A method of treating wastewater using an aeration tank of the type having an interior divided into at least a first chamber and a last chamber with each of the chambers having a lower portion and an upper portion and having liquid passageways in the lower portions of each of the chambers and gas passageways in the upper portions of each of the chambers for allowing liquid and gas to pass sequentially through each of the chambers from the first chamber to the last chamber, having an effluent outlet associated with the last chamber of the aeration tank for allowing treated water to exit the aeration tank; and having agitator means to enhance oxygen transfer from the upper portion to the lower portion of the aeration tank. The method includes maintaining the pressure in the upper portion of the first chamber of the aeration tank to at or slightly below existing atmospheric pressure; and maintaining the dissolved oxygen in the effluent from the last chamber by removing gas by vacuum from the last chamber of the aeration tank.
