A germicidal lamp having amalgam for controlling mercury vapor pressure contained in a location facilitating efficient high output operation. A low pressure mercury vapor discharge lamp has an amalgam container containing an amalgam positioned behind an electrode out of the arc path or space. The amalgam position is retained during high wall loading of the lamp preventing the amalgam from moving out of position. Efficient operation with high current loads and resulting high wall loading and temperatures is possible. The germicidal lamp is particularly suited to being positioned vertically in a waste water treatment system.