We have all learnt about Osmosis in 6th grade biology, but what exactly is reverse osmosis? You must be thinking that it is a reversal of the process of osmosis, but not quite! Simply put, reverse osmosis is the process of pushing a liquid with microscopic contaminants through a filter to clean it. It is used all around the world to filter out drinking water from salty water to sewage. Let's return to our 6th grade Biology class for a moment and recall what osmosis is. It is the net movement of water molecules from a region of high water potential (low concentration of solute) to a region of low water potential (high concentration of solute) across a partially permeable membrane. Now, you might be wondering how this is even remotely linked to drinking water?
Since the molecules of water move towards regions of high concentration (dirty water for our purpose) the molecules would move from clean water into the dirty water - the exact opposite of what needs to happen. So how do we prevent this from happening? We apply a mechanical force (pressure) to the water to push it through the membrane. For this to work, the pressure applied must exceed the “osmotic pressure” that is created by the concentration gradient between the salty and freshwater. Since the contaminant cannot cross the filter, only water is pushed through by the exerted force. And Voila, we have clean water!
A mobile reverse osmosis system
The key mechanism of reverse osmosis- applying the osmotic pressure
Mystery behind the membranes
A reverse osmosis system contains multiple stages in a single series but the most crucial part of any RO system is its membranes. It is the structure of the membranes which enables the process to occur. Excited to know how? Let's start by simplifying the complexity of the structure of these membranes to better understand their mechanisms. Imagine a sheet of fabric with holes in it, which allows some molecules to pass through but not others. Somewhat like a barrier right? That’s exactly what a membrane is all about. However, semipermeable membranes can be a bit more complicated because of conditions like pressure and temperature that affect the movement of the particles on either side.
Membranes used in reverse osmosis are made of a thin polyamide layer, (<200 nm) deposited on top of a polysulfone porous layer (about 50 microns) on top of a non-woven fabric support sheet. Let’s focus on the polyamides. Essentially, a polyamide is any polymer ( a chemical compound with molecules made up of repeating units) in which the repeating units are linked together by amide groups. The pictures below are a diagrammatic representation of these chemical structures.They are chosen mainly for their permeability to water and relative impermeability to impurities like salt ions.
But polyamides can’t serve its purpose without the polysulfones. Polysulfones come under the category of high performance thermoplastics -materials that soften when heated and harden when cooled. They have a very high resistance to “compaction” meaning that they cannot be compressed easily, allowing them to be used in high pressure environments that exist in a reverse osmosis machine. They are also not affected by pH and are resistant to the surfactants* used in detergents. Moreover, their resistance to oxidation means they can be easily cleaned with bleach solutions. Below is an image of the repeating unit of a polysulfone.
In today’s world, water scarcity remains a global threat due to unequal distribution, droughts and increasing population. Reverse osmosis helps fight against this scarcity by purifying previously unavailable water and forming drinking water by removing various impurities. These impurities consist mostly of heavy metals like iron, potassium and zinc. Thanks to this technology, clean drinking water reaches millions of homes. All praise to the Chemical Engineers who have developed these complex membranes that run this process! There’s more mystery left to discover, but for today this is all. Look out for our next article to gain invaluable knowledge that you can show off to your friends! (Hehe)
*surface active agents which lower the surface tension between two liquids. This is a primary component of cleaning detergents as it helps to trap dirt and removes it from the surface.
Written by : Mark Monie and Malaya Parwani
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