A water filter is a device that removes unwanted substances from your drinking water by physically blocking particles, chemically attracting contaminants, or both. Filters range from simple pitcher cartridges that improve taste to multi-stage systems that strip out lead, pesticides, and even bacteria. The type you need depends on what’s actually in your water and what you want to remove.
How Water Filters Work
Every water filter relies on one of two basic principles: mechanical filtration or chemical adsorption. Most home filters use a combination of both.
Mechanical filtration is the simplest concept. Water passes through a physical barrier, like a mesh screen or a tightly packed cartridge, that traps suspended particles on its surface or within its structure. Think of it like a very fine sieve. This approach catches sediment, rust flakes, and other visible debris, but it cannot remove dissolved chemicals or extremely small particles on its own.
Chemical adsorption works differently. Materials like activated carbon have an enormous internal surface area covered in microscopic pores. As water flows through, dissolved contaminants stick to those surfaces through a chemical attraction. This is how carbon filters pull out chlorine, certain pesticides, and compounds that cause bad taste and odor. The carbon doesn’t just block these substances; it grabs and holds them.
Common Filter Types
Activated Carbon
Activated carbon is the most widely used filter media in home systems. It excels at removing taste and odor compounds, chlorine, and volatile organic compounds (VOCs) like solvents and fuel byproducts. Carbon filters are found in pitchers, faucet attachments, refrigerator cartridges, and under-sink units. Their limitation is that they don’t effectively remove dissolved minerals, salts, or most heavy metals unless specifically designed and certified to do so.
Reverse Osmosis
Reverse osmosis (RO) forces water under high pressure through a semi-permeable membrane with pores so tiny that most dissolved contaminants can’t pass through. Water molecules form hydrogen bonds with the membrane material and squeeze through, while larger molecules of lead, arsenic, fluoride, and many other dissolved substances get left behind and flushed down the drain. Most residential RO systems include multiple stages: a sediment prefilter, an activated carbon stage to remove chlorine (which would damage the membrane), the membrane itself, and sometimes a carbon post-filter to polish the taste. RO is one of the most thorough filtration methods available for home use, but it produces wastewater and removes beneficial minerals along with harmful ones.
Ion Exchange
Ion exchange filters use beds of synthetic resin beads to swap harmful ions in your water for harmless ones. In a common setup, negatively charged contaminants like nitrates trade places with chloride ions on the resin surface, while positively charged contaminants like lead or calcium swap with sodium ions. This is the technology behind traditional water softeners, which replace calcium and magnesium (the minerals that cause hard water scale) with sodium. Specialized ion exchange resins can also target specific pollutants like PFAS.
UV Purification
Ultraviolet purification doesn’t filter water in the traditional sense. Instead, UV-C light damages the DNA of bacteria, viruses, and other microorganisms so they can no longer reproduce or function. The organisms don’t die instantly; rather, their ability to grow and replicate is permanently disrupted, rendering them harmless. UV systems are often paired with a physical filter, since UV light handles biological threats but does nothing about chemicals, heavy metals, or sediment.
What Filters Actually Remove
No single filter removes everything. What a filter can handle depends on its type and design. Activated carbon systems target chlorine, bad taste, odor, and organic chemicals. RO membranes remove the broadest range of contaminants, including dissolved salts, heavy metals, and fluoride. Both activated carbon, ion exchange, and RO systems have been shown by the EPA to significantly reduce PFAS levels, the group of industrial “forever chemicals” that have contaminated drinking water supplies across the country.
The size rating of a mechanical filter, measured in microns, tells you how small a particle it can catch. A 50-micron filter blocks particles visible to the naked eye, roughly the diameter of a human hair. A 10-micron filter catches much finer sediment but won’t stop bacteria. To physically block bacteria, you generally need a filter rated below 1 micron, and even then, viruses are too small to be caught by anything short of an RO membrane or UV treatment.
Whole-House vs. Single-Tap Systems
Water filters come in two broad categories based on where they connect to your plumbing. A point-of-entry system installs at your main water line, treating every drop that enters your home. Every faucet, shower, and appliance receives filtered water. These whole-house systems typically handle sediment, chlorine, and hard water minerals, protecting your pipes and fixtures as well as your drinking water.
A point-of-use system filters water at a single tap, usually the kitchen sink. Under-sink RO systems are the most common example. Pitcher filters, faucet-mounted units, and refrigerator cartridges are also point-of-use devices. If your main concern is the quality of your drinking and cooking water, a point-of-use system is usually sufficient and far less expensive than treating your entire home’s supply.
How Filters Affect Minerals
Tap water naturally contains minerals like calcium, magnesium, and potassium, which are nutritionally beneficial. Whether your filter strips them out depends on the technology. A 2025 study testing multiple pitcher filter brands found that most caused a significant drop in calcium levels, and several also reduced magnesium. Some brands that claimed to preserve healthy minerals, including fluoride, actually removed them.
RO systems are especially thorough at removing minerals. If you use an RO system and want to retain mineral content, some units include a remineralization stage that adds calcium and magnesium back after filtration. For most people, dietary sources of these minerals far outweigh what you’d get from water, so this is more of a taste preference than a health concern.
Certifications to Look For
The most reliable way to know what a filter actually removes is to check for NSF/ANSI certification. These are independent testing standards, and each number covers a different category of contaminant:
- NSF/ANSI 42 covers aesthetic improvements: chlorine, taste, odor, and particulates. If a filter carries this certification, it’s been tested and verified to make your water taste and smell better.
- NSF/ANSI 53 covers health-related contaminants. This standard includes over 50 specific claims, with the most common being lead, the parasite Cryptosporidium, VOCs, and chromium. If removing a specific harmful substance is your goal, this is the certification to look for.
- NSF/ANSI 58 applies specifically to reverse osmosis systems, including their ability to reduce PFAS.
- NSF/ANSI 401 addresses emerging contaminants, chemicals that have been detected at trace levels in drinking water supplies but aren’t yet broadly regulated. This includes certain pharmaceuticals and herbicides.
A filter without any of these certifications isn’t necessarily ineffective, but there’s no third-party verification backing up the manufacturer’s claims.
Maintenance and Replacement
Every water filter loses effectiveness over time as its media becomes saturated with the contaminants it has captured. Using a filter past its lifespan doesn’t just stop filtering; it can release trapped contaminants back into your water.
Carbon filters, whether in a pitcher, faucet mount, or under-sink housing, generally need replacement every 6 to 12 months. Reverse osmosis systems require more attention: prefilters should be swapped every 6 to 18 months, while the RO membrane itself typically lasts about two years. The actual timeline depends on how much water you use and how contaminated your source water is. Harder, dirtier water exhausts filters faster.
Filters rated at very fine micron levels (below 5 microns) tend to clog more quickly because they catch smaller particles. If you have water with a lot of sediment, using a coarser prefilter upstream protects the finer, more expensive filters downstream and extends their useful life.