Some membrane processes such as RO, UF, MF, and electrodialysis are used commercially while others such as thermo-osmosis, pervaporation, and membrane distillation (MD) are still in the research and developmental stages. Membrane distillation is an emerging technology for separations that are normally accomplished via conventional distillation or RO [22]. As applied to desalination, MD involves the transport of water vapor from a saline solution through the pores of a hydrophobic membrane.

Membranes used in MD are commonly hydrophobic polymers with pore sizes on the order of micrometers. The large contact angle of water with the hydrophobic membrane prevents liquid water from penetrating the pores, and water vapor is transported across the membrane in response to a change in partial pressure due to a thermal gradient. Permeate flux can be as high as 120kg/(m2h) [for direct contact MD (DCMD)], comparable to RO membranes, and salt rejection is typically >99%.

The efficiency of an MD process depends highly on membrane and module design, and thermal management. Heat and mass transport across the membrane must be optimized to obtain maximum permeate flux with minimal energy loss, and heat recovery from the permeate stream is essential for optimal operation [23]. Although energy consumption is quite high, the process is typically run at relatively low temperature (~70 °C) and thus can make use of waste heat or other relatively low-grade heat sources.

Potential advantages of MD are the ability to use low-grade and inexpensive heat sources, smaller plant footprint, and lower capital costs than conventional distillation processes. Membrane fouling is a problem, but is thought to be less severe than conventional RO [24]. Membrane degradation (loss of hydrophobicity) is also known to occur, but composite hydrophilic/hydrophobic membranes may overcome this problem [25].

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