The Evolution and Importance of the Desalination Process
Frequently referred to as the blue planet, this is a reference to the fact that 71% of the Earth’s surface is covered by water. Remarkably, a mere 3% of that is fresh water, and of this, almost 70% is trapped in glaciers. Of the remainder, less than 1% is to be found in our swamps, rivers, and lakes, while the rest is hidden in underground aquifers. As the population has grown along with the number of uses for which water has now become essential, it has become increasingly difficult to meet our needs from the dwindling sources of freshwater alone and so we seem destined to become more dependent on the desalination process.
It is somewhat ironic that, for millennia, man has boiled seawater or allowed it to evaporate in the heat of the sun to extract its salt to preserve their meat, treat wounds, and flavour their cooking, discarding the unwanted water, while today we perform a similar operation to retain the water and discard the salt. Although there are references to experiments with distillation by Aristotle, it is the third president of the United States, Thomas Jefferson, who is generally regarded as the pioneer in the removal of salt from seawater. He described a simple technique that could be used by sailors as a means to obtain potable water from the sea in case of emergency.
Since this simple desalination process offered a limited yield, it has been necessary to investigate more productive options and the first to be adopted on a large scale is distillation. In this type of plant, seawater is evaporated by boiling and the resulting steam is cooled to form a condensate free of dissolved salts. While this method successfully solves the problem of limited yield, it introduces a problem of its own – cost. To heat the water sufficiently for it to evaporate requires electrical energy, adding both to the cost of production and the plant’s carbon footprint. Early plants employed multi-stage flash distillation (MSF) for the desalination process and were subsequently replaced by a more economical option known as multi-effect distillation (MED), in which each stage or effect reuses the energy from the previous stage, thus reducing energy demands.
First applied for the treatment of brackish water, a new technique employing the use of membrane technology has since been widely adopted, developed in parallel with the improvements in thermal distillation techniques. Known as reverse osmosis, or RO, for use as a desalination process, it employs a semi-permeable membrane made from cellulose acetate to filter out dissolved solids from seawater. The filtration requires the assistance of applied pressure to overcome the osmotic gradient that would otherwise form naturally between regions of high and low solute concentrations when separated by a semi-permeable membrane. Without that applied pressure, the process would only continue until the concentrations on each side of the membrane are equal.
While even in this type of desalination process, electrical energy is necessary to create the pressure, less is required than for thermal distillation. It is, therefore, not surprising that more than 60% of current plants make use of RO technology. Reverse osmosis has gained widespread appeal not just because it is efficient and reasonably economical, but because it is also highly scalable. Furthermore, while it is a great way to remove the salt from brackish or seawater, it is just as effective at removing a lot of other chemicals from solutions.
To be effective as a desalination process, or for the removal of salts from liquids in general, the use of advanced membrane technology is crucial. The membranes employed need to meet critical performance criteria such as high flow rates and high salt rejection. In addition to reliable performance, they should also have a long lifespan to ensure continuity of production as well as to limit the plant’s operating costs. These qualities are not just crucial to ensure the efficiency of the desalination process, however; they are equally important to the many industries and municipalities that employ reverse osmosis technology in the treatment of their wastewater.
Some industries employ this technology not to purify a solvent, but to concentrate the solute present. For example, to recover valuable chemicals in order to reuse them or to prepare fruit concentrates. For quality desalination and other RO processes, WaterIcon offers world-class products.