Coagulation Followed By Flotation

Total pressurization is used when the material to be separated is not broken down by shearing forces in the pres-surization system or the floc reforms quickly after the pressure is released. This system (see Figure 7.34.18) produces a maximum amount of air bubbles and requires a small flotation compartment. Wastes with heavy solid loads or those that rapidly form strong flocs are compatible with this method.

Chemical additives can be added upstream or in the pressurization system itself. The system operates by pressurizing the wastewater stream to 30 to 60 psig and mix

= Mean velocity gradient (sec x) M

where:

FIG. 7.34.17 Combined coagulation and settling apparatus. (Reprinted, with permission, from Permutit Co.)

ing it with compressed air. A DO level approaching saturation is achieved in the retention tank. The pressure of the air-saturated stream is reduced in a backpressure valve on the tank discharge. This reduced pressure initiates the formation of microscopic air bubbles which assist in the flotation process.

Partial pressunzation is used when the solid load is light. As shown in Figure 7.34.18, only a fraction of the waste stream is pressurized by air. This scheme frequently requires a separate flocculation chamber in the bypassed stream. In this system, the pressurized stream solids must be absorbed on the preformed floc when mixed in the inlet compartment. Otherwise, a secondary addition of floc-culant solutions is required in the inlet compartment of the flotator.

Recycling pressurization is used when the floc formed cannot be pressurized and large quantities of air must be dissolved. In this scheme (see Figure 7.34.18), the addition of coagulants and flocculants precedes the flotation step. A side stream of the clarified effluent is air-pressurized. When extended floc formation time is required, this method is particularly applicable. The pressurization components are less prone to solids build-up in this system.

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