Stirring device

A stirring device configured for a biogas plug-flow fermenter for stirring a fermentation material comprising:

• a shaft rotatable about an axis of rotation;
• a stirring element attached to the shaft, the stirring element having a recess;
• the recess having a surface portion and an edge, the edge defining an opening which provides access to the surface portion;
• the recess being spaced from the shaft, wherein a rotation of the shaft defines a moving direction of the recess in which the edge forms a leading edge of the recess, the moving direction being always tangential to a moving path of the recess about the axis of rotation during a rotation of the shaft;
• the surface portion defining a depth of the recess with regard to the edge;
• wherein in a direction radially inwardly the depth of the recess is decreasing;
• the surface portion extending straight in a first direction and being inclined with regard to a plane which is perpendicular to the moving direction;
• wherein the first direction is parallel to the axis of rotation;
• wherein the surface portion extends straight in a direction from its radially inner part to its radially outer part;
• wherein the stirring element comprises a further recess pointing in the direction opposite to the recess;
• and
• wherein the recess and the further recess are configured symmetrically with regard to a plane of symmetry.