Primary treatment removes suspended solids from the wastewater by sedimentation. Sedimentation uses gravity to let the solid particles settle into a sedimentation basins (also known as clarifiers or settling tanks, Figure 1). The accumulated settled solids are removed by mechanical scrapers and pumps. These "solid" waste products are referred to as raw sludge. The oil and grease (or "scum") that floats to the surface is skimmed and removed for additional processing.
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