These sample values illustrate typical ranges for brackish and seawater applications.
| Case | Feed (Cf) | Permeate (Cp) | Rejection | Comment |
|---|---|---|---|---|
| Brackish | 5,000 mg/L | 150 mg/L | 97.0% | Good performance for polishing. |
| Seawater | 35,000 mg/L | 350 mg/L | 99.0% | Typical for well-operated RO. |
| Fouling trend | 10,000 mg/L | 500 mg/L | 95.0% | Investigate cleaning and integrity. |
- Salt Rejection (%) = (1 − Cp/Cf) × 100
- Salt Passage (%) = (Cp/Cf) × 100
- Recovery (%) = (Qp/Qf) × 100 (when flows are provided)
Use consistent concentration units for Cf and Cp. The ratio Cp/Cf is unitless.
- Measure feed and permeate concentrations from stable sampling points.
- Enter Cf and Cp using the same unit basis.
- Optionally add feed and permeate flow to compute recovery.
- Click Calculate to display results above the form.
- Use CSV for logs, or PDF for quick reporting.
Why salt rejection matters for construction water systems
On construction sites, desalination units, polishing skids, and temporary water treatment trains often run under variable feed quality. Salt rejection indicates how effectively a membrane separates dissolved salts from the product stream, helping teams protect mixers, It supports planning for corrosion control and concrete durability onsite. curing water lines, boilers, and equipment that is sensitive to chlorides and total dissolved solids.
Interpreting Cp, Cf, and the Cp/Cf ratio
Feed concentration (Cf) represents the salinity entering the membrane, while permeate concentration (Cp) represents the salinity of treated water. The ratio Cp/Cf is a compact indicator: a smaller ratio means stronger separation. Because the ratio is unitless, the calculator remains valid as long as Cp and Cf share the same unit basis.
Connecting rejection and passage to quality targets
Rejection and passage are complementary metrics. A rejection of 99.0% corresponds to a passage of 1.0%. If your target permeate is below 500 mg/L and the feed is 35,000 mg/L, a 99% rejection predicts Cp near 350 mg/L. When rejection trends downward, confirm sampling points and validate conductivity or TDS conversion factors.
Using recovery to manage scaling risk
Recovery (Qp/Qf) describes how much feed becomes product water. Higher recovery improves water utilization but increases concentration in the reject stream, elevating scaling and fouling risk. Use recovery trends with pressure and temperature records to identify when cleaning is needed or when operating limits are being approached.
Example data for quick field checks
Example A: Cf = 10,000 mg/L, Cp = 250 mg/L → rejection ≈ 97.5%, passage ≈ 2.5%. Example B: Cf = 35,000 mg/L, Cp = 700 mg/L → rejection ≈ 98.0%, passage ≈ 2.0%. Example C: Qf = 50 m³/h, Qp = 20 m³/h → recovery = 40.0%.
Use these examples to sanity-check instruments before logging long runs.
1) What does a “good” salt rejection value look like?
Many reverse osmosis systems operate around 95–99% rejection, depending on membrane type and feed. Compare to your specification and trend results over time for the same unit.
2) Can I use conductivity instead of TDS for Cf and Cp?
Yes, if both values use the same conversion basis. Use a consistent factor for the site water matrix and avoid mixing meter settings or temperature compensation modes.
3) Why is my permeate concentration higher than expected?
Common causes include sample contamination, incorrect sampling location, membrane damage, channeling, or inadequate flushing. Verify instruments, then check operating pressure, recovery, and differential pressure trends.
4) Does temperature affect rejection?
Temperature changes water viscosity and membrane transport, often increasing flux as temperature rises. Rejection can shift slightly, so log temperature with each sample for fair comparisons.
5) What if Cp is near zero?
Enter the measured value. Very low Cp yields very high rejection. Confirm that the meter range and calibration are appropriate and that the permeate sample is collected after stabilization.
6) How should I interpret recovery for site operations?
Recovery shows how efficiently feed becomes usable water. Higher recovery saves intake water but may raise scaling risk. Balance recovery with pretreatment, antiscalant control, and cleaning schedules.
7) Is this calculator suitable for design approval?
No. It is intended for operational checks and trending. Use manufacturer projections, complete water chemistry, and detailed hydraulic analysis for final design and compliance documentation.