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Microwave Uv: a New Wave of Tertiary Disinfection WEFTEC®.06 MICROWAVE UV: A NEW WAVE OF TERTIARY DISINFECTION Richard L. Gutierrez1, Keith N. Bourgeous1, Andrew Salveson2, Jeremy Meir3, and Allan Slater3 1Carollo Engineers 2500 Venture Oaks Way Sacramento, CA 95833 2Carollo Engineers, Walnut Creek, CA 3Quay Technologies Ltd. Cookham Dean, Berks, UK ABSTRACT Currently UV disinfection technologies available for use in drinking water and water reuse applications typically utilize one of three types of mercury UV lamps. These three types of mercury UV lamps include low pressure, low-pressure high-output, and medium-pressure lamps. All of these lamps contain electrodes that facilitate in the generation of UV radiation. These electrodes are of delicate construction and their deterioration is the primary source of failure in UV disinfection systems. An emerging UV disinfection technology that eliminates the need for electrodes is the microwave powered electrodeless mercury UV lamp. The objectives of this study were to review available literature on commercially available UV disinfection technologies used in drinking water and water reuse applications, and to provide a detailed explanation of the lamp properties and characteristics for comparison with the microwave powered electrodeless UV lamp. A description of traditional mercury UV lamps, the technology and application of microwave powered electrodeless mercury UV lamps, the advantages and disadvantages of the use of the microwave powered electrodeless mercury UV lamps, and a commercially available Microwave UV system that is currently undergoing validation testing as per the NWRI/AwwaRF 2003 Ultraviolet Disinfection Guidelines will be presented in this paper. KEY WORDS UV Disinfection, microwave UV, electrodeless UV lamps INTRODUCTION The first application of Ultraviolet (UV) radiation for disinfection dates back to the 1890’s with the development of the mercury vapor lamp and quartz tube (Phillips, 1983). However, the development of this technology was not realized with the success of chlorination which was a cheaper, more reliable, disinfection source at the time (Hijnen 2006). In recent years, the use of UV disinfection systems at wastewater treatment plants for the disinfection of tertiary treated effluent has become more common and the technology has reemerged. The impetuses for the increase in the use of UV disinfection systems at wastewater treatment plants are and have typically been 1) stringent new discharge permit limits for trihalomethanes and other chlorine generated disinfection byproducts, and 2) safety concerns over the use of gaseous chlorine. UV disinfection systems have proven to be a safe and effective way to disinfect wastewater without producing the traditional chlorine generated disinfection byproducts. UV radiation disinfects water by damaging the DNA of pathogens such as viruses, bacteria, and protozoa, so that they Copyright © 2006 Water Environment Foundation. All Rights Reserved 2853 WEFTEC®.06 can no longer reproduce and are thus incapable of infecting a host. UV disinfection systems typically utilize one of three types of mercury UV lamps. These three types of mercury UV lamps include low pressure, low-pressure high-output, and medium-pressure lamps. All three of these mercury UV lamps contain electrodes that facilitate in the generation of UV radiation. The following topics will be discussed in this paper. First, a description of traditional mercury UV lamps which utilize electrodes and are currently being used in UV disinfection systems will be presented. Second, the technology and application of microwave powered electrodeless mercury UV lamps will be discussed. Third, a description of a commercially available Microwave UV system that is currently undergoing validation testing as per the NWRI/AwwaRF 2003 Ultraviolet Disinfection Guidelines will be presented. Fourth, the advantages and disadvantages of the use of the microwave electrodeless mercury UV lamps will be presented. MERCURY UV LAMP CHARACTERISTICS AND OPERATION Most UV lamps, including all lamps used for disinfection, generate radiation through gas discharge. They contain a filling composed of mercury and an inert gas, typically argon, enclosed in UV transparent (no phosphorus) envelopes. An electrical field of one up to 100 volts per centimeter (V/cm) is used to accelerate electrons and cause them to collide with the mercury and argon atoms (Heering 2004). Initially, few collisions occur between the electrons and mercury atoms due to the low vapor pressure of mercury at ambient temperatures. The argon, which has a high ionization energy, is readily excited to a metastable (non-radiative) state and therefore many collisions occur between the electrons and the argon atoms. Argon atoms in the metastable state can only loose energy through collision with other atoms. When metastable argon atoms with high ionization energy collide with mercury atoms, ionization of the mercury atoms occurs. UV radiation is emitted by the ionized mercury atoms as they return to their lower energy state (Phillips 1983). Mercury is the ideal element for this application for several reasons as described by Phillips (1983): • It has a low ionization energy, enabling the domino effect that occurs between the molecules to happen more readily. • It is the most volatile metal and has sufficient vapor pressure at ambient temperature to provide the optimum conditions for producing resonance radiation. • Its energy state is at a level that produces resonance radiation at useful wavelengths for disinfection (200 to 300 nanometers (nm)). • It is chemically inert to the other materials used in the lamp assembly. The purpose of the argon is to aid in startup of the lamps by reducing the starting voltage required through the atomic interactions discussed above. Argon is the most commonly used inert gas in the fill material for UV lamps because it is the least expensive (Phillips 1983). A critical component in the operation of mercury UV lamps is the pressure of the mercury. Operation at low pressures results in insufficient collisions between electrons and mercury atoms, which makes starting the lamps difficult and reduces efficiency. At higher pressures, collisions are increased but efficiency is reduced due to increased elastic collision losses and reabsorption of the UV radiation within the lamp. Reabsorption occurs because the process through which a photon is emitted from an excited mercury atom is reversible and the photon can therefore be reabsorbed by other atoms. The phenomenon makes the selection of the lamp Copyright © 2006 Water Environment Foundation. All Rights Reserved 2854 WEFTEC®.06 diameter important since photons emitted by atoms at the center of the lamp have a greater likelihood of being reabsorbed by other atoms as the diameter increases (Phillips 1983). ELECTRODED UV LAMPS All of the UV disinfection systems that are listed in the Treatment Technology Report for Recycled Water (December, 2005) that have now or at one time received conditional acceptance utilize electroded mercury UV lamps. There are three types of mercury UV lamps that are conditionally accepted for use in tertiary UV disinfection systems. These three types are the low- pressure (LP) lamp, the amalgam or low-pressure high output (LPHO) lamp, and the medium- pressure (MP) lamp. Although all these lamps use electrodes (see Figure 2a for a picture of an electrode in a LPHO lamp), they differ in several important aspects including their emission spectrum, mercury pressure in the lamp, operating temperature, power requirements, efficiency (defined as the amount of input electrical power that is converted into UV light emitted in the germicidal spectrum range), and lamp life. A summary of typical characteristics for each of these lamps is presented in Table 1. In all electroded UV lamp systems, electricity from an external power source flows through the electrodes and is conducted directly into the gas discharge. UV lamp electrodes are a delicate construction composed of a coil of fine tungsten wire. Between the coils, there is a mixture of earth oxides (calcium, barium, and strontium), which aid in emission of the electrons. The lamp is configured with an electrode at each end, which take turns acting as a cathode, each with a single connection to the power supply. When the lamp is switched on, the electrodes heat up rapidly and transition from a “glow discharge” to what is referred to as an “arc discharge”. The arc is formed when heating of the cathode results in ejection of electrons through thermionic emission. The lamps require a high starting voltage to initially heat the electrodes (Phillips 1983). Electrode mercury lamps have negative resistance and thus require a ballast for stable operation. There are two major types of ballasts currently used in UV disinfection systems, the electromagnetic ballast and the electronic ballast. The electromagnetic ballast is the more conventional technology that has been available longer, is more durable, and has a longer rated life. Electronic ballasts, coming into more frequent use, are more energy efficient, allowing UV operation at a wider range of ballast power (and thus lamp intensity). Ballasts can have significant impacts on the power input requirements of electroded UV lamps (Dussert 2005). Electroded Lamp Life Electrode deterioration is the most common method of failure for electroded lamps. In normal tungsten filament light bulbs, the mode of failure is usually catastrophic and occurs when the filament breaks and the bulb can no longer
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