Civil Engineering Reference
In-Depth Information
characteristics centrifuge results (Fig. 23-15).
56,57
At very low residence times the only
solids captured in the cake are those of high density, such as calcium carbonate. Solids
of a lighter, more flocculent nature can pass into the centrate, yielding a high centrate
solids concentration and a corresponding low solids capture. As the solids residence
time in the centrifuge increases, a greater percentage of the lower density solids is
incorporated into the cake (see curve B in figure), often yielding a decreased cake
solids concentration. It is only after increased time of centrifugation that these solids
are sufficiently compressed to yield a further increase in cake solids concentration.
When lime is added to waters with high magnesium content, large quantities of
magnesium hydroxide are precipitated. If lime recalcining is practiced, the resulting
magnesium oxide will appear as recycled inert material; so there will be an ever-
increasing amount of solids in the system. A centrifuge can be used to classify the
sludge into its calcium carbonate and noncalcium carbonate components because of
differences in specific gravities. The ability of the centrifuge to provide magnesium
classification is a major advantage of centrifuges over belt filter presses or pressure
filters when recalcining is practiced.
Belt Press Filtration
Belt filter presses employ single or double moving belts to
continuously dewater sludges. All belt press filtration processes include three basic
operational stages: chemical conditioning of the feed sludge; gravity drainage to a
nonfluid consistency; and shear and compression dewatering of the drained sludge.
Figure 23-16 depicts a simplified schematic of a modern belt press.
58
The press
uses two endless belts of synthetic fiber that pass around a system of rollers at constant
speed and perform the function of conveying, draining, and compressing. Sludge feed
is pumped or delivered to the free gravity drainage zone, the low-pressure zone, the
higher-pressure zone, and finally to an independent high-pressure section.
Good chemical conditioning is important for successful and consistent performance
of the belt filter press. A flocculant (usually an organic polymer) is added to the sludge
A
100
44
96
40
92
36
88
32
84
26
80
24
B
0
5
10
15
20
25
Solids retention time - seconds
Fig. 23-15.
Effect of solids retention time on centrifuge efficiency (
Source:
Reference 12; re-
printed from
Journal AWWA,
Vol. 73, No. 11 (November 1981), by permission. Copyright
1981, American Water Works Association.)
