Sedimentation equipment

Of the various groups shown two should be mentioned specifically, as they are somewhat anomalous. First of aU the screen is not a sedimentation system per se but is listed here since there is a good chance that a slurry which behaves as indicated would also be amenable to screening, in which case this may be a preferred method. Similarly, the inclined belt, being somewhat of a hybrid, could be considered by some to be a filter using its own solid mass as a medium. The same, of course would apply for inclined vibrators. 

Whereas an expert would be able to design and calculate the performance of most pieces of equipment from laboratory tests a word of warning should be sounded about centrifugally operated systems, which include hydrocyclones. In our experience these pieces of equipment are best tested on a pilot scale rather than on a laboratory scale. 

There are a large number of processes in the chemical industries that handle a variety of suspensions of solid particles in liquids. The application of filtration techniques for the separation of these heterogeneous systems is sometimes very costly. If, however, the discrete phase of the suspension largely contains settleable particles, the separation can be effected by the operation of sedimentation. The process of sedimentation involves the removal of suspended solid particles from a liquid stream by gravitational settling. This unit operation is divided into thickening, 

increasing the concentration of the feed stream, and clarification, removal of solids from a relatively dilute stream. 

The total solids in municipal wastewaters exist in a distribution of sizes from individual ions up to visible particles. Specific analytical procedures have been established to distinguish the suspended fraction of the total solids and to further distinguish the settleable fraction within the suspended solids. A typical concentration of SS (suspended solids) for raw domestic wastewaters is 200 mg/1, but this can vary substantially from system to system. The lower limiting size for the SS fraction (about 1.5 microns) is arbitrarily defined by the test procedures and it should be noted that variations in test procedures themselves can also lead to 

Fraction Size Range, microns Raw Wastewater Secondary Effluent  

Biological wastewater treatment processes also affect solids characteristics and hence solids separation. Activated sludge solids have been found to have a distinct bimodal distribution with one mode in the supracolloidal to settleable range and another near the border between the colloidal and supracolloidal fractions. The concentrations and size limits in each range are affected by conditions in the 

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Sedimentation equipment can be divided into batch-operated settling tanks and continuously operated thickeners or clarifiers. The operation of the former is simple. Whereas use has diminished, these are employed when small quantities of Hquids are to be treated, for example in the cleaning and reclamation of lubricating oil (see Recycling, oil). Most sedimentation processes are operated in continuous units. 

Sedimentation Equipment. Centrifugal sedimentation equipment is usually characterized by limiting flow rates and theoretical settling capabihties. Feed rates in industrial appHcations may be dictated by Hquid handling capacities, separating capacities, or physical characteristics of the soHds. Sedimentation equipment performance is illustrated in Figure 8 on the basis of nominal clarified effluent flow rates and the appHcable values. The... 

Fig. 8. Sedimentation equipment performance where the particles have a A5 value of 1.0 g/cm and a viscosity, p., value of 1 mPa-s(=cP). The value of is twice the settling velocity at G = 1, and Q = overflow discharge rate in measurements given.

Centrifugal Sedimentation Equipment. Commercial sedimentation centrifuges are characterized priacipaHy by how soHds are discharged, and the general dryness of these soHds. There are batch and automatic batch soHd bowl machines which coUect the soHds at the bowl wall. SoHds are removed very dry. Almost any soHd is coUectable, even those that are very soft and compressible. 

Liquid/Liquid, Liquid/Solid Gravity Separations, Decanters, and Sedimentation Equipment... 

Figure 10.19. Performance of sedimentation equipment (after Lavanchy et al., 1964)...

Figure 11.6 is of two types of sedimentation equipment, and Figure 12.2(e) of another. They are used for clarifying a valuable liquid or for preparing a concentrated slurry for subsequent filtration. They depend on gravitational sedimentation. Removal is assisted by rake action, or by the conical sides of the vessel of Figure 11.6(b). 

Sedimentation. This is a general term for an operation wherein suspended sulids are removed from a liquid by gravitational settling. The two major forms of sedimentation equipment are (I) thickeners, and (2) clarifiers. The term decanting also is sometimes used In designate sedimentation. 

Some of the Classical Sedimentation Equipment as well as other Sizing Equipment is Available from Gilson Company Inc., P.O. Box 200, Lewis Center, OH 43035. 

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