RO Vs UV Vs UF: How To Choose A Water Filter For YOUR Needs

Since I was a kid, we had a UV purifier at home, as most Indian homes in the 90's had one. In the late 2000's , as RO purifiers became popular, we started using those instead. 

Most people in countries like the US and UK won't know a lot about water purification since over there, the tap water is good enough to drink. That's not the case in India though, and you naturally end up knowing a lot about RO, UV water purifiers since you use them everyday.

Additionally, at a previous job, I was in charge of designing a portable water purifier- so I got to learn a lot about Ultrafiltration(UF) and hollow fibre membranes as well. 

So, since I know a lot of people have questions regarding the differences between RO, UV and UF online, and there wasn't a good enough article- I decided to create this resource.

Let's get started.

RO Vs UV Vs UF- Quick Summary

Specifications Overview

Reverse Osmosis(RO)

Ultraviolet(UV)Purification

Ultrafiltration(UF)

Requires Electricity?

Removes all contaminants from water?

What contaminants can't it remove?

              -

Cysts and turbidity

Doesn't remove small viruses

Makes Hard Water Soft?

Eco Friendly?

(~60% water wastage)

Suitable For?

Water that's hard, contains heavy metals or other dissolved impurities

Soft water without heavy metals or dissolved impurities

Soft water without heavy metals or dissolved impurities

What is Reverse Osmosis and How Does It Work?

Reverse Osmosis is the most comprehensive water filtration process out of these 3. And that isn't necessarily a great thing.

But more on that in a minute. First- how does an RO filter work?

Well, a RO purifier basically has two chambers separated by the RO membrane or sheet. The first chamber- containing hard water- is subjected to pressure inside the purifier, due to which the water migrates across the membrane into the other chamber. 

The hard water minerals and other impurities- such as viruses, bacteria, etc are unable to cross the membrane barrier, get left behind and voila! You get pure water.

This is why the Reverse Osmosis process needs electricity- because pressure needs to be formed inside the machine to desalinate the water. 

As for what material an RO membrane is made of- According to this paper published by the University of Bath-it usually is Thin Film Composite polyamide

RO Water Purifier

The RO Water Purifier At My Home

How To Maintain A RO Water Purifier

The RO filter membrane will need to be changed every couple of months or so because the membrane gets clogged over time, as impurities are filtered out. 

I've been using a RO purifier for the last 10 years, and on average, I'd say a filter cartridge should last you 3-4 months, depending on how hard your water actually is.

I also get the pre-filters changed as well whenever I'm getting the RO filter changed. The pre-filters are responsible for extending the life of the RO membrane and bear the brunt of the filtration load.

Usually, the prefilters used will be a Microfilter(MF) + Carbon filter(useful for enhancing taste and to remove chlorine), but in some RO purifiers-the MF might be replaced by a UF(Ultrafilter) membrane.

Maintaining a RO water purifier is costlier than a UV or UF water purifier. You can expect to shell out ~70-100$ or about 4000-6000 Rs every year, depending on how high-end your purifier is, and how many replacable parts it has.

I also find that getting paying your purifier company for a year's worth of servicing in advance turns out to be cheaper than paying for every single visit by the service guys. They also tend to treat you better.

Apart from this- it's also good to sanitize the filter body by washing it with water on the inside while the parts are getting serviced. But if you're lazy, maybe do it twice a year, if not every time.

Advantages and Disadvantages of RO Water Purifiers

The biggest advantage of an RO purifier is that it's very effective at making hard water soft by reducing Total Dissolved Salts(TDS- a measure of how many minerals are dissolved in water, i.e: how hard is the water) as well as removing viruses, bacteria and any other contaminants that may be present in your water.

However

RO water purifiers can reduce TDS(Total Dissolved Salts) too much. So much so, that the water becomes unhealthy to consume(anything below TDS 50).

As a solution to this - See if there is a mineralizer in your RO purifier, which adds some minerals back and makes the water fit to consume. Or a TDS adjuster, which allows you to manually choose the TDS of your purified water.

Secondly, as mentioned above, the maintenance costs are way higher than a UV or UF water purifier.

Third, RO purifiers waste a lot of water. Roughly 60% of the input water gets rejected as a part of the reverse osmosis process. That means you'll waste 6 L of water for every 4 L you drink- which, by the way is how much water an average person consumes daily.

This means that in a year- a single person will end up wasting 2190 L of water!

If you have hard water supply and need something to ease your conscience- here's what I do:

I have a small bucket to collect the reject water as it flows out. This water can be used for a variety of things such as:

  1.  Mopping Floors
  2.  Watering Plants
  3. Flush water for the toilet
  4. Filling up water coolers

What you should not do with the reject water is wash clothes or utensils as the hard water minerals tend to leave a calcified residue over time.

You shouldn't bathe in it as well, as some online articles suggest, because hard water can be harmful for your hair and skin(I personally use a shower filter to protect myself)

So, if you have a RO purifier at your place- make sure you're collecting the waste water and putting it to good use.  Here are some of my recommendations for the Best RO purifiers that are currently on the market.

What is UV Water Treatment and How Does It Work?

There are basically 3 types of UV light - UVA, UVB and UVC.

  1. UVA has the lowest frequency and is the least effective in killing germs
  2. UVB is found in sunlight and is slightly more germicidal. It also causes tanning.
  3. UVC has the highest frequency and is used for purifying water.

UV water purifiers contain a mercury vapor bulb, also called germicidal bulb, that emanates UVC light.

And while UV rays are not visible to the human eye, they are very effective at killing viruses and bacteria in water. According to Scientific American, UV light damages the DNA of viruses and bacteria in water, which eventually makes their cells unable to function and leads to their death.

Let me make something clear here- UV purifiers do not physically filter out viruses and bacteria in water- they just kill them, making the water safe for consumption.

Advantages and Disadvantages of UV Water Purifiers

Let's talk about the biggest disadvantage first because it's more important here:

For a UV water purifier to be truly useful in your home, it needs to have  a pre-filter attached that can remove visible dirt or turbidity in water. UV light becomes increasingly inefficient at purification if the water is cloudy or contains mud, silty, sand, etc.

Also, the pre-filter needs to have a pore size smaller than 0.1 micron(Thousandth of a metre) in order to be effective enough to remove cysts- the only contaminant that UV rays cannot kill due to the hard protective shell around them. 

As long as your UV purifier has an adequate pre-filter(such as a microfilter or ultrafilter) you should be good

Advantages of UV water purifiers:

Lower maintenance costs as compared to RO purifiers. You can expect to spend 1/5th of an RO purifier's yearly maintenance costs with a UV system. On average, you'll just need to change the UV bulb once a year and it also uses less electricity as compared to the RO purifiers.

Here are some of my favorite UV UF water purifiers on the market.

What is UF and How Does It Work?

Ultrafilters(UF) are a type of hollow fibre membrane. In simple terms- hollow fibre membranes are a physical filtration media that contain small pores on their outer surface.

Anything bigger than these pores gets filtered out- anything that's smaller- passes through.

Not all hollow fibre membranes are made equal though. There are 3 types based on pore size:

1. Microfilter(MF):

  • Has a pore size of roughly 10 micron(10 thousandth of a metre)
  • Can filter out all contaminants except viruses. 
  • Generally used in portable water filters and as a pre-filter.

2. Ultrafilter(UF):

  • Pore size of about 1 micron(millionth of a metre).
  • More efficient than MF, it can even remove some viruses
  • Used in gravity filters for home, portable straw and hiking filters.

3. Nanofilter(NF):

  • Pore size of about 0.005 micron(5 billionth of a metre).
  • The most efficient hollow fibre membrane, it can filter out all contaminants and even reduce TDS by 80-90%.
  • Used as a pre-filter for RO membranes, for water softening in some applications and in the dairy industry.
Chart showing efficacy of hollow fibre membrane water filters

UF Maintenance

As with any filtration membrane, over time, the particles that get filtered out will start clogging the filter. This decreases the filtration efficiency and flow rate of water as it gets purified.

Once you start to notice a decreased flow rate- it generally is time to backflush your UF filter membrane. 

Here's how you do it:

Open your UF water purifier and remove the filter membrane. Now place it back upside down and pour purified water into the input end of the water purifier.

What this does is, since the filter is placed in reverse, the pure water carries away all the clogged particles with it and you'll see a stream of dirty water pouring out of the output end of your water purifier. 

You should repeat this backflushing process 2-3 times until the stream of water coming out clears up, i.e: all the clogged particles have been removed.

After this, you can place your filter back in the normal position and resume normal usage. 

So, whether you have a Microfilter or Ultrafilter- backflushing periodically is important. 

Also, every 3-4 months or so, you should change the filter cartridge. Over time you'll notice that even backflushing won't help increase the flow rate, and that's when you'll know it's time to change the filter.

Advantages and Disadvantages of UF Water Purifiers

Pros:

  1. No chemicals used
  2. No electricity needed
  3. Low maintenance costs- only the UF membrane needs to be changed every 3-4 months

Cons:

You should only use a UF purifier if you're confident your water won't contain viruses. Most urban water supplies across the world don't but where I live in India, you can't count on it.

Also, if you're interested in getting a UF water purifier, here are my Top 3 Gravity Water Purifiers(non-electric and mostly use hollow fibre membrane technology)

Conclusion: RO Vs UV Vs UF- Which Water Purifier Should You Get?

If you have a hard water supply or water that contains dissolved impurities(heavy metals, chlorine, etc), you pretty much have no choice but to buy a RO water purifier.

But if you get soft water, I implore you to get a UV+UF or a UF water purifier. It's way better for the environment and it's going to cost you less as well.

And while it's nice to know the top purifier brands, don't go crazy on a particular brand-choose a purifier according to your needs and it'll serve you well.

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