Solids Contacting

Solids contact processes combine chemical mixing, flocculation and clarification in a single unit designed so that a large volume of previously formed floe is retained in the system. The floe volume may be as much as 100 times that in a "flow-through" system. This greatly increases the rate of agglomeration from particle contacts and may also speed up chemical destabilization reactions. Solids contact units are of two general types: slurry-recirculation and sludge-blanket. In the former, the high floe volume concentration is maintained by recirculation from the clarification to the flocculation zone, as illustrated in Figure 11. In the latter, the floe solids are maintained in a fluidized blanket through which the wastewater under treatment flows upward after leaving the mechanically stirred-flocculating compartment, as illustrated in Figure 12. Some slurry-recirculation units can also be operated with a sludge blanket. Solids contact units have the following advantages: Reduced size and lower cost result because flocculation proceeds rapidly at high floe volume concentration.; Single-compartment flocculation is practical because high reaction filtration .

Pulsant With Contact Sludge

Figure 12. Solids contact clarifier with sludge blanket filtration.

Figure 11. Solids contact clarifier without sludge blanket rates and the slurry effects overcome short circuiting; Units are available as compact single packages, eliminating separate units; Even distribution of inlet flow and the vertical flow pattern in the clarifier improve clarifier performances.

Figure 11. Solids contact clarifier without sludge blanket filtration .

Figure 12. Solids contact clarifier with sludge blanket filtration.

Equipment typically consists of concentric circular compartments for mixing, flocculation and settling. Velocity gradients in the mixing and flocculation compartments are developed by turbine pumping within the unit and by velocity dissipation at baffles. For ideal flexibility it is desirable to independently control the intensity of mixing and sludge scraper drive speed in the different compartments.

Operation of slurry-recirculation solids contact units is typically controlled by maintaining steady levels of solids in the reaction zone. Design features of solids contact clarifiers should include:

1. Rapid and complete mixing of chemicals, feedwater and slurry solids must be provided. This should be comparable to conventional flash mixing capability and should provide for variable control, usually by adjustment of recirculator speed,

2. Mechanical means for controlled circulation of the solids slurry must be provided with at least a 3:1 range of speeds. The maximum peripheral speed of mixer blades should not exceed 6 ft/sec.

3. Means should be provided for measuring and varying the slurry concentration in the contacting zone up to 50 % by volume.

4. Sludge discharge systems should allow for easy automation and variation of volumes discharged. Mechanical scraper tip speed should be less than 1 fpm with speed variation of 3:1.

5. Sludge-blanket levels must be kept a minimum of 5 feet below the water surface.

6. Effluent launders should be spaced so as to minimize the horizontal movement of clarified water.

Further considerations include skimmers and weir overflow rates. Skimmers should be provided on all units since even secondary effluents contain some floatable solids and grease. Overflow rates and sludge scraper design should conform to the requirements of other clarification units.

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