Reverse Osmosis – Plant-Derived Filtration

An adequate intake of water is every bit as important for sustaining the life of plants as it is in the case of animals and humans. They have no hearts to pump liquid through their structure and must rely, instead, on capillary action to extract water present in the soil via their stems, further boosted by the effects of evaporation that takes place from the surface of their leaves. Once inside the plant, water must enter the cells to provide rigidity and nutrition. It is the reverse of the process, known in plants as osmosis, that has been adopted by humans as a powerful form of filtration.

To understand how this filtration process works, it is necessary to take a closer look at its origin in the plant kingdom. The water within a plant contains dissolved solids and their concentration may differ between that inside the cells and that in the external space that surrounds them. To ensure that the water is distributed evenly between the two, it must be able to move in or out of the cells as required. Unlike the reverse osmosis (RO) process used in domestic, commercial, and industrial applications, the natural process is designed to achieve an equal distribution of solids and liquid rather than to separate them. In practice, each solid in solution exerts an osmotic pressure and the higher its concentration the greater will be that pressure. If the concentration of dissolved solids on either side of the plant’s cell wall differ, water will move through the cell wall from the area of lower concentration to that of the higher concentration until both are equal and there is no longer a pressure difference. By contrast, in reverse osmosis, external pressure is applied to ensure that the migration of water proceeds continuously in the opposite direction until the liquid and solid phases have become completely separated.

At the heart of this process is a semi-permeable membrane. Made from thin-film composites or synthetic polymers such as cellulose triacetate, they represent man’s attempt to reproduce the physical properties of the plant’s cell wall and are an indispensable component of reverse-osmosis technology. As is the case with any form of filtration, the overall effectiveness of RO is determined by the porosity of the filter medium. In this case, it is a variable property that depends on the material used and the method employed in its manufacture. The average pore size of an RO membrane is around 0,001 microns but can be as small as 0,0005 microns. In the latter case, it would not only be capable of removing dissolved solids but bacteria and viruses as well.

It should, therefore, come as no surprise that one of the most common applications for reverse osmosis is water purification, both on a commercial scale and for personal use. A household water purifier is made up of various layers, starting with sediment filters that trap larger particles, followed by activated charcoal to extract chlorine and organic materials that might otherwise degrade the RO membrane that completes the composite structure. A portable version is often issued to troops as a means to render local water sources potable.

However, one of the most recent applications for reverse osmosis is potentially its most valuable to date, as anyone who may have experienced 2018’s severe drought in the Western Cape is likely to agree. That application is desalination – the process of extracting drinking water from the sea. Where once this process relied on evaporation and required copious quantities of costly energy for heating, the use of RO membranes to remove the salt has substantially reduced the cost of desalination and is rapidly becoming the technology of choice. In practice, reverse-osmosis technology can be found wherever water with a high degree of purity is essential. This, of course, includes the pharmaceutical and food-and-beverage industries. Interestingly, in the latter case, the goal is not always to produce a pure solvent. With the use of RO, it is also possible to produce concentrates such as that used to prepare fresh fruit beverages while, in the metallurgical field, a similar procedure can be used to recover expensive or reusable metals and minerals from liquid waste.

So, modern uses for reverse osmosis are manifold and extend to the domestic, commercial, and industrial sectors. However, while RO has proved invaluable to mankind, it’s just an idea we borrowed from plants.

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