Out of the Dark

• Mar 01, 2002

Ultraviolet radiation (UV), long a staple disinfectant in wastewater treatment, is gaining recognition as a beneficial technique in disinfection systems for municipal drinking water. Focusing on the North American market, this article reviews basics of this technology and provides up-to-date selection and application guidelines for its use in municipal drinking water treatment plants.

To ensure the highest measure of safety to the nation's water supply, regulations governing water disinfection are always being reviewed and improved. Disinfection methods for water treatment evolve, and it is essential that municipal plant managers keep abreast of trends in treatment alternatives. Although the U.S. Environmental Protection Agency (EPA) approval for ultraviolet (UV) light for disinfection of potable water is only now emerging, the current domestic industry trend is positioning UV as a viable primary disinfectant.

Because of its simplicity, easy installation and retrofit, coupled with low operating and capital costs, UV disinfection is attractive. In Europe, UV has been used for years in producing potable water. Learning the basics of UV technology is the first step for a plant manager when recognizing this trend. Today, only about one percent of the 40 billion gallons of drinking water produced daily by public utilities in the United States employs UV disinfection.¹ However, over the next ten years, this figure is expected to explode.

Evolution of UV Use

The highly effective germicidal effect of ultraviolet light has been known for over 100 years. ² The first full-scale use of UV for disinfection was in 1910, for a wastewater treatment system in Marseilles, France. The invention of neon tubes in the 1940s provided a more practical source of UV light: the low-pressure mercury vapor lamp. A significant number of installations utilizing UV disinfection systems for the municipal wastewater market began taking place in the United States during the early 1950s.

By the early 1980s, UV disinfection of wastewater had gained popularity due in part to the heightened regulatory requirements affecting the use of chlorine within the wastewater treatment process. The new regulations required dechlorination prior to discharge, the installation of chlorine scrubbers to protect against accidental release of chlorine gas (Uniform Fire Code), and the development of risk management plans in case of accidental release (Occupational Safety and Health Act).

Due to the overall effectiveness and popularity of UV disinfection within the wastewater market, there has been a shift toward applying the same technology for the disinfection of drinking water treatment systems.

How UV Disinfects Water

Basically, UV disinfection works by exposing waterborne microorganisms to UV light at a specified intensity for a specified period of time. This process of producing germicidal UV light renders the microorganism, in effect, "microbiologically dead." It does so by penetrating the cell wall and affecting the DNA in such a way that the cell cannot reproduce. The UV effect is generally referred to as "inactivating" the microorganism.

The light wavelength most efficient for absorption by the cell is about 260 nanometers (nm). Common lamp technologies used today -- e.g., low-pressure, standard intensity (LP) and low- pressure, high output (LPHO) -- generate UV light at 253.7 nm; 90 percent efficient in producing germicidal UV light.

The intensity of UV light generated by a specific lamp is a function of the power applied to the lamp and the arc length of the lamp. UV dose is a function of UV intensity over a certain length of time, given a specific flow rate for the water being treated and a known reactor size. Its unit of measurement is microwatt seconds per centimeter squared (m ws/cm²). Most UV systems used for disinfection today were sized to provide a minimum dose of 30,000 m ws/cm² .

In 1969, the U.S. Department of Health, Education and Welfare, Public Health Service, Division of Environmental Engineering and Food Protection issued a guideline that UV systems should provide a minimum dose of 16,000 m ws/cm² when used to treat drinking water.

However, the current regulatory trend appears to be establishing a minimum acceptable dose of 40 milliJoules per squared centimeter (mJ/cm²) for UV use in potable water supplies. (See the below chart to reference UV dose translations for different units of measurement. The industry trend denotes UV dose as mj/cm²). Municipal water treatment typically requires a two to four log reduction by any specific UV dose.

40,000 µWs/cm²- microwatt seconds/ squared centimeter

40 mWs/cm²- milliwatt seconds/ squared centimeter

40 mJ/cm² - milliJoules/ squared centimeter

1 Joules is equal to 1 watt-second

Basic Design of a UV Disinfection System

There are a number of physical configurations in which UV can be used for disinfection of water⁵. A quartz sleeve surrounds the UV lamp, acting as a barrier to protect the UV lamp from exposure to moisture. In the type illustrated, the jacketed lamp is contained in a closed vessel (chamber) and the whole assembly, with power source (ballast/transformer) and other accessories, is called a " UV system."

For wastewater designs, the jacketed lamps are immersed vertically or horizontally in an open vessel or channel, with the water flow perpendicular or parallel to the lamp axis.

As a key requirement of all possible system configurations, a certain amount of turbulence is necessary to enable the full amount of water to come sufficiently close to the lamp for treatment and inactivation. Such considerations are referred to as the "hydraulics" of a UV treatment unit.

Over time, the lamp sleeve will quite likely become fouled with a coating from biological or chemical materials. As a result, the effectiveness of the UV light to penetrate microorganisms will be impacted. To ensure efficient use, the sleeve must be periodically cleaned manually or automatically by a suitable cleaning mechanism.

Type of Lamp Affects Type of Light

Common lamp technologies currently in use for both water and wastewater applications include

Low-Pressure, standard intensity (LP) and Low-Pressure, high output (LPHO): Both types of low pressure lamps produce monochromatic light with a wavelength of 254 nm for germicidal purposes; they can be adjusted, however, to produce light with a wave length of 185 nm for total organic carbon (TOC)-reduction. Pressure refers to the internal vacuum pressure applied to a lamp during the manufacturing process.

A higher amount of power is required to operate an LPHO lamp (800 mA) as compared to an LP lamp (400 mA). As a result, the LPHO lamp can release higher outputs of UVC light, in turn reducing the number of lamps needed to achieve a required dose. Typically, LP lamps are used for lower flow rates, while LPHOs are used for medium flow rates.

Medium-Pressure, high output (MP): This type of lamp produces a polychromatic light with a broad spectral band output ranging from 200 nm to long wavelength visible light. MP lamps are produced with a deeper internal vacuum pressure and require higher amounts of energy to operate. The polychromatic light produced is enhanced in the germicidal region. Research on the microbial effects of polychromatic wavelengths indicates that other wavelengths can be as effective against bacteria as the UV 254 emitted from LP lamps (4). Polychromatic lamps also reduce the ability of bacterium to repair themselves (photo-reactivation) after exposure to UV light. MP lamps are typically used for higher flow rates.

Pulsed UV (P-UV) Systems: These operate by converting alternating current (ac) to direct current (dc) and storing the electrical energy in a capacitor.² The energy is then released through a high-speed switch to generate intense radiation and UV light. Pulsed UV, as well as MP, has been found effective in inactivating persistent pathogens.⁶

Effects of Water Quality on UV Disinfection

Malley⁴ cites key factors that effect UV system design and application. He includes: number and type of lamps; reactor hydraulics; target organisms and level of inactivation required; location of the UV unit in the treatment process; and water quality characteristics.

Direct effects on water quality can come from suspended solids that have not been removed by filtration. As total suspended solids (TSS) increase, the UV disinfection of the water may be less efficient due to the possible shadowing effect of the suspended solids. The microorganisms may not receive the proper dose. In general, maximum TSS is recommended to be 30 milligrams per liter (mg/l).

Typically, indirect effects due to the water condition are those that can affect lamp performance or foul the quartz sleeve. Dissolved iron, water hardness or minerals can influence fouling of the quartz sleeve. Lamps that operate at higher temperatures, such as MP, amplify the fouling effect.

LP lamps operate most efficiently at 100 degrees Fahrenheit. As water temperature decreases, LP lamps tend to reduce their output unless compensated by the installation of additional lamps. If water temperature rises above 80 degrees Fahrenheit, the lamps should be cooled. One cooling method is to use a fan to blow air through the sleeve. MP lamps are not sensitive to temperature fluctuations and are found effective from zero to 80 degrees Celsius. Any UV system can have a temperature-sensing component to alert operators that a UV reactor is overheating.

The term transmittance is used to describe the ability of UV energy to pass through water, based upon a standard one-centimeter distance. It can be measured offline by a

spectrophotometer capable of reading a 254-nm wavelength of light. On certain UV reactor models, an online monitor measures transmittance of the water being treated; this is valuable because surface water can usually have a very wide fluctuation in transmittance.

A recommended transmittance value for water being treated is 85 percent or better. This value guides the selection of flow rate or number of lamps and/or distance between the lamps within the reactor chamber. For example, a lower transmittance value could be compensated by: decreasing flow rate, increasing the number of lamps and/or decreasing the distance between the lamps.

UV Monitoring and Control

All types of UV lamps require monitoring and control. UV monitors measure the fluence being emitted by each lamp. They are essential to maintain the integrity of the system.

(UV Fluence= Intensity x Time x % Transmittance)

UV fluence is the calculation of UV dose with transmittance taken into account. Flow meters provide information on the plant flows. Transmittance meters provide the calculations on the transmittance value of the water. UV monitors either measure the intensity of lamp output in absolute units of mw/cm² or the relative intensity in a percentage value. Data logging systems can be added to demonstrate the adequacy of treatment, and to provide a permanent record of the disinfection.

Benefits of UV Disinfection in Potable Water Systems

In citing proven or potential benefits of UV disinfection in municipal drinking water systems, we must assume first that the system designer has addressed and accounted for all the factors affecting UV use. Properly used, UV disinfection offers a number of advantages in municipal water treatment systems³; these may be summarized as follows:

1. With continued research currently being conducted to further quantify its effects, UV has been proven at a specified wavelength to inactivate Giardia Lambia cysts and Cryptosporidium Parvum oocysts -- harmful parasites that are difficult to render harmless by other disinfectants. And UV does this using a low, cost-effective dose. Recent findings, moreover, have confirmed similar effectiveness of UV for many other human pathogens, including resistant viruses⁴. UV is capable of effectively treating certain bacteria found to be unaffected by chlorine disinfection.

2. As a result of these recent findings, EPA has proposed that UV can be used as a best available technology (BAT) for treating drinking water supplies, thus removing it from the current category of an "emerging technology."

3. UV treatment itself leaves no residual and may need an additional terminal disinfectant, such as chlorine, to achieve a residual. The amount of chlorine required for residual is reduced post UV, reducing costs. Moreover, UV does not pose any of the safety concerns for management and handling when compared to traditional disinfectants such as chlorine, which can be extremely dangerous to workers who are exposed to accidental releases.

4. The contact times for UV disinfection systems are generally quite short - within seconds. The resulting compact UV units having a small footprint, allowing them to be easily added as part of an existing multiple-barrier system. A multiple-barrier system refers to a treatment process that consists of various stages of filtration and disinfection that water must travel through prior to reaching the point of distribution.

5. The potential is low for UV reactions to produce undesirable organic disinfection by-products (DBPs) that would be present in the treated water. UV intensities involved are less than those that can cause photochemical effects. Further, since there are no halogens like chlorine in this physical process, there are no direct by-products to worry about, such as THMs (trihaloamines) or HAAs (haloacetic acid). Similarly, UV cannot form undesirable bromates.⁴

Deciding on Use and Placement

How does a municipality decide whether or not to adopt this technology and, if so, where in the train of treatment processes would the UV system be placed?

Malley⁴sums up the approach taken by many drinking water organizations today who see the potential benefits of UV disinfection: they are employing bench, pilot and full-scale research on applying UV to help meet drinking water disinfection requirements.

Getting a Start

Westford's experience with UV started on a small scale with one installation at its Cote Well site. The Water Department hired a local engineering firm, Fay, Spofford & Thorndike, Inc. (FST) of Burlington, Mass., to help address a key disinfection concern: coliform bacteria detected in the distribution system near the well. FST conducted a technical feasibility evaluation of disinfectant alternatives, including UV. On the basis of the study, Westford's water department decided on moving forward with UV disinfection.

FST then developed a pilot plan and submitted it to the Massachusetts Department of Environmental Protection (MA DEP), which approved it. Note that such local regulatory bodies are a vital step in adopting a new technology and their standards vary widely throughout the United States.

FST then coordinated the installation of a 10 gallons per minute (gpm), low-pressure pilot UV system. Water Department personnel gathered samples of untreated and treated water on a daily basis for two weeks. The results indicated that UV was doing the job of inactivating the bacteria and MA DEP approved full-scale use of UV disinfection - a first for this type of installation in New England.

FST then proceeded with the installation of a 24-lamp, low-pressure UV system with a design dose of 60 mJ/cm² . The system was installed in March 1999 and operated successfully until presence of other microorganisms from surface water required that the UV treated drinking water undergo additional post-treatment with chlorine. The original UV system will be replaced when the centralized water treatment plants described below come into operation.

UV in a Multiple-Barrier Treatment System

In 1998, at a special town meeting, the water department appropriated $250,000 for feasibility studies and pilot testing to determine the optimum treatment method to use at the two new plants. Because of the success of the UV system at the Cote well, the department planned to apply for a waiver from the MA DEP to use UV technology for primary treatment in place of chlorine.

Westford's multiple-barrier treatment systems are expected to be in operation at two new water treatment plants summer 2003. These plants, designed and engineered by Dufresne-Henry Inc., will treat five million gallons of well water per day. They will be the culmination a decade of extensive investigation, field-testing, evaluation, planning and engineering. All the while, as an excellent public relations tool, the Water Department maintained and updated a Web site (www.westford.com) that kept townspeople well informed. Much of the following information came from that Web site.

The multiple barrier approach, to be used at both new treatment plants, was designed to cover all contingencies. Its series of treatment steps are expected to handle viruses, Giardia, Cryptosporidium, and other contaminants that could be introduced into the well water by surface water infiltration. While none of these contaminants has been detected in Westford's water supply, they intend to be prepared if contaminants ever do infiltrate the wells.

Uniquely, Westford arrived at the final multiple barrier design of as the result of a special value engineering workshop. The objective was to develop the optimum treatment method, given the groundwater under the influence of surface water (GUI) conditions present or future and have it meet DEP approval. In attendance were representatives of the Westford water department, the Westford Board of Health, Massachusetts Water Resources Authority and the department's consulting engineers, Dufresne-Henry Inc., as well as Dr. James Malley from the University of New Hampshire, an expert in water treatment and UV technology.

The Future of Ultraviolet Disinfection

The 2000 McIlvaine study reports that the UV disinfection industry will grow to over a $1 billion per year industry. UV is an attractive disinfection alternative because it's safer than the popular chlorination method, it requires low start up and capital costs, it deals effectively with microbial contamination; it's simple to install and retrofit. In a move to actualize this growth, major UV manufacturers will continue product development, improving the efficiency and viability, of this soon to be commonly used disinfection method of treatment for potable water.

References

1. "Service International Expects New Regulations for the Disinfection of Drinking Water," Business Wire, July 2001.

2. Hunter, G. "The History of UV Disinfection in the Last 20 Years," I UVA News, 2.3 2000. Note: This paper contains a comprehensive list of 23 references, including Ref. 7 listed below

3. Cosman, J, and H. Wright, "UV Disinfection for Drinking Water," IUVA NEWS" 2.3 2000. Covers benefits of using UV in some detail.

4. Malley, J, P. Jr, Ph.D., "Engineering of UV Disinfection Systems for Drinking Water," IUVA NEWS 2.3 20003.

5. Letterman, R. D., Technical Editor, "Water Quality & Treatment - A Handbook of Community Water Supplies", Fifth Edition, guided by an AWWA Committee, published by McGraw-Hill, Inc. (1999)

6. Linden, K.G, Gwy-Am Shu and Mark D. Sobsey. "Comparison of Monochromatic and Polychromatic UV Light for Disinfection Efficacy", 2000 AWWA Water Quality Technology Conference Proceedings

7. LaFrens, R., "High Intensity Pulsed UV for drinking Water Treatment," Proceedings AWWA Water Quality Conference, Denver, CO, November 1997.

e-sources

Web sites for many suppliers of UV equipment have basic background information on UV technology; also trade and technical journals have Web sites that provide access to articles on this subject. Here are a few such sites:

Hanovia Limited, with which Severn Trent Services Inc. has reached an agreement to market and jointly develop advanced UV disinfection systems for customers throughout the United States, Canada and Mexico - www.hanovia.co.uk

Severn Trent Services, Inc: www.severntrentservices.com

International Ultraviolet Association: www.iuva.org. The public area has an excellent two-page exposition on "What Is Ultraviolet?" and a color slide presentation "The Emerging Role of Ultraviolet (UV) Disinfection in Drinking Water Treatment in North American" by James P. Malley, Jr., PhD Members can go to another site area which provides access to articles published in the association's publication, IUVA News. Reference 2 is one such article.

Environmental Protection Agency: www.epa.gov.

This article originally appeared in the March 2002 issue of Environmental Protection, Vol. 13, No. 3, p. 46.

This article originally appeared in the 03/01/2002 issue of Environmental Protection.