Civil Engineering Reference
In-Depth Information
Fig. 11-8.
Trac-Vac sludge collectors used with tube-style settlers (Courtesy of EIMCO Process
Equipment)
Solids Contact Units
Solids contact units combine coagulation, flocculation, and sedimentation in a single
basin. These units typically have two main features. They allow:
Settled sludge to be recirculated either internally or externally.
•
Formation of a sludge blanket.
•
Some combination flocculation-sedimentation basins do not provide for solids re-
circulation but have a similar configuration.
Solids contact units are frequently used in softening applications. The chemical and
physical reactions of lime-soda softening are enhanced by contact with previously
precipitated calcium carbonate. Treatment of surface waters to remove turbidity or
organic matter is very site specific, and sludge recirculation may or may not improve
performance. Similarly, operation with a sludge blanket may or may not improve
performance.
Solids contact reactors are usually sized for a 1 gpm / ft
2
(2.4 m / h) or higher loading
rates. When they are used in softening, it is customary to size them for a settling
velocity of 1.5 to 2 gpm / ft
2
(3.7 to 4.9 m / h), and they may be operated at even higher
rates. There are two types of solids contact units: sludge blanket units and slurry
recirculation units.
Figure 11-10 illustrates a typical solids contact unit. In the solids contact mode of
operation, previously settled sludge is recycled to the central mixing zone. The slurry
