Favorable results from studies of the membrane distillation (MD) process, revealing the broad scope of its applications and the possibility of achieving separation in cases where other methods fail, have contributed to the growing interest in the implementation of this process. Several pilot tests are being conducted on new ones (Gryta, 2003; Zakrzewska-Trznadel et al., 1999; Wirth and Cabassud, 2002; Banat and Simandl, 1998).
Membrane distillation (MD) is a process of water evaporation through a porous lyopho – bic membrane which forms a non-selective physical barrier (Tomaszewska et al., 1995; Tomaszewska, 1996). The hydrophobic membrane usually separates aqueous solutions with different temperatures and compositions. The process is driven by the vapor pressure difference resulting from the temperature difference across the membrane, mass transport taking place towards the lower-temperature flow. The principle behind the MD process is presented in Figure 7.6.
Depending on the way how vapor pressure difference as driving force and vapor condensation are provided, four different configurations of MD are currently known (Susanto, 2011): 
Figure 7.6. Direct contact membrane distillation (C: concentration, T: temperature, p: vapor pressure, Q: heat, F: flux) (modified from El-Dessouky and Ettouney, 2000; Bodzek, 1999; Strathmann, 2004).
• Sweeping gas membrane distillation (SGMD). To minimize heat loss in DCMD and mass transfer resistant in SGMD, a cold gas (inert) is used in permeate side to sweep the vapor molecules and carry to outside the membrane module for condensation. Nevertheless, operational cost will definetly increase due to external condensation system.
• Vacuum membrane distillation (VMD). In this type, permeate side is vacuumed yielding lower vapor pressure than in the feed side. Consequently, heat loss can be reduced and permeate flux can be increased (it should be noted that the applied vacuum pressure should not exceed the saturation pressure of volatile molecules).
Water desalination by MD is less economic than other methods, e. g. state-of-the-art RO membranes (Tomaszewska, 2001; Scott 1997). The situation changes where water with a high salt content is to be desalinated. In this case, a combination of RO with MD proves advantageous, allowing for high water recovery (Gryta, 2003). In order for 1 m3 of water to evaporate in an MD installation, 600-690 kWh of energy is required (Gryta, 2003; Zakrzewska-Trznadel et al, 1999; Godino et al, 1996). Using heat recovery modules, this amount can be reduced to 150-180 kWh (Zakrzewska-Trznadel et al, 1999). Due to the high energy consumption of the MD process, the cost of water produced strongly depends on the cost of the energy supplied and the temperature at which the process takes place.
Solar energy is the most interesting alternative energy for MD. Recently, Band and Jwaied (2008) reported an economic assessment of solar powered MD for potable water production in arid area. Based on the calculations, the estimated cost of potable water produced by the compact unit is 15 US$m-3, and 18 US$m-3 for the large unit (Susanto, 2011).