Elsevier

Desalination

Volume 356, 15 January 2015, Pages 294-313
Desalination

Scaling and fouling in membrane distillation for desalination applications: A review

https://doi.org/10.1016/j.desal.2014.06.031Get rights and content

Highlights

  • Review: temperature, flow rate, SI, feed type, specific salt effects in MD fouling

  • Module type, and temperature and concentration polarization greatly affect fouling.

  • High temperature prevents MD biofouling.

  • Particulate fouling can be easily avoided by microfiltration.

  • Reducing feed pH, membrane superhydrophobicity, and antiscalants prevent MD scaling.

Abstract

Membrane distillation (MD) has become an area of rapidly increasing research and development since the 1990s, providing a potentially cost effective thermally-driven desalination technology when paired with waste heat, solar thermal or geothermal heat sources. One principal challenge for MD is scaling and fouling contamination of the membrane, which has gained growing attention in the literature recently as well. The present paper surveys the published literature on MD membrane fouling. The goal of this work is to synthesize the key fouling conditions, fouling types, harmful effects, and mitigation techniques to provide a basis for future technology development. The investigation includes physical, thermal and flow conditions that affect fouling, types of fouling, mechanisms of fouling, fouling differences by sources of water, system design, effects of operating parameters, prevention, cleaning, membrane damage, and future trends. Finally, numerical modeling of the heat and mass transfer processes has been used to calculate the saturation index at the MD membrane interface and is used to better understand and explain some of trends reported in literature.

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 (dp0.20.4μm) 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.

References (153)

  • M. Khayet

    Membranes and theoretical modeling of membrane distillation: a review

    Adv. Colloid Interface Sci.

    (2011)
  • A. Antony et al.

    Scale formation and control in high pressure membrane water treatment systems: a review

    J. Membr. Sci.

    (2011)
  • R.B. Saffarini et al.

    Technical evaluation of stand-alone solar powered membrane distillation systems

    Desalination

    (2012)
  • J. Koschikowski et al.

    Solar thermal-driven desalination plants based on membrane distillation

    Desalination

    (2003)
  • F. Banat et al.

    Desalination by a “compact SMADES” autonomous solar powered membrane distillation unit

    Desalination

    (2007)
  • F. Banat et al.

    Performance evaluation of the “large SMADES” autonomous desalination solar-driven membrane distillation plant in Aqaba, Jordan

    Desalination

    (2007)
  • E. Guillen-Burrieza et al.

    Effect of dry-out on the fouling of PVDF and PTFE membranes under conditions simulating intermittent seawater membrane distillation (SWMD)

    J. Membr. Sci.

    (2013)
  • E. Curcio et al.

    Membrane distillation operated at high seawater concentration factors: role of the membrane on CaCO3 scaling in presence of humic acid

    J. Membr. Sci.

    (2010)
  • M. Gryta

    Effect of iron oxides scaling on the MD process performance

    Desalination

    (2007)
  • M. Krivorot et al.

    Factors affecting biofilm formation and biofouling in membrane distillation of seawater

    J. Membr. Sci.

    (2011)
  • M. Gryta

    Polyphosphates used for membrane scaling inhibition during water desalination by membrane distillation

    Desalination

    (2012)
  • S. Srisurichan et al.

    Humic acid fouling in the membrane distillation process

    Desalination

    (2005)
  • M. Gryta et al.

    Membrane distillation of NaCl solution containing natural organic matter

    J. Membr. Sci.

    (2001)
  • B. Jiao et al.

    Recent advances on membrane processes for the concentration of fruit juices: a review

    J. Food Eng.

    (2004)
  • A.S. Al-Amoudi

    Factors affecting natural organic matter (NOM) and scaling fouling in NF membranes: a review

    Desalination

    (2010)
  • S. Salvador Cob et al.

    Silica and silicate precipitation as limiting factors in high-recovery reverse osmosis operations

    J. Membr. Sci.

    (2012)
  • M. Gryta

    Alkaline scaling in the membrane distillation process

    Desalination

    (2008)
  • K. Al-Anezi et al.

    Scale formation in desalination plants: effect of carbon dioxide solubility

    Desalination

    (2007)
  • J. Peña et al.

    The vaterite saturation index can be used as a proxy of the S&DSI in sea water desalination by reverse osmosis process

    Desalination

    (2010)
  • A.G. Xyla et al.

    The inhibition of calcium carbonate precipitation in aqueous media by organophosphorus compounds

    J. Colloid Interface Sci.

    (1992)
  • H. Elfil et al.

    Role of hydrate phases of calcium carbonate on the scaling phenomenon

    Desalination

    (2001)
  • M. Gryta

    The influence of magnetic water treatment on CaCO3 scale formation in membrane distillation process

    Sep. Purif. Technol.

    (2011)
  • F. He et al.

    Studies on scaling of membranes in desalination by direct contact membrane distillation: CaCO3 and mixed CaCO3/CaSO4 systems

    Chem. Eng. Sci.

    (2009)
  • T. Abraham et al.

    Socio-economic technical assessment of photovoltaic powered membrane desalination processes for India

    Desalination

    (2011)
  • R. Beck et al.

    The onset of spherulitic growth in crystallization of calcium carbonate

    J. Cryst. Growth

    (2010)
  • M.G. Lioliou et al.

    Heterogeneous nucleation and growth of calcium carbonate on calcite and quartz

    J. Colloid Interface Sci.

    (2007)
  • L.-F. Olsson

    Induction time of precipitation of calcium carbonate

    J. Mol. Liq.

    (1995)
  • O. Pokrovsky

    Precipitation of calcium and magnesium carbonates from homogeneous supersaturated solutions

    J. Cryst. Growth

    (1998)
  • M. Gryta

    Long-term performance of membrane distillation process

    J. Membr. Sci.

    (2005)
  • K. Karakulski et al.

    Water demineralisation by NF/MD integrated processes

    Desalination

    (2005)
  • L.D. Nghiem et al.

    A scaling mitigation approach during direct contact membrane distillation

    Sep. Purif. Technol.

    (2011)
  • A. Hoch et al.

    Calcite crystal growth inhibition by humic substances with emphasis on hydrophobic acids from the Florida everglades

    Geochim. Cosmochim. Acta

    (2000)
  • R. Sheikholeslami

    Mixed salts—scaling limits and propensity

    Desalination

    (2003)
  • S. Lee et al.

    Effect of operating conditions on CaSO4 scale formation mechanism in nanofiltration for water softening

    Water Res.

    (2000)
  • G. Greenberg et al.

    Limits of RO recovery imposed by calcium phosphate precipitation

    Desalination

    (2005)
  • S.P. Chesters

    Innovations in the inhibition and cleaning of reverse osmosis membrane scaling and fouling

    Desalination

    (2009)
  • A. Zach-Maor et al.

    Diagnostic analysis of RO desalting treated wastewater

    Desalination

    (2008)
  • R. Ketrane et al.

    Efficiency of five scale inhibitors on calcium carbonate precipitation from hard water: effect of temperature and concentration

    Desalination

    (2009)
  • N. Prihasto et al.

    Pre-treatment strategies for seawater desalination by reverse osmosis system

    Desalination

    (2009)
  • M. Gryta

    Desalination of thermally softened water by membrane distillation process

    Desalination

    (2010)
  • Cited by (665)

    View all citing articles on Scopus
    View full text