How to Test Membrane Salt Rejection Rate: Methods and Calculations

How to Test Membrane Salt Rejection Rate: Methods and Calculations

This comprehensive guide covers everything researchers and engineers need to know about how to test membrane salt rejection rate. From fundamental principles to practical applications, we provide actionable insights based on the latest research and industry best practices.

Why Salt Rejection Testing Matters

Salt rejection is the primary performance metric for RO and NF membranes. It quantifies the membrane's ability to remove dissolved salts from water, directly determining whether the treated water meets quality specifications. Accurate salt rejection measurement is essential for membrane development, quality control, and system performance monitoring.

Calculating Salt Rejection

Salt rejection (R) is calculated from the concentrations of feed (Cf) and permeate (Cp): R(%) = (1 - Cp/Cf) × 100. For example, if feed conductivity is 4000 μS/cm and permeate conductivity is 80 μS/cm, rejection = (1 - 80/4000) × 100 = 98%.

Standard Test Conditions

• RO membranes: 2000 ppm NaCl, 15.5 bar (225 psi), 25°C, 15% recovery per ASTM D4194

• NF membranes: 2000 ppm MgSO₄, 4.8 bar (70 psi), 25°C

• Brackish water RO: 2000 ppm NaCl, 10.3 bar (150 psi), 25°C

• Seawater RO: 32,000 ppm NaCl, 55.2 bar (800 psi), 25°C

Equipment Required

• Crossflow membrane test cell rated for the appropriate pressure

• High-pressure pump with stable, pulsation-free flow

• Conductivity meters for feed and permeate (minimum 2 instruments)

• Temperature control system (±0.5°C accuracy)

• Precision balance for flux measurement

• Calibration standards for conductivity meters

Tech Inc. crossflow test cells provide the precision and reliability needed for accurate salt rejection testing. Our cells feature uniform flow distribution and leak-free operation at pressures up to 70 bar.

Common Errors and How to Avoid Them

• Concentration polarization: Maintain adequate crossflow velocity (>0.2 m/s) to prevent salt buildup at the membrane surface, which artificially lowers apparent rejection

• Temperature effects: Normalize results to 25°C. Rejection decreases with increasing temperature

• Inadequate compaction: Run DI water for at least 2 hours before testing to stabilize the membrane

• Leaks: Even small O-ring leaks contaminate permeate and reduce apparent rejection. Check all seals before testing

Frequently Asked Questions

What is considered good salt rejection for RO?

Commercial RO membranes achieve 99.2-99.8% NaCl rejection under standard test conditions. High-rejection membranes exceed 99.7%. Rejection below 99% may indicate membrane defects or damage.

Does salt rejection change over time?

New membranes often show slightly increasing rejection during the first few hours of operation as the membrane compacts. Over long-term operation, rejection gradually decreases due to membrane aging and chemical degradation.