Microbial Indicators for the Efficiency of Disinfection Processes

Fundamental studies have been conducted to find new microbial indicators for the effective inactivation of micro-organisms resistant to disinfectants and UV irradiation. Escherichiacoli, Salmonella typhimurium, Staphylococcusaureus, Candidatropicalis and spore-forming bacteria (isolated from sewage) were exposed to free chlorine or free ozone at concentrations in the range 10⁻² to 10⁰ mg/l, and 253.7 nm UV at light intensity levels of 10¹ to 10³µW/cm². For all disinfectants examined, the relation cⁿ·t = const., where c is the disinfectant concentration and t the contact time for 99% reduction of microorganisms, was found to be valid. The order of resistance was about the same in all cases and was as follows: spore-forming bacteria » C.tropicalis » Staph.aureus,E.coli,S.typhimurium. However, at the same disinfectant concentration, the contact time required to obtain the same inactivation efficiency was reduced effectively by ozone. UV light inactivates spore-forming bacteria and C.tropicalis more effectively than chlorine, although the intensity of UV light and the concentration of chlorine studied have similar effects on E.coli. These results are compared with results for viruses already obtained by other researchers. Spore-forming bacteria may be the ideal micro-organisms to evaluate the completeness of disinfection, but they require a long contact time to be inactivated by chlorination. However, when ozonation or UV irradiation is used as the method of disinfection, spore-forming bacteria can be used as an indicator, because they are then inactivated under usual field conditions in contact or irradiation time.