Abstract: Apparatus for continuously treating solids, with at least one liquid includes a first screw conveyor having first agitators positioned along the peripheral margin of each section of the thread of the screw conveyor for agitating the solid pieces being conveyed in the area between adjacent sections of the conveyor, and second agitators positioned along the shaft of the first screw conveyor for mixing the liquid with the solids within the area between adjacent sections of the thread of the first screw conveyor, and a second screw conveyor having agitators positioned along the peripheral margin of each section of the thread of the conveyor for agitating the solids being conveyed thereby, in the area between adjacent sections of the thread of the second screw conveyor, and liquid passages associated with the peripheral margin of each section of the thread of the second screw conveyor for allowing liquid introduced to the second screw conveyor to pass through the sections of the conveyor.