rise.

To check the air pressure within the storage tank, shut off the RO unit and drain the tank fully. If some water remains within the tank, add a small quantity(s) of air into the tank's Schrader valve until all water is pushed out from the tank. While the tank is completely empty, add air until the precharge is 7 psi. You may want to recheck the precharge again after 30-minutes to confirm the 7 psi does remain. The 7 psi will be sufficient to push out water at a reasonable rate, but remains low enough so as to not displace much water which will maximize the quantity of RO water the tank can store.
 
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ditttohead

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The proper way to test the RO membrane would be to turn off the tank valve and let the RO run for 5 minutes, then test. Let me know what happens.
 

Reader90

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I performed both recommended tasks as outlined by both @Bannerman and @ditttohead.

Performing the RO only test produced a TDS reading of 18 as compared to city tap water TDS reading of 519.

Looking at the RO tank produced an interesting find for me. I turned the city water into the RO system off and opened the RO faucet to drain the RO tank (or so I thought). When the water stopped running from the RO tank, I went to check/fill the psi pressure but noticed there was still a lot of water in the 4 gal tank. I proceeded to push air pressure into the RO tank for several minutes to empty the tank. When that was completed, set the PSI to 7 (ish). Waited ~20 mins and retested the psi and it was at 7 still. Turned on all valves and let the RO system produce water and fill the tank as designed. Came back a couple hours later, opened the RO faucet for 10 seconds, then took new water sample. TDS for this sample was at 24. After a day, and knowing RO water was used by other family members, ice maker, coffee, etc., tested a few more times and readings were between 21 and 24.

I feel much better that the system is performing as I would expect. Thank you for the tips and test procedures! I will be paying muck closer attention to the RO tank moving forward during my filter/RO maintenance.
 
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