UV Water Treatment for NYC?

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.