Eqs. (6.1) and (6.2). These products form at different pH (particularly, HOCl is at
pH – 5.0 and OCl
À is at pH À10.0). These free chlorine-based compounds react with
both inorganic and organic matter present in water to form chlorinated compounds.
Cl 2 þ H 2 O $ HOCl þ H
þ
þ Cl
À
ð6:1Þ
HOCl $ H
þ
þ OCl
À
ð6:2Þ
Hypochlorous acid reacts with ammonium ion to produce monochloramine,
dichloramine, and trichloramine according to Eqs. (6.3), (6.4) and (6.5). The formation of different types of chloramine depends on the pH value and the concentration
of NH 3 in water.
NH 3 þ HOCl ! NH 2 Cl þ H 2 O
ð6:3Þ
NH 2 Cl þ HOCl ! NHCl 2 þ H 2 O
ð6:4Þ
NHCl 2 þ HOCl ! NCl 3 þ H 2 O
ð6:5Þ
The term “free chlorine” is the combination of hypochlorous acid and the
hypochlorite, whereas chloramines are called combined chlorine. Moreover, these
chlorinated compounds have disinfection ability to oxidize sulfides (S
À
), ferrous
(Fe
2+ ), and manganese (Mn
2+ ) ions. As a result of impurities in water, chlorine
demand is assessed based on the concentration of residual chlorine in the water. The
breakeven point occurs when the amount of chlorine increases beyond the point
where the residual chlorine is detected. It generally occurs in the pH range of
7.0–7.5.
The effective dosage of chlorine concentration is at the breakpoint or slightly
higher. The concentration should be enough to break chemical bonds of contaminants as well as destroy pathogens. It is important to keep free available chlorine
residual by supplying an excess quantity of chlorine for disinfection. Figure 6.2.
shows the breakpoint chlorination curve in different stages. In the initial stage, there
is no free residual chlorine as complete oxidation occurs (Point A). Next, the curve is
linear until point B as the formation of chloramines and chloro-organic occurs at this
stage. Furthermore, oxidative destruction occurs with an increase in the concentration of chlorine. After that, the number of residual chlorine decreases and reaches a
point called breakpoint (Point C). Then, the residual chlorine keeps on increasing as
the concentration increases (Point D). The breakpoint concentration varies with the
quality of raw water (Al-Abri et al. 2019).
Table 6.1 shows the concentration of chlorine-based disinfectants and contact
time required for the inactivation of E. coli. In the water treatment plant, chlorine is
added in the form of elemental chlorine (chlorine gas), sodium hypochlorite, and
calcium hypochlorite. Each form has distinct advantages and disadvantages of water
disinfection (Table 6.2).
120
N. R. Srinivasan et al.
pH – 5.0 and OCl
À is at pH À10.0). These free chlorine-based compounds react with
both inorganic and organic matter present in water to form chlorinated compounds.
Cl 2 þ H 2 O $ HOCl þ H
þ
þ Cl
À
ð6:1Þ
HOCl $ H
þ
þ OCl
À
ð6:2Þ
Hypochlorous acid reacts with ammonium ion to produce monochloramine,
dichloramine, and trichloramine according to Eqs. (6.3), (6.4) and (6.5). The formation of different types of chloramine depends on the pH value and the concentration
of NH 3 in water.
NH 3 þ HOCl ! NH 2 Cl þ H 2 O
ð6:3Þ
NH 2 Cl þ HOCl ! NHCl 2 þ H 2 O
ð6:4Þ
NHCl 2 þ HOCl ! NCl 3 þ H 2 O
ð6:5Þ
The term “free chlorine” is the combination of hypochlorous acid and the
hypochlorite, whereas chloramines are called combined chlorine. Moreover, these
chlorinated compounds have disinfection ability to oxidize sulfides (S
À
), ferrous
(Fe
2+ ), and manganese (Mn
2+ ) ions. As a result of impurities in water, chlorine
demand is assessed based on the concentration of residual chlorine in the water. The
breakeven point occurs when the amount of chlorine increases beyond the point
where the residual chlorine is detected. It generally occurs in the pH range of
7.0–7.5.
The effective dosage of chlorine concentration is at the breakpoint or slightly
higher. The concentration should be enough to break chemical bonds of contaminants as well as destroy pathogens. It is important to keep free available chlorine
residual by supplying an excess quantity of chlorine for disinfection. Figure 6.2.
shows the breakpoint chlorination curve in different stages. In the initial stage, there
is no free residual chlorine as complete oxidation occurs (Point A). Next, the curve is
linear until point B as the formation of chloramines and chloro-organic occurs at this
stage. Furthermore, oxidative destruction occurs with an increase in the concentration of chlorine. After that, the number of residual chlorine decreases and reaches a
point called breakpoint (Point C). Then, the residual chlorine keeps on increasing as
the concentration increases (Point D). The breakpoint concentration varies with the
quality of raw water (Al-Abri et al. 2019).
Table 6.1 shows the concentration of chlorine-based disinfectants and contact
time required for the inactivation of E. coli. In the water treatment plant, chlorine is
added in the form of elemental chlorine (chlorine gas), sodium hypochlorite, and
calcium hypochlorite. Each form has distinct advantages and disadvantages of water
disinfection (Table 6.2).
120
N. R. Srinivasan et al.