REPAIR OF ESCHERICHIA-COLI AND ENTEROCOCCI IN SEA-WATER AFTER ULTRAVIOLET DISINFECTION QUANTIFICATION USING DIFFUSION-CHAMBERS

Ultraviolet (u.v.) light is now considered one of the best alternative s to chlorination for disinfecting treated wastewaters. Field experime nts were conducted to analyze the repair extent of Escherichia coli an d enterococci after their disinfection by u.v. Secondary treated efflu ent was irradiated to u.v. doses ranging from 34.5 to 95 mWs/cm(2) and was subsequently incubated from 1 to 5 h in diffusion chambers placed in a large open-air sea-water tank. The bacterial concentrations befo re u.v. were of the order of 4 log units. No repair was observed for t he enterococci. On the other hand, an increase in the number of Escher ichia coli after 3 h incubation was observed in 82% of counts. The rep air rate (RR) of E. call decreased when the u.v. dose increased. The R R were always less than 0.001 for doses above 44 mWs/cm(2). For such d oses, the increase in the number of E. call after the disinfection is low (between 0.1 and 0.6 log units). No relationship was observed betw een the repair rate and the luminous dose. (300-800 nn) received durin g the incubation. From this work we can conclude that, 1) under condit ions approaching actual post-disinfection situations, the repair of E. coli after disinfection by uv. is small and 2) the parameter determin ing the repair rate of E. coli is the u.v. irradiation dose. Copyright (C) 1996 Elsevier Science Ltd