Modeling Inactivation of Pathogens in Effluent Particles

A modeling approach was taken to study the inactivation of pathogens occluded in effluent particles. Model calibration was performed using samples collected from six full-scale facilities located in the U.S., representing a range of treatment strategies. Independent model calibration was performed for each of the six facilities using particle and disinfection data collected in laboratory tests. Model parameters selected as a result of calibration were then used to run simulations with varying inactivation coefficients that were selected based on a literature search to represent values expected for pathogens. The particle size was also varied to eliminate the effects of differing particle size distributions at the six facilities. The results suggest that effluents that are disinfected using total coliform as an indicator organism may still contain occluded pathogens that are viable. The results demonstrate some differences between inactivation with free and combined chlorine although there are site-specific disinfection characteristics that also affect the results. To clarify differences between free and combined chlorine to inactivate occluded microorganisms, additional laboratory experiments and modeling efforts are planned that will involve simultaneous disinfection experiments using free and combined on a sample collected from a nitrifying facility.