Patent ID: 8029680

Claim:
A filtration apparatus comprising: a filter medium which filters raw water through openings; a first filter layer in which an upstream side of the filter medium is precoated with a first filter aid by bridging phenomenon in which multiple pieces of particles of the first filter aid are linked to form bridges; and a second filter layer in which an upstream side of the first filter layer is precoated with a second filter aid, wherein the first filter aid has a size which is smaller than the openings, and which, when the particles thereof are aggregated at the openings due to a flow during a precoating, enables the multiple pieces to be linked to form the bridges over said openings, whereas the second filter aid has a size which is smaller than the first filter aid and which itself cannot form bridges over the openings, wherein the openings of the filter medium and particles of the second filter aid are adjusted in size by the bridging phenomenon of the first filter aid, and the filtration apparatus causes the raw water to flow from a side of the second filter layer to the filter medium to carry out a filtration, and wherein, a third filter layer is provided in which an upstream side of the second filter layer is precoated with a third filter aid having a larger particle diameter than the second filter aid, wherein an increase in a pressure loss in the course of the filtration through the second filter layer is suppressed by the third filter layer.