Water is one of the most important substances on the planet, it covers 71% of the Earth’s surface and the human body can contain as much as 75% of the stuff. Water is vital to a huge number of applications including agriculture, science, medical, transportation, heating, recreation and food processing as well as washing and perhaps most important of all: 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 disinfect the water and keep it free of germs and bacteria. Depending on where you live, you may also find that your mains water causes limescale deposits to form which can block pipes and damage appliances. These issues, chlorine taste / odour and limescale formation are just two among a host of other common water problems which can be solved by water filtration. But how do water filters actually work?
The 5 Types of Filter
Subject to your application, i.e. what you’re trying to remove or in some circumstances trying to stop, there are 5 types of water filter:
- Mechanical Filters
- Absorption Filters
- Sequestration Filters
- Ion Exchange Filters
- Reverse Osmosis Filters
Each one of these addresses a different water problem and many filters actually use a combination of these methods to perform multiple levels of filtration.
How Do They Work?
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.
A filter that utilises mechanical filtration will usually be given a micron rating which indicates how effective the filter is in terms of the size of the particles it is capable of removing. Common ratings you might see include:
- 5 micron – Will remove most particles visible to the naked eye.
- 1 micron – Will remove particles which are too small to see without a microscope.
- 0.5 micron – Will remove cysts (giardia and cryptosporidium).
Absorption in a water filter is most commonly carried out by carbon, which is highly effective at capturing water-borne contaminants. The reason carbon absorbs contaminants so readily is that 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 carbon block filter elements which are generally more effective and usually carry a micron rating for particle removal.
A variety of different substances can be used to make carbon for filters including wood and coconut shell, with coconut shell being more effective but also more expensive.
Sequestration is the action of chemically isolating a substance. Food grade polyphosphate is commonly used in water filters to sequester the calcium and magnesium minerals which cause limescale and corrosion. However, polyphosphate is generally only introduced in very small amounts and it only inhibits scale rather than eradicating it. This means that polyphosphate does not soften the water but instead works to keep the minerals within the solution, preventing them forming as scale on any surfaces they come into contact with.
Due to the hard minerals still being present in the water, scale inhibition isn’t suitable for all applications. Instead, water softening using a process such as ion exchange is usually recommended in water areas with alkalinity levels of 180ppm or more (very hard water) and applications where water is kept at a constant temperature of 95°C or more.
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 physically removes the hard minerals, reducing limescale and making water suitable for applications where it is kept at a constant high temperature e.g. in commercial coffee machines.
Ion exchange is most commonly carried out using an ion exchange resin which normally comes in the form of small beads. A similar type of resin is used in some water softeners and in the case of a water softener the resin utilises sodium ions which need to be periodically recharged to prevent the resin becoming ineffective. As water filters are usually sealed units you would simply replace the filter with a new one though it should be noted that calcium treatment units (CTUs) can be returned to the supplier and regenerated.
Resins that utilise sodium ions aren’t usually used in drinking water filters as the amount of salt (sodium) that can be present in drinking water is legally limited to 200 milligrams/litre. As sodium ion exchange increases salt levels, a hydrogen based ion exchange resin is the preferred option for water filters.
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 under pressure so that the water passes through but most of the contaminants are left behind.
Reverse osmosis is a highly effective way of purifying water and is usually combined with a number of other filters such as a mechanical (sediment) filter and an absorption (activated carbon) filter in order to return water with few contaminants remaining.
Reverse osmosis systems use water pressure to force water through the membrane so it uses no electricity, though a certain amount of waste water is produced that has to be sent to the drain. The extra filters involved in multi-stage water filtration can make a reverse osmosis unit more expensive than other filtration methods but in applications where 99.9% pure water is required, RO offers the finest level of filtration available.
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. To give an example, a household jug filter will generally use 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 and of course reverse osmosis depending on how many stages the RO system has.
By understanding of the five different methods by which water can be filtered and the way they can be combined, you should hopefully find it easier establishing which kind of filter you need for any given application.