Retention Rate Test Report: A Complete Technical Guide to Membrane Rejection Testing

A retention rate test report is a formal technical document that verifies the rejection performance of filtration membranes, filter cartridges, and separation media under controlled laboratory conditions. This guide explains the test principle, sample types, testing items, standard test methods, and practical applications of retention rate testing, helping manufacturers and buyers interpret results with confidence. All testing described herein can be performed by accredited third-party laboratories in accordance with ISO, ASTM, EN, GB, and JIS standards.

Technical Overview

The retention rate, also known as the rejection rate or interception rate, is the most critical performance indicator for any filtration or membrane separation product. It quantifies the percentage of a target solute, particle, or microorganism that a membrane or filter is able to remove from the feed stream. The retention rate is calculated using the standard formula: R (%) = (1 − Cp / Cf) × 100, where Cp is the concentration of the target substance in the permeate and Cf is the concentration in the feed solution.

A retention rate test report presents the measured rejection performance of a specimen together with all relevant test conditions, including feed concentration, applied pressure, operating temperature, recovery ratio, and analytical method. Because rejection performance is highly sensitive to these parameters, a professional report must document every condition in full so that results can be reproduced and compared. Reports of this type are routinely used for product qualification, incoming material inspection, quality control, research and development benchmarking, and regulatory submission support.

Retention rate testing is typically performed in accredited third-party laboratories or in manufacturer laboratories operating under ISO/IEC 17025 management systems. The applicable methods draw on internationally recognized standards published by organizations such as ASTM International, ISO, EN, GB, and JIS, depending on the product category and end-use industry.

Testing Samples

Reverse Osmosis (RO) Membrane Elements

Reverse osmosis membranes are the most commonly tested specimens for salt rejection evaluation. Both brackish water RO elements and seawater RO elements are tested in flat-sheet coupon form or as full-size spiral-wound elements. Standard specimens include 8-inch and 4-inch industrial elements as well as household membrane elements used in point-of-use drinking water systems.

Nanofiltration (NF) Membrane Elements

Nanofiltration membranes occupy the separation range between RO and ultrafiltration. Test specimens are evaluated for the rejection of divalent salts such as magnesium sulfate and calcium chloride, as well as small organic molecules. NF testing is particularly important for water softening, color removal, and partial desalination applications.

Ultrafiltration (UF) Membranes

Ultrafiltration membranes are characterized by their molecular weight cut-off (MWCO) rather than by salt rejection. Test specimens include hollow fiber bundles, tubular membranes, and flat sheets. Rejection is verified using marker molecules such as polyethylene glycol (PEG), dextran, globular proteins, or latex microspheres of certified particle size.

Microfiltration Membranes and Filter Cartridges

Microfiltration products include pleated filter cartridges, melt-blown depth filters, string-wound cartridges, and membrane discs with nominal ratings from 0.05 to 10 micrometers. Retention testing for these samples measures particle removal efficiency at challenge particle sizes and, for sterilizing-grade filters, bacterial retention capability.

Air Filtration Media and Protective Materials

Filtration efficiency specimens also include HVAC filter media, HEPA and ULPA filter media, and medical face mask materials. These samples are tested for particle interception efficiency at the most penetrating particle size (MPPS) or for bacterial filtration efficiency using biological aerosol challenge methods.

Testing Items

A comprehensive retention rate test report may include some or all of the following test items, depending on the specimen type and the applicable product standard:

• Sodium chloride rejection rate for RO membranes under standard brackish water or seawater test conditions
• Magnesium sulfate and calcium chloride rejection rates for nanofiltration membranes
• Stabilized salt rejection after mandatory membrane conditioning or pre-soaking periods
• Molecular weight cut-off verification for ultrafiltration membranes using PEG, dextran, or protein markers
• Particle retention efficiency at specified challenge particle diameters for microfiltration media
• Bacterial retention and log reduction value (LRV) for sterilizing-grade and virus-retentive filters
• Bacterial filtration efficiency (BFE) for medical face mask materials
• Most penetrating particle size efficiency for HEPA and ULPA class filter media
• Permeate flux and pure water permeability measured simultaneously with rejection data
• Recovery ratio, concentration polarization control, and system pressure drop verification
• Comparative rejection testing of feed, permeate, and concentrate streams

Testing Methods

Salt Rejection Testing for RO and NF Membranes

Salt rejection tests follow established methods such as ASTM D4194 for the standard practice for operating, controlling, and analyzing RO and nanofiltration systems, and ASTM D6908 for ranking the rejection capability of RO membranes. A typical brackish water RO test conditions the membrane, then challenges it with a 2,000 mg/L sodium chloride feed solution at 225 psi (1.55 MPa) applied pressure, 25 °C feed temperature, pH 7, and 15% recovery. Seawater elements are tested with approximately 32,000 mg/L sodium chloride at 800 psi (5.52 MPa). Nanofiltration elements are commonly challenged with 2,000 mg/L magnesium sulfate at 70 to 100 psi. Feed and permeate conductivity are measured per ASTM D1125, or chloride concentration is determined by ion chromatography or titration, and the rejection rate is calculated from the concentration ratio.

Molecular Weight Cut-Off Testing for UF Membranes

UF retention characterization follows ASTM E1343, which covers the evaluation of the molecular weight cut-off of ultrafiltration membranes using challenge solutions of polyethylene glycol, dextran, or globular proteins of known molecular weight. The permeate and feed concentrations of the marker molecules are measured by total organic carbon analysis, gel permeation chromatography, HPLC, or gravimetric methods. The resulting rejection-versus-molecular-weight curve defines the MWCO, conventionally reported as the molecular weight at which rejection reaches 90%.

Particle Retention Testing for Microfiltration Products

Microfiltration retention efficiency is measured by challenging the filter with suspensions of monodisperse latex microspheres or ISO medium test dust at defined concentrations and pressures. Particle counts upstream and downstream of the specimen are determined by optical or liquid particle counting techniques consistent with ISO 21501. For hydraulic and lubrication filters, the multi-pass filtration efficiency test under ISO 16889 establishes the filtration ratio (beta ratio) and retention efficiency at particle sizes of 4, 6, 10, 14, and 21 micrometers.

Biological Retention Testing

Sterilizing-grade filters are evaluated for bacterial retention using challenge organisms such as Brevundimonas diminuta at minimum challenge levels defined in recognized pharmaceutical and filtration standards, with results expressed as a log reduction value. Medical face mask materials are tested for bacterial filtration efficiency in accordance with EN 14683 and ASTM F2100, which specify the biological aerosol challenge method using Staphylococcus aureus. Air filtration media are tested under ISO 29463 and EN 1822, which define counting efficiency measurements at the most penetrating particle size.

Structural Integrity Supporting Tests

Because physical defects directly degrade retention, a complete report often includes supporting integrity tests such as the bubble point and mean flow pore test in accordance with ASTM F316, pressure hold testing, and diffusion flow measurement. These non-destructive methods confirm that the specimen structure is intact and correlate with destructive retention results.

Test Report Content Requirements

• Complete identification of the specimen, including product model, serial number, dimensions, and nominal rating
• Reference to the applied test standard and any documented deviations from the standard method
• All controlled test conditions: feed solution composition, concentration, pressure, temperature, flow rate, recovery, and pH
• Identification and calibration status of the analytical instruments used
• Measured feed, permeate, and concentrate concentrations with calculated retention rates
• Simultaneously measured flux or permeability data for performance correlation
• Uncertainty considerations, replicate results, and a clear statement of conformity to the specified acceptance criteria

Applications

Retention rate test reports serve a wide range of industries in which separation performance directly affects product quality, process reliability, and regulatory compliance:

• Drinking water treatment and residential RO systems, where salt rejection determines product water quality and compliance with national drinking water standards such as GB standards
• Seawater and brackish water desalination plants for membrane procurement qualification and periodic performance verification
• Pharmaceutical and bioprocessing industries for sterilizing-grade filtration, virus clearance studies, and batch release documentation
• Food and beverage processing, including dairy concentration, protein fractionation, juice clarification, and sterile bottling filtration
• Ultrapure water production for semiconductor and electronics manufacturing, where trace ionic contamination is unacceptable
• Power generation, oil and gas, and hydraulic systems requiring verified filtration efficiency for lubricant and coolant protection
• Medical device and personal protective equipment sectors requiring documented bacterial filtration efficiency for masks and protective media
• Manufacturer research and development programs for new membrane formulations, quality control of production lots, and benchmarking of competitive products

FAQ

What is the difference between retention rate, rejection rate, and interception rate?

These three terms are essentially synonymous and describe the same quantity: the percentage of a target substance removed by the filter or membrane. The preferred term varies by industry, with rejection rate most common in RO and NF applications, retention rate common in ultrafiltration and bioprocess filtration, and interception rate frequently used in general filter media testing.

Why do reported rejection values depend on test conditions?

Rejection performance is strongly influenced by feed concentration, applied pressure, temperature, pH, and recovery ratio. Higher recovery increases concentration polarization at the membrane surface, which can reduce apparent rejection, while higher pressure generally improves salt rejection for RO membranes. This is why a valid report must state every test condition alongside the result.

What is a typical acceptance criterion for RO membrane salt rejection?

Commercial brackish water RO elements commonly specify stabilized minimum NaCl rejection of 99.0% to 99.5% under standard test conditions, while seawater elements typically specify 99.6% or higher. Acceptance criteria are defined by the manufacturer specification sheet and must be verified against identical test conditions.

How is molecular weight cut-off different from a rejection percentage?

The MWCO is a characterization parameter for ultrafiltration membranes expressed in daltons, conventionally the molecular weight at which the membrane rejects at least 90% of a specific marker molecule. A rejection percentage, by contrast, is a direct measurement for a specific solute under specific conditions.

How often should retention rate testing be performed?

Testing frequency depends on the quality system in place. Typical practice includes full testing for new product qualification, periodic lot-release testing during manufacturing, verification testing after cleaning or regeneration cycles in service, and annual revalidation as part of quality management programs.

Can a single report be used for regulatory submission?

Reports issued by accredited third-party laboratories operating under ISO/IEC 17025 with documented traceability are widely accepted for regulatory purposes, tenders, and buyer verification. The report must clearly identify the specimen, the applied standard, all test conditions, and the conformity conclusion to be suitable for such uses.