Comparative Rates of Disinfection of Microbial Indicator Organisms in Chlorinated Sewage Effluents

The densities of several microbial indicator organisms, including fecal coliforms, enterococci, Clostridiumperfrinaens, and a male-specific bacteriophage group, were determined in the pre- and post-chlorinated effluents of three wastewater treatment plants during a rainfall event when the plants were operating near maximum flow capacity. These studies were undertaken to determine the effects of elevated flow rates on the relative rates of inactivation of the microbial indicators due to chlorine disinfection.

Three distinctly different responses to chlorine treatment were observed. The vegetative bacterial organisms (fecal coliforms and enterococci) were most sensitive, the male-specific bacteriophage group was considerably more refractory, and c. perfringens, a bacterial spore-former, was highly resistant to inactivation by the disinfecting agent. These findings are significant for two reasons. First, the vegetative bacterial indicators did not reliably index the decay rates of the bacterial virus under the field conditions experienced. This has important public health ramifications to consumers of certain seafood products and to marine recreationists. Second, because of the highly refractory nature of the c. perfrinaens spore, this microbial indicator has practical use in assessing the magnitude of the impacts (source strengths) of sewage contaminated wastewaters on estuarine and marine environments.