RO water pressure as a factor for choice of flow restrictor

[heybob]

The Filmtec TW30-1812-50HR membrane appears to be the most efficient "1812"-size reverse osmosis membrane. The product data sheet claims Typical Stabilized Salt Rejection of 99% and Minimum Salt Rejection of 98% (versus 96% for the similar 75 gpd BW60-1812-75 membrane). It is rated at 50 gpd based on 50 psi. But according to a chart on the product data sheet (pictured below), it produces about 95 gpd at 90 psi.

What is the best flow restrictor size for this RO membrane when used at 90 psi (95 gpd)?

Waste-to-product water ratio	50 gpd (131.6 ml/m) Product Water	95 gpd (250 ml/m) Product Water
400 ml/m (waste water via Flow Restrictor)	3.04 to 1 ratio	1.60 to 1 ratio
550 ml/m (waste water via Flow Restrictor)	4.18 to 1 ratio	2.20 to 1 ratio
600 ml/m (waste water via Flow Restrictor)	4.56 to 1 ratio	2.40 to 1 ratio
800 ml/m (waste water via Flow Restrictor)	6.08 to 1 ratio	3.20 to 1 ratio

My understanding is that the flow restrictor size selection is a trade-off of RO-membrane TDS reduction versus Carbon Filter toxin reduction:
*Larger restrictor = more waste water / lower TDS / less carbon filter contact time
*Smaller restrictor = less waste water / higher TDS / more carbon filter contact time

In my case, my municipal source water is 400 TDS and 11 grains/gallon, but it will run through a softener before the RO system, so it will be high-TDS soft water.

Membrane Product Literature:
https://www.dupont.com/content/dam/...O-FilmTec-TW30-1812-50HR-PDS-45-D01750-en.pdf

https://www.filterwater.com/docs/filmtec/dow-filmtec-tw1812-50hr-data-sheet.pdf

 

[Shirak]

The flow restrictor is also there to provide enough water passing over the membrane to flush the ions out with the waste water and not put too much pressure on the membrane and damage it or clog it with too little flow out the waste line. The water softener will help. I think if you stay over 2:1 waste:RO ratio you are good running softened water at 90psi I would use either the 550ml/min or the 600ml/min restrictor.