Ceramic Filter

A ceramic filter is a porous element of fired clay, often with silver worked into it, that water is made to pass through. The clay is riddled with openings too small for bacteria or for the parasites Cryptosporidium and Giardia to pass, so they are held back on the surface. Nothing is killed; they are strained out, the way a sieve keeps back whatever will not fit. Gravity alone will drive water through, which makes it the one treatment device in a household water train that still works when the power is off and the pump has stopped.

Pore size is the entire specification. The element has to carry an absolute rating of 0.3 micron or smaller - finer than anything the eye can resolve - to hold back bacteria and those parasites, and anything coarser is not doing this job however it is sold. What it cannot do matters just as much: viruses are smaller than the openings and pass straight through. So a ceramic element belongs in front of a disinfection stage, an ultraviolet lamp or a chemical dose, and never in place of one - and the clear water it delivers is exactly what an ultraviolet lamp needs. It is a safeguard on a supply already believed sound rather than a licence to drink from one known to be foul, and a coarse sediment filter ahead of it keeps silt off the fine surface.

Upkeep is cleaning rather than replacement, and that is where the trap lies. Flow falls away as the pores load with silt, and the answer is to scrub the element back to bare ceramic - but every scrub takes material off and thins the wall until it cracks. A cracked element passes water at full flow and looks exactly like a working filter, and clear, good-tasting water is easily mistaken for safe water. So the element is inspected for cracks after every cleaning, and the water is sampled above and below the device rather than judged by how it runs or how it tastes. No settled figure says how long an element lasts, because that depends on how often it has had to be scrubbed.