Page 667 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

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

19.3.4 CHLORAMINES comparison, H(Cl 2 ,208C) ¼ 7,283 mg Cl 3 =L water=atm Cl 3 ;
and from formula in Lide (1996, p. 6-5), H(ClO 2 ,
Some municipalities use chloramines for primary disinfection.
208C) 82,524 mg ClO 2 =L water=atm ClO 2 .
Reasons include (1) taste and odor effects are essentially
The power requirement to generate ozone is about 13–22
nonexistent, (2) the chloramines are persistent in the
kWh=kg ozone, when dry air is the source of oxygen and about
distribution system, and (3) disinfection by-products are not
half when the source is pure oxygen (Doull, 1980, p. 42).
formed. On the negative side, the Ct requirement for 2-log The reactor may be set up with diffuser in a counter-current
inactivation of a given organism is much higher for the
mode, or some other method, such as described in Chapter 18.
chloramines than for chlorine.
Commercial equipment is available to supply ozone in the
range 0.002 < J(O 3 ) < 40 kg=day (Doull, 1980, p. 43) at a
19.3.4.1 Chlorine–Ammonia Reactions
voltage of up to 20,000 V is applied (ASCE-WPCF, 1977,
When chlorine reacts with ammonia, chloramines are formed. p. 400). Ozone is produced at about 1% by weight when air is
The sequence of reactions related to the formation of chlor- the oxygen source and about 2% when pure oxygen is the
amines is enumerated. source gas. Its effectiveness is independent of pH but the
range 6 < pH < 7 appears most favorable. Table 19.3 shows
1. The free ammonia, on the left side, reacts with HOCl that ozone is effective in the inactivation of Giardia, and
to give a monochloramine, Cryptosporidium, that is, Ct(Giardia, 2-log) 1.9 and Ct
(Cryptosporidium, 2-log) 5.
NH 3 þ HOCl , NH 2 Cl þ H 2 O (19:25)
The species of ozone in water, in addition to O 3 , include
hydroxyl radicals, OH; hydroperoxyl radicals, HO 2 ; oxide
.
.
2. The monochloramine reacts also with HOCl, to give
radicals, O ; ozonide radicals, O 3 ; and possibly free oxygen,
.
.
a dichloramine,
.
. O (Doull, 1980, p. 44). The hydroxyl radical is considered
NH 2 Cl þ HOCl , NHCl 2 þ H 2 O (19:26) the most reactive.
3. The dichloramine is further oxidized to give trichlor- 19.3.6 CHLORINE DIOXIDE
amine,
This section reviews some of the salient as aspects of chlorine
NHCl 2 þ HOCl , NCl 3 þ H 2 O (19:27) dioxide, that is, its effectiveness, characteristics, and methods
of synthesis.
4. The equilibrium distributions between monochlora-
mine and dichloramine are at pH ¼ 5.0, 0.84 fraction 19.3.6.1 Effectiveness of Chlorine Dioxide
is NHCl 2 ;atpH ¼ 6.0, 0.62 fraction is NHCl 2 ;at as a Disinfectant
pH ¼ 7.0, 0.35 fraction is NHCl 2 ;atpH ¼ 8.0, 0.15 On bactericidal effectiveness Ct values from Doull (1980,
fraction is NHCl 2 (Fair et al., 1948, p. 1054). p. 59) were,

19.3.4.2 Chloramine Disinfection
Disinfectant Organism pH T (8C) log R Ct
The effective form of chloramines is the dichloramine
(NHCl 2 ) form. The relative bactericidal effectiveness of ClO 2 E. coli 6 5 2 0.4
dichloramine (NHCl 2 ) to monochloramine (NH 2 Cl) is about 10 0.25
20 0.18
35:1 but varies with the organism (Fair et al., 1948, p. 1054).
Polio virus 7.0 5 5.5
Ct values for monochloramines are given in Table 19.3, along
7.0 15 1.3
with chlorine and other disinfectants. The data show (1)
Coxsackievirus 7.0 15 0.3
dichloramine is confirmed as the most effective form of chlor-
Giardia lamblia cysts a 7.0 5 3 26
amines; (2) for 2-log inactivation of various organisms, very b
Chlorine Giardia lamblia cysts 5 3 149
high Ct values are required. b
Ozone Giardia lamblia cysts 5 3 1.9
19.3.5 OZONE CHEMISTRY
In other words, comparing Ct’s, chlorine dioxide is similar to or
stronger than chlorine in the inactivation of microorganisms.
Ozone gas has the formula, O 3 ,MW ¼ 48 g=mol, r(O 3 ) ¼
2.154 g=Lat T ¼ 08C, p ¼ 1.00 atm (Hill and Rice, 1982, The maximum residual ClO 2 concentrations permitted for drink-
p. 42). The half-life of ozone in distilled water is about ing water were Belgium, 0.25 mg=L; Germany, 0.2 mg=L;
25 min at 208C. In natural waters, the half-life is much Switzerland, 0.15 mg=L (Masschelein, 1992, p. 191). For
less, for example, about 10 min for filtered water from the the United States, the maximum concentrations permitted by
Bodensee (Hill and Rice, 1982, p. 4), which is a large lake the EPA regulations for the distribution system (Federal
near Zurich. The reaction rate of ozone with most compounds Register, December 16, 1998) were 0.8 mg=L. For these
is fast, particularly with organics. From Table H.5, Henry’s maximum concentrations, the Ct(log R 2) values can be
constant is, H(O 3 ,208C) ¼ 482 mg O 3 =L water=atm O 3 .By achieved with t 20 min.