Bulletin 67

Water Purification – Bulletin 67

  The word sterilization indicates that all life in the world of microbiology is destroyed. Complete sterilization of potable water is not always necessary. However, the water must comply with the drinking water standards of the Public Health Service or those of the agency that is the governing regulatory agency. Mormally, the water must contain less than 2.2 per 100 ml of E. coli to be considered safe to drink. The coliform group of microorganisms, generally associated with fecal matter, is one of the many pathogenic or disease causing organisms. Note that if 100% sterilization is required, certain sizing dictates must be used.

Factors Involving Water Purification

 The ultraviolet output of the lamp is dependent upon the primary voltage output supplied to the transformer or ballast used. The effect of varying line voltage can be seen in the following chart: 

 Likewise, lamp wall temperature can have a serious effect on the output of 253.7 nm energy. For instance, a wall temperature of about 56° F. will reduce the relative percentage output to about 22%. Typical design dictates the use of a high transmission clear fused quartz jacket so there is no direct contact between the lamp and the water. This allows the optimum temperature of 104° F. to achieve 100% ultraviolet output.

Another important factor to be considered is the useful life of the lamp. The rated life of the UV lamp deteriorates slowly with use since the glass loses its ability to transmit the UV energy. This is called solarization, and although the lamp will continue to light, this created a false impression that the germicidal UV rays are working. All UV lamps have a useful life of about 10,000 hours, which is about one year of continuous operation. A scheduled periodic lamp replacement program is essential to ensure correct and safe operation. A UV monitor is available on most models to determine the UV output of the lamp on a continuous basis.

Procedures for Sizing UV Liquid Purifiers

 Assuming a proper voltage source, the proper Liquid Purifier can be determined for your application if the following points are known:

1.      The peak flow rate of the liquid in gpm, gph, gpd, or m³/h.

2.      Absorption coefficient of the liquid to be treated.

3.      Physical characteristics of the liquid.

4.      Ultraviolet energy level required for microorganism destruction.

Exposure

 About 95% of the ultraviolet radiation from the UV lampis emitted at 253.7 nanometer wavelength, which is the region in the spectrum nearest the peak of germicidal effectiveness. The power of a UV lamp is described in two ways: that of intensity and output.

1.      Intensity is the density, or strength of the ultraviolet rays at a point a specific distance from the lamp. Intensity is usually expressed in microwatts per square centimeter, which is a product of energy, time, and area.

2.      Output is the volume, or total amount, of ultraviolet light rays being emitted by the lamp in all directions. Output is usually expressed in watts of 253.7 radiation.

 

 High intensities of UV for a short period of time, or low intensities for a long period of time, are fundamentally equal in lethal action on micro-organisms. Fuller Liquid Purifiers typically use high intensity lamps to achieve their lethal action quickly and efficiently.

UV Transmission Characteristics

 Successful treatment of water and other transparent liquids with UV depends on a number of factors, such as clarity, absorption of ultraviolet, amount of suspended matter, flow rates, temperature and, of course, the type or kind of microbial contamination.

 The transmission characteristics of UV on liquids are dependent on the depth of the liquid, as well as the absorption coefficient of the liquid. It’s interesting to note that the ultraviolet intensity through liquids decreases logarithmically with the distance from the lamp. In addition, the absorption coefficient further relates itself to the presence of minute traces of iron compounds and organic matter.

  The physical requirements of less than 10 NTU of turbidity, 15 TCU of color and 0.02 ppm of iron should be met prior to installing a Fuller UV Liquid Purifier. In addition, pre-filtering of suspended matter to at least 5 microns in size is recommended for residential water supplies.

  As a matter of interest, for average tap water the absorption coefficient is .008. Cloudy water can be 0.50 or less. Obviously, the transmission capability of the liquid to be purified must be known for proper sizing.

  Most significant, waterborne micro-organisms are destroyed by dosage of less than 10,000 microwatt seconds/ cm² (mw/sec/cm²), however, the U.S. Public Health Service has published criteria stating that acceptability of an ultraviolet disinfecting unit is contingent on imparting a dosage of 16,000 mw/sec/cm². Fuller UV Liquid Sterilizers nearly double this dosage.

 When used as directed, Fuller Liquid Purifiers will provide a minimum dosage of 30,000 mw/sec/cm². The Fuller product line is manufactured in various sizes to accommodate different flow rates for a single pass through or a recirculation system, as in industrial storage tanks, hot tubs, spas, and swimming pools.

  In order for the ultraviolet rays to kill bacteria, they must directly strike the microorganism, and each microorganism must absorb a specific amount of energy to be destroyed. The dosage necessary to inactivate a microorganism is a product of time and intensity.  

Percent Transmission of 253.7nm for Water, Various Absorption Coefficients*

* As a measured percent of transmission with GL-100 UV Monitor. 
(Note: Monitor meets the US Bureau of Standards requirements.)

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