A method of filtering a liquid sample that includes passing a sample comprising at least one biological organism through a filter membrane at a passive water volume flux of at least 10 L/m2·h·psi, wherein the filter membrane comprises a Bubble Point pore size of no more than 1.0 μm, thereby retaining at least one biological organism on the surface of the membrane; and detecting the at least one biological organism retained on the surface of the filter membrane.