Forget chlorine, says M&E water treatment process expert Larry VandeVenter. Ultraviolet disinfection is often a much better choice.

M&E Insight talked recently to Larry VandeVenter, the firm’s leading water treatment process expert, about chlorine, ozone, and the disinfection technology that he sees as most likely to dominate the market in the future.

M&E I: So what’s wrong with chlorine, Larry. It’s been used successfully for decades.

LV: You’re right – chlorine has long been the disinfectant of choice for water treatment operations. But in recent years, I think we’ve concluded that chlorine’s negatives may outweigh its positives. It generates halogenated compounds (or disinfection by-products) with aquatic toxicity as well as carcinogenic and mutagenic activity that’s problematic for potable water supplies.

M&E I: So is this why we hear more about ozone these days?

LV: I’d say that ozone certainly looked to be the most promising alternative disinfection agent for inactivation of bacteria, viruses, Giardia, and Cryptosporidium. Recently, however, ozone’s been shown to be not as effective as previously thought for inactivation of Cryptosporidium, especially in cold water, requiring higher dosages and longer contact times.

M&E I: So you’re keener on what?

LV: Ultraviolet disinfection. Recent tests show it to be more effective than previously thought for inactivation of Giardia and Cryptosporidium. And it generates negligible by-products. When you couple that with small space needs and competitive costs, UV starts to looks like a genuine alternative

M&E I: Would this hold for wastewater treatment too?

LV: Very much so. The problem with chlorine when it’s used to disinfect wastewater effluent is that even at extremely low concentrations it’s toxic to aquatic organisms. This is what prompts treatment plants to add dechlorination facilities to reduce those chlorine concentrations. However, the regulatory environment is getting much tighter and residual chlorine in treatment plant effluent is being limited to incredibly low concentrations. Particularly in the long-term, if you want to ensure meeting the regulatory criteria, you’ve got to be looking at alternatives to chlorine.

M&E I: And UV is one of those alternatives?

LV: Definitely. A proper dosage of ultraviolet radiation is an effective bactericide and virucide while not contributing to the formation of toxic compounds. There’s also a security issue – at least in comparison to using gaseous chlorine. With UV, you don’t have to transport and store potentially lethal gases.

M&E I: Care to explain how UV works?

LV: Here’s a summary version: Ultraviolet light is a physical (as opposed to a chemical) disinfecting agent, generated by a mercury vapor lamp that’s been charged by striking an electric arc. It penetrates the cell wall of the microorganism and is absorbed by cellular materials producing changes in DNA and RNA that either prevent replication or cause the cell to die. Cellular damage proceeds via a series of photochemical reactions. The distance over which ultraviolet light is effective is very limited; microorganisms which pass near an ultraviolet lamp are more likely to be killed or inactivated than those that don’t come near a lamp. So typically, multiple lamps are grouped horizontally or vertically in modular units, either immersed in a flow through tank for wastewater applications or in-line in pressure pipe for drinking water disinfection. Computational fluid dynamic modeling (CFD) is being used to optimize the hydrodynamics of UV contactors and confirm disinfection contact time.

M&E I: Thanks, Larry. Finally, anything to stop UV taking over?

LV: Well, on the water side, you’re not going to need a separate disinfectant (whether that be chlorine, ozone, or UV) if you use membrane filtration. But although membrane treatment is increasingly popular, it’s just not necessary for everyone – so UV plus conventional filtration looks like a good bet in many communities for the foreseeable future.

M&E I: Thanks, Larry. If anyone has questions about UV or other treatment technologies and they’d like to contact you, what should they do?

LV: The best way is to write a question to the M&E web site mailbox. As long as you mention my name, I’ll get in within a day or so.