Pump for pumping liquid comprising solid matter, the pump comprising an impeller seat and an open impeller, - the open impeller having a cover plate, a centrally located hub and at least two spirally swept blades connected to the cover plate and to the hub, wherein each blade of the impeller comprises a leading edge adjacent the hub and a trailing edge at the periphery of the impeller and a lower edge, wherein the lower edge extends from the leading edge to the trailing edge and separates a suction side of the blade from a pressure side of the blade, wherein the impeller is displaceable back and forth in the axial direction in relation to the impeller seat during operation of the pump, - the impeller seat having an axial inlet defined by an inlet wall and an upper surface located downstream the axial inlet, wherein the impeller seat has an inlet radius measured from an axially extending centre axis to a circular intersection between the inlet wall and the upper surface of the impeller seat, said impeller seat comprising a guide pin connected to and extending radially inwards from said inlet wall, wherein the leading edge of the blade is configured to cooperate with the guide pin of the impeller seat during operation of the pump and wherein the lower edge of the blade is located opposite the upper surface of the impeller seat, the guide pin having a tip radius (r) measured from the axially extending centre axis to the radially innermost part of the guide pin, wherein an imaginary 15%-circle is offset radially inwards from said circular intersection fifteen percent of the difference between said inlet radius and said tip radius (r), and wherein an imaginary 85%-circle is offset radially inwards from said circular intersection eighty-five percent of the difference between said inlet radius and said tip radius (r), characterized in that the impeller is rotatable in the forward direction and in the backward direction, the backward direction being opposite the direction of rotation of the impeller during normal operation of the pump, in that the guide pin extends in the upstream direction from the inlet wall, seen in the forward direction of rotation of the impeller, and in that a trailing edge line is an axially projected straight line extending between the intersection between the 15%-circle and a trailing edge of the guide pin and the intersection between the 85%-circle and the trailing edge of the guide pin, wherein a trailing edge angle (α) between a radius of the impeller seat intersecting the trailing edge of the guide pin at the 15%-circle and the trailing edge line is equal to or more than 10 degrees and equal to or less than 30 degrees. - the open impeller having a cover plate, a centrally located hub and at least two spirally swept blades connected to the cover plate and to the hub, wherein each blade of the impeller comprises a leading edge adjacent the hub and a trailing edge at the periphery of the impeller and a lower edge, wherein the lower edge extends from the leading edge to the trailing edge and separates a suction side of the blade from a pressure side of the blade, wherein the impeller is displaceable back and forth in the axial direction in relation to the impeller seat during operation of the pump, - the impeller seat having an axial inlet defined by an inlet wall and an upper surface located downstream the axial inlet, wherein the impeller seat has an inlet radius measured from an axially extending centre axis to a circular intersection between the inlet wall and the upper surface of the impeller seat, said impeller seat comprising a guide pin connected to and extending radially inwards from said inlet wall, wherein the leading edge of the blade is configured to cooperate with the guide pin of the impeller seat during operation of the pump and wherein the lower edge of the blade is located opposite the upper surface of the impeller seat, the guide pin having a tip radius (r) measured from the axially extending centre axis to the radially innermost part of the guide pin, wherein an imaginary 15%-circle is offset radially inwards from said circular intersection fifteen percent of the difference between said inlet radius and said tip radius (r), and wherein an imaginary 85%-circle is offset radially inwards from said circular intersection eighty-five percent of the difference between said inlet radius and said tip radius (r),