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How Do Water Filters Work?

Water filters can be extremely effective at removing contaminants from water and leaving you with cleaner, better tasting results. But how do water filters work? There are several types of water filters available, including backwashing filters and cartridge filters, and each type works a little differently. Read on to understand how filters work and what the different types of filtering media do.

Water Filter Types

There are two main types of water filters used in most homes and businesses: cartridge filters and backwashing filters. Cartridge filters have the filtering media in a removable cartridge that can be pulled out and replaced once it has been exhausted. Backwashing filters are made up of a large tank that holds a filtering media that can be cleaned and reused. The answer for "how do water filters work" depends on which type of filter you have.

A diagram of the different parts of a cartridge water filter

How Do Cartridge Water Filters Work?

Cartridge filters are some of the most common water filter types, and they are pretty simple to understand. In most cases, this type of filter is made up of a filter housing that has a cap and a sump; the actual filter cartridge sits inside the sump. Water flows into the filter through a valve in the cap, and it is directed down through the filtering cartridge. Once it has passed through, the water then flows back out through the cap to be used or treated further.

How Do Water Filters Work in Backwashing Systems?

An illustration of how the backwashing cycle works in a water filter

A backwashing water filter works by allowing water to flow through a loose filtering media. This type of system is made up of a control valve that’s attached to a large tank filled with the media. The control valve governs the flow of water through the filter, telling you how many gallons have been treated and the flow rate. This controller also is what determines when the system will go into its "regen" or backwashing cycle.

How does filtration work in this type of system?

  1. Water flows into the control valve and out over the top of the media.
  2. It slowly makes its way down through the media, which captures various contaminants as the water passes through.
  3. Once the cleaner water has reached the bottom of the tank, it travels up through a central tube and out into the home.


So how do water filters work when the media is backwashed? Once the filtering media has reached its exhaustion point – it’s holding as much of the contaminants as it can – the control valve starts the regen cycle:

  1. First, the media is "backwashed" by running water through the tank backwards. Basically, what this means is that fresh water is run through the tank from the bottom – rather than the top – and rinses all of the particles and contaminants off of the media.
  2. This backwash water is then rinsed down the drain, taking the contaminants with it.
  3. Then fresh water is sent through the system from the top, flushing out any remaining unfiltered or contaminated water, and rinses it all down the drain.

Regeneration cycle time can vary depending on what type of filtering media you’re using, but it typically doesn’t take more than about 30 minutes, after which, the system goes back into regular service. Most control valves are set to start the regeneration cycle at 2 AM so there is less risk of interrupting the normal water flow.

Water Filtering Media

While the majority of cartridge filters and backwashing filters work in the same way, what makes them special is what’s actually in the media. Whether it’s loose in a tank or contained within a cartridge, the media is what acts to remove contaminants from the water. Media can either be physical or chemical, meaning that it either prevents contaminants from physically moving through the filter, or it interacts with the contaminants in the water in some way to remove them.

How do water filters work when using these two processes? Most filters actually use a combination of the two to clean your water. Chemical filters – often called "active" filters – in particular often work to change the contaminant in some way, then physically prevent it from passing out of the system.

How Does Filtration Work for Physical Media?

Harmsco WB-20W WaterBetter Pleated Polyester Water Filter CartridgePhysical water filters are typically used to remove larger particles, like sand or sediment, from the water. Cartridge sediment filters are made from materials including pleated polyester, spun polypropylene, and polypropylene string. When water flows through these materials, the larger particles of dirt, sand, and silt get caught in the filter and trapped there. Some of these filers can trap particles as small as 0.35 microns; by comparison, a human red blood cell is about 8 microns in diameter.

In a backwashing sediment filter, the tank can be filled with filter sand, clinoptilolite ore (Filter-Ag Plus), zeolite (a group of very porous minerals), or a multimedia combination. These materials work in much the same way as a cartridge filter does, capturing particles as the water passes through.

Reverse osmosis systems are also a type of physical water filter. These systems differ from standard water filters in that they use a special membrane rather than a filter cartridge or loose media. Untreated water is pushed through the membrane, which prevents nearly all contaminants from passing through it. In order for reverse osmosis to be effective, the water should pass through both physical and chemical water filters first to eliminate sediment, chlorine, and other contaminants that could damage the membrane.

Active Filtration

How do water filters work with active filtration? This method works by changing the contaminant in some way, making it easier to capture. Some of the most common types of active filtering media include carbon, greensand, and ion exchange media. Other media, such as calcite, actually add compounds to the water rather than removing them.

Carbon filters are available in three main types: granulated activated carbon (GAC), catalytic carbon, and bone char. GAC is an extremely popular and common type of filtering media, and it’s particularly effective at removing chlorine and many organic contaminants. It has been specially treated to increase its surface area and promote adsorption. Catalytic carbon, typically made from coconut shells, is similar to GAC, but the electronic structure of its surface has been modified to make it extremely effective at capturing and holding chloramines, a type of disinfectant. Bone char is a special type of carbon made from select animal bones, and it’s very good at attracting fluoride ions.

Greensand is a filtering media made from sandstone that has high levels of the mineral glauconite. In a water filter, the greensand is coated with manganese oxide, which allows it to remove dissolved iron, manganese, hydrogen sulfide, and other contaminants from the water. A combination of catalytic carbon and hydrogen peroxide (as an oxidant) can also be very effective for iron removal.

Perhaps more commonly known for its use in water softeners than in water filters, ion exchange can be used to remove contaminants as well. Ion exchange works by using either a cation or anion media (or sometimes a combination of the two). Cation media attracts positively charged ions, while anion media attracts negative ions. Since nearly all contaminants in water have a positive or a negative charge, the right type of ion exchange media can be used to treat many different water problems. This type of system is often much more expensive than other water filters, however, which can be nearly as effective for many specific water issues.