Activated sludge (AS) is a process dealing with the treatment of sewage and industrial wastewaters and developed around 1912-1914. There is a large varity of design, however, in principle all AS consist of three main components: an aeration tank, which serves as bio reactor; a settling tank ("final clarifier") for seperation of AS solids and treated waste water; a return activated sludge (RAS) equipment to transfer settled AS from the clarifier to the influent of the aeration tank.