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UV Light for Drinking Water Disinfection By: Sam Jeyanayagam, Ph.D., P.E., DEE Associate & |
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1.0 Introduction
Ultraviolet (UV) light was discovered approximately 150 years ago. The first commercial UV lamp was made in the early 1900s and this was soon followed by the manufacture of the quartz sleeve. These technological advances allowed for the first application of UV light for water disinfection in 1907 in France. In the mid 1980s, UV disinfection was named as a Best Available Technology (BAT) for wastewater disinfection in the United States. Fueled by the recent findings that UV disinfection can inactivate key pathogens at cost-effective doses, the drinking water industry in North America is actively pursuing its widespread application. 2.0 Why UV Disinfection? The primary regulatory drivers for the widespread interest in UV disinfection are the Long Term 2 Surface Water Treatment Rule (LT2SWTR) and the Stage 2 Disinfectants/Disinfection By-Product Rule (D/DBPR). The LT2SWTR targets Cryptosporidium and specifies various levels of log removal/inactivation based on the concentration of the pathogen in the raw water supply. The Stage 2 D/DBPR will stipulate annual average limits of 80 mg/L THM and 60 mg/L HAA at every sampling location within the distribution system. Studies conducted over the last five years have confirmed that Cryptosporidium can be inactivated by relatively low UV doses with no increase in known disinfection by-products. The US Environmental Protection Agency (USEPA) believes that UV disinfection is an available, feasible and cost-effective disinfection technology for achieving compliance with the LT2SWTR and Stage 2 D/DBPR. Table 1 presents a comparison of commonly used drinking water disinfectants. It shows that no one disinfectant is capable of providing all benefits. An optimized disinfection strategy entails combining disinfectants to: •Inactivate the target pathogens. •Minimize the formation of DBPs. •Provide multiple barriers. •Provide a secondary disinfectant. 3.0 Knowlege Gap The technical feasibility of UV disinfection has been conclusively demonstrated. To allow full-scale implementation of UV systems, issues related to engineering and operations need to be addressed. This entails understanding optics, hydraulics and microbiology. This paper attempts to close the knowledge gap by outlining some practical issues associated with the use of UV technology. 4.0 Inactivation Mechanism UV light has a wavelength band of 100 to 400 nm. The germicidal wavelength range lies only between 200 to 300 nm. UV disinfection is a physical/biochemical inactivation process. The electromagnetic radiation restructures the DNA of the microorganism and destroys its ability to replicate. An organism that cannot replicate cannot infect. The DNA absorbance characteristic is wavelength dependent with peak absorbance occurring at approximately 260 nm. 5.0 The Concept of UV Dose Pathogen inactivation is directly linked to UV dose. In theory, UV dose is defined as follows: UV Dose = I x t Where, I = UV Intensity (mW/cm2); t = Exposure Time (s); and UV Dose is commonly expressed in mW-s/cm2 or mJ/cm2 In reality, the delivered UV dose is a complex function of: •Water quality •Lamp type •Lamp age •UV intensity distribution in the reactor •Reactor design •Reactor hydraulics •Sensor performance •Dose-response characteristics of the target pathogen(s) 5.1 Water Quality Factors The impacts of water quality on delivered UV dose is illustrated in Figure 1 and outlined below: •Complete penetration of the target pathogen by UV light represents the ideal condition since it causes maximum DNA damage. •Limited cellular damage may result when the amount of UV light reaching the target pathogen is diminished due to: •Shielding caused by suspended solids. •UV light scattering by colloidal solids. •UV absorbance by dissolved organics. •Sleeve fouling by inorganic constituents (Ca, Mg, Al, Fe, Mn, CO3, SO4, and PO4) UV abosrbance (UVA) is a gross indicator of water quality parameters and is related to UV transmittance (UVT) by the following mathematical expression: UVA = log (1/UVT) The units for UVA and UVT are cm-1 and percent, respectively. When UVA is high (low UVT), the UV intensity necessary to achieve a target UV dose should be high enough to satisfy the UV "demand" of the water. Consequently, capital and O&M costs would be high. 5.2 UV Lamp Technology The most commonly used UV technologies in drinking water treatment are Low Pressure (LP), Low Pressure High Output (LPHO), and Medium Pressure (MP). Table 2 compares the key features of the three systems. Approximately 85 to 90-percent of the low pressure lamp output is close to the optimum germicidal wavelength of 260 nm (monochromatic). The output of the medium pressure lamp is polychromatic and is distributed over the entire germicidal range. The use of LP technology is limited to very small systems. The majority of large municipal drinking water facilities use LPHO or MP units. A detailed site-specific evaluation is necessary for the selection of the most feasible UV technology. 5.3 Lamp Age When a UV lamp is first activated, it will have a very high output. During the first few hours of operation, as the impurities within the lamp are burned off, the output will diminish rapidly. This initial "burn-in" period is conservatively assumed to be 100 hours and is the time required for the lamp to stabilize. Beyond the burn-in period, the lamp output will continue to slowly diminish over its life. The lamp life, typically expressed in hours, is the time taken for the lamp output to reach a specified percent (generally 70-percent) of the output at the end of the "burn-in." UV system design must account for this temporal variation of UV output; and the system design should be based on the end-of-lamp-life output. Operating the UV system when lamp output has diminished to below end-of-lamp-life output could compromise disinfection. The design also should consider the spectral "shift" that may occur in medium pressure lamps as they age. Both the UV lamp output and DNA absorbance of UV light are wavelength dependent. Hence any wavelength shift in UV lamp output could potentially lower inactivation due to reduced DNA absorbance. 5.4 Reactor Design Figure 2 illustrates dose distribution in UV reactors. Plug flow characteristics that exists in an ideal reactor is difficult to achieve in practice. Consequently, a spatial variation of UV intensity (I) and exposure time (t) occurs within all UV reactor, resulting in a distribution of doses. Nonetheless, using computational fluid dynamic (CFD) modeling, the UV manufacturers design efficient reactors that approach plug flow conditions. For reasons outlined above, the UV dose delivered is path-dependent. As illustrated in Figure 3, a microorganism flowing close to the UV lamps (A) will receive a higher dose compared to the one that flows further away from the lamps (B). 5.5 UV Intensity Sensor Performance UV intensity sensors provide the only means of ensuring that adequate disinfection is being achieved in a UV reactor. Hence, sensor reliability is critical for demonstrating regulatory compliance and is enhanced by: •Providing an adequate number of sensors. •Placing sensors at optimum locations. •Routine checking with a reference sensor. •Factory calibration of sensors to control linearity and drift. •Sensor (or sensor window) cleaning. 5.6 Microorganism's Dose-Response Characteristics UV dose requirements are pathogen-specific and will be set by the USEPA. In general, viruses require significantly higher UV doses than Giardia and Cryptosporidium. The UV dose-response is determined by using bench-scale, collimated beam tests conducted in the laboratory. 6.0 The Importance of Validation and Monitoring Both reactor validation and monitoring are crucial for achieving future regulatory compliance. 6.1 Equipment Validation Reactor validation is a method of determining and certifying the performance of a UV reactor. Ideally, manufacturers will be able to validate a specific model reactor under a wide array of conditions (e.g., flow, UV dose, UV absorbance, lamp power settings, inlet and outlet hydraulic conditions) so that they can be applied to any utility. However, it may be necessary for some utilities to perform on-site validation testing to ensure that their specific installation and operating conditions are taken into consideration. UV reactor validation will affect the design and operation of the UV system because the UV reactors must be operated under the validated conditions to assure disinfection efficiency and to obtain regulatory approval. Therefore, UV facilities must be designed with the appropriate number of UV reactors to be able to operate within the wide range of flowrates experienced by water treatment facilities. In a nutshell, in order to ensure proper disinfection, it is important to validate the UV reactor as it is to be used and use the reactor as validated. The USEPA will soon release UV dose (It) tables for various levels of Cryptosporidium inactivation. These doses should be used in UV reactor validation after applying the appropriate safety factors to account for the following: •Reactor hydraulic inefficiencies •Differences in dose-response of the "target" pathogen and "challenge" organism •UV intensity sensor uncertainties (e.g. shift and linearity) •Type of sensor. A safety factor may be required if sensors without optical filters are used. •End-of-lamp-life output •Quartz sleeve absorbance 6.2 Monitoring Monitoring ensures that the validated reactor is operating within the validated conditions. At a minimum, UV intensity, water flowrate and lamp outage should be monitored. UV absorbance (or UV transmittance) and lamp power may also need to be monitored, depending on the validated conditions and how the UV reactor is controlled. In addition, UV facilities, may be required to monitor off-specification (operation outside the validated conditions) and reactor shutdown (e.g. power failure). Off-specification operation refers to UV reactor operation outside the validated conditions. For example, it takes 5 to 5 minutes for UV lamps to reach their optimum operating temperature at start-up. During this time disinfection may be compromised. Other key parameters (e.g. water turbidity, electrical power draw, equipment age, etc.) may also be monitored to ensure consistent and reliable operation. 7.0 Key Design and Operation Considerations Site-specific conditions impact the design and operation of UV systems. The following is a list of key considerations: Design Considerations •The best UV system location is downstream of the filters where the organics concentration (UV absorbance) and turbidity levels are the lowest. Table 3 lists the advantages and disadvantages of the different post-filter locations. •Reliable water quality data is paramount to UV system design. Ideally, a year of data including turbidity, UV254, and a UV scan is recommended. •Adequate space should be provided for UV reactors and electrical components. •Electrical ballast must be located within the specified distance from the UV reactor. •Compatibility with other processes (ozone, chlorine, softening, etc.) should be analyzed on a case-by-case basis and adequate stand-by UV reactors provided. •Redundancy needs •Uninterruptible power supply (UPS) may be necessary to improve power quality, which is central to UV reactors operating within validated limits. 8.0 UV System Cost The cost of UV system is very site specific and is impacted by factors such as water quality, inactivation goal, type of UV system, design flow, location of the UV reactor, retrofit constraints, and system redundancy. Costs for selected full scale UV facilities are presented in Table 4. From a utility perspective, the impact of implementing UV disinfection on the ratepayer is of interest. For the City of Newark, OH, incorporating UV disinfection would result in an estimated annual cost of $83,400 (annualized capital + O&M costs). For the approximately 18,500 customers of this community, this represents about $0.38 per month per customer to provide a very high degree of assurance that waterborne disease outbreak similar to the 1993 Milwaukee incident would never occur. Generalized cost data developed by Cotton et al. (2001) for a wide range of water quality and flow confirm that UV disinfection is a low cost technology with significant health benefits. 9.0 Conclusion Recent findings that Cryptosporidium can be inactivated by cost-effective UV doses have triggered a widespread acceptance of this technology. As critical design, monitoring, and reliability issues are resolved, it is anticipated that the use of UV disinfection will become widespread in North America. The drinking water industry, however, must be mindful of emerging pathogens such as Mycobacterium avium, Toxoplasma gondii, Heliobacter, microsporidium, adenovirus, and aspergilus, to only name a few. Consequently, while the current results for UV inactivation of Cryptosporidium are promising, viruses or other emerging pathogens may control future disinfection strategies. References Anderson, J. 2002. Personal Communication with Christine Cotton. Cotton, C.A., D.M. Owen, G.C. Cline, and T.P. Brodeur. 2001. UV Disinfection Costs for Inactivating Cryptosporidium. Journal of AWWA 93:6:82. Cotton, C.A., R.S. Cushing, and D.M. Owen. 2002. The Impact of the Draft UV Disinfection Requirements on UV Facility Design and Operation. Presented at the AWWA Annual Conference, New Orleans. June 2002. Fiorante, R, et al. 2002. Design and Construction of One of North America's Largest Drinking Water UV Disinfection Facilities. Presented at the British Columbia Water & Waste Association Annual Conference, Whistler, BC., Canada. April 2002. Swaim, P.D., et al., 2001 The Science Behind Achieving Approval of a Full-Scale UV Disinfection Facility. WQTC Proceedings, AWWA. Nashville, TN. Nov. 2001.
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