PURPOSE: To prevent abrupt change in flow rate by making a filtration flow rate control in which a given total filtration flow rate is proportionally distributed according to the filtering capacity of each filtration basin and determine the set values of filtration flow rate for each filtration basin.

CONSTITUTION: Effluent water from the sedimentation basin 1 is sent to the filtration basin 2 to filter it through the filter layer 3, where the flow meter 4 and the converter 5 are provided on the filtrate discharge side and the flow rate signal is sent to the regulator 6. Using the operating condition of each filtration basin as an input, the filtration continuing time of each filtration basin is measured by the time recorder 12, and also using the filtration continuing time as an input, the filtering capacity of each filtration basin is determined by the operator 11 from a preset filtering capacity curve. The filtering capacity of filtration basin not in use is zero. According to the filtering capacity of the filtration basins 3, a given total filtration flow rate from the manual setter 9 is distributed proportionally to obtain the filtration flow rate set values for each filtration basin to control the regulating valves of each filtration basin.