Paddle flocculators

Clarifiers. The largest user of clarifiers is probably the water-treatment industry. The conventional one-pass clarifier uses horizontal flow in circular or rectangular vessels (Eig. 2) with feed at one end and overflow at the other. The feed is preflocculated in an orthokinetic (paddle) flocculator... 

Figure 7.13 Conventional DAF luiit with paddle flocculator ODegremont, 1979]...

The power P can be calculated in terms of the head lost in flow through a pipe, a tube module flocculator, lamella separators or fixed (deep bed) or fluidized bed flocculators, whether in laminar or turbulent flow. It can also be estimated for paddle flocculators and stirred beakers (jar test apparatus) on the basis of paddle drag, or torque on the drive shaft. Such practical cases are dealt with in section 4.10. 

Figure 4.17 Velocity gradient produced by various shaft speeds in paddle flocculator of Figure 4.15...

Bearing in mind the problem that scale-up is not meaningful from the jar test apparatus, an alternative test method is to avoid paddle stirrers (although they appear superficially like the full-scale paddle flocculators) and use small-bore tubes. These can be related to practice in the sense that they are flow-through devices, and their mean velocity gradients (G) can be easily measured or calculated. 

Calculate the mean velocity gradient for the four-bladed paddle flocculator shown in Figure 4.19, in a tank containing 100 m of water at 20 °C ... 

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. 

Oscillating or reciprocating paddle flocculators may be on shafts mounted vertically, which move vertically up and down in the direction of the shaft axis, or they may be on frames swinging to and fro in a pendulum-like motion. Both types impart a harmonic motion to the blades. 

This is a very low value compared with conventional paddle flocculators (G = 20 75 s ). The mean residence time in practical fluidized bed floc-culators (t) is usually 20-30 min (1200-1800 s). So the product Gt = (0.24 to 0.36) X 10. This is less than the range for flow-through flocculators with no solids retention (10 —10 ). But the low value of Gt is compensated by the high value of solids concentration C, which results from the criterion GCt of the theory (equation 4.78). Taking typical values ... 

Starting with the assumption that in a flow-through paddle flocculator all the primary particles are identical (i =7=1) equation 4.44 can be integrated... 

It seems, therefore, that the values of GCt are comparable even in very different types of flocculation systems, such as the floe blanket and the flow-through paddle flocculators. 

With the advent of flotation as a useful treatment process in water and waste water treatment, made possible by the creation of finer bubbles by dissolved air precipitation, and electrolysis, the concept of bubble flocculation can be reconsidered. In the flotation process in water treatment, flocculation takes place prior to bubble flotation, usually by mechanical (paddle) flocculator with a coagulant salt (such as alum). It seems possible that the flocculation and flotation processes could be combined with the bubbles providing the flocculating power. 

The combination of a paddle flocculator within a fluidized bed produces much denser floes by the turbulent eddies which provide pressure gradients on the floes, squeezing out their interstitial water. The resultant particles are... 

The natural process of bringing particles and polyelectrolytes together by Brownian motion, ie, perikinetic flocculation, often is assisted by orthokinetic flocculation which increases particle coUisions through the motion of the fluid and velocity gradients in the flow. This is the idea behind the use of in-line mixers or paddle-type flocculators in front of some separation equipment like gravity clarifiers. The rate of flocculation in clarifiers is also increased by recycling the floes to increase the rate of particle—particle coUisions through the increase in soUds concentration. 

Orthokinetic flocculation is induced by the motion of the Hquid obtained, for example, by paddle stirring or any other means that produces shear within the suspension. Orthokinetic flocculation leads to exponential growth which is a function of shear rate and particle concentration. Large-scale one-pass clarifiers used in water installations employ orthokinetic flocculators before introducing the suspension into the settling tank (see Water,... 

MUNICIPAL WATER treatment). Scale-up of orthokinetic flocculators, generally in the form of paddle devices, is based on the product of mean velocity gradient and time, for a constant volume concentration of the flocculating particles. 

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. 

Figure 10. Mechanical flocculator vertical shaft-paddle type.

Graham (10) flocculated porous silica particles, diameter 7.6 pm, in a paddle-stirred vessel with an average shear rate of... 

POWER REQUIREMENTS AND PADDLE AREA FOR A WASTEWATER FLOCCULATOR 16.14... 

Consider this equation as the flocculation equation. ApP-is, the sum of the projected areas of the blades. Vp is the tangential velocity corresponding to r however, the location of the blades requires that there be several v s. To use only one Vp, the paddle... 

Also, excessive velocity gradients can simply break the floes to pieces. To prevent excessive velocity gradients between paddle tips, a minimum distance of 0.3 m should be provided between them. Also, a minimum clearance of 0.3 m should be provided between paddles and any structure inside the flocculator. Paddle tip velocity should be less than 1.0 m/s (Peavy et al., 1985). 

Example 6.7 A flocculator tank has three compartments, with each compartment having one paddle wheel. The ratio of the length of the paddle blades of the longest compartment to that of the length of the paddle blades of the middle compartment is 2.6 2, and the ratio of the length of the paddle blades of the middle compartment to that of the length of the paddle blades of the shortest compartment... 

Paddle wheel—The paddle configuration in a flocculator compartment. 

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