Among all performance parameters of Reverse Osmosis (RO) systems, one stands out as the decisive factor for determining final water quality - the salt rejection rate. It determines whether the system can effectively remove salts and impurities. Whether it's household purifiers, food processing, or seawater desalination, the rejection rate functions as a "system health report," clearly telling us whether the water meets quality requirements. The higher the rejection rate, the purer and safer the water. It also reveals the membrane's separation performance and the overall system condition.

1. The Essence of Salt Rejection Rate

The salt rejection rate is a key indicator used to evaluate the ability of a reverse osmosis (RO) membrane to remove dissolved salts from water, and it is typically expressed as a percentage (%). An RO membrane is a highly selective semi-permeable membrane that allows water molecules to pass through, while rejecting the majority of dissolved salts and ions such as Na⁺, Cl⁻, Ca²⁺, and Mg²⁺.

A higher salt rejection rate indicates stronger ionic rejection, lower salt content in the product water, and therefore a higher degree of water purity.

For example - If the feed water contains 100 ions and only 1 passes into the product water after RO treatment, the system has a salt rejection rate of 99%. In advanced seawater desalination applications, RO membranes can achieve salt rejection rates of over 99.8%, enabling the efficient conversion of high-salinity seawater into potable or industrial - grade freshwater.

2. Calculation Method and Key Considerations

1) Basic Calculation Method

In practical operation, total salt concentration is rarely measured directly. Instead, conductivity is used as an indirect indicator because it correlates closely with the concentration of dissolved ions.

Salt Rejection (%) = (1 − Product Water Conductivity ÷ Feed Water Conductivity) × 100%

Conductivity is typically expressed in μs/cm (microsiemens per centimeter).

2) Example Calculation

Example RO system data:

- Feed Water Conductivity: 400 μs/cm

- Product Water Conductivity: 4 μs/cm

Applying the formula:

Salt Rejection = (1 − 4 ÷ 400) × 100% = 99%

This indicates that 99% of the dissolved ions are removed, resulting in a very high water purity.

3) Key Considerations

Conductivity ≠ TDS: Conductivity represents the ability of dissolved ions to conduct electricity, while TDS (Total Dissolved Solids) includes all dissolved substances, including those that do not conduct electricity.

Temperature Compensation Is Required: Conductivity is highly temperature - dependent: for every 1°C increase in water temperature, conductivity increases by approximately 2~3%. Therefore, conductivity of both feed and product water must be corrected to 25°C before calculating salt rejection, otherwise the results may be misleading. Furthermore, higher temperature increases ion mobility and reduces water viscosity, increasing membrane flux while potentially reducing salt rejection.

Apparent vs. True Salt Rejection: Conductivity - based calculation is referred to asthe apparent rejection rate, which is suitable for daily monitoring and system performance evaluation. When detailed ionic removal performance is required, laboratory chemical analysis should be used to determinethe true rejection rate.

3. Salt Rejection: The "Lifeline" of RO Systems

Salt rejection is not only a performance indicator of RO systems, but also a key parameter that reflects system health and operational economics.

Its importance can be summarized in two major aspects:

1) Indicator of Membrane Health

Every RO membrane has an initial factory-rated salt rejection value (for example, typical 8040 brackish water membranes have a nominal rejection rate of 99.0~99.8%).

Changes in salt rejection directly reflect membrane health:

- Slow decline (e.g., from 99.5% to below 98%)

This may indicate natural aging or mild oxidation of the separation layer. Feed pretreatment and water quality should be examined.

- Sudden drop (more than 1% within one day)

This usually indicates mechanical failure or exposure to oxidants, such as membrane damage, broken center tube, seal or O-ring failure, or chlorine / ORP shock. Immediate shutdown and inspection are required.

2) Impacts Operating Cost and System Economics

A decline in salt rejection affects not only water quality, but also increases operating costs:

- Increased downstream treatment load

Additional polishing steps (such as a second RO stage or ion exchange) may be required, increasing capital cost and system footprint.

- Higher energy and chemical consumption

Lower rejection is often accompanied by membrane fouling. Higher feed pressure and more frequent chemical cleaning increase power and chemical usage.

- Reduced membrane lifespan

Continuous fouling and performance degradation shorten membrane service life and increase replacement cost.

4. How to Maintain Stable Salt Rejection

The stability of salt rejection directly affects water quality, system safety, and operational cost. Through systematic management and routine maintenance, stable and efficient operation of the RO system can be ensured.

1) Regular Monitoring and Recording

Measure the conductivity of feed and product water daily, calculate the salt rejection rate, and plot trend graphs to detect abnormalities promptly.

2) Standardized Sampling and Measurement

Sampling points should be representative, instruments must be regularly calibrated, and conductivity readings corrected to 25°C to ensure reliable data.

3) Strengthen Pretreatment

Proper pretreatment - including multimedia filtration, activated carbon, water softening, and antiscalant dosing - helps prevent membrane fouling and chemical damage, extending membrane life.

4) Develop Cleaning and Maintenance Plans

Establish routine chemical cleaning schedules based on pressure differential, product water flow, and salt rejection trends to avoid prolonged operation under compromised conditions.

5) Abnormal Response Mechanism

If a sudden drop in salt rejection is detected, immediately inspect membrane integrity. Use endoscopic examination or replace damaged elements as necessary to restore system performance quickly.

5. The Core "Performance Anchor" of RO Systems

Salt rejection is not only a technical parameter of RO systems but also a critical reference for design, operational monitoring, and maintenance decisions. It directly impacts system economics, safety, and long-term water quality.

For water treatment professionals, managing salt rejection requires a "Three Understands, Three Safeguards" approach:

- Understand it - grasp the underlying principles and factors affecting salt rejection.

- Monitor it - implement a systematic monitoring mechanism for early detection of deviations.

- Protect it - carry out proactive maintenance to prevent performance deterioration.

Only by understanding, monitoring, and protecting salt rejection can RO systems reliably and efficiently deliver high-quality water over the long term, fully realizing their central role in modern water treatment.