Downfall Bubble Contact Aeration
RICHARD E. SPEECE, Professor
MARCOS MADRID, Graduate Student
KENNETH NEEDHAM, Graduate Student
Department of Civil Engineering
New Mexico State University
Las Cruces, New Mexico
INTRODUCTION
Commercial oxygen has "come of age" as a technique for water quality management. For large oxygen utilization systems, oxygen can be economically supplied
for conventional on-site oxygen plants. For smaller systems, small, automatic oxygen
plants based on proprietary adsorption technology, have recently been developed to
supply low-cost oxygen. A simple system has been developed by the authors to
efficiently absorb commercial oxygen and air into water. It has been descriptively
referred to as Downflow Bubble Contact Aeration (DBCA). DBCA provides for the
passage of water downward through an open-bottomed, expanding cross section
hood. The water thus enters at the highest velocity at the top and leaves at the lowest
velocity from the open bottom of the hood. The inlet velocity is designed to be
greater than the buoyant velocity of the bubbles. The exit velocity is designed to be
less than the buoyant velocity of the bubbles. Thus, the bubbles are trapped inside
the hood as the water flows through it.
The hood can be filled with bubbles providing a high ratio of bubble interfacial
area to water volume. There is also considerable turbulence, resulting in high gas
transfer rates. Bubbles injected into a ten ft deep tank of water rise to the surface
and escape in about ten sec or less. By comparison, bubbles can be maintained in
contact with water in DBCA for an indefinite period. Thus, efficient absorption of
commercial oxygen is possible due to the prolonged contact times achieved.
Stripping and absorption occur simultaneously in DBCA. For example, passing
well water, which is devoid of dissolved oxygen (DO) and saturated with dissolved
nitrogen, through the system while injecting commercial oxygen causes oxygen to be
absorbed by the water and nitrogen gas to be stripped from the water. This results in
a change in the gas composition of the bubble from 100 per cent oxygen initially to
approximately 100 per cent nitrogen eventually when a batch injection of oxygen is
made. If used as the aeration system is activated sludge, carbon dioxide would be
stripped out in the DBCA. If the water contained no dissolved gases, the gas composition of the bubbles would remain 100 per cent oxygen, but the bubbles would be
completely absorbed and disappear eventually.
The hydraulic head required to move the water through DBCA may be available naturally when a DBCA system is used for river reaeration. Where no head is
available, as in a tank, an industrial propeller can be used to recirculate water through
the system.
EXPERIMENTS
Two types of experiments were conducted to evaluate the performance of
DBCA. First a bench scale system, shown in Figure 1, was studied to observe the
changes after a batch injection of oxygen and air. It consisted of a submerged,
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