Search Menu
Photonics Media Photonics Buyers' Guide Photonics EDU Photonics Spectra BioPhotonics EuroPhotonics Industrial Photonics Photonics Showcase Photonics ProdSpec Photonics Handbook
More News
Email Facebook Twitter Google+ LinkedIn Comments

UV Water Treatment for NYC?

Photonics Spectra
Oct 2001
Richard Gaughan

To ensure the quality of the nation's drinking water, the Environmental Protection Agency is enforcing increasingly strict treatment regulations. In the case of New York City's water supply, this could mean a filtration plant in the upstate Catskill/Delaware watershed costing anywhere from $4 billion to $8 billion. But an improved understanding of the effectiveness of ultraviolet water treatment has led the agency to provide provisional approval for a UV system that would cost only about $150 million.

New research reveals that ultraviolet-based water treatment can be effective against Giardia and Cryptosporidium, making the inexpensive technique an even more inviting solution for large cities such as New York.

"UV is very effective against difficult-to-kill organisms, and it doesn't add by-products like chlorine and ozone treatments can. In addition, if you're building a new facility, UV is extremely inexpensive," said Orren Schneider, a senior principal engineer with Hazen and Sawyer, a New York City firm that has been designing water treatment systems for 50 years and is now advising the city. Schneider emphasized that UV is only one tool available for water treatment.

UV irradiation is effective against bacteria at fluence levels of 3 to 30 mJ/cm2 and against viruses at 30 to 100 mJ/cm2. However, it was thought that protozoa such as Giardia and Cryptosporidium were insusceptible to this approach. This inaccurate perception was linked to assays that evaluated UV treatments based on the physical damage they caused to the organisms' outer surfaces. Studies measuring infectivity, however, confirm that UV radiation penetrates the outer walls of Giardia and Cryptosporidium, causing internal damage and eliminating their threat, even when they are exposed to fluence levels in the tens of millijoules per square centimeter.

A typical UV treatment system flushes water through a reactor vessel, where a UV lamp irradiates the flowing water. Ideally, uneven distribution is engineered out by radial mixing within the reactor, so that all objects moving through it receive equivalent irradiation, according to Thomas Hargy, a senior scientist with Clancy Environmental Consultants in St. Albans, Vt., the group primarily responsible for re-evaluating the effectiveness of UV against protozoa.

Light sources can vary widely. They encompass high- and low-vapor- pressure lamps, broadband sources covering the germicidal region from 200 to 300 nm, and narrowband emitters targeting sensitive wavelengths.

But, generally, the effectiveness of any method appears related only to total delivered dose. "For most organisms, 10 mW/cm2 for 10 seconds is as effective as 1 mW/cm2 for 100 seconds," Hargy said.

Research & TechnologyTech Pulse

Terms & Conditions Privacy Policy About Us Contact Us
back to top
Facebook Twitter Instagram LinkedIn YouTube RSS
©2019 Photonics Media, 100 West St., Pittsfield, MA, 01201 USA,

Photonics Media, Laurin Publishing
x Subscribe to Photonics Spectra magazine - FREE!
We use cookies to improve user experience and analyze our website traffic as stated in our Privacy Policy. By using this website, you agree to the use of cookies unless you have disabled them.