Scaling and fouling in membrane distillation for desalination applications: A review
Introduction
Membrane distillation (MD) is a promising thermally driven desalination technology still in its infancy in terms of development and commercial deployment [1], [2]. The technology purifies water using a hydrophobic membrane, which is permeable to water vapor but which repels liquid water. In seawater desalination applications of MD, as hot saline feed solution flows over the membrane, the increased water vapor pressure from the higher temperature drives vapor through the pores () of the hydrophobic membrane, where it is collected on the permeate side [3]. MD possesses unique advantages over other desalination technologies, including pressure-driven methods such as reverse osmosis (RO) and thermally-driven methods such flash distillation. MD is free of the specialized requirements of high-pressure RO systems, which includes heavy gauge piping, complex pumps, and maintenance demands [1]. Since MD is not a pressure driven process and only vapor is allowed to cross through the membrane, MD is more fouling resistant than RO [4] and has a potential 100% rejection of ions and macromolecules. MD can be run at lower temperatures than other thermal systems making untapped sources of waste heat usable, it requires significantly fewer parts, and can have a much smaller footprint as result of reduced vapor space [3]. Additionally, recent theoretical and computational work claims potential multistage DCMD configurations with efficiencies greater than that of other thermal technologies [5], [6], [7], assuming very large available heat exchanger areas. In practice, GOR values of practical state of the art MD systems with limited exchange areas are more modest [8]. Summers [9] has subsequently shown that multi-stage vacuum MD is thermodynamically identical to MSF, indicating that equivalent energy efficiencies can be achieved. The comparative simplicity makes MD more competitive for small-scale applications such as solar-driven systems for remote areas, especially in the developing world [3], [10], [11], [12]. However, significant advancements are needed in membrane technology for MD to reach the theoretical cost competitiveness and develop market share growth [13]. Fouling in MD is of particular importance, as fouling increases costs of energy consumption, downtime, cleaning, required membrane area, required membrane replacement, and creates problems with product water contamination from pore wetting [14], [15].
The first patents on MD were granted in the late 1960s, but it wasn't technologically feasible until ultrafiltration membranes in the 1980s enabled sufficiently high trans-membrane fluxes [3]. Currently, most MD work is done in the laboratory, although a number of test beds across the world for small-scale solar thermal MD have already been deployed, and a few other projects exist [3], [11], [16].
While increased research interest in MD is relatively recent [17], scaling under high temperature conditions has been a key problem in systems with water heating since the advent of the steam engine. Research in the area, especially for metal heat exchangers, originated well before 1900 [18]. However, with respect to thermal efficiency, these studies mainly focus on conductive resistance due to scale formation, and often do not address the type of transport phenomena that are important in the context of fluid–membrane systems [18]. A somewhat more relevant area of scaling research is that for RO. However, RO membranes are not specifically hydrophobic, are virtually non-porous, are comprised of different materials, and operate at much lower temperatures but much higher pressures. Hence, RO membranes exhibit significantly different fouling characteristics than MD membranes [3], [14], [18], [19].
Studies focused on scaling in MD largely originated in the 1990s, and since then have become more numerous [13], [17]. Between 1991 and 2011, sixteen solar-driven MD systems were tested at the pilot or semi-pilot scale [20]. Limited fouling data from those plants constitute most of what we know about the fouling potential of MD membranes and the damage they may sustain under actual field operation conditions. Parallel to those pilot studies, a number of dedicated lab-scale studies were also conducted to understand fouling in MD. For many years, it was believed that the hydrophobic nature of the membrane, the maximum pore size and the low feed pressure in the MD process are sufficient to prevent the feed solution from penetrating the membrane pores (often referred to as pore wetting), and from causing significant scaling on its surface. For example, in 2003, Koschikowski et al. [21] stated that “the membranes used in MD are tested against fouling and scaling. Chemical feed water pre-treatment is not necessary. Intermittent operation of the module is possible. Contrary to RO, there is no danger of membrane damage if the membrane falls dry.” Indeed, for years it was widely accepted that MD has this described ability to withstand dryout from intermittent operation. In fact, this is how most solar-powered MD plants operated, intermittently (shutting down overnight) and allowing the membranes to fall dry for hours every day [21], [22], [23], [24]. Intermittent operation can also result from unstable solar conditions or an uneven distribution of flux [24]. In contrast, the present review shows that while MD membranes are relatively resistant to fouling, they remains vulnerable to it and often require well engineered designs and operating methods to avoid and mitigate damage or destruction of the membranes by fouling. These design choices, especially in the case of inorganic scaling, are often related to maintaining the concentration of ions and the temperature at the membrane interface within limits where crystallization is not favored. Understanding temperature and concentration polarization effects (relative reduction in temperature and increase in solute concentration at the membrane interface compared to the feed bulk, due to the removal of energy and water mass through the membrane) therefore becomes key. Section 5.7 considers these factors in further detail while interpreting scaling data available in the literature.
Importantly, current MD membranes are adapted from microfiltration and similar markets, as yet there are no commercially available membranes specifically made for MD desalination [17]. An aim of this paper is to summarize differences in membrane properties for desalination from the literature so as to provide a background for the development of future, specialized membranes. The paper also aims to better understand fouling mitigation methods, and the effects and risks of different foulants.
Section snippets
Types of fouling in MD
Fouling is commonly defined as the accumulation of unwanted material on solid surfaces with an associated detriment of function. The types of fouling that can occur in membrane systems and therefore potentially found in MD systems can be divided into four categories: inorganic salt scaling or precipitation fouling, particulate fouling, biological fouling, and chemical membrane degradation [25], [26], [27], [28]. The appropriate mitigation methods vary dramatically for each of these [29], [30].
Temperature
Temperature is among the most dominant factors related to scaling and fouling of MD membranes. In particular, the solubility and crystal formation of salts vary widely over the temperature range relevant to the MD systems. Importantly, the solubility of individual salts may be positively or negatively correlated with temperature. For example, the solubility of sodium chloride increases with temperature, whereas those of calcium carbonate, magnesium hydroxide, and calcium phosphate decrease with
Wetting and permeate water quality change
An important requirement for the MD process to perform well is that the membranes have to remain hydrophobic, thus allowing only vapor and not liquid water to pass through. Wetting refers to the process whereby the membrane starts allowing liquid water to flow into the membrane pores. While wetting can be caused by the pressure in the feed channel exceeding the liquid entry pressure (LEP), fouling induced wetting is the concern for real MD systems. Hydrophobicity of the membrane material is the
Fouling mitigation in MD
The main scaling prevention tools employed in MD are feed pretreatment and chemical cleaning [1], [58]. Other fouling prevention methods attempted include increasing the feed flow rate, hydraulic cleaning, reducing surface roughness, changing the hydrophobicity of the membrane, magnetic water treatment, and changing surface charges on the membrane [16], [125]. The effects of the filtration and antiscalants have been studied in MD, as well as less commonly used technologies like feed heating or
Trends in scaling in MD
Future developments in MD technology and industrial applications will create new issues and areas needing investigation for MD fouling. Multi-stage designs of MD promise much better efficiency, with some theoretical studies claiming lower energy consumption that the existing state-of-the-art thermal technologies, such as MSF and MED [7]. Fouling in staged systems and for energy recovery devices that recirculate fresh feed into later stages needs further research [26], [84]. As MD is developed
Conclusion
Scaling and fouling in MD are found to be pervasive, but design and mitigation methods have proven effective at making MD technology resistant to scaling and fouling. Four principal types of fouling and membrane damage have been found in MD: inorganic salt scaling or precipitation fouling, biofouling, particulate fouling, and chemical degradation. Inorganic scaling risk, the primary focus of academic studies, varies greatly with the salts present. Alkaline salts such as CaCO3, the most common
Acknowledgments
This work was funded by the Cooperative Agreement between the Masdar Institute of Science and Technology, Abu Dhabi, UAE and the Massachusetts Institute of Technology (MIT), Cambridge, MA, USA, Reference no. 02/MI/MI/CP/11/07633/GEN/G/00.
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