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10.58 CHAFFER TEN

uum, drawing in the gas. In most applications, this is conducted in a sidestrearn, which is
subsequently blended with the remaining liquid flow in a static mixer. In small applica-
tions, this may all occur in a pipeline; for larger applications, injection would normally be
followed by a reaction chamber. The primary disadvantages of this system are the energy
input required to move the liquid in the sidestream and poor turndown characteristics.

Off-Gas Disposal
One of the principal design problems in ozone contact systems is the disposal of off-gases
from ozone contactors. Assuming that ozone contactors use from 90% to 100% of the
ozone that is applied, the air exiting from the contactor may have ozone concentrations
as high as 0.5% by volume. This compares with a threshold odor level of 0.05 ppm for
ozone and an 8-h OSHA standard of 0.1 ppm.
To date, regulations have not been established on the levels of ozone that may be dis-
charged to the atmosphere, but there is no question that large volumes of air containing
0.5% ozone cannot be casually discharged. Five principal methods of off-gas disposal
may be considered:
• Reinjection
• Heating to cause autodecomposition
• Chemical reduction with a reducing agent
• Catalytic reduction with a metal oxide
• Dilution
Reinjection generally involves the construction of two ozone contact basins. The fresh
ozone is introduced into the downstream contact basin, and the off-gases are then re-
pumped and reinjected into the upstream contact basin. Given the efficiencies of ozone
consumption in each contact stage and the loss of ozone during the repumping process,
the ozone residual in the air exiting from the reinjection stage can be as low as 0.001%
or 10 ppm. Thus injection alone does not completely solve the problem. Rather, reinjec-
tion must be used in tandem with some of the other techniques described.
Chemical reduction is another method for removing ozone residuals from off-gases.
The chemical reduction could be accomplished by passing the off-gases from the ozone
contact chamber in countercurrent flow with an ozone-specific reducing agent in a scrub-
ber much like those used for removing fumes from industrial off-gases. The key to this
method is the selection of an inexpensive reducing agent that is not also oxidized by the
oxygen present in the air. No uniformly satisfactory reducing agent has been developed
to date.
Ozone rapidly dissipates when it is heated. Consequently, in some designs the ozone
contactor off-gases are heated to a temperature at which decomposition of the ozone is
nearly instantaneous. Temperatures as high as 250 ° C have sometimes been indicated. The
obvious disadvantage of this method is the amount of heat required. In some European
designs, the hot air exiting from the ozone decomposer is recycled to a preheater to warm
the air that is about to enter the decomposer. This reduces energy requirements but in-
creases capital costs.
Most recent designs have used thermal/catalytic destruct units for off-gas treatment.
These consist of a vessel containing the catalyst preceded by a heater. Catalytic reduction
involves passing the ozone off-gases across a surface that catalyzes the decomposition of
ozone to elemental oxygen. Most catalytic compounds shown to be effective for ozone
reduction are proprietary and are based on iron or manganese oxides.