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The main goal of sedimentation is to remove readily settleable solids and floating materials (not removed in the upstream treatment phases) thus reducing the suspended solids content; so quiet conditions are set up in the sedimentation basin: collected solids are subsequently sent to the sludge treatment processes, and (in case of secondary sedimentation) partially recycled.

The sedimentation process takes place in a settling tank, which is a circular or rectangular basins made of concrete or iron, having the bottom lightly sloped towards a zone where the sludge is conveyed by appropriate withdrawal devices.

Figure 5: Example for a circular sedimentation tank (Source: Universit├Ąt Stuttgart)

Sedimentation tanks are designed to operate continuously. Primary sedimentation tanks may provide the principal degree of wastewater treatment, or may be used as a preliminary step in further treatment of the wastewater. When used as the only means of treatment (not authorized in most developed countries), these tanks provide for removal of settle able solids and much of the floating material. When used as a preliminary step to biological treatment, their function is to reduce the load on the biological treatment units. Efficiently designed and operated primary sedimentation tanks should remove 50 to 65 percent of the suspended solids and 25 to 40 percent of the biochemical oxygen demand.

Tab. 3.3.1 Typical operational data (HRT and overflow rate) for different type of clarifier

The BOD and TSS removal efficiency depends on the clarifier characteristics and on the above mentioned parameters. Basically, in a primary clarifiers, removal efficiency for BOD and TSS are mainly related to the hydraulic retention time (HRT) and the its influent concentration.

In table 3.3.2 a typical clarifier operation sheet is reported; the reporting frequency depends on the WWTP size; details on routine inspections, lubrication and adjustment, performed by the operator, should be reported as well.

Tab. 3.3.2 Clarifier operation sheet

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