Abstract: A cartridge filter assembly includes a depth filter element and a downstream second filter element. The depth filter element has a mass of melt-blown polymer filaments. The depth filter removes 0.5 to 10 micron sized contaminants at an efficiency of 90% or more. The depth filter preferably comprises multiple zones occupying different depths, with one or more melt-blown polymer filaments traversing two or more of the zones. The second filter element comprises nano-fibers having a diameter of 1 micron or less, and removes a material percentage of contaminants that are less than 1 micron in size, preferably less than 0.5 microns in size. The depth filter element may be in the form of a tube and the second filter element may be in the form of a pleated sheet located inside of the depth filter. The cartridge filter assembly may be used to pre-treat a feed water upstream of an RO membrane. The SDI of the feed water may be reduced to 3 or less or 2 or less.