ReviewFouling and cleaning of ultrafiltration membranes: A review
Introduction
Modern ultrafiltration was originally developed as a fractionation technique in the late 1960s [1]. Since then, this technology has enjoyed continuous development, and its applications have spanned a wide variety of fields, from chemical recovery, water treatment, wastewater reclamation, juice concentration, dairy making, medical usage, to the harvesting of cells [1]. However, membrane fouling is still a severe problem limiting the potential of this technique. Fouling may result in an increase in operational costs, due to an increased energy demand, additional labour for maintenance, cleaning chemical costs, and shorter membrane life. It requires effective and efficient methods for its control and minimisation.
It may be possible to prevent fouling before its occurrence by methods such as pre-treatment of the feed streams, chemical modification to improve the anti-fouling properties of a membrane, and optimisation of the operational conditions. However, periodic membrane cleaning is still currently inevitable. It is indeed an integral part of most membrane processes in modern industries, and must be regularly carried out to remove the fouled materials and restore the productivity of the operation [2].
Study of membrane cleaning has always been a complement to developing deeper knowledge of fouling. However, the dedicated literature on membrane cleaning is notably less than that on fouling studies [3]. Many previous cleaning studies were actually subsidiary to that of relative fouling, and for which the study was far from comprehensive. However, owing to the greatly improved understanding of fouling in the last two decades, there have been an increasing number of dedicated studies on membrane cleaning. In particular, systematic studies have been made in many respects in recent years.
A quick scan of bibliographic databases shows that the number of research papers with respect to membrane cleaning has boomed in the last decade. This corresponds to the large, simultaneous expansion of UF processes in industries such as water, wastewater, food and biotechnology. The up-to-date information on membrane cleaning is constantly in demand because it is a vital part for the operation of most membrane systems.
There exist a few excellent summary works and pioneering early reviews regarding membrane cleaning. Many of them are parts of more general reviews of fouling and its control technology [4], [5], [6]. Full reviews on membrane cleaning were written about two decades ago [7], including one on reverse osmosis [8]. There are also works dedicated to specific areas in this realm such as conventional cleaning [9], ultrasonic cleaning [10], membrane cleaning in the food industry [2] and chemical cleaning in the water industry [3], [11]. However, the increasing number of UF applications and the rapid development in UF cleaning constantly brings out new ideas and results. An updated review is therefore timely and useful.
In our opinion, the comprehension of membrane cleaning involves gaining knowledge of many separate aspects and making the links between them. It should include the target (common fouling problems in these industries), removal (various cleaning methods), results (cleaning effectiveness and any side effects such as membrane damage) and optimisation (effect of operational parameters). Thence, the scope of this paper is to produce a review in a comprehensive manner about current cleaning processes and techniques for UF fouling in various industries. We have restricted the coverage to major UF applications in solid–liquid separation. We have also included some innovative cleaning techniques which are not yet in common practice.
A brief introduction is given on the understanding of UF fouling to know better what the problem is. A discussion of membrane cleaning, including physical, chemical, conventional and non-conventional methods, is followed by the cleaning processes, factors and optimisation. Finally, the side effects of cleaning are discussed.
Section snippets
Membrane fouling
Optimisation of membrane cleaning protocols requires in-depth understanding of the complex interactions between the foulant and the membrane. Most cleaning studies reported are based on trial-and-error methods [7], [12]. A more systematic approach is required to study the various aspects of fouling control [13]. In addition, it is important to consider the economic impact of cleaning procedures, including the costs of the cleaning process itself along with the effect of the procedures on
Definition and principle
Cleaning can be defined as “a process whereby material is relieved of a substance that is not an integral part of the material [7]”. A membrane cleaning should result in a membrane that is physically, chemically and biologically clean, and thus can provide adequate flux and separation [73]. It should do so, while also meeting the following criteria: (1) restoration of the initial flow through a pristine membrane without adversely changing its surface; (2) keeping dislodged foulants in
Membrane properties
Membranes possess mechanical strength, thermal stability and chemical resistance, which depend on the material of construction. But it is not necessarily the case that a membrane that is stable against one of the factor can perform well against all of them. The mechanical, thermal and chemical factors regarding these materials are listed in Table 4.
CA membranes are naturally hydrophilic with low binding to proteins, but have a limited range of operational pH. A lower pH can degrade the
Conclusions
The current paper presents a comprehensive review of membrane fouling and cleaning in UF applications. Firstly, membrane fouling, the reason for cleaning in the first place, is addressed in terms of its cause, forms and the major types of foulants and effective parameters. Secondly, the goals, methods, mechanisms and processes of membrane cleaning are discussed in great detail:
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Membrane cleaning is a vital step in maintaining the permeability and selectivity of the membrane. It is also necessary
Acknowledgements
Galit Tal and Xiafu Shi would like to thank their advisors Vitaly Gitis and Nicholas Hankins for their professional support and guidance. All authors express their thanks for financial support from the UK-Israeli Science Network Development Scheme, facilitating the collaboration which led to this review article.
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