Flotation

Flotation

	Flotation - Small size, low density particles in water and wastewater have low settling velocities and cannot be settled by conventional processes. An alternative clarification process for them is to allow the particles to float to the surface where they can be removed as a scum. If the density of the suspension is close to that of the fluid in which they are suspended they may float without any encouragement. Even if the density of the particles is greater than that of the suspending fluid they can be made to float by the addition of a flotation agent which produces a positive buoyancy. Air bubbles are the effective flotation agents. A number of techniques are available to generate air bubbles and the commonly used one is the dissolved air flotation.
In dissolved air flotation process a portion of the effluent from the flotation tank is recycled through a saturator where air is injected at high pressure. The pressurized recycle flow is introduced to the inlet at the bottom of the tank where it is mixed with the incoming flow. This sudden drop in the pressure produces super saturation of the water with air and a cloud of air bubbles, which attach themselves to the particles in the suspension, are released. These air bubbles along with the particles float at the top of the tank as scum and can be removed. The cycle is repeated to produce an effective removal of suspended particles. Dissolved air flotation has certain advantages over the sedimentation tank processes. They are effective in water treatment when chemical coagulation is used to remove dissolved substance such as color, iron and manganese. They produce lower turbidity output.
Dissolved air flotation units can be brought into full operation within a short period of time whereas the conventional sedimentation tanks take several days. The scum removed from flotation units is normally of a higher solid content than that would be produced by settlement of the same suspension. Thus, the volume of the sludge is significantly reduced. The capital cost of flotation units is less than that of the equivalent settling tank but operating costs are higher because of the power required for pumping and compressed air injection. However total cost of dissolved air flotation will be less than that of the conventional sedimentation tank.

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