Chlorine Tolerance and Inactivation of Citrobacter Species Recovered from Wastewater Treatment Plants

摘要

The increasing tolerance of some microbial pathogens in wastewater effluents to chlorine at the recommended dosages has threatened water security and public health safety. This study investigated the chlorine tolerance of some Citrobacter species recovered from two wastewater treatment plants in the Eastern Cape Province, South Africa. Effluent samples were collected from the clarifier of two wastewater treatment plants in the Eastern Cape Province, South Africa over a period of three months and were processed for isolation of Citrobacter species using standard methods. Presumptive bacterial isolates were identified by 16S rRNA gene sequencing (n=20) and the Basic Local Alignment Search tools (BLAST) analysis. Bacterial isolates at initial population of 8.3 log were screened at the recommended free chlorine of 0.5 mg/L and the chlorine lethal dose and inactivation kinetics at lethal doses were examined. Data was analysed using analysis of variance (IBM SPSS Statistics 23) and linear regression (Origin Pro 2017). The BLAST results revealed a 99% similarity of bacterial isolates to Citrobacter genus. The test bacteria (n=20) at the recommended chlorine dose of 0.5 mg/L gradually declined from 4 - 5 log with chlorine residuals between 0.12 - 0.46 mg/L after 30 min. Treatment at higher chlorine doses (0.75 -1.5 mg/L) revealed a significant reduction (p < 0.05) in viability of bacterial population and complete inactivation of Citrobacter species at lethal doses of 0.75 and 1.0 mg/L in 30 min. Increase in bacterial inactivation significantly correlated (p < 0.5) with increase in contact time and R2 values ranged between 0.73 - 0.92 for the three Citrobacter species. Treatment at 0.5 mg/L dosage showed a poor removal efficiency by chlorine while an increase in chlorine concentration at 0.75 -1.0 mg/L resulted in higher inactivation of Citrobacter sp. and is hereby proposed as an alternative dose to check bacterial tolerance.