Floe blanket clarifier

In a floe blanket clarifier as used in the treatment of potable water with ahimmium sulphate, a fluidised bed of aluminium hydroxide floes is formed in the body of the vessel and the chemically treated feed water is led directly into this bed fi om below [Ives, 1968], The effect of the floe blanket is twofold. First, the presence of a high concentration of floes rapidly increases the rate of coagulation of the particles in the feed stream by orthokinetic coagulation and second there is a particle capture mechanism operating in which the smaller floes, which might otherwise pass throu the clarifier, collect on the larger ones. 

It has long been known (see Ives ) that this is an insufficient criterion to cover all cases of flocculation, particularly those of floe blanket clarification, and that integration of equation 4.45 yields a dimensionless group GtC, where C is the floe volume concentration. Consequently, the very low G values (typically < 5 s ) in floe blanket clarifiers, are compensated by much higher C values (typically 0.1-0.2). It appears that the optimum GtC value lies between 100 and 500. This would agree with the value of Gt= 10 multiplied by C= 10 (GfC = 10 ) for paddle-type flow-through flocculators. 

In the case of fluidized bed flocculators the power dissipated is the energy of drag past the fluidized bed particles per unit time. This occurs in the floe blanket clarifier where the previously formed floe particles comprise the fluidized bed. For a fluidized bed the drag force past the floe particles is equal to their weight in the liquid. 

In constant plan (flat-bottom) clarifiers the upflow velocity should be uniform at all levels and the floe blanket (fluidized bed) should have a uniform concentration and uniform particle size. In expanding upflow clarifiers (conical, or hopper-bottom) the lower levels should have greater upflow velocities and, therefore, lower floe volume concentrations or particles. In practice, such expanding upflow is highly unstable and massive vertical eddies cause recirculation, with almost uniform concentrations and particle sizes throughout the fluidized bed. A definite zone of separation forms at the surface with a clean water zone above it, relatively free from primary particles and floes. 

Occupying a position between sedimentation and flotation are the floe blanket and adsorption clarifiers, in which a layer of material is kept suspended (by fluidization) in an upward flow of water carrying solid particles. This flow is dosed with coagulant, so as the water flows through the blanket, the particles grow and are held in the blanket, which thus increases in size, until some is withdrawn for disposal or further treatment. 

In the floe blanket version, the blanket consists just of flocculated particles, whereas in the adsorption clarifier, the floes are attached to granular plastic media. These clarifiers are used for raw water treatment. 

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