The use of plume dispersion modelling for viable aerosols from an activated sludge sewage treatment plant

Predictions of the emissions of airborne total viable particle (TVP) concentrations from sewage are of concern due to possible adverse human health effects. The work presented here is an assessment of the feasibility of using environmental data to develop a plume dispersion model for the prediction of airborne TVP concentrations in the vicinity of an activated sludge sewage treatment plant. The monitoring data used were collected during an epidemiology study of the health of residents in the vicinity of an activated sludge sewage treatment plant located in a suburb of Chicago. Two types of modelling approaches were explored: dispersion modelling such as the Gaussian plume dispersion model and statistical modelling. The results of the statistical model have been reported elsewhere. The Gaussian modelling approach was limited because emission rates and rate of microbial die-off could not be measured, and techniques for their estimation proved inadequate. The die-off rate is normally expressed as a negative rate of change, but only positive coefficients were found. Although microbial growth in air is possible under certain conditions, it is thought that high background TVP concentrations due to many undefined sources of TVP in the study area were chiefly responsible for the positive coefficients. Until a better method is found for estimating the ambient total viable decay rate and defining background concentrations, the Gaussian plume dispersion modelling technique does not appear to provide a more useful approach for estimating TVP concentrations downwind from an activated sludge sewage treatment plant than does statistical modelling. Rather, the modelling process served to identify deficiencies in the application of the Gaussian plume model to ground level source aerobiological emissions when there is a significant background concentration.