The Goal
The goal of "dissolved air flotation" is to take suspended particles out of their liquid environment, and transport them to the surface of the liquid, and then create a floating layer of these particles until they are concentrated enough to be skimmed off the surface.

The Theory
In chemistry, we learned that Henry's Law states that a liquid can absorb an amount of air (or other gas) that is directly proportional to the pressure of the air (or gas) being forced into it. As we increase the pressure, we can increase the amount of air being forced into water. When we release the pressure, the air or gas is released from the liquid solution, usually taking the form of millions of very small bubbles, which rise to the surface. An excellent example of this is a carbonated soft drink. If we shakeup a can of "Coke" or "Pepsi," and then open the can, well, that's an EXTREME example of gas being liberated from a liquid! If there are small particles in the solution during the time that the bubbles form, these particles will adhere to the surface of the bubbles as they rise to the surface of the liquid.

If we continue adding more liquid to this vessel, containing both pressurized air and floatable particles, a buoyant gas-solid mixture will continue to form at the surface. This is referred to as "the float." The float will increase in concentration to the point at which some of its contents must be skimmed from the surface. The tank effluent may be removed from the process by the use of a baffle to contain the float and an effluent weir/launder arrangement; or by removing it from below the float; or near the bottom of the tank. Provisions are usually made for the removal of more dense solids that are not captured and floated to the surface, by the use of flights or scrapers on the floor of the tank.

The process has basically three manners in which this can be accomplished:
1) By pressurizing and forcing air directly into the ENTIRE flow stream to be treated.
2) By pressuring a PORTION of the flow stream and forcing air into it, and then combining this pressurized stream with the rest of the stream to be treated in the tank.
3) By RECYCLING a portion of the dissolved air flotation process effluent back to be pressurized, and then combined with the entire stream to be treated.

Most processes are designed to be operated with and/or without the use of a coagulant such as alum or a polymer. (More on this in Part 2).

The "Application" in Water and wastewater Treatment
When applied in our field, the tanks may be either rectangular or circular. The process application is quite varied. Some of the uses are:

Potable water:
The separation of particles from a raw water source, prior to floc/sedimentation and/or filtration .
The thickening of a sludge flow stream from a sedimentation process.

Wastewater
The thickening of wastewater treatment primary sludge prior to sending to a solids handling process such as anaerobic digestion.
The thickening of waste activated sludge from an activated sludge process (used quite frequently for this), trickling filter/solids contact, etc.
Used in many cases for "co-thickening" of a flow stream of combined primary sludge and secondary treatment process sludge prior to further solids treatment.

This process is often used in paper mills, dairy processing operations, refineries, food processing (like canning tomatoes), etc, to separate paper, oils, greases, and other small suspended solids as a process unit, or as a pretreatment unit prior to discharging to a wastewater collection system.

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