Civil Engineering Reference
In-Depth Information
Ozone
Ozone is an unstable gas that is manufactured on site at concentrations of 1 percent
to 3 percent for air-supplied generators and 3 percent to 6 percent for oxygen-supplied
units. The ozone-rich gas is then dissolved in the liquid stream. A variety of dissolution
methods may be used, with the most common being a baffled bubble contactor.
Ozone is unstable in aqueous solution and decomposes spontaneously by a com-
plex mechanism that involves the formation of hydroxyl radicals. As shown in Figure
16-4, ozone reacts in two modes in aqueous solution:
17
Direct oxidation of compounds using molecular ozone (O
3(aq)
)
•
Oxidation of compounds by hydroxyl radicals produced during the spontaneous
decomposition of ozone.
•
Direct reactions involving molecular ozone are very selective, with ozone reacting
rapidly with some species but slowly for others. In contrast, the hydroxyl radical is
nonselective, reacting rapidly with a large number of species. The OH radical is the
more powerful oxidant.
Decomposition of ozone may be initiated by a number of water constituents, in-
cluding hydroxide ion (high pH), natural organic matter (NOM), ferrous iron, and
others. In fact, the hydroxyl radical reacts rapidly with molecular ozone, contributing
to the autocatalytic rate of ozone decomposition. Decomposition may also be initiated
by the addition of hydrogen peroxide or irradiation with UV light, as discussed in the
section on advanced oxidation processes.
In natural waters, the lifetime of molecular ozone is extended by the presence of
bicarbonate and carbonate ions, measured as alkalinity. These constituents scavenge
hydroxyl radicals and form carbonate radicals. Therefore, waters with high concentra-
tions of bicarbonate and carbonate alkalinity will maintain molecular ozone residuals
for longer periods than will low-alkalinity waters. The scavenging activity of the car-
bonate species increases at higher pH, partially offsetting the rapid rate of hydroxide-
induced ozone decomposition at higher pH values.
Ozonation of natural organic material in water forms by-products, particularly for
surface waters. Typical products include aldehydes, aliphatic and aromatic carboxylic
acids, quinones, and peroxides. Generally, these products are not considered to cause
significant health effects at the concentrations produced.
Ozone can form bromate and brominated by-products when applied to waters with
moderate concentrations of bromide. One study found that 7 percent of the raw-water
Direct Oxidation of Substrate
O
3
Indirect Oxidation of Substrate
by Hydroxyl Radical
Byproducts
Ozone Decomposition
via Hydroxyl Radical
Radical Consumption by
HCO
3
-
, CO
3
-2
, etc.
Byproducts
Fig. 16-4.
Two modes of ozone oxidation in water
