National foreword

This British Standard is the UK implementation of ISO 20304‑4:2026.

The UK participation in its preparation was entrusted to Technical Committee LBI/50, Fine Bubble Technology (FBT).

A list of organizations represented on this committee can be obtained on request to its committee manager.

Contractual and legal considerations

This publication has been prepared in good faith, however no representation, warranty, assurance or undertaking (express or implied) is or will be made, and no responsibility or liability is or will be accepted by BSI in relation to the adequacy, accuracy, completeness or reasonableness of this publication. All and any such responsibility and liability is expressly disclaimed to the full extent permitted by the law.

This publication is provided as is, and is to be used at the recipient’s own risk.

The recipient is advised to consider seeking professional guidance with respect to its use of this publication.

This publication is not intended to constitute a contract. Users are responsible for its correct application.

Compliance with a British Standard cannot confer immunity from legal obligations.

This British Standard was published under the authority of the Standards Policy and Strategy Committee on 31 August 2026.

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 has been established has the right to be represented on that committee. International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely 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) patent(s) which may be required to implement this document. However, implementers are cautioned that 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.

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.

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

3 Terms and definitions

3.1 Terms

4 Abbreviations

5 Principle

5.1 General

5.2 Crossflow membrane fouling index

6 Apparatus

6.1 General

6.2 Water pump

6.3 Membrane module

6.4 Measuring device

6.5 Data recording and storage device

6.6 Pressure sensor installed on the membrane loop

6.7 Pre-membrane pressure sensor

6.8 Pressure-reducing valve

6.9 Temperature sensor

6.10 Water tanks

6.11 Concentrate container

6.12 UFB generator

6.13 Precision electronic balance

7 Procedure

7.1 Equipment setup

7.2 Data collection

7.3 Crossflow membrane fouling index calculation

7.4 Membrane fouling, cleaning, and reuse

8 Assessment of test results

9 Measurement errors

10 Test report

Bibliography