Gravity Sedimentation Thickeners And Clarifiers

Sedimentation involves the removal of suspended solid particles from a liquid stream by gravitational settling. This unit operation is divided into thickening, i.e.,

Clarifier And Thickener Drive Units
Figure 4. Cross-sectional view of a thickener.

Thickeners can be operated in a countercurrent fashion. Applications are aimed at the recovery of soluble material from settleable solids by means of continuous countercurrent décantation (CCD). The basic scheme involves streams of liquid and thickened sludge moving counter-currently through a series of thickeners. The thickened stream of solids is depleted of soluble constituents as the solution becomes enriched. In each successive stage, a concentrated slurry is mixed with a solution containing fewer solubles than the liquor in the slurry and then is fed to the thickener. As the solids settle, they are removed and sent to the next stage. The overflow solution, which is richer in the soluble constituent, is sent to the preceding unit. Solids are charged to the system in the first-stage thickener, from which the final concentrated solution is withdrawn. Wash water or virgin solution is added to the last stage, and washed solids are removed in the underflow of this thickener. The flow scheme for a three-stage CCD system is illustrated in Figure 5. The feed stream, F, is mixed with overflow 02 (from thickener 2) before entering stage 1. The overflow of concentrated solution, O,, is withdrawn from the first stage. The underflow from the first stage, U,, is mixed with third-stage overflow, 03, and fed to the second stage. Similarly, the second-stage underflow, U2, is mixed with wash water and fed to thickener 3.

WASH WATER

WASH WATER

Countercurrent Sedimentation

THICKENER THICKENER THICKENER NO. I NO. 2 NO. 3

Figure 5. Flow scheme for three-stage CDD.

THICKENER THICKENER THICKENER NO. I NO. 2 NO. 3

Figure 5. Flow scheme for three-stage CDD.

Standard Center-Pier Thickener

The diameters on these machines are over 400ft., with operating torques to 2,400,000ft.-Lbs. The stationary center pier supports the drive and rake mechanisms. The truss extends from the center pier to the tank periphery supports walkway, power lines and feed ¬°asunder.

Caisson Center-Pier Thickener

(,Diameters are up to 600ft with operating torques of4,000,000 ft.-Lbs. The center pier forms the control and pumping station, as well as support for the rake assembly. The underflow is pumped up through the caisson.

Caisson Center-Pier Thickener

(,Diameters are up to 600ft with operating torques of4,000,000 ft.-Lbs. The center pier forms the control and pumping station, as well as support for the rake assembly. The underflow is pumped up through the caisson.

Caisson Sediment Control

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Figure 6. A cylindrical clarifier and its features.

High-Rate Thickener

Diameters are typically 140 ft. or more with operating torques of 600,000 ft.-Lbs. This is a high throughput machine designed for large loadings.

Caisson Rake And Tie Back

Cable Torque Thickener

The arm rake automatically raises when periods of heavy sludge are encountered. Continuous raking action moves the solids down to the withdrawal point and then the arm is raised back into an optimal position automatically.

High-Rate Thickener

Diameters are typically 140 ft. or more with operating torques of 600,000 ft.-Lbs. This is a high throughput machine designed for large loadings.

Cable Torque Thickener

The arm rake automatically raises when periods of heavy sludge are encountered. Continuous raking action moves the solids down to the withdrawal point and then the arm is raised back into an optimal position automatically.

feed stream velocity as it enters the undesirable cross currents in the settling

The washed solids are removed from the third stage as the final underflow, U3. Continuous clarifiers handle a variety of process wastes, domestic sewage and other dilute suspensions. They resemble thickeners in that they are sedimentation tanks or basins whose sludge removal is controlled by a mechanical sludge-raking mechanism. They differ from thickeners in that the amount of solids and weight of thickened sludge are considerably lower. Figures 6 and 7 show examples of cylindrical clarifiers. In this type of sedimentation machine, the feed enters up through the hollow central column or shaft, referred to as a siphon feed system. The feed enters the central feed well through slots or ports located near the top of the hollow shaft. Siphon feed arrangements greatly reduces the basin proper. This tends to minimize region of the vessel.

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Figure 6. A cylindrical clarifier and its features.

Clarifier Wwtp Operation
Figure 7. Clarifier used in a wastewater treatment plant.
Drinking Water Clarifier
Figure 8. Rectangular settling basins in operation,

Most cylindrical units are equipped with peripheral weirs; however, some designs include radial weirs to reduce the exit velocity and minimize weir loadings. The unit shown also is equipped with adjustable rotating overflow pipes. Although there are a fixed number of U.S. suppliers for these unique machines, the designs are pretty much universal, and nearly identical configurations and comparable operating parameters can be found throughout the world. The photograph shown in Figure

7 is an installation at a Russian municipal wastewater treatment plant, where in fact, the operation was a part of an auto-making facility that also provided sanitary and drinking water supplies to the city. Figure 8 shows a battery of rectangular settling

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