OPEN TANK HIGH PURITY OXYGEN ACTIVATED SLUDGE
TREATMENT OF INDUSTRIAL WASTES IN JAPAN
Sharad Deshpande, Product Engineer
Richard Stetzer, Product Manager
FMC Corporation
Itasca, Illinois 60143
Yutaka Nakamura, Chief Engineer
Ataka Construction & Engineering
Tokyo,Japan
INTRODUCTION
In July 1976 and February 1977 two open tank, pure oxygen systems commenced
operating in Japan. They marked the beginning of a newer modification of the activated
sludge process in that country. The activated sludge process, which has been in operation
in the U.S. and abroad for over 50 years, has been proven to be an effective method of
biological wastewater treatment. Coupling newly found pure oxygen technology with the
proven activated sludge process produces a most effective overall system of wastewater
treatment.
High purity oxygen systems have been evaluated and applied for several years. FMC
developed its open tank system in 1971. It fdled the gap in pure oxygen technology by
allowing the economical diffusion of high purity oxygen gas in normally open mixed
liquor basins.
FINE BUBBLE THEORY
The basic principles which apply in the transfer of oxygen in conventional diffused
air systems also apply insofar as their application in the open tank pure oxygen system.
For a given dissolved oxygen saturation deficit in a mixed liquor tank, the rate of mass
transfer depends upon the total surface area of oxygen bubbles that are present in the
aeration tank. The FMC system works because it produces ultrafine bubbles and a consequently high gas surface area.
Just for comparison purposes, the ultrafine bubbles referred to here are of micron
sizes, whereas "fine bubbles" normally produced in diffused air systems are in mdlimeter
sizes. For a given volume of gas, the smaller the bubble size, the more bubbles are present,
and the more surface area that is avadable for oxygen transfer to take place. As an
example, the number of bubbles produced in the pure oxygen system would be 100,000-
200,000 times the number produced for the same volume of gas produced by a diffused
air system. The high purity oxygen system, through use of ultrafine bubbles is capable
of achieving in excess of 90% oxygen transfer (Figures 1 and 2). Further, the greater
driving force avadable utdizing high purity oxygen in water promotes oxygen transfer
efficiencies of greater than 90%.
SYSTEM COMPONENTS
The complete oxygenation system is comprised of three major components as shown
in the process schematic (Figure 3). These components are: (1) an oxygen dissolution
system comprised of rotating diffusers; (2) a source of high-purity oxygen gas, normally
an onsite oxygen generator; and (3) an oxygen control system which balances oxygen
supply with oxygen demand through use of basin-located dissolved oxygen probes and
control valves.
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