5.2.4  Sedimentation

Once large flocs are formed, they need to be settled out, and this takes place in a process called sedimentation (when the particles fall to the floor of a settling tank). The water (after coagulation and flocculation) is kept in the tank (Figure 5.6(b)) for several hours for sedimentation to take place. The material accumulated at the bottom of the tank is called sludge; this is removed for disposal.

Figure 5.6  Flocculation chambers (a) and a sedimentation tank (b) at Gondar water treatment works.