Donaldson is a leading worldwide provider of filtration systems and replacement parts. Donaldson serves customers in the industrial and diesel engine markets including in-plant air cleaning, compressed air and gas purification, power generation, disk drive filtration, off-road equipment and trucks.

Ask an Expert!
What's the difference between a primary and secondary fuel filter?
--by Brian Tucker, Engine Liquid Product Manager

Differences between primary and secondary filters vary from system to system, but in general primary filters are used to separate water and larger particles (10-50 micron efficiency). Secondary filters are for final filtration (2-10 micron efficiency). Primary filters usually have treated media to provide water separation performance. This can be either cellulose or a multi-layered synthetic media called melt-blown coupled with cellulose like Donaldson's Synteq? media. Secondary filters are usually an untreated cellulose or purely synthetic media. These differences mainly have to do with the water separation requirements placed on primary fuel filters.