DISSOLVED AIR FLOTATION PERFORMANCE
K. Larry Roberts, Graduate Research Assistant
Dennis W. Weeter, Associate Professor
Roy O. Ball, Assistant Professor
Department of Civil Engineering
University of Tennessee
Knoxville, Tennessee 37916
INTRODUCTION
Dissolved Air Flotation (DAF) is a member of a class of processes which have been
designated adsorptive bubble separation    techniques [ 1 ].   Other members of the class
have been used primarily for selective removal of a component of a particular process
or pollutant stream.   DAF has been generally applied to the nonselective removal of
suspended solids and emulsified liquids from water and wastewater.
The conventional design procedure for DAF systems has been based on a combination
of parameters such as hydraulic loading, solids loading, and amount of air available for
flotation.   In order to determine if these parameters are a reliable means of predicting
DAF performance for design purposes, a field evaluation of eight DAF systems was conducted.   The remainder of this paper consists of a brief review of DAF technology
(design process and theoretical concepts), a description of the field evaluation, and a
presentation and discussion of the results of the field evaluation.
DAF REVIEW
Figure 1 is a generalized schematic which shows most of the components which
might be found in a DAF system.   The function of each of these components is to
create or maintain some specific condition conducive to the attachment    of bubbles to
the impurities in the waste stream.
Dissolved air flotation, like many pollution control operations, benefits from receiving
an equalized feed stream both in terms of hydraulic and pollutant loading.   For streams
high in emulsified oil concentration, an API   separator frequently preceeds DAF.   In
order to enhance particle agglomeration as well as bubble-particle adhesion, chemical
conditioning is frequently utilized.   Chemicals are commonly added in a rapid mix tank
with 1-5 minutes detention time.   For a recycle system, the recycle flow is returned to
this tank.
The principal operation following feed conditioning is air introduction and dissolution.    Air may be directly injected into the appropriate stream through a separate pump
or may be metered with a rotometer into the suction side of a centrifugal pump.   The
latter method provides an economical means of air introduction but is limited to approximately 4% air by volume to prevent cavitation and subsequent damage to the pump.
Following air introduction, the stream is generally detained in a dissolution tank with
from 1-5 minutes detention time.   According to Henry's Law, the solubility of the air
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