Every sip of water we drink passes through an unseen battlefield. Whether you drink tap, bottled, or filtered water, the vast majority of it is filtered against contaminants we can’t see. But what exactly happens inside these filters as they work day in and day out? Thanks to industrial CT scanning, we can now peer inside.

The tech research company Lumafield uncovered how water filters really look on the inside — uncovering how they protect our health and what happens when they get old.

Pitcher Water Filters

Pitcher filters have become a staple in millions of homes and they’re elegantly efficient.

They use a combination of activated carbon and ion-exchange resin to remove impurities from water. As water passes through the filter, the activated carbon traps contaminants like chlorine, pesticides, and volatile organic compounds (VOCs) through a process called adsorption. Meanwhile, the ion-exchange resin works to remove metals like lead and copper by swapping harmful ions with safer ones, improving both water quality and taste.

As the CT scan highlights, when the filter is new, it’s in excellent shape. But in time, it can become saturated with trapped pollutants. It starts to exhibit clearly defined flow channels; this means water preferentially flows through that channel, which limits the filtration. The denser particles tend to migrate preferentially as well.

When the filter reaches this state, it’s already far less effective.

Refrigerated Water Filters

Refrigerator water filters are built into fridges to provide clean drinking water and ice by using a dense activated carbon block enclosed in a sturdy plastic shell.

As water flows through the filter, the carbon absorbs contaminants like chlorine, sediment, and volatile organic compounds (VOCs), improving taste and odor. Some advanced models also reduce lead, mercury, and pharmaceuticals. Over time, as impurities accumulate within the carbon structure, the filter becomes less effective.

Note the change in the relative density of the components. Initially, the plastic casing was the densest but in time, the filtration medium became denser than the shell.

Reverse Osmosis Water Filters

Osmosis is the natural process where water molecules move from an area of lower solution concentration to an area of higher concentration through a semi-permeable membrane, balancing the concentrations on both sides.

Meanwhile, reverse osmosis filters use a semi-permeable membrane with microscopic pores to remove contaminants at a molecular level. Water is forced through this membrane under pressure, allowing only pure water molecules to pass while blocking salts, heavy metals, bacteria, viruses, and chemicals. The system typically includes pre-filters to remove larger particles and post-filters to enhance taste.

These filters are typically very efficient, but in time, they have “scars” of scaling mineral deposits. These deposits clog the membrane players, restricting water flow and, like with the other filters, reducing filtration efficiency over time.

The Bottom Line: Change Your Water Filter (and CT is Useful)

If you can’t remember how long you’ve had your water filter, it probably needs a change.

The rate at which you should change your filter isn’t fixed, however. Most manufacturers recommend somewhere between 2-3 months and 6-12 months, but this depends greatly both on the type of filter and on the water you’re drinking. For instance, limestone-rich water is more likely to produce deposits that affect the filter and reduce its lifespan.

Over time, however, all of these water filters become less efficient. They can become clogged with trapped contaminants like sediment, heavy metals, bacteria, and chemicals, reducing their effectiveness and potentially allowing impurities to pass through. As the filtration media degrades, water may take the path of least resistance, bypassing key filtering components and compromising quality. In systems like reverse osmosis and pitcher filters, accumulated debris can also foster bacterial growth, making the water less safe to drink.

Regularly replacing filters ensures they continue to function as intended, maintaining optimal water purity, taste, and safety for you and your household. Of course, if you happen to have a CT scanner around, you can just test them and see what they look like on the inside. In the meantime, you’re probably better off just changing them regularly.