PURPOSE: To stabilize the quality of water to be treated by detecting TS in drained water on real time and controlling a flocculant loadings optimally.

CONSTITUTION: The concentration of total solid is calculated based on drained water amount data and electric conductivity data by loading outputs from a flow meter 1 for detecting a drained water amount, an electric conductivity meter 2 for measuring the electric conductivity of drained water, an A/D converter 4 for converting analog outputs from the flow meter 1 and the electric conductivity meter 2 to digital output. At the same time, optimal flocculant loadings to calculated solid concentration is outputted by command of a personal computer 5 and an optimal flocculant amount is discharged into the drained water by a pump 9 upon the command from the personal computer 5.