Biofouling in reverse osmosis membranes for seawater desalination: Phenomena and prevention
Highlights
► Comprehensive review of RO membrane biofouling. ► Description of the phenomenon. ► Effects on membrane processes. ► Control and prevention strategies. ► Conclusion with some recommendations.
Introduction
Water is the backbone of the global economy, with sustainable high-quality supplies being vital for agriculture, industry, recreation, energy production, and domestic consumption [1]. In the past few decades, clean water supplies have become a lot more critical due to excessive use and increasing contamination of natural water sources. Moreover, the demand for drinking water in the world is increasing and regulations on drinking water quality have become a lot more stringent [2]. Improving the effectiveness and efficiency of water purification technology, to produce clean water and protect the environment in a sustainable manner, is considered by many as perhaps the main challenge of the 21st century [3]. Therefore, intensive efforts are underway throughout the world to avert this looming crisis with conservation of the existing limited fresh water supply and conversion of the abundantly available seawater through various desalting technologies.
In recent years, reverse osmosis (RO) has become a critical technology, which promises to greatly increase the supply of clean water through the purification of nontraditional water sources such as brackish, sea, and wastewater [4]. It is a process that is inherently simple to design and operate compared with many traditional separation processes such as distillation, extraction, ion exchange, and adsorption. Thus, RO is considered as the simplest and most efficient technique for seawater desalination purposes [5]. It is reported that membrane-based desalination accounts for about 44% of the installed capacity of water desalination in the world [6].
Reverse osmosis is a pressure-driven membrane-based process, where the membrane (almost always polymers) acts as the heart of the process in separating the undesired constituents from a feed to obtain the desired pure product. An RO membrane acts as a semi-permeable barrier that allows selective transport of a particular species (solvent, usually water) while partially or completely blocking other species (solutes, such as salt). The separation characteristics depend upon the properties of the membrane which in turn depend on the chemical structure of the membrane material. The fast growing application of reverse osmosis processes in sea and brackish water desalination, and wastewater purification is attributed to the development of more sustainable membrane technologies that have lowered the cost of membrane modules and produced higher quality filtrate [7].
Membrane-based RO desalination, like other desalination technologies, is also not free from some serious concerns. A major problem related to RO applications in desalination is membrane fouling that negatively affects the performance efficiency in RO plants. Fouling is caused by solute adsorbing irreversibly or reversibly onto the surface of the membrane or within the pores of the membrane [8], [9]. It usually causes serious decline in the flux and quality of the permeate, ultimately resulting in an increase in the operating pressure with time [10]. Although the term fouling can be used to describe both reversible and irreversible solute adsorption, it is the irreversible portion that is most problematic. Irreversible adsorption produces a long-term flux decline that cannot be fully recovered by hydraulically cleaning the membrane [11].
One of the major goals of membrane research and the desalination industry has been to enhance, or at least maintain, water flux without sacrificing salt rejection over long periods in order to increase efficiency and reduce the cost of operation. Nevertheless, the decrease in performance efficiency of RO membranes due to fouling remains as one of the major concerns to the desalination industry worldwide [12].
Fouling requires frequent chemical cleaning and ultimately shortens membrane life, thus imposing a large economic burden on RO membrane plant operation (up to 50% of the total costs) [13]. The major types of fouling in RO membranes are [14]
- 1.
Crystalline: deposition of inorganic material precipitating on a surface
- 2.
Organic: deposition of organic substances (e.g. oil, proteins, humic substances)
- 3.
Particulate and colloidal: deposition of clay, silt, particulate humic substances, debris, silica
- 4.
Microbiological: biofouling, adhesion and accumulation of micro-organisms, forming biofilms [15].
While the first three types of fouling can be reduced to a great extent through pretreatment, biofouling cannot be reduced by pretreatment alone [16], because deposited microbial cells can grow, multiply and relocate. Even if 99.99% of all bacteria are eliminated by pre-treatment (e.g. microfiltration or biocide application), a few surviving cells will enter the system, adhere to surfaces, and multiply at the expense of biodegradable substances dissolved in the bulk aqueous phase. Therefore, membrane biofouling has been found to occur extensively on RO membranes even after significant pretreatment of the influent stream and the addition of disinfectants such as chlorine [17].
Many seawater desalination facilities have been affected by membrane biofouling, including the large desalination plants at Ras Abu Jarjur, Bahrain [18] and at Saint Croix, US Virgin Islands [19]. Among 70 US reverse osmosis membrane installations surveyed by Paul, 58 reported having “above average” problems with membrane fouling, with biofouling representing the most common operational problem experienced [20]. In the Middle East, the region which produces the largest amount of desalted water in the world, about 70% of the seawater RO membrane installations suffer from biofouling problems [21].
This review aims to summarize the globally important topic of membrane biofouling that has great relevance for the large-scale application of RO for water purification. First, the phenomenon of biofouling is introduced with an emphasis on biofilms; constituents, crucial events and factors controlling them. This is followed by a section on the impact of biofouling on membrane performance esp. permeate water flux and salt rejection. The mechanisms of decline are discussed in some detail. The final section deals with the different strategies being considered for the prevention and control of this troublesome phenomenon. A brief conclusion is presented at the end of the paper that summarizes the above content.
Section snippets
Biofilms
Biofouling is referred to as the unwanted deposition and growth of biofilms. A biofilm is an assemblage of surface-associated microbial cells that is irreversibly associated (not removed by gentle rinsing) with a surface and enclosed in a matrix of extracellular polymeric substances. The formation of biofilms may occur on a wide variety of surfaces including living tissues, indwelling medical devices, industrial or potable water system piping, or natural aquatic systems [22].
Micro-organisms are
Effects on membrane processes
On a separation membrane, the biofilm matrix is a secondary membrane that participates dominantly in the separation process (Fig. 5). The gel-like structure of the EPS matrix reduces the efficiency of convectional transport processes and causes a transmembrane pressure (TMP) drop that results in flux decline. Moreover, the rough, viscoelastic surface of the biofilm increases fluid frictional resistance and causes a feed–brine–pressure (FBP) drop.
Ridgway [52], [53], after collecting several case
Prevention and control
Control of membrane biofouling is necessary not only during continuous plant operation, but also during extended periods of plant inactivity due to system repair or modifications. Biofouling must also be controlled when newly manufactured membrane modules are packaged and stored for long periods prior to shipping or installation. The figure below (Fig. 17) shows schematically the potential points of intervention in the membrane biofouling process.
There are two strategies that are usually
Conclusions
Biofouling is a universal phenomenon occurring in a wide variety of situations and is inevitable wherever a non-biological surface e.g. membrane comes into contact with a fluid medium such as seawater. Micro-organisms are present everywhere and so the nutrients required for their growth, multiplication and ultimate formation of biofilms that represent a very stable configuration of micro-organisms embedded and entrenched in a dense EPS matrix that is difficult to dislodge or dismantle.
Reverse
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