After 2018’s devastating events in the Western Cape, no South African should continue to harbour any doubts about the seriousness of the water crisis that is facing our country. It will be some time before 4 million Capetonians are able to forget how the city’s taps came so close to running dry. While a change in the weather may have brought the eventual reprieve, it was only in time to prevent a disaster because a technology known as reverse osmosis (RO) succeeded in filling the gap in the city’s rapidly dwindling supply.
RO is a modern-day technology that, in recent years, has been applied to a process that dates back to ancient Greece, if not earlier. At that time, it was observed that, when boiling seawater, the condensed steam no longer contained any salt and was suitable for drinking. The process of distilling seawater has persisted over the ages and was used aboard ships during the 17th century. In 1881, long before reverse osmosis and just over 50 years after the natural process upon which RO is based was discovered by French scientist René Dutrochet, the world’s first commercial desalination plant was established in the Maltese port of Sliema. It was built to provide potable water for the British soldiers garrisoned at Fort Tigné.
Seawater distillation became less urgent with the development of treatment plants that provided piped water directly to consumers. The focus shifted to applications such as the shipboard devices, making it unnecessary to carry vast reserves of potable water. Today, with the help of reverse osmosis, desalination plants have become an increasingly important component of water security in South Africa and many other countries. Unfortunately, water shortage is not the only problem we currently face. Of equal concern is the universal dependence on burning fossil fuels to provide energy which, in turn, contributes to climate change. Most commercial desalination plants still rely on boiling seawater and distilling it, which requires a lot of energy. By contrast, desalination plants that employ reverse osmosis technology consume far less. So how is this possible?
RO is a modification of a process that occurs naturally in plant cells. When the concentration of dissolved salts differs between the water inside a cell and that of the water surrounding it, a pressure gradient is produced. Its effect is to drive water from the region of lower concentration through the cell wall, to dilute the liquid with the higher concentration of salts. The process continues until the concentration in each compartment is equal. The cell wall acts as a semi-permeable membrane permitting water molecules to pass, but nothing larger. In reverse osmosis, synthetic materials such as cellulose acetate are used to create a semi-permeable membrane. In addition, external pressure must be applied to prevent the migration of water molecules from ceasing once the point of equilibrium has been reached. In this way, the dissolved salts are retained by the membrane while the clear filtrate passes through it. The big plus in the contest between distillation and RO as the technology best suited for commercial desalination plants is that the latter process consumes a lot less energy.
That said, desalination based on reverse osmosis results in a product that is still more expensive than that produced by conventional water-treatment plants. Energy is required to provide the pressure and, although the seawater is not boiled, the process is faster if it is warmed. Accordingly, research into methods to make RO desalination plants more cost-effective is a top priority and has already led to improvements in the performance of membranes, while showing promise of pre-treatments that could help to reduce energy consumption. The fact is that humanity has painted itself into a corner and, whether or not it is sufficiently cheap, reverse osmosis currently appears to be the best way to escape from it. Quite apart from the economics of desalination, and regardless of the underlying technology, it also has environmental consequences.
What do we do with the countless tonnes of residual sodium chloride and other salts that are the unwanted by-product of the process? It cannot be returned to the sea, as hypersalinity would threaten marine life. Hopefully, human ingenuity will solve this problem too. In the meantime, WaterIcon offers South Africans a range of world-class reverse-osmosis units and membranes for water softening and potable-water production.