A combined degassing and flotation tank for separation of a water influent containing considerable amounts of oil and gas. A rotational flow is created in the tank which forces the lighter components such as oil and gas droplets towards an inner concentric cylindrical wall where they coalesce and rise to the surface of the liquid and are removed via the outlet. The heavier parts are forced down where the heavy particles sink to the lower part where they can be removed as a sludge. The water is discharged via an outlet in the lower part of the tank. The combined degassing and flotation tank is particular suited for use in oil production at sea for removal of oil and gases from water streams before the water is returned to the sea.