Ultraviolet disinfection shines on drinking water
"The reported waterborne disease outbreak in Walkerton, Ontario,……in which there were 11 deaths and more than 1,000 people infected in a population of 5,000……may very well illustrate the potential hazard of a single disinfection barrier for municipal water supplies susceptible to microbial contamination." – Dr. R.D. Samuel Stevens, UV Workshop held June 10, 2000.
By Frederick W. Pontius, P.E., Pontius Water Consultants Inc.
Contents
Developing the concept of ultraviolet disinfection
Many water systems are using UV technology, and more will follow
Benefits of UV disinfection
Additional research is still needed
UV may be used to meet regulatory compliance objectives
A water utility's worst nightmare come true is a waterborne disease outbreak. At a time when the need to ensure the microbial quality of drinking water is greater than ever, ultraviolet disinfection is emerging as an important treatment option.
UV has been applied for many years for wastewater disinfection. After years of research and development, the major barriers to the use of UV for drinking water disinfection have been removed; now UV may make sense for water systems considering additional disinfection barrier.
Developing the ultraviolet disinfection technology (top)
The germicidal effects of radiant energy from the sun was first reported in 1878. Practical application of UV was ushered in by the development of the mercury vapor lamp as an artificial UV source in 1901and recognition of quartz as the ideal lamp material in 1905. The first experiments applying UV to disinfect water were conducted in Marseilles, France in 1910.
Many water systems are using UV technology, and more will follow (top)
In 1996 about 2,000 water systems in Europe, and about 1,000 water systems in the U.S. reportedly used UV disinfection technology. Since then, the number of applications of UV technology in the U.S. has increased, and its use is expected to escalate even more. What has changed to kindle this interest in UV?
Prior to 1998, the perception was prevalent that UV was ineffective against Protozoan cysts such as Cryptosporidium. Research by Bolton et al.1 presented at the June 1998 American Water Works Association conference changed this perception forever.
Cryptosporidium oocysts are inactivated by medium pressure UV (i.e., broadband emission) with efficiencies similar to that observed with E.coli 4 log inactivation with fluences (UV doses) of 10-20 mJ/cm2. Subsequent research has demonstrated that low pressure UV (i.e., monochromatic emission; 254 nm) is equally effective for Cryptosporidium inactivation.
Benefits of UV disinfection (top)
UV has several positive attributes. UV photons are absorbed by DNA in bacteria and other pathogens, preventing replication of the organism—an organism that cannot replicate cannot infect. UV disinfection is effective for a wide variety of virus and bacteria over a relatively narrow dose range. Microbial inactivation rates by UV are neither pH nor temperature dependent. In general, UV disinfection systems only require a short residence time hence they have a small footprint. Commercial systems are modular, and can be easily expanded or upgraded.
UV may allow less use of oxidizing chemicals and perhaps produce less disinfection byproducts. In addition, the use of multiple disinfectants that act in different mechanistic ways (such as ozone-UV-chlorine; or UV-chlorine-chloramines) may provide the best protection against known and emerging waterborne pathogens. Capital and operating costs, though site-specific, are generally very compelling compared to other treatment alternatives for equivalent levels of microbial protection. UV will be a reasonable disinfection option, especially for small or very small drinking water systems.
Additional research is still needed (top)
Additional research and development of UV is still needed to fine-tune the technology. The design of inlet and outlet hydraulic conditions must be improved to ensure proper performance at large scale installations. Also, a good U.S. validation protocol and test facilities that handle both UV reactors and UV sensors are needed.
Large water plants may be able to cost effectively manifold together pre-tested modular UV reactors. Where this is not possible, custom UV system designs and costs will be very site specific. Designers of customized large UV systems must pay particular attention to ensure proper hydraulics and correctly operating sensors. UV system designs must ensure that an on-line failure (lamp explosion) cannot occur leading to mercury release into the drinking water.
The principal limitation of UV disinfection is the lack of a residual. Chemical disinfection will be needed following UV to provide residual protection in the water distribution system. Unfiltered surface water systems are allowed a 5 NTU turbidity exceedance allowance. Natural particles present at 5 NTU turbidity may shield or enmesh organisms, thereby affecting UV disinfection performance.
UV may be used to meet regulatory compliance objectives (top)
Although UV is not designated as a best available technology under U.S. drinking water regulations, it may be used to meet regulatory compliance objectives, with approval of the State Primacy Agency. Hence, UV will be included in the compliance strategy for many water systems in meeting the objectives of the forthcoming Ground Water Rule and Long Term 2 Enhanced Surface Water Treatment Rule, both rules now under development by USEPA.
A suspected factor in the Walkerton, Ontario, incident was a breakdown of the chemical disinfection barrier. UV will not eliminate the need for other disinfectants, nor should it be added at the expense of proper operations and maintenance. But could the addition of UV disinfection have prevented the tragic outbreak at Walkerton? The answer to this question will forever remain a matter of speculation. Even so, the proper role for UV is emerging as an important drinking water treatment option within the overall objective of public health protection. UV is now a technology that water utilities should consider.
References
About the Author:
Frederick W. Pontius, P.E., is president of Pontius Water Consultants, Inc., Lakewood, CO, specializing in drinking water regulatory affairs, compliance, water quality and treatment. Fred has 20+ years in the water and wastewater industries. He is a frequent conference and seminar speaker, was a former staffer with the American Water Works Association from 1982 to 1999, and is a contributing editor to the Journal of the American Water Works Association (since 1991). He may be reached at fredp@pontiuswater.com.
See what other topics our drinking water regulatory expert has addressed in his column, Getting the Inside Edge with Fred Pontius.