Civil Engineering Reference
In-Depth Information
TABLE 19-13. Injection Contacting Advantages and Disadvantages
Advantages
Disadvantages
Injection and static mixing have no
moving parts
Additional headloss (energy usage) due to static
mixers that may require pumping
Very effective ozone transfer
Turndown capability limited by injection
system
Contactor depth less than bubble diffusion
More complex operation
Source:
Adapted from Reference 18.
contactor. In other words, 90 percent of the liquid remains in the contactor for longer
than T
10
.
Off-Gas Destruction
The concentration of ozone in the off-gas from a contactor
can readily exceed 1,000 ppm of ozone. This is above the fatal concentration. Off-gas
must therefore be collected and the ozone destroyed prior to release to the atmosphere.
Ozone is readily destroyed by a catalyst operating at high temperature to prevent
moisture buildup. The off-gas destruct unit is designed to reduce the concentration to
0.1 ppm by volume, the current limit set by OSHA for exposure in an 8-hour period.
A blower is used on the discharge side of the destruct unit to pull the air from the
contactor, placing the contactor under a slight vacuum to ensure that no ozone escapes.
Instrumentation
Instrumentation must be provided for ozone systems to protect
both personnel and the equipment. The typical instrumentation includes the following:
Gas phase ozone detectors in spaces such as generator rooms where ozone gas
and personnel are routinely present
•
A gas phase ozone detector on the outlet from the off-gas destruct unit
•
A dew point detector on the feed gas supply
•
Flow switches on the cooling water supply to ensure continued supply
•
A pressure switch to prevent overpressurization
•
Temperature monitoring of cooling time and ozone gas
•
Ozone residual concentration in the contactor. Residual measurements can be
taken at several points to calculate
CT
values
•
Distribution System Impacts
Ozonation of organic compounds increases the bio-
degradability of the organic compounds. This is a benefit if GAC or some other filter
follows the ozone application for organic removal. This will reduce the organic con-
centrations in the water and reduce the potential for producing DBPs in subsequent
chlorination. However, the increase in biodegradable organics also increases the po-
tential for biological growth in the distribution system. The designer must be aware
of these impacts and take appropriate action to manage the bacterial regrowth. In
organic-containing waters, ozone is typically applied upstream of granular filters de-
signed to remove AOC. The design of filters to remove AOC is discussed in Chapter
12, ''Filtration.''
