A unit for purifying water contaminated by droplets of hydrocarbonaceous liquid has an essentially vertically positioned filter, an inlet chamber for the contaminated water that is partially defined by a first vertical surface of the filter, supply means for contaminated water debouching in the lower portion of the inlet chamber, and outlet means connected to the upper portion of the inlet chamber for discharging hydrocarbonaceous liquid separated from the water in the inlet chamber. The operational period between filter cleaning and/or the hourly capacity of the filter is increased by injecting fine gas bubbles into the contaminated water in least one location selected from locations in the lower portion of the inlet chamber and locations in the supply means to produce a stream of gas bubbles upwards along the first vertical filter surface.