A prefilter is defined by what it protects rather than by what it is, and the specification changes with the stage behind it. The same housing can hold four different cartridges depending on whether it is standing in front of a membrane, a resin bed, an ultraviolet chamber or a carbon block, because each of those is destroyed by something different.
Ahead of a membrane
Two prefilters, in a fixed order. Sediment first, because particles abrade the membrane surface and block its feed channels. Then carbon, because chlorine degrades thin film composite material permanently and no cleaning reverses it. That carbon stage is not there to improve the water: it exists to keep the expensive component alive, which is why its replacement interval is a calendar rather than a taste judgement and why skipping it is the most costly habit in reverse osmosis ownership.
Ahead of a softener
Sediment, at a moderate rating. Grit scores the control valve's seals and seats, produces the classic fault of water running continuously to drain, and blinds the resin bed so capacity falls without anything being reported. On a chlorinated supply, carbon as well, since chlorine degrades resin over time. What a prefilter cannot do here is deal with dissolved iron, which passes through and fouls the resin regardless, and which needs its own oxidizing stage.
Ahead of an ultraviolet chamber
Sediment, because suspended particles physically shade organisms from the light and the dose collapses without anything reporting a fault. Then, where the water carries them, iron and manganese treatment and softening, since all of those film the quartz sleeve and cut transmission gradually and invisibly. And carbon where the water is tannin stained, because dissolved organics absorb the working wavelength even when the water looks perfectly clear.
Ahead of a carbon block, and sizing any of them
Sediment again, because a block's face blinds and the media inside is then wasted on particles rather than spent on chlorine and taste. In every case the rating should be matched to the load rather than set as fine as possible, since a very fine prefilter blocks quickly and defeats its own purpose, and staging coarse before fine outlasts a single fine element several times over. Our whole house filter guide covers ordering the stages.
Questions people ask about prefilter water filter
What does a prefilter protect a membrane from?
Sediment, which abrades and blocks it, and chlorine, which degrades the membrane material permanently.
Does a softener need a prefilter?
Yes on any supply carrying grit, which scores the valve seals and blinds the resin bed without reporting anything.
Why does an ultraviolet system need one?
Particles shade organisms from the light, and iron, manganese and hardness film the quartz sleeve and cut the dose.
How fine should a prefilter be?
Matched to the load rather than as fine as possible. A very fine prefilter blocks quickly and defeats its own purpose.