Abstract: Arrangement for the biological cleaning of water, said arrangement being suitable for large urban stations for cleaning sewage or for the cleaning of waste waters of industrial plants. The arrangement comprises an activating system with pneumatic aeration and a separating system communicating therewith, the arrangement operates on the basis of fluid filtration and automatic return of activated sludge into the activating system by gravitation. Suitable partition walls in a large container form a number of part activating spaces and part separating spaces; by suitable interconnections, by rectifying walls, by the supply of raw water, and by aeration a convenient streaming flow of the liquid and an advantageous operation of the arrangement are obtained.