# CHLORINE DIOXIDE

Since the beginning of the twentieth century, when it was first used at
a spa in Ostend, Belgium, chlorine dioxide has been known as a powerful
disinfectant of water. During the 1950s, it was introduced more
generally as a drinking water disinfectant since it provided less
organoleptic hindering than chlorine. Approximately 700 to 900 public
water systems use chlorine dioxide to treat potable water (Hoehn, 1992).
Today, the major uses of chlorine dioxide are:

-   CT disinfection credit;

-   Preoxidant to control tastes and odor;

-   Control of iron and manganese; and

-   Control of hydrogen sulfide and phenolic compounds.

## Chlorine Dioxide Chemistry

### Oxidation Potential

The metabolism of microorganisms and consequently their ability to
survive and propagate are influenced by the oxidation reduction
potential (ORP) of the medium in which it lives (USEPA, 1996).

Chlorine dioxide (ClO~2~) is a neutral compound of chlorine in the +IV
oxidation state. It disinfects by oxidation; however, it does not
chlorinate. It is a relatively small, volatile, and highly energetic
molecule, and a free radical even while in dilute aqueous solutions. At
high concentrations, it reacts violently with reducing agents. However,
it is stable in dilute solution in a closed container in the absence of
light (AWWA, 1990). Chlorine dioxide functions as a highly selective
oxidant due to its unique, one-electron transfer mechanism where it is
reduced to chlorite (ClO~2~^-^) (Hoehn et al., 1996). The pKa for the
chlorite ion, chlorous acid equilibrium, is extremely low at pH 1.8.
This is remarkably different from the hypochlorous acid/hypochlorite
base ion pair equilibrium found near neutrality, and indicates the
chlorite ion will exist as the dominant species in drinking water. The
oxidation reduction of some key reactions are (CRC, 1990):

*ClO*~2(aq)~ + *e*^-^ = *ClO*~2~^-^ E° = 0.954V

Other important half reactions are:

*ClO*~2~^-^ + 2*H*~2~*O* +4*e*^-^ = *Cl*^-^ + 4*OH* ^-^ E° = 0.76V

*ClO*~3~^-^ + *H*~2~*O* + 2*e*^-^ = *ClO*~2~^-^ + 2*OH* ^-^ E° = 0.33V

*ClO*~3~^-^ + 2*H^+^* + *e*^-^ = *ClO~2~* + *H*~2~*O* E° = 1.152V

In drinking water, chlorite (ClO~2~^-^) is the predominant reaction
endproduct, with approximately 50 to 70 percent of the chlorine dioxide
converted to chlorite and 30 percent to chlorate (ClO~3~^-^) and
chloride (Cl^-^) (Werdehoff and Singer, 1987).

## Generation

### Introduction

One of the most important physical properties of chlorine dioxide is its
high solubility in water, particularly in chilled water. In contrast to
the hydrolysis of chlorine gas in water, chlorine dioxide in water does
not hydrolyze to any appreciable extent but remains in solution as a
dissolved gas (Aieta and Berg, 1986). It is approximately 10 times more
soluble than chlorine (above 11°C), while it is extremely volatile and
can be easily removed from dilute aqueous solutions with minimal
aeration or recarbonation with carbon dioxide (e.g. softening plants).
Above 11 to 12°C, the free radical is found in gaseous form. This
characteristic may affect chlorine dioxide\'s effectiveness when
batching solutions and plumbing appropriate injection points. Other
concerns are the increased difficulty in analyzing for specific
compounds in the presence of many interfering compounds/residual
longevity and volatility of gaseous compounds. In the gaseous form, the
free radicals also react slowly with water. The reaction rate is 7 to 10
million times slower than that of the hydrolysis rate for chlorine gas
(Gates, 1989).

Chlorine dioxide cannot be compressed or stored commercially as a gas
because it is explosive under pressure. Therefore, it is never shipped.
Chlorine dioxide is considered explosive at higher concentrations which
exceed 10 percent by volume in air, and its ignition temperature is
about 130°C (266°F) at partial pressures (National Safety Council Data
Sheet 525 -- ClO~2~, 1967). Strong aqueous solutions of chlorine dioxide
will release gaseous chlorine dioxide into a closed atmosphere above the
solution at levels that may exceed critical concentrations. Some newer
generators produce a continuous supply of dilute gaseous chlorine
dioxide in the range of 100 to 300 mm-Hg (abs) rather than in an aqueous
solution (National Safety Council, 1997). For potable water treatment
processes, aqueous solutions between 0.1 and 0.5 percent are common from
a number of current generation technologies.

Most commercial generators use sodium chlorite (NaClO~2~) as the common
precursor feedstock chemical to generate chlorine dioxide for drinking
water application. Recently, production of chlorine dioxide from sodium
chlorate (NaClO~3~) has been introduced as a generation method where in
NaClO~3~ is reduced by a mixture of concentrated hydrogen peroxide
(H~2~O~2~) and concentrated sulfuric acid (H~2~SO~4~). Chlorate-based
systems have traditionally been used in pulp and paper applications, but
have recently been tested full-scale at two U.S. municipal water
treatment plants. This is an emerging technology in the drinking water
field and is not discussed in this guidance manual.

### Chlorine Dioxide Purity

Chlorine dioxide generators are operated to obtain the maximum
production (yield) of chlorine dioxide, while minimizing free chlorine
or other residual oxidant formation. The specified yield for chlorine
dioxide generators is typically greater than 95 percent. In addition,
the measurable excess chlorine should be less than 2 percent by weight
in the generator effluent. Generator yield is defined as (Gordon et al.,
1990):

$$\text{Yield} = \frac{\left\lbrack \text{ClO}_{2} \right\rbrack}{\left\lbrack \text{ClO}_{2} \right\rbrack + \left\lbrack \text{ClO}_{2}^{-} \right\rbrack + \left( \frac{\text{67}\text{.}\text{45}}{\text{83}\text{.}\text{45}} \right)\left\lbrack \text{ClO}_{3}^{-} \right\rbrack} \times \text{100}$$

Where: \[$\text{ClO}_{2}$\] = Chlorine dioxide concentration, mg/L.

\[$\text{ClO}_{2}^{-}$\] = Chlorite concentration, mg/L.

\[$\text{ClO}_{3}^{-}$\] = Chlorate concentration, mg/L.

$\left( \frac{\text{67}\text{.}\text{45}}{\text{83}\text{.}\text{45}} \right)$
= Molecular weight ratio of ClO~2~^-^ to ClO~3~^-^.

Since any chlorite ion fed to the generator may result in the formation
of ClO~2~, ClO~2~^-^, or ClO~3~^-^, the purity of the resultant mixture
can be calculated using the concentrations of each of the species from
appropriate analytical measurements. The determination of purity
requires neither flow measurement, mass recoveries, nor
manufacturer-based methods to determine production "yield," "theoretical
yield," "efficiency," or conversion for any precursor feedstock. This
approach does not require flow measurements that can introduce up to 5
percent error in the calculations.

Utilities that use chlorine dioxide should measure excess chlorine (as
FAC) in the generator effluent in addition to the ClO~2~^-^ related
species. FAC may appear as false ClO~2~ residuals for CT purposes, or
result in the formation of chlorinated DBPs if high, relative to the
ClO~2~ level in the generated mixture. Excess chlorine is defined as:

$$\text{Excess Cl}_{2} = \frac{\lbrack\text{Cl}_{2}\rbrack}{\lbrack\text{ClO}_{2}\rbrack + \lbrack\text{ClO}_{2}^{-}\rbrack + \left( \frac{\text{67}\text{.}\text{45}}{\text{83}\text{.}\text{45}} \right)\ \lbrack\text{ClO}_{3}^{-}\rbrack \times \frac{\text{70}\text{.}\text{91}}{2\  \times \ \text{67}\text{.}\text{45}}} \times \text{100}$$

Where:$\frac{\text{70}\text{.}\text{91}}{(2\  \times \ \text{67}\text{.}\text{45})}$=
stoichiometric and molecular weight ratio of Cl~2~ to ClO~2~^-^.

The following represents a summarily simpler equation that substantially
resolves the problems of different equipment-specific calibration
methods, chlorine-contaminated ClO~2~, or low efficiency conversion of
either chlorite- or chlorate-based precursor material.

$$\text{Purity} = \frac{\lbrack\text{ClO}_{2}\rbrack}{\lbrack\text{ClO}_{2}\rbrack + \lbrack\text{FAC}\rbrack + \lbrack\text{ClO}_{2}^{-}\rbrack + \ \lbrack\text{ClO}_{3}^{-}\rbrack} \times \text{100}$$

This practical (weight-based) calculation permits a variety of approved
analytical methods (discussed in section 4.6) to be used to assess
generator performance on unbiased scientific principles, rather than
non-standardized manufacturer specifications.

### Methods of Generating Chlorine Dioxide

For potable water application, chlorine dioxide is generated from sodium
chlorite solutions. The principal generation reactions that occur in the
majority of generators have been known for a long time. Chlorine dioxide
can be formed by sodium chlorite reacting with gaseous chlorine
(Cl~2(g)~), hypochlorous acid (HOCl), or hydrochloric acid (HCl). The
reactions are:

2NaClO~2~ + Cl~2(g)~ = 2ClO~2(g)~ + 2NaCl \[1a\]

2NaClO~2~ + HOCl = 2ClO~2(g)~ + NaCl + NaOH \[1b\]

5NaClO~2~ + 4HCl = 4ClO~2(g)~ + 5NaCl + 2H~2~O \[1c\]

Reactions \[1a\], \[1b\], and \[1c\] explain how generators can differ
even though the same feedstock chemicals are used, and why some should
be pH controlled and others are not so dependent on low pH. In most
commercial generators, there may be more than one reaction taking place.
For example, the formation and action of hypochlorous acid as an
intermediate (formed in aqueous solutions of chlorine) often obscures
the "overall" reaction for chlorine dioxide production.

Table 4-1 provides information on some types of available commercial
generators. Conventional systems react sodium chlorite with either acid,
aqueous chlorine, or gaseous chlorine. Emergent technologies identified
in Table 4-1 include electrochemical systems, a solid chlorite inert
matrix (flow-through gaseous chlorine) and a chlorate-based emerging
technology that uses concentrated hydrogen peroxide and sulfuric acid.

[]{#__RefHeading___Toc450036532 .anchor}Table 4-1. Commercial Chlorine
Dioxide Generators

+-------------------+---------------------+----------------------------+
| **GENERATOR       | **MAIN REACTIONS\   | **SPECIAL ATTRIBUTES**     |
| TYPE**            | Reactants,          |                            |
|                   | byproducts, key     |                            |
|                   | reactions, and      |                            |
|                   | chemistry notes**   |                            |
+===================+=====================+============================+
| ACID-CHLORITE:    | 4HCl + 5NaClO~2~ è  | Chemical feed pump         |
|                   | 4ClO~2(aq)~ +       | interlocks required.       |
| (Direct Acid      | ClO~3~^-^           |                            |
| System)           |                     | Production limit \~ 25-30  |
|                   | -   Low pH          | lb/day.                    |
|                   |                     |                            |
|                   | -   ClO~3~^-^       | Maximum yield at \~80%     |
|                   |     possible        | efficiency.                |
|                   |                     |                            |
|                   | -   Slow reaction   |                            |
|                   |     rates           |                            |
+-------------------+---------------------+----------------------------+
| AQUEOUS           | Cl~2~ + H~2~O è     | Excess Cl~2~ or acid to    |
| C                 | \[HOCl / HCl\]      | neutralize NaOH.           |
| HLORINE-CHLORITE: |                     |                            |
|                   | \[HOCl/HCl\] +      | Production rates limited   |
| (Cl~2~ gas        | NaClO~2~ è          | to \~ 1000 lb/day.         |
| ejectors with     |                     |                            |
| chemical pumps    | ClO~2(g)~ +         | High conversion but yield  |
| for liquids or    | H/OCl^-^ + NaOH +   | only 80-92%                |
| booster pump for  | ClO~3~^-^           |                            |
| ejector water).   |                     | More corrosive effluent    |
|                   | -   Low pH          | due to low pH (\~2.8-3.5). |
|                   |                     |                            |
|                   | -   ClO~3~^-^       | Three chemical systems     |
|                   |     possible        | pump HCl, hypochlorite,    |
|                   |                     | chlorite, and dilution     |
|                   | -   Relatively slow | water to reaction chamber. |
|                   |     reaction rates  |                            |
+-------------------+---------------------+----------------------------+
| RECYCLED AQUEOUS  | 2HOCl + 2NaClO~2~ è | Concentration of \~3 g/L   |
| CHLORINE OR       | 2ClO~2~ + Cl~2~ +   | required for maximum       |
| \"FRENCH LOOP\"™  | 2NaOH               | efficiency.                |
|                   |                     |                            |
| (Saturated Cl~2~  | -   Excess Cl~2~ or | Production rate limited to |
| solution via a    |     HCl needed due  | \~ 1000 lb/day.            |
| recycling loop    |     to NaOH formed. |                            |
| prior to mixing   |                     | Yield of 92-98% with \~10% |
| with chlorite     |                     | excess Cl~2~ reported.     |
| solution.)        |                     | Highly corrosive to pumps; |
|                   |                     | draw-down calibration      |
|                   |                     | needed. Maturation tank    |
|                   |                     | required after mixing.     |
+-------------------+---------------------+----------------------------+
| GASEOUS           | Cl~2(g)~ +          | Production rates 5-120,000 |
| CHLORINE-CHLORITE | NaClO~2(aq)~ è      | lb/day.                    |
|                   | ClO~2(aq)~          |                            |
| (Gaseous Cl~2~    |                     | Ejector-based, with no     |
| and 25% solution  | -   Neutral pH      | pumps. Motive water is     |
| of sodium         |                     | dilution water. Near       |
| chlorite; pulled  | -   Rapid reaction  | neutral pH effluent. No    |
| by ejector into   |                     | excess Cl~2~. Turndown     |
| the reaction      | -   Potential       | rated at 5-10X with yield  |
| column.)          |     scaling in      | of 95-99%. Less than 2%    |
|                   |     reactor under   | excess Cl~2~. Highly       |
|                   |     vacuum due to   | calibrated flow meters     |
|                   |     hardness of     | with min. line pressure \~ |
|                   |     feedstock.      | 40 psig needed.            |
+-------------------+---------------------+----------------------------+
| GASEOUS CHLORINE- | Cl~2(g)~ +          | Cl~2~ gas diluted with     |
| SOLIDS CHLORITE   | NaClO~2(s)~ è       | N~2~ or filtered air to    |
| MATRIX            | ClO~2(g)~ + NaCl    | produce \~8% gaseous       |
|                   |                     | ClO~2~ stream. Infinite    |
| (Humidified Cl~2~ | -   Rapid reaction  | turndown is possible with  |
| gas is pulled or  |     rate            | \>99% yield. Maximum rate  |
| pumped through a  |                     | to \~1200 lb/day per       |
| stable matrix     | -   New technology  | column; ganged to \>10,000 |
| containing solid  |                     | lb/day.                    |
| sodium chlorite.) |                     |                            |
+-------------------+---------------------+----------------------------+
| ELECTROCHEMICAL   | NaClO~2(aq)~ è      | Counter-current chilled    |
|                   | ClO~2(aq)~ + e^-^   | water stream accepts       |
| (Continuous       |                     | gaseous ClO~2~ from        |
| generation of     | -   New technology  | production cell after it   |
| ClO~2~ from 25%   |                     | diffuses across the gas    |
| chlorite solution |                     | permeable membrane. Small  |
| recycled through  |                     | one-pass system requires   |
| electrolyte cell) |                     | precise flow for power     |
|                   |                     | requirements (Coulombs     |
|                   |                     | law).                      |
+-------------------+---------------------+----------------------------+
| ACID/             | 2NaClO~3~ +         | Uses concentrated H~2~O~2~ |
| PEROXIDE/CHLORIDE | H~2~O~2~ +          | and H~2~SO~4~.             |
|                   | H~2~SO~4~ è         |                            |
|                   | 2ClO~2~ + O~2~ +    | Downscaled version; Foam   |
|                   | NaSO~4~ + H~2~0     | binding; Low pH.           |
+-------------------+---------------------+----------------------------+

Source: Adapted from Gates, 1998.

#### Commercial Generators

The conventional chlorine-chlorite solution method generates chlorine
dioxide in a two-step process. First, chlorine gas is reacted with water
to form hypochlorous acid and hydrochloric acid. These acids then react
with sodium chlorite to form chlorine dioxide. The ratio of sodium
chlorite to hypochlorous acid should be carefully controlled.
Insufficient chlorine feed will result in a large amount of unreacted
chlorite. Excess chlorine feed may result in the formation of chlorate
ion, which is an oxidation product of chlorine dioxide and not currently
regulated.

***Acid-Chlorite Solution*** - Chlorine dioxide can be generated in
direct-acidification generators by acidification of sodium chlorite
solution. Several stoichiometric reactions have been reported for such
processes (Gordon et al., 1972). When chlorine dioxide is generated in
this way, hydrochloric acid is generally preferred (Reaction \[1c\]).

***Aqueous Chlorine-Chlorite Solution*** - Chlorite ion (from dissolved
sodium chlorite) will react with hydrochloric acid and hypochlorous acid
to form chlorine dioxide in these systems, commonly referred to as
conventional systems (Reaction \[1b\]):

Figure 4-1 shows a typical chlorine dioxide generator using aqueous
chlorine-chlorite solution (Demers and Renner, 1992).

[]{#_Ref419604249 .anchor}If chlorine gas and chlorite ion are allowed
to react under ideal conditions (not usually formed in aqueous chlorine
type systems), the resulting pH of the effluent may be close to 7. To
fully utilize sodium chlorite solution, the more expensive of the two
ingredients, excess chlorine is often used. This approach lowers the pH
and drives the reaction further toward completion. The reaction is
faster than the acid-chlorite solution method, but much slower than the
other commercial methods described in the following discussion.

***Recycled Aqueous Chlorine or "French Loop"™*** - In this aqueous
chlorine design, shown in Figure 4-2, chlorine gas is injected into a
continuously circulating water loop. This eliminates the need for a
great excess of Cl~2~ gas to be fed to the generator since the molecular
chlorine will dissolve in the feed water, and thus maintain a low pH
level of the feed water. Loop-based generators keep chlorine at or above
saturation levels. The low pH condition results in high yields of
chlorine dioxide (greater than 95 percent at design production rate)
(Thompson, 1989). Chlorine in the generator effluent may react with
chlorine dioxide to form chlorate if allowed to stand in batch storage
too long. The "French Loop" type of generator is more difficult to
operate due to system start-up and control of sodium chlorite feed rate
(meter pump), chlorine feed rate (rotameter), and the recirculating loop
(pump). Newer designs incorporate a second batching tank for continuous
aqueous chlorine storage, thus removing many of these startup or
recycling difficulties.

***Gaseous Chlorine-Chlorite Solution*** - Sodium chlorite solution can
be "vaporized" and reacted under vacuum with molecular gaseous chlorine.
This process uses undiluted reactants and is much more rapid than
chlorine solution:chlorite solution methods (Pitochelli, 1995).
Production rates are more easily scaled up, and some installed systems
have reported producing more than 60,000 pounds per day.

![](media/image1.wmf){width="6.245833333333334in"
height="5.228472222222222in"}

Source: Demers and Renner, 1992.

Figure 4-1. Conventional Chlorine Dioxide Generator When Using

Chlorine-Chlorite Method

The acid-sodium hypochlorite-sodium chlorite method of generating
chlorine dioxide is used when chlorine gas is not available. First,
sodium hypochlorite is combined with hydrochloric or another acid to
form hypochlorous acid. Sodium chlorite is then added to this reaction
mixture to produce chlorine dioxide.

#### pH Effects on Chlorine Dioxide Generation

If hypochlorous acid is formed, one of the byproducts of its reaction
with sodium chlorite in solution is sodium hydroxide. Since sodium
hydroxide is also a common stabilizer of sodium chlorite feedstock, the
resulting pH of the mixture can be too high. A high pH slows the
formation of chlorine dioxide and impels less efficient chlorate-forming
reactions. This is the same process in which chlorite and hypochlorite
ions react in drinking water to form chlorate ion. This neutralizing
effect of caustic may be influenced by different stabilities used in
each of the types and sources of sodium chlorite which are approved for
use in drinking water under AWWA Standard B303-95 (AWWA, 1995).

![](media/image2.wmf){width="6.2444444444444445in" height="4.4125in"}

[]{#_Ref419604411 .anchor}Source: Demers and Renner, 1992.

Figure 4-2. Chlorine Dioxide Generation Using Recycled Aqueous Chlorine
Method

In very low pH aqueous chlorine solutions, chlorous acid (and not the
chlorite ion) may be directly oxidized to chlorine dioxide as shown in
reaction \[1d\]. At this low pH, gaseous chlorine remains \"dissolved\"
in the water at concentrations higher than the normal occurrence, and
allows reaction \[1a\] to proceed.

2*HClO*~2~ + *HOCl* = *HCl* + *H*~2~*O* + 2*ClO*~2~ \[1d\]

#### Chlorate Byproduct Formation

One of the most undesirable byproducts in generators is the chlorate ion
(ClO~3~^-^). Chlorate production is possible through reactions with the
intermediate dimer, {Cl~2~O~2~}. Rather than the chlorite ion being
simply \"converted\" to chlorine dioxide, reactions \[1a\] through
\[1d\] can result in the supposed formation of the unstable,
unsymmetrical intermediate dimer, {Cl~2~O~2~} or {Cl^-^-ClO~2~} as shown
in reaction \[2\] (Emmenegger and Gordon, 1967).

*Cl*~2~ + *ClO*~2~^-^ = {*Cl*-*ClO*~2~} + *Cl*^-^ \[2\]

In some generators that operate with relatively low initial reactant
concentrations, a significant amount of chlorate is formed by reactions
with {Cl~2~O~2~}, as shown in reactions \[3a\], \[3b\], and \[3c\].

{*Cl*~2~*O*~2~} + *H*~2~*O* = *ClO*~3~^-^ + *Cl*^-^ + 2*H*^+^ \[3a\]

{*Cl*~2~*O*~2~} + *HOCl* = *ClO*~3~^-^ + *Cl*^-^ + *H*^+^ \[3b\]

{*Cl*~2~*O*~2~} + 3*HOCl* + *H*~2~*O* = 2*ClO*~3~^-^ + 5*H*^+^ +
3*Cl*^-^ \[3c\]

Highly acidic (pH \<3) reaction mixtures force the degradation of
{Cl~2~O~2~} to chlorate rather than chlorine dioxide, as well as the
direct oxidation of chlorite to chlorate.

The overall reactions that describe chlorate ion formation are:

*ClO*~2~^-^ + *HOCl* = *ClO*~3~^-^ + *Cl*^-^ + *H*^+^ \[4a\]

and

*ClO*~2~^-^ + *Cl*~2~ + *H*~2~*O* = *ClO*~3~^-^ + 2*Cl*^-^ + *H*^+^
\[4b\]

The following conditions may also produce the chlorate ion:

-   Excessively high ratios of Cl~2~ gas:ClO~2~^-^.

-   Presence of high concentrations of free chlorine at low pH in
    aqueous solutions.

-   Dilute chlorite solutions held at low pH.

-   Base-catalyzed disproportionation of chlorine dioxide at high pH
    values (pH \>11).

-   Reaction mixtures that are highly acidic (pH \<3).

-   An excess of hypochlorous acid will directly oxidize chlorite ions
    to chlorate ions rather than to chlorine dioxide (independent of the
    rapid formation of the {Cl~2~O~2~} intermediate).

### Generator Design

As hypochlorous acid is formed under acidic conditions, the lowering of
optimal concentrations of precursor reactants will also increase
chlorate levels in the generator by promoting reaction \[3b\].
Therefore, if weak precursor feed stocks or high amounts of dilution
water are added to the generator, chlorate will be more prevalent
(according to reaction \[3a\]). These limitations explain why generators
most often use \~25 percent chlorite solutions and gaseous (or
near-saturated aqueous) chlorine. Higher strength solutions of sodium
chlorite (e.g., 37 percent) also are more susceptible to crystallization
or stratification at ambient temperatures as high as 25°C(78°F).

Due to these dilution effects, some systems function best as
\"intermittent batch\" generators, (that produce high concentrations of
chlorine dioxide) rather than as \"continuous\" generators (that produce
lower concentrations (\< 1g/L) of chlorine dioxide). The stored
solutions are pumped or injected from the storage tank. Cycling
frequently avoids long-term (over 24 hour periods) storage of the
generated solution.

Chlorine loop-type systems can obtain high conversion rates if excess
chlorine is always present. Excess chlorine permits the molecular
chlorine reaction mechanism (described above) to proceed. The low pH of
the mixture also minimizes the contribution of OH^-^ formed via equation
\[1b\] by neutralizing it. These solutions may still be contaminated
with excess chlorine needed to drive the conversion of chlorite ion, but
not to the same degree as found in simple aqueous chlorine systems when
operated under dilute conditions. Chlorine-loop generators run best at
high capacity since the chlorite ion is most available in this
production mode.

Conventional or acid-enhanced generators produce chlorine dioxide
through the intermediate {Cl~2~O~2~} as long as relatively high
concentrations of reactants (\~above 20--30 g/L) are maintained in the
reaction chamber prior to dilution. Vapor-phase, recycled loop, and
solid chlorite-type generators that minimize dilute aqueous reaction
conditions can obtain high efficiencies by preventing any chlorite ion
from reacting in the \"slower\" steps described above. This is
accomplished by establishing conditions that force the immediate
reaction between chlorite ions and gas-phase or molecular chlorine at a
rate hundreds of times faster than the Cl~2~ hydrolysis in water. This
essentially minimizes the impact of competitive chlorine hydrolysis or
acidification on the dominant \[ClO~2~^-^:Cl~2~ gas\] mechanism, and
prevents the chlorite ion reacting with hypochlorous acid directly.

In all generators, large excess amounts of Cl~2~ may result in the
over-oxidization of chlorite and directly form chlorate in aqueous
solution (reaction \[4b\]). Precursor chemical feed rates for the
generators should always be adjusted to chart settings supplied with
generators, notably with the continuous flow, direct gas injection
systems. Re-calibration of these systems is sometimes needed on-site if
feed stock sodium chlorite is not of the correct strength, or if
pre-calibrated flow devices have been replaced.

If aqueous chlorine solutions are mixed with sodium chlorite feed stock
solutions, the following mechanisms are dominant, which may affect the
formation rates of chlorine dioxide:

-   Chlorine gas reacts with water to form hypochlorous and hydrochloric
    acids, rather than directly with chlorite to form chlorine dioxide.
    (Water and chlorite both compete for the Cl~2~ molecule
    simultaneously) (see equations \[4.1a-c\] and Section 6.1.1).

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-   Chlorate ion is formed (reactions \[3a\], \[3b\], and \[3c\]).

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-   Only 4 moles of chlorine dioxide are obtained from 5 moles of sodium
    chlorite via direct acidification (reaction \[1c\]). This may become
    important at low pH and high chloride ion levels.

The practical side of all of this is that different generators operate
under different optimal conditions. For example, reactor columns should
not be continuously flooded with excess water in vapor-phase systems. It
is the main reason why dry chlorite-based generator reactor columns
should not get wet. Over-dilution of the precursor reactants themselves
will lower conversion efficiencies due to the favored formation of
chlorate over chlorine dioxide. Batch-type generation should always be
carried out at maximal ClO~2~ concentration with appropriate adjustments
at the pump (located downstream of the reactor at the batch tank) for
dosage or flow. Changes in chlorine dioxide concentrations in the batch
tanks would then be minimized, and pump calibration does not need to
include a broad range of chlorine dioxide levels. For the newer gas
chlorine generators using dry sodium chlorite in an inert matrix, small
amounts of humidifying water in the mixture do not interfere
significantly with the simple gaseous Cl~2~:ClO~2~^-^ reaction. These
small traces of water allow for continuous exposure of ClO~2~^-^ on the
inert surfaces within the reactor column.

Chlorine dioxide generators are relatively simple mixing chambers. The
reactors are frequently filled with media (Teflon chips, ceramic or
raschig rings) to generate hydraulic turbulence for mixing. A sample
petcock valve on the discharge side of the generator is desirable to
allow for monitoring of the generation process.

The *Recommended Standards for Water Works* (GLUMRB, 1992) and drinking
water design textbooks such as *Unit Processes in Drinking Water
Treatment* by Masschelein (1992) are excellent sources for chlorine
dioxide generation design criteria and application.

### Chemical Feed Systems

Fiberglass Reinforced vinyl ester Plastic (FRP) or High Density Linear
Polyethylene (HDLPE) tanks with no internal insulation or heat probes
are recommended for bulk storage of 25 to 38 percent solution sodium
chlorite. Nozzles should include truck unloading vents and local level
and temperature indication. Transfer pumps should be centrifugal with
316 stainless steel, fiberglass, Hypalon, wetted Teflon parts, or
epoxy resins. The pump should be sealless or equipped with double
mechanical seals. The recommended piping material is CPVC, although
vinyl ester or Teflon piping systems are acceptable. Carbon steel and
stainless steel piping systems are not recommended.

Depending upon system size, sodium chlorite can be purchased in
55-gallon drums, 275-gallon non-returnable totes, or in bulk quantities.
A 30-day storage supply of sodium chlorite can easily be met for most
small systems by using 55-gallon drums. A 55-gallon drum weighs
approximately 600 lbs. Equipment should be provided such that one person
can easily handle a drum. All gaseous chlorine or hypochlorite
solution-related plumbing should follow Chlorine Institute directives.

Storage and chlorine dioxide systems typically include the following:

-   Storage and feeding in a designated space.

-   Use of non-combustible materials such as concrete for construction.

-   Storage in clean, closed, non-translucent containers. Exposure to
    sunlight, UV light, or excessive heat will reduce product strength.

-   Avoid storage and handling of combustible or reactive materials,
    such as acids or organic materials, in the sodium chlorite area.

-   Secondary containment for storage and handling areas to handle the
    worse case spill with sumps provided to facilitate recovery.

-   A water supply near storage and handling areas for cleanup.

-   Inert material should be used in contact with the strong oxidizing
    and/or acid solutions involved in chlorine dioxide systems.

-   Storage tanks with vents to outside.

-   Adequate ventilation and air monitoring.

-   Gas masks and first aid kits outside of the chemical areas.

-   Reactor with glass view ports if it is not made of transparent
    material.

-   Flow monitoring on all chemical feed lines, dilution water lines,
    and chlorine dioxide solution lines.

-   Dilution water should not be excessively hard in order to avoid
    calcium deposits and should be near neutral pH.

-   On-site and frequent testing of chemical solution strengths should
    be practiced to achieve efficient process control.

-   Air contact with chlorine dioxide solutions should be controlled to
    limit the potential for explosive concentrations possibly building
    up within the generator. Chlorine dioxide concentrations in air
    higher than 8 to 10 percent volume should be avoided. Two methods
    can be applied: operation under vacuum or storage under higher
    positive pressure (45 to 75 psig) to prevent buildup of gas-phase
    ClO~2~ in the head space. Bulk storage (batch) tanks containing
    ClO~2~ should be suitably vented to atmosphere.

Sodium chlorite solution feed pumps are commonly diaphragm-metering
pumps for liquid feed rate control. If centrifugal pumps are used, the
only acceptable packing material is Teflon. If lubrication is needed,
minimum quantities of fire-resistant lubricants should be used. Pump
motors should be totally enclosed, fan-cooled (TEFC) with
sealed-for-life bearings. Couplings should be of the greaseless type.
Water lines for mechanical seals should have a pressure gauge and
throttling valve on the outlet side. Visual means should be provided to
verify adequate water flow. Each pump should include a calibration
chamber.

Pipes carrying sodium chlorite should be provided sufficient support to
minimize risk of overstressing joints. Flexible connections to pumps
should also be provided to minimize risk of vibration damage. Pipe
should be sloped to drainage points and valved hose connections provided
at strategic points for efficient flushing and draining. Service water
for flushing feed lines should be introduced only through temporarily
connected hoses protected by a backflow preventer. Service water lines
should include check valves. Hose connections from service water lines
should have a vent valve to release pressure before the hose is
disconnected after use.

Flows are frequently measured with magnetic flow meters, mass flow
meters, or rotameters for precise control. Provisions should always be
made for back-flow prevention. Sodium chlorite is extremely reactive,
especially in the dry form, and care should be taken to protect against
potentially explosive conditions.

Chlorine dioxide solution concentrations below about 10 g/L will not
produce sufficiently high vapor pressures to present an explosion hazard
under most ambient conditions of temperature and pressure. In water
treatment, chlorine dioxide solution concentrations rarely exceed 4 g/L
for temperatures less than 40°C, and treatment levels generally range
from 0.1 to 5.0 mg/L. If temperatures exceed 50°C, storage tanks should
be suitably vented due to the higher levels of ClO~2~ possible. As cold
service/potable water is typically used as generator dilution water,
these conditions are rarely encountered.

### Generator Power Requirements

Generator power requirements are similar to those for chlorination
systems. For all generators (20 to 12,000 lb/day) power can be supplied
from 120 VAC single phase, to 480 VAC three phase. Power demand will
vary based upon make-up water pressure available to operate the venturi.
Fractional horsepower metering pumps are required, based upon system
configuration.

## Primary Uses and Points of Application for Chlorine Dioxide

The calculation of CT for chlorine dioxide is similar to other
disinfectants, with accurate determinations of residual concentrations
being a prerequisite for effective disinfection. Primary disinfectant
credit is achieved by the residual concentration and the effective
contact time. It has been found in practice that because of the volatile
nature of the gas, chlorine dioxide works extremely well in plug flow
reactors such as pipe lines. It can be easily removed from dilute
aqueous solution by turbulent aeration in rapid mix tanks or purging in
recarbonation basins. CT credits should not be expected through a filter
because the likelihood that no residual remains in the filtered water
(DeMers and Renner, 1992). For post CT disinfection credit, chlorine
dioxide can be added before clearwells or transfer pipelines. Ample
sampling points should be included to allow close monitoring of residual
concentrations. It is well known that chlorine dioxide is commonly
destroyed by UV in basins exposed to sunlight or bright fluorescent
lights. Therefore, protective design elements should be incorporated if
such exposure is anticipated.

### Disinfection

Before chlorine dioxide is selected for use as a primary disinfectant an
oxidant demand study should be completed. Ideally, this study should
consider the seasonal variations in water quality, temperature, and
application points. Table 4-2 shows typical results for a single sample
of a demand study completed on a surface water source.

The MRDL for chlorine dioxide is 0.8 mg/L and the MCL for chlorite is
1.0 mg/L per the D/DBP rule. This means that if the oxidant demand is
greater than about 1.4 mg/L, chlorine dioxide may not be used as a
disinfectant because the chlorite/chlorate ions byproduct, might exceed
the maximum level allowed, unless inorganic byproducts (e.g., chlorite)
are subsequently removed. There are numerous means to reduce excessive
chlorite levels prior to chlorination in conventional water plants.

[]{#__RefHeading___Toc450036533 .anchor}Table 4-2. Surface Water
Chlorine Dioxide Demand Study Results

  -------------- -------------- -------------- -------------- --------------
  **Dose         **Time (min)** **ClO~2~       **ClO~2~^-^    **ClO~3~^-^
  (mg/L)**                      (mg/L)**       (mg/L)**       (mg/L)**

  1.4            3              0.47           0.76           0.05

                 10             0.30           0.98           0.06

                 20             0.23           1.08           0.07

                 40             0.16           1.11           0.07

                 60             0.11           1.11           0.07
  -------------- -------------- -------------- -------------- --------------

Source: DeMers and Renner, 1992.\
Note: \*Raw water sample, 23°C, 8.5 pH.

Typical dosages of chlorine dioxide used as a disinfectant in drinking
water treatment range from 0.07 to 2.0 mg/L. For plants using chlorine
dioxide, median concentrations of chlorite and chlorate were found to be
0.24 and 0.20 mg/L, respectively in an EPA survey (USEPA, 1998), the
standard is 1.0 mg/L.

### Taste and Odor Control

A common application of chlorine dioxide in drinking water in the United
States has been for control of tastes and odors associated with algae
and decaying vegetation. Chlorine dioxide is also effective in
destroying taste and odor producing phenolic compounds. The recommended
location for application of chlorine dioxide for this purpose will
depend on raw water quality, the type of treatment plant and any other
purposes for chlorine dioxide addition. In conventional treatment
plants, it is recommended that chlorine dioxide be added near the end of
or following, the sedimentation basin. If the raw water turbidity is low
(for example, less than 10 NTU), chlorine dioxide can be added at the
beginning of the plant. Some utilities follow this practice because
chlorine dioxide is effective in controlling algae growth in
flocculation and sedimentation basins that are exposed to sunlight
(DeMers and Renner, 1992). Such application during periods of darkness
may be more successful for nuisance algae control.

### Oxidation of Iron and Manganese

Chlorine dioxide can be used to oxidize both iron and manganese.
Chlorine dioxide reacts with the soluble forms of iron and manganese to
form precipitates that can be removed through sedimentation and
filtration. Chlorine dioxide reduces to chlorite ion in this reaction
(Knocke et al., 1993). About 1.2 mg/L of chlorine dioxide is required to
remove 1.0 mg/L of iron, and 2.5 mg/L of chlorine dioxide are required
to remove 1.0 mg/L of manganese. For high concentrations of iron and
manganese, the use of chlorine dioxide is limited to the 1.0 mg/L
chlorite/chlorate ion byproduct, as described before. Ferrous iron may
be added prior to conventional coagulation to chemically reduce chlorite
ion (to chloride ion) and improve the overall flocculation process.

## Pathogen Inactivation and Disinfection Efficacy

For water treatment, chlorine dioxide has several advantages over
chlorine and other disinfectants. In contrast to chlorine, chlorine
dioxide remains in its molecular form in the pH range typically found in
natural waters (Roberts et al., 1980). Chlorine dioxide is a strong
oxidant and disinfectant. Its disinfection mechanisms are not well
understood, but appear to vary by type of microorganism

### Inactivation Mechanisms

Gross physical damage to bacterial cells or viral capsids has not been
observed at the low concentrations of chlorine dioxide typically used to
disinfect drinking water. Therefore, studies have focused primarily on
two more subtle mechanisms that lead to the inactivation of
microorganisms: determining specific chemical reactions between chlorine
dioxide and biomolecules; and observing the effect chlorine dioxide has
on physiological functions.

In the first disinfection mechanism, chlorine dioxide reacts readily
with amino acids cysteine, tryptophan, and tyrosine, but not with viral
ribonucleic acid (RNA) (Noss et al., 1983; Olivier et al., 1985). From
this research, it was concluded that chlorine dioxide inactivated
viruses by altering the viral capsid proteins. However, chlorine dioxide
reacts with poliovirus RNA and impairs RNA synthesis (Alvarez and
O'Brien, 1982). It has also been shown that chlorine dioxide reacts with
free fatty acids (Ghandbari et al., 1983). At this time, it is unclear
whether the primary mode of inactivation for chlorine dioxide lies in
the peripheral structures or nucleic acids. Perhaps reactions in both
regions contribute to pathogen inactivation.

The second type of disinfection mechanism focuses on the effect of
chlorine dioxide on physiological functions. It has been suggested that
the primary mechanism for inactivation was the disruption of protein
synthesis (Bernarde et al., 1967a). However, later studies reported the
inhibition of protein synthesis may not be the primary inactivation
mechanism (Roller et al., 1980). A more recent study reported that
chlorine dioxide disrupted the permeability of the outer membrane (Aieta
and Berg, 1986). The results of this study were supported by the
findings of Olivieri et al. (1985) and Ghandbari et al. (1983), which
found that the outer membrane proteins and lipids were sufficiently
altered by chlorine dioxide to increase permeability.

### Environmental Effects 

Studies have been performed to determine the effect of pH, temperature,
and suspended matter on the disinfection efficiency of chlorine dioxide.
Following is a summary of the effects these parameters have on pathogen
inactivation.

#### pH

In comparison to chlorine, studies have shown that pH has much less
effect on pathogen inactivation for viruses and cysts with chlorine
dioxide than with chlorine in the pH range of 6 to 8.5. Unlike chlorine,
studies on chlorine dioxide have shown the degree of inactivation of
poliovirus 1 (Scarpino et al., 1979) and *Naegleria gruberi* cysts (Chen
et al., 1984) increase as the pH increases.

The results of studies on *E. coli* inactivation are inconclusive. It
has been found that the degree of inactivation by chlorine dioxide
increases as pH increases (Bernarde et al., 1967a). However, an earlier
study found that the bactericidal activity of chlorine dioxide was not
affected by pH values in the range of 6.0 to 10.0 (Ridenour and Ingols,
1947). A recent study on *Cryptosporidium* found that inactivation of
oocysts using chlorine dioxide occurred more rapidly at a pH of 8.0 than
6.0. At a similar CT value, the level of inactivation at pH of 8.0 was
approximately twice that at a pH of 6.0 (Le Chevallier et al., 1997).
Another study found that chlorine dioxide efficacy increases for
*Giardia* inactivation at higher pH levels and that this may be the
result of chemical or physical changes in *Giardia* cyst structure
rather than pH effects on chlorine dioxide disproportionation (Liyanage
et al., 1997). More research is needed to further clarify how pH impacts
the effectiveness of chlorine dioxide.

#### Temperature

Similar to chlorine, the disinfection efficiency of chlorine dioxide
decreases as temperature decreases (Ridenour and Ingols, 1947). This
finding is supported by the data from Chen et al. (1984) shown in Figure
4-3 for the inactivation of *Naegleria gruberi* cysts. The curve shows
the CT required to achieve 99 percent inactivation for temperatures
between 5 and 30°C.

In a more recent study, LeChevallier et al. (1997) found that reducing
the temperature from 20°C to 10°C reduced the disinfection effectiveness
of chlorine dioxide on *Cryptosporidium* by 40 percent, which is similar
to previous results for *Giardia* and viruses. Gregory et al. (1998)
found that even under the most favorable conditions (i.e., at a pH of
8.5), required doses to achieve 2-log *Cryptosporidium* inactivation do
not appear to be a feasible alternative, requiring doses of more than
3.0 mg/L with a 60 minute detention time. At neutral pH levels, the
required doses may be more than 20 mg/L.

![](media/image3.wmf){width="5.517361111111111in"
height="3.7215277777777778in"}

[]{#__RefHeading___Toc450036541 .anchor}Figure 4-3. Effect of
Temperature on *N. Gruberi* Cyst Inactivation at pH 7

#### Suspended Matter

Suspended matter and pathogen aggregation affect the disinfection
efficiency of chlorine dioxide. Protection from chlorine dioxide
inactivation due to bentonite was determined to be approximately 11
percent for turbidities equal to or less than 5 NTUs and 25 percent for
turbidities between 5 and 17 NTUs (Chen et al., 1984).

Laboratory studies of poliovirus 1 preparations containing mostly viral
aggregates took 2.7 times longer to inactivate with chlorine dioxide
than single state viruses (Brigano et al., 1978). Chen et al. (1984)
also found that clumps of *Naegleria gruberi* cysts were more resistant
to chlorine dioxide than unclumped cysts or clumps of smaller size.

### Disinfection Efficacy

Several investigations have been made to determine the germicidal
efficiency of chlorine dioxide since its introduction in 1944, as a
drinking water disinfectant. Most of the investigations were carried out
as a comparison to chlorine; some studies have compared chlorine dioxide
and ozone. Chloride dioxide is a more effective disinfectant than
chlorine but is less effective than ozone.

#### Bacteria Inactivation

Quantitative data were published as early as the 1940s demonstrating the
efficacy of chlorine dioxide as a bactericide. In general, chlorine
dioxide has been determined to be equal to or superior to chlorine on a
mass-dose basis. It was demonstrated that even in the presence of
suspended matter, chlorine dioxide was effective against *E. coli* and
*Bacillus anthracoides* at dosages in the range of 1 to 5 mg/L
(Trakhtman, 1949). Ridenour and Armbruster (1949) reported that an
orthotolidine arsenite (OTA) chlorine dioxide residual of less than 1
mg/L was effective against *Eberthella typhosa*, *Shigella dysenteriae*,
and *Salmonella paratyphi* B. Under similar pH and temperature slightly
greater OTA residuals were required for the inactivation of *Pseudomonas
aeruginosa* and *Staphylococcus aureus*.

Chlorine dioxide was shown to be more effective than chlorine at
inactivating *B. subtilis*, *B. mesentericus*, and *B. megatherium*
spores (Ridenour et al., 1949). Moreover, chlorine dioxide was shown to
be just as effective or more effective than chlorine at inactivating
*Salmonella typhosa* and *S. paratyphi* (Bedulivich et al., 1954).

In the early 1960s several important contributions were made by Bernarde
et al. (1967a and 1967b). Chlorine dioxide was found to be more
effective than chlorine at disinfecting sewage effluent and the rate of
inactivation was found to be rapid.

A comprehensive investigation of chlorine dioxide as disinfectant was
performed by Roberts et al. (1980). The investigation was performed
using secondary effluents from three different wastewater treatment
plants. One of the objectives was to determine the relationships between
dosages and contact times and bactericidal efficiency. Dosages were
compared for 2, 5, and 10 mg/L of chlorine dioxide and chlorine. The
contact times selected were 5, 15 and 30 minutes. Results of the
investigation are shown in Figure 4-4. As shown, chlorine dioxide
demonstrated a more rapid coliform inactivation than chlorine at the
shortest contact time of 5 minutes and higher concentrations. However,
after 30 minutes of contact time, chlorine dioxide was equal or slightly
less efficient than chlorine as a bactericide.

Oliveri et al. (1984) studied the effectiveness of chlorine dioxide (and
chlorine) residuals in inactivating total coliform and f2 coliphage
virus in sewage introduced to a water distribution system. Initial
chlorine dioxide residuals between 0.85 and 0.95 mg/L resulted in an
average 2.8--log inactivation of the total coliform and an average
4.4-log inactivation of the f2 coliphage virus, over a contact time of
240 minutes.

![](media/image4.png){width="5.559722222222222in" height="5.1in"}

Source: Roberts et al., 1980.

[]{#__RefHeading___Toc450036542 .anchor}Figure 4-4. Comparison of
Germicidal Efficiency of Chlorine Dioxide and Chlorine

#### Protozoa Inactivation

The disinfection efficiency of chlorine dioxide has been shown to be
equal to or greater than chlorine for *Giardia* inactivation. Based on a
60 minute contact time, chlorine dioxide doses in the range of 1.5 to 2
mg/L are capable of providing a 3-log *Giardia* inactivation at 1°C to
25°C and pHs of 6 and 9 (Hofmann et al., 1997). Depending on the
temperature and pH, *Cryptosporidium* has been found to be 8 to 16 times
more resistant to chlorine dioxide than *Giardia* (Hofmann et al.,
1997). Although some *Cryptosporidium* oocysts remained viable, one
group of researchers found that a 30-minute contact time with 0.22 mg/L
chlorine dioxide could significantly reduce oocyst infectivity (Peeters
et al., 1989). In contrast, other researchers have found that CT values
in the range of 60 to 80 mg·min/L were necessary to provide 1- to
1.5-log inactivation (Korich et al., 1990; Ransome et al., 1993). Finch
et al. (1995) reported that the CT values for 1-log inactivation was in
the range of 27 to 30 mg·min/L. For 2-log inactivation, the CT value was
approximately 40 mg·min/L, and 70 mg·min/L for 3-log inactivation. Finch
et al. (1997) found 3-log inactivation of *Cryptosporidium* oocysts with
initial chlorine dioxide residual concentrations of 2.7 and 3.3 mg/L for
contact times of 120 minutes, at pH of 8.0 and a temperature of 22ºC.

Both Chen et al. (1985) and Sproul et al. (1983) have investigated the
inactivation of *Naegleria gruberi* cysts by chlorine dioxide. Both
studies concluded that chlorine dioxide is an excellent disinfectant
against cysts and that chlorine dioxide is better than or equal to
chlorine in terms of inactivation. Chlorine dioxide was found to be
superior to chlorine at higher pHs. However, the authors cautioned that
the CT required for 2-log inactivation was much higher than normally
employed for water treatment at that time.

#### Virus Inactivation

Chlorine dioxide has been shown to be an effective viricide. Laboratory
studies have shown that inactivation efficiency improves when viruses
are in a single state rather than clumped. It was reported in 1946 that
chlorine dioxide inactivated *Poliomyelitis* (Ridenour and Ingols,
1946). This investigation also showed that chlorine dioxide and free
chlorine yielded similar results. Other studies have verified these
findings for poliovirus 1 (Cronier et al., 1978) and *Coxsackie* virus
A9 (Scarpino, 1979). At greater than neutral pHs (where hypochlorite ion
is the predominant species) chlorine dioxide has been found to be
superior to chlorine in the inactivation of numerous viruses such as
echovirus 7, coxsackie virus B3, and sendaivirus (Smith and McVey,
1973). Sobsey (1998) determined CT values based on a study of Hepatitis
A virus, strain HM 175. The study found 4-log inactivation levels are
obtainable at CT values of less than 35 at 5°C and less than 10 at a
temperature of 25°C.

#### CT Values

Chlorine dioxide is regarded as a strong disinfectant that is effective
at inactivating bacterial, viral, and protozoan pathogens. CT values for
*Giardia* and virus inactivation are shown in Figure 4-5 and Figure 4-6,
respectively (AWWA, 1991).

CT values shown in Figure 4-5 are based on disinfection studies using in
vitro excystation of *Giardia muris*. Average CT values for 2 log
removal were extrapolated using first order kinetics and multiplied by a
safety factor of 1.5 to obtain the CT values for other log removal CT
values. Due to the limited amount of data available at pH values other
than 7, the same CT values are used for all pHs. Because chlorine
dioxide is more effective at a pH 9 than at a pH of 7, the CT values
shown in Figure 4-5 are more conservative for higher pHs than for lower
pHs. A lower safety factor was used to derive the CT values for chlorine
dioxide than for ozone due to the fact that the chlorine dioxide values
were derived from *Giardia* *muris* studies, which are more resistant
than *Giardia lamblia*.

![](media/image5.wmf){width="5.88125in" height="3.8270833333333334in"}

Source: AWWA, 1991.

[]{#__RefHeading___Toc450036543 .anchor}Figure 4-5. CT Values for
Inactivation of *Giardia* Cysts by Chlorine Dioxide

CT values shown in Figure 4-6 were obtained by applying a safety factor
2 to the average CT values derived from the studies on hepatitis A
virus, strain HM 175 (Sobsey, 1988). CT values at temperatures other
than 5°C were derived by applying a twofold decrease for every 10°C
increase in temperature.

Figure 4-7 and Figure 4-8 show the relationship between CT products and
log inactivation of *Cryptosporidium* at 20 and 10°C, respectively, and
pHs of 6 and 8. CT values shown in Figure 4-7 and Figure 4-8 indicate
that oocysts were more rapidly inactivated at pH 8 than 6 and that
temperature does impact the disinfection efficiency of chlorine dioxide.
Reducing the temperature from 20 to 10°C reduced the disinfection
effectiveness by 40 percent. Finch (1997) is studying *Cryptosporidium*
inactivation under laboratory conditions using a variety of different
disinfectants, one of which is chloride dioxide.

![](media/image6.wmf)

Source: AWWA, 1991.

[]{#__RefHeading___Toc450036544 .anchor}Figure 4-6. CT Values for
Inactivation of Viruses by Chlorine Dioxide

## Chlorine Dioxide Disinfection Byproducts

Byproducts from the use of chlorine dioxide include chlorite, chlorate,
and organic DBPs. This section discusses the formation of these
byproducts and methods to reduce or remove these DBPs. The use of
chlorine dioxide aids in reducing the formation of TTHMs and HAAs by
oxidizing precursors, and by allowing the point of chlorination to be
moved farther downstream in the plant after coagulation, sedimentation,
and filtration have reduced the quantity of NOM.

### Production of Chlorite and Chlorate

Chlorite and chlorate are produced in varying ratios as endproducts
during chlorine dioxide treatment and subsequent degradation. The
primary factors affecting the concentrations of chlorine dioxide,
chlorite, and chlorate in finished drinking water involve:

-   Dosage applied/oxidant demand ratio.

-   Blending ratios of sodium chlorite and chlorine during the chlorine
    dioxide generation process.

-   Exposure of water containing chlorine dioxide to sunlight.

![](media/image7.wmf){width="5.596527777777778in"
height="3.8270833333333334in"}

Source: LeChevallier et al., 1996.

[]{#__RefHeading___Toc450036545 .anchor}Figure 4-7. *C. parvum*
Inactivation by Chlorine Dioxide at 20°C

![](media/image8.wmf){width="5.7868055555555555in"
height="4.021527777777778in"}

Source: LeChevallier et al., 1996.

[]{#__RefHeading___Toc450036546 .anchor}Figure 4-8. *C. parvum*
Inactivation by Chlorine Dioxide at 10°C

-   Reactions between chlorine and chlorite if free chlorine is used for
    distribution system residual maintenance.

-   Levels of chlorate in sodium chlorite feedstock.

Incomplete reaction or non-stoichiometric addition of the sodium
chlorite and chlorine reactants can result in unreacted chlorite in the
chlorine dioxide feed stream. Dilute chlorine dioxide solutions are
stable under low or zero oxidant-demand conditions. The quantity of
chlorate produced during the chlorine dioxide generation process is
greater with excess chlorine addition. Likewise, a low or high pH can
increase the quantity of chlorate during the chlorine dioxide generation
process. See Section 4.2, "Generation," for a detailed discussion of the
chemistry of chlorine dioxide generation.

Numerous inorganic and biological materials found in raw water will
react with chlorine dioxide (Noack and Doerr, 1977). Chloride (Cl^-^)
and chlorite (ClO~2~^-^) ions are the dominant degradation species
arising from these reactions, although chlorate (ClO~3~^-^) can appear
for a variety of reasons when chlorine dioxide is used (Gordon et al.,
1990; Werdehoff and Singer, 1987). The immediate redox reactions with
natural organic matter play the dominant role in decay of chlorine
dioxide into chlorite in drinking water (Werdehoff and Singer, 1987).
Chlorite ion is generally the primary product of chlorine dioxide
reduction. The distribution of chlorite and chlorate is influenced by pH
and sunlight. Approximately 50 to 70 percent of the chlorine dioxide
consumed by oxidation reactions is converted to chlorite under
conditions typical in water treatment (Rav-Acha et al., 1984; Werdehoff
and Singer, 1987). The application of 2 mg/L chlorine dioxide is
expected to produce 1 to 1.4 mg/L of chlorite (Singer, 1992).

Chlorite is relatively stable in the presence of organic material but
can be oxidized to chlorate by free chlorine if added as a secondary
disinfectant (Singer and O'Neil, 1987).

*ClO*~2~^-^ + *OCl*^-^ = *ClO*~3~^-^ + *Cl*^-^

Chlorate is therefore produced through the reaction of residual chlorite
and free chlorine during secondary disinfection.

In addition, chlorine dioxide also disproportionates under highly
alkaline conditions (pH\>9) to chlorite and chlorate according to the
following reaction:

2*ClO*~2~ + 2*OH*^-^ = *ClO*~2~^-^ + *ClO*~3~^-^ + *H*~2~*O*

In water treatment processes that require high pH, such as softening,
chlorine dioxide should be added after the pH has been lowered (Aieta et
al., 1984).

The occurrence of photochemical decomposition of chlorine dioxide can
affect the ultimate concentrations of chlorine dioxide, chlorite, and
chlorate in water treated with chlorine dioxide. Moreover, generally,
sunlight may increase chlorate concentrations in uncovered storage
basins containing water with chlorine dioxide residuals. Exposure to
ultraviolet light will also change the potential reactions between
chlorine dioxide and the bromide ion.

### Organic DBPs Produced by Chlorine Dioxide

Chlorine dioxide generally produces few organic DBPs. However, Singer
(1992) noted that the formation of non-halogenated organic byproducts of
chlorine dioxide has not been adequately researched, and expected that
chlorine dioxide will produce the same types of oxidation byproducts
that are produced through ozonation. The application of chlorine dioxide
does not produce THMs and produces only a small amount of total organic
halide (TOX) (Werdehoff and Singer, 1987).

A study was conducted in 1994 by Richardson et al., to identify
semivolatile, organic DBPs produced by chlorine dioxide treatment in
drinking water. Samples were taken from a pilot plant in Evansville,
Indiana that included the following treatment variations:

-   Aqueous chlorine dioxide;

-   Aqueous chlorine dioxide, ferrous chloride, (FeCl~2~), chlorine
    (Cl~2~), and dual media filtration (sand and anthracite);

-   Gaseous chlorine dioxide; and

-   Gaseous chlorine dioxide, ferrous chloride (FeCl~2~), chlorine
    (Cl~2~), and dual media filtration (sand and anthracite).

Using multispectral identification techniques, more than 40 different
DBPs (many at sub-nanogram/L \[ng/L\] levels) were identified including
carboxylic acids and maleic anhydrides isolated from XAD™ concentrates,
some of which may be regulated in the Stage 2 DBPR. THMs were not found
after chlorine dioxide was added to the water; however, THMs did show up
during subsequent chlorination.

### Chlorine Dioxide DBP Control Strategies

EPA recommends that the total concentration of chlorine dioxide,
chlorite, and chlorate be less than 1.0 mg/L as Cl~2~ (USEPA, 1983). In
addition, chlorine dioxide concentrations exceeding 0.4 to 0.5 mg/L
contribute to taste and odor problems (AWWA, 1990). Due to these issues,
the use of chlorine dioxide to provide a disinfectant residual is
somewhat limited in moderate to high TOC water. In low oxidant-demand
water, however, ClO~2~ residuals may last several days.

Once formed, chlorate is stable in finished drinking water. No known
treatment exists for removing chlorate once it is formed. However, three
strategies (Gallagher et al., 1994) that have been proven effective for
chlorite removal are:

-   Adding reduced-sulfur compounds such as sulfur dioxide and sodium
    sulfite (not recommended).

-   Applying either granular activated carbon (GAC) or powdered
    activated carbon (PAC).

-   Adding reduced iron salts, such as ferrous chloride and ferrous
    sulfate.

Chlorite removal from drinking water through sulfur dioxide and other
sulfur-based reducing agents has been reported effective, but not
desirable. A study of chlorite removal by sulfur dioxide indicates that
a lower pH level yields higher chlorite removal, and chlorite removal
efficiencies increase as the sulfur dioxide dose increases.
Unfortunately, this removal process forms significant levels of chlorate
when sulfur dioxide and metasulfite are utilized. Therefore, it is
concluded that treatment with sulfur dioxide and metasulfite is not
desirable for chlorite removal (Dixon and Lee, 1991). In addition,
sodium thiosulfate results in effective chlorite reduction, but the
degree of removal is highly dependent upon pH and contact time and
relatively high dosages are required. Again, this application of sodium
thiosulfate is not desirable because the required dosages are too high
(Griese et al., 1991).

The addition of ferrous iron in drinking water is effective for chlorite
removal, with chloride the expected byproduct. Chlorite reduction occurs
quickly in the pH range of 5 to 7, and complete reduction occurs within
3 to 5 seconds. Excess reduced iron remaining in solution reacts with
dissolved oxygen at neutral pH, but under acidic conditions (pH \< 6.5)
the stability of the soluble iron can create aesthetic (staining)
problems if excess iron is used. Special consideration should be given
to ferrous iron dosage requirements so that the secondary MCL for iron
is not exceeded (Knocke and Iatrou, 1993).

Chlorite can be controlled by PAC at relatively high dosages (10 to 20
mg/L) and low pHs (5.5 to 6.5). Unless PAC is used for other purposes,
such as odor control, it requires large doses and is not cost effective.
PAC brands can differ in their capacity to reduce chlorite.

GAC can remove chlorite but breakthrough may occur relatively early. The
removal of chlorite by GAC appears to be a result of adsorption and
chemical reduction (Dixon and Lee, 1991). There is an initial high
removal efficiency due to chlorite adsorption. As the adsorptive sites
are occupied, chemical reduction on the GAC surface becomes the primary
removal mechanism. This results in an initial high removal efficiency.
Although chlorite levels exiting the GAC filters are low, the chlorate
levels are high, most likely a result of reactions in the GAC filters
between chlorite and free chlorine. According to studies, the capacity
of GAC beds is low, and if free chlorine and chlorite ion are present in
the GAC influent, chlorate ion will form. The most effective way to
operate GAC for chlorite reduction and avoid chlorate is to minimize
production run times and have no chlorine present in the filter.

## Status of Analytical Methods

In addition to the monitoring requirements that apply regardless of the
disinfectant used, the DBPR requires that water systems that use
chlorine dioxide for disinfection or oxidation must also monitor their
system for chlorine dioxide and chlorite.

### Chlorine Dioxide and Chlorite Analytical Methods

For compliance monitoring for chlorine dioxide, systems must use one of
the two methods specified in 40 CFR §141.131(c), including (1) DPD,
Standard Method 4500-CLO~2~ D, or (2) Amperometric Method II, Standard
Method 4500-CLO~2~ E. Where approved by the state, systems may also
measure residual disinfectant concentrations for chlorine dioxide by
using DPD colorimetric test kits.

For compliance monitoring for chlorite, systems must use one of the
three methods specified in 40 CFR §141.131(b), including (1)
Amperometric Titration, Standard Method 4500-CLO~2~ E, (2) Ion
Chromatography, EPA Method 300.0, or (3) Ion Chromatography, EPA Method
300.1. The regulations specify that Amperometric Titration may be used
for routine daily monitoring of chlorite at the entrance to the
distribution system, but that Ion Chromatography must be used for
routine, monitoring of chlorate and monthly additional monitoring of
chlorate in the distribution system.

Details of these analytical procedures can be found in:

-   *Standard Methods for the Examination of Water and Wastewater*,
    19^th^ Edition, American Public Health Association, 1995.

-   *Methods for the Determination of Inorganic Substances in
    Environmental Samples*. USEPA. 1993. EPA/600/R-93/100.

-   *USEPA Method 300.1, Determination of Inorganic Anions in Drinking
    Water by Ion Chromatography, Revision 1.0.* USEPA. 1997.
    EPA/600/R-98/118.

Table 4-3 summarizes the analytical methods approved for use for
chlorine dioxide and chlorite and provides some background information
for each method.

### Chlorine Dioxide Monitoring for Systems Using Chlorine Dioxide

For chlorine dioxide monitoring, community, non-transient non-community,
and transient non-community water systems that use chlorine dioxide for
disinfection or oxidation, are required to take daily samples at the
entrance to the distribution system. For any daily sample that exceeds
the chlorine dioxide MRDL of 0.8 mg/L, the system must take additional
samples in the distribution system the following day at the locations
specified in the DBPR, in addition to the daily sample required at the
entrance to the distribution system.

Additional sampling is to be performed in one of two ways, depending on
the disinfectant that is used to maintain a disinfectant residual in the
distribution system. If chlorine dioxide or chloramines are used to
maintain a disinfectant residual, or if chlorine is used to maintain the
residual and there are no disinfection addition points after the
entrance to the distribution system (i.e., no booster chlorination), the
system must take three samples as close to the first customer as
possible, at intervals of at least six hours. If chlorine is used to
maintain a disinfectant residual and there are one or more disinfection
addition points after the entrance to the distribution system, the
system must take one sample at each of the following locations: (1) as
close to the first customer as possible, (2) in a location
representative of average residence time, and (3) as close to the end of
the distribution system as possible (reflecting maximum residence time
in the distribution system). Chlorine dioxide monitoring may not be
reduced.

If any daily sample taken at the entrance to the distribution system
exceeds the MRDL, and on the following day one (or more) of the three
samples taken in the distribution system exceed the MRDL, the system is
in violation of the MRDL. The system must take immediate corrective
action to lower the level of chlorine dioxide below the MRDL, and must
notify the public of the [acute]{.underline} violation pursuant to 40
CFR §141.32. The system must also report to the State pursuant to 40 CFR
§141.134.

If any two consecutive daily samples taken at the entrance to the
distribution system exceed the MRDL, the system is also in violation of
the MRDL and must notify the public of the [non-acute]{.underline}
violation pursuant to 40 CFR §141.32. The system must also report to the
State pursuant to 40 CFR §141.134.

### Chlorite Monitoring for Systems Using Chlorine Dioxide

For chlorite monitoring, community and non-transient non-community water
systems that use chlorine dioxide for disinfection or oxidation are
required to take daily samples at the entrance to the distribution
system. For any daily sample that exceeds the chlorite MCL of 1.0 mg/L,
the system must take additional samples in the distribution system the
following day at the locations specified in the DBPR. These additional
samples are to be collected at: (1) a location as close to the first
customer as possible, (2) a location representative of average residence
time, and (3) a location as close to the end of the distribution system
as possible (reflecting maximum residence time in the distribution
system).

In addition, systems using chlorine dioxide must take a three-sample set
each month in the distribution system similar to the three locations
required if the chlorite MCL is exceeded in the sample collected at the
entrance to the distribution system. Specifically, these three-sample
sets are to be collected: (1) in a location near the first customer, (2)
in a location representative of average residence time, and (3) at a
location reflecting maximum residence time in the distribution system.
Any additional routine sampling must be conducted in the same
three-sample sets at the specified locations. This monthly sampling
requirement may be reduced to quarterly after one year of monitoring
where: (1) no individual chlorite sample taken in the distribution
system has exceeded the MCL and (2) the system has not been required to
conduct follow-up monitoring as a result of a daily sample collected at
the entrance to the distribution system. These systems can remain on an
annual schedule until either the daily sample or any of the three
individual quarterly samples exceed the MCL, at which time, the system
must revert to monthly monitoring.

If the arithmetic average of any three-sample set exceeds the chlorite
MCL of 1.0 mg/L, the system is in violation of the MCL and must notify
the public pursuant to 40 CFR §141.32, in addition to reporting to the
State pursuant to 40 CFR §141.134.

## Operational Considerations

As with all disinfectant selections, the primacy agency should be
consulted when selecting disinfectants. Certain states have their own
operational, maintenance, and monitoring requirements for the
application of chlorine dioxide. California prohibits the use of
chlorine dioxide in ground water systems, according to Merkle et al.,
1997. Also, in Texas, the Texas Natural Resources Conservation
Commission (TNRCC) requires the public water supply to sign a bilateral
agreement which outlines a detailed operator qualifications requirement,
testing methods, and procedures, monitoring locations, testing frequency
and reporting procedures. The chlorine dioxide concentration leaving the
water treatment plant must be less than 0.8 mg/L and the chlorite
concentration in the distribution system must be less than 1.0 mg/L.

State requirements must be reviewed to determine the cost-effectiveness
of utilizing chlorine dioxide as part of the overall water treatment
scheme. Analytical testing and reporting requirements may have
significant labor and cost impacts.

[]{#__RefHeading___Toc450036534 .anchor}Table 4-3. Analytical Methods
for Chlorine Dioxide and Related Compounds

+------------------+------------------+------------------+------------+
| **Method**       | **Basis**        | *                | **Limits** |
|                  |                  | *Interferences** |            |
+==================+==================+==================+============+
| DPD as Test      | Colored          | Mn~2~^+^, other  | \> 0.1     |
| Kits\            | oxidation        | Cl~2~, related   | mg/L       |
| Colorimetric\    | product.\        | oxidants.        |            |
| (S               | Use of color     |                  |            |
| M-4500-ClO~2·~G) | comparator is    |                  |            |
|                  | not recommended. |                  |            |
|                  | Use instrument   |                  |            |
|                  | detection.       |                  |            |
+------------------+------------------+------------------+------------+
| DPD-glycine      | Colored product, | ClO~2~^-^        | \> 0.1     |
| Method\          | free Cl~2~ is    | slowly; other    | mg/L       |
| Colorimetric\    | masked with      | oxidants.        |            |
| (SM 4500-        | glycine as       |                  |            |
| ClO~2·~D)        | chloraminacetic  |                  |            |
|                  | acid.            |                  |            |
+------------------+------------------+------------------+------------+
| DPD-FAS\         | DPD color        | Iron, other      | \> 0.1     |
| Titrimetric      | titration with   | oxidants.        | mg/L       |
| method\          | standard FAS     |                  |            |
| (SM 4500-        | until red color  |                  |            |
| ClO~2~.D)        | is discharged.   |                  |            |
+------------------+------------------+------------------+------------+
| 5-Step           | I^-^ oxidation;  | Suitable for     | \~ PQL     |
| Amperometric\    | pH control and   | ClO~2~ generated | C          |
| Method           | gas purging      | solution.\       | lO~2~^-^:\ |
| 4500-ClO~2~.E    | steps. Skilled   | Low levels not   | 0.1-0.0.5  |
|                  | analyst needed.  | okay.            | mg/L;\     |
|                  |                  |                  | ClO~3~^-^  |
|                  |                  |                  | at 0.5     |
|                  |                  |                  | mg/L       |
+------------------+------------------+------------------+------------+
| Ion              | Must use AS9     | No other         | \~ 0.05    |
| Chromatography\  | column, ext.     | oxidants.        | mg/L       |
| (EPA Method      | standards &      | Chloramines,     |            |
| 300.0 or 300.1)\ | suppression.     | ClO~2~; OCl^-^ & |            |
| Conductivity     |                  | HOCL             |            |
|                  |                  | undetectable.    |            |
+------------------+------------------+------------------+------------+
| Two-step         | I^-^ Oxidation;  | Cu~2~+, Mn~2~+,  | \> 0.1     |
| Amperometric     | pH control.\     | NO~2~^-\         | mg/L,\     |
|                  | Amendable to     | ^Accounts for    | not        |
| Method           | operator-based   | free Cl~2~,      | ClO~3~^-^  |
| 4500-ClO~2~.E    | dosage control.\ | NH~2~Cl,         |            |
|                  | Practical        | ClO~2~^-^        |            |
|                  | method.          | species.         |            |
+------------------+------------------+------------------+------------+

Source: Gates, 1998.\
Notes: SM = Standard Methods

### Process Considerations

The basic components of chlorine dioxide generation systems include:

-   Aqueous hypochlorite solution storage and feed system;

-   Sodium chlorite storage and feed system;

-   Acid storage and feed system (for Direct-Acidification generators);

-   Chlorine storage and feed system;

-   Chlorine dioxide generator; and

-   Chlorine dioxide feed piping and dispersion equipment.

Sodium chlorite storage and feed systems are basically liquid systems
that consist of a storage tank(s) and solution feed pumps. Outside
storage of 25 percent solutions (or greater) of sodium chlorite is not
recommended in cold climates since stratification may occur below 4°C
(40°F). Any ice formation may also damage the storage tanks. In some
cases, storage might be separated into bulk tanks and smaller
operational or day tanks that are filled periodically. Storage of dark
drums for long periods in hot climates should be avoided since sodium
chlorite decomposition will occur. In the storage area, light fixtures,
switches, wiring, and conduit runs should be located to avoid the risk
of sodium chlorite spilling on them.

### Generator Operation

A manual chlorine dioxide feed system may be used where the chlorine
dioxide dose remains fairly constant. The reagent chemicals are manually
set for the desired chlorine dioxide capacity at a ratio of chemicals
optimized for maximum chlorine dioxide yield. Some generating systems
can produce 95 percent pure chlorine dioxide solutions at full design
capacity, but purity can vary when the feed rate is changed. Turndown
capacity may be limited by precision of the flow metering devices,
typically 20 percent of rate capacity. Purity can vary when the feed
rate is changed significantly. Feed water alkalinity, operating
conditions, and pH also can affect yield. The ratio of reagent chemicals
should be routinely adjusted for optimum operation. Chlorine dioxide
generators can be provided with automated control to provide modulation
of chlorine dioxide feed rates based upon changes in flow (flow paced)
and chlorine dioxide demand (residual control). The automatic modulation
of the generators to meet a demand setpoint varies with manufacturer.
Generally, vacuum and combination systems are limited by the hydraulic
requirements of the venturi and the optimum reaction conditions for
chlorine dioxide generation. A chemical metering pump or injector system
is then used with a batch production system to control the applied dose
of chlorine dioxide.

### Feed Chemicals

Chlorine dioxide is generated when sodium chlorite is either oxidized or
acidified, or both, under controlled pH and temperature conditions.
Commonly, solutions of 25 percent active sodium chlorite or less are
used in chlorine dioxide generators. The major safety concern for
solutions of sodium chlorite is the unintentional and uncontrollable
release of high levels of chlorine dioxide. Such levels may approach
detonation or conflagration concentrations by accidental acidification.

The feedstock acid used by some of the generators is only one source of
accidental chemical acidification. Accidental mixing with large amounts
of any reducing agent or oxidizable material (such as powdered activated
carbon or flammable solvents) also represents a significant hazard. The
AWWA Standard B303-95 (a) includes an outline of some of these materials
(AWWA, 1995).

Another concern when handling and storing sodium chlorite solutions is
crystallization, which occurs as a result of lower temperatures and/or
higher concentrations. Crystallization will plug pipelines, valves, and
other equipment. Sodium chlorite solution should not be allowed to
evaporate to a powder. If dried, this product becomes a fire hazard and
can ignite in contact with combustible materials. A sodium chlorite fire
may result in a steam explosion if too much water and inappropriate
fire-fighting techniques are used to quench such a fire. As the
temperature of burning sodium chlorite is around 2200°C, water quickly
turns to steam. Because thermal breakdown products of sodium chlorite at
high temperatures include molecular oxygen, appropriate techniques are
required to correctly extinguish closed containers or large amounts of
dry material that has been ignited.

Stratification of sodium chlorite in holding tanks may also occur and
would influence the chlorine dioxide yield. If stratification occurs in
the bulk tank, sodium chlorite changes from high density to low density
as it is fed. The density will continue to change until the material is
re-mixed. In stratified tanks, excess chlorite would be fed to the
generator since the bottom of the tank will have denser material, and
this material would have more chlorite than required. Similarly, the
bulk tank would later discharge too little chlorite.

Although infrequent, such stratification is not readily apparent and may
likely remain unnoticed by operations unless the generator performance
is evaluated frequently. If stratification or crystallization occurs in
bulk delivery trucks, the entire content should be warmed prior to
delivery so that the sodium chlorite is re-mixed. Operators should be
aware of the possibility of stratification and crystallization during
delivery conditions.

Sodium chlorite is commercially available as a 38 percent or 25 percent
solution. Chemical and physical properties are given in Table 4-4.

[]{#__RefHeading___Toc450036535 .anchor}Table 4-4. Properties of Sodium
Chlorite as Commercially Available

  ---------------------------------------- --------------- --------------
                                           **38%           **25%
                                           Solution\***    Solution\***

  Sodium Chlorite, (%) NaClO~2~            38              25

  Sodium Chloride, (%) NaCl                1.5-7.5         1-4.5

  Inert Ingredients, mixture of other      3-4             3-4
  sodium salts (%)                                         

  Water (%)                                55-61           68-74

  Appearance                               Slightly        Clear, pale
                                           cloudy, pale    yellow
                                           yellow          

  Density @ 35°C (lb/gal), typical         11.4            10.1

  Crystallization Point (°C)               25              -7
  ---------------------------------------- --------------- --------------

\* Source: Vulcan Chemicals

For systems handling the 38 percent solution, storage tanks, piping and
pumps will require a heated enclosure, or heat tracing and insulation.
The 25 percent solution may not require any special protection except in
cold climates.

The ideal production of 1.0 pound of chlorine dioxide requires 0.5
pounds of chlorine and 1.34 pounds of pure sodium chlorite. Chlorine gas
is available as a nearly 100 percent pure chemical on a weight basis.
Gas flow metering devices are typically limited to +/- 5 percent
accuracy at full rated capacity. For example, a 100 pound per day flow
tube would allow between 20 and 30 pounds of chlorine to flow if set at
25 pounds per day (i.e., 25 +/- 5 percent of maximum flow capacity).
Sodium chlorite is supplied commercially as a pre-mixed aqueous solution
of various strengths. The 25 percent solution is the most commonly used
grade for potable water treatment.

Pure chlorine dioxide solutions (very dark amber and oily in appearance)
are very dangerous and are likely to detonate if exposed to oxidizable
materials or vapors, or even to bright lights. They are extremely
uncommon except perhaps in very specific laboratory setup systems using
concentrated sodium chlorite and concentrated acid mixtures. Such
laboratory generation methods are not recommended for the uninitiated
laboratory analyst or operator. Inexperienced personnel should not mix
strong acid and strong sodium chlorite solutions together unless they
are familiar with the purgeable extraction methods for sodium chlorite
and have a safely designed setup under a fume hood.

## Summary

### Advantages and Disadvantages of Chlorine Dioxide Use

The following list highlights selected advantages and disadvantages of
using chlorine dioxide as a disinfection method for drinking water
(Masschelein, 1992; DeMers and Renner, 1992, Gallagher et al., 1994).
Because of the wide variation of system size, water quality, and dosages
applied, some of these advantages and disadvantages may not apply to a
particular system.

**Advantages**

-   Chlorine dioxide is more effective than chlorine and chloramines for
    inactivation of viruses, *Cryptosporidium*, and *Giardia*.

-   Chlorine dioxide oxidizes iron, manganese, and sulfides.

-   Chlorine dioxide may enhance the clarification process.

-   Taste and odors resulting from algae and decaying vegetation, as
    well as phenolic compounds, are controlled by chlorine dioxide.

-   Under proper generation conditions (i.e., no excess chlorine),
    halogen-substituted DBPs are not formed.

-   Chlorine dioxide is easy to generate.

-   Biocidal properties are not influenced by pH.

-   Chlorine dioxide provides residuals.

**Disadvantages**

-   The chlorine dioxide process forms the specific byproducts chlorite
    and chlorate.

-   Generator efficiency and optimization difficulty can cause excess
    chlorine to be fed at the application point, which can potentially
    form halogen-substitute DBPs.

-   Costs associated with training, sampling, and laboratory testing for
    chlorite and chlorate are high.

-   Equipment is typically rented, and the cost of the sodium chlorite
    is high.

-   Measuring chlorine dioxide gas is explosive, so it must be generated
    on-site.

-   Chlorine dioxide decomposes in sunlight.

-   Chlorine dioxide must be made on-site.

-   Can lead to production noxious odors in some systems.

### Summary Table

Table 4-5 summarizes considerations and descriptions for chlorine
dioxide use.

[]{#__RefHeading___Toc450036536 .anchor}Table 4-5. Summary for Chlorine
Dioxide

  -----------------------------------------------------------------------
  **Consideration**    **Description**
  -------------------- --------------------------------------------------
  Generation           Chlorine dioxide must be generated on-site. In
                       most potable water applications, chlorine dioxide
                       is generated as needed and directly educed or
                       injected into a diluting stream. Generators are
                       available that utilize sodium chlorite and a
                       variety of feedstocks such as Cl~2~ gas, sodium
                       hypochlorite, and sulfuric or hydrochloric acid.
                       Small samples of generated solutions, up to 1
                       percent (10 g/L) chlorine dioxide can be safely
                       stored if the solution is protected from light,
                       chilled (\<5°C), and has no unventilated
                       headspace.

  Primary Uses         Chlorine dioxide is utilized as a primary or
                       secondary disinfectant, for taste and odor
                       control, TTHM/HAA reduction, Fe and Mn control,
                       color removal, sulfide and phenol destruction, and
                       Zebra mussel control.

  Inactivation         Chlorine dioxide rapidly inactivates most
  Efficiency           microorganisms over a wide pH range. It is more
                       effective than chlorine (for pathogens other than
                       viruses) and is not pH dependent between pH 5-10,
                       but is less effective than ozone.

  Byproducts Formation When added to water, chlorine dioxide reacts with
                       many organic and inorganic compounds. The
                       reactions produce chlorite and chlorate as
                       endproducts (compounds that are suspected of
                       causing hemolytic anemia and other health
                       effects). Chlorate ion is formed predominantly in
                       downstream reactions between residual chlorite and
                       free chlorine when used as the distribution system
                       disinfectant. Chlorine dioxide does not produce
                       THMs. The use of chlorine dioxide aids in reducing
                       the formation of TTHMs and HAAs by oxidizing
                       precursors, and by allowing the point of
                       chlorination to be moved farther downstream in the
                       plant after coagulation, sedimentation, and
                       filtration have reduced the quantity of NOM.

  Point of Application In conventional treatment plants, chlorine dioxide
                       used for oxidation is fed either in the raw water,
                       in the sedimentation basins, or following
                       sedimentation. To limit the oxidant demand, and
                       therefore chlorine dioxide dose and the formation
                       of chlorite, it is common to add chlorine dioxide
                       following sedimentation. Concerns about possible
                       taste and odor complaints have limited the use of
                       chlorine dioxide to provide a disinfectant
                       residual in the distribution system. Consequently,
                       public water suppliers that are considering the
                       use of chlorine dioxide for oxidation and primary
                       disinfectant applications may want to consider
                       chloramines for secondary disinfection.

  Special              An oxidant demand study should be completed to
  Considerations       determine an approximate chlorine dioxide dosage
                       to obtain the required CT value as a disinfectant.
                       In addition to the toxic effects of chlorine,
                       chlorine dioxide gas is explosive at levels \> 10%
                       in air. The chlorine dioxide dosage cannot exceed
                       1.4 mg/L to limit the total combined concentration
                       of ClO2, ClO2-, ClO3-, to a maximum of 1.0 mg/L.
                       Under the proposed DBP regulations, the MRDL for
                       chlorine dioxide is 0.8 mg/L and the MCL for
                       chlorite is 1.0 mg/L. Regulations concerning the
                       use of chlorine dioxide vary from state-to-state.
  -----------------------------------------------------------------------

## References

1.  Aieta, E., and J.D.Berg. 1986. "A Review of Chlorine Dioxide in
    Drinking Water Treatment*." J. AWWA.* 78(6):62-72.

2.  Aieta, E.M., P.V. Roberts, and M. Hernandez. 1984. "Determination of
    Chlorine Dioxide, Chlorine and Chlorate in Water." *J. AWWA.*
    76(1):64-70.

3.  Alvarez, M.E. and R.T. O'Brien. 1982. "Mechanism of Inactivation of
    Poliovirus by Chlorine Dioxide and Iodine." *Appl. Envir.
    Microbiol.* 44:1064.

4.  AWWA (American Water Works Association). 1995. AWWA Standard
    B303-95: Sodium Chlorite.

5.  AWWA. 1991. *Guidance Manual for Compliance with the Filtration and
    Disinfection Requirements for Public Water Systems Using Surface
    Water Sources*.

6.  AWWA. 1990. *Water Quality and Treatment,* fourth edition.
    McGraw-Hill, Inc., New York, NY.

7.  Bedulivich, T.S., M.N. Svetlakova, and N.N. Trakhtman. 1954. "Use of
    Chlorine Dioxide in Purification of Water." *Chemical Abstracts.*
    48:2953.

8.  Bernarde, M.A., et al*.*1967a. "Kinetics and Mechanism of Bacterial
    Disinfection by Chlorine Dioxide." *J. Appl. Microbiol.* 15(2):257.

9.  Bernarde, M.A., W.B. Snow, and V.P. Olivieri. 1967b. "Chlorine
    Dioxide Disinfection Temperature Effects." *J. Appl. Bacteriol.*
    30(1):159.

10. Chen, Y.S.R., O.J. Sproul, and A.J. Rubin. 1985. "Inactivation of
    *Naegleria gruberi* Cysts by Chlorine Dioxide." *Water Res*.
    19(6):783.

11. Chen, Y.S.R., O.J. Sproul, and A.J. Rubin. 1984. "Inactivation of
    *Naegleria Gruberi* cysts by Chlorine Dioxide." EPA Grant
    R808150-02-0, Department of Civil Engineering, Ohio State
    University.

12. *CRC* *Handbook of Chemistry and Physics*. 1990. D.L. Lide (editor),
    Seventy-first edition, CRC Press, Boca Raton, FL.

13. Cronier, S., et al. 1978. *Water Chlorination Environmental Impact
    and Health Effects*, Vol. 2. R. L. Jolley, et al. (editors) Ann
    Arbor Science Publishers, Inc. Ann Arbor, MI.

14. Demers, L.D., and R. Renner. 1992. *Alternative Disinfectant
    Technologies for Small Drinking Water Systems*. AWWARF, Denver, CO.

15. Dixon, K.L. and R.G. Lee. 1991. "Disinfection By-Products Control: A
    Survey of American System Treatment Plants." Presented at AWWA
    Conference, Philadelphia, PA.

16. Emmenegger, F. and G. Gordon. 1967. "The Rapid Interaction Between
    Sodium Chlorite and Dissolved Chlorine." *Inorg. Chem.* 6(3):633.

17. Finch, G.R., L.R. Liyanage, M. Belosevic, and L.L. Gyürek. 1997.
    "Effects of Chlorine Dioxide Preconditioning on Inactivation of
    *Cryptosporidium* by Free Chlorine and Monochloramine: Process
    Design Requirements." Proceedings 1996 Water Quality Technology
    Conference; Part II. Boston, MA.

18. Finch, G.R., L.R. Liyanage, and M. Belosevic. 1995. "Effect of
    Disinfectants and *Cryptosporidium* and *Giardia*." Third
    International Symposium on Chlorine Dioxide: Drinking Water, Process
    Water, and Wastewater Issues.

19. Gallagher, D.L., R.C. Hoehn, A.M. Dietrich. 1994. *Sources,
    Occurrence, and Control of Chlorine Dioxide By-Product Residuals in
    Drinking Water.* AWWARF, Denver, CO.

20. Gates, D.J. 1998. *The Chlorine Dioxide Handbook; Water Disinfection
    Series*. AWWA Publishing, Denver, CO.

21. Gates, D.J. 1989. "Chlorine Dioxide Generation Technology and
    Mythology." Conference proceedings, Advances in Water Analysis and
    Treatment, AWWA, Philadelphia, PA.

22. Ghandbari, E. H., et al. 1983. "Reactions of Chlorine and Chlorine
    Dioxide with Free Fatty Acids, Fatty Acid Esters, and
    Triglycerides." *Water Chlorination: Environmental Impact and Health
    Effects*, R. L. Jolley, et al. (editors), Lewis, Chelsea, MI.

23. Gordon, G., G.L. Emmert, and B. Bubnis. 1995. "Bromate Ion Formation
    in Water When Chlorine Dioxide is Photolyzed in the Presence of
    Bromide Ion." Conference proceedings, AWWA Water Quality Technology
    Conference, New Orleans, LA.

24. Gordon, G., et al. 1990. "Minimizing Chlorite Ion and Chlorate Ion
    in Water Treated with Chlorine Dioxide." *J. AWWA.* 82(4): 160-165.

25. Gordon, G., W.J. Cooper, R.G. Rice, and G.E. Pacey. 1987.
    *Disinfectant Residual Measurement Methods*, AWWARF, Denver, CO.

26. Gordon, G., R.G. Kieffer, and D.H. Rosenblatt. 1972. "The Chemistry
    of Chlorine Dioxide." *Progress in Organic Chemistry*, vol. 15. S.J.
    Lippaer (editor). Wiley Interscience, New York, NY.

27. Great Lakes Upper Mississippi River Board of State Public Health
    (GLUMRB) and Environmental Managers. 1992. *Recommended Standards
    for Water Works*, Health Research Inc., Albany, NY.

28. Gregory, D. and K. Carlson. 1998. "Applicability of Chlorine Dioxide
    for Cryptosporidium Inactivation." Proceedings 1998 Water Quality
    Technology Conference, San Diego, CA.

29. Griese, M.H., K. Hauser, M. Berkemeier, and G. Gordon. 1991. "Using
    Reducing Agents to Eliminate Chlorine Dioxide and Chlorite Ion
    Residuals in Drinking Water." *J. AWWA.* 83(5):56.

30. Hoehn, R.C. 1992. "Chlorine Dioxide Use in Water Treatment: Key
    Issues." Conference proceedings, Chlorine Dioxide: Drinking Water
    Issues: Second International Symposium. Houston, TX.

31. Hoehn, R.C., A.A. Rosenblatt, and D.J. Gates. 1996. "Considerations
    for Chlorine Dioxide Treatment of Drinking Water." Conference
    proceedings, AWWA Water Quality Technology Conference, Boston, MA.

32. Hofman, R., R.C. Andrews, and Q. Ye. 1997. "Chlorite Formation When
    Disinfecting Drinking Water to *Giardia* Inactivation Requirements
    Using Chlorine Dioxide." Conference proceedings, ASCE/CSCE
    Conference, Edmonton, Alberta, July.

33. Knocke, W.R. and A. Iatrou. 1993. *Chlorite Ion Reduction by Ferrous
    Ion Addition*. AWWARF, Denver, CO.

34. Korich, D.G., et al. 1990. "Effects of Ozone, Chlorine Dioxide,
    Chlorine, and Monochloramine on *Cryptosporidium* *parvum* oocyst
    Viability." *Appl. Environ. Microbiol.* 56:1423-1428.

35. LeChevallier, M.W., et al. 1997. "Chlorine Dioxide for Control of
    *Cryptosporidium* and Disinfection Byproducts." Conference
    proceedings, 1996 AWWA Water Quality Technology Conference Part II,
    Boston, Massachusetts.

36. LeChevallier, M.W., et al. 1996. "Chlorine Dioxide for Control of
    *Cryptosporidium* and Disinfection Byproducts." Conference
    proceedings, AWWA Water Quality Technology Conference, Boston,
    Massachusetts.

37. Liyanage, L.R.J, et al. 1997. "Effects of Aqueous Chlorine and
    Oxychlorine Compounds on *Cryptosporidium* Parvum Oocysts."
    *Environ. Sci. & Tech.* 31(7): 1992-1994

38. Masschelein, W.J. 1992. "Unit Processes in Drinking Water
    Treatment." Marcel Decker D.C., New York, Brussels, Hong Kong.

39. Merkle, J.C. and C.B. Reeverts. 1997. "Ground Water Treatment: What
    Are the States Doing Now?" AWWARF, Denver, CO.

40. Noack, M.G. and R.L. Doerr. 1977. "Reactions of Chlorine, Chlorine
    Dioxide and Mixtures of Humic Acid: An Interim Report." Conference
    proceedings, Second Conference on the Environmental Impact of Water
    Chlorination. R.L. Jolley, H. Gorchev, and D. Heyward (editors),
    Gatlinburg, TN.

41. Noss, C.I., W.H. Dennis, V.P. Olivieri. 1983. "Reactivity of
    Chlorine Dioxide with Nucleic Acids and Proteins." *Water
    Chlorination: Environmental Impact and Health Effects.* R. L.
    Jolley, et al. (editors), Lewis Publishers, Chelsea, MI.

42. Olivieri, V.P., et al. 1985. "Mode of Action of Chlorine Dioxide on
    Selected Viruses." *Water Chlorination: Environmental Impact and
    Health Effects*. R. L. Jolley, et al. (editors), Lewis, Chelsea, MI.

43. Olivieri, V.P., et al. 1984. Stability and Effectiveness of Chlorine
    Disinfectants in Water Distribution Systems. USEPA, Cincinnati, OH.

44. Peeters, J. E. et al. 1989. "Effect of Disinfection of Drinking
    Water with Ozone or Chlorine Dioxide on Survival of
    *Cryptosporidium* *parvum* oocysts." *Appl. Environ. Microbiol*.
    r5:1519-1522.

45. Pitochelli, A. 1995. "Chlorine Dioxide Generation Chemistry."
    Conference proceedings, Third International Symposium, Chlorine
    Dioxide: Drinking Water, Process Water, and Wastewater Issues. New
    Orleans, LA.

46. Ransome, M.E., T.N. Whitmore, and E.G. Carrington. 1993. "Effect of
    Disinfectants on the Viability of *Cryptosporidium* *parvum*
    Oocysts." *Water Supply*. 11(1):103-117.

47. Rav-Acha, C., A. Serri, E. Choshen, B. Limoni. 1984. "Disinfection
    of Drinking Water Rich in Bromide with Chlorine and Chlorine
    Dioxide, While Minimizing the Formation of Undesirable Byproducts."
    *Wat. Sci. Technol*. 17:611.

48. Richardson, S.D. et al. 1994. "Multispectral Identification of
    ClO~2~ Disinfection Byproducts in Drinking Water." *Environ. Sci. &
    Technol.* 28(4):592-599.

49. Ridenour, G.M. and E.H. Armbruster. 1949. "Bactericidal Effects of
    Chlorine Dioxide." *J. AWWA.* 41:537.

50. Ridenour, G. M. and R.S. Ingols. 1947. "Bactericidal Properties of
    Chlorine Dioxide." *J. AWWA.* 39.

51. Ridenour, G.M., and R.S. Ingols. 1946. "Inactivation of
    Poliomyelitis Virus by Free Chlorine." *Amer. Public Health*.
    36:639.

52. Ridenour, G.M., and R.S. Ingols, and E.H. Armbruster. 1949.
    "Sporicidal Properties of Chlorine Dioxide." *Water & Sewage Works*.
    96(8):279.

53. Roberts, P.V., E.M. Aieta, J.D. Berg, and B.M. Chow. 1980. "Chlorine
    Dioxide for Wastewater Disinfection: A Feasibility Evaluation."
    Stanford University Technical Report 251. October.

54. Roller, S. D. et al. 1980. "Mode of Bacterial Inactivation by
    Chlorine Dioxide." *Water Res.* 14:635.

55. Singer, P.C. 1992. "Formation and Characterization of Disinfection
    Byproducts." Presented at the First International Conference on the
    Safety of Water Disinfection: Balancing Chemical and Microbial
    Risks.

56. Singer, P.C., and W.K. O'Neil. 1987. "Technical Note: The Formation
    of Chlorate from the Reaction of Chlorine and Chlorite in Dilute
    Aqueous Solution." *J. AWWA.* 79(11):75.

57. Smith, J. E., and J.L. McVey. 1973. "Virus Inactivation by Chlorine
    Dioxide and Its Application to Storm Water Overflow." Proceeding,
    ACS annual meeting. 13(2):177.

58. Sobsey, M. 1988. "Detection and Chlorine Disinfection of Hepatitis A
    in Water." CR-813-024, EPA Quarterly Report, December.

59. Sproul, O. J. et al. 1983. "Comparison of Chlorine and Chlorine
    Dioxide for Inactivation of Amoebic Cyst." *Envir. Technol.
    Letters*. 4:335.

60. Thompson, A.L. 1989. "Practical Considerations for Application of
    Chlorine Dioxide in Municipal Water Systems." Conference
    proceedings,, Chlorine Dioxide Workshop. AWWARF, CMA, EPA. Denver,
    CO.

61. Trakhtman, N.N. 1949. "Chlorine Dioxide in Water Disinfection."
    *Chemical Abstracts*. 43:1508.

62. USEPA (U.S. Environmental Protection Agency). 1983. "Trihalomethanes
    in Drinking Water: Sampling, Analysis, Monitoring, and Compliance."
    EPA 570/9-83-002, August.

63. USEPA. 1979. "Effect of Particulates on Disinfection of
    Enteroviruses and Coliform Bacteria in Water by Chlorine Dioxide."
    EPA-600/2-79-054.

64. USEPA 1978. "Effect of Particulates on Inactivation of Enteroviruses
    in Water by Chlorine Dioxide." EPA-600/9-79-018, Cincinnati, OH.

65. Werdehoff, K.S, and P.C. Singer. 1987. "Chlorine Dioxide Effects on
    THMFP, TOXFP and the Formation of Inorganic By-Products." *J. AWWA.*
    79(9):107.

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4\. CHLORINE DIOXIDE [4-1](#chlorine-dioxide)

4.1 Chlorine Dioxide Chemistry [4-1](#chlorine-dioxide-chemistry)

4.1.1 Oxidation Potential [4-1](#oxidation-potential)

4.2 Generation [4-2](#generation)

4.2.1 Introduction [4-2](#introduction)

4.2.2 Chlorine Dioxide Purity [4-3](#chlorine-dioxide-purity)

4.2.3 Methods of Generating Chlorine Dioxide
[4-4](#methods-of-generating-chlorine-dioxide)

4.1.4 Generator Design [4-9](#generator-design)

4.1.5 Chemical Feed Systems [4-11](#chemical-feed-systems)

4.1.6 Generator Power Requirements [4-13](#generator-power-requirements)

4.3 Primary Uses and Points of Application for Chlorine Dioxide
[4-13](#primary-uses-and-points-of-application-for-chlorine-dioxide)

4.3.1 Disinfection [4-13](#disinfection)

4.3.2 Taste and Odor Control [4-14](#taste-and-odor-control)

4.3.3 Oxidation of Iron and Manganese
[4-14](#oxidation-of-iron-and-manganese)

4.4 Pathogen Inactivation and Disinfection Efficacy
[4-15](#pathogen-inactivation-and-disinfection-efficacy)

4.4.1 Inactivation Mechanisms [4-15](#inactivation-mechanisms)

4.4.2 Environmental Effects [4-15](#environmental-effects)

4.4.3 Disinfection Efficacy [4-17](#disinfection-efficacy)

4.5 Chlorine Dioxide Disinfection Byproducts
[4-22](#chlorine-dioxide-disinfection-byproducts)

4.5.1 Production of Chlorite and Chlorate
[4-22](#production-of-chlorite-and-chlorate)

4.5.2 Organic DBPs Produced by Chlorine Dioxide
[4-25](#organic-dbps-produced-by-chlorine-dioxide)

4.5.3 Chlorine Dioxide DBP Control Strategies
[4-25](#chlorine-dioxide-dbp-control-strategies)

4.6 Status of Analytical Methods [4-26](#status-of-analytical-methods)

4.6.1 Chlorine Dioxide and Chlorite Analytical Methods
[4-27](#chlorine-dioxide-and-chlorite-analytical-methods)

4.6.2 Chlorine Dioxide Monitoring for Systems Using Chlorine Dioxide
[4-27](#chlorine-dioxide-monitoring-for-systems-using-chlorine-dioxide)

4.6.3 Chlorite Monitoring for Systems Using Chlorine Dioxide
[4-28](#chlorite-monitoring-for-systems-using-chlorine-dioxide)

4.7 Operational Considerations [4-28](#operational-considerations)

4.7.1 Process Considerations [4-30](#process-considerations)

4.7.2 Generator Operation [4-31](#generator-operation)

4.7.3 Feed Chemicals [4-31](#feed-chemicals)

4.8 Summary [4-33](#summary)

4.8.1 Advantages and Disadvantages of Chlorine Dioxide Use
[4-33](#advantages-and-disadvantages-of-chlorine-dioxide-use)

4.8.2 Summary Table [4-34](#summary-table)

4.9 References [4-35](#references)

Table 4-1. Commercial Chlorine Dioxide Generators
[4-5](#__RefHeading___Toc450036532)

Table 4-2. Surface Water Chlorine Dioxide Demand Study Results
[4-14](#__RefHeading___Toc450036533)

Table 4-3. Analytical Methods for Chlorine Dioxide and Related Compounds
[4-30](#__RefHeading___Toc450036534)

Table 4-4. Properties of Sodium Chlorite as Commercially Available
[4-32](#__RefHeading___Toc450036535)

Table 4-5. Summary for Chlorine Dioxide
[4-34](#__RefHeading___Toc450036536)

Figure 4-1. Conventional Chlorine Dioxide Generator When Using
Chlorine-Chlorite Method [4-7](#_Ref419604249)

Figure 4-2. Chlorine Dioxide Generation Using Recycled Aqueous Chlorine
Method [4-8](#_Ref419604411)

Figure 4-3. Effect of Temperature on *N. Gruberi* Cyst Inactivation at
pH 7 [4-17](#__RefHeading___Toc450036541)

Figure 4-4. Comparison of Germicidal Efficiency of Chlorine Dioxide and
Chlorine [4-19](#__RefHeading___Toc450036542)

Figure 4-5. CT Values for Inactivation of *Giardia* Cysts by Chlorine
Dioxide [4-21](#__RefHeading___Toc450036543)

Figure 4-6. CT Values for Inactivation of Viruses by Chlorine Dioxide
[4-22](#__RefHeading___Toc450036544)

Figure 4-7. *C. parvum* Inactivation by Chlorine Dioxide at 20°C
[4-23](#__RefHeading___Toc450036545)

Figure 4-8. *C. parvum* Inactivation by Chlorine Dioxide at 10°C
[4-23](#__RefHeading___Toc450036546)