This invention relates generally to the field of separation of heterogeneous mixtures and, more particularly to a solid-liquid separating tank and system.A settlement tank including: a tank wall defining a tank inner space; an internal wall structure in the inner space and defining a fluid flow path from a center of the inner space to a fluid outlet at the tank wall; and a plurality of drainage pipes having an open internal end and an outer end connected to a drainage manifold, wherein the open internal end is positioned at a bottom of the inner space to remove gravity decanted sediment and hydrostatically decanted water from the inner space.