Photons as Disinfectants
Ultraviolet light at 254 nm wavelength — the germicidal peak — penetrates microbial cell walls and is absorbed by nucleic acids. The energy creates covalent bonds between adjacent thymine bases in DNA, forming dimers that block transcription and replication. The organism cannot reproduce and is effectively inactivated. This physical mechanism works against bacteria, viruses, and protozoa without any chemical addition to the water.
Dose Equals Intensity Times Time
UV dose (fluence) is the product of average UV intensity throughout the reactor and the exposure time. It is measured in mJ/cm². The simulation calculates dose from lamp intensity, water transmittance, and path length. In a real reactor, computational fluid dynamics models track millions of particle trajectories through the non-uniform UV field to determine the dose distribution.
Water Quality and UV Transmittance
UV transmittance (UVT) measures how much 254 nm light passes through the water. Dissolved organic carbon, iron, manganese, and nitrate all absorb UV. As UVT drops, more powerful lamps or more lamps are needed, and the dose distribution widens — some water parcels may receive insufficient dose. The simulation uses Beer-Lambert absorption to show how UVT affects average intensity across the reactor path length.
UV and the Multi-Barrier Approach
UV excels at inactivating chlorine-resistant pathogens like Cryptosporidium (needing only 10 mJ/cm² for 3-log) but provides no residual disinfectant. Modern treatment plants combine UV as primary disinfection with chloramine as secondary residual. This multi-barrier strategy addresses each pathway's weakness while minimizing disinfection byproduct formation — a superior approach to relying on any single disinfectant.