Industrial Experience with Dissolved Air Flotation
M.G. BIESINGER, Research Engineer
T.S. VINING, Research Specialist
G.L. SHELL, Director
Sanitary Engineering Research and Development
Eimco Processing Machinery Division
Envirotech Corporation
Salt Lake City, Utah 84110
INTRODUCTION
Dissolved air flotation is being used by a growing number of industries to aid in the
achievement of stringent wastewater standards. Wastewater containing light or oily
suspended matter is difficult to treat with conventional gravity sedimentation equipment.
Dissolved air flotation has proven to be an effective and economical method of treating
such wastewater. However, because of a lack of operational field data, its full potential has
not been realized.
THEORY OF DISSOLVED AIR FLOTATION
The mechanism for flotation of suspended and oily matter is similar to gravity
sedimentation in that the separation is caused by a difference in the specific gravities of the
gas-suspended solids phase system and the wastewater. Figure 1 shows a simplified
flowsheet for a recycle dissolved air flotation system.
Figure I — Dissolved air flotation unit
with recycle flow pressuri-
zation.
As the name implies, the gas bubbles used in the flotation process of DAF (Dissolved
Air Flotation) are obtained by dissolving gas in the wastewater at an elevated pressure then
lowering the pressure to atmospheric conditions producing a supersaturated solution. The
gas in excess of saturation precipitates out of solution as micron-sized bubbles.
Three mechanisms are possible for attachment of the bubbles onto the suspended
matter, as can be seen in Figure 2. The flotation process commences after: 1) The collision of
a rising gas bubble with a suspended particle, or the precipitation of air bubble directly on
the solid; 2) Rising gas bubbles being trapped by a floe structure; or 3) The adsorption of gas
bubbles in the forming floe structure.
The quantity of gas soluble in wastewater follows Henry's Law; that is, the solubility of
gas varies with its partial pressure. Equation 1 can be used to calculate the amount of gas
released in the flotation process:
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