**2. Membrane processes by nanofiltration**

The nanofiltration technique is located between ultrafiltration (UF) and reverse osmosis (OI), it is used to separate components of size close to the nanometer order.

This type of membrane does not retain non-ionized organic compounds with a molar mass of less than 200 g/mol and monovalent salts. On the other hand, nonionized organic compounds with a molar mass greater than 250 g/mol and multivalent ionized salts (calcium, magnesium, aluminum, sulfates, etc.) are retained [31].

Nanofiltration has lower electrical energy consumption, because it requires lower pressures than reverse osmosis [32].

The nanofiltration technique (**Figure 1**) is based on two separation mechanisms, separation under the effect of electrical repulsion for charged species, and separation under the effect of size for uncharged solutes [33].

**Figure 1.** *Selective membrane [15].*
