A system and method for filtering is disclosed, which is capable of accomplishing a filtering operation at a high recovery rate of 96% or more, and realizing a compact and simplified system structure, the system comprising a water bath including an inlet and a discharging hole, wherein feed water to be treated is supplied to the inside of the water bath through the inlet, and concentrated water is discharged out through the discharging hole; and plural membrane cassettes including first and second membrane cassettes submerged into the feed water contained in the water bath, wherein the first membrane cassette is positioned nearest to the inlet, and the second membrane cassette is positioned nearest to the discharging hole, wherein the first membrane cassette treats the feed water with a first impurity concentration; the second membrane cassette treats the feed water with a second impurity concentration; and the first impurity concentration is smaller than the second impurity concentration.