Photobiological effects of polychromatic medium pressure UV lamps.

Ultraviolet (UV) light has become widely accepted for the disinfection of potable water, process water and wastewater as an alternative to chlorination. To avoid the failure of a UV disinfection system due to the recovery of micro-organisms, certain additional wavelengths in the UV area are emitted by newly developed UV lamps. To reduce the chance of microbial recovery after ultraviolet irradiation, damage must be inflicted in as many areas of the micro-organism as possible. The effective killing of micro-organisms by improved polychromatic medium pressure UV lamps is due to their exceptionally high UV energy output at specific wavelengths across a broad section of the UV spectrum. The combination of these properties results in several different lethal effects in small and large micro-organisms. Important biological molecules other than DNA are likely to be damaged, which helps to prevent the recovery of irradiated micro-organisms. Absorption line spectra of absorbing nucleotide bases, DNA and other biological molecules, including proteins and enzymes, show how effective UV light can be. Recent findings on the biological effects of short wavelengths on Bacillus subtilis, Cryptosporidium parvum and Escherichia coli confirm the effect of short wavelengths. Practical comparisons with conventional low pressure UV lamps at equal UV dosages show better killing rates from polychromatic medium pressure lamps, without formation of disinfection byproducts (DBPs).