RO reject water ratio computation from TDS

km9830 · 02/12/2018, 12:43 PM

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.

TDS of input
TDS of permeate
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.

mcgyvr · 02/12/2018, 02:26 PM

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...

02/12/2018, 12:43 PM	#1
km9830 Registered Member 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. TDS of input TDS of permeate 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.

02/12/2018, 02:26 PM	#2
mcgyvr Registered Member Join Date: Sep 2003 Location: North Carolina Posts: 20,050	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... __________________ Who me?