Replacement germicidal ultraviolet lamp for the Wallace & Tiernan WTL3500 UV disinfection system, original part number AAC9421. A germicidal lamp loses output steadily through its life while continuing to strike normally, so it is replaced on hours run rather than on failure — running past the rated life is the most common reason a system quietly drifts below its required dose. Wallace & Tiernan is an Evoqua Water Technologies brand, and was part of Siemens Water Technologies before that, so the same equipment is catalogued under all three names and its parts carry Evoqua W3T/W2T and legacy AAC/AAD numbers. Item number 4101-0508. Original part number AAC9421. Supplied by cBallast on quote-to-invoice terms; lead time and freight are quoted per RFQ against the destination Incoterm.