Depth filters are specially constructed "melt-blown" (polypropylene) sediment removal cartridges that are made of several layers which have different micron ratings. This allows the outer layer to catch large particles, the second layer to catch slightly smaller particles, and so on, until the inner level traps the particle size for which the filter is rated.
These filters work especially well and have greater capacity than standard sediment cartridges when treating water that contains a variety of particle sizes.
(whole house & well units)