Page 253 - Industrial Wastewater Treatment, Recycling and Reuse

Novel Technologies for the Elimination of Pollutants 227

In addition to the above-mentioned scope, absorption measurements
were carried out with aqueous solutions of salts and soluble liquids. They
have shown that the O 2 input is strongly influenced by these additives.
When pure liquids are aerated, the primarily generated fine gas bubbles
coalesce into bigger ones as soon as they leave the region of high shear stres-
ses. This gas coalescence can be prevented by adding salts or miscible liquids
to the system. The higher the concentration of the additives, the higher the
degree of non-coalescence.
In a coalescence system (pure liquid) the average diameter of the gas bub-
bles is 3–5 mm, and it is almost independent of the gas dispersion device. In a
non-coalescent system with approximately 20 g NaCl/L, gas bubble coales-
cence is already significantly prevented, and the average bubble diameter is
only 0.5–1.0 mm. Because of this partial preservation of the primary bubble
size, it is very important in such systems to use a gas dispersion device that
readily generates very fine gas bubbles.
The decrease in the final bubble diameter in the non-coalescence system
results in a considerable increase in interfacial area and a longer bubble rising
time. Both effects considerably enhance mass transfer, and this has to be
taken into account by using different absorption characteristics.

5.3.2 Optimum Design of Injectors (Bayer, n.d.)
For the optimization of the Bayer injector or the Bayer Tower Biology on
the basis of the characteristics and relationship described above, the follow-
ing parameters are usually given:
- O 2 input
- Average concentration of dissolved O 2 in the wastewater
- Average temperature of the wastewater, and hence the degree of O 2 sat-
uration in the wastewater
- Concentration of additives (salts, etc.) in the wastewater
For these conditions, the required O 2 input can be achieved by an infinite
number of pairs of values for the gas and liquid throughput, but there is only
one pair of values where the required O 2 is transferred into the wastewater
with a minimum of mechanical power. Under these process conditions effi-
ciency is at a maximum.

5.3.3 Examples of Tower Biology (Zlokarnik, 1985)
The Bayer Tower Biology process is generally more economical than
other treatment systems in terms of both investment and operating costs