Reverse Osmosis Performance Calculator
Use this tool to estimate permeate flow, concentrate flow, salt rejection, concentration factor, net driving pressure, and specific energy consumption for an RO system.
What reverse osmosis calculation actually means
Reverse osmosis (RO) calculation is the process of turning basic operating data into useful performance metrics. Instead of guessing whether a system is healthy, you can calculate flows, salt rejection, concentration levels, and energy usage. These numbers help with design, troubleshooting, membrane protection, and operating cost control.
At a minimum, most operators track feed flow, permeate quality, pressure, and recovery. Once these values are known, a few simple equations reveal how hard the membranes are working and whether the unit is trending toward scaling, fouling, or high energy consumption.
Core RO formulas used in practice
1) Flow balance and recovery
Where Qf is feed flow, Qp is permeate flow, and Qc is concentrate flow. Recovery is the fraction of feed converted into product water. Higher recovery improves water yield but increases concentrate salinity and scaling risk.
2) Salt rejection
Cf is feed TDS and Cp is permeate TDS. A high rejection value usually indicates good membrane condition. A sudden drop can signal membrane damage, poor sealing, or chemical upset.
3) Concentration factor and concentrate TDS
As recovery increases, dissolved solids in the brine side become more concentrated. This is why recovery targets should always be linked to scaling potential and antiscalant strategy.
4) Net driving pressure (NDP)
NDP is the pressure available to push water through the membrane. If NDP drops too low, permeate production can fall. If NDP is negative, the entered operating data are physically inconsistent for RO production.
5) Specific energy consumption (SEC)
SEC is a useful benchmark for optimization. If SEC rises over time with similar feed conditions, look for membrane fouling, pressure losses, or pump performance decline.
How to use the calculator
- Enter measured plant data from the same time window.
- Use realistic recovery (typically below 85% for many brackish systems unless designed otherwise).
- If you do not know osmotic pressure, leave it blank and the calculator will estimate it from feed TDS.
- Review warnings, especially if NDP is very low or rejection appears unrealistic.
Worked example
Assume feed flow is 10 m³/h, recovery is 70%, feed TDS is 2000 mg/L, permeate TDS is 50 mg/L, feed pressure is 16 bar, and pump efficiency is 75%.
- Permeate flow: 7.0 m³/h
- Concentrate flow: 3.0 m³/h
- Salt rejection: 97.5%
- Estimated concentrate TDS: roughly 6550 mg/L (mass-balance estimate)
- Estimated SEC: around 0.85 kWh/m³ permeate
These are the kinds of numbers you can compare daily to detect drift. Even before lab reports arrive, simple trend calculations give an early warning system.
Practical design and troubleshooting notes
Watch trends, not single points
One odd reading may be a sensor issue. A multi-day trend in pressure, rejection, or SEC is much more reliable for diagnosis.
Normalize when feed conditions change
Temperature and feed salinity affect RO behavior strongly. For detailed engineering analysis, normalize data to standard conditions before comparing performance across seasons.
Balance recovery with scaling risk
Higher recovery reduces reject volume but raises concentration factor. If scaling ions are present, pushing recovery too high can increase cleanings, reduce membrane life, and erase energy savings.
Final takeaway
Reverse osmosis calculation is not just academic math. It is a practical operating discipline. With a few inputs and consistent tracking, you can optimize water production, protect membranes, and control power cost with confidence.