The dosage of ultraviolet (UV) light required to inactivate adenoviruses serves as the basis for virus inactivation requirements in the Long Term 2 Enhanced Surface Water Treatment Rule. The rule increases the required UV dose from the standard practice of 40 mJ/cm2 to 186 mJ/cm2 for 4‐log inactivation. Ensuring this delivered dose in the UV reactor requires accounting for uncertainties in the reactor validation testing, which results in an applied UV dose of 200–300 mJ/cm2 for 4‐log virus inactivation credit for a given UV reactor. Concerned about the potential effect of this action on drinking water treatment, a group of experts met to assess the state of the science with respect to adenoviruses in drinking water. This working group reviewed the current science on adenoviruses and identified the effects—positive and negative—on public health protection arising from the elevation of UV design target doses.
