Civil Engineering Reference
In-Depth Information
TABLE 19-15. Calculated CT Values for the Inactivation of
Giardia muris
with Ozone
and Peroxone
Ozone
C
1
T
1
*
Ozone
C
2
T
2
*
Peroxone†
C
1
T
1
*
Peroxone
C
2
T
2
*
Inactivation
(%)
(mg
min/L)
(mg
min/L)
(mg
min/L)
(mg
min/L)
90
1.6
2.8
1.2
2.6
99
3.4
5.4
2.6
5.2
Source:
See Reference 20. Results at 14
C. (Reprinted with permission from
Environ. Sci. Technol.,
Vol. 23,
No. 6, p. 744. Copyright
1989 American Chemical Society.)
*
C
1
, ozone residual;
C
2
(ozone dose
ozone residual) / 2;
T
1
and
T
2
time (in minutes) to reach 10 percent and
50 percent breakthrough, respectively.
† The H
2
O
2
/O
3
ratio for all results was 0.2.
is required. However, there have been some public health concerns with respect to the
overall efficiency of UV to disinfect potable water.
Based on the available research literature, it appears that although exceptional for
disinfection of small microorganisms such as bacteria and viruses, UV doses required
to inactivate larger protozoa such as
Giardia
and
Cryptosporidium
are several times
higher than for bacteria and virus inactivation.
4,19
Recent improvement in protozoan
enumeration techniques suggest, however, that protozoa can be inactivated by disrup-
tion of their reproductive process at much lower UV doses than was previously
thought.
UV radiation quickly dissipates into water, to be absorbed or reflected off material
within the water. As a result, no residual is produced. This process is attractive from
a DBP formation standpoint; however, a secondary chemical disinfectant is required
to maintain a residual throughout the distribution system.
UV radiation energy waves are the range of electromagnetic waves 100 to 400 nm
long (between the X-ray and visible light spectrums). The division of UV radiation
may be classified as Vacuum UV (100-200 nm), UV-C (200-280 nm), UV-B (280-
315 nm) and UV-A (315-400 nm). In terms of germicidal effects, the optimum UV
range is between 245 and 285 nm. UV disinfection utilizes either low-pressure lamps
that emit maximum energy output at a wavelength of 253.7 nm; medium-pressure
lamps that emit energy at wavelengths from 180 to 1370 nm; or lamps that emit at
other wavelengths in a high-intensity ''pulsed'' manner.
UV Disinfection Reactions
The degree to which the destruction or inactivation of
microorganisms occurs by UV radiation is directly related to the UV dose. The UV
dosage is calculated as:
D
I
t
(19-16)
where:
D
UV Dose, mW
s/cm
2
I
Intensity, mW / cm
2
t
Exposure time, s
