A water filter is a device that removes contaminants from water by passing it through a material that traps, absorbs, or neutralizes unwanted substances. Most home water filters use one or more techniques: physically straining out particles, chemically attracting pollutants to a surface, or swapping harmful ions for harmless ones. The type you need depends on what’s actually in your water and what you want to remove.
How Water Filters Actually Work
At the most basic level, filtration means pushing water through a porous material that lets water molecules pass while blocking larger particles. But modern filters do much more than simple straining. A typical home filter combines mechanical straining with chemical adsorption and sometimes sedimentation, all happening simultaneously inside a single cartridge or housing.
The key distinction is between filters that physically block contaminants (like a mesh catching sand) and those that chemically attract and hold them (like activated carbon pulling chlorine out of water). Many systems layer both approaches. An under-sink reverse osmosis unit, for example, often includes a sediment prefilter, an activated carbon stage, and a membrane, each targeting different types of contaminants.
Activated Carbon: The Most Common Type
The filter inside your pitcher, refrigerator, or faucet attachment almost certainly uses activated carbon. This material works through adsorption: contaminants are attracted to and held onto the surface of tiny carbon particles as water flows past them. It’s not just trapping things by size. The carbon’s enormous internal surface area acts like a magnet for certain chemicals.
Activated carbon is effective at removing chlorine, chloramines, and disinfection byproducts that give tap water an unpleasant taste or smell. It also reduces volatile organic compounds (VOCs) like benzene, trichloroethylene, and carbon tetrachloride, along with some pesticides. The EPA has studied carbon filtration for “forever chemicals” (PFAS) and found that granular activated carbon can cost-effectively treat 76 to 87 percent of the more than 400 commercially available PFAS chemicals studied.
One important limitation: carbon filters retain most minerals in the water. That’s generally a good thing, since calcium and magnesium in drinking water aren’t harmful. But it also means carbon alone won’t soften hard water or remove dissolved salts.
Reverse Osmosis Membranes
Reverse osmosis (RO) pushes water under pressure through a semipermeable membrane with pores so small they’re measured in nanometers, typically 0.5 to 1.5 nm. For perspective, that’s thousands of times smaller than a human red blood cell. This allows water molecules through while blocking nearly everything else.
RO systems remove 95 to 100 percent of inorganic salts and charged organic compounds. In testing, some systems have achieved 98.9 percent removal of total dissolved solids. That includes heavy metals like lead and arsenic, fluoride, nitrates, and most bacteria and viruses. The trade-off is that RO strips beneficial minerals too, produces wastewater (typically 2 to 4 gallons for every gallon filtered), and works slowly enough that most home units include a storage tank.
Ion Exchange and Water Softening
If you’ve ever had a water softener, you’ve used ion exchange. These systems contain resin beads loaded with sodium or potassium ions. As hard water flows over the beads, calcium and magnesium ions swap places with the sodium or potassium, effectively removing the minerals that cause scale buildup on faucets and in pipes.
Ion exchange can also be configured for demineralization, where the resin is loaded with hydrogen ions instead. In that form, it removes all positively charged ions from water, not just hardness minerals. This is more common in industrial settings than in homes. Water softeners need periodic regeneration with salt to recharge the resin, which is why softener systems have a brine tank.
UV Purification: Not a Filter, but Often Bundled With One
Ultraviolet purification doesn’t filter anything in the traditional sense. Instead, it exposes water to UV-C light at a wavelength around 254 nm, which destroys the DNA of bacteria, viruses, and parasites. The damaged DNA can no longer replicate, killing the microorganism. At the right dose, UV-C irradiation can disinfect bacteria at concentrations up to 100 million colony-forming units.
UV systems are often paired with a carbon or sediment filter because they don’t remove chemicals, sediment, or dissolved metals. They’re particularly useful for well water or any supply where microbial contamination is a concern, since most standard carbon filters aren’t designed to remove germs.
Distillation: A Different Approach Entirely
A water distiller boils water into steam, then condenses the steam back into liquid, leaving behind salts, heavy metals like lead and mercury, hardness minerals, and most bacteria and viruses. It’s one of the most thorough purification methods available for home use.
The catch is that some volatile organic compounds and pesticides evaporate at roughly the same temperature as water. Without an additional carbon filter or vent, those chemicals can carry over into the distilled water. Distillation also removes all minerals, produces water slowly (typically a few liters per hour), and uses significant electricity.
Whole-Home vs. Single-Tap Systems
Water filters fall into two broad categories based on where they’re installed. Point-of-use systems filter water at a single tap. This includes pitcher filters, faucet-mounted units, refrigerator filters, and under-sink systems. They’re the right choice if you only need to filter water you drink and cook with.
Whole-home filters, also called point-of-entry systems, treat all the water coming into your house before it reaches any tap. The CDC recommends this type when you need to remove a harmful substance like certain VOCs from all your water, including what you bathe in and clean with, not just what you drink. Whole-home systems are also recommended for radon removal, since radon in water can become airborne during showers.
What Certification Standards Tell You
When shopping for a filter, look for NSF/ANSI certification numbers on the packaging. Each number corresponds to a specific category of contaminants the filter has been independently tested to reduce:
- NSF/ANSI 42 covers aesthetic improvements: chlorine, taste, odor, and particulates.
- NSF/ANSI 53 covers health-related contaminants with over 50 possible claims, including lead, the parasite Cryptosporidium, VOCs, and chromium.
- NSF/ANSI 401 covers emerging contaminants like prescription drugs, over-the-counter medications, herbicides, and pesticides, with up to 15 specific reduction claims.
A filter certified to Standard 42 will make your water taste better. A filter certified to Standard 53 has been verified to actually remove substances that pose health risks. These are not interchangeable, and many inexpensive pitcher filters only carry the 42 certification.
When Filters Stop Working
Every filter has a lifespan. Carbon eventually runs out of surface area to adsorb contaminants, RO membranes develop buildup, and ion exchange resins become saturated. Using a filter past its replacement date doesn’t just mean it stops working. It can become a breeding ground for bacteria.
Warning signs that a filter has failed or developed bacterial growth include a musty odor from the filtered water, slimy residue inside the filter housing, cloudy water even right after filtration, and noticeably reduced flow rate (which can indicate biofilm clogging the pores). For most healthy adults, brief exposure to small amounts of bacteria isn’t dangerous. But for infants, elderly people, and anyone with a compromised immune system, bacteria like E. coli or Legionella in an expired filter can cause serious illness.
Replacement schedules vary by filter type. Pitcher filters typically last two to three months, faucet-mounted cartridges three to four months, and under-sink carbon filters six to twelve months. RO membranes last longer, often two to three years, but their prefilters need changing more frequently. Following the manufacturer’s schedule is the simplest way to keep a filter doing what it’s supposed to do.