Ozone transfer efficiency, by whatever method of ozone contacting will never be 100% unless the water is treated with a
volume of ozone substantially below the demand. This can not normally be done and achieve treatment objectives, so due
to limitations in dissolving and the need to be sure treatment objectives are met, their will always be some ozone in the
vent gas from a ozone water treatment system.
In most cases this excess ozone needs to be reduced from typical values of 1% to 10% to less than 0.1 ppm. This decomposition can be achieved thermally or thermocatalytically. Before discussing the specific types of ozone desruct units, it is important to understand the nature of the vent gas stream. Ozone emissions are covered by the Clean Air At and subsequent regulations associated with that act (see EPA Ozone Emission Fact Sheet). In some cases low concentration ozone monitors are employed to insure the tart levels are met.
The ozone vent gas will likely come from either a contact/reactor vessel or from a degas tank. In both cases the gas will be fully saturated with water at the temperature of the water in the vessel. In all ozone destruct systems the gas is heated, so the water will remain in the gas phase past the heater and catalyst bed. As it cools down however, the water will begin to condense. Provisions have to be made to prevent this water from ozone destruct unit to prevent damage from the catalyst bed and/or the heater element. In addition, condensation will cover the catalyst particles and prevent them from destroying the ozone.
If the contact/reactor vessel is not pressurized as is the cases for most bubble diffuser system, a blower will need to be fitted onto the ozone destroyer to pull a slight vacuum( -2 to -4 inches of water) to draw the vent gas into the destroyer. If a degas tank is being employed, as in a side stream injection system, these tanks normally operate under pressure and there should be sufficient pressure to push the vent gas through the ozone destroyer/destruct system.
There are three basic types of ozone destroyer (ozone destruct) system. Spartan provides all three types of ozone destroyers (see our data sheet on ozone destroyers).
Ozone decomposes spontaneously at elevated temperature to oxygen. Thermal ozone destroyers simply heat the vent gas to the necessary temperature. The systems are the simplest and least expensive on a capital cost basis. They do use more energy than other systems so they are usually only applied to relatively small ozone water treatment systems. Essentially all of the gas that passes through the contact/reaction vessel must be heated to the decomposition temperature (gas from the ozone generator, water vapor from the reactor vessel and any air that leaks into the contact tank).
In the presence of a catalyst the temperature needed to decompose ozone is greatly reduced. This reduces the energy required to for the ozone destruct system. However, the capital cost for the catalyst based system is higher than for a pure thermal system. The additional capital cost is easily paid for as the size of the system increases. The catalyst is almost always manganese dioxide. Activated carbon can decompose ozone, but it also burns, so it is not used for this purpose.
Heat Recovery Systems
Another way to reduce energy cost in a thermal based ozone destroyer is to heat the incoming vent gas with the treated vent gas. This recaptures a portion of the energy used to heat the vent gas and reduces overall energy consumption. These systems are only employed on the largest ozone water treatment systems.