Different water separation media operate under different principles. Hydrophobic barrier media, such as silicone treated cellulose, rejects water and causes it to bead up on the upstream surface. As the beads become larger, they run down the face of the element into a cup under the force of gravity. Hydrophilic depth coalescing media, such as glass micro-fibre, has a high affinity for water. The water in the fuel associates with the glass fibres and over time as more water enters from the upstream side, massive droplets are formed. The water moves through the filter with the fuel and on the downstream side, falls out of the fuel flow into a collection cup.

Increased use of surface active fuel additives and fuel components such as biodiesel have rendered conventional separating media less effective and filter manufacturers have needed to develop new approaches such as composite media and ultra-high surface area coalescing media [Stanfel 2009][Pangestu 2009][Bessee 2009]. Methods of quantifying fuel/water separation performance have also been affected [Stone 2009].