RO vs UV vs UF-Guide on Water Purification Part 2

Image describing reverse osmosis

Welcome to RO Vs UV Vs UF-Part 2 of the Guide on Water Purification. In case you landed up here straightaway, you might like to read Part 1 in this series first.

This post is for the buyer that wants to be make informed purchasing decisions on water purifiers. This’ll help you make a decision on deciding amongst RO vs UV vs UF water purifier.

 

Let’s get started.

Summary Table- RO Vs UV Vs UF:

ProductReverse Osmosis(RO)Ultraviolet Purifier(UV)Ultrafiltration(UF)
Image describing reverse osmosisImage showing UV water purifierImage showing hollow fibre membrane filter
Requires ElectricityYesYesNo
Suitable for turbid(muddy) waterYesNoYes
Kills all virusesYesYes(but doesn't physically remove the dead cells)Almost, but not all
Kills all bacteria, cysts and protozoaYesYes(but doesn't physically remove the dead cells)Yes
Decreases Total Dissolved Salts(TDS)YesNoNo
Wastes waterYes(~60%)NoNo

What you’ll learn

RO Vs UV Vs UF- Guide on Water Purification Part 2

#1 Reverse Osmosis(RO)

What is Osmosis?

Ever seen dried raisins kept in water? How, a day later, the same dried raisins swell in size because they magically absorb the water? Congratulations, you’ve seen osmosis in action.

A natural osmosis process occurs when you place water containing a lot of dissolved particles(present inside the raisins) across a semi-porous membrane(the raisin skin) with pure water, which doesn’t have any dissolved particles. The natural tendency of pure water to migrate across the membrane(raisin skin) towards the concentrated water containing dissolved particles is called osmosis.

The reverse of this operation is called Reverse Osmosis.

Image describing reverse osmosis(Img 1 RO Vs UV Vs UF)

How Does Reverse Osmosis Work?

Imagine a few swollen raisins being put in pure water and losing their size. While this may not naturally occur, it occurs artificially by applying a certain amount of pressure at the side of the concentrated solution, basically pushing the pure water towards the other side of the membrane and retaining the dissolved particles at the other side.

This is how total dissolved salt (TDS) content in water is lowered by RO purifiers. The act of putting pressure to push pure water from the high TDS solution across the RO membrane is what takes energy. This is why RO purifiers are electricity driven.

Advantages and Disadvantages of RO Water Purifiers?

Pros

  • Only water purifier that can decrease TDS level of water.
  • Can remove other dissolved impurities such as chemicals, heavy metals, etc.

Cons

  • Wastes 60% of input water. So basically, for every 10L of water consumed by the system, you will only get 4L of drinkable water.

#2 UV Water Purification

Image showing UV water purifier(Img 2 RO Vs UV Vs UF)

What is UV?

UV or Ultraviolet light is a part of the spectrum of light that humans cannot see with the naked eye.

How Does UV Purify Water?

A specific range of the UV light spectrum has germ killing properties. This property is used in UV water purifiers to disinfect. UV light affects the DNA of water contaminants such as viruses, bacteria, protozoa and cysts and renders them as well as their reproductive capacities, ineffective.

Let me be clear, UV light does not physically remove contaminants from water. It just renders them harmless.

Advantages and Disadvantages of UV Water Purifiers?

Pros

  • Gives odor free and tasty water.
  • Uses no chemical additives to purify water.
  • Low maintenance. Only the UV bulb has to be changed periodically.

Cons

  • Cannot remove visible contaminants such as mud, silt, etc. They have to be removed by a physical filter before using a UV light. In fact, if this is not done, the efficacy of UV light against bacteria, viruses, cysts and protozoa goes down.

#3 Ultrafiltration

What is Ultrafiltration(UF)?

Ultrafilters are water filters based on hollow fibre membrane technology. This includes Microfilters and Nanofilters as well. So the bigger question here would be:

What is a Hollow Fibre Membrane?

A single hollow fibre membrane resembles a long strand of noodle. In membrane based systems, there are multiple hollow fibres laid out parallel to each other and tethered between the water inlet and outlet.

Image showing hollow fibre membrane filter(Img 3 RO Vs UV Vs UF)

How Does a Hollow Fibre Membrane Work?

Each strand of noodle or hollow fibre has multiple small pores along it’s wall. The contaminants in dirty water get physically trapped in these holes and out comes purified water. The actual trajectory of water flow may be from the outside-in or inside-out of the hollow fibre. Both methods have their pros and cons. Anyway, any end consumer need not get into so many technicalities.

What Are The Types of Hollow Fibre Membranes?

The different types of hollow fibre membrane filters are categorized based on the pore size of the membrane walls. The smaller the pore size, the more efficient the filter is in removing contaminants. The three types of hollow fibre membrane filters are:

  • Microfilters(MF): Have a pore size of around 0.1 micron.
    • Can remove bacteria, protozoa and cysts.
  • Ultrafilters(UF): Have a pore size of around 0.01 micron.
    • Can remove bacteria, protozoa, cysts and viruses.
  • Nanofilters(NF): Have a pore size of around 0.001 micron
    • Can remove bacteria, protozoa, cysts and viruses.

Here’s a handy chart describing the efficacy of different hollow fibre filters

Chart showing efficacy of hollow fibre membrane water filters

Advantages and Disadvantages of Hollow Fibre Membrane?

Pros

  • Requires no electricity.
  • Requires no chemicals.
  • Removes viruses, bacteria, protozoa and cysts(ultra and nanofilters).

Cons

  • Does not remove heavy metals such as Arsenic, Lead, etc.
  • Does not decrease TDS level of water. Not suitable for brackish or hard water.
  • Require a minimal level of maintenance(backwashing).

That’s it! Hope you liked our comparison piece on RO vs UV vs UF water purifiers and that it helps you make a better choice.

Any questions? Jump into the comments section below!

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