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CLARIFICATION 7.19

New facilities should be designed with sludge viewing pits that permit the operator to
observe the consistency of the sludge during blowdown. Direct observation provides a means
of optimizing withdrawal rates and reduces excessive loading of sludge handling facilities.

Circular Basins
Circular sedimentation basins became more prevalent in water clarification when periodic
manual cleaning of long, rectangular basins became unpopular. The top-drive circu-
lar mechanisms used for sludge cleaning have no bearings under water, resulting in lon-
gevity with little maintenance. In reasonable sizes--not exceeding 125 ft (38 m) in
diameter--the circular center-feed clarifiers perform as well as long, rectangular basins
provided there is a reasonably well-balanced radial flow from the center well with sub-
stantial water depth maintained at the center.
Some circular basins are designed for rim feed with clarified water collected in the
center. However, most circular basins used today are the center-feed type. Included in this
category are square tanks with center feed that are used for their feature of lower cost by
means of common wall construction. A typical circular clarifier is shown in Figure 7.11.
A circular clarifier with a center flocculation zone is shown in Figure 7.12.
Basin Dimensions. Circular basins, like rectangular basins, are designed based on sur-
face overflow rates, and rates used are typically the same as those for rectangular units.
Circular basins may be of any diameter but are usually sized based on the commercially

ILLUSTRATIVE SECTION
FIGURE 7.11 Typical circular clarifier. (Courtesy of Eimco Water Technologies.)