33    ECONOMIC SENSITIVITY OF THE DISSOLVED AIR
FLOTATION PROCESS WITH RESPECT TO
OPERATIONAL VARIABLES
Beth Pascual, Graduate Research Assistant
Berrin Tansel, Assistant Professor
Department of Civil and Environmental Engineering
Florida International University
University Park, Miami, Florida 33199
Robert Shalewitz, Project Engineer
U.S. Army Belvoir Research, Development and Engineering Center
Ft. Belvoir, Virginia 22060
INTRODUCTION
Flotation is a physical treatment of solid/liquid separation that conforms with the goals of RCRA
for removal and recovery of oily wastes. The use of Dissolved Air Flotation (DAF) has been applied to
treating oil-bearing wastes in many industries for the past several decades. With the expanding use of
DAF, more investigation into kinetics and the effects on operational variables is needed for optimal
design.
The DAF process follows mass transfer principles for removal of soluble substances. Air is introduced into a contaminated water stream under pressure. The liquid becomes supersaturated as pressure is reduced and excess air is released in the form of minute air bubbles within the water column.
Dissolved material adheres to the bubbles and rises to the surface where it forms a layer of "float" that
is easily removed. The effluent, drawn from the clarified zone near the bottom perimeter of the main
tank, is discharged or recycled back into the system.
The stipulations of this research include the ability to remove a major portion of emulsified oil and
yet retain certain operational and size criteria. The need for operational analysis is essential in
optimizing the PHC removal and cost efficiency. An extensive literature review phase and experimental phase preceded the economic analysis portion of this research in order to determine optimum
operating conditions. A general description of experimental procedure and results is presented. For a
more detailed description of experimental procedure, the reader is referred to previous work by the
authors.9"'2
EXPERIMENTAL PHASE
Experimental Design
The effects of source water type, coagulant usage, contamination concentration, recycle ratio, flow
rate, detention time, and mode of operation (batch or continuous, full-pressurization or effluent-
recirculation) on organics removal were studied. A 60-liter bench scale DAF unit designed to operate
at an ideal pressure of 50 psi was run in a series of 100% recycle tests and 50% recycle tests to
determine the effect of each parameter on the PHC removal. At 100% recycle, the DAF system was
filled with contaminated water and allowed to run in batch mode for 30 minutes. At 50% recycle, the
system was modified to run in a continuous mode in order to replace the water exiting the system.
Figure 1 presents a schematic of the experimental set-up.
The contaminant mixture used in the experiments was a 1:1:1 mixture of regular unleaded gasoline,
diesel, and jet fuel. Water samples were prepared with 150 ppm and 1500 ppm contaminant concentrations. The experiments were conducted using natural pond water and synthetic brackish (10,000
TDS) water.
Coagulation with a low molecular-weight aluminum-sulfate based cationic polymer was used as a
pretreatment prior to the DAF process. The coagulation process involved the addition of the polymer
49th Purdue Industrial Waste Conference Proceedings, 1994 Lewis Publishers, Chelsea, Michigan 48118. Printed in
U.S.A.
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