Repair of wastewater microorganisms after ultraviolet disinfection under seminatural conditions

Repair of wastewater microorganisms after ultraviolet (UV) disinfection was studied by incubating secondary effluent previously irradiated to UV doses of 40 to 60 mW·s/cm 2 in diffusion chambers for 2 to 24 hours. For somatic coliphage repair estimations, the effluent was incubated in flasks for 2 hours. There was only slight repair of fecal streptococci, and it did not depend on the UV irradiation dose. Repair of the thermotolerant coliforms was a function of the UV dose. Repair corresponded to a maximum increase of 1 log unit after irradiation to 40 mW·s/cm 2 , and to smaller increases after 50 or 60 mW·s/cm 2 . No relationship was observed between repair and suspended solids content of the effluent in the range of 4 to 12 mg/L. For the Salmonella , no repair was observed after a 60‐mW·s/cm 2 irradiation and a 24‐hour incubation. No repair was observed for the viruses in nine in‐flask incubations, with coliforms showing increases between 1 and 2 log units in the same flasks. Compared with the coliforms, this absence of repair of microorganisms that cause the greatest health risk in bathing and shellfish‐breeding zones brings interesting perspectives in the debate of post‐UV‐irradiation repair.