Precise Determination of Rejection Rate: Principles, Methods, and Applications in Membrane Testing

Rejection rate, also known as retention rate, is the single most important performance indicator for filtration membranes, quantifying how effectively a membrane separates solutes, salts, or particles from a liquid stream. This article explains the scientific principles behind rejection rate, the standard test methods and reference standards used for its precise determination, and the quality controls required to produce accurate, reproducible results in accredited third-party laboratories.

Technical Overview

Rejection rate (retention rate) is a dimensionless performance parameter that expresses the fraction of a target solute or particle that a membrane prevents from passing into the permeate stream. It is conventionally calculated from measured concentrations on the feed side and the permeate side: R = (1 − Cp / Cf) × 100%, where Cf is the solute concentration in the feed and Cp is the solute concentration in the permeate. A rejection rate of 99.5% for sodium chloride on a reverse osmosis membrane, for example, means that only 0.5% of the feed salt concentration appears in the filtrate.

Accurate determination requires distinguishing between observed rejection, which is based on the bulk feed concentration, and intrinsic rejection, which is based on the solute concentration at the membrane surface. Under crossflow conditions, concentration polarization elevates the wall concentration above the bulk value, causing observed rejection to underestimate true membrane selectivity. Precise testing therefore combines controlled hydrodynamic conditions, validated analytical chemistry, and rigorous data normalization to minimize systematic bias and deliver results with quantified measurement uncertainty.

• Rejection rate is the primary specification metric for reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF), and microfiltration (MF) membranes.
• It underpins molecular weight cut-off (MWCO) assignment, salt passage specifications, and sterilizing-grade filter validation.
• Reference frameworks include ASTM D4194 for RO and NF devices, ASTM D6161 terminology, ASTM D4516 performance normalization, ASTM F838 for bacterial retention, and ISO 16889 for multipass filtration efficiency of hydraulic filter elements.

Testing Samples

Flat-Sheet Membrane Discs

Flat-sheet samples of polymeric RO, NF, UF, or MF membranes, typically cut into discs of 47 to 90 mm diameter, are tested in stirred dead-end cells or laboratory crossflow cells. This format is ideal for material screening, research and development, and incoming quality control of membrane coupons.

Spiral-Wound and Hollow-Fiber Elements

Complete membrane elements, including 2.5-inch and 4-inch spiral-wound elements and hollow-fiber modules, are evaluated on single-element test skids under standard pressure, recovery, and temperature conditions. Element-level testing reflects real hydrodynamics, feed spacer effects, and end-cap sealing quality.

Filter Cartridges and Capsules

Pleated liquid filter cartridges, membrane capsules, and sterilizing-grade filter assemblies are challenged under defined flow and differential pressure conditions to verify particle retention, bacterial retention, or log reduction values relevant to pharmaceutical and bioprocess applications.

Ceramic and Specialty Membranes

Tubular ceramic membranes, carbon membranes, and chemically resistant polymeric membranes used in aggressive solvent or high-temperature service are tested with compatible challenge solutes and corrosion-resistant test hardware.

Testing Items

Precise rejection determination programs typically cover the following measurands, each paired with an appropriate challenge solute and analytical technique:

• Monovalent salt rejection (sodium chloride) for reverse osmosis membranes.
• Divalent salt rejection (magnesium sulfate, calcium chloride) for nanofiltration membranes.
• Molecular weight cut-off verification using polyethylene glycol, dextran, or protein ladder standards.
• Neutral organic solute rejection (sucrose, raffinose, glycols, trace organics) for NF and tight UF characterization.
• Particle retention efficiency at defined size thresholds, expressed as a beta ratio or percent retention.
• Bacterial retention and log reduction value for sterilizing-grade membrane filters.
• Flux and permeability measured concurrently, since rejection must always be reported together with the driving operating conditions.

Testing Methods

Salt Rejection by Conductivity

Feed and permeate streams are monitored with calibrated conductivity sensors or analyzed by ion chromatography following protocols aligned with ASTM D4194 and ASTM D4195 guidance. Brackish RO membranes are commonly verified at a 2,000 mg/L sodium chloride feed, approximately 1.55 MPa (225 psi) applied pressure, 25 °C, and 15% recovery, with performance normalized per ASTM D4516 to eliminate temperature and pressure drift between runs.

Molecular Weight Cut-Off Determination

Ultrafiltration membranes are challenged with either a ladder of narrow-distribution polyethylene glycol standards or a polydisperse dextran mixture. Permeate and retentate samples are analyzed by gel permeation chromatography to construct a retention curve of rejection versus molecular weight; the MWCO is reported as the molecular weight at which rejection reaches 90% or 95%, as agreed with the client specification.

Particle Retention and Beta Rating

For filter elements and microfiltration media, a multipass test consistent with ISO 16889 continuously injects test contaminant while online particle counters measure counts upstream and downstream across defined size bands. The beta ratio is calculated as the quotient of upstream to downstream particle counts at a given size, and retention efficiency follows as (1 − 1/β) × 100%, providing a statistically robust efficiency metric.

Microbial Challenge Testing

Sterilizing-grade filters are subjected to bacterial retention challenge testing in accordance with ASTM F838, using a defined suspension of Brevundimonas diminuta applied at a minimum challenge level of 10⁷ colony-forming units per square centimeter of effective filtration area. Complete retention with a documented log reduction value supports claims of a sterilizing-grade 0.2-micron rating.

Analytical Instrumentation and Quality Assurance

Depending on the target analyte, quantification employs conductivity meters, total organic carbon analyzers, UV-Vis spectrophotometry, high-performance liquid chromatography, ion chromatography, or gel permeation chromatography, all calibrated with certified reference materials. Samples are collected only after steady state is reached, membranes are preconditioned and compacted with deionized water until flux stabilizes, and recovery is kept low to limit concentration polarization bias. Repeat measurements, control charts, and measurement uncertainty budgets evaluated at approximately 95% confidence level are essential elements of a defensible test report, and testing is best performed in ISO/IEC 17025-accredited third-party laboratories.

Applications

Precise rejection rate data directly support engineering decisions, regulatory submissions, and commercial specifications across every major membrane market:

• Seawater and brackish water desalination, where salt rejection governs product water quality and energy consumption.
• Ultrapure water production for semiconductor and power generation industries, requiring verified ion and particle retention.
• Pharmaceutical and bioprocess sterilizing filtration validated through bacterial retention and integrity correlation.
• Food, beverage, and dairy processing, including protein fractionation and cold sterilization of clear liquids.
• Municipal and industrial water reuse, where trace organic and salt rejection determine treatment train design.
• Membrane manufacturing quality control, research and development benchmarking, and failure analysis of fouled or damaged elements.

FAQ

What is the difference between rejection rate and retention efficiency?

The two terms are largely interchangeable for dissolved solutes; rejection rate is more common in RO and NF contexts, while retention is more common in UF and MF contexts. For rigid particles in filter elements, retention is often expressed as a beta ratio derived from particle counts rather than as a concentration-based percentage.

Why can reported rejection values differ between laboratories?

Differences arise from test pressure, temperature, crossflow velocity, recovery, feed concentration, membrane preconditioning, and the analytical method used to quantify the solute. Standardized procedures and ISO/IEC 17025-accredited quality systems are the best safeguard for comparability.

How is molecular weight cut-off precisely assigned?

A retention curve is generated by challenging the membrane with polyethylene glycol or dextran standards and analyzing permeate versus feed by gel permeation chromatography. The MWCO is defined as the molecular weight at which rejection reaches an agreed threshold, most commonly 90% or 95%.

What sample quantity is required for testing?

Flat-sheet characterization typically requires only a few membrane discs, while element-level or cartridge testing requires one complete element. Microbial challenge testing requires non-sterile but intact product samples with documented effective filtration area.

How long does a rejection determination take?

Single-salt rejection tests are usually completed within a few days including membrane compaction and steady-state sampling. MWCO profiling, multipass beta rating, and microbial challenge programs may require one to several weeks because of preconditioning, replicate runs, and biological incubation periods.