How do Water Filters work?

The Water Filter Family

You can find a wide variety of water filters on the market and each one of them filters water in its specific way and with its individual limitations. The following blog gives you an overview of the most common members of this family and explains you how they work.

Number 1. The Sediment Filter

The most important characteristic that defines a filter of any kind is its “hole size” or its porosity due to the tiny dimensions of those holes. This refers to the particle size that will be trapped by the filter. A fifty micron filter traps particles of 50 microns or larger, while a 2 micron cartridge traps particles of two microns or larger. To give you a better understanding of those dimensions, 1 micron (micrometer) equals 0.000001m or 0.001mm which is way to small to see with your naked eye and about the tenth of a size of a hair. Bacteria for example can be between 0.2 microns and 3 microns in size whereas a grain of sand can have dimensions down to 5 microns.

If you have a reverse osmosis drinking water unit, 5 micron filtration is the normal size for the sediment prefilter. There’s no reason to use a tighter cartridge, and it might cut pressure to the membrane and have a negative result.

Sediment filters can be made of a variety of materials. Wound string or cord, polypropylene, polyester, cellulose, ceramic, glass fiber, and cotton are among the most common.

What does a sediment filter remove and reduce?

In your water supply the filtered particles may be rust flakes from the water pipes, sand grains, small pieces of organic matter, clay particles, or any other small particles drifting in the water.
Water that has a high sediment level can change the aesthetic value of the finished beverage. It also can have a detrimental effect on the performance of your equipment. Sediment can cause blockages in the strainers, flow controls and even the solenoids inside your equipment.

What does a sediment filter not remove?

The important thing to know about sediment filters is that they only reduce sediment. They don’t remove chemicals or heavy metals or make the water taste or smell better.

Number 2. The Activated Carbon Filter

Since the Egyptians discovered that storing water in charcoal made it stay fresher and taste better, carbon has been a standard feature in water treatment. Today It is manly used to remove odor and bad taste from your tap water.

Activated carbon filters are small pieces of carbon, in granular or block form, that have been treated to be extremely porous. Just one gram of activated carbon has a surface area of 500-3000m2. 4 grams is the equivalent of a football field. It’s the massive surface area that allows active carbon filters to be very effective in adsorbing (essentially removing) contaminants and other substances. Carbon attracts certain chemicals at the molecular level much in the way that a magnet attracts and holds metals. When the surfaces are full, the filter must be discarded and replaced.

How Activated Carbon adsorption works –

In addition to the surface area active carbon filters have different capabilities in terms of the size of contaminants they remove. They range from around 50 microns to 0.5 microns. The smaller the more effective but smaller pores also may reduce water flow.

When the water flows through active carbon chemicals stick to the carbon, resulting in cleaner water output. The effectiveness depends on the flow and temperature of the water. Therefore most smaller active carbon filters should be used with low pressure and cold water.

Activated carbon is usually made of coconut shells, wood or coal by Carbonisation. It has been treated by steam and high temperature in the absence of oxygen.

What does activated carbon remove and reduce?

Active carbon is very effective in removing chlorine.

According to EPA (the Environmental Protection Agency in the United States) Activated Carbon removes all 32 identified organic contaminants including THMs (by-products from chlorine). The same is true for all 14 listed pesticides and 12 herbicides. This does not include fluoride, nitrates, and sodium.

Chlorine removal

Most public tap water in Europe and North America is highly regulated, tested and certified for drinking. However, to make it safe, chlorine is added which may make it taste and smell bad. Activated Carbon filters are excellent at removing chlorine and related poor taste and odor. High quality activated carbon filters can remove 95% or more of the chlorine.

For more details on this read about total and free chlorine.

Chlorine should not be confused with Chloride which is a mineral combined by sodium and calcium. Chloride may actually increase slightly when the water is filtered with activated carbon.

Chlorine byproducts

The most common concern about tap water is by-products (VOCs) from chlorine such as THMs that are identified as potentially cancerous. Activated carbon is more effective than any other filter technology in removing these.

What does an activated carbon filter not remove?

Despite the contaminants Activated Carbon filters there is also many materials it doesn’t remove

  • Dissolved solids including minerals, salts or metals such as iron that are not considered contaminants (this means TDS does not reduce with activated carbon)
  • Most microbiological contaminants incl cysts, coliform and bacteria
  • Inorganic contaminants such as arsenic and asbestos
  • Radionuclides 

Activated carbon water filters generally does not reduce minerals or TDS (Total Dissolved Solids).

More specifically charcoal filters may not be sufficient in removing the following substances:

Microbiological contaminants

This is one of the most common drinking water issues in less developed countries. Especially for waterborne gastrointestinal diseases (e.g. diarrhoea that visitors not used to the local water get). Activated carbon filters on their own are generally not sufficient to remove such contaminants.


Common in some places where the groundwater has been contaminated. Activated Carbon removes 30-70% of arsenic but is not sufficient in places where this is highlighted as a real problem.

If your local water contains one or more of these substances then you should ensure that the filter reduces them to a safe level. Most of the time this means combining activated carbon with other types of filters such as Ion Exchange.

What does and what doesn’t Activated Carbon remove –

Microbiological contaminants –

Number 3. Reverse Osmosis Membrane

A reverse osmosis membrane is a semi-permeable membrane that allows the passage of water molecules but not the majority of dissolved salts, organics, bacteria and pyrogens. It operates down to an unimaginably tiny pore size of 0.0001 microns. As for bacteria and viruses, the smallest virus is about ten times as big as the largest possible pore in an RO membrane. And by comparison, bacteria are giants.

The membrane is continually flushed and contaminants are carried out of the vessel on a separate line which reduces waste buildup and accumulation in the element. This feature helps to protect the membrane from clogs and obstructions caused by bio-fouling, scale hardness and contaminant saturation. This is the main difference between a membrane filter and the filters described before therefore the lifetime of the membrane is much higher then of the average filter.

What does reverse osmosis remove?

If there’s lead in your water, a typical thin-film membrane will remove 94% to 96% of it. If your water has a high amount of sodium, the same thin-film membrane will remove 90% to 95% of it. Sodium is not removed by normal filtration. In addition to the ions listed, other EPA-regulated contaminants that RO is a preferred treatment for include nitrates and nitrites, arsenic, asbestos, and all of the cancer-causing “radionuclides.”

It also has a very high effectiveness in removing bacteria (for example, Campylobacter, Salmonella, Shigella, E. coli) and viruses (for example, Enteric, Hepatitis A, Norovirus, Rotavirus).

RO removes minerals because they have larger molecules than water. The subject of minerals and RO created controversy and disagreement among water and health professionals.  The World Health Organization (WHO) made clarification that majority of healthy minerals are needed for human body is from food or dietary supplementary sources and not from drinking tap water. In addition, minerals found in water can be harmful to human health.  The evidence is strong that calcium and magnesium are essential elements for human body.  However, its a weak argument to suggest that we should make up this deficiency through water consumption. Tap water presents a variety of inorganic minerals which human body has difficulty absorbing. Their presence is suspect in a wide array of degenerative diseases, such as hardening of the arteries, arthritis, kidney stones, gall stones, glaucoma, cataracts, hearing loss, emphysema, diabetes, and obesity. What minerals are available, especially in “hard” tap water, are poorly absorbed, or rejected by cellular tissue sites, and, if not evacuated, their presence may cause arterial obstruction, and internal damage. (read more)

What does reverse osmosis not remove?

Even though RO removes most impurities from water and makes it safe and reliable for use, the process doesn’t remove some impurities.

Contaminants not removed from water by RO filters include dissolved gases, some pesticides, solvents and volatile organic chemicals.

It is important to know, that no one piece of treatment equipment manages all contaminants. All treatment methods have limitations and often situations require a combination of treatment processes to effectively treat the water. Activated carbon filtration and sediment filtration is commonly used in conjunction with RO filter. In particular activated carbon filters remove chlorine and certain pesticides and organic solvents that the RO membrane is not as effective in removing.

Number 4. Remineralisation Filter

The remineralisation filter basically enhances purified water with essential minerals, which are important for it taste.

This filter replicates the process that occurs in nature when water flows through rocks. The water flows through the different layers of the filter and slowly picks up essential minerals.

There is a vide variety of filters available, with different ingredient combinations, which create remineralised waters with different characteristics, pHTDS and taste.

Used by the WHO as an indication of water purity, we refer to the Total Dissolved Solids (TDS) present in water, expressed as parts per million (ppm). It is the total amount of mobile charged ions – minerals, salts or metals – that are dissolved in a given volume of water. It is important to know that the TDS it self does not tell you anything about the actual content of dissolved solids in your water. But its fair to say that the smaller the TDS value, the purer the water. As a form of comparison, tap water has an average TDS value of 350 ppm, RO filtered water is at 5-10 ppm, which is basically pure water.

What does a remineralisation filter add to the water?

Depending on the filter you choose it can add some of the following minerals and many more.

  • Calcium
  • Magnesium
  • Potassium
  • Trace minerals: Iron, Manganese, Selenium, Zinc
  • Antioxidants

What does a combination of all these filters look like?

There are a few systems available on the market where you can find these filters combined. They achieve a superb water quality but only if the filters are changed regularly and the system can filter on peak performance. The only system on the market today with a true consumption tracking and wifi connection is the Osmo from Osmosys. This futuristic filter system enables real insides into your water consumption and true filter status. Osmosys even offers a worry free monthly subscription service in which all maintenance is included and you can be sure your water consumption is safe and of the highest quality all the time. Check out more about the machine and the service here

Global Water Crisis