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Old 02/12/2018, 12:43 PM   #1
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Join Date: Feb 2018
Posts: 1
RO reject water ratio computation from TDS

I haven't see this information in these forums, so wanted to share.

The recommended brine to permeate ratio for RO membranes is 4:1. This is inconvenient to measure accurately in some setups, and impossible when you are pumping into a pressure tank for drinking water. The back pressure from the tank changes the operational characteristics as the pressure increases or if you have a permeate pump.

After a bit of thought, I realized that by measuring the TDS at three places you can compute the reject ratio quite accurately.
  1. TDS of input
  2. TDS of permeate
  3. TDS of brine

Reject ratio x = (TDS_in - TDS_permeate) / (TDS_brine - TDS_in)

i.e x:1 --- For every (x+1) input water volume, you will get (x) brine and (1) permeate output.

If you have a triple probe TDS meter, you can obtain the reject ratio without any re-plumbing and using measuring jugs.

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Old 02/12/2018, 02:26 PM   #2
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Location: North Carolina
Posts: 17,971
Most systems are set from the manufacturer for a 4:1.. But yes if you plan to change restrictors/add dual membranes, etc.. then have at it...

Being blunt and having fun
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