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July 1998OZONE DISINFECTION OF PRODUCE WASHWATERWHAT IS OZONE?Ozone (O3) is a gas that is used for water disinfection. Ozone has been recently approved (July, 1997) for use on fresh-cut produce. It can not be stored or transported, so it is made on-site (at the point of use) with commercially available equipment. It can be used as a disinfectant for washwater, flumes, dumptanks, and hydrocoolers. HOW DOES OZONE WORK?Like chlorine and chlorine dioxide, ozone disinfects by oxidation. It undergoes oxidation to affect the reproduction and metabolism of microorganisms. This results in a reduction of pathogen populations and disinfected water and produce surfaces. Ozone's oxidation power is higher than both chlorine dioxide and chlorine. Materials may be broken down by direct reaction with ozone or by reacting with secondary products of ozone. A microorganism is any yeast, mold, bacteria, protozoa, or virus. A pathogen is a disease or injury causing microorganism. Pathogens of interest in the fresh produce industry include E. coli, Cyclospora, Salmonella, and Hepatitis. A cyst is a protective coating for a dormant microorganism, and an ova is a female germ that is capable of reproducing. HOW EFFECTIVE IS OZONE?Ozone will effectively destroy bacteria and viruses. Unlike chlorine or chorine dioxide, ozone will kill spores and cysts so that they do not need to be filtered. A 5 micron filter will significantly reduce spores and cysts. Also unlike chlorine and chlorine dioxide, transition metals such as iron and manganese are oxidized to a stable high oxidation state. Ozone will not react with metal ions with one oxidation state. Heavy metals are left in least soluble form (further filtration or settling process must be used for removal). All halides except fluorine will be oxidized by ozone. WHAT CONDITIONS ARE BEST?In order for ozone to be effective, the treated water must be filtered before treatment to reduce the amount of organic material. The pH of solution has little or no effect on the effectiveness of ozone, but a neutral pH (7.0) is recommended. Organics will affect both the reaction time and concentration of ozone. Organics may also combine with metals. Cool temperatures are desired to increase contact times. Warm water will cause ozone to be used up rapidly because the solubility decreases. WHERE IS OZONE ADDED?Ozone is usually injected with commercially available equipment into a produce dump tank or water reservoir. Ozone is effective and safe at concentrations 3 to 8 ppm in water (parts per million or mg/L). Dump tanks can be treated at 0.75 ppm and contact times can vary from 1 to 10 min.1 A general rule is to have a dump tank turnover of 20 minutes and have the water treated three times per hour. It is important to monitor the concentration and contact time of solutions containing ozone. Complete mixing will ensure that all of the solution will have the same ozone dose and contact time. Increasing the concentration and contact time of ozone increases the sanitizing power. After a 10 minute contact time of 0.75 ppm solution, microbial loads can undergo at least a 2 log (> 99%) reduction.1 If ozone is to be applied to any part of the wash system, the injection area must use filtered water. HOW DO I CONTROL MY DOSAGE?Monitors are available from most companies that sell ozone generating equipment. It is important to have, understand, and use monitors for ozone dosing to avoid off-gases. These gases are irritable to the nose and throat. IS OZONE A HEALTH HAZARD?Ozone is irritable to the nose and throat above 0.1 ppm in air. The OSHA maximum allowable short-term ( < 15 minutes) exposure level to ozone is 0.3 ppm; 0.1 ppm is the maximum concentration for allowable long term ( > 8 hours) exposure to ozone. Long term exposure to concentrations above this limit may cause temporary headaches or fatigue for several days. At concentrations slightly above this, it will cause burning and watering eyes. It will cause difficulty breathing, shortness of breath, coughing, and choking at 0.5 to 1.0 ppm. Symptoms of extended exposure to this concentration are pain and tightness in the chest, headache, upset stomach, dizziness, shock, or unconciousness. Concentrations from 1 ppm to 10 ppm may cause pulmonary edema (fluid in lungs), congestion, hemorrhage, or possible death. Most by-products of ozone rapidly break down to biodegradable organic materials. Few by-products of ozone are toxic. Pesticides may form some toxic by-products when reacted with ozone. Bromide, a naturally occurring element in many water sources, will turn to bromate (a suspected carcinogen) when activated with ozone. Ozone is explosive under pressure so it can not be compressed or stored in any way. Ozone will turn to a liquid below -12 oC, as is explosive in it's liquid state. HOW DO I IDENTIFY OZONE?Ozone is a colorless gas that is soluble up to 8800 ppm in water. It is possible to identify ozone by a slightly pungent sulfer-like smell at concentrations above 0.01 ppm in air. Ozone is heavier than air. DISPOSE OF TREATED WATER?Because of its short residual life, ozone poses few problems to the receiving environment. WHAT WILL I NEED TO BUY?Proper ozone use requires a filter, mixer, generator, injector, monitor, and ozone-safe plumbing (some companies that offer monitoring devices and ozone products are listed at the end of this article). Most companies that distribute ozone equipment sell generators, injection systems, monitors, and ozone-safe plumbing, and they may sell mixers and filters as well. If filters or mixers are not supplied by the chosen ozone company, companies that deal specifically with these products should be contacted. Ozone is corrosive to rubber and mild steel, so stainless steel is usually used. This increases the cost of using ozone in the packing line. REFERENCES
WHERE CAN I PURCHASE OZONE GENERATING EQUIPMENT?Ozone equipment and information can be obtained from the following companies. These companies were chosen only for reference. This article is not intended to be an advertisement nor provide a product recommendation. Analytical Technology, Inc. - Sell a portable units for sensing concentration of ozone, chlorine and chlorine dioxide in air, and a unit for measuring dissolved ozone in water |
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