Media migration is the phenomenon in which a portion of the filter (media) itself comes loose from the filter and travels downstream with the liquid or gas being filtered.
An example of this is a piece of fiber from a string-wound filter being carried downstream in the liquid being filtered. Most commodity-type filters, as well as many more sophisticated membrane or membrane pre-filters, experience media migration of some kind or another. To minimize this migration of media, more sophisticated filters are frequently pre-flushed with filtered, deionized or RO (reverse osmosis) water. Typically, a pre-flushing procedure like this removes most of the loose media present in a filter.
With commodity filters, the detrimental effects of media migration can be minimized by re-circulating the material being filtered for several minutes. By doing this, any media that migrates downstream is redirected on the exterior surface of the filter, which filters the media, which has migrated.