Page 252 - Industrial Wastewater Treatment, Recycling and Reuse

226 Industrial Wastewater Treatment, Recycling, and Reuse

way that one injector covers 10–20 sq ft of the floor area. The injectors in
the cluster are directed downward, so that their jets reach the bottom and
prevent solid particles from settling. This arrangement has yet another
advantage: The gas–liquid jet that leaves the injector fans out into a bubble
swarm after about 80 characteristic nozzle diameters. With this arrangement
of the injectors, the bubble swarm starts to rise to the water surface right from
bottom of the tank, and the total height of the liquid column is utilized. This
gives maximum possible residence time for the gas bubbles in the water. The
injector works under optimum conditions when the required oxygen input
(kg or lb O 2 /h) is achieved with a minimum of mechanical energy (kW or
hp) for the pumps and compressors of liquid and gas, that is, when the effi-
ciency (kg O 2 /kWh or lb O 2 /hp-h) is at a maximum.
It operates economically from a depth of about 7 m. The specific data of
an injector, for example, quantities of air and pressure water, pressure drops,
level of efficiency, and residual oxygen content in the off-gas, are calculated
by computer on the basis of the effluent data for each project.

5.3.1 Process Design Characteristics (Bayer, n.d.)
For the determination of the respective optimum operating conditions, the
following process characteristics are necessary:
- The absorption characteristics of the injector combines the O 2 input of
the injector with the height of the liquid column, the characteristic con-
centration difference, and the two independent process parameters, gas
and liquid, throughout. In addition, information about the influence of
liquids and/or salts dissolved in the wastewater on the O 2 absorption
has to be inherent in this characteristic.
- The pressure drop characteristic of the injector is needed in order to size
the pump and the compressor correctly. Knowledge of the pressure drops
permits calculation of the “kinetic” energy of the water jet that is utilized
for the dispersion of the gas into gas bubbles.
- The relationship for the adiabatic compression of the gas throughput takes
into account the influence of the height of the liquid column on the
power to compress the gas.
Disregarding this last relationship, which can be taken from an appropriate
textbook, all the other characteristics were determined by Bayer using a
Bayer injector of original size. For greater experimental accuracy, the O 2
input was measured under stationary conditions, using the “hydrazine
method,” which was specially developed for this purpose.