PURPOSE: To permit extraction of sludge only when sludge turbidity is above a set value by detecting a sediment water by means of a sampling pump through 'open' operation of a sampling valve on a periodic base, and inputting the value of the turbidity obtained by measurements of a turbidity meter as a control process value.

CONSTITUTION: In sedimentation facilities consisting of a sedimentation pond 1 for settlement of foul materials of water to be processed, a sludge discharging valve 2 for extraction of a sediment in the sedimentation pond and a transport pump 3 for sending the discharged sludge to a sludge treatment equipment, a sampling pump 4 for detection of a sediment water, a turbidity meter 6 for measurement of turbidity of the sediment water detected, and valves 5a, 5c for determining a sampling point in the sedimentation pond are provided. Then the afore-mentioned sludge discharging valve 2 and the sludge transport pump 3 are controlled based on the turbidity process value of sediment water obtained through the control of the sampling valves 5a, 5c. Thus it is possible to detect the concentration of sludge precipitated, and extract the sludge only when the turbidity value is above a set level.