**2.2 Air-gap membrane distillation process**

In the technical of air-gap membrane distillation (**Figure 1**), only the effluent to be treated is in contact with the active membrane layer. After crossing the membrane, the vapor permeates passing through the membrane and is condensed on the cold outer surface. There is an air distance between the outer surface of the membrane and the cold surface to reduce the loss of energy by thermal conduction through the membrane. The main male of the air space is that there is an additional resistance to mass transfer. In this technique of distillation, permeate is not in contact with the outer surface of the membrane, there is no fear of wetting the membrane on the permeate side in this process.
