An improved sewage treatment plant is described having a primary waste receiving and mixing vessel, an aeration vessel and a settling tank. Material from the primary mixing vessel is pumped into a bottom region of the aeration vessel by means of a pump and a venturi aerator is provided for introducing air into the stream of sewage being pumped into the aeration vessel. The aeration vessel is of unique design and contains a series of downwardly inclined baffles which are arranged to trap pockets of air and to circulate the sewage along a tortuous path across these air pockets. A recycling system is provided for recycling a portion of aerated sewage from a top region of the aeration vessel back into the primary mixing vessel for mixing with the raw, unaerated sewage and a second recycling system is provided for recycling a portion of aerated sewage from an upper region of the aeration vessel into the stream being pumped into the lower region of the aeration vessel.