Abstract: A plant for the biological processing by means of activated sludge, by oxidation with pure oxygen or oxygen-enriched air, followed by a settling step, of a liquid suspension containing oxidizable matters, includes, in addition to the settling tank, a plurality of oxidation tanks disposed in series, communicating with each other, and adapted to be supplied with oxygen or oxygen-enriched air and the suspension to be treated, while recycling the decanted sludge, the suspension to be treated flowing in the direction of from the upstream tank to the downstream tank.