Abstract: An arrangement for treating wastewater containing soluble organic matter through the proliferation of aerobic microorgnisms to purify the wastewater. In an apparatus having a wastewater tank, a water pervious drum rotatably mounted in the tank and partially submerged in wastewater therein, and a multiplicity of biological contactors packed in the drum, the improvement comprising the biological contactors being of hollow structure, each having at least two small openings, a subsidiary biological contactor being provided in the interior of each biological contactor where required, the contactors being compactly packed in the drum so as to be prevented from movement therein.