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 alternatives to chlorination for disinfecting treated wastewaters. Field experiments were conducted to analyze the repair extent of Escherichia coli and enterococci after their disinfection by u.v. Secondary treated effluent was irradiated to u.v. doses ranging from 34.5 to 95 mWs/cm2 and was subsequently incubated from 1 to 5 h in diffusion chambers placed in a large open-air sea-water tank. The bacterial concentrations before u.v. were of the order of 4 log units. No repair was observed for the enterococci. On the other hand, an increase in the number of Escherichia coli after 3 h incubation was observed in 82% of counts. The repair rate (RR) of E. coli decreased when the u.v. dose increased. The RR were always less than 0.001 for doses above 44 mWs/cm2. For such doses, the increase in the number of E. coli after the disinfection is low (between 0.1 and 0.6 log units). No relationship was observed between the repair rate and the luminous dose (300–800 nm) received during the incubation. From this work we can conclude that, 1) under conditions approaching actual post-disinfection situations, the repair of E. coli after disinfection by u.v. is small and 2) the parameter determining the repair rate of E. coli is the u.v. irradiation dose.