Filter apparatus with sand filter bed

A filter apparatus for removing foreign substances contained in raw water by passing the raw water through a filter layer (8) made up of a particulate filter medium disposed in a filter tank (1), said apparatus comprising:

• a partition wall (2) provided inside said filter tank (1) so as to extend from an upper end of said filter tank (1) downward while leaving a space to a bottom surface (7) of said filter tank (1) such that said filter tank (1) is divided into a supply portion (4) having a raw water supply port (3) and a discharge portion (6) having a backwashed water discharge port (5);
• wherein the bottom surface (7) of said filter tank (1) is inclined from the discharge portion (6) downward toward the supply portion (4), said bottom surface (7) having the filter medium spread thereon in a manner to cover both said supply portion (4) and said discharge portion (6), the filter medium being filled to such a depth that an upper end of the filter medium reaches at least a bottom end of said partition wall (2);
• water collecting pipe (10) embedded inside the filter medium for collecting filtered water;
• and
• backwashing means (9) for backwashing the filter medium,
• wherein said backwashing means (9) comprises an air lift pipe (9a) having a suction port which is embedded into the filter medium in said supply portion (4) and a discharge port which opens above the filter medium in the discharge portion (6), and a backwashing pipe (9b) for supplying backwashing water into said suction port of said air lift pipe (9a).