UV offers hope for safer drinking water

centered on 254 nm, and sometimes referred to as ‘germicidal UV’ lamps. But the new EPA rules call for higher doses, and attention has shifted to medium-pressure sources, which produce a much broader UV spectrum, including wavelengths below 240 nm, and offer potential energy savings. But the effects of the shorter wavelengths on pathogens have not been well characterized. For some microbes, only a single study has been done until now.”

Those data suggest that there is a dramatic disparity in the inactivation of various microbes at different sub-250 nm wavelengths. Earlier this year, a water-project research group charged with studying those effects asked Larason if PML could provide precise UV doses from NIST-calibrated devices to various bacteria and viruses to determine their action spectra. Larason took that request to PML’s SIRCUS (Spectral Irradiance and Radiance Responsivity Calibrations Using Uniform Sources) facility, which employs continuously tunable lasers as irradiation sources. Within a short time, SIRCUS staff took a portable laser and associated apparatus to the project test lab in Vermont for studies scheduled to conclude at the end of this year.

The release notes that the SIRCUS equipment emits radiation from 210 nm throughout the experimental range of interest in the form of a nearly collimated beam that strikes the microbial samples, which are kept in Petri dishes placed below the beam exit.

“At this stage,” Larason says, “we’re providing the equipment and expertise to help the project find the real dose-response characteristics for different microbes at short wavelengths. Among other things, that will determine how much power you need in the MP lamp, which in turn influences energy costs. After that, we might end up getting involved in devising calibration and validation standards for sources and sensors in the range of 200 nm to 300 nm. But it’s too soon to say where all this will lead.”

It is not, however, too soon for the American Water Works Association to express its appreciation.

In a September 2012 letter to PML director Katharine Gebbie, the association praised the “unique expertise and tools” brought to the project by Larason along with Keith Lykke, Steven Brown, Ping-Shine Shaw, and Mike Lin of SIRCUS. Their work “is providing information critical to our

understanding of pathogen inactivation by low-wavelength UV spectrum” that will “define treatment design for medium-pressure UV treatment in drinking water across the United States,” the letter said.

Thanks to the contribution of the NIST researchers and SIRCUS equipment, the collaboration has determined the wavelength responsivity of specific pathogens and associated surrogates with greater accuracy.

“Using NIST’s tunable UV laser, we have developed the gold standard on measuring the wavelength response of test microbes and waterborne pathogens for UV disinfection applications across the U.S.,” says Harold Wright of Carollo Engineers, Inc. in Boise, ID, a contributor to the research project.

“I have worked with Tom Larason and the folks from NIST on two UV disinfection projects sponsored by the Water Research Foundation. With both projects, they brought to the table a level of expertise in the application and measurement of ultraviolet light that is unparalleled in our industry.”

The collaboration may also have repercussions beyond the issue of drinking-water safety. “It expands current research areas with minimal investment in new equipment and manpower,” Larason says. “But it is also applicable beyond microbiology to other technological areas such as material processing (UV curing), medical (testing devices that measure UV exposure), and expanded calibration capabilities for irradiance and dose.”