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Postharvest Information Network

Friday, April 18, 2014

WSU-TFREC/Postharvest Information Network/Chlorine Dioxide

Chlorine Dioxide


Chlorine dioxide is a biocide used in three separate areas of the packing plant: treating process water, sanitizing hard surfaces, and sanitizing fruit wash. To understand why chlorine dioxide might be used instead of hypochlorite for water treatment, we must delve into basic chlorine chemistry.

Chlorine is lethal to fungi and bacteria at very low concentrations, like a few parts per million. Yet the recommended dosage rates for process waters are in the neighborhood of 100 parts per million. When you add sodium hypochlorite to a fresh dump tank on Monday morning, some of the hypochlorite will take on a hydrogen ion and become hypochlorous acid. The amount that becomes hypochlorous acid and the amount that stays as hypochlorite ion depends on the pH of the water. At a pH of about 7.5 the two forms will be present in equal concentrations. Chlorine test kits are designed to measure either free chlorine or total chlorine. Both hypochlorous acid and hypochlorite ion are considered to be free chlorine. They are not equally effective as disinfectants, however. Depending on the target organism, hypochlorous acid may be 100 times more effective than hypochlorite ion.

Free chlorine reacts with some organic compounds to form trihalomethanes, the best known of which is chloroform. Chloroform is a priority pollutant and its presumed presence is the basis for some of the free chlorine effluent limits in the proposed wastewater discharge permit.

Chlorine reacts with ammonia and ammonia-like compounds to form chloramines. Chloramines in their various forms constitute combined chlorine, the difference between free chlorine and total chlorine. In well-used process waters, such as a five-day-old dump tank, all of the chlorine will probably be in a combined form. Although some chloramines can be disinfectants, their activity is much slower than free chlorine.

The point is that, for a constant level of total chlorine, as water quality changes it becomes increasingly difficult to determine just what form the chlorine will take and whether it is adequately disinfecting the tank.

Chlorine Dioxide

Chlorine dioxide differs from chlorine in that it dissolves in water but does not react with water as chlorine does, so it is less affected by pH. Unlike chlorine, you may use chlorine dioxide in combination with acid. Chlorine dioxide does not form trihalomethanes or other chlorine-additive products. Chlorine dioxide does not form chloramines. There is little ambiguity about the concentration of chlorine dioxide. If an adequate residual of chlorine dioxide exists, the water is being disinfected. Because it is less involved in competing reactions, it is much easier to maintain an effective residual of chlorine dioxide. Maintaining a residual disinfectant throughout the water stream destroys spores as soon as they enter the tank, an efficient, practical method.

Dump tanks and flumes are built to recirculate large quantities of water. If each gallon of water could be disinfected as it passed through the pump and re-entered the tank free of viable spores, all fruit entering the tank would be introduced to sterile water. Because of volumes involved, this is impractical.

Chlorine dioxide has a few drawbacks. It is not stable enough to be manufactured off-site and shipped. It must be created at the point of use and that requires a generator of some sort. The type of generator depends on the reagents used, and these vary with application. The control systems used for monitoring and feeding chlorine dioxide into water streams such as hydrocoolers are based on oxidation reduction potential electrodes. The relationship between ORP and chlorine dioxide concentration changes with water quality, so it must be routinely checked against a wet test using a test kit and corrected.

Difficulty of Using Chlorine Dioxide Indoors

Chlorine dioxide can be difficult to use indoors. When a treated water stream is agitated or aerated, some of the chlorine dioxide comes out of solution and enters the atmosphere. There is a permissible exposure level of 0.1 ppm in the air, but workers will respond to the odor before that level is reached. For this reason the recommended treatment rate for indoor applications is 1 ppm or less. A dosage of 2 ppm is more effective, but the potential for worker discomfort at that level is too high. Anyone constructing a new packing facility should provide adequate ventilation of the wet end.

Reactions Used to Generate Chlorine Dioxide

Two different reactions are used to generate chlorine dioxide. For use as a water treatment, chlorine dioxide is generated by reacting sodium chlorite with chlorine gas. This takes place in a Rio Linda C30I generator. Rio Linda holds the patent for combining chlorite and chlorine gas under vacuum. The yield of chlorine dioxide from chlorite is 95% to 98% and the product is almost neutral in pH.

An acid reaction is used to produce sanitizing detergent solutions. This reaction is not balanced because the actual products depend on the type of acid employed. The acid and the detergent come combined in one drum and the sodium chlorite in another drum. The contents of the two drums are metered together and allowed to react for a few minutes before being diluted with water.

Hard Surface Sanitation Using Chlorine Dioxide

For hard surface sanitation the chlorine dioxide detergent is usually blown into a foam to aid application. This foam helps control the formation of chlorine dioxide gas. Foaming systems can either be small and portable or large and permanently plumbed. The foam cart provides a degree of portability to an acid/chlorite generator. Once the cart is connected to a water source, an electrical supply and compressed air, it can produce large quantities of sanitizing foam. With sufficient incoming water pressure it is possible to reach a 30-ft. ceiling from the floor.

A Different Detergent for the Fruit Wash System

A different detergent is used for the fruit wash system, one that is approved for food contact. Instead of a foam application, the diluted detergent is applied as a line spray. This takes the place of conventional soap. The water treatment generator uses an eductor to create a vacuum. The vacuum pulls chlorine gas through a vacuum regulator and sodium chlorite solution from the drum. However, even the smallest generator is too big for packing applications. To accommodate the high flow rate of the generator, we capture the strong chlorine dioxide solution in a reservoir and feed out of the reservoir at a lower flow rate on demand.

The fruit wash system is a self-contained wall-mounted generator. The water is filtered, pressure regulated, and metered for volume. The reagents are pumped into the reaction column in proportion to the water flow rate. The reacted chemicals are diluted with water and sprayed onto the fruit. By installing these systems only where a ventilation hood exists, the problem of worker discomfort from gases is eliminated.

The fruit wash generator solution effectively kills fungi. In my study, I added Penicillium conidia to a sample of the chlorine dioxide detergent solution and, after a few seconds, neutralized the chlorine dioxide with a reducing agent. I found 80% of the colony-forming units were dead within three seconds, and the remainder were dead within five seconds.

Gary Apel

Michelsen Packaging, Yakima WA

Tree Fruit Postharvest Journal 4(1):12-13
June 1993

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