Among MD processes, variations exist as to the method by which the vapor is recovered once it has migrated through the membrane. MD systems can be configured in a number of ways, depending on the nature of the cold side of the membrane. In direct contact MD (DCMD) the membrane is in direct contact with the feed on one side and permeate on the other. While in air gap MD (AGMD) an air gap is interposed between the membrane and a cold condensation surface are perhaps most appropriate for desalination applications. Other configurations, such as vacuum MD (VMD) and sweeping gas MD (SGMD) methods, are typically used for stripping of volatile organics or dissolved gases.
The major requirement of MD membranes is that they should not be wetted by the process liquids. To avoid liquid invasion of the pores, highly hydrophobic membranes with an appropriate pore size are used. The liquid surface tension also affects wetting. Organic solutes present in an aqueous solution reduce the surface tension to the point where spontaneous membrane wetting may occur. At this point, the surface tension is called the critical surface tension at which MD is no longer possible. Therefore, aqueous solutions containing inorganic solutes or low concentrations of volatile organic compounds can be treated while solutions with surface active components cannot.
The second major consideration in membrane selection for this process is pore size and porosity. High porosities are of special interest since the area available for evaporation is directly related to flux. However, high porosities are usually associated with large pore sizes, which are undesirable as they increase the risk of membrane wetting.
MD vs Distillation
The potential advantages of MD, in comparison with conventional separation processes, are found primarily in the lower working temperature and pressure, and thus the lower energy costs and less stringent mechanical properties. In contrast to distillation, supply solutions can be separated at a temperature well below the boiling point and under atmospheric pressure. Typical supply temperatures around 30-60°C permit re-use of residual heat flows, and the use of alternative energy sources such as sun, wind and geothermic.
In comparison with traditional distillation, MD possesses typical basic advantages of membrane separation, namely simple up-scaling, simple operations, possibility for high membrane surface/volume ratios, possibility to treat flows with heat-sensitive components and/or a high suspended particle-content at atmospheric pressure and a temperature below the boiling point of the supply.
MD vs. RO
Since permeate flux in a RO process is controlled by the osmotic pressure of treated water, a high-water recovery cannot be achieved with an elevated salinity. While in MD process, the osmotic pressure is not a limiting factor, allows for a significant increase in water recovery. In addition to that, MD is less susceptible to flux limitations caused by concentration polarisation. Theoretically, MD offers 100% rejection for non-volatile dissolved substances with no limit on the feed concentration.
The main disadvantage of MD compared to RO are the relatively low permeate flux, flux reductions caused by concentration polarisation, membrane pollution, water loss due to conduction through the polymeric membrane and/or pore wetting, and the high thermal energy consumption, for which the economic costs are determined by the MD configuration and the specific application.
MD with RO
There are two ways in which MD and RO can be integrated. The first is by using the RO brine as feed to the MD, or RO permeate as feed to the MD.
Using RO brine as a feed to MD has a great potential for MD utilization. This directly addresses the upper concentration limit of RO at around 70,000 mg/L, as MD is far less influenced by salt concentration. Typically, the need for an RO-MD process to increase water recovery is for inland applications where disposal of the brine is an issue. Testing of MD on RO groundwater concentrates revealed that the concept is indeed viable, but suffers from practical issues such as scaling on MD membranes. Membrane scaling led to flux declines, but flux was easily restored using an acid clean. Scaling was found to be effectively managed by cleaning or the addition of scale inhibitor.