Pathogens and indicators in United States Class B biosolids: national and historic distributions

This paper reports on a major study of the incidence of indicator organisms and pathogens found within Class B biosolids within 21 samplings from 18 wastewater treatment plants across the United States. This is the first major study of its kind since the promulgation of the USEPA Part 503 Rule in 1993, and includes samples before and after the Part 503 Rule was promulgated. National distributions collected between 2005 and 2008 show that the incidence of bacterial and viral pathogens in Class B mesophilic, anaerobically digested biosolids were generally low with the exception of adenoviruses, which were more prevalent than enteric viruses. No Ascaris ova were detected in any sample. In contrast, indicator organism numbers were uniformly high, regardless of whether they were bacteria (fecal coliforms) or viruses (phage). Indicators were not correlated with pathogen loads. Historic distributions were collected between 1988 and 2006 at one location in Tucson, AZ. By comparing data collected before and after 1993, the influence of the USEPA Part 503 Rule on indicator and pathogen levels within Class B biosolids can be inferred. In general, the bacterial indicators total and fecal coliforms decreased from the 1980s to present. Enteric virus concentrations after 1993 are much lower than those reported in other studies in the 1980s, although our values from 1988 to 1993 are not significantly different from our values obtained from 1994 to 2006. Presumably this is due to better and more consistent treatment of the wastewater, illustrating that the Part 503 Rule has been effective in reducing public exposure to pathogens relative to 17 yr ago. The percent reduction of both indicators and pathogens during anaerobic mesophilic digestion was between 94 and 99% for all organisms, illustrating that such treatment is effective in reducing pathogen loads.