In Water and Wastewater Treatment
Chlorine dioxide is composed of an atom of chlorine and two atoms of oxygen: ClO2

It is generated by combining sodium chlorite and chlorine dissolved in water.
The chemical equation that illustrates this is:
2NaClO2 + Cl2 à 2ClO2 + 2 NaCl
Two molecules of sodium chlorite plus chlorine yields two molecules of chlorine dioxide and two molecules of sodium chloride (rock salt).

CDG Technology, Inc., of Bethlehem, PA has a propriety process for generating chlorine dioxide from a combination of "solid sodium chlorite" and gaseous chlorine. This process produces a high purity concentration of chlorine dioxide.

Most others combine a liquid solution of sodium chlorite and chlorine dissolved in water.

Chlorine dioxide is generated onsite principally due to two factors:
· it is an unstable gas
· it is an explosive gas

Chlorine dioxide:
· is better at inactivating viruses than chlorine.
· usually costs more than chlorine to install and to use (sometimes two or three times more expensive). There are some industrial and possibly some wastewater exceptions to this. It is site and water constituent based.
· forms chlorates and chlorites which, in higher concentrations, are potentially toxic.
· does not react to form appreciable amounts of THMs (trihalomethanes).
· does not react with ammonia to form chloramines.
· does not react to form appreciable amounts of haloacetic acids (HAAs) like chlorine does.
· is second to ozone in biocidal efficiency, and ahead of chlorine and chloramines.
· in many instances, is an increase in person-hours of labor to use when compared to gaseous chlorine alone; especially when gaseous chlorine is used onsite to generate the chlorine dioxide. (I know of an exception to this.)
· may be dechlorinated (neutralized) by using sulfur dioxide.
· after generation, and injection, chlorine dioxide remains a true gas dissolved in the water.

The ClO2 ion is the disinfecting ion.

Its oxidizing and disinfecting properties remain constant over the pH range of 4 to 10 (as compared to free chlorine residuals that are strongly affected by the pH value of the water being treated.

Chlorine reacts with more compounds than chlorine dioxide, primarily through oxidation reactions, but also because chlorine is able to react via addition and substitution reactions. Chlorine dioxide is NOT as reactive as chlorine with organic compounds, and therefore has a greater percent of the dosage remaining as a residual than chlorine (means: ClO2 does NOT have as great of a "chlorine demand" as chlorine dioxide.)

WATER TREATMENT
In water treatment chlorine dioxide has been used
· for disinfection (there are reportedly thousands of installations in Europe).
· able to inactivate cysts of Giardia and Cryptosporidium.
· for removal of color-causing substances in the raw water.
· for oxidation of iron and manganese (creates precipitates for subsequent removal in sedimentation and filtration processes).
· for removal of some taste and odor compounds in the raw water.

WASTEWATER TREATMENT
"There are no known chlorine dioxide installations in the United States at this time." (1) While I am writing this section on chlorine dioxide, a certain set of wastewater treatment plants in California are being given wastewater discharge permits that essentially are requiring that the wastewater discharge EXCEED the potable drinking water standard for THMs and other constituents. The potable water for these towns, if dechlorinated and discharged into the receiving waters, like an agricultural drainage ditch, would be in severe violation of these new standards. Regulations such as these will obviously drive facilities to explore other disinfection options, like chlorine dioxide. (Ok, I would say more, but alas, I am not retired yet!!)

SAFETY ISSUES
· Generate onsite with provisions that insure that ClO2 gas CAN NOT accumulate at any point, anywhere. Insure that backup equipment (such as dilution and feedwater pumps, analyzers) and safety features and alarms are installed and KEPT operable at all times.
· Insure proper attention to maintenance and operation of the equipment and the process.
· Must insure the proper storage and handling of the sodium chlorite solution. (The majority of safety-related issues/accidents involve this phase of the operation.)
· "If sodium chlorite solution dries in contact with combustibles, it can ignite. For example, sodium chlorite solution spilled on wood can dry and then burst into flames, the wood serving as a fuel source. Similarly, contact with oil or grease, paper products, textiles, leather, and the like can cause dried sodium chlorite to combust."(1)

References:
1) Geo. Clifford White Handbook of Chlorination and Alternative Disinfectants 4th Edition. Wiley Interscience, New York, NY 1998

2) Promotional Literature from: CDG Technology, Inc., of Bethlehem, PA

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