Submitted to: Environmental Science and Technology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/4/2004
Publication Date: 4/14/2005
Citation: Bolster, C.H., Bromley, J., Jones, S. 2005. Recovery of chlorine - exposed escherichia coli in estuarine microcosms. Environmental Science and Technology. Vol. 39, 3083-3089 Interpretive Summary: Chlorination is one of the most widely used forms of disinfection for wastewater effluent, however, studies have shown that in many case chlorination results in the injury, rather than death, of fecal-borne microorganisms. Injured cells, although incapable of growing on standard growth media may retain their virulence. In addition, chlorine-injured cells are capable of resuscitating upon removal from chlorine stress and introduction to a more favorable environment. Therefore, an underestimation of the number of potentially harmful microorganisms being released into the environment by wastewater treatment facilities is likely to occur. In this study we investigated the ability of chlorine-injured Escherichia coli to increase in numbers when introduced into laboratory microcosms containing estuarine water. Chlorine-exposed bacteria were introduced into nine 2-L microcosms each containing estuarine water collected from a different location in the Seacoast region of New Hampshire. Our results show that following chlorination, E. coli concentrations can increase significantly over a 3-day period if discharged into estuarine waters. Our findings also suggest that nutrients (total dissolved nitrogen and ammonia) and dissolved organic carbon play a major role in this increase in bacterial numbers. These findings suggest that chlorine-treated bacteria may pose a greater water quality and public health risk than previously thought and that the regular monitoring of wastewater effluent may underestimate the true impact on estuarine water quality. Failure to detect injured bacteria in wastewater effluent may give a false impression of the effectiveness of disinfection processes. Monitoring wastewater effluent for injured bacteria could provide better confirmation of the effectiveness of disinfection processes and the potential for impacts on receiving water quality.
Technical Abstract: Chlorination is one of the most widely used forms of disinfection to treat wastewater effluent. Studies have shown, however, that exposure to chlorine may result in cellular injury (defined as the inability to culture viable cells on commonly used selective growth media), rather than death, for fecal-borne bacteria. In addition, a small number of bacteria pass through the disinfection process relatively unharmed. Microbial contamination of aquatic habitats by wastewater effluent therefore may occur by the reacquisition of culturability by injured cells and/or the growth of the few cells that escaped the chlorination process unharmed. Laboratory microcosm experiments were performed to determine to what extent conditions found within estuarine waters are sufficient for chlorine-exposed Escherichia coli to recover (i.e., increase in numbers of culturable cells due to reacquisition of culturability and/or growth) and the water quality parameters responsible for this recovery. Suspensions of E. coli were exposed to 0.5 mg/L of chlorine for 5 minutes followed by dechlorination with sodium thiosulfate. The chlorine-exposed bacteria were introduced into nine 2-L microcosms each containing estuarine water collected from a different location in the Seacoast region of New Hampshire. The number of culturable cells in the microcosms was enumerated at 0, 10, 24, 48, and 74 hours. In all estuarine microcosms the number of culturable cells increased by factors ranging from 40 to 170 over the 74-hour incubation period. Linear regression analyses indicated that dissolved organic carbon followed by total dissolved nitrogen and ammonium were most significantly correlated with the amount of recovery of E. coli over the 74-hour incubation period. The extensive recovery observed in our study indicates that chlorine-exposed E. coli may increase significantly in numbers when discharged into estuarine waters posing a water quality and public health risk and that the regular monitoring of wastewater treatment effluent may underestimate its true impact on estuarine water quality.