Retention Rate Analysis Solution for Membrane and Filter Performance Evaluation

Retention rate analysis quantifies the ability of a membrane or filter medium to reject dissolved salts, particles, macromolecules, and microorganisms from a feed stream, making it a core indicator of separation performance. This solution covers sample types, testing items, standardized test methods, and typical applications across water treatment, bioprocessing, pharmaceuticals, and air filtration. All testing is carried out in accredited third-party laboratories in accordance with ISO, ASTM, EN, and GB standards.

Technical Overview

Retention rate, also referred to as rejection rate or removal efficiency, is the fundamental performance metric used to characterize membranes, filter media, and separation devices. It expresses the percentage of a target species that is retained by the barrier rather than passing through it into the permeate or filtrate. The basic calculation is expressed as: R = (1 − Cp / Cf) × 100%, where Cf is the concentration of the target species in the feed and Cp is its concentration in the permeate.

In practice, a distinction is made between apparent retention, which is calculated from directly measured feed and permeate concentrations, and intrinsic retention, which is corrected for concentration polarization at the membrane surface. Accurate retention data is essential for membrane grading and classification, incoming quality control, research and development of new membrane materials, process design, and regulatory compliance verification. This solution applies to microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO), depth and pleated filter media, and high-efficiency air filtration materials.

Testing Samples

Flat Sheet Membranes

Flat sheet coupons and discs cut from polymer or ceramic membranes, covering the full spectrum from MF to RO. Samples are typically pre-cleaned, conditioned, and pre-wetted (for example with a water-miscible alcohol for hydrophobic membranes) to ensure stable and reproducible results.

Membrane Modules and Elements

Spiral-wound elements, hollow fiber modules, tubular modules, and ceramic monoliths in laboratory, pilot, and industrial formats. Testing verifies the rejection performance of the assembled element under realistic hydraulic conditions rather than the flat sheet alone.

Filter Cartridges and Filter Media

Depth filter cartridges, pleated cartridges, melt-blown elements, nonwoven filter fabrics, woven wire meshes, and battery separator materials. Retention testing is combined with structural integrity checks to confirm the nominal or absolute rating claimed by the manufacturer.

Air Filtration Materials

HEPA and ULPA filter media, general ventilation media, and cabin air filter materials. Particle removal efficiency is evaluated across the most penetrating particle size (MPPS) range using standardized aerosol challenge methods.

Testing Items

The scope of testing is selected according to the membrane class and end-use application. Common testing items include:

• Inorganic salt rejection: NaCl, MgSO4, CaCl2, and Na2SO4 rejection for RO and NF membranes, measured by conductivity or ion chromatography.
• Molecular weight cut-off (MWCO) determination: retention curves for polyethylene glycol (PEG), polyethylene oxide, or dextran markers, defining the molecular weight at which retention reaches 90%.
• Particle retention efficiency: removal efficiency and beta ratio for filter media challenged with defined test dust or standardized particles.
• Organic matter rejection: rejection of natural organic matter, dyes, sugars, or micropollutants quantified as TOC, COD, UV254, or compound-specific concentrations.
• Heavy metal rejection: retention of Pb, Cd, Cr(VI), As, and other metal ions determined by ICP-OES or ICP-MS.
• Microbial retention: bacterial challenge testing and log reduction value (LRV) determination for sterilizing-grade membranes, plus endotoxin retention where applicable.
• Simultaneous flux and permeability measurement: pure water permeability and test-liquid flux recorded together with retention to give a complete performance profile.

Testing Methods

Dead-End Stirred Cell Filtration

Small-area membrane coupons are tested in pressurized stirred cells under controlled transmembrane pressure and agitation. This configuration minimizes concentration polarization artifacts and is the standard approach for MWCO evaluation of flat sheet UF membranes per ASTM E1343, using mixed PEG or dextran feed solutions analyzed by gel permeation chromatography or total organic carbon.

Crossflow Filtration Testing

Crossflow test rigs circulate the feed across the membrane surface at controlled flow rate, pressure, temperature, and recovery. RO and NF elements are characterized for salt rejection under typical conditions such as a 2000 mg/L NaCl feed at 1.55 MPa and 25 °C for brackish RO, following the operating and calculation principles of ASTM D4194. Conductivity-based feed and permeate measurements are performed per ASTM D1125.

Challenge Tests with Surrogate Particles

Filter elements are evaluated by single-pass or multi-pass challenge testing with calibrated test dust or monodisperse particles. The multi-pass method of ISO 16889 determines the beta ratio (β = upstream count / downstream count) at defined particle sizes, from which retention efficiency is derived as (1 − 1/β) × 100%. Bacterial retention of sterilizing-grade membranes is verified by challenge testing per ASTM F838 using Brevundimonas diminuta at challenge levels of at least 10⁷ CFU per square centimeter.

Air Filtration Efficiency Testing

Flat sheet media and assembled filters are challenged with submicrometer aerosols to measure fractional efficiency versus particle size. HEPA and ULPA products are tested at the most penetrating particle size in accordance with EN 1822 and ISO 29463, while general ventilation media are classified by ePM efficiency per ISO 16890.

Analytical Quantification

Feed and permeate concentrations are quantified with validated instrumentation selected to match the analyte: conductivity meters and ion chromatography for salts, TOC analyzers per ISO 8245, UV-Vis spectrophotometry for dyes and PEG, HPLC or GC-MS for trace organics, ICP-OES/ICP-MS for metals, COD digestion per HJ 828, and laser particle counters or membrane microscopy for particulates. Supporting water quality parameters follow GB/T 5750 where applicable.

All tests include membrane compaction and stabilization stages, controlled temperature (typically 25 ± 1 °C), replicate sampling, and mass balance verification. Pore size characterization by bubble point and mean flow pore testing per ASTM F316 can be added to corroborate retention results.

Applications

Retention rate analysis supports product development, quality control, and acceptance testing across a wide range of industries:

• Water and wastewater treatment: RO and NF membrane qualification for desalination, brackish water treatment, and water reuse projects.
• Pharmaceutical and bioprocessing: verification of sterilizing-grade filters, virus filters, and UF membranes used in buffer exchange and protein concentration.
• Food and beverage: validation of cold sterilization, clarification, and concentration membranes for dairy, juice, and brewing processes.
• Electronics and ultrapure water: confirmation of ion and particle rejection in polishing loops for semiconductor fabrication.
• Hydraulics and lubrication: beta ratio certification of filter elements for hydraulic systems and fluid cleanliness control.
• Cleanrooms and HVAC: efficiency classification of HEPA, ULPA, and general ventilation air filters for contamination control.
• Material research: comparative evaluation of novel membrane chemistries, coatings, and antifouling modifications.

FAQ

What is the difference between retention rate and rejection rate?

The two terms are used interchangeably and are calculated with the same formula R = (1 − Cp/Cf) × 100%. Membrane literature tends to use rejection for dissolved solutes such as salts, while retention is more common for particles and macromolecules.

How is molecular weight cut-off determined?

MWCO is determined by filtering a series of PEG, dextran, or protein markers and plotting retention against molecular weight. The MWCO is defined as the molecular weight at which the membrane retains at least 90%, following the approach of ASTM E1343.

What feed conditions are used for salt rejection testing?

Typical conditions for brackish RO elements are a 2000 mg/L NaCl solution at 1.55 MPa applied pressure, 25 °C, and controlled recovery. NF membranes are commonly evaluated with MgSO4 or CaCl2 solutions at lower pressure. Custom feed compositions and pressures can be applied to simulate specific site conditions.

Why can measured retention vary between laboratories?

Variations arise from differences in test pressure, crossflow velocity, recovery, temperature, and the degree of concentration polarization. Standardized conditioning, stabilization, and calculation procedures, as specified in ASTM and ISO methods, are therefore essential for comparable results.

How long does a retention test take?

Single-point salt rejection or particle efficiency tests on conditioned samples typically require one to three working days, while full MWCO curves, bacterial challenge tests, or element qualification programs may require one to two weeks depending on the test matrix.