**3.1 Conventional thin-film composite membrane structure**

The RO membrane which is used widely today are composed a semipermeable thin film (0.2 um), made of either CA or PA, supported by a 0.025- to 0.050-mm microporous layer that in turn is cast on a layer of reinforcing fabric (**Figure 7**). Maintaining and reinforce the membrane structural integrity and durability is the main functions of the two support layers underneath the thin film [31].

In the dense semipermeable polymer film that is made up from a random molecular structure (matrix), there is no any pores. Water molecules are transported through the membrane film by diffusion and travel on a multidimensional curvilinear path within the randomly structured molecular polymer film matrix [12, 31].

#### **3.2 Thin-film nanocomposite membrane structure**

Thin-film nanocomposite (TFC) consisting from two main structure; inorganic nanoparticles in traditional membrane polymeric film structure (**Figure 8**) and highly structured porous film consisting of a densely packed array of nanotubes (**Figure 9**). In **Figure 8**, part A shows the thin film of a conventional PA membrane that supported by the polysulfone support layer. Part B shows the same type of membrane with embedded nanoparticles.

In nanocomposite membrane the specific water permeability, at comparable salt rejection, is higher than the conventional RO membrane. In addition, the fouling rates in TFC membrane, at the same operation conditions, is lower in comparison to conventional TFC RO membrane. In other words, in case of production of tubular membranes with completely uniform size, theoretically the membrane could produce up to 20 times more water per unit surface area than the common RO membrane commercially available on the market today.

**Figure 7.** *Structure of a typical reverse osmosis RO membrane with ultrathin PA film.*

**Figure 8.** *Polyamide reverse osmosis RO membrane with nanoparticles.*

**Figure 9.** *The RO membrane with carbon nanotubes [32].*

## **3.3 Cellulose acetate CA membrane**

For the first time in the late 1950s the thin semipermeable film as the first membrane element from cellulose acetate (CA) polymer was made at the University of California, Los Angeles [33]. Although the CA membrane is similar to the aromatic polyamide (PA), but, because of the existence of the top two layers (the ultrathin film and the microporous polymeric support) in the main structure of the CA that are made of different forms of the same CA polymer, the CA is different from PA [34]. In PA membrane unlike the CA these two layers consist of two completely different polymers, the polyamide and polysulfone form the semipermeable films and microporous supports, respectively. In CA membrane similar to PA membrane, thickness of the film layer is typically about 0.2 μm, but the thickness of the entire membrane in CA membrane is different (about 100 μm) from the PA membrane (about 160 μm) [35].

One of the important advantages of CA membrane is its surface very little charge, which is considered practically uncharged, while in PA membrane, because of negative charge in the surface of the membrane, with use of cationic polymers for water pretreatment, the potential for fouling increases dramatically. Furthermore, due to the smoother surface in CA membrane than the PA membrane, the CA membrane less clogged [34].

Some disadvantages of the CA membrane are; low operation temperatures 35°C (95°F) and narrow pH working range (4–6). Operation outside of this pH range can cause hydrolysis of the membrane, also, exposure to temperatures above 40°C (104°F) causes membrane compaction and failure [33]. Due to these limitations, the pH in feed water interring to the CA membrane has to be reduced and maintain between 5 and 5.5, which, in order to normal plant operation, the use of acid increases. in addition, the requires reverse osmosis RO permeate adjustment by addition of a base (typically sodium hydroxide) to achieve adequate boron rejection [36].

Since CA membrane has a higher density than PA membrane, it creates a higher head loss and has to be operated at higher feed pressures, which results in increase in energy consumption. Despite their disadvantages, due to their high tolerance to oxidants (chlorine, peroxide, etc.) than the PA membrane, CA membrane is used in municipal applications for ultrapure water production in pharmaceutical and semiconductor industries and for saline waters with very high fouling potential (mainly in the Middle East and Japan).

### **3.4 Aromatic polyamide membrane**

The aromatic polyamide (PA) membrane widely used in RO membrane structure and production of potable and industrial water at today. The thin polyamide film of this type of semipermeable membrane is formed on the surface of the microporous polysulfone support layer. For production of PA membrane uses the interfacial polymerization of monomers containing polyamine and then immersion of it in the solvent containing a reactant to form a highly cross-linked thin film. Because of some properties such as lower working pressure, lower salt passage than CA membrane and higher productivity (specific flux), the PA membranes have wider application at today [37, 38].

By changing pH, the surface charge of PA and CA membrane is also changes. For example, CA membrane has a neutral charge while, PA membrane in pH greater than 5 has a negative charge, and for this reason, co-ion repulsion amplified and therefore salt rejection is higher than CA membrane. However, when pH is lower


#### **Table 4.**

*Comparison between polyamide PA membrane and cellulose acetate CA membrane [37].*

than 4, the charge of the PA membrane changes to positive and rejection reduces significantly to lower than the CA membrane [38]. One another of the most important advantage of the PA membrane is much wider operation pH range (2–12). This allows easier maintenance and cleaning. Furthermore, the PA membrane has resistant to biodegradation and have a longer useful life (5–7 years) compare to usually membrane (3–5 years). From Aromatic polyamide membrane is used in order to production of membrane elements for nanofiltration, seawater desalination and brackish water [33, 37].
