Abstract: There is provided an apparatus for automatically controlling and regulating the freedom from suspended matter of quiescent reservoir overflow liquid in mechanical separatory processes for liquid-solid phases based on the sedimentation of suspended particles in quiet or slow flowing liquids. The apparatus includes a quiet reservoir 2 below an overflow outlet 1, a withdrawal tube 6 for removal of liquid or suspension from the reservoir, a free falling interval below the outer end of the tube 6, a light ray 3 between a light source 4 and a photoelectric cell 5 arranged across the free falling interval. The tube 6 enters the reservoir at the height of the uppermost suspension material boundary level. The photocell 5 is operatively connected with the driving circuit of a slurry pump 8 connected to the outlet 12 at the bottom of the quiet reservoir 2.There is also disclosed the process of using the apparatus.