Civil Engineering Reference
In-Depth Information
m) orifices with an opening ratio of 6 to 8 percent and a headloss of 0.08 to 0.12 in.
(2 to 3 mm) have been found effective.
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In cases where extreme wind currents, density
flows, or large variations in flow rate occur, an intermediate diffuser wall at the basin
midpoint will improve basin efficiency. Unfortunately, such a wall limits the type of
sludge collection system that can be used.
In circular, center-feed tanks, the water is introduced into a center influent well by
an inlet pipe brought up from underneath the basin or suspended from the clarifier
walkway. The velocity in the inlet pipe should not exceed 1 ft / sec (0.3 m / s). Dis-
charging the pipe horizontally into the feed well can introduce undesirable currents,
so it is better instead to discharge vertically upward from the pipe into the influent
well (Fig. 11-5). The influent well may be perforated to assist in dissipating inlet
velocities. One approach for energy dissipation is to use an inlet diffusion well that
utilizes ports to develop tangential flow in an outer zone formed by a skirt. This
approach uses inlet energy to distribute flow evenly across the tank. The diameter of
the outer skirt is a minimum of 25 percent of the tank diameter, which represents 6
percent of the tank area. Larger skirts are used in tanks with flocculator center wells.
The basin outlet system should collect the water uniformly across the width of the
basin to prevent localized high velocities from carrying floc out of the basin. Sub-
merged weirs, or effluent ports, are sometimes used to avoid the breakup of floc that
can occur with a freely discharging weir. Perforated launders with ports, commonly
submerged 1 to 2 ft (0.3 to 0.6 m) below the surface, are useful in minimizing problems
of floating trash passing to the filters. They are also useful when it is desired to vary
the water level in the basin during operations that cannot be done with weirs (i.e.,
greater basin level variation can be achieved). Sometimes a utility finds it helpful to
use the storage in the basins to permit some temporary differences between the inflow
to the plant and the discharge from the plant.
In northern climates, launders should be placed at such a depth as to avoid problems
with icing. Fluctuating levels may also minimize ice attachment to basin walls.
An adequate effluent weir length is necessary to avoid excessive velocity currents.
In rectangular basins, adequate weir length usually cannot be obtained with a single
weir across the end of the tank, so weirs are provided in the outlet quarter or third of
the tank. The weirs or launders may be aligned either parallel or transverse to the
direction of flow. In center-feed circular clarifiers, weirs or launders around the pe-
riphery of the tank usually provide sufficient weir length. In some cases, an inboard
annular trough located about 10 percent of the radius in from the periphery is also
used to provide low weir overflow rates for very light flocs. Commonly used weir
overflow rates are shown in Table 11-3.
The use of inboard launders to provide added weir length has been reported to
adversely affect circular clarifier performance.
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As shown in Figure 11-6A, typical
circular clarifier designs establish a current that moves across the basin floor, up the
wall, and into the launders. A proposed approach to overcome the effects of such
currents is shown in Figures 11-6B and 11-6C. Plant-scale tests using a 24-in. (0.610-
mm) baffle beneath a simple peripheral weir (see Fig. 11-6C) showed improved clar-
ifier performance.
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It was also found that the larger-diameter inlet well design used
in flocculating clarifiers provided better flow distribution and energy dissipation than
the inlet wells in standard clarifiers.
Sludge Removal
Modern sedimentation basins are equipped with sludge collection
and removal mechanisms to eliminate the need to shut them down for cleaning. In
rectangular tanks, the bottom is usually sloped gently downward about 5 percent from
