Charcoal Filter

A charcoal filter passes water through activated carbon, charcoal treated so that its interior is riddled with pores. That gives a little material an enormous internal surface, which dissolved chemicals stick to as the water passes. The sticking is called adsorption, and it is all the device does: it pulls out chlorine, the compounds behind bad taste and smell, dissolved organic chemicals, trihalomethanes and radon gas. It serves one tap or line, which is why it is called point-of-use.

What carbon cannot do matters more. It leaves bacteria, most metals and other inorganic chemicals untouched, reduces nitrate only partly, and disinfects nothing. Worse, the rough pores that make it a good adsorbent are sheltered ground for bacteria, and the filter strips the residual chlorine that was holding them back. On untreated water, what comes out can be worse than what went in. So it belongs only on water already continuously disinfected or known to be free of bacteria, and it goes last in the treatment train, after a mechanical or ceramic filter has removed the particles. It is never the only treatment between a source and a drinking tap.

Hydrogen sulphide, the rotten-egg gas, is a partial exception: ordinary carbon handles it below about 1 milligram per litre; above that the right medium is catalytic carbon, which oxidises the gas rather than merely holding it. Upkeep is done blind. Carbon works from the first flow, reducing chlorine within seconds in the first centimetres of a new bed, but it fills up with use and gives no sign when it has, so the element is changed on a schedule rather than when something tastes wrong. Published intervals run from a few months to a year, since what matters is how much water has passed and how dirty it was. After five days or more unused, flush it with chlorinated water for thirty seconds. Changing the element needs no tools or skill.