Page 669 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

624 Fundamentals of Water Treatment Unit Processes: Physical, Chemical, and Biological

The solutions from the two tanks, that is, HOCl and
Na =ClO 2 , are fed to a ‘‘reactor,’’ which facilitates BOX 19.4 THE INTERNATIONAL

þ
the following reaction (Doull, 1980, p. 190): ULTRAVIOLET ASSOCIATION (IUVA)
The International UltraViolet Association became visible
HOCl þ 2ClO 2 þ H ! 2ClO 2 þ Cl þ H 2 O

þ
at the AWWA Annual Conference in Chicago in 1999.
(19:30)
Also, at the trade show, UV was evident at a level not
seen in previous years. It became evident later that the
3. Chlorine gas reacting directly with solid sodium
high level of activity in 1999 was the 1998 finding of
chlorite: According to White (1999, p. 1160) a spe-
Dr. Jennifer Clancy and associates that UV could inacti-
cially processed solid sodium chlorite (Saf-T-chlort,
vate Cryptosporidium parvum oocysts at much lower
CDG Environmental, LLC) reacts with chlorine gas
dosages than found by previous investigators. After Giar-
to produce a high purity chlorine-free chlorine diox-
dia, which emerged as an issue in the 1970s, Cryptospor-
ide gas (quoting a patent by Rosenblatt et al. 1992).
idium came on the scene in the late 1986 and became one
chlorine dioxide gas (quoting a patent by Rosenblatt
of the major issues in the drinking water industry. Until
et al. 1992), according to the following reaction,
UV, ozone was the only known effective disinfectant.
Cryptosporidium was also the regulatory issue (i.e., the
Cl 2 (g) þ 2NaClO 2 ! 2ClO 2 (g) þ 2NaCl (19:30 )
0
EPA ‘‘surface water treatment rule’’) that would eventually
forcemunicipalitieswithunfilteredwatersuppliestoimple-
On a mass basis, 1.97 kg ClO 2 reaction product is
ment filtration. The UV finding was a ‘‘way out’’ for these
formed per kg Cl 2 (1.97 Lb ClO 2 =Lb Cl 2 ). The chlor-
utilities. Also, the finding gave other utilities that had filtra-
ine gas is diluted approximately 90% (by volume)
tion a second positive barrier to the oocysts, that is, an
with air to maintain a safe partial pressure of ClO 2 in
alternative to ozone or microfiltration. In addition, UV
the product stream. The system is operated under
was found to be effective in inactivating other organisms,
vacuum produced by chlorine sidestream injector,
for example, viruses, bacteria, bacterial spores, and cysts.
similar to vacuum chlorination systems. One potential
Finally, UV did not cause the formation of disinfection
advantage of this process for water treatment facilities
by-products. Thus, the interest in UV picked up from rela-
is that, because the reaction product is high-purity tively ‘‘casual’’ to what might be termed, ‘‘high-profile.’’
ClO 2 gas, excess chlorite ion, which does not exist
The IUVA was formed in the spring of 1999, largely
as a gas, cannot be added to the main transmission
in the context of the foregoing circumstances. The organ-
line. This feature can be significant given the USEPA
ization started with a quarterly newsletter, IUVA News,
MCL of 1.0 mg=L for chlorite ion (Gregory, personal
and has held a biennial congress, the fifth being in
communication, January, 2010).
Amsterdam in 2009. Dr. James P. Malley, University
of New Hampshire, was the first president and Dr. James
19.3.7 ULTRAVIOLET RADIATION R. Bolton was named Executive Director, while Dr. Rip
G. Rice became Editor of the newsletter (Malley, 1999).
The effectiveness of ultraviolet radiation has been known
since about 1910 (see Section 19.2.3), with the technology
being largely dormant until about the 1980s. An upsurge of
interest started during the 1980s, mostly in disinfection of Integration gives the relation (Masschelein, 2002, p. 68),
wastewater effluents, with more growth during the 1990s.
N
The impetus for its use in drinking water disinfection came ln ¼ kIt (19:32)
in 1998 with the finding by Clancy et al. (1998, 2000) that low N 0
doses of UV inactivated Cryptosporidium parvum oocysts The product, It,isdefined as the ‘‘dose,’’ that is,
(Box 19.4).
Dose ¼ It (19:33)
2
19.3.7.1 Disinfection Rate by UV where Dose is the energy quantity per unit area (J=m ).
2
In general, inactivation of organisms by UV follows the The ‘‘dose’’ is given usually as mW s=cm ; the conversion
2
2
mathematical model of Chick, that is, is: 1 kJ=m ¼ 100 mW s=cm . Several notes relevant to UV
in practice are
dN
¼ k I N (19:31)
dt . UV radiation in the wavelength range 200 l 300
where nm causes damage to the DNA, which prevents
N is the # microorganism=m 3 replication and hence inactivates the organism.
t is the elapsed time of contact (s) Prescott et al. (1993) mention that l 260 nm is
k is the kinetic constant, a function of transmittance, micro- the most lethal to microorganisms. The predominant
2
organism, etc. (m =Js) wavelength emitted by ‘‘low-pressure’’ UV lamps is
2
I is the intensity of UV radiation (W=m ) l 254 nm, which is highly effective, though