Chlorination has been used on an emergency basis from
as far back as 1850. Continuous chorination of public water
supplies began in 1904 in England, and was followed by the U.S.
around 1909. After the facilities for gaseous chlorine were developed
in 1912, chlorination grew rapidly. In that time, infectious
waterborne diseases have decreased dramatically. Chlorine may have saved more lives than antibiotics.
Nevertheless, chlorine has had its share of controversy. Chlorine
use was delayed until 1909 in the U.S. because of a lawsuit arguing
it was unsafe. Indeed, concentrated chlorine gas is extremely toxic.
As recently as the mid-1990's, the Clinton Administration proposed a
ban on chlorine (the "Chlorine Zero Discharge Act"). In many ways it
is understandable, because chlorine helps makes up dioxins and PCB's,
not to mention the CFC's role in stratospheric ozone depletion.
However, the law was not passed when a group of scientists pointed
out that chlorine is already found in nature [salt (sodium chloride)
comes to mind...]. Moreover, chlorine plays a huge role in the safety
of our water supplies.
To better understand the issues, we need to take a closer look at
chlorine chemistry. Chlorine can be found in gas, liquid, and solid
forms as shown below. The key compunds are shown in red, and are
further discussed below.
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C12
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+
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H20
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<-->
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HCL
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+
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HOCl
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<-->
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OCL-
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+
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H+
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(Elemental Cl gas)
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(Hypochlorous Acid)
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(Hypochlorite Ion)
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2. Liquid: sodium hypochlorite
(bleach)
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NaOCl
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+
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H20
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<-->
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Na+
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+
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OCL-
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< H+>
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HOCl
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(Sodium
Hypochlorite)
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(Hypochlorite Ion)
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(Hypochlorous Acid)
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3. Solid: calcium hypochlorite (as powder
or tablets)
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Ca(OC1)2
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+
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H20
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<-->
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Ca++
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+
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2
OCL-
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< H+>
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HOCl
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(Calcium
Hypochlorite)
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(Hypochlorite Ion)
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(Hypochlorous Acid)
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HCl (hydrochloric acid) is actually ineffective in killing
microbes at the concentrations used in water treatment. HOCL
(hypochlorous acid) is the active form that kills the microbes
(concentrations of about 0.2-1.2 ppm are needed for drinking water
supplies). OCl- (hypochlorite ion) also is an active killing
ingredient, and along with HOCl makes up what we refer to as
"free available chlorine." HOCL is
roughly a eighty times more effective in killing power than
OCl-, largely because the uncharged HOCL is more effective in
penetrating cell walls.
How much chlorine do we need to add? It depends... on the quality
of the water. Not all of the chlorine is converted to free available
chlorine (i.e., the HOCl and OCl- that makes up the active killing
ingredients). The chlorine can participate in a variety of reactions,
yielding one of the strangest curves we see in our field:
For a graph like the one above, we normally expect a straight line
(i.e., "what you add is what you get" in the form of a residual). But
a number of reactions are acting to reduce the free chlorine
residual, and we can analyze them by referring to each of the letters
in the graph (a, b, c, and d):
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a.
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Fe++ --> Fe+++
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Mn++ --> Mn+++
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NO2- --> NO3-
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Because of these three reactions, chlorine is
reduced to HCl. The more iron, manganese, and
nitrites in the water, the more of these reactions
that occur.
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b. Combined chlorine
b1.
Chloro-organics
In the first part of this curve (labelled
b1), chlorine is combining with various organics in the
water.
b2. HOCL + NH3 --> NH2Cl (monochloramine)
HOCL + NH2Cl --> NHC12 (dichloramine)
HOCL + NHC12 --> NC13 (trichloramine)
In the second half of this curve (labelled
b2), chlorine is combining with ammonia in the water,
forming what we collectively refer to as combine chlorine,
or chloramines.
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c. --> --> -->
HCl + N2
A variety of
reactions occur starting with letter c on the curve,
ultimately releasing HCl and nitrogen gas. We sometimes
refer to this as "burning off the chloramines."
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d. Free available chlorine
Once we have "burned off the chloramines,"
any additional chlorine is converted directly into free
available chlorine. Letter d
is also referred to as the "breakpoint", and this type of
chlorination is referred to as "breakpoint chlorination."
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