Safety issues related to chlorine disinfectants

By Doug Kelley

Both chlorine and chlorine dioxide have been misapplied in the tree fruit industry, resulting in "off gassing," where a cloud of noxious gas rapidly fills an entire packing house room. Most of the issues with "off gassing" pertain to water temperature, pH, and oxidant dosage.

The most common occurrence of "off gassing" with chlorine is when the pH of a dump tank is rapidly reduced in order to use the tank for de-scaling hardness from the surface of apples. Another mode for chlorine to "gas-off" involves chloramine formation. Chloramines are much more volatile than chlorine, and are readily "gassed off" at reduced pH.

Being a dissolved gas in water, chlorine dioxide is much more volatile than chlorine or chloramines. For this reason, the primary mechanisms for chlorine dioxide to "gas off" must be well understood in order to maintain a safe and effective chlorine dioxide application.

First, chlorine dioxide should never be applied to a heated water system in a poorly ventilated room. As a dissolved gas in water, chlorine dioxide increasingly releases from solution with increasing temperature. At dump tank water temperatures greater than 80oF, a sufficient chlorine dioxide residual cannot be maintained in the water. Attempts to do so will result in "gassing off" and a release of noxious gas into the packing house. Chlorine dioxide residuals of 0.5 to 1.0 parts per million can be safely maintained at 70oF, and residuals of 3 to 5 ppm can be safely maintained in 50oF water. At these dosages, very effective microbial control can be achieved.

Chlorine dioxide also will "gas off" from a water system if the concentration in the water gets too high. A situation that can cause a high chlorine dioxide concentration can occur when a neutral pH dump tank is rapidly acidified for the purpose of de-scaling apples. This is due to the acid-driven conversion of chlorite, the chlorine dioxide byproduct, back into chlorine dioxide.

Finally, unlike chlorine, chlorine dioxide is excellent at removing biofilms, the microorganism populations living on the walls of all water systems, and on the fruit. Typically, more than 99% of all microorganisms live on the wetted surfaces and not in the bulk water. Chlorine cannot effectively penetrate biofilms.

Chlorine dioxide is more chemically selective than chlorine, and effectively penetrates biofilms via targeted reactions on sulfide-containing amino acids or protein di-sulfide linkages. These targeted reactions allow for effective biofilm destruction at very low chlorine dioxide doses.

It may require a long time to completely remove an existing biofilm. During this time, it may be very difficult to achieve a chlorine dioxide residual. Therefore, another mechanism of how chlorine dioxide may "gas off" from a dump tank water system involves feeding chlorine dioxide at a dosing rate that for the first few hours of a day does not provide a residual. Once the biofilm has been significantly removed, suddenly the chlorine dioxide concentration will increase in accordance with the feed rate.

In order to minimize the risks of "off- gassing," use of an on-line Oxidation Reduction Potential monitor and automated feed control system is recommended, which can detect any rapid increases in disinfectant dosage and turn off the feed system.