After being used for oxidizing the water under treatment, the air expelled from the ozone generator will continue to hold residual ozone. The proportion of residual ozone in the outgoing feed gas will range from 1 to 15%, depending on the type of ozone generator used and the proportion of residual ozone maintained in the water. Ways must be found for avoiding the release of high ozone concentrations into the atmosphere. It may be possible to recycle the off-gas ozone into the treatment stage, though it should be remembered that this will require considerable energy input (for recompression and recycling) while giving unspectacular results, especially in drinking water treatment; the ozone dissolution efficiency will not exceed 50% at the low concentration at which it occurs in the recycled air (0.2 to 2 g.Nm-3). Ozone recycling does not, therefore, avoid the need for destroying off-gas ozone, which must not be released into the atmosphere at concentrations above 0.1 ppm.
As it is not possible to simply dilute the off-gas ozone directly into the atmosphere, a method must be found for systematically destroying it. In theory, ozone can be destroyed in one of three ways.
Chemical destruction : may prove suitable for small laboratory-scale production units but not for medium- or large-scale plant, as the cost of reagent would be prohibitive.
Catalytic destruction : has been tried, but the catalysts used are not specific to ozone destruction and are highly sensitive to poisons such as nitrogen oxides, chlorides and sulphides. Further, they have a short life-span and require frequent recharging.
The most reliable method is thermal destruction, with or without recovery of the heat energy applied. For small-scale plant, with less than 200 Nm3.h-1 of air to be treated, ozone is destroyed by heating the gas to about 320°C by means of special easy-to-install high-surface-area expanded metal heating elements. No attempt is made to recover the heat energy in the outgoing gas. The heaters are powered from a thermostatically-controlled thyristor power supply. For medium- and large-scale plant, with more than 200 Nm3.h-1 of air to be treated, ozone is again destroyed by heating, but this time a heat-exchanger system recovers up to 80% of the heat energy as the gas leaves the furnace. This means that, under steady-state conditions, the electrical energy input required for ozone destruction will only be about 20 to 25% of the total energy required for heating the gas up to 320°C. Air containing the residual ozone to be destroyed (1) is sucked in by a fan (2) located in the upper part of the contact column and taken into the secondary circuit of a high-efficiency plate-type heat exchanger (5), where, under steady state conditions, its temperature is brought up from 15 to 260°C. The air then goes through the furnace as such (3), where its temperature is further increased to about 320°C. After spending about 3 seconds in the reactor (4), the purified air comes back through the primary circuit of the exchanger, where, flowing in countercurrent, it yields its heat energy to the new incoming air. Purified air leaves the exchanger at about 70°C and is released into the atmosphere (6).