21 Chlorine residual efficiency.pmd

1Department of Microbiology; 2Department of Environmental Health, Isfahan University of Medical Sciences, Isfahan, Islamic Republic of Iran (Correspondence to A. Tavakoli: [email protected]). Received: 05/10/03; accepted: 22/03/04 ABSTRACT To investigate the inactivation of potential pathogens, we evaluated survival rates for Escherichia coli, Enterococcus faecalis, Aeromonas hydrophila and Salmonella typhi in samples taken from the Isfahan drinking water system. Chlorine residual, pH, temperature and total organic carbon levels were measured. The organism most sensitive to chlorine was A. hydrophila. It was inactived in < 100 minutes at chlorine levels of 0.11 mg/L to 0.90 mg/L. The other 3 organisms showed higher resistance. E. coli tolerated 0.30 mg/L chlorine for > 1000 minutes while Ent. faecalis and S. typhi survived at total chlorine concentration of 0.50 mg/L for 100 minutes. We concluded that total chlorine levels of less than 0.71 mg/L in water supply systems cannot provide the recommended safety levels.