In recent years, the increasing threat to groundwater quality due to human activities has become a matter of great concern. The groundwater quality problems present today are caused by contamination and by overexploitation, or by combination of both, which are faced by many Indian states. Today, reverse osmosis (RO) membranes are the leading technology for desalination of groundwater because of their strong separation capabilities and exhibiting a great potential for treatment of waters worldwide. However, the RO process had some problems due to the formation of polarization films because high pressure operation and by-products which may generate bacteria and fouling. Also, high energy consumption and brine disposal problem is faced in RO process due to the limited recovery of water. These problems may be overcome by other membrane thermal process such as a membrane distillation (MD). This paper addresses the outline of RO and MD process for desalination. RO has developed over the past 40 years and MD is an emerging technology for brackish water desalination and yet is not fully implemented in industry. The MD is the better alternative to RO for desalination theoretically found in the literature.

A B S T R A C TMembrane distillation (MD) is a non-isothermal separation process driven on the vapor pressure difference, induced by the temperature difference across the hydrophobic membrane. This paper offers the review of the potentability of MD process for purification application and water desalination. It covers the basic fundamental of MD process, MD modules, membrane materials, heat and mass transfer phenomena, operating parameters, and performance of MD process. It also covers the review of MD processes driven by renewable energy sources and current innovations in the process. The recent research results in these different areas are presented and discussed. The multi-effect MD process is found to be a new generation MD process and attractive research area in the wastewater treatment and purification application for the commercial approach.

Vacuum membrane distillation (VMD) process received a great deal of attention by many investigators because of its promising applications in several separation areas. It is a rising technology for seawater or brine desalination process. The process simply consists of a flat sheet hydrophobic microporous PTFE membrane and diaphragm vacuum pump without a condenser for the water recovery or trap. In this work, VMD performance was investigated for aqueous NaCl solution. In order to enhance the performance of the VMD process in desalination, that is, to get more flux, it is necessary to study the effect of operating parameters on the yield of distillate water. The influence of operational parameters such as feed flow rate, feed temperature, feed salt concentration and permeate pressure on the membrane distillation (MD) permeation flux have been investigated. The VMD performance showed that this device could reach a desalting degree of 99.99% which was not affected by feed concentration. The membrane distillation flux reached 14.62 kg/m 2 h at 333 K bulk feed temperature, 1.5 kPa permeate pressure, 54 l/h feed flow rate, and 30,000 mg/l feed concentration. With these chosen operating conditions, experiments with concentrated salt water showed a permeate flux decreases with time, but these reduction is less than 14% over a long term experimentation. However, this fouling is reversible and easily removed by a water washing. This study promotes the research attention in apply of VMD for over-concentrated salt water means rejected brines of reverse osmosis process.

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