AGING EVALUATION OF MEDIUM-PRESSURE MERCURY LAMPS UNDER TYPICAL OPERATING CONDITIONS FOR DRINKING WATER DISINFECTION APPLICATIONS

Little is known regarding how medium pressure (MP) ultraviolet (UV) lamps age under typical operating conditions, yet a number of assumptions regarding lamp aging are utilized when UV reactors are validated, often as unfounded approximations. This study was designed to evaluate the aging of MP UV lamps utilized with a closed-vessel UV reactor technology for drinking water disinfection applications. Beginning in May 2002, measurements of MP UV lamps were recorded to track the total UV output, the germicidal output and the spectral wavelength distribution of four lamps under normal operating conditions. Of the four lamps, one aged to its end-of-life output (dropping below 70% efficiency) at 6,600 hours. The second lamp reached its end of lamp life at 13,600 hours. Two lamps lasted the full span of testing (16,500 h), ending at 68% and 71% efficiency. These data show that MP lamps are capable of lasting well beyond their guaranteed 4,000 hour lifetime. No appreciable changes in the spectral wavelength distribution could be discerned until the efficiency dropped below approximately 70%. These results demonstrate that appropriate use of sensor readings are essential for tracking lamp aging and with these readings, it may be unnecessary to presume a lamp's lifetime or to constantly track a lamp's spectral characteristics.