Abstract: A device for the biological treatment of liquids, especially sewage, including at least two compartments between which a vertical wall is inserted so that the compartments communicate with each other by free passages above and below the wall. The liquid to be treated is introduced into the compartments which contain particles which are supports for microorganisms exhibiting a density that is lower than that of the liquid to be treated. A gas is injected into the lower part of one of the compartments in order to establish, in the two compartments, a concurrent, continuous and orderly circulation of the multiphase mixture consisting of the liquid, solid and gaseous phases. In the lower part of the compartment not having gas injection, and above the recovery of the treated liquid, in the region where the change takes place in the direction of the flow passing from one compartment to the other, a system is provided for separating the less dense phases (particles, gas) from the multiphase mixture.