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6.1 Introduction
Good water and its availability are the key features contributing to habitable ecosystem, acceptable human health, and admirable socioeconomic development.
However, inflated population, poor sanitation, rapid industrialization, and limited
environmental concerns have led to the degradation of water cycle. It is estimated
that 80% of the wastewater merges into the ecosystem without any pre-treatment or
being reused (UNESCO 2017). UNDESA (2014) states that by 2050, about 70% of
the world’s population will live in cities (Barnett et al. 2005; Hoekstra et al. 2012;
Purkait and Singh 2018; Rockström et al. 2009; Singh and Purkait 2019; Yadav
et al. 2014); most of the cities lack requisite infrastructure and resources to address
wastewater management in an efficient and sustainable way. With the ever-growing
population and degraded ecosystem, ensuring safe and sufficient water supply
becomes increasingly challenging. Thereby, a considerable solution to provide less
polluting and enhanced techniques for wastewater treatment is of global importance.
Although a wide variety of techniques including adsorption, advanced oxidation,
ion exchange, and activated sludge processes have been used, membrane treatment
technology has progressively remained prominent in the wastewater industry.
Among the membrane treatment technologies, reverse osmosis (RO), forward
osmosis (FO), membrane filtration (MF), nanofiltration (NF), and ultrafiltration
(UF) provide a conservative, economical, scalable, and comparatively simpler treatment strategies (Khawaji et al. 2008; Masafu et al. 2016; Ray et al. 2018c). To the
contrary, membrane distillation has been less adopted due to its complex and formidable synthesis techniques. The process results in sequestration of the target
compound(s) based on membrane-contaminant surface interaction, usually with the
help of a porous membrane with good selectivity for the extract. Additionally, in the
non-isothermal separation technique, vapors are transported (thermally driven)
through a non-wetted hydrophobic membrane. The propulsive force of the process
is the difference of vapor pressure between the two sides of the membrane pores
accompanied by heat and mass transfer phenomena through the membrane
(Alkhudhiri et al. 2012; Lawson and Lloyd 1997; Martı́ nez-Dı́ ez and VazquezGonzalez 1999; Yadav et al. 2014).
Although membrane distillation (MD) has shown promising results in water
treatment, the evidences of membrane fouling and wetting are major concerns.
Membrane fouling in a simplistic approach can be described as the accumulation of
the foulants (dissolved materials and particles) into the pores of membrane, while
the feed stream passes through the membrane during filtration. Consequently, the
decrease in water flux at constant operating parameters indicates the phase of membrane fouling. In general, the colloidal foulants interact among each other, followed
by membrane surface interaction to form depositions. Fouling formation mechanism can be understood by examining the forces of interaction between the particles
(foulants) and the membrane surface (Gryta 2008b; Tijing et al. 2015). Correlating
to Mayer et al. (2006), membrane fouling is very evident in membrane distillation,
for a hydrophobic membrane is used to treat feed with abundance of hydrophobic
contaminants (Mayer et al. 2006). Similarly, material of construction, membrane
6 Fouling and Wetting: A Major Challenge for Membrane Distillation
6.1 Introduction
Good water and its availability are the key features contributing to habitable ecosystem, acceptable human health, and admirable socioeconomic development.
However, inflated population, poor sanitation, rapid industrialization, and limited
environmental concerns have led to the degradation of water cycle. It is estimated
that 80% of the wastewater merges into the ecosystem without any pre-treatment or
being reused (UNESCO 2017). UNDESA (2014) states that by 2050, about 70% of
the world’s population will live in cities (Barnett et al. 2005; Hoekstra et al. 2012;
Purkait and Singh 2018; Rockström et al. 2009; Singh and Purkait 2019; Yadav
et al. 2014); most of the cities lack requisite infrastructure and resources to address
wastewater management in an efficient and sustainable way. With the ever-growing
population and degraded ecosystem, ensuring safe and sufficient water supply
becomes increasingly challenging. Thereby, a considerable solution to provide less
polluting and enhanced techniques for wastewater treatment is of global importance.
Although a wide variety of techniques including adsorption, advanced oxidation,
ion exchange, and activated sludge processes have been used, membrane treatment
technology has progressively remained prominent in the wastewater industry.
Among the membrane treatment technologies, reverse osmosis (RO), forward
osmosis (FO), membrane filtration (MF), nanofiltration (NF), and ultrafiltration
(UF) provide a conservative, economical, scalable, and comparatively simpler treatment strategies (Khawaji et al. 2008; Masafu et al. 2016; Ray et al. 2018c). To the
contrary, membrane distillation has been less adopted due to its complex and formidable synthesis techniques. The process results in sequestration of the target
compound(s) based on membrane-contaminant surface interaction, usually with the
help of a porous membrane with good selectivity for the extract. Additionally, in the
non-isothermal separation technique, vapors are transported (thermally driven)
through a non-wetted hydrophobic membrane. The propulsive force of the process
is the difference of vapor pressure between the two sides of the membrane pores
accompanied by heat and mass transfer phenomena through the membrane
(Alkhudhiri et al. 2012; Lawson and Lloyd 1997; Martı́ nez-Dı́ ez and VazquezGonzalez 1999; Yadav et al. 2014).
Although membrane distillation (MD) has shown promising results in water
treatment, the evidences of membrane fouling and wetting are major concerns.
Membrane fouling in a simplistic approach can be described as the accumulation of
the foulants (dissolved materials and particles) into the pores of membrane, while
the feed stream passes through the membrane during filtration. Consequently, the
decrease in water flux at constant operating parameters indicates the phase of membrane fouling. In general, the colloidal foulants interact among each other, followed
by membrane surface interaction to form depositions. Fouling formation mechanism can be understood by examining the forces of interaction between the particles
(foulants) and the membrane surface (Gryta 2008b; Tijing et al. 2015). Correlating
to Mayer et al. (2006), membrane fouling is very evident in membrane distillation,
for a hydrophobic membrane is used to treat feed with abundance of hydrophobic
contaminants (Mayer et al. 2006). Similarly, material of construction, membrane
6 Fouling and Wetting: A Major Challenge for Membrane Distillation