Water is one of the most important compounds in existence, it covers 71% of our planet’s surface and the human body can contain as much as 75% of the stuff. Uses of water include agriculture, science, medical, transportation, heating, recreation, food processing, washing and most importantly drinking.

For the majority of us, drinking water comes from a treated municipal supply, which is safe to drink but will often feature unpleasant tastes and odours from chemicals such as chlorine, which are used to treat the water. Depending on where you live, your water may be a hard water supply that causes limescale build-up, which in turn can block pipes and damage appliances. These 2 problems along with a number of other common water problems often result in people utilising a water filter to improve the quality of their water but how water filters actually work?

The 5 Types of Filter

Subject to your application, what you’re trying to remove or in some circumstances trying to stop, there are 5 types of water filter:Household Jug Water Filter

  1. Mechanical Filters
  2. Absorption Filters
  3. Sequestration Filters
  4. Ion Exchange Filters
  5. Reverse Osmosis Filters

Each one of these addresses different water problems and many filters actually use a combination of these methods to perform multiple levels of filtration.

How Do They Work?

Mechanical

The basic idea of mechanical filtration is to physically remove sediment, dirt or any particles in the water using a barrier. A mechanical filter can be anything from a basic mesh to filter out large debris to a ceramic filter, which has an extremely complex pore structure for ultra-fine filtration of pathogenic organisms.

Any filter that utilises mechanical filtration will usually be given a micron rating, which indicates the ability the filter has to remove contaminants by the size of the particles in the water. Common ratings you might see include:

  • 5 micron – Will remove most particles visible to the naked eye.
  • 1 micron – Will remove fine particles.
  • 0.5 micron – Will remove cysts (giardia and cryptosporidium).
Wound Sediment Filter for Mechanical Filtration

Wound sediment filter with a 100 micron rating for mechanical filtration

Absorption

Absorption in a water filter is most commonly carried out by carbon, as carbon is probably the most absorbent material known to man. The reason carbon is so absorbent is because it has a huge internal surface which is jam packed with nooks and crannies, that can trap chemical impurities such as chlorine.

Most common domestic water filters contain granular activated carbon (GAC), which reduces unwanted tastes and odours by absorption. More expensive filters use a more effective carbon block, which also carry a micron rating for particle removal.

A variety of different substances are used to make carbon for filters, including wood and coconut shell, with coconut shell being more effective but also more expensive.

Activated Carbon for Absorption Filtration

Granular activated carbon and a carbon block for absorption filtration

Sequestration

Sequestration is the action of chemically isolating a substance. Food grade polyphosphate is commonly used in water filters to sequester calcium and magnesium minerals which cause limescale and corrosion.

The polyphosphate is introduced in very small amounts but it only inhibits scale. The polyphosphate does not soften the water but attempts to keep the minerals in the solution and stop them forming as scale on any surfaces they come into contact with.

Due to the minerals still being present in the water, scale inhibition isn’t suitable for all applications. In areas with alkalinity levels of 180ppm or more (very hard water) and applications where the water is kept at a constant temperature of 95°C or more, actually softening the water with a process such as Ion Exchange is a better option.

Ion Exchange

Ion exchange is a process used to soften hard water by exchanging the magnesium and calcium ions found in hard water with other ions such as sodium or hydrogen ions. Unlike scale inhibition, ion exchange actually softens the water so it can effectively be used in areas where alkalinity levels are 180ppm or more and applications where the water is kept at a constant temperature of 95°C or more e.g. commercial coffee machines.

Ion exchange is most commonly carried out using an ion exchange resin, which normally comes in the form of small beads. This is same resin that is used in some water softeners and in the case of a water softener the resin utilises sodium ions and these need to be periodically recharged or the resin will become ineffective. As water filters are usually sealed units you would simply replace the filter with a new one but some filters such as calcium treatment units (CTUs) can be regenerated.

Resins that utilise sodium ions aren’t usually used in drinking water filters as there is a limit set on the recommended level of salt (sodium) in drinking water of 200 mg/l. As sodium ion exchange increases salt levels, a hydrogen based ion exchange resin is the preferred option for water filters.

Reverse Osmosis

Reverse osmosis (RO) is the process of removing dissolved inorganic solids (such as magnesium and calcium ions) from water by forcing it through a semipermeable membrane at pressure so the water passes through but the unwanted contaminants are left behind.

Reverse osmosis is a highly efficient way of purifying water but depending on the level of filtration required an RO filter is usually combined with a number of other filters such as a mechanical (sediment) filter and an absorption (activated carbon) filter.

Reverse osmosis systems force water through the membrane and depending on the pressure of the feed water an electronically powered pump (booster pump) may be required, which can make running a reverse osmosis unit more expensive than other filtration methods.

Reverse Osmosis System

4 stage domestic drinking water reverse osmosis system

Combinations

Each filter method has limitations on what it can filter out so most water filters or filtration systems use a combination of methods to achieve a specific level of water purity. For this reason, saying exactly how a water filter works is difficult as it really depends on what’s inside it.

To give you a better idea, something like a household jug filter will utilise mechanical, absorption and ion-exchange whereas an inline filter will utilise mechanical and absorption with the possible inclusion of sequestration if the filter is designed to inhibit scale. Reverse osmosis systems can utilise mechanical, absorption, sequestration and of course reverse osmosis depending on how many stages the RO system has.

Every filter can work in a different way but now you have a basic understanding of the 5 ways water can be filtered, working out what’s going on in a particular filter should be pretty straightforward.