The present invention provides a method of purifying a contaminated liquid flow (A) comprising the steps of: introducing a first type of gas bubbles (8) in the contaminated liquid flow (A), 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; introducing a second type of gas bubbles (9) to the contaminated liquid flow (A) 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″); extracting a reject stream (C) comprising aggregates formed by the interaction of contaminants from the contaminated liquid flow (A) with both the first and the second type of gas bubbles; and obtaining a purified liquid flow (D) downstream of the introduction of the second type of gas bubbles; as well as a system and a vessel for use in such a method.