Evidence of emerging challenge of chlorine tolerance of Enterococcus species recovered from wastewater treatment plants

This study assessed the survival of some Enterococcus species recovered from the clarifier of two selected wastewater treatment plants in the Eastern Cape Province, South Africa, in the presence of different concentrations of chlorine disinfectant. The bacterial survival, lethal dose and inactivation kinetics at lethal doses were examined. Inactivation of the test bacteria (n = 20) at the recommended dose of 0.5 mg l−1 progressively increased from 3.0-5.8 log reduction within the range of chlorine residual of 0.14-0.45 mg l−1 after 30 min. The bactericidal activity of chlorine increased at higher chlorine dosages with a significant reduction in viability of the bacteria and complete sterilization of the bacterial population was achieved at a lethal dose of 0.75 mg l−1 and 1.0 mg l−1 in 30 min. For the inactivation kinetics, bactericidal activity of chlorine increased with time showing a 3.0-6.8 log reduction in 10 min, 4.0-8.3 log reduction in 20 min and above 7.9 log reduction in 30 min for all test Enterococcus isolates used in this study. Chlorine dose of 0.75 mg l−1-1.0 mg−1 showed a better disinfecting capacity to effectively reduce the tolerance of Enterococcus species.