Extending the Risk Assessment Framework for Pathogens in Biosolids

Over the past decades the field of microbial risk assessment has advanced to the point where a framework for quantitative assessments of the microbial risk associated with the land application of biosolids is now available. Within this framework, a variety of models have been used for different exposure pathways. This study reviewed these options and proposed models for different exposure pathways. For aerosol exposure, existing approaches have been successful at reproducing downwind average exposures but have generally not reproduced the distinct spikes in downwind concentrations observed by field researchers. A Gaussian plume model with a time dependent source location is evaluated and found to be a suitable means to capture the dynamics of aerosol exposure, while still using a straightforward mathematical model that can be programmed in a spreadsheet environment. For both ground water and surface water pathways, the effect of wet weather events on transport of pathogens from biosolids is identified as a key gap which will require the use of more detailed hydrologic models in the risk assessment framework in order to provide realistic risk estimates. The first step in including wet weather events in the framework is developing an appropriate model of infiltration and runoff. In this study an explicit approach to estimating infiltration is compared to an implicit model. The explicit approach can be readily implemented in a spreadsheet environment, while the implicit approach requires complex numerical solution algorithms. The explicit approach is found to correspond closely with the implicit approach over a wide range of rainfall intensities and soil conditions. This indicates that the explicit approach is suitable for use in risk assessments. These modifications will extend the scope of the existing risk assessment framework while enabling it to be implemented in a spreadsheet environment.