PURPOSE: To enable having low water content to be discharge stably by discharging sludge from a thickener at a time when equivalent accumulative addition quantity of alkaline agents, which are added to produce hydroxides of heavy metals contains in waste liquids, has reached a certain value or more.

CONSTITUTION: When sludge is discharged from a thickener 5 in a process for treating waste liquids containing heavy metals, equivalent accumulative addition quantity of alkaline agents which are added to produce hydroxides of heavy metals contained in the waste liquids is compared with equivalent accumulative addition quantity of alkaline agents set in advance in a computer 8. In response to the output from the computer 8, a discharge 9 of the thickener 5 is actuated to discharge sludge from the thickener 5, while concentration of the sludge discharged therefrom is always measured by means of settled sludge concentration meter 11. Based on the measured values, set value of the computer 8 of equivalent accumulative addition quantity of alkaline agents is changed to discharge the sludge. Consequently, storing a large quantity of sludge in the thickener can be avoided, enabling thereby sludge having low water content to be discharged stably.