A pump for pumping liquids containing entrained solids. The pump includes a volute surrounding an impeller comprising vanes that are self-cleaning. The outer surfaces of the vanes are coplanar and define a first plane and have a leading edge. The volute includes a planar mating surface defining a second plane parallel to the first plane of the rotary impeller. The planar mating surface is proximate to the outer surfaces of the vanes and includes a plurality of channels extending radially from the inner perimeter to the outer perimeter thereof. Each channel includes a forward edge in the direction of impeller rotation. The channels are oriented such that when the impeller is rotated within the volute, for any vane, the leading edge of the vane traverses each channel progressively from the inner end of the channel to the outer end of the channel.