The media trap is to be installed after your backwashing filter. Its function is to protect your home plumbing fixtures from particles of filter media that might escape from the backwashing filter. This usually doesn't happen, but the media trap will provide protection if it does.
It has no cartridge to change, although you might eventually need to remove the clear bowl and clean the metal screen. When installing, please note the directional flow arrow.
Please call if there are questions.
(whole house & well units)