Dissolved Air Flotation Application
to
Industrial Wastewater Treatment
JERRY L. BOYD, Chief Process Application Engineer
GERALD L. SHELL, Director
Sanitary Engineering Research and Development
Eimco Processing Machinery Division
Envirotech Corporation
Salt Lake City, Utah
INTRODUCTION
Dissolved air flotation processing of industrial wastewater has been used for many
years. However, its application has been limited until recently by a lack of understanding of
application principles and performance data. With increased emphasis on pollution control
and man's total environment, more engineers have turned to flotation as a useful tool in
helping to solve many difficult wastewater treatment problems. Wastewaters that contain
fats, greases or oily material float naturally and present a difficult removal problem to
conventional clarifying equipment. Dissolved air flotation treatment of wastewater
containing these types of pollutants has proven to be an effective and economical approach.
THEORY OF DISSOLVED AIR FLOTATION
The flotation process consists of attaching fine gas bubbles to suspended or oily
material causing a reduction of specific gravity of the S.S.-air mass in relation to the
suspending liquid. The micron-size bubbles are produced by dissolving gas into the
wastewater at elevated pressures followed by subsequent release to atmospheric pressure.
When the pressurized wastewater is reduced to atmospheric conditions, the dissolved gas
(in excess of saturation) is released as extremely fine gas bubbles.
Figure I shows a simplified flowsheet of a dissolved air flotation treatment system. The
wastewater enters the flotation vessel and the air-oil or air-solids mixture rises to the liquid
surface because the air-solids mixture has a specific gravity less than the liquid. Any solids
having a specific gravity greater than the liquid tend to settle to the bottom. Dissolved air is
applied by recycle flow pressurization in Figure 1. Types of pressurization systems will be
discussed later.
Figure I - Dissolved air flotation
unit with recycle flow
pressurization.
The amount of gas that can be dissolved in water follows Henry's Law (C= KP). This
law states that for gases of low solubility, the mass dissolved in water varies with its partial
pressure. The quantity of gas which will theoretically be released from solution when the
pressure is reduced to atmospheric conditions is calculated from the following equation:
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