Civil Engineering Reference
In-Depth Information
Most centrifuge installations have the centrifuge mounted a few feet above the floor,
and use a belt conveyor to move dewatered cake away. Other methods of installing a
solid-bowl centrifuge are to put the centrifuge on the second floor of a two-story
building and drop the dewatered cake into either trucks or a storage hopper on the
first level; to mount the centrifuge about a foot off of the floor and drop the cake into
a screw conveyor built into the floor; or to let the centrifuge cake drop into an open-
throated progressive cavity-type pump for transfer of the cake to a truck, incinerator,
or storage.
Centrifuge performance is measured by the percent solids of the sludge cake, the
percent solids capture, the overall quality of the centrate, the solids loading rate, and
the polymer requirement. The performance of a particular centrifuge unit will vary
with the sludge feed rate and the characteristics of the feed sludge, including percent
solids and sludge temperature.
Centrifuge performance is also affected by polymer type, the dosage utilized, and
its point of introduction. Centrifuge performance on a particular sludge will also vary
with bowl and conveyor design, bowl speed, differential speed, and pool volume.
The feed rate to the centrifuge is always a critical factor. The best performance
data has been obtained at about 75 to 85 percent of the total solids or hydraulic
capacity of the centrifuge, where the lowest polymer consumption is observed and the
driest cake is obtained.
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Solid-bowl centrifuges have the largest sludge throughput capability for single units
of any type of mechanical dewatering equipment. The larger centrifuges are capable
of handling 300 to 700 gpm (19 to 44 1 / s) per unit.
Solid-Bowl Centrifuge Performance Data.
Feed solids concentrations of alum sludges
dewatered by centrifuges generally range from 2 to 6 percent solids; however, 0.4 to
1.0 percent alum sludges have been successfully dewatered. Well-controlled feed con-
centration usually produces polymer savings and good performance.
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Overall raw-water characteristics affect the dewatering property of coagulant sludge.
For alum sludge generated from processing raw water with a turbidity of 4 to 8 NTU,
cake dryness will generally reach 15 to 16 percent, which is considered good perform-
ance for a centrifuge.
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Alum sludge is dewatered at the City of Phoenix, Arizona,
Verde Water Treatment Plant, using centrifuges located on a platform above the floor
(Fig. 23-14).
This water treatment plant is operated frequently as a peaking plant or an emergency
standby plant, but is not operated all the time. The centrifuge produces a cake dryness
of 25 to 30 percent solids, from a feed solids concentration of 3 to 4 percent. In fact,
the centrifuges could produce a cake that was so dry it was difficult to remove from
loadout hoppers, so the polymer dosage was reduced to 5 to 6 lb per ton of solids, to
limit cake solids to 25 percent. The alum dose is typically 12 mg / L, and normal raw-
water turbidity is 10 to 15 NTU.
Alum sludges containing high raw-water turbidity, clay additives, or lime may be
expected to produce higher cake solids concentrations with lower polymer require-
ments than pure alum sludges.
Lime softening sludge dewaters with relative ease because of its calcium carbonate
content. It was reported that a thickened lime sludge could be dewatered in a solid-
bowl centrifuge to a cake solids concentration of 55 percent, with 78 to 93 percent
solids capture.
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An improvement in solids capture efficiency (90 to 100 percent solids
capture) was produced by increasing the sludge residence time; however, a slightly
wetter cake of 45 percent suspended solids resulted.
