Backwashable depth filter

A hollow cylindrical depth filter formed of fibers of a thermoplastic resin and having a thickness of a hollow cylindrical filter medium of 5 to 25 millimeters, wherein

• the depth filter has an outer surface of the hollow cylindrical filter medium on a fluid inflow side and an inner surface of the hollow cylindrical filter medium on a fluid outflow side, and the depth filter can be backwashed by flowing liquid from the inner surface to the outer surface,
• the hollow cylindrical filter medium has a compression ratio of 0.2 or less when a load of 0.5 MPa is applied thereto,
• the hollow cylindrical filter medium has a fiber layer of at least three layers from the outer surface on the fluid inflow side toward the inner surface on the fluid outflow side,
• a porosity of a first layer on the fluid inflow side is in a range of 0.70 to 0.85,
• a porosity of a second layer on the fluid inflow side is in a range of 0.60 to 0.80,
• a porosity of a third layer on the fluid inflow side is in a range of 0.70 to 0.85,
• wherein the porosity of the first layer is greater than the porosity of the second layer,
• wherein the porosity of the third layer is greater than the porosity of the second layer,
• intersection points of the fibers forming the hollow cylindrical filter medium are bonded,
• a mean interval between the intersection points is 2 to 100 times a mean fiber diameter of the fibers in a length direction,
• a ratio of the mean fiber diameter on a surface on an upstream side to the mean fiber diameter on a surface on a downstream side of the hollow cylindrical filter medium is 0.9 to 1.2,
• a portion of a thickness of 0.5 to 3 millimeters from an outer surface of the hollow cylindrical filter medium is wound and thermally press-bonded to have a strong adhesion strength that the fiber layer cannot be peeled without being broken, while other portion of the hollow cylindrical filter medium is wound with a low adhesion strength that the fiber layer can be peeled without being broken, and
• the portion of the thickness of 0.5 to 3 millimeters from the outer surface of the hollow cylindrical filter medium is a portion of the first layer of the fiber layer of the hollow cylindrical filter medium.