|
|
July 1998ULTRAVIOLET DISINFECTION OF PRODUCE WASHWATERWHAT IS ULTRAVIOLET RADIATION (UVR)?Ultraviolet (UVR) is produced naturally by the sun and artificially by mercury vapor lamps. At specific frequencies, UVR lamps act as an effective germicide that are used in the produce industry. Produce washwater is disinfected on-site (at the point of use) with commercially available UVR equipment. Produce will experience a longer shelf life and value and chemical disinfectants for water will no longer need to be purchased. HOW DOES UVR WORK?The maximum germicidal power of UVR occurs at a wavelength of 253.7 nm (2537 Angstroms). At this wavelength, the energy of light is greater than the bond energies of most chemicals encountered in the produce industry. This means that the chemicals will be broken down to less harmful forms. Microorganisms are also affected by UVR. When an organism passes through germicidal UVR, its DNA is destroyed to a point that the organism can be killed or mutated so that it cannot reproduce. This results in disinfection of water through a reduction of pathogens. 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 UVR?When used properly, UVR treatment is effective against a wide variety of pathogens. UVR will effectively destroy bacteria and viruses. As with chlorine and chorine dioxide, spores and cysts are relatively resistant to UVR. They must be filtered out before UVR treatment. A 5 micron filter will significantly reduce spores and cysts. WHAT CONDITION ARE BEST?In order for UVR to be effective, the treated water must be filtered before treatment to reduce the amount of particulate material. Produce washwater must be filtered in order to allow UVR to penetrate. Even particles as small as 15 micron can affect UVR disinfection. After filtration, target molecules will readily absorb UVR. Along with filtration, the water to be treated by UVR must approach the generating unit with uniform velocity. Turbulent flow conditions will cause some target molecules to experience low intensity and low contact times. Contaminants are most efficiently broken down by alkaline (pH > 7.0) water conditions, although UVR disinfection is not greatly affected by pH. UVR is largely unaffected by temperatures. UVR concentration is measured in terms of intensity (mW/cm2), which is dependent on the power of the lamps and the area exposed to the lamps. The dose, along with the effectiveness of UVR, will increase with increased intensity and contact time. Although pathogen inactivation is virtually immediate, adequate contact time is needed in case some target pathogens are shaded. WHERE IS UVR ADDED?UVR disinfection can be implemented in several parts of the wash system as long as the water is filtered before disinfection. It can be used in hydrocoolers, dump tanks, recirculation tanks, makeup water sources, rinse water sources, and directly in-line of the water circuit. The dosage of UVR should be between 40 and 80 mW-s/cm2. Because UVR has no residual life, it is desired to place the disinfection as close to the point of use as possible. HOW DO I CONTROL MY DOSAGE?Dosage of UVR is dependent on intensity and contact time. Intensity can increase dosage by increasing the number of lamps or decreasing the area they need to disinfect. Dose can also be increased by increasing the contact time. Dosage will decrease as the life of the lamps increases. Lamps must be replaced annually. Dosage will also decrease with the life of the lamps because of fouling on the lamp housing. This fouling is caused by hard water or alternating wet/dry processes and will block a significant amount of radiation. Some UVR companies offer automatic lamp wipers to eliminate fouling. Under-dosed organisms can recover and continue to reproduce. IS UVR A HEALTH HAZARD?There are no federal exposure limits to germicidal UVR. The lamps must be operated in a closed container and must not be viewed while in operation. UVR lamps can damage vision if they are directly exposed to the eyes. All fluorescent lamps are fragile and give off poisonous gas when broken. Broken lamps should be replaced and disposed immediately. HOW DO I IDENTIFY UVR LAMPS?Lamps that generate germicidal action also emit a pale blue color. Germicidal lamps are available in standard sizes and power of normal fluorescent lamps. The medium pressure mercury lamps that produce UVR frequencies have a life of about 7500 hours. The lamps will give off visible light after this time, but will not give off enough UVR to result in germicidal action. DISPOSAL OF TREATED WATER?Because there is no chemical residual in the wastewater, disposal is not an issue of properly dosed irradiated water. WHAT DO I NEED TO BUY?Proper UVR usage requires a filter for the water to remove particulate material, UV lamps in an opaque container mounted such that the water to be treated can be pumped past the surfaces of the lamps, and pumps and associated piping. UVR generators require minimal maintenance. Most UVR companies recommend that the lamps be replaced annually under normal operating conditions. WHERE CAN I PURCHASE UVR EQUIPMENT?Chlorine dioxide 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. Many other companies exist that make and sell similar products. Aquanics Inc. -- Design and sell UVR disinfection equipment |
|
|
