Ultraviolet radiation is a type of light, unlike visible light, that cannot be seen. Its wavelengths, expressed in Angstrom units (one Angstrom unit wavelength equals one hundred - millionth of a centimeter), are shorter than the wavelengths of visible light and carry more energy. Because of this high concentration of energy, UV radiation has the unique ability to kill microorganisms with which it comes in contact.
HOW DOES ULTRAVIOLET RADIATION WORK?
Ultraviolet radiation sterilizes water. Sterilization implies that life such as bacteria, viruses, yeasts, molds, and algae are destroyed. For UV radiation to work, a 2537 Angstrom unit (254 nanometers) wavelength must come in contact with microorganisms to inactivate it. When ultraviolet rays reach the microorganism, they strike the heart of the organism destroying the DNA (deoxyribonucleic acid) and preventing it from reproducing. Table 1 gives the amount of UV necessary to kill various microorganisms.
The Public Health Service does not require water to be completely sterilized to be potable, but water must conform to the departments drinking water standards or those of the agency governing your supply. To meet drinking water standards, the supply must contain less than 2.2 coliforms per 100 ml. The microorganisms in the coliform group are usually associated with fecal matter or human and animal wastes and suggest the presence of pathogenic (disease - causing) organisms such as typhoid and dysentery. If 100% sterilization is required, a different sizing formula must be used.
WHAT ARE THE FACTORS AFFECTING STERILIZATION?
Energy and exposure. The amount of power delivered by the lamp and the amount of time the water is exposed to the UV radiation are principal factors in UV water sterilization. The germicidal spectrum of the ultraviolet wavelength, which peaks at 2537 Angstroms, ranges from 2000 to 3000 Angstroms.
The total UV energy emitted from all sides of the UV lamp is expressed in watts. Over time, a lamps intensity decreases, and as a result, the UV output gradually decreases. Because of this, lamps must be replaced periodically for optimum efficiency. The performance of various types of lamps is shown in Table II. As options, some units have light detectors or UV monitors. Light detectors detect if the bulb is on (light), and monitors detect if there are enough Angstroms to kill the most significant amount of waterborne pathogenic microorganisms. The total exposure of the liquid is measured in micro-watt seconds per centimeter square (microwatts/cm2). In other words, exposure is a product of the energy produced by the lamp over a certain amount of time and within a given area. A short exposure at a high intensity UV and a long exposure at a low intensity UV produce the same number of micro-watt seconds per centimeter square.
Transmission. The amount of energy available to any microorganism from a given ultraviolet source is dependent on the UV transmittability of the liquid. The transmission of UV rays is effected by the waters depth (the amount of water through which UV travels) and the waters absorption coefficient (the amount and type of dissolved and suspended matter in the water, which absorbs the UV before it reaches the microorganism). Because the efficiency of the sterilizer is determined by the quality of the water, the physical requirements of less than 10 NTU of turbidity, 15 TCU of color, and 0.2 ppm of iron should be met before and UV unit is installed. Table III illustrates the percentage of transmission of ultraviolet for water at various absorption coefficients. The absorption coefficient must be known for proper sizing.
Other factors. The amount of ultraviolet energy the UV lamp produces is also dependent upon the primary voltage output and the lamp wall temperature. Table IV shows the effect of line voltage on UV output, and the effect of temperature is shown on Table V. As the table indicates, the lamp will be only about 22% efficient in generating bactericidal radiation at 56.6 Fo (12 Co). ATS uses high intensity UV lamps inside a high transmission clear fused quartz jacket so that an optimum temperature of 104 Fo (40 Co) is maintained for 100% UV output.
HOW IS SIZING DONE?
If a proper voltage source exists, ATS can properly size an ultraviolet sterilizer if the following conditions are known:
1. The peak flow rate required in gpm, gph, gpd, or m3/h.
2. The transmission and physical makeup (the absorption coefficient) of the water.
3. The ultraviolet energy level required to destroy various microorganisms (see Table I).
The Public Health Service requires that UV disinfection have a minimum dosage of 16,000 uW sec./cm2 (microwatt-seconds per centimeter square). ATS sterilizers are manufactured in standard sizes 6 to 150 gpm to impart a dosage of 30,000 +uW sec./cm2. Most significant waterborne pathogenic microorganisms are destroyed by under 10,000 uW sec./cm2. Industrial high purity water may require higher radiation levels depending on the type of microorganism to be destroyed.
If 100% sterilization is required, the flow rate through the sterilizer can be computed, depending on the energy level required. For a particular problem or application, consult an authorized GNC Water Well Representative.
Modified from Aqua Treatment Service, Inc. UV Technical Bulletin.
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