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  • Writer's pictureSamhita Pokkunuri

An Introduction to Osmosis, Reverse Osmosis, and Water Filtration Systems

By Samhita Pokkunuri, Grade 8

By the title, I presume you think that this blog has something to do with osmosis, reverse osmosis, and water filtration systems — and right you are! Though these water conservation methods might not seem important, they actually portray an essential aspect of our lives today.


Well, what is osmosis? Based on a classical science textbook, osmosis is the movement of water molecules from an area of high concentration to an area of low concentration of water molecules, through a cell’s semipermeable membrane (1). Hold up — what? In simpler terms, osmosis is the net movement of water molecules through a membrane that only allows certain molecules to pass through. Water molecules with a HIGHER concentration indicate that water molecules are more abundant relative to the overall volume, and water molecules with a LOWER concentration indicate that water molecules are less abundant relative to the overall volume. In other words, there are more water molecules in a higher concentration than in a lower concentration — therefore, particles will diffuse, or move from an area of higher concentration to lower concentration. While transferring from a high to low concentration, these water molecules pass through a partially permeable membrane. This membrane basically only allows certain particles to pass through it, depending on certain conditions. What are these “conditions?” While they often vary, in the case of osmosis, it allows the passage of water, but not ions, nor larger molecules. This process tends to continue (the passage of water molecules through the semipermeable membrane) until the concentrations become equal on both sides of the membrane (2). This process is also passive, meaning it doesn’t require any extra expenditure of energy to occur.

There are two types of osmosis: endosmosis and exosmosis. Endosmosis occurs when there is the movement of the water INSIDE cells, where it’s placed in a hypotonic solution (a solution where the outside of the cell has a higher solute than the inside), and the cell begins to swell. Exosmosis occurs when there is movement of the water OUTSIDE of cells, where it’s placed in a hypertonic solution, and these cells become “flabby.” Well then — what’s the difference? The key differentiation you can make between the two is that endosmosis has water movement inside of the cell, and exosmosis has water movement outside of it, therefore causing two different results.

Well then, you might be wondering, what does osmosis do? Why does it exist? And to answer that is relatively simple; this process helps the movement of essential materials inside and outside cells. Osmosis is important because we wouldn’t be able to access nutrients, water, and other solutes without this process.

Osmosis can take place in the large and small intestine; when our body processes food, osmosis is what helps you obtain nutrients, and get waste products out of your blood. Even as we digest food, this moves from the esophagus to the stomach to the small intestine, where your body absorbs such nutrients through osmosis. And as this food leaves to the large intestine, osmosis can occur here as well.

Osmosis can also occur in plants. When plants absorb water from the soil, the roots have a higher solute concentration than the surrounding soil, so water flows inside of these roots through osmosis. Through guard cells, cells disperse along the surfaces of leaves, so when pairs of guard cells surround a pore of these leaves, osmosis allows it to control the opening and releasing moisture.

Osmosis even can occur in the simplest things like soaking up a water spill in your house using a towel; when the paper towel becomes less absorbent as it sucks up water, you are seeing osmosis in action!

Reverse Osmosis

Well, if we know what osmosis is, what’s reverse osmosis? And if you are thinking it’s something along the exact opposite of osmosis, you are correct! To be exact, reverse osmosis is the process of osmosis, except in reverse.

By the textbook, reverse osmosis is the process where you dematerialize water by putting it under pressure, through a semipermeable membrane. Osmosis usually doesn’t require any extra energy, but to reverse this process means that you need to apply energy to the solution, making it an active process. The membrane it passes through, once again, only allows the passage of certain molecules through certain conditions. In this case, the conditions wouldn’t include a majority of organics, bacteria, pyrogens, and dissolved salts. Throughout this process, water is continually being “pushed” through the membrane by applying a greater amount of pressure (than in osmosis), which finally allows pure water through, and taking out MAJOR contaminants. Basically, if the pressure greater than the osmotic pressure is being applied to the higher concentration, the direction of the water flow through the membrane can be reversed (4). So, in short, this natural process allows water to flow from a concentrated solution to a dilute solution. For example, reverse osmosis may start with salt water, and as applied pressure leads water to flow through a RO (Reverse Osmosis) semipermeable membrane, contaminants are taken out, and freshwater comes out.

The exact difference between reverse osmosis and osmosis is that osmosis travels from a higher concentration to a lower one, and requires no external processes of energy. Reverse osmosis goes from a lower concentration to a higher concentration and therefore requires extra energy.

So, what are some applications of reverse osmosis in the real-life world? Why even is this important? In its most famous usage, reverse osmosis helps conserve water for its domestic and industrial usages. It helps us remove many types of contaminants, and remove considerable bacterias and impurities in water.

Regarding real-life applications, reverse osmosis can be used in the desalination of water, as we mentioned before. Usually, reverse osmosis is seen upon membrane modules, which are made of polyamide hollow fibers, allowing the measurement of the rejection coefficient to be achieved with a high percentage of 99.3% (5). *A rejection coefficient is the measure of the solute in a solution fed to a semipermeable membrane*

Reverse osmosis can also be used in places such as the concentration of fruits and vegetable juices, the pre-concentration of milk and whey, and even the alcoholization of alcoholic beverages.

Water Filtration Systems

We are now well-aware of the processes of osmosis and reverse osmosis. Now, what are water filtration systems?

(Note: Reverse osmosis and water filtration systems are NOT the same things! Filtration systems are more effective when it comes to removing or reducing impurities or contaminants in water such as chlorine, sediment, and poor tastes; reverse osmosis, however, removes viruses, bacteria, and parasites (6). But, contextually, reverse osmosis can potentially be declared as a TYPE of water filtration systems.)

In the simplest form (no more textbook definitions), a water filtration system helps decontaminate water either by physical barriers, chemical processes, or biological processes (7). There are various types of water filtration systems; some of the more well-known types include activated carbon filters (which uses activated carbon granules that can trap chemical impurities through absorption) and ion exchange (split apart atoms that are contaminated to make ions, then trap the bad ones and release the good ones).

But how does the water filtration process work, overall? Since water is a solvent, it is able to dissolve other substances easily — this can lead to it being contaminated by chemicals and dirt, making it unsafe for drinking or other purposes. Water filtration can either be a physical or a chemical process (8). In a physical process, the water is strained through a gauze-like membrane to remove the larger particles. In a chemical process, the water is treated with technology to remove impurities.

Regarding applications of water filtration systems, it’s practically used everywhere! People use water filters to remove bacterial contaminants and lead to better drinking water. Water filtration occurs under your sink, with Under Sink Water Filtration Systems (that’s pretty self-explanatory). Its implementations in the real world are endless.

Key Takeaways

Osmosis, reverse osmosis, and water filtration systems clearly all serve a significant impact on our world; they are in places we see and use in mundane tasks we complete daily. Without them, you might be drinking nasty water, or leaving all your floors wet.

Cover photo from: Michigan State University


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