Also known as tubular-flow reactor. Plug-flow reactors are elongated reactors where the influent (e.g. wastewaters) enters on one side, and constantly moves with minimal longitudinal dispersion to the other side comparable with a train that passes a train station. Fluid particles pass through the tank and are discharged in the same sequence in which they enter. The particles retain their identity and remain in the tank for a time equal to the theoretical hydraulic retention time.