How Ceramic Water Filters Work: Science of Pore Filtration

A ceramic water filter is not a fine sieve with one hole size. It is fired clay packed with a maze of tiny channels 0.2 to 0.5 micron wide, so narrow that bacteria and parasites physically cannot fit through.

That maze of pores holds the whole story of what a ceramic filter stops, what slips past it, and why scrubbing the candle restores the flow. This guide covers the pore science, the kiln chemistry that builds the pores, contaminant size data, flow rates, cleaning steps, field study results, and how to choose a system.

What Is a Ceramic Water Filter and How Does Pore Filtration Work?

A ceramic water filter is a cartridge of fired clay whose connected pores measure about 0.2 to 0.5 micron across. Water drips through those pores under gravity or line pressure, and the passages physically trap bacteria, protozoa, rust, and sediment while dissolved minerals pass through untouched.

The straining is purely mechanical and needs no chemicals. That is why a candle keeps working with no power, no moving parts, and no electronics.

A ceramic candle is a type of depth filter, meaning capture happens inside the wall itself, not just on its surface. A complete cartridge consists of a porous ceramic shell, colloidal silver worked into the pore structure, a plastic or steel mount, and sometimes a carbon core slipped inside the shell.

Henry Doulton fired the first commercial ceramic candles from Lambeth clay in England in 1827, according to the company’s documented history. Those early elements proved the concept that a properly fired clay wall could purify drinking water.

Modern replacement ceramic candle elements sell for roughly $20 to $60, and the shell alone does the straining. The mount hardware and housings simply hold the shell in place.

Key Specifications:

• Pore rating: 0.2 to 0.5 micron for quality elements
• Typical bacteria claim: 99 to 99.99 percent for tested candles
• Replacement cost: $20 to $60 per element
• Feed options: gravity drip or 40 to 60 psi pressurized housings

How these pore mazes compare with carbon and UV matters, because the wrong stages waste money. In the end, the pore network itself, not any coating or chemical, is the entire cleaning system.

What Makes Ceramic Pore Filtration Different from Every Other Filter Method?

Ceramic is the only common filter medium that is mechanically cleanable, fully passive, and mineral-preserving at the same time. Carbon saturates without any warning sign, UV lamps need electricity, and reverse osmosis sends three to four liters to the drain for every clean liter, while a scrubbed candle announces its own condition through its flow rate.

A clogging candle slows to a drip first, so the material signals when it needs care. A spent carbon block filters at full speed right up until it quietly stops adsorbing.

Ceramic differs from carbon in mechanism, and that difference matters for maintenance. The pores exclude particles by physical size, while carbon uses adsorption, a surface chemistry process that pulls dissolved molecules onto internal surfaces.

Development agencies rely on that passive reliability. The New York Times (2008) profiled the late Ron Rivera, whose Potters for Peace pot filter design brought microbial protection to village families with no plumbing or power.

One clarification before the deeper science: ceramic in this article means fired clay, not the silica nano films sold for car paint. Our guide to how automotive ceramic coatings build their slickness covers that other meaning.

For most buyers, ceramic earns its place because it can be scrubbed back to health instead of thrown away.

How the Kiln Builds Sub-Micron Pores: The Firing Science

Manufacturers create sub-micron pores by pressing fine clay mixed with combustible filler into shape, then firing it hot enough to bond the clay particles but not hot enough to seal the body shut. The burnt-out filler leaves a huge connected network of channels that water can travel through.

Sintering means clay particles fuse at their contact points. Vitrifying means the body melts dense, like porcelain, and that is exactly what a filter must avoid.

Porosity by Design: Clay, Sawdust, and Diatomite

Potters for Peace workshop recipes combine local clay with roughly equal volumes of sieved sawdust or rice husk. The organic filler burns out between about 300 and 600 degrees C (572 to 1112 F), and the remaining clay sinters at around 900 degrees C (1652 F), near cone 010 on the Orton scale.

The Digitalfire reference library, maintained by ceramic materials author Tony Hansen, describes how studios measure fired porosity with a boiling-water absorption test. A drinking vessel wants low absorption, under 1 percent for vitrified ware, while a filter body must stay openly porous.

Many commercial candle makers press diatomaceous earth, a sediment of fossil diatom skeletons naturally loaded with micron-scale voids. The fossil structure gives those shells a head start on fine, even porosity.

Overfire the body and the pores sinter shut, so flow dies. Underfire it and the shell stays soft, warps, and sheds clay particles into the water.

Tortuous Paths and Depth Filtration

Tortuosity is how twisted and indirect a pore path is. Water forced through thousands of tiny turns gives every particle repeated chances to collide with a wall and stick.

In plain terms: the filter works like a maze with walls narrower than the things trying to get through, and even small wanderers eventually bump into something and stop.

This capture only works within the rated pore distribution and only if water actually travels through the wall. A crack, or a mount without a snug washer, lets water bypass the pores entirely, and a bypassing filter passes everything the gap allows.

Why Colloidal Silver Is Baked into the Ceramic

Silver ions interfere with the enzymes bacteria need to metabolize, a biocidal effect that works at microgram levels. Filter makers bake that silver into the shell or brush it on after firing to keep trapped organisms from multiplying inside the walls.

That bacterial growth control is called bacteriostatic protection, meaning it stops growth inside the candle during storage and use. Potters for Peace applies its colloidal silver as a final production step on every pot.

Without silver, trapped bacteria can colonize the pore network as biofilm. A growing biofilm can later slough cells back toward the clean side, undoing the filter’s own work.

In one sentence: the kiln makes the maze, the maze makes the water clean, and the silver keeps the maze itself sterile.

What Gets Caught and What Slips Through: The Size Data

A quality ceramic element removes bacteria and protozoa almost completely, typically 2 to 4 log reduction, which means 99 to 99.99 percent, while most viruses pass because they are far smaller than the pores. Dissolved chemicals, chlorine, and dissolved minerals are unaffected by physical straining of any kind.

A 2007 World Health Organization review of household water treatment options reported ceramic filters achieving good bacterial removal with much weaker virus performance. The review noted that results varied widely with source turbidity and how carefully owners maintained the units.

Peer-reviewed field work supports the same pattern. In a trial published in the Journal of Water and Health (2009), Joe Brown and Mark Sobsey measured locally made Cambodian pot filters cutting E. coli counts by more than 90 percent in real household use.

Use the table below to match each contaminant type to what a plain ceramic element does with it, and to see which problem needs a second treatment stage.

Contaminant	Typical size	Removed by ceramic alone	Why it behaves that way	Add-on stage for coverage
Bacteria (E. coli, Salmonella)	0.5 to 5 microns	Yes, 2 to 4 log	Cells larger than the pore throats cannot fit through	None needed for bacteria
Protozoa (Giardia, Cryptosporidium)	4 to 14 microns	Near complete	Cysts far larger than a 0.5 micron channel	None needed
Viruses (polio, norovirus)	0.02 to 0.3 micron	Limited, about 1 log	Small enough to pass, though some stick in tortuous paths	Boiling, UV, or chlorination
Sediment, rust, microplastics	1 to 100+ microns	Yes, improves as it clogs	Particles cake on the surface and are strained	Sediment prefilter extends life
Chlorine, taste, odor	Dissolved	No	Pores catch particles, not dissolved molecules	Activated carbon stage
Lead and heavy metals	Dissolved ions	No, on plain shells	Ion sizes are far below any physical pore	Carbon core with metal media, or RO
Calcium and magnesium minerals	Dissolved ions	Pass through by design	Straining cannot touch dissolved solids, which keeps water mineralized	None, retention is a feature

The chart below sets the rated ceramic pore size against the actual sizes of the organisms and particles a candle is asked to stop.

Two honest caveats follow the data. A partly caked filter can trap slightly more viruses by adsorption onto captured debris, while a freshly scrubbed candle may trap fewer.

In plain terms: ceramic stops the living organisms that make people sick, and chemicals plus viruses need a different stage or treatment.

How Fast Does Water Move Through Ceramic, and Why Do Filters Clog?

A gravity candle filters roughly 0.5 to 1 liter per hour, while the same ceramic in a pressurized housing delivers 1.5 to 2.5 liters per minute. The height of the water column sitting above the element, called head pressure, sets the drip rate.

As debris collects, it forms a cake layer that actually improves particle capture while it strangles the flow. That is why a slowing filter is often filtering its best, just not fast enough to be convenient.

Every pot filter is flow-tested against a standard before shipping. Producers working to the Potters for Peace design expect 1 to 3 liters per hour from a new unit.

Backpacking gear shows the same physics in miniature. The MSR MiniWorks EX pushes water through a 0.2 micron ceramic element with a hand pump, and the Katadyn Ceradyn drip filter is rated by its maker for up to 50,000 liters through its ceramic cylinder.

Key Specifications:

• Element: 0.2 micron cleanable ceramic (MSR MiniWorks EX)
• Output: about 1 liter per minute by hand pump
• Cartridge life: about 2,000 liters for the MiniWorks element
• Field maintenance: scrub the ceramic surface, no tools needed

If sediment cakes deep into the wall, or a crack opens a bypass, force never fixes the problem. Cleaning fixes the cake, and replacement fixes the crack.

In one sentence: water height drives the drip, and accumulated dirt sets the timer for your next scrub session.

What Types of Ceramic Water Filters Can You Actually Buy?

Four formats dominate the market: gravity countertop units that hold multiple candles, two-bucket DIY kits, humanitarian pot filters, and pressurized under-sink or countertop housings. All of them use the same fired ceramic science, so the differences come down to feed pressure and mounting.

Gravity Countertop Systems with Candles

These units stack an upper reservoir over a lower one, with two or more candles bridging the gap. Berkey-class gravity filter systems are the best known example of the housing style.

More candles mean proportionally more flow, since each element contributes its own drip. A two-candle unit roughly doubles a single candle’s liters per hour.

Two-Bucket Kits

A bucket kit mounts a single candle through the floor of an upper bucket that drips into a lower one. Complete ceramic bucket filter kits start around $40 and suit cabins, travel, and emergency shelves.

A one-hour drip of about half a liter to one liter is normal. Owners simply top up the upper bucket when needed.

Ceramic Pot Filters

A pot filter is a pressed clay bowl that sits like a flowerpot inside a larger receptacle. Workshops across Latin America, Africa, and Asia produce them to the Potters for Peace design, where each unit is flow-tested and painted with colloidal silver.

Retail shoppers in the United States rarely see these on shelves. They matter most in humanitarian programs, where a low local production cost is the entire point.

Pressurized Housings, Under Sink and Countertop

A standard 10-inch housing accepts screw-in ceramic cartridges and runs on line pressure. Under-sink ceramic filter systems suit permanent installs, and renter-friendly countertop diverter units tap off the faucet instead.

Inline ceramic discs and candles feed pumps, RVs, and boats too. The elements scrub clean just like their bigger cousins.

Use the table below to match a format to your plumbing situation and your patience for refilling reservoirs.

Format	Fed by	Typical flow	Typical element life	Best for
Gravity countertop, multi-candle	Gravity	1 to 4 L per hour	6 to 12 months	Households, outages, advisories
Single-candle bucket kit	Gravity	0.5 to 1 L per hour	6 to 12 months	Cabins, camps, budgets
Ceramic pot filter	Gravity	1 to 3 L per hour	6 months to 2 years	Humanitarian household use
Under-sink 10-inch housing	Line pressure, 40 to 60 psi	1.5 to 2.5 L per minute	6 to 12 months	Permanent kitchen installs
Countertop diverter unit	Faucet diverter	1.5 to 2 L per minute	6 to 12 months	Renters and travelers
Disc or inline element	Manual pump or line	About 1 L per minute	Maker rated, cleanable	Backpacking, RVs, boats

In one sentence: the plumbing you already have, or the lack of it, decides the format before price ever does.

How Do You Clean and Recharge a Ceramic Filter Element?

You restore a clogged ceramic candle by scrubbing its surface under cool running water with a stiff pad. The scrub removes the dirty outer shavings of clay along with the trapped debris, exposing fresh pore channels in about 15 minutes.

The method works because debris mostly lodges in the outer millimeter of the wall. Each cleaning consumes a thin layer of the element’s ceramic, and that consumption is the only wear a candle suffers.

In plain terms: you are not washing the dirty layer off, you are removing the dirty layer itself.

Keep the right supplies on hand, since a fresh non-scratch scouring pad costs a few dollars and does the whole job. Never substitute a wire brush, which gouges the surface and leaves pits.

The six steps below cover one full cleaning cycle, from opening the housing to logging the date.

Replace the element for good when scrubbing no longer restores an acceptable drip rate, or when the walls look thinned. Most makers set that rated life between 6 and 12 months of household use.

In one sentence: scrubbing peels off the dirty outer skin of the ceramic, and the candle regrows its flow with every clean layer you expose.

Ceramic vs Activated Carbon: Which Stage Owns Which Contaminant?

Ceramic handles organisms and particles, while activated carbon handles dissolved chemistry, meaning chlorine, volatile organics, pesticides, tastes, and odors. Neither stage substitutes for the other, so complete systems pair them, ceramic first and carbon second.

Carbon granules adsorb, a process in which contaminant molecules stick to an enormous internal surface area. One gram of activated carbon offers hundreds of square meters of that adsorbing surface.

Certification claims sort the same way. Carbon blocks typically carry NSF/ANSI 42 claims for taste and NSF/ANSI 53 claims for specific health contaminants, while ceramic cyst claims fall under NSF/ANSI 53 testing.

Use the comparison below to see which stage does which job before you buy a housing or a combined candle.

Combined candles compress both stages into one housing. A carbon-core candle such as the Doulton UltraCarb adds chlorine and lead media inside the ceramic shell, and standard 10-inch activated carbon blocks drop into most housings.

Key Specifications:

• Media: ceramic shell with an activated carbon core insert
• Added claims: chlorine, taste, and lead reduction on listed datasheets
• Replacement interval: typically six months for the carbon function

In one sentence: ceramic first, carbon second, and give the carbon its own shorter replacement clock.

What Field Programs and Studies Actually Report

Field experience confirms the lab logic: well-maintained ceramic filters consistently deliver strong bacterial reduction, while turbid water shortens element life and viruses keep slipping past. The historical record and modern public health studies tell the same story from different centuries.

In the 1897 typhoid outbreak in Maidstone, England, published public health accounts reported that households using Doulton ceramic filters were largely spared. That documented episode pushed fired-clay filtration into hospitals and schools across Victorian England.

The modern humanitarian chapter began with the pot filter. The New York Times (2008) credited organizer Ron Rivera of Potters for Peace with spreading a design now manufactured by workshops across multiple continents.

Program evaluations repeat one weak point with striking consistency. Performance drops when users stop scrubbing, crack their filters, or store treated water in dirty containers, so maintenance decides how much of the rated protection survives in practice.

For system-by-system figures we maintain a full performance breakdown of popular ceramic filter systems covering flow, lifespan, and maintenance costs. It compares the specific housings most buyers shortlist.

In one sentence: the ceramic does its job in any century, and the human holding the scrub pad decides how long the job holds up.

How to Choose the Right Ceramic Filter for Your Water

Choose by source water first: microbiological risk calls for a tested ceramic and a stated pore claim, chemical taste calls for a carbon stage, and highly turbid water calls for a sediment prefilter in front of the candle. Mount type, budget, and flow rate come after those three checks.

Buyers should demand a stated log-reduction claim or an NSF/ANSI certification rather than phrases like fine ceramic. A drinking water bacteria test kit costs roughly $20 to $50 and tells you which problem you actually own before you buy anything.

Mounting is the last big call. Our comparison of countertop and under-sink ceramic filter systems breaks down the plumbing and cost tradeoffs for each install style.

For most households on treated municipal water who want advisory-proof reliability, a countertop gravity unit with two candles plus a carbon stage is the default pick. Choose a pressurized under-sink housing only if you prefer maintenance at a valve over lifting a bucket.

In one sentence: test the water first, match the stages to the results, then pick the mounting that fits your counter or cabinet.

Troubleshooting Ceramic Filters: Slow Flow, Cracks, and Cloudy Water

Most ceramic filter problems trace back to three causes: a caked element that needs scrubbing, a cracked element that needs replacing, or a loose seal that needs re-tightening. Each problem leaves a signature you can diagnose in a few minutes.

• Slow drip or full stall: the cause is a sediment cake. Scrub the element, and fit a sediment prefilter if your source runs turbid.
• Suddenly fast flow with particles downstream: the shell has cracked, usually from impact or thermal shock. Replace the ceramic replacement candle immediately, since no glue or sealant is food-safe inside pores.
• Cloudy first liter after cleaning: fine clay dust from fresh pore walls. Flush 2 to 3 liters to waste and it clears.
• Drips at the mount: a worn or missing washer. Replace the food-grade washer and hand-tighten the wing nut snugly.
• Flat or stale taste: the carbon stage likely saturated, not the ceramic. Swap the carbon block and the taste returns.
• Odd smell in stored water: the receptacle re-infected it, not the filter. Wash the lower bucket and keep it covered.

Thermal shock deserves its own note. Kiln-fired ceramic endures steady heat for years, and the same sintering science that lets ceramic heaters survive heating cycles explains why a candle shrugs off hundreds of scrubs. Dump boiling water on a cold candle, though, and a crack is nearly certain.

In one sentence: slow flow means scrub, sudden flow means replace, and everything else is a washer.

Frequently Asked Questions About Ceramic Water Filters

Does a ceramic water filter remove lead and heavy metals?

A plain ceramic shell does not remove dissolved lead, because lead travels as dissolved ions that a 0.5 micron physical pore cannot stop. Elements with an added carbon core and lead-specific media, such as the Doulton UltraCarb line, reduce lead through chemical adsorption in the insert instead of in the clay.

Check the datasheet for an NSF/ANSI 53 lead claim before trusting any element for lead. That claim belongs to the insert media, not to the ceramic itself.

For a confirmed lead problem, reverse osmosis or certified lead-specific media remains the stronger choice. Ceramic continues earning its place ahead of those stages as microbial and sediment protection.

Is the silver inside ceramic filter candles safe to drink from?

The silver in candle walls is safe to drink from, because it is either fired into the structure or applied at microgram levels that stay far below taste thresholds. United States drinking water standards treat silver as a secondary contaminant with a 0.1 milligram per liter reference level, and the amounts used in filters operate far beneath it.

The silver does one job: it keeps trapped bacteria from multiplying inside the pore network during use and storage. Without it, a candle could become a bacteria farm between cleanings.

If silver concerns you, choose elements that publish test data for silver release. Quality makers routinely document it.

Can I drink untreated lake or river water through a ceramic filter?

Filtering untreated surface water through ceramic alone is not a guarantee of safe drinking water, because viruses and chemical pollutants pass through the pores. In a wilderness setting, ceramic plus boiling, or ceramic plus chemical treatment, covers the gaps left by the clay.

Bacteria and protozoa, the main threats in most North American backcountry water, are exactly what ceramic handles best. Giardia and Cryptosporidium cysts have no chance against a 0.5 micron wall.

Cloudy water complicates the plan by clogging the element fast. Pre-filter through cloth, then let the candle work.

Ceramic or reverse osmosis: which is better for my kitchen?

Choose ceramic if microbe safety and mineral retention matter most, and reverse osmosis if dissolved contaminants drive the purchase. Ceramic runs on gravity and wastes no water, while reverse osmosis strips healthy minerals along with the contaminants and sends three to four liters to the drain per clean liter.

A reverse osmosis system also needs line pressure, a storage tank, and regular membrane care. A gravity candle sits on a counter and works during a boil advisory or a blackout.

The two pair well when a membrane exists: a sediment or ceramic stage ahead of the membrane shields it from particles. Pick RO first only when the water test names dissolved enemies like fluoride, arsenic, or nitrates.

Should I buy a candle with a built-in carbon core?

A carbon-core candle is worth it on municipal water, because one element then handles both bacteria-scale particles and chlorine inside a single housing. The ceramic stands guard against organisms while the insert polishes the chemistry.

The carbon function saturates faster than the clay, often inside six months, so the combined element goes on the calendar more often than a plain shell. The convenience of one housing buys you a shorter replacement clock.

On rainwater or well water with little chlorine, a plain ceramic candle plus separate treatment usually makes more sense. Doulton’s UltraCarb and similar carbon-core models fit the chlorinated tap crowd.

Do ceramic filters remove fluoride?

No, ceramic pores do not remove fluoride, because fluoride is a dissolved ion that passes through any physical straining barrier. Fluoride reduction requires adsorbent media such as activated alumina or bone char, or a reverse osmosis membrane.

Products claiming fluoride reduction always list the specific media doing the work. The claim never belongs to a ceramic shell alone.

Test before and after if fluoride is your concern. A fluoride water test kit costs about $20 to $40 and settles the question for your specific element.

Can I clean my ceramic filter with dish soap or hot water?

No to both: dish soap soaks into the open pores and later flavors your water, and hot water can crack fired ceramic through thermal shock. Scrub under cool running water with a stiff, clean pad only.

Detergent residue cannot be rinsed out of a microporous wall, it can only be scrubbed away with the clay around it. Manufacturers commonly advise replacing an element after detergent exposure rather than gambling on it.

Boiling to sanitize is equally dangerous for the shell. Sanitize the housing and washers separately in mild bleach solution, rinse them well, and let the ceramic clean itself the low-tech way.

Why is my filtered water cloudy right after I scrubbed the element?

The first flush after scrubbing is often slightly cloudy, because cleaning releases fine ceramic dust from newly exposed pore walls. Run 2 to 3 liters to waste and the cloudiness clears within that volume.

Persistent cloudiness after a full flush points to a different problem. Suspect over-scrubbing that thinned the wall, or hairline cracks shedding clay into the stream.

Inspect the shell under bright light for spider-line cracks at that point. A candle that keeps clouding water needs replacement, not another scrub.

How do I prime a brand new ceramic candle?

Prime a new element by rinsing it under cool running water, scrubbing lightly if the maker includes a pad, then running the first 2 to 3 liters to waste before use. Gravity kit manuals call this a purge cycle, and it clears manufacturing dust from the Fresh pore walls.

New candles often drip slowly for the first day while the wall fully wets. That slow start is normal, and the flow stabilizes after the pores saturate fully.

Log the install date on a taped label before the housing goes back together. You are building the maintenance record that every later decision uses.

How do I know my ceramic filter is actually removing bacteria?

You cannot see bacterial removal, so rely on three checks: a certified log-reduction claim from the maker, periodic laboratory tests of source and filtered water, and a physical inspection of the shell. A total coliform lab test costs roughly $20 to $50 and closes the loop.

A bright-light inspection should show no dark spider lines, chips, or grease films. Physical integrity plus a published test claim is the best assurance a homeowner can assemble.

Owners on private wells should retest annually or after any flood or plumbing work. The filter only protects what the housing, seals, and storage containers allow it to.

Can I make my own ceramic pot filter in my pottery kiln?

A studio potter can press and fire a pot filter body, because the recipe is simple: roughly equal volumes of plastic clay and sieved sawdust, fired near 900 degrees C (1652 F), close to cone 010. The hard part is validation, since a home kiln batch comes with no proof of pore consistency or bacterial performance.

low-fire earthenware clay costs about $20 to $35 and presses into dozens of small test pots for flow trials.

Workshop programs flow-test every pot at 1 to 3 liters per hour and brush colloidal silver on each fired unit before release. Without that testing, treat your homemade pot as unverified for drinking water.

Is the ceramic in a water filter the same material as a ceramic knife?

No, the two are opposite ends of ceramic engineering: a knife blade is fully dense sintered zirconia, hard enough to hold an edge near 8.5 on the Mohs scale, while filter ceramic is a deliberately under-vitrified clay body whose porosity is the entire point. Our use-case guide to ceramic kitchen knives covers the dense side of the family.

The knife wants zero porosity, and the filter wants 30 to 50 percent open volume. Sintering science serves both goals by controlling temperature and body composition.

The same disambiguation applies to cars. The coatings marketed for vehicles are thin silica films, not fired clay, and our article on how automotive ceramic coatings work explains that chemistry.

Do ceramic filters need electricity or tap pressure?

A ceramic filter needs no electricity and no line pressure, because gravity feed is standard on drip systems. That independence is why candles and buckets serve cabins, campsites, aid programs, and homes during boil advisories or power outages.

Pressurized housings remain a convenience option rather than a requirement. The same element that drips at 0.5 to 1 liter per hour by gravity flows at 1.5 to 2.5 liters per minute under 40 to 60 psi.

Keep a spare candle on the shelf for emergencies. A stored, dry ceramic element stays ready for years with no power draw at all.

Wrapping Up the Science of Pore Filtration

A ceramic water filter cleans water with nothing but a fired-clay wall of 0.2 to 0.5 micron channels that physically block bacteria, protozoa, and sediment while leaving healthy minerals alone. Scrub the candle when the flow halves, replace it at 6 to 12 months, and add a carbon stage when chlorine or lead matters.

The technology traces to 1827 and still outperforms much powered gear for microbial safety. Test your water, pick the format that fits, and let the pores do the physics.