Comparative Microbial Risks of Land Applied Biosolids and Animal Manure

The transmission of pathogens by land application of untreated human and animal wastes has been known for more than 100 years. In the United States there are more than 450,000 (EPA) animal feeding operations producing more than 100 times as much manure as sewage sludge produced by municipal wastewater treatment plants. In addition, grazing animals also deposit large quantities of manure on range land. This study attempts to look at the relative risks of pathogens in biosolids vs. animal manure (feces) applied to land using a quantitative microbial risk assessment approach. This process involves four basic steps: pathogen identification, exposure assessment, dose-response and risk characterization. Several examples which were evaluated included: risk to workers, consumption of produce crops, and children playing in a field. While some of these exposures are less likely for biosolids applications because of site and application restrictions, they can commonly occur in areas where manure is applied. The risks are largely determined by the degree of treatment that the manure receives before land application, and risks compared to biosolids are often greater or comparable, depending upon assumptions used in the risk model. Generally, annual risks of infection were greater than the 1:10,000 chance of infection for bacteria (manures) and viruses (biosolids) when land applied and assuming an 1 month decay time period. Using more appropriate conditions of pathogen die-off in the soil (> 6 months), land application of either residual resulted in risks less than 1:10,000 per year. Assuming USDA-AMS and USEPA recommended guidelines and regulations are followed for crops grown on waste-applied land, risks associated with manure or biosolids land application are well below the 1:10,000 per year risk of infection standard. This analysis provides insight on the relative risks of animal waste and biosolid land application that allows for a better understanding of the risks to the general public and the industry.