Osmotic pressure estimate
Estimate osmotic pressure from TDS and temperature for NaCl-dominated waters, the starting point for RO feed pressure and membrane selection.
Results
| Estimated osmotic pressure | 1.4 bar |
|---|---|
| In psi | 20.3 psi |
| Rule of thumb | ≈ 0.7 bar per 1 000 mg/L TDS (NaCl basis, 25 °C) |
Feed pressure must exceed osmotic pressure plus the membrane’s net driving pressure and system losses before any permeate flows. As recovery rises, the concentrate-side osmotic pressure climbs well above this feed-side figure. The estimate assumes NaCl-dominated water; waters high in divalent ions run somewhat lower. Use a full analysis and the membrane manufacturer’s projection software for design.
Formula used
π = 0.0007 × TDS × (T + 273.15) ÷ 298.15
- π
- osmotic pressure, bar
- TDS
- total dissolved solids, mg/L
- T
- water temperature, °C
Industry rule of thumb (≈0.7 bar per 1 000 mg/L NaCl at 25 °C) scaled by absolute temperature. Note where the 0.7 comes from: ideal van 't Hoff behaviour for dilute sodium chloride gives about 0.85 bar per 1 000 mg/L, and the working figure is lower because the osmotic coefficient falls below unity at real feed concentrations. It is calibrated for brackish feeds in roughly the 1 000 to 10 000 mg/L range and it drifts outside that: it understates dilute feeds and is not a seawater figure. Waters dominated by divalent ions (Ca, Mg, SO4) run 10–30% lower again. Treat the result as an order-of-magnitude check with an uncertainty of tens of per cent, not a design number: for design, use the membrane manufacturer's projection software with a full ionic analysis.
The background
- How reverse osmosis works: pressure, rejection and recoveryRO pushes water backwards through nature's preferred direction. Three numbers (osmotic pressure, rejection and recovery) describe everything an RO system does.
- TDS, EC and what conductivity actually tells youTotal dissolved solids and electrical conductivity are the first numbers on any water analysis. Here is what they measure, how they relate, and what they cannot tell you.
- The DuPont FilmTec families: BW, LE/XLE, ECO, SW and the specialistsDuPont's element names encode a design philosophy (rejection-first, energy-first or duty-specific). A map of the families and how to choose between them.
Equipment for this duty
Send the water analysis and duty point, and the branch confirms exact selections, pricing and lead times, usually within one working day.
Request a quoteResults are indicative engineering estimates based on standard formulas and typical assumptions. They are not a design service; confirm any design against a full water analysis and the relevant manufacturer’s data.