How RO Water Purifier Works? Complete Guide

If you have landed here, chances are that you are planning to purchase an RO water purifier.

RO water purifiers have become quite common, but not many people are aware about how RO water purifier works. Don’t worry, we have given a detailed explanation of working of RO water purifiers along with all of its key parts.

What is Reverse Osmosis?

For you to understand how reverse osmosis works, it is necessary to understand what is osmosis.

For osmosis (or reverse osmosis) to take place, three things are required. Impure water, pure water and semi-permeable membrane.


Osmosis means (automatic) movement of water through a semipermeable membrane (such as RO membrane) into a solution of higher concentration.

This means that the pure water will pass through the membrane towards impure water automatically. This is a property of water (and other solvents)!

Reverse Osmosis

Reverse Osmosis is exact opposite of this process. It means that the impure water is pushed with force towards the semi-permeable membrane so that water flows through it, while the impurities are left behind.

This technique is commonly used for purification of water as well.

RO Water Purifier Working Explained With Diagram

First of all, reverse osmosis process itself does not need anything else apart from RO membrane. But these RO membranes cannot clean very dirty water. This means that water has to purified sufficiently before it is fed to the RO membrane.

Also, other filters and controllers are added for making the water drinkable.

RO Water Purifier working

As soon as you switch on the water purifier, this water reaches the sediment prefilter first.

Step 1: Sediment Filtration

Sediment filtration is usually the first stage. It separates larger impurities such as dirt, sand, rust etc. and a few dissolved solids. As the name suggests, the physical impurities are deposited at the bottom the filter in the form of sediment. It is typically made of polyester, glass fibre or cotton.

Step 2: Activated Carbon Filtertration

Second stage is activated carbon filtration. Activated carbon is typically powdered form of carbon that is treated to make it very porous. This increases its surface area exponentially to filter out far more impurities as compared to traditional carbon. Activated carbon is very useful in removing volatile organic impurities, chlorine and chlorine by-products. Since many organic compounds are responsible for foul taste and smell in water, this filter helps remove bad odour and smell.

Step 3: Ultrafiltration (UF)

Some part of the water from activated carbon filter is allowed to pass through UF membrane. Thus, Ultrafiltration takes place with the help of a semipermeable membrane. This filter removes bacteria, suspended impurities and other small impurities from water. This process makes water free from impurities that could clog the RO membrane. Thus, it increases the life of RO membrane.

Step 4: RO Filtration

Another part of water from activated carbon is passed through RO membrane. Pore size of an RO membrane is very small at 0.0001 micrometres. Which is sufficient to remove bacteria, viruses, ions, pesticides, heavy metals and almost all other impurities. Most commonly used RO membranes are thin film composite membranes that are spirally wound inside a casing. Now let’s see why some of the water is diverted to UF membrane.

Step 5: TDS Controller

A TDS controller is very important component of a water purifier. This helps regulate the TDS level of water. RO purification tends to reduce TDS of water by as much as 90%. And low TDS can be bad for health because low TDS means that water does not essential nutrients such as magnesium, calcium, potassium etc. Therefore, to maintain decent level of essential trace elements, small amount of impure water is added back to the RO purified water through the TDS controller. This TDS controller comes with a screw that can be rotated clockwise or anti-clockwise to increase or decrease the TDS.

In short, UF purified water is added back to RO purified water and TDS controller acts as a valve to regulate the volume of UF water that is added to the RO water.

Step 6: UV Purification

UV purification is usually the last process. This is because UV purification can take place efficiently in in comparatively clean water. UV purification is a process that deactivates bacteria and viruses still present in the water. This purifier releases UV-C radiation to kill microorganisms by denaturing their DNA. After this process, the water is sent to water tank.

Additional Step:

Many RO water purifier also come with activated post carbon filter that gives better tase and ‘polishes’ the water.

Parts of RO Water Purifier

RO water purifier diagram

Now that we have understood how RO water purifier works, let’s see all the major parts of an RO water purifier. It has following parts:

Filtration unit containing Sediment Filter, Activated Carbon Filter, RO Membrane, UF Membrane, TDS Controller, UV Purifier, Post Activated Carbon Filter.

Booster Pump: As discussed, RO filtration requires water to be fed into the RO unit with great force. This cannot take place without the help of a booster pump. This booster pump is thus, an essential part of an RO water purifier.

SMPS: SMPS in a water purifier converts AC current to DC current. This is necessary for booster pump to function.

Float Valve / Microswitch: This valve prevents water from overflowing in the water tank by turning off the water purifier. If this valve malfunctions, it can damage the water purifier. It can cost as low as Rs 50.

Low-Pressure Switch: This switch is responsible for cutting off the water supply in case of low water pressure.

Solenoid Valve: This valve stops RO waste-water from flowing when you turn off the RO water purifier.

Storage Tank: Almost all RO water purifiers come with a storage tank because RO purification is a slow process and once the purifier is switched on, it can take one hour to purify 10 to 20 litres.

Drain Line: The Drain line discharges RO waste-water containing high level of impurities. You should always try to recycle RO waste water.

Faucet: Faucet or water tap controls output of water.

Reading Suggestion:

Best RO Water Purifier

FAQs- How RO Water Purifier Works

Is RO water purifier bad for health?

RO purification removes most of the natural nutrients along with other impurities. Therefore, it’s necessary to buy an RO water purifier that comes with a TDS controller so that you can set regulate the TDS of water, which will add back essential elements into the water.

What TDS level is best?

TDS levels of 150 to 250 is considered the healthiest. As a rule of thumb, anything below 500 PPM can be consumed.

Should I buy an RO Water Purifier?

This depends on the TDS of your water. If the TDS of water is higher than 500 PPM, then you should go for RO water purifier. Also, please keep in mind that if your water contains heavy metals then you will have to buy RO purifier only. This is because no other technology can remove such minute impurities and heavy metals are toxic even in small quantities. If TDS is low and there are not heavy metals, then you can go with UV water purifiers.

How can I check the TDS of water?

TDS of water can be easily checked with a TDS meter. You can buy it online for as low as Rs 200-300.

How much water is rejected by RO water?

Typical RO water purifier rejects 65-80% water.

Is RO water hard or soft?

RO purifiers remove ions and minerals that are responsible for hardness of water. Thus, RO water is soft.


Reverse Osmosis water purifiers filter water in several stages.

Sediment filter does basic filtration by removing visible impurities, carbon activation filter removes bad smell and taste, UF purification removes suspended particles, while RO removes almost all impurities. Finally UV kills viruses and bacteria.

Reverse Osmosis process, by itself is not complicated, but this process removes trace elements and makes the taste of water unnatural. Due to this, a TDS regulators are added to RO purifiers. Also, it necessarily requires a high pressure pump to push water with high force to pass water through RO membrane. Different types are valves are also present in the water purifier for added safety of the device. Additionally, all RO water purifiers come with water tank, drain pipe and faucet.

We have presented a detailed explanation of how RO water purifier works. We hope that this post helped you enhance your understanding of working of RO water purifier. You can ask your doubts in the comments and we will make sure to answer them.


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