Water filtration, explained

Pressure pushes water through a membrane whose pores pass water molecules and hold back nearly everything dissolved in them. What gets through is the filtered water; what does not leaves as concentrate.

Four questions decide it: what is in the water you start with, how much filtered water you need a day, how much room there is under the sink, and what pressure the mains gives you.

It depends entirely on the stage. Sediment filters catch particles, activated carbon takes chlorine and taste, and a reverse-osmosis membrane is the stage that removes dissolved salts, metals and most of what carbon cannot.

Reverse osmosis takes things out of the water. Ultraviolet takes nothing out — it inactivates microorganisms and leaves every dissolved substance exactly where it was. They answer different problems.

A whole-house filter treats everything entering the building, usually for sediment and chlorine. An under-sink system treats only what you drink, and treats it much further. Most homes want both, for different reasons.

In order: a sediment filter takes the grit out so the later stages last, carbon takes the chlorine out so the membrane is not damaged by it, the membrane does the separation, and a final carbon stage polishes the taste.

Pre-filters are replaced several times for every membrane, because they do the coarse work. The interval depends on your feed water and how much you draw, so the figure that matters is the one in your model's manual.

Separation by pore size, across a spectrum. Microfiltration and ultrafiltration hold back particles and microorganisms; nanofiltration and reverse osmosis go further and hold back dissolved matter.

Treating water that has already been used so it can be used again, for irrigation, industry or supply. It matters because it makes the same freshwater go around more than once.

It lowers demand on fresh sources, makes reuse possible at all, and takes bottled water and its transport out of the picture. A system that needs no pump takes the electricity out too.