settling. Other flow patterns are shown in parts C, D, and E in Figure 7.17.1.

Basins are usually made of steel or reinforced concrete. The bottom slopes slightly to make sludge removal easier. In rectangular tanks, the bottom slopes toward the inlet end, whereas in circular or square tanks, the bottoms are conical and slope toward the center of the basin.

The selection of any shape depends on the following factors:

• Size of installation

• Regulation preference of regulatory authorities

• Local site conditions

• Preference, experience, and engineering judgement of the designer and plant personnel

The advantages and disadvantages of rectangular clar-ifiers over circular clarifiers follow.


• Less area occupied when multiple units are used

• Economic use of common walls with multiple units

• Easy covering of units for odor control

• Less short circuiting

• Lower inlet-outlet losses

• Less power consumption for sludge collection and removal mechanisms


• Possible dead spaces

• Sensitivity to flow surges

• Collection equipment restricted in width

• Multiple weirs required to maintain low-weir loading rates

• High upkeep and maintenance costs of sprockets, chains, and fliers used for sludge removal

Square clarifiers combine the common-wall construction of rectangular basins with the simplicity of circular sludge collectors. These clarifiers have generally not been successful (Montgomery 1985). Because effluent launder-ers are constructed along the perimeter of basins, the corners have more weir length per degree of radial arc. Thus, the flow is not distributed equally, resulting in large sludge

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