I have a First Need water purifier. I like it because it purifies water without chemicals. (There are water filters that work without chemicals, but I think the First Need is the only purifier that does so. Purifiers need to meet certain higher standards to be called that.)
When I replaced the cartridge, I opened up the old one to see what was inside. General Ecology calls its purification technology "structured matrix". For some reason, that name made me think the inside would be all white and crystalline. It's actually black and kind of boring.
Here's the unopened cartridge. I've just started sawing through it.
With the top removed, you can see part of a hollow black cylinder with grooves cut along its circumference. There's a translucent plastic cap glued on the top.
Here the plastic cap is removed. You can see that some of the black stuff is still stuck to it. You can see straight down the cylinder to daylight on the other end. The little plastic cap is all that keeps water from passing through untreated.
Cutting straight down the middle, we see that the hollow cylinder is the same all the way through.
These are the two halves of the cylinder, pried out from their casing. Water passes from the side you see here through to the hollow center. What looks like an extra-deep groove is actually a mark from my saw.