DEEP TANK AERATION/FLOTATION
FOR FERMENTATION WASTEWATER TREATMENT
Melbourne L. Jackson, Professor
Department of Chemical Engineering
University of Idaho
Moscow, Idaho 83843
INTRODUCTION
The limitations of conventional biological growth processes for wastewater treatment
and some fermentations with respect to oxygen supply and land space requirements led to a
series of investigations of oxygen transfer in very tall columns and tanks [1-6]. These included equations to properly describe the oxygen driving force effecting transfer. A process
utilizing tall tanks, completely mixed, followed by flotation utilizing the inherent dissolved
gases, has been the subject of two patents [7]. Installations utilizing the process operating
for a number of years have been described [8,9]. Potential applications have been considered for the growth of single cell protein [ 10,11 ].
DEEP TANK AERATION/FLOTATION SYSTEM
The initial work showed, contrary to some comments in the literature, that tall liquid
columns resulted in increased oxygen transfer efficiencies (percentage of the inlet oxygen
which is transferred). Such efficiencies increased linearly with depth up to 50-60 ft for air in
clean water for oxygen transfer at zero dissolved oxygen in the liquid [1,2]. This results
from the increased pressure and oxygen solubility as the liquid column height is increased.
In addition, the deeper liquid provides for corresponding increases in the bubble residence
and exposure times. The higher efficiencies result although a substantial depletion of oxygen
in the bubbles occurs. At 50 feet of liquid depth, the percentage of oxygen transferred was
observed to be 50% and decreased only somewhat for heights of 75 feet. Much work has
been reported in the literature using relatively narrow columns of one or two inches, and
heights of only six to ten feet. Under such conditions oxygen transfer on bubble formation
can be a substantial proportion of the total transfer. In a wider bubble column and in a wide
tank, the transfer efficiency is least during the first six to ten feet of bubble rise, probably
because of breakup and formation of smaller bubbles. After this initial rise height, the
bubble population and size distribution appear to remain relatively constant with little
coalescence unless populations in the bubble swarms become quite high.
The high oxygen transfer efficiencies suggested a number of advantages of the use of tall
(deep) tanks for microbial growth processes. The volume of air which needs to be supplied
can be reduced by factors of up to ten times [5 ] for 50 feet of depth (vs about 5% transfer
efficiency in shallow basins). Also, increased energy for compression and air supply is not
indicated; the reduced volumes for the several stages of increasing pressure can result in a
nominal reduction in the energy requirement compared to the larger volumes at lower
Pressures required in conventional systems. Air supply lines can be very short, and of small
diameter, which reduces both pressure losses in lines and capital costs.
It was observed early for wide tanks that supersaturation occurred with respect to the
residual dissolved gases, particularly nitrogen. This results from the fact that mixing in the
tanks occurs over the entire depth and in local zones around the bubble swarms. The high
hydrostatic pressures at the lower levels result in absorption of the gases at a rate which is
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