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Formula reference
Salt rejection
R = (1 − Cp / Cf) × 100Conductivity rejection
R = (1 − Kp / Kf) × 100Salt passage
SP = Cp / Cf × 100 = 100 − RWater recovery
Recovery = Qp / Qf × 100Flow balance
Qf = Qp + QcSalt mass balance
CfQf = CpQp + CcQcConcentration factor
CF = 1 / (1 − recovery as a decimal)Required rejection
Rrequired = (1 − Ctarget / Cfeed) × 100How to use this calculator
- Choose concentration or conductivity mode based on your available measurements.
- Select units before entering feed, permeate, concentrate, flow, pressure, and temperature data.
- Enter feed and permeate values from samples taken under stable operating conditions.
- Add feed and permeate flow to calculate recovery and complete the flow balance.
- Enter concentrate data when available for salt mass-balance checking.
- Add rated membrane data and custom thresholds for practical performance classification.
- Use the ion table when laboratory results are available for individual species.
- Add baseline values to identify meaningful performance changes.
- Review warnings, charts, and mass-balance closure before exporting or printing the report.
Understanding salt rejection performance
What salt rejection means
Salt rejection describes the percentage reduction between feed-water concentration and permeate concentration. A membrane with high rejection allows only a small fraction of dissolved salt into the product stream. The complementary result is salt passage, which shows the percentage crossing the membrane.
Rejection is not a universal membrane constant under every operating condition. It can change with feed composition, pressure, temperature, recovery, pH, membrane age, pretreatment, fouling, scaling, oxidation, mechanical damage, and sampling quality. Comparisons are strongest when measurements are normalized to consistent reference conditions.
Concentration and conductivity methods
Laboratory concentration data provides a direct basis for salt rejection. Conductivity is often faster and easier for routine plant checks. Conductivity-based rejection assumes the relationship between ionic concentration and conductivity remains reasonably comparable between the sampled streams.
Conductivity should not be treated as an exact replacement for complete ionic analysis. Different ions contribute differently to conductivity. Temperature compensation, instrument calibration, dissolved gases, pH, and sample handling can also affect measured values.
Recovery and concentrate behavior
Recovery measures how much feed water becomes permeate. Higher recovery reduces concentrate flow and increases the concentration of retained salts. This can raise osmotic pressure and scaling risk. It may also change observed rejection and element loading.
The ideal concentration factor is calculated from recovery. Real systems may depart from that ideal because of salt passage, blending, recirculation, sampling position, unequal stage loading, or measurement error. The mass-balance section helps expose such inconsistencies.
Individual ion rejection
Total dissolved solids can hide important chemical behavior. Calcium, sulfate, silica, boron, nitrate, and other species may have different passage rates. Nanofiltration membranes can show especially large differences between monovalent and multivalent ions.
Ion-specific results help evaluate product-water limits and scaling risks. They also support process troubleshooting when total conductivity appears acceptable but one regulated or operationally important species remains high.
Normalization and trend interpretation
Raw performance values should be compared carefully. Temperature, pressure, salinity, recovery, and membrane condition all influence output. Normalized performance attempts to remove expected operating-condition effects so long-term trends become clearer.
This application provides a simplified temperature adjustment and baseline comparison. It is intended for screening, education, and routine operational review. Manufacturer software or a validated engineering model should be used for warranty, design, contractual acceptance, and safety-critical decisions.
Common causes of declining rejection
Declining rejection may result from oxidant damage, abrasion, seal leakage, telescoping, poor interconnector seating, membrane aging, excessive pressure, biological attack, pH exposure, incompatible cleaning chemicals, or instrument problems. A sudden change often suggests leakage, damage, sampling error, or analyzer error.
A gradual change may indicate fouling, scaling, compaction, or aging. Review normalized flow and pressure drop alongside rejection. A complete diagnosis should include pretreatment records, differential pressure, cleaning history, feed chemistry, permeate quality, and vessel-level sampling.
Data-quality recommendations
Use calibrated instruments and consistent sampling points. Allow the system to stabilize before sampling. Record temperature, pressure, flow, recovery, pH, and conductivity at the same time. Rinse sample containers and avoid contamination.
Compare results against the correct membrane specification and its stated test conditions. A rated rejection value measured with one salt, concentration, pressure, temperature, pH, and recovery may not match field performance under different conditions.