RO performance can be described using a few simple mass-balance calculations. They are useful for daily checks, although detailed membrane performance evaluation should also normalize for temperature, pressure and feed concentration.
Recovery
Recovery (%) = Permeate flow ÷ Feed flow × 100
If feed flow is 100 m³/hr and permeate is 75 m³/hr, recovery = 75%.
Concentrate flow
Concentrate flow = Feed flow − Permeate flow
For the same example, concentrate = 25 m³/hr.
Salt rejection
Rejection (%) = (1 − Permeate TDS ÷ Feed TDS) × 100
If feed TDS is 2,000 mg/L and permeate TDS is 40 mg/L, rejection = (1 − 40/2000) × 100 = 98%.
Salt passage
Salt passage is simply 100 − rejection. In the example, salt passage is 2%.
Why raw numbers can mislead
Permeate flow increases with temperature, while required pressure changes with feed salinity and membrane condition. For long-term performance analysis, compare normalized data rather than only daily raw values.
Use the RO Recovery Calculator and RO Salt Rejection Calculator for quick checks.