A method of purifying a contaminated liquid flow comprising the steps of: a. providing a vessel comprising a cylindrical wall, a first inlet for the contaminated liquid flow, a second inlet for a second gas (B"), a first outlet for a reject stream, and a second outlet for obtaining a purified liquid flow, wherein the first inlet is arranged in an upper half of the vessel such that a flow of the contaminated liquid will enter the vessel in a direction being substantially tangential to the cylindrical wall at the point of entry of the flow, the second inlet is fluidly connected by a conduit to a distributor unit arranged in a lower half of the vessel, the second outlet is arranged in the lower half of the vessel below, or at the same height of, the distributor unit, and the first outlet is arranged in the upper half of the vessel above the first inlet; b. introducing a first type of gas bubbles in the contaminated liquid flow, the first type of gas bubbles obtained by at least partly saturating a liquid with a first gas (B'), at a first pressure, followed by a lowering of the first pressure to a second pressure; c. introducing the second type of gas bubbles to the contaminated liquid flow downstream of the introduction of the first type of gas bubbles, the second type of gas bubbles formed by sparging, entrainment or attrition of a second gas (B"); d. extracting the reject stream comprising aggregates formed by the interaction of contaminants from the contaminated liquid flow with both the first and the second type of gas bubbles; and e. obtaining the purified liquid flow downstream of the introduction of the second type of gas bubbles; wherein the vessel comprises an internal sleeve having a frusto-conical shape with an upper end and a lower end, wherein the upper end has a diameter which is smaller than the diameter of the lower end, and the diameter of the lower end is smaller than the diameter of the circular wall, such that a flow area is provided between the circular wall and the lower end, and the lower end is arranged above the distribution unit and the first inlet of the vessel is arranged below the upper end of the internal sleeve and above the lower end of the internal sleeve a. providing a vessel comprising a cylindrical wall, a first inlet for the contaminated liquid flow, a second inlet for a second gas (B"), a first outlet for a reject stream, and a second outlet for obtaining a purified liquid flow, wherein the first inlet is arranged in an upper half of the vessel such that a flow of the contaminated liquid will enter the vessel in a direction being substantially tangential to the cylindrical wall at the point of entry of the flow, the second inlet is fluidly connected by a conduit to a distributor unit arranged in a lower half of the vessel, the second outlet is arranged in the lower half of the vessel below, or at the same height of, the distributor unit, and the first outlet is arranged in the upper half of the vessel above the first inlet; b. introducing a first type of gas bubbles in the contaminated liquid flow, the first type of gas bubbles obtained by at least partly saturating a liquid with a first gas (B'), at a first pressure, followed by a lowering of the first pressure to a second pressure; c. introducing the second type of gas bubbles to the contaminated liquid flow downstream of the introduction of the first type of gas bubbles, the second type of gas bubbles formed by sparging, entrainment or attrition of a second gas (B"); d. extracting the reject stream comprising aggregates formed by the interaction of contaminants from the contaminated liquid flow with both the first and the second type of gas bubbles; and e. obtaining the purified liquid flow downstream of the introduction of the second type of gas bubbles;