An arrangement for biological cleaning of organically polluted liquids composed of building units including at least two substantially cylindrical horizontal bodies which are placed alongside the other, with the upper part of their mantles forming the bottom of a separating space of the cleaned liquid and of the coagulated suspension, said separating space being provided between the upper parts of the mantles of said cylindrical bodies. The cylindrical bodies communicate with each other, one of the bodies communicates with the separating space which has a trough to discharge clarified liquid.