So here's some early morning jargon and math for you.
If your Feedwater is 280 ppm,
and your permeate (RO water) is 85 ppm,
then your rejection rate is
1 - (85/280) =
1 - (0.30) =
0.70, or 70%
You want the rejection rate to be something north of 95% - meaning the membranes would be rejecting...