Sedimentation and Other Clarification Processes - Public Water Systems - page 314

the effluent end of the tank to a sludge hopper located at the tank inlet. Three types
of sludge removal mechanisms are typically used in rectangular basins:

• Chain and Flight. These units consist of a series of chains and sprockets, with
the sprockets mounted at the top and bottom of the tank (Fig. 11-7). Wood, plastic,
or steel cross flights are fitted between the chains. The flights scrape along the basin
bottom slowly (less than 1 ft / min [0.3 m / min]) to avoid resuspending the settled
sludge dragging the sludge to a hopper. The standard width is 20 ft (6.1 m) for this
type of mechanism, although other widths can be custom made.
• Traveling Bridge. This type of mechanism is mounted on a carriage that travels
along the top of the wall of the tank. The sludge scrapers move slowly (less than 1
ft / min [0.3 m / min]) along the bottom to a hopper or, in some models, sludge is
removed by vacuum to an external hopper.
• Suction Type. Several models are available from various manufacturers that re-
move sludge by suction pumping to an external hopper. One example of this type of
unit is shown in Figure 11-8.

The bottom slope of circular clarifiers is typically about 8 percent from the outer
wall to a central sludge hopper, although steeper slopes up to 15 percent may be used
with very high turbidities or heavy sludges, such as lime-softening sludges. Usually,
a rotating sludge collector pivots around the center of the tank and scrapes the sludges
toward the center hopper (see Fig. 11-9).

Basin Geometry Rectangular basin widths are 20 ft (6.1 m) or less with a single
sludge collector and are wider with parallel sludge collectors. Length-to-width ratios
of 3:1 to 5:1 are typical and typical depths are 12 to 16 ft (3.6 to 4.9 m). Circular
clarifier sludge removal mechanisms are typically available in 1-ft (0.3-m)-diameter
increments between 10 and 30 ft (3 and 9 m); in 2-ft (0.6-m)-diameter increments
between 30 and 50 ft (9 and 15 m), and in 5-ft (1.5-m)-diameter increments above 50
ft (15 m).

I I I I I I I I I I I I

Sludge Discharge Line

Fig. 11-7. Rectangular sedimentation basin with flight and chain sludge removal system (From
Culp, Gordon, and Williams, Robert, Handbook of Public Water Systems. Copyright 1986 by
John Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc.)