Abstract: A method and device for processing sewage which contains coating paste from a paper or board mill with a device operating on the principle of a double action impact mill. The device includes a first rotor (11) equipped with blades; a second rotor (12) concentric with the first rotor, equipped with blades and arranged to rotate in a direction opposite to the direction of the first rotor, or a stator concentric with the first rotor and equipped with blades. The sewage is led to a feed opening (14) which opens out to the nave of rotors (11, 12) or to the nave of the rotor and the stator. In the device, the material flow is brought to flow as a flow-off through the blades of the rotors (11, 12) arranged within each other, or of the rotor and the stator arranged within each other.