Removal of water from liquid Sewage sludge is divided into 3 different processes:

• thickening (up to approx. 9% TS)

• dewatering (up to approx. 35% TS)

• drying (up to approx. 100% TS)

At low TS contents the volume changes significantly with varying TS contents. If the TS goes up from 1 to 3%, the resulting volume is one third!

Gravity thickening shown in figure 4 is one of the most common methods in used and is accomplished in a tank similar in design to a conventional sedimentation tank. Normally, a circular tank is used, and dilute sludge is fed to a center feed well. The feed sludge is allowed to settle and compact and the thickened sludge is withdrawn from the bottom. Vertical pickets stir the sludge gently, thereby opening up channels for water to escape and promoting densification. The supernatant flow that results is drawn off and returned to either the primary settling tank, the influent of the treatment plant or a return-flow treatment process.

Figure 4: Gravity thickner

In gravity thickeners two zones can be distinguished: above the sludge blanket there is the sedimentation zone. Below the sedimentation zone is the compaction zone located, where the increased density of the sludge helps to “squeeze out� water. The mechanical compression rises with the height of the compaction zone.