By Nadia Abboud

he U.S. water industry is highly regulated by federal, state and local governments to promote public health. To comply with regulations, water systems often incur significant costs. Of the 170,000 public water systems regulated under the Safe Drinking Water Act, small water systems have the most difficulty complying. Defined as serving a population of 500 to 3,300, a small water system typically has a small customer base and less access to outside federal and state capital to finance infrastructure improvements.

Small systems with little capital resources, however, are just as responsible for producing and distributing safe drinking water to the public as a large municipality. Typically, small water systems have adopted scaled down models of purification technologies commonly used at larger drinking water treatment facilities. But all systems aren’t alike, and what works on a large scale may not be as efficient or economical on a small system. One disinfection alternative that can be effective without forcing a large-scale technology on a small-scale system is ultraviolet (UV) disinfection.

A practical alternative
The use of UV light for disinfection has become an increasingly popular, economical and safe alternative to traditional disinfection technologies. A UV system is comprised of a simple design—a stainless steel chamber, a power supply, a protective quartz sleeve and a UV lamp. The system works by emitting a wavelength at or near the peak germicidal wavelength of 260 nm into the flowing water. As UV light penetrates the cell wall of passing microorganisms, it’s absorbed by their DNA or RNA, their nucleic acid strands—or genetic sequence—and prevents the reproduction process. Without the ability to reproduce, a microorganism is considered “microbiologically dead” or inactivated. UV doesn’t affect the chemistry of the water and doesn’t remove any beneficial minerals from the water; however, UV lacks a residual effect, making it necessary to add chemicals such as chlorine to achieve a residual for continued disinfection in the distribution system.

UV considerations
When evaluating UV technology, a small water system operator—or professional water treatment dealer serving the system—should consider the following characteristics of their water:

  • Transmittance—The ability of UV energy or germicidal wavelengths to pass through water based on a one-centimeter distance. Typically, a transmittance value of 85 percent or higher is recommended for the use of UV to disinfect water.
  • Flow rate—Since every UV system is designed to treat a maximum flow rate based on a specific transmittance value, it’s important to size and install a system of adequate size to accompany maximum flow rates while achieving the required UV dose.

A typical UV drinking water system is rated to produce a UV dose of about 40 milliJoules per square centimeter (mJ/cm²) at the end of lamp life. Standard lamp life is approximately 8,000 to 12,000 hours. It’s important to install a unit that produces the proper dose in order to deactivate target bacteria and other microorganisms and produce a log inactivation in compliance with local, state and federal regulatory standards (see Table 1).

Cost consciousness
For the financially conscious small water system operator, the ease of maintaining and supporting UV disinfection is attractive. Most systems are designed with instrumentation and other features such as quartz sleeve wiper mechanisms, lamp out alarms, heat sensors and UV intensity monitors. Able to run with minimal operator intervention, a UV system requires a small system plant operator to perform routine annual maintenance such as changing out the lamps and the quartz sleeves when nearing their recommended life usage.

Conclusion
The combination of an effective, economical and safe alternative disinfection technology has made UV an attractive choice for small water systems. Through continued education and exposure, this once emerging technology has recently been upgraded to a best available technology (BAT) with the potential of becoming a standard disinfection choice in a small water system’s effort to produce and distribute safe drinking water to the public.

About the author
Nadia Abboud is marketing coordinator for Filtration, Disinfection and Bottling products for Severn Trent Services, of Fort Washington, Pa. Severn Trent offers UV disinfection solutions through its UltraDynamics® low pressure UV and FrontLine™ medium pressure UV product lines. She can be contacted at (215) 997-4000, (215) 997-4062 (fax), email: info@severntrentservices.com or website: www.severntrentservices.com

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