Flocculation clarifier performance

Clarifier performance depends on area, which is determined by the flocculation nature of the feed suspension. When the overflow clarity is... 

Dilution. In many appHcations, dilution of the flocculant solution before it is mixed with the substrate stream can improve performance (12). The mechanism probably involves getting a more uniform distribution of the polymer molecules. Since the dosage needed to form floes is usually well below the adsorption maximum, a high local concentration is effectively removed from the system at that point, leaving no flocculant for the rest of the particles. A portion of the clarified overflow can be used for dilution so no extra water is added to the process. 

Control philosophies for clarifiers are based on the idea that the overflow is the most important performance criterion. Underflow density or suspended sohds content is a consideration, as is optimal use of flocculation and pH control reagents. Automated controls are of three basic types (I) control loops that optimize coagulant, flocculant, and pH control reagent additions (2) those that regulate underflow removal and (3) rake drive controls. Equahzation of the feed is provided in some installations, but the clarifier feed is usually not a controlled variable with respect to the clarifier operation. 

Chemical addition is typically used with clarification to improve both the utility and performance of the unit operation. Coagulants and flocculants are generally used to improve the ability to settle particles in the clarifier. Jar tests are used to determine the proper dosage of chemicals and streaming current detectors or turbidity monitors are used to monitor performance and control chemical dosage. Chlorine is often used to improve the removal of organics and color in the clarifier. Chlorine also provides disinfection of the make-up water to prevent the clarifiers from going septic. 

These fines air bubbles are produced by recycling a fraction of the water clarified by flotation (20 to 50%) at a pressure of 4 to 6 bar. Compressed air is dissolved in the water to saturation (in what is called a pressurizing drum), then the water is expanded in contact with the flocculated raw water and at flotation cell backpressure (0.2 to 0,4 bar). Expansion and mixing with flocculated water should be performed with care. The delicate floe must in no case be sheared. This is why the design of potable water flotation cells (where the use of organic flocculants is sometimes prohibited, whereas the floe is labile), is very different from the design of WW flotation cells. 

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