Dissolved Air Flotation (DAF) has become an accepted process for the removal of suspended
solids, oil, grease, and other dissolved matter streams from municipal water treatment systems.
It is a process which relies on uniting air bubbles coming from solution with suspended
particles and subsequently, raising the suspended particles to the surface for removal.
The DAF process employs the principle of increased solubility of gas in solution at elevated
pressures (Henry's Law). In the flotation process, the stream to be treated is saturated with
air at several times atmospheric pressure.
When the pressure is released, air in excess of atmospheric saturation comes out of solution
in the form of tiny air bubbles which attach to the suspended solids and float them to the surface.
Flotation units typically operate at 30 to 70 pounds per square inch.
The released air bubbles become attached to the suspended particles by one of the following
1. In the adsorption mechanism, air in excess of atmospheric saturation comes out of solution
by formation on the surface of the suspended particle. This is not the dominant mechanism. . Hence,
it is necessary to first achieve near complete air release by turbulence before attachment to the particles.
2. The entrapment mechanism is perhaps the most significant in the flotation process. In this
mechanism, the air-to-solids bond is created by collision during random motion. Air bubble and particle
size must be controlled to some extent to ensure that there is a sufficient radius of attachment to maintain
the bond until separation. Particle size is controlled by the amount of energy added during the enflocculation process.
3. The absorption mechanism, which provides a "permanent" air-to-solids bond, can be the predominant
mechanism when the final step of chemical flocculation occurs after air release. This occurs as the air
bubbles become embedded in the floc mass.
After the air-to-solids bond is complete, flotation will occur if the net combined specific gravity of
the air-to-solids agglomerate is less than 1.0. Rise rate of the undisturbed agglomerate is governed by Stokes' Law.
Actual separation of the suspension in the flotation unit will also be governed by the solids and air concentration; and,
the degree of turbulence.
The flotation process is employed where separation of particles having specific gravities close to that of water
is desired. The flotation process will provide faster separation and higher ultimate solids concentration.
Sludge volume generated by the DAF process will be nearly equal to other system processes, but the DAF will have
more air volume entrapped with the sludge; therefore, less water entrapment.
There are three general parameters involved in sizing DAF cells:
1. Hydraulic Loading (gpm/ft2)
2. Solids Loading (lbs/hours/ft2)
3. Air-to-Solids Ratio (lbs of air/lbs of solids)
Generally, one of the above parameters will be the controlling design point.
1. Hydraulic Loading: The relationship of the surface area of the flotation cell (ft2) versus the influent rate into the flotation cell (gpm).
2. Solids Load: The relationship of the surface area of the flotation cell (ft2) versus the amount of solids (lbs) entering
the system per hour.
3. Air-to-Solid Ratio: The ratio of the number of pounds of air to the number of pounds of solids. to 0.1.
Advantages of Dissolved Air Flotation
1. Purchase cost is lower than any other system of comparable waste extraction performance.
2. Installation cost is low. The unit is typically delivered fully prefabricated. Normal concrete pad installation.
3. Space requirements are minimal.
4. Capability to treat a wide variety of organic and inorganic solids and dissolved waste streams.
5. Low retention time from wastewater stream to effluent ejection.
6. Superior clarification of most waste streams.
7. Easy to clean and maintain.
8. Higher density sludge with low water content.