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.

A wastewater purification apparatus includes an elongate tank, which has an inflow for feeding wastewater, a first vertical agitator with a first hyperboloid agitator body mounted on a vertical first agitator shaft and provided in a first treatment portion downstream of the inflow on the first narrow side, a second vertical agitator with a second hyperboloid agitator body mounted on a vertical second agitator shaft and provided in a second treatment portion downstream of the first vertical agitator, an aeration device with a fan for aerating wastewater received in the tank, a first drive device for rotating the first hyperboloid agitator body in a first rotation direction, a second drive device for rotating the second hyperboloid agitator body in a second rotation direction opposite the first rotation direction, and a decanter for discharging purified wastewater in a third treatment portion on a second narrow side opposite the first narrow side.