Microfiltration media are available in a wide variety of materials and methods of manufacture. They can be rated either 'absolute' or 'nominal' depending upon the percentage of capture of particles of the same size or larger than the retention rating of the media. Membrane filters are generally rated as absolute media. They can be manufactured of various polymeric materials, metals and ceramics. Nominal media includes filters made of glass fibers, polymeric fibers, discrete particles (diatomaceous earth), ceramics, etc. However, even absolute media can be considered absolute only within a finite time span because of the possibility of bacterial grow-through. Microfiltration membranes can be divided into two broad groups based on their pore structure. These are membranes with capillary-type pores, hereafter called screen membranes, and membranes with tortuous-type pores, hereafter called depth membranes.