Abstract: A grit tank (1), the inside of which is partitioned into an upper chamber (2), intermediate chamber and lower chamber (5), the intermediate chamber being partitioned into a left intermediate chamber (3) and a right intermediate chamber (4), is connected to a transport pipe (16) via a discharge port (9) of the lower chamber (5). The intermediate chambers (3, 4) are respectively communicated with the upper chamber (2) and the lower chamber (5) via a left upper communicating port (6) and a right upper communicating port (10) and also via a left lower communicating port (8) and a right lower communicating port (12). The communicating ports are respectively provided with a left filling valve (7), right filling valve (11) and changeover type feed valve (13).