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OXIDATION AND DISINFECTION 10.41

• o ~, of organic substrate, microorganism or NH 3
x~">v/ • Products
o+\°/ Slow, selective

/

~'9_ / "~. Organic radical
N~or/ /~" ~ organic substrate, microorganisms or NH 3 7
09 / ~-
~%,. / ~o'-/¢ Fast nonselective ~.
%" %~06. / ~b'/ Products
o.~ . , 0
~t organic radical )/Oxidation of bicarbonate ion
~ ( • Bicarbonate radical
Hydroxyl ~O+.. Moderate
radical \'~¢.
~'':~¢~ of carbonate ion
~ Carbonate radical
Fast
FIGURE 10.19 Reaction pathways of ozone in water. (Source: J. Hoigne and H. Bader, "Role of Hy-
droxyl Radical Reactions in Ozonation Processes in Aqueous Solutions," Water Resources Bulletin, vol.
10, 1976, p. 377.)

nonselective in its oxidation of organic compounds, but at the same time, it is scavenged
by carbonate and bicarbonate ions to form carbonate and bicarbonate radicals.
These carbonate and bicarbonate radicals are of no consequence in organic reactions.
Furthermore, the hydroxyl radicals and organic radicals produced by autodecomposition
become chain carriers and reenter the autodecomposition reaction to accelerate it. Thus
low-pH conditions favor the slow, direct oxidation reactions involving 03, and high-pH
conditions or high concentrations of organic matter favor the autodecomposition route.
High concentrations of bicarbonate or carbonate buffer, especially carbonate buffer, re-
duce the rate of autodecomposition by scavenging hydroxyl radicals. This means that
ozone residuals last longer at low pH and in highly buffered waters.
The rate of autodecomposition is highly dependent on many factors, including pH, UV
light, temperature, ozone concentration, and the presence of radical scavengers. Decom-
position can be expressed, assuming pseudo-first-order kinetics and in terms of ozone
residuals, as
Ct/C0 = e -kr
where Ct = ozone concentration at time t, mg/L
Co = ozone concentration at time 0, mg/L
k = decay rate constant, L/min
t = time, rain
For design purposes, the decay rate constant must be determined for the water to be
treated under expected operating conditions, including temperature, pH, and water
quality.

Ozone Systems
Ozone systems are typically comprised of four basic subsystems: ozone generation, feed
gas preparation, contacting, and off-gas disposal.