Civil Engineering Reference
In-Depth Information
Fig. 11-18.
Rectangular sedimentation basin with tube module support beams spanning basin
width (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.)
On the other hand, the width should be as narrow as possible so as to block only a
minimum number of tube openings. The support members should be located a mini-
mum of 6 in. (0.15 m)—preferably 1 ft (0.3 m)—in from the module end to enable
maximum support of the module.
Flocculation.
In expanding a plant's capacity with settling tubes, a utility must closely
study the existing flocculation facilities to ensure that the capacity of those facilities
will not be overtaxed. The tubes are settling devices—they cannot remove material
that has not been flocculated to the point of being settleable.
Some plants may have nonmechanical baffle flocculators that generally perform
well over a limited flow range. When these facilities are expanded, the flocculation
compartment may have to be replaced, supplemented, or modified using mechanical
units. The exact flocculation time needed depends in part on the energy input of the
mixing device. A rough rule of thumb is that flocculation facilities provide 20 min of
detention at water temperatures below 45
F (7.2
C) and 15 min with warmer temper-
atures. These detention times assume that the basins are designed well enough that
they are free of short-circuiting and that a polymer is used as a coagulant aid.
Sludge Removal Facilities.
Sometimes when the raw water is of very low turbidity,
plants may include settling facilities designed to be manually cleaned. These basins
may be cleaned infrequently, perhaps only once a year, if the sediment load is very
light. In other cases, they may be cleaned every 30 days. Although it is usually de-
sirable to have continuous mechanical sludge collectors in basins equipped with tubes,
tube modules have been successfully used in manually cleaned basins. The frequency
