Disinfection of drinking water is a challenging and controversial issue. Chemical and ultraviolet (UV) methods are used to deactivate bacteria. Some of the chemical disinfection causes threats to public health since they produce toxic chemical compounds. Recently, several researchers conducted experiments and mathematical models to deactivate pathogenic bacteria from wastewater and drinking water using UV. The intensity of UV is very effective in deactivating bacteria. The authors compared the radiation intensity of a single and double lamp in a UV reactor for water treatment using computational fluid dynamics. The results indicated that the mean volume emission rate of single lamp and double lamp in the reactor was similar. However, the energy distribution of single lamps was much better than the double lamp in the reactor. The sensitivity analysis using three different UV transmittances (UVT), 70, 80 and 90%, indicated that the increase in % UVT value improved the energy distribution in the UV reactor.