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 .