Removal and inactivation of microorganisms occluded in effluent wastewater particles using filtration and chlorine disinfection

Within the wastewater treatment industry there is a trend toward increasingly stringent discharge standards resulting from the discharge of treated wastewater into sensitive water bodies and the practice of water recycling. One concern is that effluent wastewater particles can harbor microorganisms and shield them from disinfectants such as ultraviolet light and chlorine. To ensure occluded microorganisms are sufficiently reduced, particles can either be removed using filtration or sufficiently disinfected such that the occluded microorganisms are inactivated. In this work a combined experimental and modeling approach was taken for improving understanding of the removal and inactivation of occluded microorganisms in plant effluents. First, a model was developed for quantifying the presence of occluded microorganisms in plant effluents and demonstrated by collecting samples from an extended aeration treatment facility. The results indicate that heterotrophic bacteria were present in most particles at high concentrations, while aerobic endospores and total coliform bacteria were only present in a small fraction of particles. In addition to the model for occluded microorganisms, an integrated inactivation and radial pore diffusion model was used to estimate the efficacy of chlorine disinfection for inactivating microorganisms occluded in different sized particles. The integrated model was calibrated using disinfection and particle data collected from six operational facilities in the U.S. and model simulations were conducted to analyze the effects of altered particle counts and initial chlorine concentrations. The results indicate that chloramines were more effective for disinfecting occluded microorganisms than free chlorine products. The results also indicate that facilities that are designed and operated based on inactivating total coliform may be producing treated effluents that still contain viable occluded pathogens. The approach presented herein addresses an important problem: existing practice and regulatory frameworks do not directly address the issue of occluded microorganisms and the performance metrics used (e.g. turbidity) are not directly related to health risks. This work provides a significant contribution for providing a more meaningful use of particle counts for understanding the fate of occluded microorganisms in filtration and disinfection systems as well as the true concentration in treated effluents.