**3. Principle of membrane distillation**

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forward osmosis membranes [10].

**2. Principle of forward osmosis**

protein and peptide modifications. This type of PEG derivatives is commonly used as acylating agents to modify amino groups of lysine residues and also has some reactivity with histidine imidazole and tyrosine hydroxyl groups. For example, PEG succinimidyl succinate (PEG-SS) produced by reacting mPEG with succinic anhydride followed by carboxylic acid activation to form succinimidyl ester is an NHS ester that has been successfully coupled to the enzyme asparaginase [4, 9]. Also, PEG derivatives such as trichlorophenyl carbonate and carbonylimidazole were synthesized by reacting mPEG hydroxyl group with chloroformates or carbonylimidazole. Alkylating reagents derived from mPEG include PEG tresylate and PEG dichlorotriazine. Other examples of monofunctional PEG derivatives that are specific to sulfhydryl groups include PEG-maleimide, PEG-vinylsulfone, and PEGiodoacetamide [10]. In addition to protein modifications, monofunctional PEG derivatives were also used in osmosis membrane-related applications. For instance, PEG conjugated to fatty acid and PEG monolaurate was used as draw solutes to test

According to the abovementioned chemistry and properties of PEG, it has been widely used in many different areas such as biomedical, biotechnology, and membrane technology-based applications. The main focus of this chapter is to discuss the usage of PEG, its derivatives, and copolymers (**Figure 1**) in emerging membrane technologies, such as forward osmosis and membrane distillation, as their

Forward osmosis is an emerging technology using a semipermeable ultrathin membrane to treat water or wastewater, and the membrane is typically thinner than RO membranes. Similar to the structure of thin film RO membranes, an FO membrane typically consists of an active layer and a support layer. In a commercially available thin film composite (TFC) FO membrane, the active layer is polyamide (PA), and the support layer is mainly made of polysulfone (PSf) or polyethersulfone (PES). An FO system generally consists of an FO module/cell for holding FO membranes, a draw solute recovery unit and pumps for circulating feed and draw solution (**Figure 2**). The FO module/cell is classified into flat sheet, hollow fiber, and spiral wound configurations, mainly depending on the operating scale. Although FO takes some advantages of an osmotically driven process, such as less membrane fouling, low energy requirement and operating cost, over

applications relate to wastewater treatment and desalination.

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**Figure 2.**

*Schematic of a forward osmosis system with draw solute recovery.*

Membrane distillation (MD) is another emerging technology in membrane separation, which uses a hydrophobic, microporous membrane to treat water or wastewater through a thermally driven separation process. In an MD process, the

**Figure 3.** *Configurations of membrane distillation. (a) DCMD. (b) AGMD. (c) SGMD. (d) VMD.*

feed solution is usually heated up, and the produced vapor passes through the pores of the membrane and condenses on the distillate side by cooling. MD is considered cost-effective and promising because it can achieve almost 100% dissolved solid rejection specifically in desalination. Similar to an FO module, an MD module can also be constructed as flat sheet, hollow fiber, and spiral wound forms. According to different configurations of the distillate side [14], MD can be classified into direct-contact MD (DCMD), air-gap MD, sweeping-gas MD (SGMD), and vacuum MD (VMD) shown in **Figure 3**. Conventional microporous membrane with pore size (0.1–1 μm) can be used for MD, and there also are some membranes specifically designed for MD. PEG and its derivatives can be used to improve the hydrophilicity of the membrane surface facing the feed solution during the fabrication of some specific MD membranes [15, 16].
