United States Environmental Protection Agency Office of Health and Environmental Assessment Washington, D.C. 20460 Research and Development EPA/600/S6-90/002 Apr. 1990 &EPA Project Summary Pathogen Risk Assessment for Land Application of Municipal Sludge Volume I: Methodology and Computer Model Volume II: User's Manual This document describes a methodology and associated com- puter model for assessing the risk to humans of pathogens in treated municipal sewage sludge applied to land. Land application of sludge in this methodology refers to the dis- tribution of sludge on or just below the soil surface where it is employed as a fertilizer or soil conditioner for growing human food-chain and non- food-chain crops. The two categories of land application addressed in this model are (1) agricultural utilization and (2) distribution and marketing (D&M), and the source of microbial pathogens is (1) liquid or (2) dried or composted municipal sewage sludge. This Project Summary was developed by EPA's Environmental Criteria and Assessment Office, Cincinnati, OH, to announce key findings of the research project that is fully documented in two separate volumes of the same title (see Project Report ordering information at back). Introduction Section 405 of the Clean Water Act requires the U.S. Environmental Protec- tion Agency to develop and issue regulations that identify: (1) uses for sludge including disposal; (2) specific factors (including costs) to be taken into account in determining the measures and practices applicable for each use or disposal; and (3) concentrations of pollutants that interfere with each use or disposal. To comply with this mandate, the U.S. EPA has embarked on a program to develop four major technical regulations: land application, including distribution and marketing; landfilling; incineration and surface disposal. The development of these technical regula- tions requires a consideration of pathogens as well as chemical constitu- ents of sludge. Public concern related to the reuse and disposal of municipal sludge often focuses on the issue of pathogenic organisms. The purpose of this report is to describe a proposed methodology and associated computer model designed to assess the potential risks to human health posed by pathogens in municipal sewage sludge applied to land as fertilizer or soil conditioner. Volume I: Methodology and Computer Model describes the conceptual frame- work of the risk assessment methodology and the structural organization, including assumptions and components, of the computer model. Volume II: User's Manual contains background information to provide the user with an understanding of the actual functioning of the model. This information includes descriptions of operating variables and their default values, explanations of the various subroutines, and the mathematical basis for process and transfer functions. ------- Approach The approach used for the model provides a structure capable of support- ing both stochastic and deterministic mathematical relationships, i.e., it is a dynamic model that can incorporate site- specific data while allowing process functions to be dependent on environ- mental factors, such as temperature and rainfall. The model structure provides a flexibility that permits addition and/or deletion of sludge management practice compartments as well as modifications in process and transfer functions. The model is designed to run on a personal computer with a minimum of 540 KB of free memory. Currently limited by a lack of data, the model will be able to utilize data gathered in the future to enhance its predictive accuracy. The purpose of the model is to determine the probability of infection of the human receptor from pathogens in the land-applied sludge. The ultimate objective is to use the model to assist EPA in its regulatory activities, but the immediate uses include (1) further development of the pathogen model as a research and risk assessment tool and (2) the application of the methodology in the performance of actual pathogen risk assessments. The five municipal sewage sludge management practices addressed by the model are: application of liquid treated sludge (1) for production of commercial crops for human consumption, (2) to grazed pastures, and (3) for production of crops processed before animal consump- tion; and application of dried or composted sludge, (4) to residential vegetable gardens, and (5) to residential lawns. Model Structure The computer model represents the compartments and transfers among compartments of the five management practices. The compartments are the various locations, states, or activities in which sludge or sludge-associated path- ogens exist; they vary to some extent among practices. In each compartment, pathogens either increase, decrease, or remain the same in number with time, as specified by "process functions" (growth, die-off or no population changes) and "transfer functions" (movement between compartments). The population in each compartment, therefore, generally varies with time and is determined by a combination of initial pathogen input, "transfer functions" and "process functions." The populations of pathogens in the compartments representing human exposure locations, together with ap- propriate intake and infective dose data, are used to estimate human health risk. Considering modern disposal prac- tices, almost any pathogenic organism can be found in municipal sewage. Because of the difficulty of designing a model that could accurately simulate the survival arid environmental movement of more than a few microbial species, organisms or organism groups were se- lected to represent the enteric pathogens most commonly found in sludge. The current version of the model deals with only three of these selections: Salmonella spp. representing the bacteria; Ascaris lumbricoides the parasites (both hel- minth worms and protozoa); and enteroviruses (a grouping of several animal viruses), the enteric viruses. Exposure of an individual to enteric pathogens can lead to (1) no effect, (2) a subclinical (asymptomatic) infection or (3) a clinical (symptomatic) infection. Although subclinical infections are not clinically detectable, that individual by either direct or indirect transmission of the pathogenic organisms may cause disease to develop in others. In this methodology, infection rather than disease is used to measure risk. Exposure pathways, i.e., migration routes of pathogens from or within the application or disposal site to a target organism or receptor, for sludge applied to land include the following: • inhalation or ingestion of emissions from application of sludge or tilling of sludge/soil; • inhalation or ingestion of windblown or mechanically generated particulates; • swimming in a pond fed by surface water runoff; • direct contact with sludge-contam- inated soil or crops (including grass, vegetables, or forage crops); • drinking water from an offsite well; inhalation and subsequent ingestion of aerosols from irrigation; • consumption of vegetables grown in sludge-amended soil; • consumption of meat or milk from cattle grazing on or consuming forage from sludge-amended fields. Since the model provides only an approximation of environmental transport mechanisms, it does not represent every possible exposure pathway. It does, however, trace the flow of pathogens through the major routes leading to possible human exposure. Infection The dose required to cause infection is based on the virulence or infectivity of the pathogenic organism and on the susceptibility of the exposed population or individual receptor. The "minimum infective dose" or MID is typically the dose required to infect 50% of the population. The uncertainty in measuring infectious doses greatly weakens the power of any quantitative risk assess- ment. The model is designed, therefore, so that the user can supply a best estimate of infectious dose for the particular pathogen and practice being modeled. Risk assessments ordinarily proceed from source to receptor. That is, the source, or sludge disposal/reuse practice, is first characterized, and contaminant movement away from the source is then modeled to estimate the degree of exposure to the human receptor. Health effects are then predicted based on the estimated exposure and dose-response relationships. This computer model sums the hourly exposures of a human receptor to pathogens in each exposure compartment and computes the daily (24-hour) probability of the human recep- tor receiving an exposure exceeding an infective dose (e.g., for Salmonella the default MID = 10). Uncertainty Analysis Many factors contribute to the uncertainties associated with the present risk assessment model. Chief among these is the lack of quantitative data describing the processes involved. Even when available, data are highly variable with regard to (1) the initial concentrations of microbial pathogens in wastewater and sludge; (2) processes of microbial transport and inactivation; (3) dose-response relationships; and (4) exposure levels and receptor susceptibility. A sensitivity analysis was performed, but because of the large number of input parameters and the uncertainty related to the values of parameters, it should be viewed as preliminary. However, the analysis does indicate that the model is very sensitive to the inactivation rate of microorganisms in soil, as well as to the parameters used to calculate the frac- tions of pathogens transferred from surface soil to subsurface soil, from subsurface soil to groundwater and from surface soil to surface runoff water. Accordingly, these parameters should be selected with great care, especially as ------- they are all likely to be site-specific. Because the data available to support choices of the values are limited, research efforts should be directed to these areas in order to increase the accuracy of the model. Model Use The user of the model has the option of modifying many of the operating variables used by the program to calculate the number of organisms present in the compartments. These variables include initial pathogen concen- tration; organism type; environmental parameters such as soil temperature and moisture, air temperature, windspeed, rainfall amount and frequency; and parameters descriptive of the land application practice modeled such as size of field or garden, type of crop, irrigation method and frequency, timing and yield of harvest or size of cattle herd. There are also variables for the subroutines that represent rainfall, groundwater and crop preparation practices. Operation of the model is governed by a series of compartment differential equations, trans- fer factors and times, and process functions. These equations calculate the increase or decrease of pathogens within each compartment and the transfer of pathogens from one compartment to another at intervals during the model run. Transfers may be conditional on a particular applications option such as spray irrigation, on time after initiation of the practice or on physical processes such as rainfall or dust storms. The more complex transfer functions are those resulting from generation of particulate emissions by wind erosion or tilling, from generation of liquid aerosols during spray irrigation using wastewater, from transport of soil-bound microorganisms by surface runoff associated with rainfall or from subsurface transport of pathogens associated with groundwater movement. Conclusions Preliminary conclusions drawn from sample runs of the model suggest that the most likely candidate for infection is an onsite human receptor who ingests pathogens from direct contact or from contaminated surface runoff confined onsite; the probability of infection of an offsite human receptor seems relatively low if surface runoff does not move offsite. ------- Worm Kowal is the EPA Project Officer (see below). The complete reports, entitled "Pathogen Risk Assessment for Land Application of Municipal Sludge, Volume I: Methodology and Computer Model," (Order No. PB 90-171 901/AS; Cost: $23.00) and "Pathogen Risk Assessment for Land Application of Municipal Sludge, Volume II: User's Manual," (Order No. PB 90-171 919; Cost $31.00) (costs sub/ect to change) will be available only from: National Technical Information Service 5285 Port Royal Road Springfield, VA22161 Telephone: 703-487-4650 The EPA Project Officer can be contacted at: Environmental Criteria and Assessment Office U.S. Environmental Protection Agency Cincinnati, OH 45268 United States Center for Environmental Research Environmental Protection Information Agency Cincinnati OH 45268 Official Business Penalty for Private Use $300 EPA/600/S6-90/002 ------- |