Abstract: Described are a method and apparatus for the biological treatment of wastewater in an activated sludge process that use a primary separator to produce a pretreated wastewater, a pressurized aeration tank which has a headspace pressure of between about 1 and about 10 psi, and a secondary separator to separate mixed liquor from the aeration tank into an activated sludge component and a clarified liquor component. The aeration tank has a rectangular or square base and may be cuboid. Embodiments contemplate making a package plant, and may use a screening tank, a membrane separator, air eductors to aerate a return activated sludge, and multiplier nozzles to introduce the return activated sludge into the aeration tank. The apparatus has a small footprint, is simple in design, and is low maintenance.