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Why UV Water Purification?

History – UV is not a new technology: UV disinfection is an established technology supported by decades of use in applications from drug manufacturing to waste water treatment. The germicidal properties of sunlight were discovered by Downes and Blunt (1877). Once it was understood that UV light was the wavelength responsible for this germicidal activity, the development of mercury lamps as artificial UV light sources in 1901, and the use of quartz as a UV transmitting material in 1906, paved the way for the technology to be developed and used in a controlled and meaningful way. The first drinking water disinfection application took place in Marseilles, France in 1910 and considerable research on the mechanisms of UV disinfection and the inactivation of microorganisms has since been completed.

Why use UV?: UV is effective at inactivating bacteria, viruses, and protozoa such as Cryptosporidium and Giardia which may be present in water supplies from all sources. Many people believe that well water is pristine, glacier water is pure and Municipal water is treated to safety standards specified by regulatory bodies. While all of this is generally true, even these “good” sources of water may be contaminated.

Groundwater quality can be degraded by failing septic systems, animal farms and many other sources. Groundwater in the aquifer is continuously moving, which makes contamination intermittent. It can test “good” today but fail tomorrow. People who fall sick tend to blame it on the food they ate or some other explanation because they believe their water is safe.

Maps of areas where the micro-organism contaminants are highest show that it can be at the base of mountains where the “pure” water from mountain streams is collected. Yet the water picks up contaminants on its journey down the mountain that can create a need for disinfection.

Cryptosporidium and Giardia protozoa have become more evident in drinking water sources. The occurrence of Cryptosporidium parvum in drinking water sources is recognized as a significant threat to private and public water supplies throughout the world (Rose et al., 1991; Lisle and Rose, 1995; Messner and Wolpert, 2000). Water treatment plants cannot usually guarantee the removal of all Cryptosporidium  from water because the oocysts are very small (4-5 micrometers in diameter) and are resistant to chlorine and other disinfectants. (Omar A. Khan) It is for this reason that many Municipal treatment plants are installing UV systems.

A report published by the USEPA, (EPA-822-R-01-009, March 2001) indicates that “…Cryptosporidium is not only a Surface water problem.” In Canada and the US, 60.2% of surface water samples contained oocysts in a study done by LeChevallier and Norton in 1995. The same report also cites a study done by Hancock et al (1998), reporting a study of 199 ground water samples tested for Cryptosporidium. They found that 5% of vertical wells, 20% of springs, 50% of infiltration galleries, and 45% of horizontal wells tested contained Cryptosporidium oocysts. The significance of this is that normal water testing does not test ground water for oocysts.

The most challenging water source is the dug well. As runoff enters the well, it can carry with it such challenges as surface animal waste and septic drainage from the aquifer.

Municipal water can be deemed perfectly safe when it leaves the treatment plant. Nonetheless, “boil water alerts” happen frequently as a result of the unexpected. Many times the “BWA” is issued 24 – 48 hours after the contamination occurred.

Residential POE (point of entry) UV can be a primary barrier to protect people from contamination in a well that has failed a water test and it can be inexpensive insurance to others that think the water is safe all the time, but want to be sure that their family is fully protected.

How does it work?: Ultraviolet light, in the 200 to 300 nm (UV-C) range is most effective at destroying bacteria and viruses by altering their DNA.  This natural, non-chemical method of treatment penetrates and permanently alters the DNA of the microorganisms in a process called thymine dimerization. The microorganisms are “inactivated” and rendered unable to reproduce or infect.

Typically, UV light is generated by applying a voltage across a gas mixture, resulting in a discharge of photons. Nearly all UV lamps currently designed for water treatment use a gas mixture containing mercury vapor. Mercury gas is advantageous for UV disinfection applications because it emits light in the germicidal wavelength range. The UV light output from mercury-based lamps depends on the concentration of mercury atoms, which is directly related to the mercury vapor pressure. Low-pressure (LP) UV lamps contain mercury at low vapor pressure which produces primarily UV light at 253.7 nm. Variations of LP lamps are LPHO and Amalgam UV lamps which operate at higher current and use different gas mixtures to increase the amount of germicidal UV that is emitted. An easy way to think of the relative lamp power is, LPHO produces approximately 2 times as much UV light as an LP lamp and an Amalgam lamp produces about 4 times as much UV light as an LP lamp.

The advantages of the higher output UV lamps are that they allow reactors to be smaller and higher flow rates to be treated with one lamp.

Factors Affecting UV Performance: Once the UV light is generated, it begins its journey towards the micro-organisms that are its intended target. Along the way a certain amount of the UV energy is lost due to absorption, and scattering of the light.

Absorption happens in two significant ways. First a certain amount of the UV light is absorbed by the quartz that the lamp envelope and the sleeve are made from. It is important to be sure the UV manufacturer is using high quality quartz and that you replace lamps and sleeves with OEM parts to ensure consistent performance of your system.

Second, minerals like iron and manganese and, more especially, organic compounds like tannins, from decay of organic material in the water, will absorb the UV light, reducing transmitted energy and its ability to alter the DNA in the micro-organism.

Scattering is primarily caused by particles in the water. Particles can cause shadowing and some micro-organisms may pass through the UV reactor without receiving enough UV exposure to inactivate them.

Water Quality (Hardness and Iron) Depending on the concentrations of the minerals, the water’s hardness can also affect the performance of the UV system as water hardness can cause scale to form on the lamp’s sleeve. This can reduce UV light transmission and the inactivation of pathogens.

Flow rate The combination of the amount of UV energy and the flow rate of the water determine the contact time (CT) or UV dose. Each micro-organism has a different susceptibility to UV light and therefore requires a different dose of UV to be inactivated. To achieve successful UV disinfection, the UV equipment capacity must match the target microorganism’s UV dose requirement.

Micro-organism susceptibility to UV: Numerous studies have been done that indicate the UV dose that a micro-organism needs to receive to inactivate it. The attached chart is a sample of some of this work. Many universities and research companies continue to add to the literature as new information is discovered.

Pathogen

LampType

UV Dose (mJ/cm2)

Reference

3 log

4 log

Escherichia coli C LP

4.5

6

Tosa and Hirata 1999
Salmonella Typhi ATCC 19430 LP

6.4

8.2

Wilson et al. 1992
Shigella dysenteriae ATCC29027 LP

2

3

Wilson et al. 1992
Cryptosporidium parvum LP

<5

Shin et al. 2001
Giardia lamblia LP

<4

Mofidi et al. 2002

(UV Dose Required to Achieve Incremental Log Inactivation of Bacteria, Protozoa and Viruses1., revised and expanded by: Gabriel Chevrefils B.Ing, and Eric Caron, B.Sc., 2006 IUVA News/ Vol. 8 No.1)

Summary of Water Quality Parameters: Listed below are the common water quality parameters to ensure the best performance from a UV system in a residential or commercial application. If your water is outside these parameters you should contact a water treatment dealer for equipment recommendation.

  • < 0.3 PPM iron
  • < 7 grains per gallon (120 PPM) hardness
  • > 75% UVT (UV transmission through water sample)

UV light advantages:

  • Low lifetime ownership cost
  • No moving parts
  • Minimal maintenance requirements
  • No disinfection by-products which can occur using chlorine
  • No risk to children from having quantities of chlorine stored at your home
  • No change to the taste or smell of the water, as can happen with chlorine
  • No handling or mixing of chemicals

Specifications: Watch the specifications and be sure the system is capable of operating in a UVT range that your application may encounter. Be sure the system will provide a minimum UV dose of 30 mJ/cm2, at the end of the lamp life and pay attention to the UVT that this dose is quoted at. Many people do not know the UVT of their water. Class A certified systems are tested at 70% UVT or lower, and must provide 40 mJ/cm2, to meet the requirements for certification. If you compare UV systems, keep in mind this is a science, similar lamp power and water layers with similar chamber designs should have similar UV dose. Look at the industry leader specs as a check to see if what you are reading is “too good to be true”.

References:

1) US EPA report EPA 815-R-06-007 November 2006 ULTRAVIOLET DISINFECTION GUIDANCE MANUAL FOR THE FINAL LONG TERM 2 ENHANCED SURFACE WATER TREATMENT RULE

2) USEPA Report EPA-822-R-01-009, March 2001 Cryptosporidium: Drinking

3) Water Health Advisory A Review of Cryptosporidiosis – Omar A. Khan, Associate Faculty, Dept. of International Health,Johns Hopkins University School  of  Public Health,  Suite 310, 111 Market Place Baltimore, Maryland 21202-4024 USA