How to use this calculator
- Read feed and permeate flow meters.
- Measure feed and permeate TDS (or conductivity).
- Enter the values to get rejection, recovery and reject quality.
How do you calculate RO rejection and recovery?
Rejection (%) = (1 − Cp ÷ Cf) × 100
Recovery (%) = Qp ÷ Qf × 100
Cr = (QfCf − QpCp) ÷ Qr
Concentration factor = 1 ÷ (1 − recovery)
Worked example
With feed flow 10 m³/h, permeate flow 7 m³/h, feed tds 1,500 mg/L, permeate tds 40 mg/L:
Salt rejection: 97.33 %
The calculator also gives recovery 70 %; salt passage 2.67 %; reject flow 3 m³/h.
Assumptions and limits
- Mass balance on TDS; conductivity can be used in place of TDS if consistent.
- Normalise for temperature and pressure before comparing with the startup baseline.
Frequently asked questions
What is a good RO rejection rate?
New brackish water membranes reject 99 to 99.7% of salts. Plants usually act when system rejection falls by more than 1 to 2% from the startup baseline.
What is the recovery of an RO plant?
Domestic purifiers recover 20 to 50%. Industrial brackish water RO usually runs at 65 to 80%, limited by scaling.
What can be done with RO reject?
Reject can be reused for floor washing or cooling towers if quality allows, concentrated further with a second RO, or sent to an evaporator in a ZLD plant.
References
- DuPont FilmTec, Reverse Osmosis Technical Manual
- ASTM D4516, Standard Practice for Standardizing RO Performance Data