eng Science and Education Publishing American Journal of Water Resources 2333-4819 2022-07-27 10 2 54 58 10.12691/ajwr-10-2-3 AJWR20221023 article J.J. Dragoni-Rosado jason.dragoni@gmail.com 1 G. Tav¨˘rez-Hern¨˘ndez 1 A. Gonz¨˘lez-Mederos 1 A.E. P¨Śrez-Matos 2 Department of Science and Technology, Inter American University of Puerto Rico, San Germ¨˘n, Puerto Rico Biotechnology and Agrobiotechnology Research and Learning Center, Department of Natural Sciences, Pontifical Catholic University of Puerto Rico, Ponce, Puerto Rico Wastewater treatment plants are essential in reducing the microbial load of water discharged into the ecosystems by UV light. However, it has been found that some pathogenic bacteria have developed mechanisms to reverse the damage caused to their DNA by UV light, with possible adverse effects on the environment and human health. Therefore, this research evaluated if fecal indicator bacteria that use photoreactivation or dark repair are present in the wastewater from a treatment plant in Puerto Rico. Samples of wastewater treated with UV light were collected and exposed to two treatments: fluorescent light (photoreactivation) and darkness (dark repair). The number of colonies of total coliforms, fecal coliforms, and enterococci was determined every hour of exposure. Results show that after exposure to fluorescent light, the number of colonies of total and fecal coliforms increased, being able to repair and reverse the damage caused to their DNA when exposed to visible light but not in darkness, possibly through the mechanism of photoreactivation. However, enterococci showed no increase in colonies when exposed to fluorescent light and kept in darkness. These results suggest reviewing the disinfection process considering photoreactivation and dark repair mechanisms. The new considerations can reduce pollution of watersheds when large amounts of treated wastewater are released into the environment. http://pubs.sciepub.com/ajwr/10/2/3/ajwr-10-2-3.pdf bacteria sewage disinfection water enterobacteria