PURPOSE: To detect the liquid boundary between org. resins and unnecessary waste liquid surely by measuring the conductivity of the waste liquid between a recovering tank for waste resin liquid and a three-way valve for recovering of org. resins, and controlling the three-way valve and a two-way valve for discharging the unnecessary waste liquid in accordance with the measured value.

CONSTITUTION: Waste phenolic resin liquid is separated and settled in a tank 2 to unnecessary waste liquid 9 such as water and recovered resins 10. A three-way valve 3 for recovering resins is connected to the tank 2, and a two-way valve 4 for discharging the unnecessary waste liquid is connected to the three-way valve. A conductivity meter 5 is provided between the tank 2 and the three-way valve 3 and the conductivity of the waste resin liquid is measured. The measured value is put into a deciding device 6 in which the upper and lower limits of the conductivity are set. The value obtd. by adding a safety factor to the difference between the min. conductivity of the liquid 9 and the max. conductivity of the resin 10 is set in an operator 7 for change rate. The outputs of the device 6 and the operator 7 are inputted to a sequence controller 8 which controls the valve 3 and the valve 4. The boundary surface is detected surely by the above-mentioned control and the org. resins and the unnecessary waste liquid are separated and recovered.