A Scientist Proclaims There Is Much More to UV Than Meets the Eye

Award-winning researcher Karl Linden sees big potential for ultraviolet light, not just for disinfection but to treat pharmaceuticals and even attack Covid.

A Scientist Proclaims There Is Much More to UV Than Meets the Eye

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The value of UV light for disinfection is well known. It’s highly effective against a broad range of pathogens, and it eliminates the chemical costs and the handling risks of disinfecting with chlorine.

But there’s more good news about UV, according to Dr. Karl Linden, a civil engineering professor at the University of Colorado Boulder. It’s already part of a treatment process for pharmaceuticals in reuse water. It could come into play against the virus that causes COVID-19. And UV LED lamps are on the horizon.

Linden last November received the Clarke Prize for Excellence in Water Research from the National Water Research Institute for his work with UV technology. He began his academic work in 1992 as a research assistant at the University of California, Davis; he received his doctorate degree in environmental engineering in 1997. He then moved to professorships at the University of North Carolina and Duke University before arriving at University of Colorado Boulder in 2008.

Linden and colleagues have made breakthroughs that influence how clean-water plants deploy UV disinfection. He has published or contributed to hundreds of papers on the topic. His studies include investigating low-cost, compact UV systems for developing countries and remote areas.

In 2012-15, Linden and his research team worked with the Bill & Melinda Gates Foundation to develop a toilet that focuses sunlight to disinfect waste and produces useful fuel called biochar. Among numerous honors, Linden earned the 2013 Pioneer Award in Disinfection and Public Health from the Water Environment Federation. He talked about his work and the role of UV in an interview with Treatment Plant Operator.

How did you become interested in UV?

Linden: It started when I was in graduate school, working on a wastewater treatment project at UC Davis. We were piloting different types of filters and disinfection systems and working with a UV system. A project opened up in UV for applications in wastewater reuse. To me it made a lot of sense, and I was really excited about the technology.

What was it about UV that you found so fascinating?

Linden: The way it works is really interesting. It uses the energy of photons in light to achieve disinfection. It doesn’t use chemicals, and there are no byproducts that form. It was an interesting field scientifically, and it had a lot of promise from a sustainability standpoint. It was fascinating to explore the interaction of light with matter, whether pathogens or chemicals.

How exactly does UV light deactivate pathogens?

Linden: The target of the photons is actually the nucleic acids — the DNA and RNA of the pathogens, whether bacteria, viruses or protozoa. Nucleic acids absorb UV light very strongly. The light energy breaks the bonds that hold the DNA double helix together. That destroys the organism’s ability to replicate itself, and so it can’t cause infection.

What are the benefits of UV from an operator’s perspective?

Linden: UV works in a matter of seconds. Instead of having a 30- to 60-minute contact time in a chlorination chamber, you’re talking about one to five seconds that the water is in the UV chamber. That’s a paradigm shift in operation and maintenance. And there is no danger of operators getting exposed to UV light, because the UV lamps are always contained in a channel or a vessel of some sort.

Does the technology use any specific type of UV light?

Linden: Not all photons get absorbed by nucleic acids. It has to be a certain wavelength of light. UV at 254 nanometers is one of the best wavelengths, and that is the typical output of a conventional UV lamp.

Does UV work effectively on Cryptosporidium and Giardia?

Linden: After the Cryptosporidium outbreak in Milwaukee in 1993, everyone was looking for solutions for its inactivation. Chlorine is completely ineffective. At the time there were some studies going on with Crypto and UV, and it turned out that UV was very effective against it.

Why was UV at first thought to be ineffective against Crypto‑­­sporidium?

Linden: It used to be “common knowledge” that UV didn’t work against Cryptosporidium and Giardia, or that it took an extremely high dose that wasn’t realistic. But basically, people were asking the wrong question. UV doesn’t kill organisms. It inactivates them. If you assay and in effect ask the organism, ‘Are you alive?’ after it got hit by UV, it might say, ‘Yes, I’m alive.’ But if you ask, ‘Can you replicate?’ it would say ‘No.’ Once people did activity studies with Crypto, they realized that UV actually worked very efficiently against it. After that point, UV was clearly the best available technology for Cryptosporidium inactivation. In fact, it’s easier to inactivate Crypto than to kill E. coli with UV.

How does UV compare with ozone for treating Cryptosporidium and Giardia? 

Linden: Ozone also works with both, but it works differently. Ozone is a chemical oxidant that tears apart the cell membrane, the cell wall and the oocyst structure. UV goes through the cell membrane and destroys the cell from the inside out. Ozone takes quite a high dose for Giardia and Crypto inactivation as compared to viruses and bacteria.

Are you saying that UV is more cost-effective than ozone for that purpose?

Linden: I definitely believe UV is more cost-effective than ozone for disinfection of Crypto, because Crypto is basically the easiest thing to kill with UV — it requires one of the lowest doses possible. But ozone and chlorine might be more cost-effective for viruses, which require a higher UV dose than for Crypto. There are tradeoffs with all technologies.

How does UV figure in treating pharmaceuticals in wastewater streams?

Linden: That’s a different process in that you have to add a precursor chemical, typically hydrogen peroxide. It’s an advanced oxidation process. UV light gets absorbed by the peroxide; it breaks down to form hydroxyl radicals that then destroy the pharmaceuticals.

Is this type of process commercially viable or still experimental?

Linden: It’s completely viable, and it’s actually one of the most standard processes in wastewater reuse, especially for potable reuse. Usually the UV and hydrogen peroxide process is added after membrane treatment, such as microfiltration, to clean up any pharmaceuticals and other organic contaminants that got through the membrane. It’s also used in drinking water to treat taste- and odor-causing chemicals.

How might UV light be used to help control the spread of COVID-19 virus?

Linden: UV is very effective against viruses, and there are wavelengths of UV that are less harmful to human skin and eyes. At wavelengths below 230 nanometers, the light actually doesn’t penetrate into the skin much farther than the very top layer, so there is less risk of causing cancer and other mutations. You can use those wavelengths in public spaces to disinfect aerosols between people, and also to disinfect surfaces.

Where and how might UV light be deployed in those applications?

Linden: It could be used inside the air-handling units of HVAC systems. It could also be used in the spaces around the ceilings of rooms. If there is good airflow so that the air gets up to the ceiling levels, you could end up killing a good portion of any pathogens that might be in the air at the time. That could be helpful for the control of respiratory viruses like the COVID-19 virus.

Would there be any limits on UV light exposure for health and safety reasons?

Linden: There are certain regulations around the amount of exposure you’re allowed to have per day. But you can effectively disinfect viruses while adhering to those maximum exposure levels. You find the sweet spot where you’re not overdosing but have enough UV light to kill the viruses on surfaces and in the air.

Where else might UV technology contribute to water and wastewater systems?

Linden: We’re looking at applications for UV LED technology. Usually UV lights are tube lamps, but LED technology is just really small points of light. You could think about distributing those LEDs throughout a network of pipes, for instance, or at certain booster spots in a system. UV in water holding tanks might be another application. In New York City, every building has a water storage tank. Lots of microbial changes can go on in those tanks, and you could have UV systems inside them. The nice thing about LEDs is that they are DC-powered, so you can run them off of solar panels. They are much more flexible in how you operate them and much more robust and less sensitive to breakage.

Is it possible that UV LEDs could eliminate the need for a chlorine residual in drinking water distribution systems?

Linden: There are a lot of questions to answer first, but that’s one of the visions I have. It certainly is possible to design a treatment system so you don’t need a chlorine residual; many countries don’t use chlorine as a secondary disinfectant. In those cases there could be a role for UV at the point of entry to a building, or at the point of use in a faucet. Using very small LEDs, you could actually protect the public from any unknown potential contaminants that might get into a distribution system where you don’t have a chlorine residual.

Will UV LEDs mean lower energy costs for disinfection?

Linden: That’s the hope. They are definitely ready for prime time for operation now. The energy efficiency is constantly improving. Regular UV lamps are pretty efficient. LEDs aren’t quite there yet, but they will be. It’s just a matter of time.   



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