Sewage treatment comes in two stages - primary and secondary treatment. The primary stage involves a process of screening solids from sewage, leaving a sludge and relatively clear water for further treatment or for disposal into rivers, the sea or on to the land. In the secondary stage the sludge is stirred constantly in vast tanks to get more oxygen into the mixture, allowing bacteria to break down the organic matter and leave a harmless residue that falls as a sediment to the bottom of the tank. After processing, the clear water on top of the tank is discharged into rivers and the sediment is used as landfill or discharged at sea.