Taper flocculation

Rotary vertical shaft turbine units as shown in Figure 10 and other rotary or reciprocating equipment are other examples. Tapered flocculation may be obtained by varying reel or paddle size on horizontal common shaft units or by varying speed on units with separate shafts and drives. In applications other than coagulation with alum or iron salts, flocculation parameters may be quite different. Lime precipitates are granular and benefit little from prolonged flocculation. 

Flocculation doesn t occur] faulty fluid dynamics into the basin/reel at wrong rpm/residence time too short/mixing not tapered/unexpected turbulence/too short a residence time between coagulant and subsequent flocculant dosage/ wrong flocculant. 

Several designs have evolved, many with horizontal axes (i.e. both ends closed) with the outer cylinder rotating and the inner cylinder fixed, which provides best hydrodynamic stability. Some horizontal-axis Couette flocculators are flow-through designs, even with a tapered gap to provide taper flocculation However, horizontal-axis apparatuses suffer from end effects which cause secondary circulation, so only a limited central zone (about one quarter of the length) is in defined laminar flow. 

Figure 4.20 shows typical arrangements of horizontal- and vertical-axis paddle flocculators. The axes should rotate in alternate directions along the tank, e.g. in Figure 4.20 the first and third paddles rotate clockwise, but the second rotates anticlockwise. This rolls the liquid containing the floes from paddle to paddle without excessive shear as the paddle blades pass each other. If taper flocculation is desired, each sequential axis will rotate more slowly. The choice between horizontal and vertical axes depends primarily on mechanical considerations, such as the power drive system and the maintenance of bearings and support frames. 

Propeller or turbine blade flocculators are difficult to assess from a theoretical standpoint as they vary greatly in form and efficiency. An example of successful taper flocculation using turbine agitators is described by Bernhardt and Schell, for the formation of ferric hydroxide floes for phosphorus removal of the water supplying the Wahnbachtal reservoir. The practical approach to such propeUer/turbine flocculators is to make torque... 

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