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OXIDATION AND DISINFECTION 10,55

improved transfer efficiency. However, the corresponding decrease in gas flow rates may
significantly change the hydrodynamics within the contact basin, making uniform con-
tacting (distribution of the ozone throughout the liquid) much less certain.
Certain characteristics of the water itself can also influence the ozone transfer rate, in-
cluding temperature, pH, and water quality. The solubility of ozone increases with in-
creasing temperatures, and thus the transfer rate will increase at higher temperatures. As
pH increases, the transfer rate will increase. The presence of ozone-reactive materials, in-
cluding organics, iron, or manganese, will increase mass-transfer efficiency.

Common Types of Contactors. Types of contactors that are commonly used include
• Conventional fine bubble
• Turbine
• Packed column
• Injectors
• Deep U-tube
The selection of contactor type depends on many factors, including site considerations
and economics, but most often the contactor should be selected based on the specific treat-
ment objective. Appropriate choices are indicated in Table 10.9.
Fine Bubble Contactors. The fine bubble diffuser contactor consists of a series of
over/underbaffled cells. Ozone is applied to some or all cells through a grid of ceramic
diffusers at the bottom of the basin. The fine bubble diffuser contact basin is the most
widely used contactor. Its prevalence is justified by many factors, including

• No moving parts
• Adaptable to both rapid and slow reactions
• Adaptable to high transfer efficiency
• Adaptable to high hydraulic efficiency (T10/HRT, where HRT is the theoretical hy-
draulic retention time)
In an effort to improve transfer efficiency, the depth of bubble contactors has increased
over the years to the current practice of 20 to 22 ft (6 to 7 m) over the diffusers. Further
increases in depth yield only marginal improvement in transfer efficiency, especially at
low ozone doses.
Distribution of gas throughout the liquid must be carefully considered, especially in
the design of disinfection applications where successful operation cannot be directly mea-

TABLE 10.9 Contactor Selection
Treatment objective Suitable contactor choices
Primary disinfection Multistage, fine bubble diffuser type
Iron and manganese oxidation Injectors, multistage fine bubble
Color removal Multistage fine bubble, injectors, deep U-tube, turbine
Taste and odor control Multistage fine bubble, deep U-tube
Algae removal Turbine
Oxidation Deep U-tube, injectors, multistage fine bubble