Researchers from around the world are working to develop alternative means to purify water through such technologies as deep-ultraviolet light emitting diodes (LEDs) and UV lasers.

These efforts would improve upon the existing water purification technologies that employ filters, chemicals, and conventional UV light sources.

UV light technology using mercury-arc lamps has been an effective means of purifying water for years, and its value has grown since it was discovered that chlorination alone couldn’t eradicate certain protozoan parasites such as Cryptosporidium, which causes a diarrheal illness.

UV light in wavelengths in the range of 250 nm to 275 nm can cause physicial changes to the DNA of a host of bacteria, rendering them inable to replicate. By using UV lamps on water containing such things as bacteria, viruses, spores and parasites, the microorganisms are rendered harmless and the water is sterilized.

Despite the effectiveness of conventional UV lamps, they have some disadvantages, which are driving the research into alternative UV technologies. The lamps do not have a long life and so maintenance is more frequent. They are also mercury-based, which creates the added concern about keeping mercury out of the waste stream.

Solid-state LEDs on the other hand are compact, robust, and last a long time. The issue holding back their use for water purification, was that it hadn’t been proven that LEDs could reach the required wavelengths to purify water.

Those concerns began to fade with the development of more powerful LEDs and the successful tests of disinfecting flowing water. LED manufacturer Sensor Electronic Technology, Columbia, S.C., following on work from University of South Carolina researchers led by Asif Khan, has developed deep UV LED in wavelengths ranging from 247 nm to 365 nm. The LEDs are produced on sapphire substrates using the company’s Migration Enhanced MOCVD growth process.

Yuri Bilenko, Optoelectronic Products Manager at SET, says that the LED technology to inactivate bacteria in flowing water is in its prototype stages, but companies are working to incorporate the technology into their products. “This technology is very young,” he says. “The first proof-of-concept devices happened 2-1/2 years ago, and since then we have made a lot of progress.”

In 2005, a team that included SET, microbiologists at the University of Maine, and researchers at Blue Hill, Md.-based Hydro-Photon, makers of the Steri-Pen portable UV water purifiers, were able to demonstrate bacterial destruction in flowing water using LEDs made from aluminium gallium nitride. This particular combination of alloys, aluminium nitride and gallium nitride, works best in generating wavelengths down to 250 nm.

The LED used in the test could emit at wavelengths of 280 nm and was able to test sterile tap water flowing at 38 ml/minute contaminated with E. coli at a concentration of 10,000 microbes per milliliter. The UV LEDS destroyed at least 95.5 percent of the microbes.

In a subsequent test using improved LEDs, Hydro-Photon was able to reduce the level of E.-coli spiked water flowing at 150 ml/minute and 300 ml/minute by 99.99 percent and 99.0 percent.

The deep-UV LEDs, those with wavelengths below 300 nm, have a lot of advantages over the mercury-arc lamps, says Bilenko. “With a standard mercury lamp that produces UV light, you have to switch it on, warm it up and then it should stay in the ‘on’ condition because the ‘turn-on’ and ‘turn-off’ time is very long,” he says. “This is a digitally controlled device and we can use it exactly when it is needed.”

LEDs also have a much longer life span; they can operate for up to 100,000 hours. UV fluourescent tubes have an estimated life span of 4,000 to 10,000 hours. This means less maintenance and fewer times when the unit is off line.

Laser point-of-use

Additional alternative research on UV-based water purification focuses on laser  technology. SALTech, Albuquerque, N.M., developed a UV laser designed to treat water at the point of use. The company says that the laser allows increased UV exposure by providing a more focal irradiation of pathogens in a smaller process stream.

As water passes through the treatment area, microorganisms contained in the fluid are subjected to light reactive at a predetermined wavelength. For instance, the DNA of Giardia has been shown to break down at doses on the order of 2mJ/cm2.

Another company, Britain-based Biotech Industries, Ltd. offers UV Excimer Laser Water Sanitizers disinfection solutions for light commercial and residential applications. These UV-based water-treatment systems are capable of treating small to mid-size flow projects at the point-of-use.

Point-of-use treatment was the main idea in recent report from the World Health Organization. The agency pointed to point-of-use (POU) technology as a key weapon in the fight against waterborne diseases.

Peter Censky, executive director of the water quality association says that the report reaffirms other findings. “Point-of-use technologies can serve as a final barrier to waterborne diseases,” he says, “as well as environmental contaminants that enter through the water supply.”

For more information, email:
Biotech Industries Ltd            : info@biotech-ind.co.uk
Hydro-Photon: info@steripen.com
SALTech Corp: saltechinfo@saltechcorp.com
Sensor Electronic Technology: sales@s-et.com