How Do Water Filters Work? (5 Popular Types Explained)

Water filters offer the promise of clean, filtered water, often directly from your kitchen faucet. But how do water filters work? What exactly happens inside a water filter to make your tap water any cleaner? We’ve shared everything you need to know in this guide.

📖 5 Types of Water Filters & How They Work

1) Mechanical Filters

Mechanical filters are designed to provide a physical barrier against contaminants like sediment, dirt, and other suspended particles.

There are numerous types of mechanical filters, including ceramic filters (which remove contaminants of all sizes, including pathogens, with a network of pores) and mesh filters (which are designed to trap suspended particles, like sand and rust, while allowing water particles to pass through).

Most mechanical filters have a micron rating, which tells you what sized contaminants can be removed. For instance:

• A 0.5-micron filter has one of the smallest micron ratings for mechanical filtration and is capable of removing even the tiniest contaminants, like cysts.
• A 1-micron filter is capable of reducing particles that are invisible to the naked eye.
• A 5-micron filter reduces contaminants that you may be able to see suspended in your water, like sediment or metal shavings.

💡 Sediment filters are perhaps the most common example of the mechanical filtration method. These filters offer physical filtration, removing large suspended particles in water. Sediment filters are often used to protect later filter stages, including adsorption and membrane filters, and extend their lifespans.

2) Absorption Filters

Adsorption filters are water filters that are capable of grabbing contaminants and holding them in their pores – a bit like a sponge – using a process called adsorption. Activated carbon filters are the most common types of adsorption filters. Activated alumina filters also reduce contaminants by adsorption.

Adsorptive filters have a large surface area, which gives the best chance of trapping all the contaminants present in water. Most adsorptive filters have a pore size of around 5-10 microns.

Only contaminants that are attracted to the adsorptive media can be removed by adsorption. In the case of activated carbon filters, these contaminants include chlorine, disinfection byproducts, pesticides, herbicides, and some VOCs. Activated alumina filters are capable of adsorbing fluoride, arsenic, and some selenium.

📌 There are several types of activated carbon adsorption filters. Granular activated carbon (GAC) and carbon block filters are two of the most common filters offering carbon filtration. Carbon block filters are typically more expensive due to their design – the media is packed together into a solid block, allowing for more thorough contaminant removal.

Coconut shells, charcoal, and wood are some of the materials that are used to make activated carbon adsorption filters. Activated alumina adsorption filters are made from an aluminum oxide ceramic compound.

Adsorption filters are cheap to buy and don’t add anything dangerous to water, which makes them a popular choice for a variety of filter systems.

3) Ion Exchange Filters

Ion exchange is a process that involves the exchanging of ions on a charged resin.

The most common use of ion exchange in water treatment takes place in a water softener, which exchanges magnesium and calcium ions with ions that don’t affect water hardness (such as potassium or sodium ions). This prevents scale formation and helps to reduce existing scale in plumbing fixtures and pipes.

Some water filters, such as water pitcher filters, combine ion exchange resin (usually hydrogen-based) with other water filter media, like carbon media, to remove substances like fluoride, arsenic, nitrates, and sulfate.

Any substance with a charge can effectively be removed with an ion exchange filter.

The ion exchange resin consists of small beads. In the case of a water softener, these beads are packed with sodium ions. As water flows through the resin, the sodium ions are released, and magnesium and calcium ions take their place. When the resin is saturated with hardness ions, the softener regenerates, flushing away these ions and replacing them with a fresh bed of sodium ions.

In the case of a water filter that uses ion exchange, the resin won’t regenerate – you’ll simply replace the filter cartridge when the resin becomes depleted.

The cost of an ion exchange filter depends on the filter size and application. Water softener systems need much more resin than a small tap water pitcher filter, and this is reflected in their cost.

4) Membrane Filters

Membrane filters use a process called membrane separation to reject the majority of total dissolved solids in a drinking water supply.

The most commonly found membrane filter is a reverse osmosis system. Ultrafiltration systems also use membrane filtration to treat water.

The main difference between the two is the membrane pore size – reverse osmosis systems have pores of around 0.0001 microns, while ultrafiltration systems have pores of around 0.02 – 0.05 microns, so they don’t remove quite as many contaminants.

📌 How do membrane filters work? They force water under pressure through a semi-permeable membrane. The membrane pores are so small that only water (and dissolved salts and solids, in the case of ultrafiltration) can pass through. The rejected contaminants are washed down a drain with a small amount of wastewater.

Membrane filters are usually combined with other filter types. For instance, in reverse osmosis filters, the reverse osmosis membrane is combined with a mechanical sediment filter, an adsorptive activated carbon filter (offering chemical filtration), and usually a post-filter, also made from activated carbon.

Membrane filtration is one of the most thorough water filtration processes, producing virtually 100% pure water and removing the majority of impurities, including metals, chemicals, minerals, salts, and microorganisms.

For this reason, reverse osmosis and ultrafiltration are often referred to as water purification systems.

However, there are a few setbacks of membrane filters, too: they’re more expensive due to their high-quality filtration, and they also waste water.

5) Deionization Filters

Deionization filters, otherwise known as “water polishing” filters, are units that exchange positive hydrogen and negative hydroxyl molecules with contaminants (both positive and negative) in water, in a process known as ion exchange.

DI water has had almost all its positively and negatively charged ions removed, including anions like sulfate and chloride, and cations like copper, calcium, sodium, and iron.

📌 The deionization process uses an ion exchange resin that’s loaded with hydrogen (H+) and hydroxyl (OH-) ions. These ions are exchanged with the positive and negative ions in the water, and the water treatment process continues until the DI resin is depleted of H+ and OH- molecules and the filter is replaced.

DI resins provide a cost-effective means of delivering deionized water on demand, but they have a few setbacks, too. Because these filters don’t have physical filtration media, they don’t capture dissolved organics, so they may require pre-filtration.

Filter Combinations

Because different types of water filters can remove different contaminants, some manufacturers combine several filter types in a single water filtration system. This allows for a more thorough cleaning process and removes a greater range of impurities.

Common types of water filters that are often combined are mechanical filters (such as sediment filters), adsorptive filters (such as carbon filters), and membrane filters (such as reverse osmosis membranes).

A whole home filtration system and an under-sink filtration system are most likely to contain filter combinations.

Other Water Filter Types

The filter types mentioned above all offer the most common traditional water filtration. However, there are other water treatment types that are often labeled as water filter systems, including:

• UV purification – UV purifiers treat water with ultraviolet light, which alters the DNA of bacteria, viruses, and other microorganisms, preventing them from reproducing or causing harm in the human body. Usually found as final-stage filters in whole-home filtration systems.
• Water distillation – A water distiller works by boiling water until it evaporates, then condensing it into a separate container, leaving the contaminants behind in the boiling chamber and producing purified drinking water. The water distillation process takes place in countertop units and needs electricity to operate.
• Air injection/ oxidation – Air injection/oxidation systems send water through an air bubble to oxidize iron, manganese, and sulfur, then removes these contaminants with a resin such as birm or manganese greensand. This filtration process is often found in well water filtration systems.
• Mineralization filters – A mineral-adding filter uses calcite or mineral beads to add minerals to water. Reverse osmosis units sometimes use mineralization filters as a final filter stage to reintroduce the minerals that were removed in the RO process. Alkaline water pitchers also use mineralization media to boost water pH and improve its taste.

🚰 Water Filter Applications

Water filters can be found in the following types of filtration systems:

Faucet Filter Units

Faucet filters usually use an adsorptive water filtration media, like activated carbon. These single-stage filtration systems attach as an extension to the end of your faucet, filtering your drinking water before it leaves the tap.

A faucet filtration system is one of the most affordable systems available today, making it a popular choice for people with small budgets.

Under-Sink Systems

Under-sink water filter systems typically consist of numerous filter stages, combining several filter types, including mechanical sediment filters, adsorptive carbon filters, and (sometimes) ion exchange filters.

Membrane filers, like reverse osmosis systems and ultrafiltration units, are also usually installed as under-sink systems.

Learn which under sink systems are worth investing this year.

Shower Filters

Shower filters are single-stage filters that usually combine numerous types of filter media within one cartridge. Adsorptive media makes up most of these filters. Ion exchange media may also be included. The purpose of these filters is to reduce chlorine and sometimes hardness minerals from shower water.

Check out the top shower filters of 2023.

Whole House Filtration Systems

Whole house water filtration systems are similar to under-sink filters, combining different types of filters to provide thorough water filtration. The difference is that they treat the water supply in your whole home, rather than just your drinking water.

There are also whole house water softeners, which use ion exchange to treat hard water, and whole house reverse osmosis systems, which use membrane filtration to purify water.

Lear more in our ultimate guide on the best whole home water filtration systems.

Fridge Filter Units

A fridge filter uses a single filter cartridge to reduce common contaminants in a water supply. Usually, a fridge filter is an adsorptive filter made from granular activated carbon or carbon block. The main purpose of a fridge filter is to improve the taste of water by filtering out chlorine.

Fridge filters may be built into a fridge (such as above the top shelf or under the back grille) or installed as inline filters along the water line leading to the refrigerator.

Dispense clean, healthy water from fridge with our recommended refrigerator water filters.

Portable/ Emergency Filter Devices

Portable or emergency filters are compact filters that make water potable (safe to drink) by filtering out microorganisms like bacteria and cysts. Usually, portable filters use mechanical filters, such as ceramic media, because they last a long time, have small enough pores to trap microorganisms, and can often be washed and reused. Some portable filters combine mechanical filters with other media types, like adsorptive media.

There are several kinds of portable or emergency filters, including straw filters, hanging gravity filters, and pump filters.

Filtered Water Bottles

Filtered water bottles are designed to improve drinking water quality on the go. There are various types of bottled water filters, including bottled straw filters (which typically use mechanical or adsorptive filtration), push bottle filters (which also use UV or adsorptive filters), and UV bottle filters.

To use a bottle water filter, you either:

• Drink through the straw to send water through the filter and out of the straw
• Fill the bottle, then push the lid (containing the filter) into the bottle to send water through the filter
• Press a button to activate the UV light

Here are the best filtered water bottles which remove the most contaminants.

Pitchers & Dispensers

Pitchers and dispensers use gravity filtration to reduce common contaminants in a water supply. Most water filter pitchers use adsorptive filtration, such as carbon filter media, which may be combined with other media types, like ion exchange and mechanical filtration.

When you add tap water to the top chamber in the pitcher, the water flows through the filter and into the bottom chamber, ready for drinking.

Check out our review on the most affordable & highest performing water filter pitcher brands.

Countertop Units

Countertop units are similar to pitchers and dispensers, but with a bigger water holding capacity and (usually) longer filter lifespans.

Most commonly, countertop filtration units use mechanical filtration (often a ceramic filter) to reduce contaminants of varying sizes, including microbiological contaminants.

Some countertop units use reverse osmosis filtration, combining various water filter types to significantly improve tap water quality.

See which countertop water filter fits your needs here

❔ How Do Water Filters Work? FAQ

Do filters really clean water?

Yes, filters really do clean water. A filter traps impurities in its media bed, preventing these impurities from passing through with water particles. This means that the water leaving the filter is cleaner than it was when it entered the system. The best water filters can reduce or remove hundreds of contaminants from water, giving it a significantly cleaner appearance, taste, and smell.

How do water filters work step by step?

A basic step-by-step of how a water filter works is this:

1. Contaminated water enters the filter.
2. The filter removes the contaminants using a dedicated treatment process. This could be by adsorbing or trapping the contaminants in its media, rejecting the contaminants from its membrane, exchanging ions, or treating the water with an electric current.
3. The water leaves the filter containing fewer contaminants than when it entered.

What does a water filter remove?

The contaminants removed by a water filter depend on the type of filter and its water filtration process. A typical adsorption or mechanical filter can usually trap chemical impurities like chlorine. Other contaminants commonly removed by a water filter are heavy metals, pharmaceuticals, nitrates, pesticides, herbicides, and volatile organic compounds (VOCs).

Is it worth it to filter your water?

It’s worth it to filter your water if you’re concerned about the effects of drinking contaminated water at home. Unfortunately, our drinking water supplies aren’t as clean as we’d like to hope. The only way to remove trace contaminants from your tap water is to use an at-home water filtration system.

What’s the difference between filtered water and bottled water?

Filtered water is water that has been treated in a water filter, while bottled water quality varies depending on the water source and its potential treatment. Not all bottled water is filtered. Some bottled water is simply bottled tap water.

What do water filters not remove?

Most water filters can’t remove microorganisms (because they’re small enough to slip through the filter pores), and minerals and salts. However, some filtration types, like mechanical filtration and membrane filtration, can remove these impurities.