Abstract: This invention relates to apparatus for continuously discharging liquid from a container, such as a waste water tank having a discharge pipe with an inlet located adjacent the container floor and an outlet located outwardly of the container. The liquid flow is controlled by a control unit which operates as a function of the liquid level in the container. The discharge pipe has a riser section between its inlet and its outlet which extends upwardly to an apex section to define an overflow for the liquid. The riser section is adjustable in height which is controlled by control means that always maintains the overflow at a constant elevation below the liquid level in the container.