A separator tank (1) for separating oil and gas from water, and comprising an essentially cylindrical vertical tank casing (3) with a plurality of separator tank units (2). The separator tank units (2, 2′, 2″) are arranged on top of one another within a central area in an inner annular enclosure (4) that divides the separator tank into an annular outer area (5) and the central area. The flow paths of fluids to inlets (7) in the separator tank units (2) and from at least one second outlet (9) in the separator tank units are arranged at least in the annular outer area.