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7.8 CHAPTER SEVEN

One approach to address the vertical velocity problem is to use a perforated end wall,
similar to the inlet distribution wall, to maintain parallel hydraulic flow along the length
of the basin (Monk and Willis, 1987). Velocity approaching the wall does not increase
except near the ports because the entire basin cross section is used. Velocity remains low
along the floor, reducing the potential for scouring. Flow uniformity approaching the end
wall helps to ensure that flow covers the entire basin surface to achieve the design over-
flow rate. The approach to designing the perforated wall is the same as for the inlet. A
small head loss must be taken to obtain uniform distribution. Velocities through the ports
may be higher than those for the inlet because the smaller floc carried over is less likely
to be sheared. In some cases it is preferable to increase the head loss through the efflu-
ent ports to obtain hydraulic flow splitting to the basins rather than taking the head loss
at the inlet. This avoids high inlet velocities that may cause breaking up of a fragile floc.
The use of effluent baffle walls may not be effective in controlling density currents in
the settled solids. Such density currents may be as high as 2.5 to 6 ft/min (0.8 to 1.8
m/min), much faster than the tank average flow-through velocity that could carry solids
through a baffle wall (Kawamura, 1991). Control of density currents in the sludge blan-
ket may be best accomplished by frequent solids removal at several locations (Taebi-
Harandy and Schroeder, 1995). Traditional mechanical sludge removal equipment, how-
ever, does not typically provide for draw-off at several locations. However, frequent or
continuous solids removal should minimize density currents in the settled solids.

Manual Solids Removal. Although no longer used in many modem water treatment
plants, basins can be designed for manual cleaning. In such cases basins must be designed
to store sludge for a reasonable period of time. An extra depth of 4 to 5 ft (1.2 to 1.5 m)
should be provided, basin floors must slope to a drain, and adequate pressurized water
must be available for flushing. Manually cleaned basins are suitable for use in develop-
ing countries with low labor rates.
Basins can be designed with hoppers in the first half of the tank, where most of the
sludge is likely to settle, and equipped with mud valves that can be frequently opened to
waste the bulk of the sludge. This design reduces the frequency of removing the basin
from service for complete cleaning. However, withdrawal of sludge from these hoppers
should be frequent and controlled. Otherwise, if the solids become too thick, flow may
"pipe" through to the drain, leaving the bulk of solids on the sides of the hoppers. Fre-
quent removal of solids through these hoppers may also disrupt density currents and im-
prove performance.
Mechanical Solids Removal. Most modern sedimentation basins are designed to be me-
chanically cleaned using a variety of mechanisms, most of which are proprietary. These
include systems that drag or plow sludge along the basin floor to hoppers and systems
that rely on hydraulic or siphon action to withdraw solids. Because each system has dif-
ferent design requirements for basin dimensions and solids draw-off, it is important that
the designer research the available equipment or systems around which to design the tanks.
The following text discusses various types of equipment commonly used. It is not a com-
plete discussion, and new devices are being developed all the time. Good references for
lists of currently available equipment and the names of manufacturers is the Public Works
Manual, an annual publication of Public Works Magazine and the A WWA Sourcebook,
annually published by AWWA.
Traditional Equipment. Traditional desludging equipment was mostly chain-and-
flight drags made up of two strands of iron chain with wooden flights attached at 10-ft
(3-m) intervals and operated at about 2 ft/min (0.6 m/min) to convey dense sludge to a
hopper. Flights were usually made of redwood in lengths up to 20 ft (6 m). Cast iron or
steel wearing shoes were attached to the wooden flights to prevent the wood from wear-