A lamp inside the pipe scrambles bacteria and viruses as they pass, and changes nothing else.
also: UV sterilizer, ultraviolet disinfection, UV water purifier
The basics
What to know about water filters & purifiers
What counts as a filter
Anything that sits between the water main and your glass to reduce a named contaminant belongs here: pitchers and dispensers, faucet mounted units, countertop gravity vessels, under sink cartridges, reverse osmosis systems, refrigerator cartridges and whole house tanks. The word filter is the least useful thing about them, because these devices share no single job. Each one removes a specific list and passes everything else through untouched. A water softener does not belong on this shelf, since it trades calcium and magnesium for sodium to stop limescale and leaves contaminants alone. Two phrases sort the rest: point of use treats one tap, point of entry treats the whole building at the pipe where water arrives.
Four mechanisms, four blind spots
Four mechanisms do nearly all the work, and each carries a blind spot you can predict. Activated carbon relies on adsorption, meaning dissolved chemicals cling to the vast internal surface of the carbon rather than being strained out, so it handles chlorine, taste, odor and many organic chemicals while ignoring dissolved salts, nitrate, fluoride and hardness. Contact time governs how much clings: a solid carbon block pushes water through a dense wall for several seconds and can earn a lead claim. Loose granules let water carve a fast channel between grains and usually manage taste alone. Reverse osmosis presses water against a membrane with pores near 0.0001 micron, rejecting nearly everything dissolved and flushing it to the drain, making it the only home method that reliably cuts nitrate, arsenic, fluoride and sodium. Ultraviolet light changes no chemistry at all and instead scrambles the DNA of bacteria and viruses so they cannot reproduce. Sediment cartridges are plain strainers rated in microns. A 5 micron one standing in front of the rest keeps sand and rust from blinding expensive media.
Test your water first
Testing comes before shopping, every time, because a unit bought against a guess treats the wrong thing at full price. Utilities on a public US system publish a Consumer Confidence Report each year by July 1, listing what was detected and at what concentration. Lead is the gap that report cannot fill, since it usually enters from your own service line, solder or fixtures. It has to be measured at your tap in a first draw sample taken after the water has stood still for at least six hours. Well owners get no report from anyone and should test yearly for total coliform bacteria and nitrate. Match the result to the format after that: chlorine taste points to carbon in any shape, a lead number points to a unit certified for lead, nitrate or arsenic points to reverse osmosis, and coliform points to ultraviolet.
Removal, speed and money
Removal, speed and money pull against each other at every step up. A pitcher asks nothing of your plumbing and charges you a 5 to 10 minute drip for 10 cups, a cartridge good for roughly 40 gallons, and the highest cost per gallon of any format. Reverse osmosis sits at the far end, rated at 50 to 100 gallons a day but delivering a trickle. So a bladder tank holds 2 to 3 usable gallons ready; older designs send 3 to 4 gallons to the drain for every gallon made. Current ones run closer to 1 to 1. Take that much out and the minerals leave with it, so the water tastes flat until a remineralizing cartridge returns some calcium and magnesium. Whole house units hold pressure only when sized for the house, since a housing rated at 6 gallons per minute in a home peaking near 12 turns every shower weak.
Read the certification wording
Certification is the only claim on a box worth your time, and the wording matters more than any logo. A real line names a standard and a contaminant together, such as certified to NSF/ANSI 53 for lead and cyst reduction, while a phrase like tested to standards or built with certified materials verifies nothing. Five numbers let you read any package. 42 covers taste and appearance issues such as chlorine and particulate. 53 covers health effects, including lead, cysts, chromium, PFOA and PFOS. 58 covers complete reverse osmosis systems. 401 covers incidental compounds such as pharmaceutical and pesticide residues. 55 covers ultraviolet, and within it only Class A, which delivers 40 millijoules per square centimeter, is meant for water of unknown safety. Find the rated capacity in gallons next, since a cartridge sold as a six month filter is really a 100 or 500 gallon filter. Four people drinking and cooking use about 120 gallons a month. Last, confirm the housing takes a standard 10 inch cartridge rather than a proprietary shape that ties you to one supplier.
A spent filter gives no warning
Maintenance is mostly replacement discipline, and the trap is that a spent filter gives no warning. Exhausted carbon keeps passing clear water at full speed long after it has stopped holding anything. A lead claim fails silently well before chlorine taste returns, so count gallons or mark a date instead of waiting for a signal. Keep hot water out of carbon units by using the bypass lever on a faucet mount, since heat above roughly 100 degrees Fahrenheit softens a block and can release what it captured. Sanitize a reverse osmosis tank and its housings at every membrane change, and swap an ultraviolet lamp yearly even while it still glows, as germicidal output at 254 nanometers drops long before the visible light does.
Buyer checklist
Before you buy water filters & purifiers
Measure the space, check compatibility and include cleaning and running costs in the decision.
01
Measure the space and access
Check the product, clearance, doorways, cabinets, sockets and storage space, not just the advertised width.
02
Confirm compatibility
Induction cookware, replacement filters, voltage, plugs and dishwasher safety can decide whether a product works in your home.
03
Count the ongoing costs
Energy, water, filters, pods, bags and replacement parts can outweigh a small difference in purchase price.
04
Check cleaning and repair
Removable parts, accessible seals and available spares usually make long-term ownership easier.
Measure the refrigerator before buying a pitcher. A 10 cup model stands 10 to 11 inches tall and about 5 inches wide at the base, and most door bins are narrower than that. It ends up on a shelf taking the space of a milk jug.
Unscrew your faucet aerator and look at the threads before ordering a faucet mount. The common sizes are 15/16 inch male and 55/64 inch female, and pull-down sprayers, touchless spouts and many designer faucets carry no usable thread at all.
Clear the under sink cabinet on paper first. A single carbon housing needs about 5 inches of width and 14 inches of height so the sump can drop free at a change, while a reverse osmosis system with a tank wants roughly 15 inches wide by 15 deep by 18 tall.
Put a gauge on an outside spigot and read your water pressure before choosing reverse osmosis. Membranes want 40 to 60 psi, and below 40 the system makes less water and sends more of it to the drain. It needs a booster or permeate pump.
Size cartridges by gallons against your real use. Drinking and cooking runs about 1 gallon per person per day, so four people pass 120 gallons a month, which works out to three pitcher cartridges, one faucet mount cartridge, or a fifth of an under sink block.
Add up peak flow before buying a whole house unit. A shower at 2.5 gallons per minute alongside a washing machine and a kitchen tap asks for 10 to 15 gallons per minute, and a housing rated below that drops pressure everywhere in the building.
Care
Caring for water filters & purifiers
Change cartridges by gallons, not by how the water looks or tastes. Spent carbon keeps passing clear water at full speed. A lead or cyst claim fails silently months before chlorine taste comes back to warn you.
Flush every new cartridge for the stated time, usually about 5 minutes of running or two full pitchers thrown away. Fresh carbon sheds fine black dust, and media that has never been wetted adsorbs poorly for its first few gallons.
Send hot water around any carbon unit by using the bypass lever on a faucet mount. Above roughly 100 degrees Fahrenheit a carbon block softens and can release chemicals it already captured. That damage does not reverse when it cools.
Never leave a wet cartridge sitting unused and warm. Damp carbon is a comfortable home for bacteria, so keep a pitcher refrigerated and pull the cartridge into a sealed bag in the fridge if you will be away more than two weeks.
Sanitize housings, storage tanks and gravity chambers with an unscented bleach solution at every filter change. Biofilm builds on the clean side of a filter where nothing is treating it, and a rinse with plain water does not lift it.
Replace an ultraviolet lamp every 12 months even though it still lights, and wipe the quartz sleeve while it is apart. Germicidal output at 254 nanometers falls around 20 percent a year while the visible glow stays exactly the same.
Questions
Common questions
Do water filters remove fluoride?
Carbon does not, in any format, because fluoride is a small dissolved ion and carbon works by holding chemicals on its surface. Reverse osmosis cuts it by 90 percent or more, and activated alumina or bone char cartridges are the other route. Any product claiming fluoride removal should name NSF/ANSI 58 or 53 with fluoride listed by name.
How do I know which filter I actually need?
Read a test before you shop. On a public US system, the Consumer Confidence Report published each year by July 1 lists what was detected and at what level. Lead comes from your own pipes, so it needs a first draw sample from your tap after six hours standing still. Well owners test yearly for coliform bacteria and nitrate. Buy against the result, not against a worry.
How often do filters really need changing?
Count gallons rather than months. A pitcher cartridge is spent near 40 gallons, a faucet mount near 100, an under sink block between 500 and 1500. A reverse osmosis membrane lasts 2 to 5 years with prefilters every 6 to 12 months. Four people use about 120 gallons a month for drinking and cooking, which turns any rating into a date.
Will a filter make well water or flood water safe to drink?
Only if it is certified against microbiological contaminants, which most are not. Look for NSF/ANSI 53 cyst reduction, NSF/ANSI 55 Class A on an ultraviolet unit, or a stated bacteria and virus claim with a standard number. During a boil water notice, boil anyway, since the notice usually means the pressure keeping contamination out of the mains was lost.
Is reverse osmosis water bad for you because it strips minerals?
No. Drinking water supplies a small share of dietary calcium and magnesium, and food supplies the rest. Taste is the real complaint, because water below about 50 parts per million of dissolved solids reads flat and slightly acidic. A remineralizing cartridge after the membrane fixes that, and most systems now include one as the final stage.
Which filters remove PFAS?
Reverse osmosis and dense carbon blocks, and only the ones certified for it. Look for PFOA and PFOS reduction listed under NSF/ANSI 53 or 58. Loose granular carbon in a typical pitcher does not qualify, since contact time is too short and water finds channels between the grains. The claim has to name the compounds, not just say forever chemicals.
Does a water filter soften hard water or stop limescale?
No. Hardness is dissolved calcium and magnesium, which carbon ignores and a sediment cartridge cannot strain out. A softener trades those ions for sodium on a resin bed and regenerates itself with salt, while a scale inhibitor changes the crystal shape so deposits will not stick. Both are separate machines from every format on this page.
Why did my water pressure drop after installing a filter?
Something is clogged or undersized. A sediment cartridge loading up with rust is normal and means it is working, so change it. On reverse osmosis a slow tap usually means tank air pressure, which should read 5 to 7 psi on a tire gauge with the tank drained. Whole house units starve fixtures whenever their flow rating sits below household peak demand.
