The Specialised Membranes Used for More Sustainable Water Treatment

Of the vast global stock of water contained in our oceans, lakes and rivers only about 2,5%* is free of dissolved salts and suitable for most human purposes. In practice, however, the bulk of this in the form of frozen snow and ice at the planet’s poles or contained in deep, inaccessible aquifers, leaving a mere 0,007%* directly available for use. Unfortunately, much of the remainder is polluted as a result of manufacturing, agriculture, mining, and oil and gas exploration. Moreover, the demands of a burgeoning population have meant that the natural cycle of evaporation, condensation, precipitation, transpiration, and runoff no longer satisfy them. Consequently, reclamation technologies like membrane filtration are now essential to ensure adequate reserves of clean water.

Furthermore, the global distribution or reserves is far from even. Certain areas, such as the deserts of North Africa and the Middle East, are seriously endangered by the lack of rainfall, while South Africa is officially designated a semi-arid region and also no stranger to droughts. In an effort to ensure long-term survival, along with countries such as Saudi Arabia, Dubai, and Israel, we will need better ways to supplement the current methods of reclamation with newer technologies such as desalination. Like several other purification processes, desalination depends predominately on the use of specialised water membranes.

The extraction of potable supplies from seawater is certainly not a new idea. In fact, it was first referred to in a work by Aristotle who recorded that, upon turning to vapour, it became sweet and, upon condensation, no longer contained salt. Nevertheless, it was not until the 1930s that the distillation of seawater was undertaken on a large scale but, by 2015, there were more than 18 000 desalination plants in operation worldwide and around 300 million people relying on their output. However, the energy required for thermal distillation makes it costly. Instead, the use of porous membranes capable of extracting drinking water from the sea and from brackish sources has made desalination plants both a more economical and a more sustainable option.

These porous structures can be prepared from a variety of materials, each differing in its porosity and chemical behaviour. While some materials are best suited for the removal of progressively smaller, undissolved contents by means of microfiltration, ultrafiltration, and nanofiltration, other membranes are designed to separate the dissolved content from water and other liquids by the process known as reverse osmosis, or RO. Although the process requires the application of hydrostatic pressure sufficient to reverse the normal movement of fluid due to osmosis, the overall energy requirement is still far less than that required for processes that rely on thermal distillation. In addition, while the latter results in a residue of concentrated brine that can have long-term ecological consequences when returned to the sea, reverse osmosis can be applied repeatedly to produce a product of far greater purity with no brine residue.

In practice, the purification of water by means of semi-permeable membranes and reverse osmosis is not limited to desalination plants. Wherever there may be a need to remove small molecules and ions from a solution, RO is one of the most efficient methods for this purpose. The food industry is a classic example of the widespread application of this technology, not only to remove unwanted solutes and particles, but also to create concentrates from dilute solutions without the need to heat them and risk denaturation.

Typically composed of cellulose compounds such as its acetates, because either purified water or concentrated solute may be retained by the use of these membranes, they are also useful for the recovery of costly chemical content, such as precious metals, for reuse. In other cases, for example, in a manufacturing plant or on a mine, the effluent from an essential process may contain chemicals that could contaminate groundwater sources if discharged untreated into the local environment. However, by applying a suitable modification of this versatile RO technology, the liquid waste can be rendered free of harmful contaminants prior to disposal. Incidentally, what works for liquids, can also work for gaseous materials. For example, membranes similar to those used for water treatment are routinely used to separate carbon dioxide from natural gas.

Whether for filtration or reverse osmosis, the established industry leader, WaterIcon, offers expert advice and world-class products to companies in South Africa.

* https://www.sciencedirect.com/science/article/pii/S2214993715300105

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