The treatment of saline and briny effluents is not possible using conventional processes. The only technology that offers a complete solution is vacuum evaporation, given that reverse osmosis or electrodialysis generate a discharge effluent that must be managed. And conventional distillation entails costs that make it financially unviable.
However, there is a technology that, though the first patent dates from 1963, its use is currently beginning to emerge harnessing all the developments in membrane engineering. It is membrane distillation.
Membrane distillation consists of a thermal process in which only steam molecules can pass through the membrane, which is hydrophobic. The feed that is to be treated is in direct contact with one of the surfaces of the membrane but it does not penetrate through the pores of the membrane since the latter is hydrophobic. The driving force for the separation is steam pressure through the membrane, not the total pressure as in reverse osmosis. On increasing the temperature of the deed, the steam pressure increases and, therefore, the steam pressure gradient that is the driving force also increases.
From the commercial point of view, it is a technology that has been widely implemented for the following reasons:
- The thermal efficiency of the process is low due to the heat loss because of the conductivity of the membranes that occurs.
- Concentration and temperature polarisation effects occur that decrease the flow of permeate through the membrane.
- The wetting effect occurs, which consists of the penetration of impurities in the feed into the pores of the membrane, thus reducing the flow of permeate.
Despite these drawbacks which, as the research progresses are overcome, the technology has a series of advantages that make it competitive in ever more applications. The most important advantages of membrane distillation are:
- As in evaporation, the process is not limited by the equilibrium, such that it is possible to achieve the necessary water and discharge concentration factors. Unlike reverse osmosis, there is no equilibrium that establishes a separation limit.
- The technology generally does not need pre-treatment of the feed to lengthen the life of the membrane.
- The efficiency of the system and the good quality of the water that is produced are practically independent of the feed salt.
- 100% discharge of non-volatile solutes.
- The possibility of treating corrosive and acidic effluents, which in conventional distillation is difficult due to the materials that are required.
- Operational flexibility since they are Independent modules.
The selection of the membrane is key for the process to function well. The characteristics of the membrane directly influence the process; the most important are: the porosity, the size of the pore, the thickness of the membrane, the thermal conductivity and the composition, which is related to the resistance to chemical attack.
The characteristics of membrane distillation make it a technology that may be satisfactorily applied in such varied areas as:
- Production of pure water.
- Brine treatment.
- Elimination of dyes and treatment of wastewater from the textile industry.
- Concentration of acids and corrosive substances, as well as separation of azeotropic mixtures in the chemical industry.
- Concentration of juices and processing of milk in the food industry.
Membrane distillation is a competitive technology in a wide variety of industrial sectors given that it makes it possible to treat complex effluents. It is a technique that, together with vacuum evaporation, are among the few technologies that make it possible to treat saline and briny effluents without producing, if necessary, a discharge flow, given that the separation is not limited by the equilibrium. However, membrane distillation it still not a technology with high energy efficiency due to the heat loss resulting from the conductivity of the membrane. As such, its application is restricted to those applications in which conventional distillation or vacuum evaporation are not viable alternatives, as is the case when wishing to concentrate acids or corrosive substances.