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How It Works
Reverse osmosis forces water through a semi-permeable membrane under pressure, leaving dissolved salts and contaminants behind. The applied pressure must exceed the osmotic pressure of the feed solution. The Van't Hoff equation estimates this osmotic pressure from solute concentration, temperature, and ion count, while the rejection equation measures how effectively contaminants are removed.
Typical seawater desalination requires 50–70 atm of pressure. Brackish water systems operate at 10–25 atm. Modern RO membranes achieve 95–99.5% salt rejection.
Example Problem
A 0.5 gmol/L NaCl solution at 298 K has φ = 0.93, and NaCl dissociates into 2 ions. What is the osmotic pressure?
- π = 0.93 × 2 × 0.5 × 0.082 × 298
- π ≈ 22.7 atm
The RO system must apply more than 22.7 atm to produce fresh permeate from this feed.
Frequently Asked Questions
What is osmotic pressure in reverse osmosis?
Osmotic pressure is the natural tendency of water to flow from a dilute solution to a concentrated one across a membrane. RO overcomes this by applying higher pressure on the concentrated side, pushing clean water through and leaving salts behind.
What does contaminant rejection mean for RO membranes?
Rejection is the percentage of a solute that the membrane blocks. For example, 98% rejection of a 500 mg/L feed means the permeate contains only 10 mg/L. High-performance RO membranes routinely achieve 99%+ rejection for dissolved salts.
How much energy does reverse osmosis use?
Modern seawater RO plants consume 3–5 kWh per cubic meter of permeate with energy recovery devices. Brackish water RO uses significantly less -- typically 0.5–2 kWh/m³.
Related Calculators
- Ultrafiltration Calculator -- design membrane pretreatment upstream of RO.
- Microorganism Disinfection Calculator -- ensure post-RO disinfection meets CT targets.
- Swimming Pool Calculator -- manage water quality in pool applications.
- Electrodialysis Calculator — size membrane stacks for alternative desalination.
- Fluid Pressure Calculator — calculate the transmembrane pressure driving RO separation.
- Pressure Unit Converter — convert between psi, bar, and kPa for membrane pressures.