PURPOSE: To perform control of effective amt. of filthy water to be contained in an aeration tank easily and quickly by constructing the tank to have adjustable effective volume by the use of hollow shell bodies which are freely chargeable and dischargeable to and from the tank and can be at least partly sunk below the level of filthy water to be treated in the tank.
CONSTITUTION: An aeration tank 2 having a manhole 12 with a lid at the top and an air dispersing device 1 in the inside is communicated with a settling tank 9 through a communication pipe 13, and the level of water in both tanks 2, 9 is regulated to same level by an overflow trough 14. Hollow shell bodies 3 can be charged or discharged freely through the manhole 12, and are constructed to submerge their lower half sections to below the water level WL in the tank 2 when they are charged to the tank 2. Each of the hollow shell bodies 3 are made of rigid polyethylene with a fixed internal volume and a grip 3B and plug 3C constituting a transparent or translucent vessel 3A wherein liquid such as water or particles such as sand is contained. The effective volume of the tank 2 is controlled by changing the number of the hollow shell body 3 to be charged.