The aerators mostly used in deep tanks are tubes or plates made of porous ceramics or synthetic materials. We refer to these reactors as deep tanks if the water depth is greater than H = 6 m. Existing tanks frequently have a depth of 4-6 m, but already 6 years ago activated sludge reactors with H = 8-12 m were planned, constructed and commissioned (Popel et al. 1998). Reactors with L = 15-25 m have been built for the treatment of industrial effluents (Turmbiologie, Bio-Hochreaktor, deep-shaft reactor). Although most reactors are dug into the ground, some have been built above ground; we speak of "deep tanks" in both cases and use L to indicate the water depth (Turmbiologie, Bayer 1991; Bio-Hochreaktor, Leistner et al. 1979; deep-shaft reactor, Lock 1982).
Let us study mass transfer in deep tanks only in such systems which are approximately characterized by completely mixed water and completely unmixed oxygen in the rising air bubbles (plug flow). Reactors with L>12 m may be excluded, because nearly complete mixing of the water is not to be expected. Mass transfer in bubble columns is a complicated process:
1. Oxygen concentration inside the rising bubbles decreases, resulting in a high Ac' at the beginning and a lower Ac' at the end before the bubbles reach the water surface.
2. The pressure caused by the water column decreases with decreasing depth, which has an effect on both:
(a) the gas concentration of oxygen and inert components inside the bubble and
(b) the bubble diameter as well as the velocity of the rising bubble, consequently the specific overall mass transfer coefficient KLa.
3. As the result of the mass transfer of oxygen into the water and the mass transfer of CO2 into the bubbles, the molar fraction of O2 changes in a complicated way and the concentration difference between dissolved and gaseous O2 is influenced.
Let us discuss these influences on mass transfer separately.
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