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A Beginners Guide To Household Water Filters

How water filters work
How water filters work

Do you want to learn how water filters work?

Have you ever looked at the huge range of water filters available and wondered what the differences are between them?

Water quality is important when it comes to healthy living. It’s the second most essential requirement for you to survive, with oxygen being the first.

It is therefore only logical to make sure that the water you drink and use is safe.

You can do that by using a water filter.

To understand the ins and outs of household water filters just keep on reading!

In this article I will discuss:

What is a household water filter?

Water is a chemical compound made up of hydrogen and oxygen.

Under normal circumstances there are always other substances present in water aside from hydrogen and oxygen.

This is also true even for water coming from water treatment facilities.

These substances and particles range from microscopic living organisms like bacteria, to soil particles and dissolved chemicals.

Most of the stuff in natural occurring water has positive health effects.

It is, however, the handful of harmful things that we need to watch out for.

These include harmful bacteria, viruses and microscopic parasites.

Some of the most common diseases caused by these harmful organisms are giardia, typhoid fever, cholera and hepatitis A and E.

Most of the times diarrhoea is the main symptom of waterborne diseases.

You also get harmful man made chemicals in water like herbicides and insecticides.

A water filter is a device or system that removes some or all of these.

How a mechanical water filter works

A mechanical water filter has a filtering medium with really small pores in it.

Water flows through this medium and anything in the water that is larger than the pores, gets stuck.

That is just the basic principle on which it works.

In reality, though, there’s a lot more to it, especially when you look at the filter pore sizes, nominal vs absolute filter ratings and types of filtering surfaces.

Filter pore sizes

The pores in the filtering medium are extremely small.

The diameter of the pores is measured in microns.

Micron is another name for micrometer. It is indicated as “µm”.

There are 1000 micrometers, or microns, in 1 millimeter.

To give you an idea of the scale of things, here are some measurements in microns.

100 µm – Grain of salt

80 µm –  Human hair

40 µm –  Limit of visibility

25 µm –  White blood cell

0.2 to 10 µm –  Bacteria

0.005 to 3 µm –  Viruses

0.0005 µm –  Oxygen

The diameter of particles in water are also measured in microns.

That way you know which particles a certain filter will be able to remove since all mechanical water filters have a filtering capability rating given in microns.

For example, if your filter is rated as 0.2 microns, it is capable of filtering out most bacteria since they range between 1 and 10 µm in length and 0.2 and 1 µm in diameter.

That is, if your filter has an absolute rating of 0.2 µm and not a nominal rating.

Nominal vs Absolute filter ratings

When a filter has an absolute rating of 0.2 µm, like in the example above, then that filter is capable of filtering out 99.99% of particles larger that 0.2 µm.

If, however, it has a nominal rating of 0.2 µm, then it would be a bit different.

Nominal ratings usually comes as a double number rating. A filter would, for example, be rated as 95% of 0.2 µm.

That means that the filter will filter out 95% of the particles that is larger than 0.2 µm.

Filter surface types

There are basically two different types of filters when it comes to the filtering medium surfaces. These are surface and depth filters.

Surface filters

Surface filters have a more flat, uniform filter surface. Particles larger than the pore sizes are all retained at the surface of the filter.

These particles accumulate and increase the fineness of the particles retained.

That’s because over time even particles small enough to pass through the pores of the filter can’t get through the layer of bigger particles.

Because the filter surface is flat, the total filter area is usually enlarged by pleating it. That way you have a larger filter surface area without taking up more room.

Depth filters

Depth filters, on the other hand, have thicker filter media or multiple layers of filtering media.

Particles of different sizes penetrate the medium to different depths. Smaller particles are generally able to penetrate deeper than the larger ones.

The difference between surface and depth filters

Think of it this way.

A surface filter is like a flat surface with similar sized holes in it. When a selection of differently sized objects is thrown on the surface, anything smaller than the holes in the surface will fall through. Any objects larger will accumulate on top of the surface.

As these larger objects keep on accumulating, it would get increasingly difficult for even the smaller objects to make it through.

On the other hand, a depth filter is like a lot of fibres pressed together and compressed.

When a selection of differently sized particles is thrown on top of the mat of fibres, particles of different sizes will be able to penetrate it from the top.

Generally, smaller particles penetrate deeper into the filter. When small enough, it will make it right through.

To increase the size of the particles that are allowed through, the density of the fibres in the filter is decreased and the fibre diameter is increased. That way you get a less dense filter medium.

The opposite is also true, to decrease the size of the particles allowed through, the fibre density is increased and the fibre diameter decreased. This will give you a more dense filter medium.

What is activated carbon

Activated carbon, also called activated charcoal, is charcoal that is chemically treated to increase the number of microscopic pores. That way the total filtering surface area is increased.

So much so that 1 gram of activated charcoal has a surface area of between 950 and 2000 square meters!

Activated charcoal is really good at absorbing toxins and certain other substances like chlorine.

Almost all household water filters have activated charcoal incorporated somewhere in the system, with the main duty of removing chlorine and odors from water.

Single vs Multistage filters

Filters get clogged gradually as they are being used.

As filtered out particles build up over time on the filter surface, the effectiveness of the filter declines because more pressure is needed to keep the water flowing through.

That is because even smaller particles that would normally go through, gets caught up between the bigger ones in the layer that forms on the filter.

This is the challenge with a single stage filter. It only has one filter with one rating.

To be effective that rating has to be high, which means quicker clogging.

The workaround for this is to use multistage filters.

A multistage filter has more than one filter. The most commonly used household filter is usually a 3 or 4 stage filter.

You do get filters with more stages, even up to 8.

The principle is that you start of with a very coarse filter to remove the biggest particles like sediment.

The filters then get finer and finer.

This way each of those filters takes longer to get clogged because the bigger particles gets removed by the filter just before.

In some instances all the stages of a filter is not just mechanical, which is the use of the porous physical barrier discussed in this article.

UV lights, magnetic stones and reverse osmosis are also sometimes included into the system as one of the stages.

Reverse osmosis (RO)

Reverse osmosis is where water is forced through a membrane using pressure.

The membrane is semi permeable, meaning it only lets through certain particles while blocking others.

In the case of water filters, it blocks solved inorganic solids and lets through water.

Examples of contaminants that reverse osmosis removes include aqueous salts and metal ions like iron, copper and lead.


We as humans will always need water. And as long as there is water, there will be contaminants in the water.

Water purification techniques at residential water treatment facilities are ever improving with new technologies becoming available regularly.

Governments are nowadays also focussing more and more on legislation for healthy living, including norms and standards for residential water supplies.

It is unfortunately highly unlikely that the water coming out of your tap, will always be safe to drink and use.

I therefore highly recommend water filters as a personal last defense for your home.

Chances are that your tap water is safe to drink. In which case your water filter will even last longer due to to reduced contaminants in the water.

But you will have at the ready your household water filter, already installed, for when the day comes that the water, for whatever reason, becomes unsafe to use.