Abstract: The downcomer through which air is supplied to a header for liberation of air deep in an aeration tank is of straight-line nature to minimize constant waste of power due to air flow friction. The downcomer nevertheless folds when the header is to be raised for inspection or servicing. The folding axis is external of the downcomer, which breaks open as the folding action starts. In one form the header is straight while initially raised, hanging from a first hoist cable, and then as the lower section is raised further by a second cable the upper section is allowed to fold over. In another form, the folding progresses as the structure is raised, the upper section being hinged to the air supply fitting.