Inactivation of bacterial and viral indicators in secondary sewage effluents, using chlorine and ozone

The inactivation rates of indigenous populations of fecal coliforms, enterococci, Clostridium perfringens, male-specific bacteriophage and somatic coliphage in secondary sewage effluents were compared using chlorine and ozone as disinfecting agents. The fecal coliform indicator was evaluated for its reliability to index viral responses to chlorine and ozone treatments in fecally polluted effluents. Effluents were collected from several wastewater treatment plants during a variety of meteorological conditions and exhibited a wide range in quality. In chlorinated effluent, a greater than 100-fold reduction in both fecal coliform and enterococci densities was observed. However, densities of the two bacteriophage groups were generally reduced less than 10-fold. In contrast, ozone treatments resulted in the inactivation of bacterial viruses, with more than 100-fold reductions of male-specific and somatic bacteriophage densities. The elimination of vegetative bacteria was less dramatic in ozonated effluents with less than a 30-fold reduction in the densities of both of these indicators. C. perfringens was relatively insensitive to inactivation by either disinfectant; populations of this indicator were stable for prolonged contact periods. These findings demonstrate that the fecal coliform indicator is inadequate for predicting viral responses to chlorine or ozone treatments. Ozone contact time and residual concentrations were increased appreciably in later trials to further determine the sensitivities of fecal coliforms and enterococci to this treatment. However, increasing these parameters, alone or in concert, did not enhance the rates at which vegetative bacterial populations declined.