Inactivation kinetics of gram-negative bacteria in the presence of residual free chlorine

Inactivation kinetics of six chlorine tolerant gram-negative bacterial strains (n = 6) were studied in a water distribution system, fed with chlorinated drinking water (0.5-1 mg/L residual free chlorine (RFC)), at a hydraulic retention time of 120 min. These strains were persistently isolated after chlorination that interestingly led to further examine their inactivation kinetics in-vitro. The data were fit into Chick-Watson's first-order kinetics model to find decay rate 'k', CT factor, 2 log reduction (T-99(%)) and 3 log reductions (T-99(.9)%) of the isolates. All the six strains were tolerant to 0.5-1 mg/L RFC at a contact time of 30 min. Morganella morganii subsp. sibonii, Proteus myxofaciens, Enterobacter and Pseudomonas aeruginosa showed the lowest inactivation at RFC 3.5 mg/L for 30 min. Providencia rustigianii (R-2 = 0.43 +/- 0.02) was the most tolerant out of all the six isolates, 2 log inactivation of Providencia rustigianii was achieved after 180 min with 5 mg/L RFC. Morganella morganii subsp. morganii was somewhat sensitive to RFC; log reduction was achieved within 30 min at 1.5 mg/L RFC. Unsatisfactory disinfection using low-level free chlorine (0.5-1 mg/L RFC at 30 min), harbors potentially tolerant pathogenic strains in a water system.