By J. E. Smith, Jr., USEPA, Cincinnati, OH


High Priority Needs


Federal sewage sludge regulations require for some uses of sludge on land the analyses of fecal coliforms, Salmonella sp., enteric viruses, and/or helminth ova.  It is essential that the methods for analyzing these organisms be standardized and validated as quickly as possible.  Secondly, the approach taken by the regulations to minimize risks to public health from infectious disease causing organisms was to combine a disinfection step with a vector attraction reduction one.  Most facilities land applying today utilize a Process to Significantly Reduce Pathogens (PSRP), Class B, Alternative 2 Disinfection, like anaerobic digestion followed by the mandatory public access, grazing, and/or crop harvesting restrictions.  This combination of a PSRP Process and allowance for natural attenuation on the land is intended to reduce all pathogens to below the detection limit.  This assumption, while tested for some situations in the 1970s, needs to be validated.  Thirdly, studies done by EPA and others suggest that the sludges of 2004 are quite different from those of the 1960s, 1970s, and even early 1980s.  There is a large need to see what the levels are of organisms like fecal coliforms, Salmonella sp., enteric viruses, helminth ova, and the protozoan parasites in today’s raw sludge as well as in the treated sludges from PSRP processes.  This data collection will help in filling another major information gap; that of getting data on what pathogens, endotoxins, etc. people are exposed to at the application site and areas adjacent to it.


Pathogenic Organisms, Their Sources, and the Risks Posed


Disease causing organisms enter a community’s wastewater from patients in hospitals, homes, schools, and nursing facilities, to name some sources.  Animal wastes can contain zoonotic organisms and enter the wastewater from farms, meat packing and processing facilities and from rats and other animals and/or vectors found in or around sewage or sewers.  Humans come in contact with the pathogens and thus potentially disease by direct contact with sewage, by eating food or drinking water contaminated with sewage, or through contact with human, animal, or insect carriers.   During the course of typical wastewater treatment, the microorganisms in sewage are reduced in number, becoming concentrated in the sewage sludge.  Some organisms of concern were noted in the plenary presentation and are listed in the table below.  







Giardia lamblia



Cryptosporidium parvum




E. coli O157:H7



Yersinia entercolitica

Coronaviruses (?)

Entamoeba histolytica

Helicobacter pylori


Toxoplasma Gondii



Sarcosystis hominis

Listeria monocytogenes




The public can be exposed to treated sludge in several ways: direct ingestion, indirect ingestion (eating contaminated food like vegetables, meat, fish, etc.), dermal contact, inhalation, drinking contaminated water and/or recreating in contaminated water. 


Little experience is available for conducting a quantified microbial risk assessment.  The application of microbial risk assessment demands better data on infectious dose and the survival and transport of specific pathogens during the land application of wastes.  Paul Gale with WRc-NSF Ltd. in the UK illustrates one approach to doing a microbial risk assessment with “A Prototype Event Tree for Risk Assessment for Salmonella on Potato Crops at Point of Harvest.”  This is shown in the figure below.  Sludge treatment destroys 99% of the Salmonella allowing 1% remaining in the treated sludge. On application to the soil, 99% decay occurs in  5 weeks leaving 1% of that applied. Undoubtedly further decay will occur.  Dilution may be modeled as the probability of a tonne of potato crops colliding with a sludge particle. The dilution factor is 900. Therefore a potato has a 899/900 = 0.99888 probability of colliding with a soil particle. The probability of collision with a sludge particle is 1/900 = 0.00111.  Since each tonne of potatoes contains 0.02 tonnes of soil/sludge at point of harvest, 0.02 / 5 = 0.004 = 4% of the 5 tonnes dry solids of sludge applied to the hectare will be transmitted to the potato on collision with a sludge particle. 4.98 tonnes of the 5 tonnes of sludge applied = 0.996 = 99.6% will remain on the soil.  So starting with a tonne of raw sludge containing 5.4 x 108 salmonellas, one ends up with a tonne of potatoes potentially containing 44 salmonellas following treatment and dilution.  Eisenberg et.



al. from the University of California-Berkeley have assembled a risk assessment framework for infectious disease agents based on the following perspectives:  1) the attributable risk associated with one particular exposure pathway, such as biosolids, requires the consideration of the complete disease transmission system consisting of a collection of interdependent transmission pathways; 2) the level of uncertainty and variability inherent in estimating risk requires an approach that provides distributional information and not just point estimates of risk; and 3) the focus of a risk assessment should be on decision making and the sensitivity of a given decision on uncertain and variable parameter values.  Their modeling in particular considers the health effects of immunity and secondary infections.  They are now looking for sites with some data to test their model and where additional data can be collected to meet the needs of their relatively complex model.


Animal Wastes


Some years ago a senate report said that five tons of animal manure were produced annually and nationwide for every person living in the United States. Today there are approximately 238,000 animal feeding operations (AFOs) producing about 350 million tons of manure, which is about  130 times the quantity of solids as municipal wastewater treatment plants.  Confined animal feeding operations create the same amount of waste solids as a city of 10,000 people. 

Typically 40% of manure is managed onsite and the rest is exported and how it is handled is unknown.  Pathogens of concern in animal wastes are shown below.  The six microbial pathogens that



account for more than 90% of disease in humans that is acquired through food and/or water come principally from domestic livestock.  These organisms are: Campylobacter spp., Salmonella spp. (non-typhoid), Listeria monocytogenes, Escherichia coli O157:H7, Cryptosporidium parvum, and Giardia lamblia.  A significant feature relating to potential transmission for each of these organisms is the relatively low infectious dose.  For E. coli O157:H7 and Cryptosporidiosis the infective dose can be as low as 10 organisms or 10 oocysts and for all others except Listeriosis the infective dose is often less than 1000 organisms.  For Listeriosis the infective dose varies greatly and can be as low as 1000 organisms or greater than 10 billion and all of the factors that relate to this variability are not fully known.  The many recent epidemics such as Walkerton, Ontario, Canada, the Washington County, NY fairgrounds episode and Cryptosporidiosis in Milwaukee, WI are quite dramatic.


Land Application of Biosolids & Aerosols


The University of Arizona has now sampled in excess of 1000 aerosol samples during the application of treated sludge and around the country.  They have been unable to find microorganisms in the samples and thus forced to conclude they stay with the solid particles falling out shortly after the sludge is applied.  When they seeded water with coliforms and coliphages and spread the mixture, a totally different situation resulted.  Air samples did contain the microorganisms at up to almost 60 m from the application site.  Obviously wind, etc. has a considerable influence.


Closing Plenary Paper Comments


Lower levels of microorganisms including Salmonells sp., enteric viruses and helminth ova are now found in untreated sludges than were found two and three decades ago; good and consistent operation of the wastewater and sludge treatment processes is critical; time and temperature continue to be critical parameters for sludge disinfection;  globalization with the associated greater movement of people internationally, changes in food production and changing demographics all contribute to our seeing more emerging pathogens, say at a rate of at least one new one a year like the SARS and avian flu viruses.  More work is needed in the areas of pathogen evaluation and source tracking.  Jan/Feb'00 JEQ article on Bioaerosol Transport Modeling and Risk Assessment in Relation to Biosolids Placement by Dowd, Gerba, Pepper and Pillai at the Univ. of Arizona considerably overestimated risks.  A multiplication factor was misapplied in the calculations; the authors will be publishing an errata commentary.  "The University of Arizona study that reports on the absence of Staphylococcus Aureus has been peer reviewed and recently published (Rusin et al. ES&T, 2003, 37: 4027-4030).  While S. aureus was found in untreated sludges it was never found in treated sludges or aerosols sampled.


Discussion Groups


The report of a 2001 jointly sponsored USEPA-US Department of Agriculture expert meeting on Emerging Infectious Disease Agents and Issues Associated with Animal Manures, Biosolids and Similar Byproducts is in press (Smith, Jr. et al., 2004).  Several issues and research needs were identified at the workshop and in related literature.  New organisms of concern were identified including the bacteria: E. coli O157:H7, Listeria, and Helicobacter; viruses: Poliovirus, Coxsackievirus, Echovirus, Hepatitis A, Rotavirus, and Norwalk agents; and parasites: Cryptosporidium, Cyclospora, Toxoplasma, Microsporidia, Balantidium, Giardia, and Entamoeba.  Already discussed in the plenary paper, here, and in the proceedings were the following needs: the microbiological methods for fecal coliforms, Salmonella spp., enteric viruses, and helminth ova require standardization and validation; work is needed to better document the presence of pathogens and other organisms in raw sewage sludge and their fate through the various treatment regimes including their survival in or on the soil or on crops after application of the treated sludge; and field verification of the efficacy of Class A and Class B treatment processes (including data to directly relate process controls to initial and final pathogen and indicator densities) is needed.  Testing should include the presence, movement, and content of any aerosols.  Animal waste associated needs identified were completing a manure related infectious disease incidents database; identifying applicable treatment technologies for reducing pathogens (and possibly vector attractiveness) from experiences with agricultural wastes and industrial and municipal wastes (for example: anaerobic digestion, aerobic digestion, lime treatment, composting); and developing information regarding the degree to which food or water borne illness pathogens can survive in manure-treated soils or migrate onto harvestable plant parts.


Bacterial pathogens of concern were ranked according to public health significance (in declining order) as Salmonella sp., E. coli strain 0157, Shigella sp., Campylobacter sp., Yersinia sp., Legionella sp., Pseudomonas sp.


Methods suggested for treating animal wastes and their expected performance is shown in the table below.  It is largely based on experience with treating sewage sludge.


Nematodes, tapeworms, protozoa, microsporidia are expected to survive if not totally, at least partially,  treatment by processes like anaerobic digestion, aerobic digestion and lime stabilization.  The ranking of parasitic organisms with regard to hazards posed to human health is shown in the table below.



































Rank is the score given to each organism based on Immune Status, Health Impact, Treatment Availability as an indication of hazard to human health.


Additional Comments made during Deliberations


In discussing the sludge survey and pathogen occurrence, concerns about antibiotics and personal care products were expressed.  In addition the need to document the effects of organisms in the soil as well as soil characteristics on any pathogens applied with the sludge were issues.  What influence does a storm have on runoff?  The need for better indicators of the presence of pathogens was also expressed. 


In addition to validated methodologies for the organisms called out in 40CFR503 was previously discussed, participants also wanted information on sampling protocols, sample preparation and a laboratory certification program.  


Several concerns were expressed about the fact that the treatment processes for sludge need to be reliable (good equipment and good operation) and when there appears to be a health effect, there needs to be a mechanism to investigate it and learn why the person became ill.  Further plant personnel as well as authorities need to know how best to communicate with the public. 


Some last minute thoughts in discussion groups included the importance of defining the roles of temperature, moisture, ammonia, and soil type in reducing pathogens; concerns in handling septage, and the role of an “Information Sharing Group.”