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How Do Water Filters Work? Understanding the Water Filtration Process


Everyone wants clean water — but the truth is that even water in nature isn’t technically “pure.” That’s why we humans have been improving the taste, odor and appearance of our drinking water since at least 4,000 B.C.

Of course, our methods — and the research that guides them — have advanced. Today’s water filtration systems work through sophisticated physical or chemical processes that address different contaminants in distinct ways. Some can even help control your water’s pH.

Read on to see how water filters work, what they do and why it matters.

A Closer Look at Water Filtration Processes

From the simplest water pitcher filter to the most comprehensive reverse osmosis (RO) or whole home system, water filtration is all around us. However, different methods have different results — and that’s an important distinction if you’re looking to solve a particular water quality problem.

Here are some of the most common water filtration processes and how they work:

Reverse Osmosis

Reverse osmosis is based on a naturally occurring process responsible for helping plants absorb water from soil. This is called “osmosis,” and it involves a semipermeable membrane separating two amounts of fluid. One side has a higher concentration of solutes than the other. To create a sort of equilibrium, fluid with fewer solutes tries to flow to the other side, moving through the membrane along the way.

In reverse osmosis filtration, this process happens backward: An outside force or pressure causes the fluid to move in the opposite direction, or against the “concentration gradient.” As this occurs, solutes like salt are left on one side of the membrane and filtered water flows into the other. That’s why RO is often used for seawater desalination as well as everyday filtration.

It’s important to keep in mind that the reverse osmosis process itself is often only one part of what’s actually happening in an RO drinking water filtration system. The best solutions combine RO with other filters and processes to catch different kinds, sizes and amounts of contaminants.* Some even have optional filter cartridges that put you in control of your water’s pH.

Activated Carbon

In this type of filtration, carbon or activated carbon forms the basis for a process called adsorption, not to be confused with absorption:

  • Adsorption: This process occurs when individual molecules, atoms or ions “stick to” a particular surface.
  • Absorption: This is what happens when something like a sponge completely “soaks up” a liquid and everything that liquid contains.

That one letter makes a big difference. Because carbon filtration uses adsorption instead of absorption, certain tiny materials stick to the carbon while the rest of the water flows through. This solution is a good fit for chlorine, sediment, certain organic compounds and some other contaminants, but it may not catch other materials that could potentially cause aesthetic and health concerns.

Activated Alumina

Activated alumina (AA) is a porous material made from aluminum oxide. An AA water filter works in much the same way as carbon filtration; however, it’s perhaps a bit more complicated — and that means its benefits can be a bit less reliable, too. For example, while AA filters are often a good fit for particular water quality problems such as fluoride or arsenic, the effectiveness of the filtration process relies on the AA material itself, plus the pH and material makeup of your water supply. Some people are also concerned that the main component of AA, aluminum, may leach into the water under certain circumstances.


Mesh filtration is perhaps the easiest to picture. It relies on tightly woven layers of wire cloth, creating a sort of trap that’s generally most effective for sediments such as sand and dirt. Mesh filtration on its own can’t catch certain contaminants or impurities, particularly those that have dissolved in the water.

You may notice that this process is somewhat similar to reverse osmosis. While it’s true that many kinds of filtration rely on the same basic concepts, keep in mind that RO membranes are designed differently and often installed in systems with other filter media.

Example: City Water Filtration

Although a simple water pitcher filter may rely on just one kind of filtration, some of the best systems unite different solutions for a more comprehensive, complementary treatment process. RO is one example of this, but a more familiar application may already be connected to your house: treated city water.

Cities have varying approaches to water treatment, sometimes because relevant governing bodies have established different water quality guidelines (like Water Quality Standards in the U.S. and the Guidelines for Canadian Drinking Water Quality in Canada). However, many cities use this combination of filtration methods:

  • Coagulation: During this first stage, water flows into the treatment center and is filled with positively charged chemicals such as salt or iron, which can neutralize negatively charged particles and help make them larger.
  • Flocculation: The water is mixed to create even larger particles called flocs, sometimes formed with chemicals.
  • Sedimentation: Because flocs are heavier than water, they settle to the bottom of the tank or container, much like sand sinks to the bottom of a lake.
  • Filtration: Next, treatment experts separate the flocs from the water by using a variety of filter types. These filters come in different sizes and materials, which means they’re able to address a wide variety of issues — from parasites and bacteria to bad odors. The plant may also use reverse osmosis during this step, particularly if it treats recycled water.
  • Disinfection: After filtration, it’s time to add chemical disinfectants such as chlorine. While this additive is intended to kill germs all the way from the plant to your tap and is not generally a health concern, it can cause unpleasant odors that you may need to address with your own filtration system.
  • Final treatment: Some plants may choose to adjust pH levels and add fluoride. Much like the inclusion of chlorine, these steps are intended to improve taste and promote better health respectively — but there are still times when you may want a water filter to take care of pH and fluoride levels.

Remember, private wells don’t have this kind of water treatment, which is why fluoride and chlorine issues are less common there while others, like high iron or hydrogen chloride levels, are more common. That’s not to say that filtration is only for wells, though; treated city water can still have problems from issues such as hard water or contaminants that are not yet federally regulated, like PFAS in water.

What Do Water Filters Do?

In many ways, filtration is a step you add to the water cycle. Think about the way water moves through ecosystems: It exists as precipitation, snowmelt and runoff and can gather in lakes, rivers, aquifers and more. In all of these stages and locations, it has plenty of opportunities to pick up minerals, sediment and other materials — and in nature, that’s not necessarily bad. It only becomes a problem when that unfiltered water ends up in your glass.

Filtration is able to address these concerns from any source or water system. After all, it’s not just nature that impacts your water quality; other elements — such as corrosion in your home’s piping or chlorine added to municipal water — can also play a role. That’s why it’s helpful to think of filtration as the last step of your water’s journey before it ends up at your tap.

Broadly speaking, a water filter’s job is to address and reduce a wide range of potential problems. Depending on the type of filter, these may include:

  • Silt, sand and sediment
  • Chlorine
  • Fluoride
  • Pesticides and other chemical runoff
  • Heavy metals
  • Bacteria

How exactly a filtration system achieves this depends on its size, structure and complexity — but the goals are generally similar:

  • Aesthetic quality: Get rid of odd odors, bad tastes and appearance issues such as discoloration or staining.
  • Health and safety: Address any contaminants that could make water unsafe to use for cooking, cleaning, bathing or drinking.
  • Maintenance: Ensure water doesn’t corrode your pipes, reduce the lifespan of certain appliances or cause staining around your home.

Do Filters Soften Your Water?

Filtration and softening aren’t interchangeable. The former addresses specific contaminants and water quality issues based on the filtration media or mechanism, and it has benefits such as clearer water and reduced taste issues. The latter removes different materials — like the calcium and magnesium that cause hard water — using resin beads and a saltwater “recharge” process that isn’t part of filtration. Softening doesn’t address taste, odor and appearance issues, but it can help solve hard water problems such as mineral deposits, spotty dishes, dry skin, brittle hair and more.

Do Filters Purify Your Water?

It’s important to note that filtration and purification aren’t the same thing, especially when it comes to at-home water treatment. Generally speaking, purification is a more involved process that makes water usable for medical, industrial and other purposes. Household tap water for drinking, cleaning, cooking and bathing is usually well-suited for regular filtration — as long as you choose the right filtration system, that is.

Types of Water Filtration

As you research house filter options, you’ll quickly notice that there are two primary types of filtration: physical and chemical. These can differ widely — not just in size, price and functionality, but also in comprehensiveness. That’s why it’s helpful to understand the basic mechanics of different filtration types and what they do in practice.

Physical Filtration

Physical filtration uses a physical barrier to block, trap or catch certain solid debris. It’s the kind of water treatment early civilizations likely used, although in a much simpler form than today’s filters, and you can see it for yourself: Just find a muddy river, dig a hole deeper than the water level and watch cleaner water filter up through the sand or dirt.

To get the job done, a physical filtration system needs certain components:

  • Barrier: This is the physical trap, somewhat like a net, that prevents some solid contaminants from passing through.
  • Fluid: This can be any fluid containing suspended solids — but in this case, it would be water from your well or municipal provider.
  • Force: Physical filtration doesn’t generally happen on its own; water needs to be forced through the filter media in some way before the process can begin.
  • System: Something needs to hold the medium, fluid and force together. This can be a natural system, like a river environment, or a mechanical one, like a home water filter.

Chemical Filtration

The biggest difference in chemical filtration is that it doesn’t use the same kind of barrier. Instead, it relies on filtration media — that is, an active material that attracts or otherwise traps some contaminants through chemical interactions. For example, in a carbon or activated carbon filter, the medium traps or “adsolves” dissolved materials as they pass through, thereby reducing their presence in the water supply.

Chemical filtration systems need many of the same components as their physical filtration counterparts. However, because each chemical medium type only reacts to or with certain contaminants, these systems may not be as comprehensive on their own.

Find Your Water Filtration Solution

Now that you understand how water filters work, it’s time to choose one that fits your needs. However, that may be easier said than done. That’s because some contaminated water problems can’t be seen, smelled or tasted — and other issues, such as discoloration or bad odors, can have multiple causes.

To ensure you get the right filtration solution for the right purpose, it’s smart to begin with a professional water test. Your local water expert will test for chlorine, iron, total dissolved solids (TDS) and more — and in about 30 minutes, you’ll have the answers you need. You’ll even get personalized recommendations to help you choose the best house water filter system.

Ready to get started? Schedule your free, in-home water test and consultation.

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