Abstract: Apparatus for treating water has a container having a clarification zone, a water entry arrangement for flowing water into the clarification zone, and skimmer elements mounted to the container for partial immersion in the water in the clarification zone. Each skimmer element has a transfer channel for transferring a flow of clarified water from a layer of the water to a predetermined depth in the clarification zone and to direct the received clarified water to a water exit arrangement. A height adjustment mechanism is used for differentially adjusting the heights of the skimmer elements and their transfer channels relative to the container so as to compensate for the clarifier being installed on sloping ground.