To suppress scum from depositing on the inner surface of a scum box, and to avoid increasing equipment costs or operation costs.

In a sedimentation device 100, a part of the scum box 8 below the water surface of the sedimentation tank 1 is provided with an inflow hole 8a through which water in the sedimentation tank 1 flows into the scum box 8. A timing when a scum scraping member 7 drops scum into the scum box 8 is detected, and air lift pumps 11, 12 are operated to discharge the scum inside the scum box 8. The water of the sedimentation tank 1 flows into the scum box 8 from the inflow hole 8a, so as to suppress the scum dropped into the scum box 8 from being deposited on the inner surface of the scum box 8. Moreover, a chute shower is provided above the scum box 8, so that dropping of water is not needed; as a result, an increase in equipment costs or operation costs can be avoided.

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