Spectral Distribution of UV Light During Aging of Medium Pressure UV Lamps

As part of a project to validate a UV reactor for disinfection of drinking water, the germicidal output of four medium pressure lamps housed in a continuously operating reactor was followed as a function of time. Spectra were measured monthly and were used in conjunction with radiometer readings to calculate germicidal fluence rates (using DNA absorbance weighting) at the radiometer sensor position with the lamps operating at full power (4 kW) during each measurement. Based on the measured fluence rates, the power was adjusted so that the UV lamps operated continuously to deliver a germicidal output of approximately 70% of that obtained at full power at the end of an initial 100 h “burn-in” period when the lamps were new. The experimental protocol was designed to ensure reproducibility of spectrometer and radiometer positioning and response. Under the conditions of this test, lamp aging has been relatively slight. The average depreciation in germicidal fluence rates for all four lamps was 17% after 3980 h of operation. Slightly more depreciation was observed in the UVC than in the UVB region (18% versus 15% for the average of all lamps). Consistent with this, the germicidal fluence rate depreciated slightly more rapidly than the total (200 to 350 nm) fluence rate. These changes indicate a more rapid depreciation of the shorter wavelengths, but the extent to which this is due to inherent changes in the lamp spectra compared to solarization of the quartz sleeves cannot be determined from the present data. Efforts will be made to separate these two effects by examining the quartz sleeves at the end of the tests. INTRODUCTION The way in which UV lamp irradiance depreciates over time has been a topic of interest ever since UV equipment was applied to pathogen inactivation (disinfection). Unfortunately, there is very little information available on the topic and even less has actually been published. In wastewater disinfection, many plants will operate the UV lamps past their expected life, because even with lower germicidal output than originally assumed, most plants will still meet their discharge permits. However, over the past several years more emphasis has been put on accurately assessing dose delivery, especially with the wide acceptance of UV disinfection for drinking water and wastewater reuse applications. This requires more careful monitoring of the UV lamps in the system and precise information regarding the depreciation of the UV lamps over time under worst-case conditions. Currently, there is no industry standard protocol for aging and measuring the depreciation of UV lamps. Several approaches are possible: the UV lamps could be aged and measured in the air; they could be aged in water and measured in air; or they could be aged and measured in water. Lamp operation may also differ during aging; the lamps may or may not be cycled on and off, and the power levels may be set to provide a range of irradiance levels during the aging test. Measurement equipment has not been standardized either. For example, some equipment allows only a narrow band of wavelengths around 254 nm to be measured while others allow measurement of a broad spectrum of wavelengths in the UV-C and UV-B range. In December 2000, a protocol for lamp aging was introduced in the NWRI/AWWARF UV Disinfection guidelines [AWWARF, 2000]. The guidelines specified a narrowly defined method for determining lamp output depreciation over time. The guidelines specified aging in water, addressed dimming and cycling and minimum measuring standards. The NWRI/AWWARF guidelines were the basis for the aging and measurement of the UV lamps outlined in this paper. MATERIALS AND METHODS Overview: The tests were designed to determine the germicidal output of the lamps over time relative to their output after a 100 h “burn-in” period during which the lamps were operated at full power (4 kW). In the first test after this period, the germicidal output was measured at full power and at 75% power (3 kW), and the data were used to determine the operating power that would provide approximately 70% of the full power germicidal output. Between tests, the lamps were operated at sufficient power to maintain 70% germicidal output (i.e., after every test the operating power was increased slightly to account for lamp depreciation as determined during the test). Tests were conducted at two places. For approximately the first 4000 h of testing, the reactor was installed at the Appomatox River Water Authority drinking water treatment plant (ARWA, Chesterfield, VA) where water was taken post-filtration. A typical absorbance spectrum of the water delivered to the reactor is shown in Figure 1. Problems with maintaining adequate flow led to relocating the reactor to Degremont North American Research & Development (DENARD) where water was taken from the tap and re-circulated through the reactor and a chiller with the addition of approximately 10% fresh tap water to ensure adequate cooling of the lamps. All the data presented here were obtained after operation at ARWA, and tests are presently continuing at DENARD. During tests and daily operation of the reactor, the water flow rate was approximately 100 gal/min, which was sufficient to keep the temperature of the reactor at that of the water (at or below 78 F). During daily operation, the lamps were subjected to one on/off cycle per day, and one automatic wiper cycle per day. UV Reactor: The UV reactor used for aging and monitoring the UV lamps was a modified Aquaray H2O 20” reactor with four medium pressure UV lamps. The UV lamps can be operated up to 4KW power and they have a 50 cm arc length. The UV lamp is designed so both electrical connections are on one side so they are accessed and maintained from one side of the reactor. The Aquaray H2O uses two state of the art electronic ballasts to power the UV lamps. The ballasts operate from 480 V, 3 phase, 50/60 Hz power and are designed to operate medium pressure discharge lamps. Each ballast has outputs for two UV lamps. In contrast to conventional ballasts (inductive lamp ballast or transformer with transductor), the lamp with an electronic ballast is operated at high frequency (approx. 250 kHz). The lamp does not flicker and dimming is infinitely adjustable to a range between 20% and 100% of the electric power. Each UV lamp in the reactor is monitored by a UV sensor. Due to the modifications required for the lamp measurement, special sensor ports were used in order to bring the sensor port closer to the UV lamp reducing the water gap to 3.8 cm (Figure 2). Figure 1: Absorbance spectrum of post-filter AWRA water (October, 2002). 0.00 0.05 0.10 0.15 0.20 0.25 200 250 300 350 400 Wavelength (nm) Ab so rb an ce (c m -1 ) Figure 2: Schematic of the sensor port arrangement. Test Equipment: Transmittance spectra of optical components used in the tests (filters and screen diffusers) were measured in the laboratory at Duke University on a Cary Bio-100 double-beam scanning spectrometer (Varian, Inc.). The same instrument was used to record the absorbance spectra of water from the reactor. An IL1700 radiometer system was used (International Light, Inc., Newburyport, MA). The detector was an SED240 equipped with a W-diffuser and a QNDS1 filter. The detector response function as determined by the manufacturer during its most recent calibration (July, 2002) is shown in Figure 3 along with the transmittance spectrum of the QNDS1 filter. Radiometer/sensor in sensor port