How to select the best water purifier?

Introduction – Thinking to buy a new water purifier but not sure how to select it ? Are these terms like RO (Reserve Osmosis), UV (Ultraviolet filtration), UF (Ultrafiltration), NF (Nanofiltration), TDS (Total Dissolved Solids) etc confuse you and you are not sure that which combination is best for you? Then this article will give you clarity for same.

To start with, it is very important to understand that it is not possible to have a single medicine for types of diseases. Likewise a particular type of water purifier will not be suitable for all households. It completely depends upon the quality of water you are using.

Identify the Water Type – Soft or Hard Water

Water can be classified as soft or hard based on the level of dissolved solids in water. Total Dissolved Solids (TDS) is regarded as the degree of hardness and is measured in units of Parts Per Million (PPM) or Milligrams per Litre (mg/L).

1 ppm = 1 mg/L

Water with TDS value ranging between 150-300 ppm is considered to be soft water, while water with TDS value higher than 500 ppm is considered to be hard or contaminated water.

What’s the source of the water you get supplied to your home? If it comes from underground water, supplied by water tankers, or drawn from deep dug bore wells, then most probably it’s hard water. Now, hard water contains pretty high percentages of dissolved solids like calcium, magnesium, heavy metals, fluorides, and arsenic. All these dissolved impurities make hard water unfit for human drinking. On the other hand, water sourced from rivers, rainwater harvesting systems, lakes, and municipality managed storage and delivery system is generally termed as soft water. Soft water, though not as dangerous, also needs purification before it can be deemed safe for human consumption over the long run.

Water Purifier Types and Mapping them to Your Water Type

  1. Reverse Osmosis (RO) Purifiers

To understand the working of RO, we need to first understand what Osmosis is.

In the normal osmosis process, the water naturally flows from an area of low solute concentration (low TDS level), through a membrane, to an area of high solute concentration (high TDS level). The pores of the membrane are very small (about 0.0001 microns), water molecules being smaller are allowed to pass through and the smallest dissolved impurities and bacteria are trapped.

As the name suggests, Reverse Osmosis (RO) does the opposite of Osmosis, that is, push the water molecules from the region of higher TDS level to the region of lower TDS level. This is achieved by applying external pressure with the help of a water pump to reverse the natural flow of water. Water with impurities or high TDS is pumped at high pressure into the RO chamber. This pushes the water molecules across the semi-permeable membrane to the other side while leaving the dissolved solids and other impurities behind. All the dissolved solids and impurities along with some input water, also known as RO waste water, are discharged through a separate outlet.

RO Purifiers are therefore always recommended for purifying water which has a high TDS level. The TDS level of the output drinking water from RO purifier is very low as compared to the input water.

Some drawbacks of RO technology:

Requires Electricity: A high-pressure electrical water pump is used to apply external pressure on the input water, hence RO purifiers cannot work without electricity.

Wastes Water: A significant part of the input water is discharged along with the dissolved impurities, which results in unnecessary wastage of water. On average, RO purifiers produce 3 litres of waste water for every 1 litre of purified water.

  1. Ultrafiltration (UF)

Like RO, Ultrafiltration also uses a semi-permeable membrane to purify water. After reading the first sentence, you must now be thinking if both RO and UF use the same method to purify the water then what is the difference between RO and UF. Ultrafiltration or UF uses a membrane with much larger pores (appx. 0.01 microns) as compared to RO which uses a membrane with very small pores (appx. 0.0001 microns).

The advantage of using Ultrafiltration is UF purifiers can work without electricity because the membrane has much larger pores and water can pass through it naturally using the force of gravity. Which means no external pressure or water pump is required. Also, since UF purifiers do not hold back any water, there is no wastage of water.

But there are some limitations of using UF purifiers, because of larger pore size, UF can only remove un-dissolved solids and larger impurities. It cannot remove the dissolved solids or reduce the TDS level. So, UF purifiers are not suitable for purification of high TDS water or hard water.

  1. Ultraviolet (UV) Purification

As the name suggests, Ultraviolet or UV purification uses ultraviolet rays for the purification of water. A UV purifier works by throwing high-intensity UV rays on the water which kills or inactivates the disease-causing bacteria and viruses.

However, UV purifiers cannot remove any dissolved or un-dissolved impurities or chemicals from water. Because of which most of the UV purifiers that are available in the market use some form of external sediment pre-filters to remove un-dissolved impurities and activated carbon filter to remove Chlorine and some dissolved impurities.

Therefore, UV water purifiers are only recommended for areas where the water source has a low level of TDS. If the water has low TDS level but it is contaminated with bacteria and viruses and appears muddy then you can use UF+UV water purifier.

  1. Tap/Faucet Mounted Filters or Gravity Based Purifiers

These types of filters or purifiers are simplest to use and provide the most basic water purification. These filters generally comprise of sediment or sediment + activated carbon filters which can remove large and un-dissolved impurities like mud and sand along with some chemicals and microorganisms. Tap/Faucet Filters are very small in size and can be directly fitted on taps.

Gravity-based storage purifiers are the advanced version of tap/faucet filters. These purifiers offer slightly advanced purification and come with in-built storage tank to store input/impure water. Most of the Gravity based water purifiers available in the Indian market now have 2 separate storage tanks for input/impure and output/purified water.

Tap/Faucet Mounted Filters and Gravity Based Purifiers are only suitable for areas where the TDS level is low and water is not highly contaminated with biological impurities like bacteria, viruses, and other disease-causing germs.

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Which Water Purifier Should You Choose?

RO technology has become synonymous with water purifiers and you must have heard a lot of people referring to water purifier as RO (like the brand Xerox has become synonymous with photocopying).

Because of this, when we talk about buying a water purifier most people think RO water purifiers should be purchased by default without considering the most important facts like the source of water and TDS level of input water.

If you have read all the above discussion then you are well aware of the fact that RO purification is required only when the water coming to your home or office has high TDS value (generally higher than 500 ppm).

These days there are different kinds of water purifiers available in the market ranging from simple tap/faucet filters and Gravity based purifiers to UF, UV, RO and their combinations. This vast range of different technologies and thousands of different water purifier models from tens and hundreds of brands have increased the complexity of buying the right purifier for your home or office.

As discussed earlier, you should buy an RO purifier only if the water to be purified has a high TDS level. There is absolutely no need to use an RO purifier if the TDS level is below 500 ppm. Because if your water source has low TDS then the RO purifier will further reduce it to a very low TDS level. Which means the purified water will then be devoid of essential minerals like calcium and magnesium which are required for our good health.

If the TDS level of incoming water is below 500 ppm then you should check the incoming water for turbidity (clarity), muddy appearance, or presence of biological impurities like bacteria and viruses.

Though water purifier makers try to differentiate their products from competitors by flaunting advanced water purification technologies, most of them actually use a mix of three major purification approaches – reverse osmosis (RO), ultraviolet (UV), and UF (ultra filtration).

If you do not want to go into the technicalities of the different purification approaches, then you can simply refer to the below checklist that helps you buy the right purifier for your home or office. Here we will list all the parameters to evaluate and choose the right water purifier according to your needs.

Below 500 ppm No No Gravity-Based Purifiers
Below 500 ppm No Yes UV
Below 500 ppm Yes No UF
Below 500 ppm Yes Yes UF or UF+UV
Over 500 ppm No No RO
Over 500 ppm No Yes RO+UV
Over 500 ppm Yes No RO+UF or RO+UV+UF
Over 500 ppm Yes Yes RO+UV+UF


The sole and biggest criteria to decide the type of water purifier you should buy is the source of water and the kind of impurities present in the water supplied to you.

  • Soft water has low TDS level; UF water purifiers work pretty well for it.
  • For soft water with a high level of biological contamination, go for a UV or UF+UV water purifier.
  • Hard water contains a higher level of TDS, apart from specific toxins; so, RO based purifiers are well suited for purifying it.
  • For hard water with a high level of biological contamination, go for a RO+UV or RO+UV+UF water purifier.
  • UF filters are best used in combination with another water purification system, as they don’t kill any microorganisms in contaminated water. UF filters only remove suspended solids from water.
  • Activated carbon based water purifiers are ideally suited to remove excess chlorine from water.

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