ISO 25175:2026

International
Standard
ISO 25175
First edition
Marine technology — Standard test
2026-06
methods for performance of reverse
osmosis (RO) and nanofiltration
(NF) membrane elements
Technologie maritime — Méthodes d’essai normalisées pour les
performances des éléments de membranes d’osmose inverse (OI)
et de nanofiltration (NF)
Reference number
© ISO 2026
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Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 RO and NF membrane elements types . 1
5 Membrane elements air tightness test . 1
5.1 General .1
5.2 Apparatus .2
5.3 Test procedure .2
6 Membrane elements performance test . 3
6.1 Apparatus .3
6.2 Test Solution .4
6.3 Test procedure .4
7 Calculation . 5
7.1 Water production .5
7.2 Membrane permeation .6
7.3 Salt rejection .6
7.4 Average salt rejection .6
8 Test report . 6
Annex A (informative) Example of a test report . 8
Bibliography . 9

iii
Foreword
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Any feedback or questions on this document should be directed to the user’s national standards body. A
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iv
Introduction
Membrane elements are key components in seawater desalination. The diversity of application scenarios
and the complexity of feed-water quality in seawater desalination give rise to requirements for the flexibility
and diversity of membrane elements. Therefore, there are currently many types of membrane elements on
the market. The comparison and use of membrane components mainly refer to the product manual, but
the testing methods for membrane elements of different brands are not the same, which inevitably brings
obstacles to the users and researchers. For instance, performance evaluations of the same type of membrane
elements are obtained from different testing methods, making the comparison of these membrane elements
inaccurate.
To solve this problem, this document establishes test methods for evaluating the performance of membrane
elements used in seawater desalination, with a focus on reverse osmosis (RO) and nanofiltration (NF)
elements. It specifies the types of membrane elements to be tested and standardizes the corresponding
testing conditions. The document also covers the required test equipment, procedures, and data processing
methods. By providing a uniform benchmark, it serves as a standard reference for users in desalination
and related industrial applications, particularly in water treatment. This aims to facilitate the selection of
membrane elements and enhances clarity in technical communication.

v
International Standard ISO 25175:2026(en)
Marine technology — Standard test methods for performance
of reverse osmosis (RO) and nanofiltration (NF) membrane
elements
1 Scope
This document provides test methods for determining air tightness and separation performance (permeation
and salt rejection) of reverse osmosis (RO) and nanofiltration (NF) membrane elements.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 10523, Water quality — Determination of pH
3 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
4 RO and NF membrane elements types
The following membrane elements are covered in this document.
— Seawater desalination RO (SWRO) membrane elements
— Brackish water RO (BWRO) membrane elements
— Low pressure RO (LPRO) membrane elements
— NF membrane elements
This includes membrane elements with the following configurations.
— Spiral wound
— Hollow fibre
5 Membrane elements air tightness test
5.1 General
The membrane performance test shall be performed after passing the air tightness test.

5.2 Apparatus
Air tightness of the membrane elements shall be tested using the vacuum decay test (VDT) method. A
diagram of the apparatus is depicted in Figure 1.
Key
1 stopper
2 membrane element
3 valve A
4 valve B
5 vacuum pressure gauge
6 valve C
7 water trap
8 vacuum pump
vacuum degree
Figure 1 — Apparatus for testing air tightness of membrane elements
5.3 Test procedure
The following steps apply for testing spiral wound membrane elements.
a) Fully wet the membrane.
b) Open valve A to drain the water trapped in membrane element, then seal one end of the permeate
collection tube and close valve A.
c) Connect the other end of the membrane element to the vacuum pressure gauge, open valve B and valve C
and start the vacuum pump.
d) Close val
...