Abstract: According to the present invention there is provided a method and apparatus for treating a contaminated liquid, the method comprising feeding said liquid to a treatment tank containing a treatment medium in the form of a filtration membrane or a reactive surface, applying a siphon to said treatment tank to draw said liquid through said treatment medium, and discharging the resultant treated liquid from the treatment tank, wherein said siphon is actuated upon said treatment tank reaching a predetermined head pressure or level. The invention also provides a treated liquid when so treated by the inventive method or apparatus.