Page 657 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

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

technology. First was the Innovative and Alternative Tech- high pH; consequently, tablets (called ‘‘globaline’’) developed
nologies program of the USEPA, which provided for replace- for the military included an acid buffer to lower the pH of the
ment if an experimental technology failed to achieve water and released 8 mg=L iodine. Some of the Ct values
compliance with a National Pollution Discharge Elimination reported for 0.99 fraction inactivation at pH ¼ 7.0, 238C
System (NPDES) permit. Then, in 1988, the Uniform Fire T 308C, were: Ct(coliforms) ¼ 0.4; Ct(poliovirus 1) ¼ 30
Code began to impact facilities undergoing expansion by (Doull, 1980, p. 68). For cultured E. histolytica cysts and for
requiring chlorine scrubbers as protection against the acciden- pH ¼ 5, Ct ¼ 200, 130, and 65 at 38C, 138C, and 238C,
tal release of chlorine gas. The foregoing helped to expand respectively.
interest in UV, particularly for the disinfection of municipal
wastewaters. In 1984, only a few installations existed, but by 19.2.5.2 Bromine
1999, the number exceeded 500. The technology developed Bromine is another halide effective in inactivating microor-
during the 1990s had horizontal as well as vertical alignment ganisms. Its chemistry in reacting with water is similar to
of lamps and were equipped with self-cleaning mechanisms. chlorine and iodine, that is,
Pulsed systems were developed also, giving radiation of high
flux density. The foregoing is from Hunter (2000, p. 5). Br 2 þ H 2 O , HOBr þ H þ Br (19:3)
þ
In the United States, DeMers and Renner in 1992 deter-
þ
mined that there were over 2000 drinking water installations HOBr , H þ OBr : (19:4)

in Europe using UV and by 1994 Montgomery–Watson
Engineers determined that about 700 wastewater plants in Bromine may also be added to water as bromine chloride gas,
the United States had UV (Snicer et al., 2000, p. 7). Massche- that is, BrCl, which reacts with water to give HOBr as a

lein (2002, p. 59) estimated the number of drinking water product (along with H þ and OBr ). Bromine reacts with
systems using UV disinfection at 3000–5000. ammonia analogous to the reactions of chlorine with ammo-
In 1998, UV was found to be effective in inactivating nia. The HOBr form is most effective as a germicide.
Cryptosporidium, which gave a final impetus to its wide- Bromine was first applied to water in the form of a liquid
spread adoption, especially with EPA regulations pending (as cited in a 1935 article by Doull, 1980, p. 72); it condenses
(Malley, 2000, p. 9). The only alternatives for Cryptosporid- as a liquid at p ¼ 1.00 atm, T ¼ 58C. Its application is usually
ium inactivation were ozone (with long detention time) or as BrCl 2 (due to the corrosive nature of Br 2 ) that is shipped
microfiltration (MF). The mechanism of UV inactivation and handled in a manner similar to chlorine, but is more
(see also Section 19.3.9.4) was found to be damaging to the reactive than chlorine with PVC plastics. A more usable
nucleic acids by absorption in the 200–300 nm range (Snicer form for small-scale application is potassium bromide, KBr.
et al., 2000, p. 32). Although bromine is considered to be an effective disin-
fectant, that is, similar to chlorine, data were sparse in the
review by Doull (1980, pp. 72–82). For 2-log inactivation of
19.2.5 OTHER DISINFECTANTS Bacillus subtilis spores, for example, Ct ¼ 280 (pH ¼ 7,
A variety of disinfectants have been used since the early days T ¼ 258C). But for 2-log inactivation of poliovirus 1,
of the science of microbiology, which was underway by about Ct ¼ 0.06 (pH ¼ 7.0, T ¼ 208C). Bromine as HOBr, remains
1880 as a science. A few that have emerged that are important the predominant form at pH < 8.5 (as contrasted with
in water treatment are reviewed here. pH < 7.5 for chlorine).

19.2.5.1 Iodine 19.2.5.3 Silver
Iodine is the only common halogen that is solid at room Silver has been applied as a disinfectant in both metallic form
temperature (Doull, 1980, p. 62). Free iodine, I 2 , is considered and as silver nitrate. Usually dosages are <50 mg=L because
an effective disinfectant, and a residual of 0.5–1.0 mg=L can of low solubility (for reference, the USEPA 1975
be maintained in a distribution system, and it does not react MCL ¼ 0.050 mg=L. Values for Ct for 0.999 inactivation of
with ammonia. The basic chemical reactions with water are E. coli were at 6.3 pH 8.7, 5 T 25, 0.01 C(Ag as
AgNO 3 ) 0.27 mg=L, then 4.6 < Ct < 14. The city of Zurich
WTP has produced packaged water (1 L plastic bags) with
I 2 þ H 2 O , HOI þ H þ I (19:1)
þ
silver used (1.0 mg Ag =L) to minimize possible growth
þ
HOI , H þ OI (19:2) during storage for use in emergencies (1982 visit).
þ
The most effective form is I 2 , which is predominant at lower
19.3 THEORY
pH values. For pH ¼ 5, 0.99 fraction of the species is I 2 with
The main contribution of theory to practice is the Ct param-
about 0.01 fraction being HOI; at pH ¼ 7, 0.5 fraction is I 2
and about 0.5 fraction is HOI; at pH ¼ 8, 0.12 fraction is eter, established by Chick (1908) and Watson (1908). Their
I 2 and about 0.88 fraction is HOI. combined theory evolved as the basis for the design of
Iodine concentrations at 5–10 mg=L were effective against reactors and for the operation and development of extensive
bacterial pathogens but were not effective as a cysticide at Ct databases.