How RO Water Filters Use Reverse Osmosis to Purify Water?

Nowadays, when we’re thirsty, we turn to a single device. What is it? The answer would be pretty obvious. A water filter!!!

As a result, we come across water purifiers i.e. water filters everywhere. There is a large variety of water filters out there.

  • The primary division is based on the process of filtration. Some filters use activated carbon to purify the water while some use reverse osmosis

That’s not all. You might also come across some water filters that use UV rays to sanitize the water. But, there’s no doubt that the RO or the reverse osmosis filters are the most common of them all.

You probably have a RO water filter installed in your living room. Why are the RO filters so popular? There is a huge scientific reason underlying this huge popularity?

  • So, in this post, we’ll be uncovering how these reverse osmosis (RO) water filters make our water clean and fresh. This can help you take better care of your water filter. So, stay tuned.

Why do we need to purify the tap water?

Let’s start with the basics. First, you need to know what a solution is. In our context, a solution is a mixture of various components.

  • For example, when we drop sugar into water and stir, we make a solution. Here, the water is a solvent as its quantity is higher. On the contrary, sugar is the solute.

So, the tap water we get regardless of the source is a solution. Why? Because, if you test your water, you will find other materials like dust, rust, salt ions, and many more.

  • All these various substances mix with water and that’s why we can say the tap water is a solution. Here, water is the solvent and the other materials are the solute.

Now, the presence of these materials is not always welcome. Not all of them offer the best interest in our health. That’s when the water filters come into the business.

What is a semipermeable membrane?

From the last section, we now know that the tap water we get each day is a mixture of many materials. Not all of those are neutral to our health.

  • So, how can we get pure and clean drinking water? In earlier days, people poured the water through filter papers which would let the water flow interrupting the clay, sand, or dust particles.

That’s how we used to purify our water. But, now technology has changed. We nowadays mostly use reverse osmosis to sanitize our water.

The process of reverse osmosis also requires something like the filter paper. But, in this case, we call that little something membrane.

So, what’s a membrane?

  • The word “Membrane” can have varied meanings in different contexts. But, in this case, it will mean a border substance that we will place between solutions with varying density.

Based on the particles at the border, these membranes can be of three types. The first one is the permeable membrane. From the name, you can likely guess what it is.

So, suppose, you place two solutions with varying concentrations on opposite sides of a membrane.

If the membrane is permeable, then both the solvent and solute molecules will travel across it.

The journey will happen until both solutions have the same density. If you use plastic as the membrane, none of the molecules will be able to travel across.

That’s why plastic is impermeable. Now, there is another type of membrane out there which is known as the semipermeable membrane.

Semipermeable, because it will only let solvent molecules or water to flow, disrupting the solute molecules or sugar. 

  • So, let’s define what a semipermeable membrane is. A semipermeable membrane allows certain materials to pass through while filtering the rest.

This selective nature is very essential in reverse osmosis. So, let’s find out why.

How does osmosis happen?

By now, you probably have seen the word “Reverse Osmosis” many times. But, to understand it first, you have to know about osmosis. 

  • So, what is osmosis? Osmosis is a process that regulates the movement of solute particles through a semipermeable membrane.

To many of us, the definition above is somewhat nerdy. So, let’s uncover it with an appropriate example, shall we?

In the earlier section, we talked about what would happen if we separate two solutions with a membrane. We explored situations for various types of membranes.

Now, what would happen if the membrane is partially selective? By that, we’re assuming that the membrane only lets water molecules pass, but not the sugar ones.

In this case, only water or the solvent will flow from the low-density side to the high-density side. Feeling puzzled? A low-density solution means it has more percentage of water than sugar.

The situation alters for a high-density solution. Therefore, water travels from the side where it is in larger quantity which would be the low-density side to the high-density side. Get it?

We call this process Osmosis. Pretty interesting, right? And, you can also identify the real player here. The semipermeable membrane.

Its selective nature paves the way for osmosis to occur. So, for how long will water keep on flowing? Can you guess?

  • As water flows to the high-density side, the low side loses some, and its density increases accordingly. So, this process will continue until both sides have the same concentration.
How do osmosis and reverse osmosis work?

Trees use osmosis to absorb water from the soil

Most of us already know that trees can produce their food through Photosynthesis. In this case, it uses water and Carbon-dioxide gas to synthesize Glucose.

There is plenty of Carbon-dioxide in the air. But, have you ever imagined how trees take in the water? They don’t have a mouth like animals to drink water.

  • So, how do trees get the water? The answer is in the roots. Yeah, trees have specialized cells in the roots to efficiently absorb water molecules.

If you’ve some concepts of biology, you know that all cells have an outer covering named cell wall. This wall controls the transport of fluids inside and outside the cell.

The cells at the tip of the roots have semi-permeable substances in the cell wall and the cell membrane. As a result, water can enter under certain circumstances.

In common cases, the roots of a tree or plant stay below the ground. Although the soil might look rigid, there are plenty of vacuums out there that get filled with water.

  • Gradually, the roots underground find out these tiny water storages. The selective nature of the cell covering then only let water flow inside avoiding the entrance of minerals.

So, you can realize how osmosis plays a huge role in the lives of trees and plants. This bears testimony that osmosis is the purest and most natural way of purifying water.

How do RO Water filters use Reverse osmosis to purify water?

Now, let’s inspect how RO filters use reverse osmosis. In all the RO filters, there is a tubelike reverse osmosis chamber with a thick semipermeable membrane.

So, tap water enters the filter through some initial blocks. Then, the purifier applies pressure and forces the water to pass through the membrane.

  • As water goes through the RO chamber, all the harmful chemicals get removed. That’s how reverse osmosis plays its role in purifying water.

Due to the selective nature of the membrane, the RO filters make sure you get the purest and cleanest water. This is applicable as long as the semipermeable membrane is fresh and working.

  • So, you need to take good care of the RO chamber from time to time. Fortunately enough, in most cases, these membranes can last up to 2-4 years.

To help you with this, we have a post here – in this one, we’ve talked about how regularly you should replace the semipermeable membrane. Check it out here.

Reverse Osmosis water filters with semipermeable membrane in real life

How does Reverse Osmosis work?

Now that we have a crystal clear idea of how osmosis works, let’s find out how the idea of osmosis paves the way for reverse osmosis.

As you see, in osmosis, the semipermeable membrane has a huge role.

  • It blocks all the minerals and only lets water pass. But, the problem is water flows from low-density solutions to high-density ones.

In our case, the tap water will be the high-density solution as it has more particles other than water. On the other hand, the purified water will be the low-density solution as it only has water molecules in it.

So, when you separate them with a semipermeable membrane, water will flow from low-density solution i.e. the clean water to the high-density solution i.e. the tap water.

But, do we want that?

  • No, we want it to happen the other way around. We want more water molecules to enter from the tap water region to the clean one.

That’s where reverse osmosis comes in. In this case, water will flow in the intended direction and as a result, you get a higher quantity of pure and clean water.

But, that will not happen automatically. You see, osmosis is a natural process. On the other hand, reverse osmosis is an artificial one.

So, how does it pan out? The idea is simple.

  • In this case, if you apply pressure on the high-density side, water molecules will eventually start conducting to produce more clean water.

We call this reverse osmosis. So, you realize this is the opposite of what happens in osmosis. 

In this fashion, you can strip the tap water of all the unnecessary chemicals through reverse osmosis.

Do I need a prefilter before the reverse osmosis chamber?

In most RO filters, you will see one or two more additional stages before the RO chamber. These stages consist of activated Carbon.

What does this activated carbon do?

  • The answer is, they take out some special chemicals like Iron, Sulphur, and Chlorine. 
  • This facilitates the function of the semipermeable membrane, i.e., the reverse osmosis chamber. The membrane can get damaged if it comes in contact with these materials particularly Chlorine.

That’s why most RO filters provide some activated carbon blocks before the RO chamber. We know them as the prefilters.

So, if your RO filter has a prefilter, you will have to run less maintenance of the RO chamber. With pre filters installed, RO membranes can last two times longer.

You can check out the product here at Amazon.

Also, click here to read our reviews of the top under sink water filters.

Do I need an ion exchanger with the RO filter?

One very important thing before we finish. As you can see, the RO chamber gets you super pure and clean water, no doubt about that.

But you also lose out on some useful mineral contents. As the semipermeable membrane only lets water pass, Calcium and Magnesium can’t seep through.

As the mineral contents can’t follow through, the purified water gets slightly acidic. This minimally acidic water can upset your stomach like others.

  • So, many people like to attach an additional ion exchanger or alkaline filter with the RO filter. The ion exchanger adds some essential minerals into the water.

As a result, the water gets its natural form back. This also causes the pH value to go to normal. As a result, this can help your health as well.

  • If you’d like to see some of the RO filters that have an ion exchanger within, you should visit our post here. Here you will find some of the best under sink water filters with ion exchangers.


Throughout the post, we tried our best to show you why reverse osmosis is the most trustworthy process of purifying water nowadays.

As we go further, it would get more and more popular. And, consequently, there will be multitudes of queries about how these filters work, which one is better, which ones last longer, and so on.

  • On our website here, we’ll be trying to answer all those questions and keep you updated on all the latest things about the water filters, water purifiers, water softeners, etc.

So, if you have any more queries that you’d like to be answered, please let us know in the comment section.

Happy purification!!! Cheers!!!

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