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Introduction to Membrane Technology

Mohammad Younas1 and Mashallah Rezakazemi2

1University of Engineering and Technology, Department of Chemical Engineering, Peshawar, 25120, Pakistan

2Shahrood University of Technology, Faculty of Chemical and Materials Engineering, Shahrood, Iran

1.1 Overview of Membrane Technology

Membrane technology is a general term used for a range of different separation processes. Membrane separation processes have been proven to be well-established technologies in a wide range of water, energy, food, and environmental applications throughout the production, purification, and formulation of useful products [1-4]. Thus, the membrane separation processes have become the leading separation technology over the past two decades. The membrane is defined as a selective thin layer of a semipermeable material that acts as a selective barrier and separates undesired species from a feed solution based on their sizes or affinity by exerting a potential gradient, such as pressure, temperature, electrical, or concentration difference (Figure 1.1). Separation is accomplished if one species of a mixture moves through the membrane faster than another species in the mixture. The main advantage of membrane technology, which differentiates it from traditional separation, purification, and formulation processes, is that it produces stable products without adding chemicals with a relatively low energy consumption with a remarkable potential for an environmental impact. Other benefits include modular and easy to scale-up, well-arranged, compact, and straightforward process in concept and operation, decreased capital and operational cost of technology applications using membrane, and environment friendly.

In general, membranes are classified based on their average pore size, driving force, morphology, and materials. The pore size of the membrane material or surface is a paramount factor in its first differentiation. Nevertheless, membrane materials can be organic and inorganic. All of the membrane separation processes are effective methods of treating the feed mixture, e.g. water, gas, and food that hardly is treated using conventional separation methods.

Figure 1.1 Typical membrane separation process.

1.2 Conventional Membrane Separation Processes

1.2.1 Microfiltration (MF)

Microfiltration (MF) is the first classification of membrane separation techniques based on pore size. The MF membrane was first developed to analyze the bacteria in the water. In the 1960s, the first commercial MF membrane was also developed in biological and pharmaceutical applications. Since then, MF membranes have been widely applied in wastewater treatment and juice technology to remove microorganisms, clarify cider and other juices, and sterilize beer and wine. The separation mechanism in MF membranes is governed by the sieving effect or size exclusion technique. Thus, the species are separated according to their size. Large pores of MF remove suspended solids, while even proteins can pass through the MF membrane easily. The MF membranes can also be used to separate sand, clays, algae, and some bacteria from aqueous feed streams. They are recommended to separate species with a diameter larger than 0.1 µm. The applied pressure in MF is low (usually <2 bar), while this is the lowest applied pressure in other pressure-driven membrane separation processes [5,6].

1.2.2 Ultrafiltration (UF)

Ultrafiltration (UF) is also included in size exclusion-based pressure-driven membrane separation processes. The pore size of UF membranes is around 0.01 µm. These membranes can prevent species in the molecular weight range of 300-500 000 Da to pass through. UF rejects protein and suspended solids. However, dissolved substances could not be removed by UF unless they are first pretreated in an adsorption column like with activated carbon or coagulated with alum or iron salts. Similarly, UF membranes cannot retain the mono- and disaccharides, salts, amino acids, organics, inorganic acids, or sodium hydroxide. They exhibit small osmotic pressure differentials due to their inability to reject salts, as compared with reverse osmosis (RO). UF processes operate at 2-10 bars. Separation efficiency will further be augmented if the difference in the sizes of the species is high enough. UF is considered nowadays to be the dominant part of membrane separation processes due to its diverse applications in water, energy, food, and the environment. UF processes are considered the most used membrane separation process next to dialysis and MF [7].

1.2.3 Nanofiltration (NF)

Nanofiltration (NF) is another pressure-driven membrane process between RO and UF pore size of around 0.001 µm. NF membranes remove most organic molecules, viruses, and a range of salts. These membranes are often applied to soften the hard water by removing divalent ions. NF membranes possess a negative charge on the surface. It demonstrates the anion repulsion, which mainly causes the species rejection. Low rejection is witnessed for salts with monovalent anion and nonionized organics with a molecular weight below 150. However, high rejection can be observed for salts with di- and multivalent anions and organics with a molecular weight above 300. NF is advantageous over RO in different aspects, such as being operated at low pressure, giving high permeate flux, retention of multivalent salt and organic solutes, and having low investment and operation and maintenance costs.

NF membrane is more suitable for ions with more than one negative charge in single charged ions pass, such as sulfate or phosphate. However, NF membranes also reject uncharged and positively charged ions according to the molecule's size and shape. For example, the same rejection of calcium chloride and sodium chloride can be observed while the rejection of sodium sulfate is the same for magnesium sulfate. Instead, the rejection of di- and multivalent anions is high compared with that for monovalent ions. The species rejection decreases with increasing concentration. The Donnan exclusion model can explain this phenomenon. The higher the species concentration, the more cations available to shield the negative charges on the membrane surface, making it easier for the anions to pass through the membrane pores. On the other hand, the charge density of ions also plays an important role in its rejection. For example, the sulfate ion has a higher charge density than the chloride ion and is almost completely repelled by the NF membrane even in a high ionic strength solution such as seawater [8].

1.2.4 Reverse Osmosis (RO)

RO demonstrates, in principle, the least possible pore structure among the membranes. Water is the only species that can pass through the RO membrane; essentially, all dissolved and suspended species are rejected. RO membranes have a pore size of around 0.0001 µm. The permeate is essentially the pure water because RO also removes most healthy minerals such as calcium, zinc, magnesium, etc. that are present in the water and are useful in a certain quantity for drinking water especially for people with inadequate diets and people living in hot climates. The water can be made healthy bypassing the RO water through calcium and magnesium beds. RO removes monovalent ions to desalinate the saline water. Both NF and RO are also termed as dense membrane separation processes because separation relies to some extent on physicochemical interactions between the permeate (species) and the membrane material. In wastewater treatment and reclamation, RO systems are typically used as the last step for removing total organic carbon (TOC). RO has been proven to remove dissolved species effectively, microbes, and neutral base compounds [9,10].

To understand the working principle of RO, it is helpful to understand first osmosis. Osmosis refers to the migration of water from a weaker solution to the stronger solution when a semipermeable membrane separates two salt solutions of different concentrations. The migration of salts continues until the two solutions reach the same concentrations, achieving the osmotic equilibrium. The semipermeable membrane allows the water species to pass through naturally, but not the salt. In RO, the two solutions are still separated by a semipermeable membrane, but the pressure is applied to reverse the water's natural flow. This forces the water species to move from the more concentrated solution to the weaker. Thus, the solute aggregate on one side of the semipermeable membrane and the pure water pass through the membrane on the other side. The concept of osmosis and RO is described schematically in Figure 1.2 where (a) and (b) illustrate the process of osmosis and (c) represents the RO. If a certain pressure (?P) applied to the concentrated solution equals the osmotic pressure difference between the two solutions (?p), the system reaches the osmotic equilibrium, and water flow stops. If the applied pressure exceeds osmotic pressure (?P > ?p), water flows from the concentrated solution to the dilute solution. A summary of pressure-driven processes is outlined in...