DISSOLVED AIR FLOTATION PROCESS

1. Condensation
2. Collision
3. Entrapment

In the condensation 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, however, since non-turbulent depressurization of the suspension results in a degree of super saturation. Hence, it is necessary to first achieve near complete air release by turbulence before attachment to the particles.

The collision 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 chemical added during the enflocculation process.

The entrapment 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. The revised Stokes' Equation for the flotation process is:

Vt = (p liquid - pa) Da²

u 18

In this formula Vt = terminal velocity agglomerate, p = density of liquid, pa = density of agglomerate, u = viscosity of the liquid, and g = acceleration due to gravity.

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.

Equipment Used in the Flotation Process

The DAF unit consists of two major components: the retention tank and the separation vat. The retention tank is a pressure vessel designed to provide sufficient time for dissolution of air into the stream to be treated. There are a variety of air introduction systems available, most employing a sparger or ductor.

From the retention tank, the stream is released back to atmospheric pressure in the separation vat. The system is usually designed so that most of the pressure drop occurs in the transfer line between the retention tank and the separation vat so that the effects of turbulence are minimized. In the separation vat, air in excess of atmospheric saturation comes out of solution in the form of tiny air bubbles, which become attached to the particles in the suspension, thus resulting in floating of the particles. The separation vat is equipped with a flight scraper mechanism, which removes the floated material to a recovered solid compartment. Clarified effluent is drawn off from the bottom of the vat. There are a number of different vat configurations and process application methods available to satisfy the conditions of a particular application.

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.

Hydraulic Loading: The relationship of the surface area of the flotation cell (ft²) versus the influent rate into the flotation cell (gpm). For full pressurization systems, the influent rate is the capacity of the pressurizing pumps. For recycle pressurization systems, the total influent is the raw influent flow plus the recycle pump rate. Effective design ranges are 1.0 to 2.5 gpm/ft², depending upon the application.

Solids Load: The relationship of the surface area of the flotation cell (ft²) versus the amount of solids (lbs) entering the system per hour. Design points for solid loadings range from 0.5 to 3.5 lbs/hr/ft², depending on the application and the type of solids involved. It should be noted that any chemical additives used to promote coagulation and flocculation are generally included as solids determining the surface loading since the chemicals used are removed with the float from the system.

Air-to-Solid Ratio: The ratio of the number of pounds of air to the number of pounds of solids. Generally, air is injected in a range of two percent to eight percent (2% to 8%) by volume. Depending upon the type of solids and application, the air-to-solids ratio ranges from 0.020 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.

Types of Pressurization Flow Schematic

Full Pressurization

The entire wastewater flow is injected with air and pressurized for dissolution of the air in the water. This flow then passes into the flotation cell where pressure is relieved. A pump equalization is used to return water to the pump in order to maintain flooded suction during periods of low flow.

Partial Pressurization

Only part of the wastewater flow is pressurized, and the remainder enters the flotation cell bypassing the pressurization step. Once inside the flotation cell, the two flows are again joined together as the air is coming out of solution. A pump equalization return line is also employed in this mode to protect the pressurizing pumps.

Recycle Pressurization

Raw influent is introduced directly into the flotation cell. A portion (generally 50% of raw flow) of the system effluent is pressurized and recycled to the flotation cell where it is blended with the raw wastewater flow.