Retention Rate Testing (Rejection Rate Testing): Principles, Methods, Standards, and Applications

Retention rate testing, also known as rejection rate testing, is a fundamental performance evaluation used to verify how effectively membranes, filter elements, and separation devices reject dissolved salts, macromolecules, particles, and microorganisms. As a critical quality control and product qualification step in water treatment, pharmaceutical manufacturing, and medical device industries, this testing determines whether a separation product meets its specified separation performance. This article explains the testing principles, applicable standards, sample types, test items, methods, and typical applications of retention rate examination.

Technical Overview

Retention rate (also called rejection rate or removal efficiency) quantifies the ability of a membrane, filter, or separation module to prevent a target substance from passing into the permeate stream. It is defined as the relative reduction of the solute or particle concentration across the separation barrier and is typically calculated as: R = (1 − Cp / Cf) × 100%, where Cf is the concentration in the feed stream and Cp is the concentration in the permeate stream. The related sieving coefficient S is defined as Cp / Cf, so that R = 1 − S. A retention rate of 99.5% for sodium chloride, for example, means the permeate contains only 0.5% of the feed salt concentration.

Retention rate is the single most important separation performance indicator for reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF), microfiltration (MF) membranes, sterile filter cartridges, and blood purification devices such as hemodialyzers. Because measured retention values are strongly influenced by operating conditions, professional testing is performed under strictly standardized conditions and reported alongside pressure, temperature, recovery ratio, feed concentration, and pH. Accredited third-party laboratories following ISO/IEC 17025 quality management systems typically conduct these evaluations with traceable instruments and calibrated reference materials.

• Key international and national reference standards include ASTM D4194 (operating characteristics of reverse osmosis and nanofiltration devices), ASTM E1343 (molecular weight cutoff evaluation of membranes), ASTM F838 (bacterial retention of membrane filters), ASTM F316 (pore size characteristics by bubble point and mean flow pore test), ISO 16889 (multi-pass filtration efficiency of hydraulic filter elements), ISO 8637-1 (hemodialyzers and hemoconcentrators), and GB/T 20103 (membrane separation technology terminology).
• Factors that significantly affect measured retention include transmembrane pressure, cross-flow velocity, feed concentration, temperature, pH, membrane compaction, and fouling or concentration polarization at the membrane surface.
• Retention performance must be distinguished from productivity: flux and retention usually exhibit a trade-off, so both parameters are characterized together during a complete performance verification.

Testing Samples

Flat-Sheet Membrane Coupons

Small membrane samples cut from production sheets are tested in laboratory stirred cells or cross-flow test cells. Flat-sheet coupon testing is the standard approach for research and development, incoming material inspection, and batch release testing, offering rapid turnaround with minimal sample requirement.

Hollow Fiber Membranes and Modules

Hollow fiber ultrafiltration, microfiltration, and membrane contactor modules are tested as complete units, with the feed applied to the shell side or lumen side depending on the module design. Full-module testing verifies both membrane retention and the integrity of potting, sealing, and end-cap assembly.

Spiral-Wound Elements and Ceramic Monoliths

Full-size 4040, 8040, or smaller spiral-wound RO and NF elements, as well as ceramic multi-channel monolith elements, are evaluated in dedicated element test stands. Testing of complete elements confirms salt rejection under realistic hydraulic conditions and validates membrane-to-membrane and glue-line integrity.

Filter Cartridges, Capsules, and Membrane Discs

Sterilizing-grade and final filter cartridges, disposable capsule filters, and membrane discs are subjected to challenge testing to verify their stated pore rating, bacterial retention capability, or particle removal efficiency. These products are widely used in pharmaceutical sterile filtration and high-purity water loops.

Medical Separation Devices

Hemodialyzers, hemofilters, plasma separators, and leukocyte reduction filters are tested for solute clearance and sieving performance of middle molecules and proteins, in accordance with ISO 8637-1 and related medical device standards, to support biological safety and performance evaluation for regulatory submission.

Testing Items

Depending on the product type and its intended separation duty, retention rate testing covers a range of specific test items:

• Salt rejection: single-salt rejection for sodium chloride, magnesium sulfate, sodium sulfate, or calcium chloride, used to characterize RO and NF membranes.
• Molecular weight cutoff (MWCO) verification: determination of the solute molecular weight at which the membrane retains at least 90%, using a homologous series of neutral macromolecules.
• Particle retention efficiency: removal ratings verified against standard test dust, latex microspheres, or counted particles at defined sizes.
• Bacterial and viral retention: log reduction value (LRV) determination against standard challenge organisms and bacteriophages for sterilizing-grade and virus-retentive filters.
• Protein and macromolecule rejection: rejection of bovine serum albumin, cytochrome c, dextran fractions, or blue dextran for bioprocess ultrafiltration membranes.
• Organic matter rejection: rejection of total organic carbon (TOC) surrogates, sucrose, or specific organic micropollutants for drinking water and reuse applications.
• Sieving coefficient curves: molecular weight sieving profiles for dialysis membranes covering solutes such as urea, creatinine, vitamin B12, and inulin.

Testing Methods

Salt Rejection Test by Conductivity

Spiral-wound RO and NF elements are tested in a recirculating cross-flow rig at standardized conditions, commonly a 2000 mg/L sodium chloride feed at 1.55 MPa (225 psi) and 25 °C for brackish water RO membranes, or a 2000 mg/L magnesium sulfate feed at lower pressure for NF membranes. Feed and permeate conductivity are measured with calibrated conductivity meters, and rejection is calculated directly from the conductivity ratio. The procedure follows ASTM D4194, with results normalized to a reference temperature of 25 °C using standard viscosity and membrane permeability correction factors.

Molecular Weight Cutoff Challenge Testing

Ultrafiltration membranes are challenged with polyethylene glycol (PEG), dextran, or protein marker solutions of known molecular weight. Permeate and feed concentrations are quantified by total organic carbon analysis, gel permeation chromatography, or spectrophotometry. A retention curve is constructed across a molecular weight range, and the MWCO is reported as the molecular weight at which retention reaches 90%, following the principles of ASTM E1343. This method verifies the manufacturer's stated MWCO rating, which is essential for bioprocessing, protein concentration, and diafiltration applications.

Microbial Retention Challenge Testing

Sterilizing-grade membrane filters are challenged with Brevundimonas diminuta at a minimum challenge level of 10^7 colony-forming units per square centimeter of effective filter area in accordance with ASTM F838. The permeate is collected on assay membranes and incubated, and the retention performance is expressed as a log reduction value. A retention of seven or more log reductions is generally accepted as evidence of sterilizing-grade performance. Similar challenge logic using bacteriophage suspensions is applied to virus-retentive filters.

Particle and Bead Challenge Testing

Hydraulic and industrial filter elements are evaluated by the multi-pass test method of ISO 16889, in which test contaminant is continuously injected and upstream and downstream particle counts are recorded to derive a beta ratio and particle removal efficiency at specified sizes. Microfiltration membranes may alternatively be challenged with monodisperse latex microspheres of certified diameter, with downstream detection by liquid particle counting or microscopy.

Test Configuration and Conditioning

Retention testing is conducted in either a dead-end (static or stirred) configuration typical for MF/UF discs and final filters, or a cross-flow configuration required for RO, NF, and process UF elements. Samples are pre-cleaned and pre-compacted with deionized water at specified pressure until stable flux is achieved, after which the challenge solution is introduced. Steady-state permeate samples are collected after a defined equilibration period to eliminate dead-volume and adsorption artifacts. All concentration measurements are performed with calibrated, traceable analytical instruments, and duplicate testing is recommended for critical qualification programs.

Applications

Retention rate testing supports product development, manufacturing quality control, incoming inspection, and regulatory compliance across many industries:

• Water and wastewater treatment: verification of RO and NF element salt rejection for desalination, brackish water plants, and water reuse systems.
• Pharmaceutical and biopharmaceutical manufacturing: qualification of sterilizing-grade filters, virus filters, and ultrafiltration membranes used in sterile filling, downstream purification, and buffer filtration.
• Food and beverage processing: validation of membrane systems for dairy protein concentration, juice clarification, whey processing, and cold sterilization.
• Medical devices: performance verification of hemodialyzers and plasma separators for middle-molecule removal and albumin retention.
• Electronics and ultrapure water: confirmation of particle and TOC retention in polishing loops supplying semiconductor fabrication facilities.
• Research and product development: membrane modification studies, fouling mechanism research, and competitive benchmarking of separation performance.

FAQ

What is the difference between retention rate and rejection rate?

The two terms are used interchangeably in membrane science and both describe the percentage of a target substance prevented from passing through the membrane. Retention rate is more common in filtration and bioprocess contexts, while rejection rate is more common in desalination and RO/NF applications. Both are inversely related to the sieving coefficient (R = 1 − S).

How is molecular weight cutoff determined in practice?

MWCO is determined by challenging a membrane with a series of neutral solutes of increasing molecular weight, such as PEG or dextran fractions, and plotting retention versus molecular weight. The cutoff value is reported at the molecular weight where retention equals 90%. Because retention curves vary with test conditions, results should always be compared at identical pressure, concentration, and temperature.

Why do retention results differ between laboratories or test runs?

Differences in feed concentration, applied pressure, cross-flow velocity, recovery ratio, temperature, and pre-compaction history can shift measured retention values. Standardization of test conditions, calibrated instrumentation, and adherence to recognized methods such as ASTM D4194 and ASTM E1343 minimize variability between laboratories.

Can a single membrane sample be tested for multiple retention targets?

Yes, in many cases sequential testing is possible, for example salt rejection followed by organic rejection on an RO element. However, strongly adsorbing markers such as proteins or dyes can foul the membrane and alter subsequent results, so independent samples are recommended for microbial challenge tests and high-accuracy MWCO studies.

How long does a typical retention rate test take?

Simple salt rejection tests on flat-sheet coupons or small elements can usually be completed within a few hours including conditioning. Full microbial challenge testing, MWCO curve determination with multiple markers, and element-level qualification programs typically require several days to two weeks, including sample preparation, incubation, and reporting by an accredited third-party laboratory.