Membranes for Water Treatment Applications - American Chemical

Chapter 10

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Membranes for Water Treatment Applications – An Overview Tilak Gullinkalaa and Isabel C. Escobar*,b aHydration

Technology Innovations, LLC, Albany, OR 97321 of Chemical and Environmental Engineering, The University of Toledo, 2801 W Bancroft St, Toledo, OH 43606, U.S.A. *E-mail: [email protected]. Phone: (419) 530-8267. Fax: (419) 530-8086

bDepartment

Water is one of the basic requirements for the survival of life on this planet. However, the total quantity of fresh water on the planet is finite while the world population and its water usage are fast increasing due to industrialization and urbanization (Kirby, A. http://news.bbc.co.uk/2/hi/science/nature/3747724.stm). Only 1% of the water available on earth can be consumed without processing, filtering or melting polar ice caps. The solution to this problem can be achieved by water conservation and guiding research and technology towards sustainable water purification. Although seventy percent of earth’s surface is covered with water, ninety seven percent of this water is contained in oceans, making it unsuitable for drinking or any other application due to its high salt content. Of the remaining three percent of fresh water, only 0.3% is found in rivers and lakes and or remains frozen. These numbers clearly indicate the necessity for exploring the waters from other than fresh water sources, i.e. ocean waters and used waters to elude the impending global water crisis. The refinement of these waters through various techniques is a requirement for the survival of life on this planet.

© 2011 American Chemical Society In Modern Applications in Membrane Science and Technology; Escobar, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


1. Water Treatment Water treatment can be defined as the practice of subjecting water to an agent or process with the objective of increasing its quality to meet the specific requirements for different applications, such as human consumption, industrial utilization, domestic operations and irrigation. Water purification can be achieved by thermally-driven process such as distillation, membrane assisted pressure-driven processes such as ultrafiltration (UF), microfiltration (MF) and reverse osmosis (RO), electrically-driven process such as electrodeionization (EDI) and other methods such as activated carbon adsorption and ion exchange, in which appropriate agents are used to adsorb specific impurities present in the water. Of all these processes, membrane-based operations are straightforward, cost effective and versatile (2).

1.1. Membrane Separations Membrane technologies are widely used in separation processes, such as water purification, protein separation, metal recovery and pigment recovery, enabling them to play a pivotal role in major industries, such as food and beverage, biotechnology, chemical and pharmaceuticals and municipal water treatment. A membrane can be defined as a very thin layer or cluster of layers that allows selective components to pass through when mixtures of different kinds of components are driven to its surface. Membranes are considered symmetric and homogeneous if they are made of single layer, or asymmetric and non-homogeneous if they consist of more than one layer (3). The flux of the asymmetric membrane is higher than that of symmetric homogeneous membrane because of the thinness of the dense selective layer. This feature of the asymmetric membranes makes them largely applicable in water purification industry so that higher production rates can be achieved. An integrally-skinned asymmetric membrane was first successfully developed by Loeb and Sourirajan (20), which led to the initial breakthrough in the use of membranes and membrane-assisted separation processes. Three general modes of construction are widely observed in polymeric membrane structures. They are homogeneous, asymmetric, and composite. Typical illustrations are shown in Figure 1. Homogeneous membranes constitute of single polymer material and uniform pore size throughout the membrane. In contrast to symmetric membranes, asymmetric membranes usually have a very thin skin layer that determines the membrane selectivity and a relatively thick porous supporting layer (21). In a composite membrane, the selective layer is made of a different polymer. Asymmetric and composite membranes can have better performance than the symmetric membranes because the selective layer in an asymmetric membrane is thinner, thereby reducing the membrane resistance relative to a symmetric membrane of similar retentive capability. These membranes consist of a thin, “dense” polymer layer supported by a thick, mechanically strong polymeric substructure. The dense layer, being close-knit in physical structure, provides maximum resistance to the flow through, so it is responsible for the membrane 156 In Modern Applications in Membrane Science and Technology; Escobar, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

selectivity. Thus, the dense selective layer provides the filtration properties, while the porous support layer provides mechanical strength to the membrane.


1.1.1. Membrane Materials Various polymeric materials are used for membrane synthesis. Polymers such as cellulose acetate (CA), cellulose diacetate, cellulose triacetate, polyamide (PA), sulfonated polysulfone and aromatic polyamide are widely used as synthesizing material for water treatment membranes. Combination of these materials is used in casting of the thin film composite (TFC) membranes. The choice of the material makes an enormous difference to the membrane performance (3), as the material plays a crucial role in interacting with feed solutions. The material determines various membrane properties, such as hydrophilicity, surface charge, chlorine tolerance limit and allowable pH range. The degree of hydrophilicity is higher for cellulosic materials and some of its ester derivatives such as cellulose acetate. Polyethylene and polypropylene are very hydrophobic in nature. Various polymers with intermediate hydrophilicity, such as the polysulfone (PS)/polyether sulfone (PES) family, polyacrylonitrile (PAN) and polyvinylidene fluoride (PVDF), are also used as selective layers for the membranes (5). Cellulose acetate-based membranes are able to tolerate moderate amounts of chlorine (0.3-1.0 mg/L), but are vulnerable to biological attack, hydrolysis, and chemical reaction with feed waters to form cellulose and acetic acid (3). Linear polyamide membranes are not tolerant to chlorine, and thus, chlorine levels must be kept below 0.05 mg/L. Linear polyamide membranes typically have allowable pH ranges between 4 and 11 (22). 1.2. Modes of Pressure-Driven Separations Water purification is a rigorous process, which requires removal of a large number of impurities of varying size, shape and solubility depending on the nature of the water source. Membranes of varying pore size distributions and molecular weight cutoffs (MWCO) are used for this purpose. MWCO is defined as the molecular weight in Daltons of the lowest molecular weight solute that is 90% retained by the membrane. Based on these distributions and type of impurities removed, water filtration processes can be classified into particle filtration, microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO). A typical pore size distribution and rejection characteristics of the membranes used for these filtration processes are shown in Figure 2. Table 1 summarizes the pore size characteristics of membranes and different application of membranes. These various filtration techniques fall into two broad categories. In the first category, RO and NF are used to remove dissolved components from water or wastewater feeds. RO is dominated by diffusion of matter and NF is characterized by both diffusion through polymer network and convection through membrane pore network. In the second, MF and UF are used for removal of fine particulates as these processes are mainly dominated by feed solution convection through membrane pore network. 157 In Modern Applications in Membrane Science and Technology; Escobar, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


Figure 1. (A): Homogeneous membrane construction; (B): Asymmetric membrane construction; (C): Thin film composite membrane construction.

Figure 2. Rejection characteristics of membrane filtration. 158 In Modern Applications in Membrane Science and Technology; Escobar, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


Table 1. Membrane pore size characteristics and application Membrane Type

Average Pore Size (nm)

Application

MF

200

Suspended solids, turbidity, and pathogens

UF

2-50

Macromolecules, viruses, colloids, and protein

NF

Membranes for Water Treatment Applications - American Chemical

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