Wastewater Treatment 175MotorluentFigure 9-7. Circular settling tank.or cZar$ers. The settling tank that immediately follows screening and grit removal is
called the primary cZur$er. The solids that drop to the bottom of a primary clarifier
are removed as raw sludge.
Raw sludge generally has a powerfully unpleasant odor and is full of water, two
characteristics that make its disposal difficult. It must be stabilized to reduce further
decomposition and dewatered for ease of disposal. Solids from processes other than
the primary clarifier must be treated similarly before disposal. Treatment and disposal
of wastewater sludge are discussed further in Chap. 10.
Settling Tank Design and Operation
Gravitational settling is, as has been noted, one of the most efficient means of separating
solids from surrounding liquid. Solidniquid separation can be achieved this way as long