RO Flux Calculation: How to Calculate Membrane Permeate Flux
Quick answer: what is the RO flux formula?
RO permeate flux = permeate flow ÷ active membrane area. Flux expresses how much permeate is produced per unit of membrane area and is commonly reported as LMH (litres per square metre per hour) or GFD (gallons per square foot per day).
Example
If an RO stage produces 2,000 litres per hour using 100 m² of active membrane area:
Flux = 2,000 L/h ÷ 100 m² = 20 LMH.
Why flux matters
Flux affects membrane loading, fouling tendency, recovery distribution and cleaning frequency. A higher flux is not automatically better. The suitable design flux depends on feed-water quality, pretreatment, membrane type, recovery, temperature and system configuration.
Why measured flux changes in operation
- Feed-water temperature changes
- Net driving pressure changes
- Feed salinity/osmotic pressure changes
- Fouling or scaling
- Recovery changes
- Membrane ageing or damage
Use normalized performance for troubleshooting
Raw permeate flow and calculated flux can change even when the membrane itself has not deteriorated. For troubleshooting, normalize permeate flow, salt passage and pressure drop to a reference condition before deciding that cleaning or replacement is required.
Frequently asked questions
Is higher RO flux always better?
No. Excessive flux can increase fouling risk and may move the system outside recommended design conditions.
Can I compare flux from two plants directly?
Only with care. Feed-water quality, temperature, pressure, membrane model, recovery and pretreatment may be different.
Does low flux always mean the membrane is fouled?
No. Lower temperature or lower net driving pressure can also reduce flux.
Reference
DuPont’s current FilmTec RO/NF Technical Manual discusses flux, normalized permeate flow and the need to compare system performance under equivalent conditions. See the FilmTec RO/NF Technical Manual.
See also ABHIRO’s Industrial RO Troubleshooting Guide, Salt Rejection Formula and Recovery Formula.
