ISO 20304-4:2026

• Abstract:

  This document specifies a method for assessing the impact of ultrafine bubbles (UFBs) on membrane antifouling using the crossflow filtration system as the evaluation setup. The evaluation compares the membrane fouling behaviour of the original wastewater (control) with that of the same wastewater after UFB generation under specified operating conditions. This document is applicable to dispersions containing UFBs, including dispersions in which microbubbles (MBs) are present in combination with UFBs. The method employs the crossflow membrane fouling index (CMFI), measured under constant transmembrane pressure during crossflow filtration as the evaluation criterion. This method is applicable to various membrane treatment systems including ultrafiltration, nanofiltration, and reverse osmosis and can be used for different types of wastewater containing particulates, colloids, and soluble organic matter. This method is primarily intended for dispersions containing UFBs (see Annexes A to C and E); however, it is equally applicable, by analogy, to dispersions containing MBs (see Annex D). This document can guide the design and optimization of industrial processes that integrate FB technologies for enhanced membrane antifouling. NOTE 1 This method is intended for waters that contain a measurable amount of either particulate matter or colloidal matter, or both. It is not applicable to essentially particle-free waters, such as permeate from reverse osmosis or ultrafiltration systems. NOTE 2 Users can record supplementary information, e.g., turbidity, total suspended solids, and particle-size distribution; however, such data are not required in this document.

• Status: Published
• Publication Date: 26-Aug-2026
• ICS: 07.030 - Physics. Chemistry
• Technical Committee: ISO/TC 281 - Fine bubble technology
• Drafting Committee: ISO/TC 281/WG 3 - Applications of fine bubble technology
• Current Stage: 6060 - International Standard published
• Start Date: 27-Aug-2026
• Due Date: 20-Feb-2028
• Completion Date: 27-Aug-2026

Overview

ISO 20304-4:2026 provides a standardized test method for evaluating the antifouling performance of fine bubble (FB) water, specifically ultrafine bubbles (UFBs), within crossflow membrane filtration systems. Developed by ISO’s Technical Committee 281, this standard is designed for practical use in the water and wastewater treatment industry, where membrane fouling remains a persistent challenge impacting operational efficiency and cost. The document enables consistent assessment of how integrating fine bubble technologies can mitigate fouling, comparing traditional membrane filtration approaches with those utilizing UFB dispersions under controlled operational conditions.

Key Topics

• Fine Bubble (FB) and Ultrafine Bubble (UFB) Technology
  The standard focuses on dispersions with UFBs and may include microbubbles (MBs) for comprehensive evaluations. UFB technology has demonstrated potential for significantly reducing membrane fouling, an issue that restricts the widespread use of membrane filtration in water treatment.

• Crossflow Filtration Systems
  ISO 20304-4 emphasizes using crossflow membrane filtration, where feed water moves tangentially across the membrane surface. This approach closely models actual industrial scenarios and is more effective for evaluating antifouling versus conventional dead-end filtration.

• Crossflow Membrane Fouling Index (CMFI)
  The method introduces the CMFI, an index calculated under constant transmembrane pressure during filtration. The CMFI quantitatively reflects fouling rates and enables direct comparison between control (untreated) and UFB-treated wastewater, supporting evidence-based process optimization.

• Membrane Types and Wastewater Variability
  The procedure is applicable to a wide range of membrane technologies, including ultrafiltration, nanofiltration, and reverse osmosis systems. It accommodates wastewater matrices rich in particulates, colloids, and dissolved organic matter, addressing multiple industrial treatment contexts.

Applications

• Industrial and Municipal Wastewater Treatment
  ISO 20304-4 provides essential guidelines for water treatment operators, engineers, and researchers to evaluate the benefits of FB/UFB integration in reducing membrane fouling, thus enhancing operational lifespan and lowering cleaning frequency and costs.

• Process Design and Optimization
  Organizations can utilize results from this standard to inform process improvements, adjust operational conditions, and optimize membrane system design when adopting fine bubble technologies. The standard supports the development of more resilient, cost-effective, and sustainable membrane filtration systems.

• Product Performance Validation
  Manufacturers of UFB generators, membranes, and associated filtration equipment can use ISO 20304-4 as a reliable performance testing framework, facilitating transparent communication of efficacy to customers and regulatory bodies.

• Comparative Studies and Research
  The reproducible method allows for the direct comparison of various bubble generation techniques and membrane types in controlled studies, promoting further advancements in antifouling solutions and water purification innovations.

Related Standards

• ISO 23015: Fine bubble technology - Measurement technique matrix for the characterization of fine bubbles
  Offers detailed guidance on fine bubble measurement techniques, which complements the characterization requirements in ISO 20304-4.

• ISO 20480-3:2021: Fine bubbles - Part 3: Generation
  Describes methods for generating UFBs and MBs referenced for setting up test apparatus and generators in accordance with this standard.

• ISO 19430:2016: Fine bubble technology - General principles for particle tracking analysis
  Provides protocols for measurement of bubble size distribution essential for reporting and characterizing FB dispersions.

• ISO 21910-1:2020 and ISO 21910-2:2025: Fine bubble technology - Microbubble characterization
  These support comprehensive bubble dispersion analyses, especially when MBs are present alongside UFBs.

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Keywords: ISO 20304-4:2026, fine bubble technology, ultrafine bubbles, crossflow filtration, membrane fouling, CMFI, antifouling performance, water treatment, membrane filtration systems, wastewater treatment, test method, standardization, ISO water standards

International
Standard
ISO 20304-4
First edition
Fine bubble technology — Water
2026-08
treatment applications —
Part 4:
Test method for evaluating the
antifouling performance of
fine bubble water in crossflow
membrane filtration systems
Technologie des fines bulles — Traitement de l'eau —
Partie 4: Méthode d'essai pour l'évaluation des performances
anti-encrassement de l'eau à fines bulles dans les systèmes de
filtration tangentielle sur membrane
Reference number
© ISO 2026
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
3.1 Terms.2
4 Abbreviations . 3
5 Principle . 3
5.1 General .3
5.2 Crossflow membrane fouling index .4
6 Apparatus . 7
6.1 General .7
6.2 Water pump .7
6.3 Membrane module . .7
6.4 Measuring device .7
6.5 Data recording and storage device .8
6.6 Pressure sensor installed on the membrane loop .8
6.7 Pre-membrane pressure sensor . .8
6.8 Pressure-reducing valve .8
6.9 Temperature sensor .8
6.10 Water tanks .8
6.11 Concentrate container .8
6.12 UFB generator .8
6.13 Precision electronic balance .8
7 Procedure . 8
7.1 Equipment setup .8
7.2 Data collection .9
7.3 Crossflow membrane fouling index calculation .10
7.4 Membrane fouling, cleaning, and reuse .10
8 Assessment of test results.11
9 Measurement errors .11
10 Test report .11
Annex A (Informative) Example of test results of the antifouling performance of fine bubble
water using ceramic membranes with coking wastewater .13
Annex B (Informative) Example of test results of the antifouling performance of fine bubble
water using polyvinylidene fluoride hollow fibre membranes with comprehensive
wastewater from the entire steel plant .20
Annex C (Informative) Example of test results for the antifouling performance of fine bubble
water using ceramic membranes with surface water .28
Annex D (Informative) Example of test results for the antifouling performance of microbubble
water using ceramic membranes with black liquor wastewater .38
Annex E (Informative) Example of test results of the antifouling performance of fine bubble
water using 47 mm mixed cellulose ester flat-sheet membranes with lens adhesive
rinse wastewater .46
Bibliography .53

iii
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee
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with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO document should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
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this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 281, Fine bubble technology.
A list of all parts in the ISO 20304 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
Introduction
Membrane separation processes exhibit high efficiency in removing various impurities from water. Hence,
they have been widely employed for pure-water production, reclaimed-water treatment, and industrial
wastewater treatment. However, the application of membrane separation technologies is limited by
membrane fouling, which directly affects the economic feasibility and sustainability of these technologies.
Conventional membrane antifouling techniques include introducing pre-filters, enhancing membrane
hydrophilicity, applying surface coatings, and frequent physical and chemical cleaning. However, these
approaches often lead to increased operational costs, environmental concerns, and limited effectiveness.
In recent years, fine bubble (FB) and ultrafine bubble (UFB) technologies have been demonstrated to
considerably reduce membrane fouling. However, there are currently no effective methods to evaluate the
efficiency of these technologies for reducing membrane fouling.
The silt density index (SDI), introduced by the American Society for Testing and Materials (ASTM), is a widely
used standard for measuring the fouling potential caused by colloidal and suspended particles. However,
the SDI has several limitations including a lack of temperature correction, insensitivity to membrane
resistance variations, and poor correlation with colloidal or suspended particle concentrations. These
shortcomings influence the accuracy and reproducibility, raising concerns about its predictive value. To
address these issues, ASTM has proposed a fouling index model based on microfiltration membranes (MFI-
0,45), which improves the accuracy of membrane fouling assessments under various operational conditions
by considering the filtration mechanism of the cake layer as well as temperature, viscosity, and membrane
area.
However, this method employs dead-end filtration, which is unsuitable for water containing FBs. FBs
adhere and accumulate on the membrane surface, decreasing the permeate flux, increasing the filtration
resistance, and reducing the effective contact area between test water and membranes, causing considerable
fluctuations in the assessment results and failure in accurately reflecting the true extent of membrane
fouling. Therefore, neither the SDI nor the fouling index model based on dead-end filtration can accurately
evaluate the effects of FBs and UFBs on reducing membrane fouling.
In practical reverse osmosis (RO) operations, membranes are typically operated under crossflow conditions,
which better mimic real-world operating scenarios and are more suitable than dead-end filtration for
evaluating the influence of FBs on reducing membrane fouling. Many researchers have adopted crossflow
filtration setups to improve the measurement of fouling indices, such as the membrane fouling index
(MFI), thereby demonstrating the feasibility and maturity of the crossflow approach. Building on these
advancements, this document improves on traditional approaches by moving away from using fixed reference
conditions, such as 0,45 μm membranes and 200 kPa pressure. Instead, it compares the fouling indices under
conditions with and without FBs to specifically evaluate the role of bubbles in fouling mitigation.
Using the crossflow filtration mode, the permeate flow through the membrane is quantitatively measured
over time, and a crossflow membrane fouling index (CMFI) is established. This testing condition is in
good agreement with the actual operation mode of membrane filtration, resulting in a more realistic and
accurate reflection of membrane fouling and enhancing the reliability and performance of membrane
fouling assessment. Therefore, CMFI is not only suitable for evaluating the effectiveness of FB technology in
reducing membrane fouling but also provides valuable guidance for the design and optimization of industrial
processes that integrate FB technology with membrane treatment applications.

v
International Standard ISO 20304-4:2026(en)
Fine bubble technology — Water treatment applications —
Part 4:
Test method for evaluating the antifouling performance of
fine bubble water in crossflow membrane filtration systems
1 Scope
This document specifies a method for assessing the impact of ultrafine bubbles (UFBs) on membrane
antifouling using the crossflow filtration system as the evaluation setup. The evaluation compares the
membrane fouling behaviour of the original wastewater (control) with that of the same wastewater after
UFB generation under specified operating conditions. This document is applicable to dispersions containing
UFBs, including dispersions in which microbubbles (MBs) are present in combination with UFBs.
The method employs the crossflow membrane fouling index (CMFI), measured under constant
transmembrane pressure during crossflow filtration as the evaluation criterion.
This method is applicable to various membrane treatment systems including ultrafiltration, nanofiltration,
and reverse osmosis and can be used for different types of wastewater containing particulates, colloids, and
soluble organic matter.
This method is primarily intended for dispersions containing UFBs (see Annexes A to C and E); however, it is
equally applicable, by analogy, to dispersions containing MBs (see Annex D).
This document can guide the design and optimization of industrial processes that integrate FB technologies
for enhanced membrane antifouling.
NOTE 1 This method is intended for waters that contain a measurable amount of either particulate matter or
colloidal matter, or both. It is not applicable to essentially particle-free waters, such as permeate from reverse osmosis
or ultrafiltration systems.
NOTE 2 Users can record supplementary information, e.g., turbidity, total suspended solids, and particle-size
distribution; however, such data are not required in this document.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
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/

3.1 Terms
3.1.1
crossflow
filtration method wherein the feed stream flows tangentially to the membrane surface rather than
perpendicular to it
Note 1 to entry: The flow orientation ensures that the fluid on the upstream side of the membrane moves parallel to
the membrane surface rather than directly through it.
3.1.2
crossflow membrane fouling index
CMFI
index calculated from the development of filtration velocity through a membrane filter under a crossflow
mode
3.1.3
concentrate
stream containing rejected particles and solutes discharged from a membrane module during crossflow
filtration
3.1.4
dead-end filtration
filtration method wherein the only exit for the upstream fluid is through the membrane
Note 1 to entry: In this configuration, the feed stream flows perpendicularly to the membrane surface and all the fluid
must pass through the membrane to exit the system
3.1.5
flux
throughput of a membrane, typically quantified as the permeate volume produced per unit time and per unit
membrane surface area
Note 1 to entry: Flux is commonly expressed in litres per square meter per hour (L/m /h), and measurements are
often standardized at a specific temperature (typically 20 °C).
3.1.6
membrane fouling
undesirable accumulation of substances on a membrane surface or within its pores during filtration
processes
Note 1 to entry: This buildup hinders the membrane’s performance by reducing its permeability or flux, increasing the
system operating pressure and decreasing the membrane separation efficiency.
3.1.7
pore size
size of openings of a porous membrane expressed using either a nominal (average) or an absolute (maximum)
value
Note 1 to entry: For microfiltration membranes, the pore size is usually expressed in µm, whereas for ultrafiltration,
nanofiltration and reverse-osmosis membranes, it is typically expressed in nm.
3.1.8
transmembrane pressure difference
pressure difference across a membrane, which is the driving force for fluids passing through it
Note 1 to entry: This transmembrane pressure difference allows the membrane to selectively exclude or permit
certain substances based on its pore sizes or other specific properties.
3.1.9
feed pressure
pressure applied to the feed side of a membrane in a membrane separation system

3.1.10
pure-water flow rate
volume of pure water passing through a membrane per unit time under specific conditions
3.1.11
permeate flux
volume of permeate produced per unit membrane area per unit time
Note 1 to entry: Permeate flux is typically expressed in L/m ·h.
4 Abbreviations
ASTM the American Society for Testing and Materials
CMFI crossflow membrane fouling index
FB fine bubble
MB microbubble
MFI modified fouling index
RO reverse osmosis
UFB ultrafine bubble
5 Principle
5.1 General
In this method, the CMFI is used to assess the effect of FBs on membrane fouling. The CMFI offers an
approach that is different from the one offered by the modified fouling index (MFI) established by the ASTM,
which determines the fouling potential of the feed water of membrane systems based on a similar principle
that involves measuring fluid flow through the membrane filter as a function of time. MFI uses dead-end
filtration, as illustrated in Figure 1(A), which is not suitable for evaluating water containing FBs because
hydrophobic bubbles accumulate on the membrane surface and adhere to it, reducing its effective filtration
area. This leads to inaccurate results; thus, the beneficial effect of UFBs on membrane fouling mitigation
cannot be accurately assessed using this method.
In practice, crossflow filtration, as shown in Figure 1(B), is typically used in wastewater treatment to control
membrane fouling. Therefore, to accurately evaluate the effect of FBs on fouling mitigation, the assessment
method should employ crossflow filtration. The CMFI used in the proposed method addresses this problem
and provides a more accurate assessment of membrane fouling.
(a)
(b)
Key
1 membrane
2 feed flow
3 applied pressure
4 membrane
5 feed flow
6 applied pressure
Figure 1 — Schematic of filtration modes (a) Dead-end filtration with indications of water flow and
pressure directions; (b) Crossflow filtration with indications of water flow and pressure directions
In the CMFI-based method, FBs pass through membranes in a crossflow manner during which changes in
the filtration rate are continuously monitored. CMFI is calculated based on the collected data regarding the
change in the flow rate over time. The FB contribution to membrane fouling prevention can be effectively
determined by comparing the CMFI values for the same test sample with and without FBs.
5.2 Crossflow membrane fouling index
To determine the CMFI, the fluid flow through the membrane filter is measured as a function of time. Thus,
the relationship between the flux and cumulative permeate volume is expressed as Formula (1)
dV t

Jt  (1)

A dt
where
Vt accumulated permeate volume (L),

A membrane area (m ),
Jt flux (L/m /h),

t filtration time (s),
dV t

−1
derivative of V with respect to time (L·s ).
dt
Darcy’s law, describing fluid flow through a porous medium (the membrane), is given as follows (Formula (2)):
P
J t  (2)

R
t
where
∆P applied transmembrane pressure (Pa),
η water viscosity (Pa s),
R total resistance, including membrane resistance and fouling resistance (s/m).
t
Combining Darcy’s law with the flux relationship gives (Formula (3)):
dV t

PA
 (3)
dt R
t
The total resistance R is the sum of the membrane resistance and fouling resistance, which is given as
t
follows (Formula (4)):
RRR (4)
tm f
where
R membrane resistance (s/m),
m
R fouling resistance (s/m).
f
The fouling resistance is linearly related to the cumulative permeate volume, as given by (Formula (5)):
IV t

R  (5)
f
A
−2
where I represents fouling potential index (m ).
Substituting Formulas (3), (4), and (5) into Darcy’s law gives Formula (6):
dV t

PA
 (6)
dt  IV t 

 R 
 
m
 
A
 
Separating the variables Vt and t�gives Formula (7):

 IV t 

 R 
 
m
 
A
 
dt  dV (7)
PA
Integrating both sides from 0 to Vt gives Formula (8):

 
IV t


 
t  RV T  (8)

m


PA 2A
 
During the cake filtration period, resistance and cumulative permeate volume V(t) are linearly related,
and the slope (b) represents the fouling tendency of the driven permeate, which is shown in the following
formulas (Formulas (9) and (10)):
R
t 1 I
m
  Vt (9)

Vt Q PA

2PA
avg
 
I d t
b (10)
 
 
dV t Vt
 
2PA
 
where Q average flow rate (L/s).
avg
The CMFI is calculated based on the slope of the t/V vs. V curve and is given by Formula (11). The minimum
value of the slope (tanα) is equal to CMFI. The CMFI is then corrected using η, A , and ∆P , as expressed in
0 0
Formula (12). Therefore, it is independent of water viscosity, membrane area, and applied transmembrane
pressure.
I
tan  , (11)
2PA
  
P A
C tan  (12)
 
 
 P A
 
where
η water viscosity at 293,15 K (Pa s),
C is crossflow membrane fouling index (CMFI),
A membrane area used in the control group during the crossflow membrane fouling index (CMFI)
measurement (m ),
∆P transmembrane pressure applied in the control group during the crossflow membrane fouling
index (CMFI) measurement (Pa).
The minimum value of tan α is equal to the CMFI because the filtration mechanism frequently blocking
filtration at the start, resulting in a high slope. Next, cake filtration begins and gradually becomes the
governing mechanism until cake compression is initiated, which increases the slope. Figure 2(A) shows
the tan α calculated based on the t/V vs. V curve. Figure 2(B) shows that tan α is dependent on time and
the minimum value of tan α is equal to the CMFI.
(A)
(B)
Key
X V(t) (L)
−1
Y t/V(t) (s·L )
X’ t (s)
−2
Y’ tanα (s·L )
1 blocking filtration stage
2 cake filtration stage
3 cake compression stage
4 CMFI
Figure 2 — Analysis of tan α during different filtration phases. (A) tan α calculated based on the t/V
vs. V curves during blocking filtration, cake filtration, and cake compression phases. (B) Relation
between tan α and filtration time
6 Apparatus
6.1 General
The apparatus required for conducting the CMFI-based membrane fouling evaluation is specified in 6.2 to
6.13.
6.2 Water pump
A water pump shall generate pressure in the range of 0,1–0,6 MPa to ensure the passing of untreated
wastewater through the membrane module.
6.3 Membrane module
Suitable membrane materials shall include ceramics, organic materials, metals, nonpolar materials, and
composites. Several membrane structures, such as single-channel tubular, multichannel tubular, hollow
fibre, and flat sheet membranes, can be used. The membrane shall include an inlet for feed water, a circulation
outlet, and a permeate outlet. The pore size and surface area of the membrane shall be within the ranges of
1–2 000 nm and 0,005–30 m , respectively.
6.4 Measuring device
A device shall be capable of measuring the mass or flow rate of the permeate at a scale interval not greater
than 0,01 g, and it shall support continuous data recording and transmission.

6.5 Data recording and storage device
A device with a recording interval of 0,01–10 s shall be used for real-time data logging and storage.
6.6 Pressure sensor installed on the membrane loop
A sensor shall be used for monitoring pressure on the circulating side of the membrane.
6.7 Pre-membrane pressure sensor
A sensor shall be used for measuring the feed pressure of the fluid entering the membrane.
6.8 Pressure-reducing valve
A valve shall be used to maintain the feed pressure of wastewater entering the membrane module at a
constant pressure.
6.9 Temperature sensor
A sensor shall measure the temperature of entering wastewater (precision = 0,1 °C).
6.10 Water tanks
Two tanks shall be used, one for the water sample to be tested and the other for the permeate.
6.11 Concentrate container
A container shall be used for storing wastewater that has passed over the membrane surface during
crossflow filtration.
6.12 UFB generator
A UFB generator shall be based on any one method or a combination of the thirteen methods described in
ISO 20480-3:2021.
6.13 Precision electronic balance
A precision electronic balance shall have an accuracy of at least 0,01 g. It shall be capable of real-time data
transmission to a computer or a host system for recording the weight in real time.
7 Procedure
The procedure involves three steps: equipment setup, data collection, and CMFI calculation.
7.1 Equipment setup
The connections and structure of the equipment are shown in Figure 3.

a) UFB Generator
b) Representative apparatus for measuring the CMFI at a constant pressure using a crossflow
filtration model
Key
1 UFB generator
2 reactor
3 inlet reservoir
4 pressure-reducing valve
5 pump assembly
6 temperature sensor
7 gauge pressure sensor
8 membrane module
9 concentrate container
10 outlet reservoir
11 precision electronic balance
12 data recording and storage device
Figure 3 — Apparatus for UFB generation and CMFI measurement using a crossflow filtration model.
7.2 Data collection
Data collection is performed as follows:
Two sets of experiments are performed sequentially: (1) in the control group, the membrane module is
directly filled with wastewater for membrane filtration experiments; (2) in the UFB-treated group, a UFB

generator is used to generate UFBs in the wastewater sample. The wastewater sample containing UFBs is
then used to fill the membrane module for membrane filtration experiments.
For industrial applications, wastewater samples can be collected directly from the inlet and outlet of an
industrial fine bubble treatment system. Samples should be collected before (control group) UFB treatment
and after (UFB-treated group) the wastewater has passed through an industrial-grade UFB generator. To
ensure the samples are representative of the entire production batch, commonly used industrial sampling
protocols should be followed, including automated or manual grab sampling at critical process points.
Cavitation may occur during the generation of UFBs. However, because the generation of UFBs and membrane
filtration are two separate steps, cavitation does not affect the membrane fouling mitigation.
The transmembrane pressure of the fluid to be tested should be selected based on the operational pressure
range recommended by the membrane manufacturer, typically 0,05–0,60 MPa. The liquid flow rate on the
membrane surface is maintained at 0,05–1 m/s. The selected membrane surface flow rate ensures that
the water flow inside the tube remains laminar, thereby minimizing inconsistencies in process conditions
caused by flow rate variations.
The test temperature and pressure, as well as the mass of the filtered liquid, are recorded at each filtration
timepoint.
Note The known conditions of the experiment are the area of the membrane, viscosity of the wastewater to
be tested, pure-water flux of the membrane under set conditions, and pure-water flux of the membrane under test
conditions. The objective of this experiment is to assess the effect of UFBs on membrane fouling, independent of any
cavitation effects.
7.3 Crossflow membrane fouling index calculation
First, systematic records of the liquid flow rate (i.e., liquid mass passing through the membrane over
time) are created. Next, a graph—the x-axis represents the filtration time and the y-axis represents the
accumulated permeate volume or mass—is constructed.
Subsequently, the ratio of the filtration time to the accumulated permeate volume is calculated, and a
graph depicting the relationship between the two is plotted, with the x-axis and y-axis representing the
accumulated permeate volume and filtration time, respectively.
To analyse local trends in the data, a moving window comprising 3–15 data points (depending on the
sampling density) is set up. Within each window, data points are fitted using polynomial regression. The
fitted polynomial regression models are then differentiated to determine the rate of change in the dependent
variable relative to the independent variable (i.e., tan α).
After data processing in each window, the window is moved forward along the time series, and the steps are
repeated to create a graph of tan α vs. time.
The minimum value of tan α (i.e., the value of the CMFI) is corrected based on the pressure and temperature
conditions of the experiments. For detailed steps of CMFI calculation using time-series data processing,
refer to Annex C.
7.4 Membrane fouling, cleaning, and reuse
During a CMFI measurement, the membrane can become progressively clogged, which is observed as a
decline in the permeate flow rate. This decline is fully accounted for in the CMFI calculation specified in 7.3;
therefore, it does not compromise the validity of the result. After test completion, the membrane shall be
handled as follows:
a) after each test, the membrane shall be backwashed under reverse pressure.
b) if the membrane is chemically resistant (e.g., ceramic membrane), chemical cleaning shall be applied
after backwashing.
c) the membrane shall be reused only if its pure-water flux is at least 95 % of the initial value.

d) if the criterion mentioned in item c) is not met, the membrane shall be replaced.
Note For single-use polymeric or flat-sheet membranes, direct replacement is recommended instead of cleaning.
8 Assessment of test results
The difference in the CMFI values of the two groups is calculated to assess the efficiency of the UFB-treated
group in reducing membrane fouling. Owing to potential variations in the uniformity of the membrane pore
size within the same batch, the CMFI should be further corrected during calculations using the pure-water
flux of the membrane. While membranes with identical pore sizes and surface areas theoretically provide
consistent pure-water flux, manufacturing variations can result in differences in membrane resistance,
leading to deviations in CMFI data. Additionally, membranes from different manufacturers—despite having
the same nominal pore size, material, and surface area—may not perform identically, complicating CMFI
comparisons. Therefore, testing the pure-water flux of membranes in both the blank and experimental
groups before conducting experiments is crucial, and to compare these values to ensure strict CMFI
comparability. Efficiency (E) is calculated as follows (Formula (13)):

 Q 
CC
 
CU
Q
 
E  100% , (13)
 
C
C
 
 
 
where
E is the efficiency,
C is the CMFI of the control group,
C
C is the CMFI of the UFB-treated group,
U
Q is the pure-water flow rate of the membrane under defined conditions (control) (mL/s),
Q is the pure-water flow rate of the membrane under test conditions (UFB) (mL/s).
The E value reflects the relative performance of the system containing UFBs in preventing membrane
fouling. For examples of UFB test results, please refer to Annexes A, B, C and E, and for examples of MB test
results, please refer to Annex D.
9 Measurement errors
The measurement uncertainties are as follows:
a) large air bubbles in the water accumulate on the membrane surface and reduce the effective membrane
area, introducing uncertainties.
b) uncertainties also arise during water sample weighing.
10 Test report
The test report shall include the following information:
1) Date and time of measurement.
2) Test method used, including a reference to this document (with its year of publication) and information
about the measuring instrument used, such as the name of instrument, type of test method, and
manufacturer’s name.
3) Characterization method used to verify the presence of UFBs in pure water, including the name of
instrument, manufacturer’s name, software version, bubble size distribution, and total bubble number

concentration of the UFB dispersion. A particle tracking analysis method (ISO 19430) should be
used. Other methods described in ISO/TR 23015:2020 that provide equivalent performance are also
acceptable.
4) If MBs are present in combination with UFBs, the characterization method used for MBs should also be
reported, including the name of instrument, manufacturer’s name, software version, MB concentration,
and MB size distribution. Methods described in ISO 21910-1:2020 and ISO 21910-2:2025 should be used;
other suitable methods that provide equivalent performance are also acceptable.
5) Nature of the water used, including the wastewater sources and water sample properties, such as
volume, dissolved oxygen concentration (DO), pH, temperature, total organic carbon (TOC), chemical
oxygen demand (COD), electroconductivity, turbidity, and estimated salt concentration.
6) Measuring conditions, including the sampling method used, temperature of the water during
measurement, atmospheric pressure during measurement, and elapsed time between the shutdown of
the bubble generator or the end of sampling and the start of the measurement test.
7) FB generator information, including the bubble generation principle; operational conditions of the
generator (such as pressure, temperature, and flow rate), and gas type of FBs or UFBs.
8) Results as per Clause 7, including the volume of the wastewater, wastewater viscosity, temperature and
pressure used in the experimental setting, pure-water flux of the test membrane under test conditions,
filtrate volume accumulated over time during the membrane filtration, t/V values for different groups as
a function of accumulated filtrate volume, membrane fouling index curve recorded during the crossflow
filtration, and efficiency of FBs or UFBs in membrane antifouling.
9) The type and extent of interaction (if any) between cavitation bubbles and the filtration membrane, and
any unusual features observed during the test.
10) All circumstances that might have influenced the result.

Annex A
(Informative)
Example of test results of the antifouling performance of fine bubble
water using ceramic membranes with coking wastewater
Water samples were collected from the deep-treatment unit of the zero-liquid-discharge section of a
coking wastewater treatment plant in Maanshan Iron & Steel Co., China. After multi-stage biological, and
flocculation processes, suspended solids are almost completely removed (TSS = 3,8 mg/L), yet a moderate
chemical oxygen demand remains (COD = 200 mg/L). The residual organic fraction is dominated by phenols,
nitrogen-containing heteroaromatics and trace polycyclic aromatic hydrocarbons. Inorganic salt levels are
elevated; chloride, sulfate and sodium are the principal ions. The water is weakly alkaline, dark brown and
carries the characteristic odor of coking operations.
A ceramic membrane (pore size = 20 nm and membrane area = 0,007 5 m ) was used for filtration. Filtration
was operated in crossflow mode; a schematic is shown in Figure A.1. In this setup, the feed flows tangentially
along the inner channel of the tubular ceramic element, the permeate passes radially through the porous
membrane wall and is collected outside the tube, and the remaining concentrate exits at the outlet.
Three test groups were used in the experiment: the first group was untreated raw water (Control group),
the second group was raw water containing air UFBs treated for 20 min (AirUFB group), and the third group
was raw water containing ozone UFBs treated for 20 min (OzoneUFB group). Moreover, the ozone dosage
was 300 mg/L.
The UFB generator was operated at a gas flow rate of 300 mL/min and a pump flow rate of 240 L/h, and a
gas/liquid ratio of 1:13. Before the main experiment, the pure-water flux of the membrane was measured to
ensure membrane cleanliness and consistency.
The UFB number concentrations of the AirUFB and OzoneUFB groups were measured under identical
7 8
conditions to be 6 × 10 and 1,2 × 10 particles/mL, respectively. However, pure water was used in the
measurement instead of wastewater because the high particle content in wastewater makes it challenging
to differentiate between UFBs and impurities.
Key
light brown feed
gray concentrate
blue permeate
Figure A.1 — Schematic of crossflow filtration with a tubular ceramic membrane used for
antifouling tests.
Key
X time (s)
Y accumulated permeate volume (pure water; mL)
Control
AirUFB
OzoneUFB
−1 −1 −1
NOTE The slopes of the linear fits are 1,23 mL·s for Control, 1,72 mL·s for AirUFB and 1,17 mL·s for
OzoneUFB.
Figure A.2 — Volume of accumulated permeate (pure water) over time for the three groups using
new membranes before the main experiment

Key
X time (s)
Y applied transmembrane pressure (10 Pa)
Control
AirUFB
OzoneUFB
Figure A.3 — Variation in the applied transmembrane pressure with filtration time for the three
test groups
Key
X time (s)
Y temperature (K)
Control
AirUFB
OzoneUFB
Figure A.4 — Variation in the temperature with filtration time for the three test groups

The pure-water flux (Figure A.2), transmembrane pressure (Figure A.3), and test temperature (Figure A.4)
for each group were obtained from sensor readings. The viscosity of the water samples was measured using
a viscometer. The experimental condition parameters are shown in Table A.1.
Table A.1 — Parameters of membrane filtration
A
η
∆P T
η
Q
Treatment
−3
Pa m K ( mL/s) −3
  (10 Pa·s)

(10 Pa·s)
Control 332 000 0,007 5 298,65 1,23 1,013 0,951
AirUFB 336 000 0,007 5 298,65 1,72 0,912 0,890
OzoneUFB 321 000 0,007 5 298,65 1,17 0,709 0,729
∆P : Applied pressure (Pa)
A: Membrane area (m )
T : Temperature (K)
Q : Pure-water flux of the membrane under test conditions (mL/s)
η : Wastewater viscosity at 293,15 K (Pa·s).
η : Wastewater viscosity at measurement temperature (Pa·s).
Note: The experiment employed a ceramic membrane with a pore size of 20 nm.
As listed in Table A.1, the pure-water flux of the membrane module was measured using pure water to
ensure the cleanliness of the membrane before the experiment. During filtration,
...