**4.2 Front filtration**

The flow of the feed solution is perpendicular to the membrane (**Figure 3**) [17, 28]. **Classification according to the separation mechanism** The processes responsible for membrane filtration are:


According to their separation mechanisms, the membranes are classified:


**Figure 2.** *Tangential filtration [34].* *Treatment of Wastewater by Nanofiltration DOI: http://dx.doi.org/10.5772/intechopen.94329*

• Ion-exchange membranes: consist of dense gels, it is a special type of nonporous membrane, they can be anion-exchange (with a positive charge) or cation-exchange (with a negative charge).
