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SECTION IV: PATHOGEN AND ODOR ISSUES

4.1 Introduction
4.2 Pathogen Reduction Methods and Vector Attractiveness Reduction
4.3 Health Risks
4.4 Evidence of Pathogenicity
4.5 Risks Posed By Bioaerosols
4.6 Odors and Atmospheric Transport
4.7 Transport into Groundwater
4.8 Summary

4.1 Introduction
There is some debate about the potential for biosolids to present a risk to public health. The Class A and B standards have only a small fraction of the number of pathogens found in raw sewage or manures. The debate has focused on whether biosolids contain viable pathogens that could be infectious. Dosing could occur if biosolids were ingested via direct contact, inhaled as bioaerosols, or possibly through exposure to soil containing biosolids residues. The magnitude of these risks is a source of contention because risk-assessment and the protection of public health is one of the objectives of the regulations; the completeness of the regulatory assessment has been questioned (Smith and Perdek, 2004).

Biosolids are derived from human wastes and Class B biosolids are allowed to contain markedly reduced populations of viable enteric bacteria and possibly-pathogenic organisms (bacteria, viruses, and parasites) that have survived the treatment process. The biosolids also contain other organisms that helped to convert the raw sewage into a sewage sludge and ultimately become incorporated into a Class A or B biosolids. The survival of pathogens during the production of biosolids and the ability of these organisms to be infectious is a fundamental public-health concern addressed by the US EPA in the federal rules (Smith and Perdek, 2004). Maine, under Chapter 419, also follows the US EPA guidelines.

Biosolids are classified on the basis of pathogen reduction (Chapter 419, Section 4 (I)). The goal for Class A pathogen reduction is to destroy or inactivate pathogens to a concentration equivalent to natural background content in soils. Class A biosolids must have a density of Salmonella bacteria that is less than three Most Probable Number (MPN) per four grams of total solids (Standard Methods). This pathogen reduction goal can be assumed to have been met if the density of fecal coliform has been reduced to less than 1000 MPN per gram of total solids. The Class B standard requires that pathogens be reduced by 90 per cent. Animal manures have many times more pathogens than even Class B biosolids (Moss et al., 2002; Pyke et al., 2002).

In addition to the question of pathogen content, sewage sludge, and some biosolids have odors. Human wastes have a distinctive odor that most people find unpleasant (Witherspoon et al., 2004). Sewage sludge (unprocessed) has a distinctive, and possibly offensive, odor. The intensity and composition of the odors in biosolids can vary from strong to nearly none. Biosolids may have odors not unlike sewage sludge, or may even smell earthy when processed into compost. Odors associated with biosolids are typically the primary cause of complaints from those living near land-spreading sites, even though Maine requires a minimum 300-foot setback because of odors. The question arises: do biosolids pose a risk if they produce an odor?

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4.2 Pathogen Reduction Methods and Vector Attractiveness Reduction
Pathogen reduction is an essential part of converting sewage sludge into biosolids. Maine recognizes nine methods to produce Class A biosolids and six methods to produce Class B biosolids (Table V). The standards allow the Maine DEP to evaluate the use of new methods that can meet the pathogen reduction goals. In Maine, composting is the most commonly employed method to produce Class A biosolids, followed by alkaline stabilization. Most of the Class B biosolids meet the pathogen reduction goal by using the lime-stabilization technique. The goal of these processing methods is to destroy or inactivate pathogenic organisms (Capizzi-Banas et al., 2004). The performance standards for Class A Biosolids include monitoring these four groups of pathogenic organisms:

• Salmonella
• Fecal coliform
• Enteric viruses
• Helminth ova.

TABLE V. Biosolids Processing Methods (06-096 CMR Chapter 419)

Closely associated with the destruction or inactivation of pathogens is vector reduction. A vector is an organism that is attracted to the biosolids and has the potential to transfer pathogens. Example vectors are flies, mosquitoes, and rodents. In Maine, the Chapter 419 (Appendix B) rules address vector reduction. The Maine rules set vector attractiveness reduction goals involving the control of volatile solids. Volatile solids are organic compounds that can be evolved from the biosolids, some of which are odor causing compounds. It is assumed that since volatile solids may include odors that attract vectors, reduction controls odors and thus attractiveness to vectors. The preferred attractiveness reduction standards for Class B biosolids are: (1) direct injection into the soil; (2) incorporation into the soil within six hours of application; or (2) use of alkalai (e.g. lime) to raise the pH to 12 for two or more hours and then have it remain at pH 11.5 for 22 more hours.

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4.3 Health Risks
Defining a health risk due to residual pathogens in biosolids is difficult because of uncertainties in determining actual exposure routes and the designation of an exposed population (Harrison and Oakes, 2002). In addition, there have been few rigorous epidemiological studies of biosolids utilization (Lewis and Gattie, 2002). Many of the researchers who have studied the exposure health risks have concentrated on wastewater-treatment-plant workers, or workers at composting facilities, where the potential for exposure to pathogens is greatest. A cohort study of treatment plant workers, using a cumulative 6,886 person-years of exposure, found no elevated cancer risks relative to the general population (Lafleur and Vena, 1991). Risks have been assessed by considering the type of pathogen that can occur in biosolids and how long they can survive under exposed conditions (stockpiles, or after spreading). The presence of pathogens in biosolids does not necessarily imply that a person would become ill after exposure (Epstein, 1998; NIOSH, 2002). Illness occurs when two events happen in sequence: (1) exposure to a sufficient quantity of pathogens from inhalation or ingestion; and (2) the dose of pathogens must be in a sufficient quantity to overwhelm the immune system’s ability to contain the pathogen. NIOSH (2002) does not view biosolids as presenting an extraordinary health risk and recommends that workers exposed to Class-B biosolids employ good environmental practices and use care in maintaining good personal hygiene.

According to the commonly accepted definition, a pathogen is any organism or genetic substance that causes disease; bacteria, viruses, parasites, cell substances, and fungi are all potential pathogens (Epstein, 1998). Some pathogens are sufficiently aggressive that they can invade and infect any healthy individual. For example, a cold virus can quickly spread through a commingled population. Many other pathogens can only affect people predisposed with weakened or suppressed immune systems.

Some pathogens do occur in biosolids and survive well past the time of land application (Epstein, 1997, 1998; Millner et al., 1994). The distinction needs to be emphasized that Class B biosolids have reduced content of pathogens while Class A biosolids have pathogen content nearly equal to natural soils. Additional pathogen reduction likely occurs in, or on the surface of, soil. Soils are full of predatory microorganisms, while the ground surface is subjected to the sterilizing effects of ultraviolet radiation (UV). These two factors work against the survival of pathogens from biosolids (Epstein, 1998). Lan et al. (2004) reported that e. coli decayed to background concentrations in biosolids in less than 96 days. Pathogens associated with biosolids and their persistence in soils are summarized in Table VI; note that local climate will affect these values significantly. The persistence of pathogens is the basis for the 30-day and one-year rules restricting access and use of fields receiving Class B biosolids. Epidemiological data contain evidence that these rules for biosolids work, along with other measures to protect food quality. Health survey data collected by the Center for Disease Control (CDC) for food transmitted pathogens show a significant decline in occurrence rates between 1996 and 2002 (http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5316a2.htm).

TABLE VI. Pathogens in Biosolids and Their Persistence in Soil (modified from Epstein, 1998).

Work continues to define the risks to human health with greater certainty. Two variables are needed to quantify the risk from pathogens: (1) a human health effect that is a function of a given pathogen; and (2) an exposure risk that is a function of a given biosolids product and the route of exposure. Combining these two variables defines the actual health risk posed by biosolids to people. Colford et al. (2003) have developed a theoretical dynamic model using these two variables. The output of the model is a matrix that differentiates exposure and risk for different scenarios. The utility of this model is limited because some of the key variables have been estimated, not measured. The model will not be capable of determining human health risks until the biosolids exposures are quantified with more precision.

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4.4 Evidence of Pathogenicity
The presence of pathogens in sewage sludge is unarguable; as is the reduction in numbers of pathogens when the sewage sludge is processed into biosolids. The ability of the remaining pathogens in biosolids (applicable to Class B) to be infectious is less certain (Lewis et al., 2002). Is there evidence that exposure to biosolids can cause illness? The summary that follows focuses on two specific routes of exposure: inhalation and ingestion in water. This is based on the current practices that limit the potential for direct human contact by keeping the general public away from land application sites. Thus, the risks posed by direct ingestion are minimized by site access control. These pathways both require the transport of pathogens through another media (air or water) before they can have human contact. Very small and biologically active particles that are transported by air currents are called bioaerosols. These particles can range in size from 0.02 to 20 micrometers in diameter (Pillei and Ricke, 2002).

Bioaerosols in Confined Composting Facility. Epstein et al. (2001) reported on two documented cases of worker reaction to bioaerosols (dust) at an enclosed composting facility:
Case No. 1: A worker reported respiratory discomfort that disappeared on weekends and,
Case No. 2: A worker developed a rash.
A survey of the facility found respirable quantities of dust, endotoxins and non-viable Aspergillus. Although there are no air quality standards for these aerosols, they appeared to have caused allergic reactions in the sensitized individuals. This problem was managed using water to minimize dust and improving ventilation; no lasting symptoms were reported.

Evidence of Infection - Farm Application. A rigorous study of biosolids and ill health was performed in Ohio between 1977 and 1983 (Ohio Farm Bureau, 1985). The Ohio study concluded that the health of residents on 47 farms receiving sewage sludge were no different from the residents of 45 control farms. The occurrence of illness was the same for both groups. A complaint response in an agricultural area studied by NIOSH (Burton and Trout, 2000) identified enteric bacteria as the source of illness. The NIOSH study did not link the illness occurrence with land spreading of biosolids. All of the pathogenic organisms identified as potential infectious agents in the NIOSH study occurred in the natural background. Gattie and Lewis (2002) conducted a retrospective analysis of health effects reported as associated with biosolids utilization and reported ill-health concerns in the general public in association with land application of biosolids (Class B).

Some authors have cited that the limited number of comprehensive health-related studies and the lack of timely investigations of complaints have prevented the development of any generalized conclusions (Harrison and Oakes, 2002; Brobst et al., 2004; Lewis and Gattie, 2004). As an example of how ad hoc studies can add to the confusion, a study of treatment plant workers found overall very good health and the authors proposed that this was due to a robust-worker effect (Bunger et al., 2000). Exposures are greater at sewerage treatment plants than at biosolids utilization sites. This can be interpreted to mean that reasonably healthy individuals in the general public have a minimal pathogen-exposure risk associated with well-managed biosolids utilization.

The research reviewed presented information about the complex interactions that define a minimum infective dose; a valuable measure of risk but nearly impossible to establish. Stated simply, this concept attempts to define how many pathogenic organisms are needed to make someone sick. Lewis and Gattie (2004) expressed a concern that the other chemicals present in biosolids could act as irritants that then allow fewer opportunistic pathogens to infect. A synergistic effect between pathogens and irritating chemicals opens a new area of risk assessment. The irritant-infection hypothesis is tempered by the very small number of documented health problems related to exposure to biosolids (Epstein, 1998).

Retrospective analyses of wastewater treatment plant workers have identified some immune system response to endotoxins and enteric pathogens (i.e. evidence of pathogens being controlled by the immune system). The endotoxins come from the breakdown of gram-negative bacteria that are produced in quantities during the sewage treatment process (Gattie and Lewis, 2002). Exposure to endotoxins in dust at a composting facility in Colorado has been cited as a potential health risk (Darragh et al., 1997). Immune system responses are how a body fights off illness or infection; this is a normal and healthy reaction. A survey of 58 workers at compost facilities found a small increase in symptoms of air passageway irritations and gastrointestinal symptoms, but the workers were found to be overall healthier than the general population (Bunger et al., 2000). The exposure of workers at a treatment plant or compost facility is very different from that of the general public. Exposures at a minimum 300-foot setback from spread fields are significantly less than the exposures to workers at treatment plants; especially since biosolids are processed to reduce their content of pathogens.

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4.5 Risks Posed by Bioaerosols
Bioaerosols are very small, biologically active particles containing viruses or bacteria that may become airborne by themselves or in attachment to other fine particles. Bioaerosols are biological particles in the size range of 0.02 to 100 micrometers. These particles are subject to many forces that cause them to be mobile and to form concentration gradients away from a source (Suresh and Pillai, 2002). Very-fine particles such as bioaerosols could move as much as a kilometer from a source, likely much less depending upon a variety of environmental conditions such as terrain roughness, wind speed and relative humidity (Dowd et al., 2000; Brooks et al., 2004). There is disagreement about the risks posed by bioaerosols, with one camp (e.g. Lewis et al., 2002) associating infections with exposure and the other camp stating that bioaerosols present negligible risks (e.g. Millner et al., 2004). The ability of pathogens to infect individuals is strongly dependent upon the specific organism, the transfer pathway, and the health of the exposed population (Eisenberg and Cicmanec, 2004).

Epstein (1998) noted that the lack of significant pathogenicity from bioaerosols is supported by the absence of independently documented dose-response illness associated with biosolids utilization. Millner et al. (2004) found that properly processed biosolids, Class A or B, maintained at a high pH will have no pathogenic bioaerosols. A survey of 15 different sites in the United States found that staphylococcus aureus was absent in biosolids and aerosols (Rusin et al., 2003). The existing rules for setbacks and access controls can be inferred to protect the general public (Gerba and Smith, 2004 and 2005; James and Perdek, 2004).

A lack of dose-response does not correspond inconclusively with the absence of pathogenicity in bioaerosols as based on a limited number of epidemiological studies (Lewis and Gattie, 2002; Brobst et al., 2004). Evidence of pathogenicity has been documented for exposures of workers at biosolids processing facilities (Epstein, 1998; Bunter, 2000). Exposure of the general population near biosolids utilization sites is based on few controlled studies. Vulnerable populations may be sensitive to certain pathogens, or they may be more likely to become ill from many causes. Associations between biosolids applications and ill health have been cited by Lewis et al. (2002) and Gattie and Lewis (2004). The Cornell Waste Management Institute (2004) maintains an incident list to track health related complaints. All of these associations of biosolids and ill-health come from anecdotal sources. Self-reporting systems are used to assess the public health issues when regulatory agencies do not track complaints. Such surveys need to be interpreted with care since they lack the needed experimental control for epidemiological analysis and self-reported data could be fabricated by unscrupulous individuals.

In an opinion paper by Lewis et al. (2002) they surveyed a total of 54 people who lived within one kilometer from one of 10 different biosolids utilization sites. The survey of health issues was self-reported and it was not clear how the communities surveyed valued the use of biosolids (i.e. were the participants randomly selected?). Although the size of the population exposed was not stated, approximately 25 per cent of those surveyed (13 people) reported symptoms ranging from skin or upper respiratory tract irritations, to staph infections, to flu-like symptoms. Gattie and Lewis (2004) characterized the health complaints as mostly irritations to the skin, mucous membranes, and respiratory tract; the type of symptoms expected from exposures to volatile compounds and dust in any setting. Symptoms of gastro-intestinal disorders were much less frequent. They interviewed affected individuals and disclosed that the reported exposed population was deeply distrustful of the existing scientific research on biosolids. Also, the same people said that their greatest concerns were about the noticeable odors, vectors (especially flies), and the potential for adverse health effects. This study highlights an important psychological response to biosolids that associates the detection of malodors with pathogens so that any sign of illness is assumed to be caused by the odor. Odors associated with other agricultural practices such as manure spreading were not described in this study. Relative to the volume of biosolids that are land-applied each year in Maine, the number of reported health problems directly related to biosolids is very small.

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4.6 Odors and Atmospheric Transport
The utilization of biosolids, and Class B biosolids in particular, brings attention to itself because of its unique odor. This is an attribute shared with other agricultural materials such as manure. Negative comments about Class B biosolids almost always include mention of their odor. Class A biosolids typically have less intense odors. The association of odors with potential pathogens is a commonly cited trigger for community response to biosolids utilization (Tyson, 2002; Witherspoon et al., 2004). Odor management is an important concern for more than just the a nuisance value. At another extreme of the waste-management field, research has cited confined animal feeding operations as generating odors that pose a health risk (Schiffman and Williams, 2004 and 2005).

What constitutes odor in biosolids? Odors are caused by various organic compounds whose occurrences change with the source of the sewage sludge and the techniques used to transform it into biosolids. The odor compounds are formed as the natural product of biological processes and the decay of organic matter, be it vegetation, manure, or municipal wastes. Common odor-forming compounds are volatile fatty acids that smell like rancid butter or vinegar; ketones that have a slightly sweet smell; aldehydes that may have a pungent odor; amines that have a fishy smell; indole and skatole that have a fecal odor; and phenol that smells like antiseptic (Rosenfeld et al., 2001). The nitrogen in biosolids may be in the form of ammonia which has a characteristic sharp odor, or the fishy odor of an amine. Odor causing compounds containing sulfur are hydrogen sulfide (rotten eggs), dimethyl sulfide (skunk), and methyl mercaptan (rotten cabbage); all of these odors may be persistent. Humans are able to detect some of these odor compounds at very small concentrations - 0.000026 to 20 parts-per-million (Amoore and Hautala, 1983; Rosenfeld and Suffet, 2004).

It is important to distinguish between odor concentration, the amount of an odor in a given volume of air, and odor intensity, the strength of the human reaction to an odor compound. Concentration can be measured using standardized laboratory methods to generate an absolute value. There is no absolute measure of intensity because of the differences in our perceptions of odor and our individual abilities to smell odors that complicate potential instrumental analysis (Gostelow et al., 2001). There has been a method for measuring the intensity of odors in air, ASTM Method D1391-57 “Measurement of Odor in Atmospheres”. This method uses the dilution-to-threshold (DT) principle to measure the intensity of an odor in air. Another method, ASTM Method E679 uses a triangular forced choice to determine specific odor thresholds in people; this is a perception rather than absolute test.

There has been some research conducted to determine the amount of dilution needed to minimize biosolids odor. Caballero et al. (1997) reported that a compost facility in California needed up to 1,400 volumetric dilutions to threshold. This means that one cubic foot of air with an odor needed to be added to 1,400 cubic feet of clean air. Rosenfeld and Suffet (2004) also reported that some fresh biosolids may have a DT greater than 7,000 when fresh, that falls to ~3,000 within one week. Although these dilution values appear to be large, since volume increases by the cube of distance, a 3,000-fold dilution could occur within 12.5 feet.

Schiffman et al. (2000) argued that odor causing compounds, primarily from confined animal feeding operations, are also capable of causing illness, even without pathogens. Runny noses, itchy eyes, sore throats, coughing, etc., are all symptoms of allergic reactions to odors. That is, some people may have a temporary allergic reaction which is different from a response to simple irritation. Typically, exposed individuals recover quickly after removal from the odors (irritation response). Sensitive individuals, such as asthmatics, may experience much more severe reactions (allergic response). It is very difficult to distinguish the potential responses of sensitive individuals exposed to biosolids odors from those of other natural antagonists.

There are exposure standards for many volatile compounds, including those detected in biosolids. Safe exposure limits expressed over an average work-day (time-weighted average) are available from the American Conference of Governmental Industrial Hygienists (ACGIH). As an example, the data from a confined composting operation are compared to the ACGIH standards. Measured ammonia concentrations in the venting system of a biosolids composting building (Caballero, 1997) were 57 parts-per-million by volume (ppmv), or approximately 43 mg/m3. This concentration was above the 8-hour time weighted average (TWA) exposure limit of 19 mg/m3. At the same time measured concentrations of carbon disulfide were approximately 7 mg/m3and the TWA is 34 mg/m3. Dilution outside of the vent reduced concentrations to below detection. The dilution volume of clean air presented by Maine’s setback distances is a factor of 786,000. This amount of vapor dilution provides sufficient protection with respect to ACGIH exposure limits for all commonly encountered volatile compounds.

Under field conditions, winds can carry odors to greater distances than are possible by molecular diffusion. The calculated risks posed by atmospheric transport from properly managed biosolids utilization are very small (Pillai et al., 1996; Dowd et al., 1997). Odor molecules can be transported greater distances than bioaerosols. A key question is: Is the presence of detectable odors indicative of the presence of pathogenic organisms?

Dowd et al. (2000) modeled the transport of bioaerosols from an actual utilization site using an advection-dispersion model that includes a time-based microbial inactivation factor. The output of the transport model was then entered into a dose-response model to derive a risk value for a given distance at a fixed wind speed. The risk to workers at a biosolids application site to viral or bacterial infections was an increase risk of 3 and 2 per cent, respectively (assumed wind speed of 2 m/s and one hour exposure). At 1,000 meters an exposed population had a bacterial infection risk of 0.46 per cent and a virus infection risk of 0.00000034 per cent (assumed wind speed of 2 m/s and 24 hour exposure). This risk may be over-stated by 1,000 to 10,000 times and risk of infection is less than 1 in a million. Brooks et al. (2004a, 2004b, 2004c) re-evaluated the initial model using newer analyses of Class B biosolids and found: (1) few bioaerosols at a biosolids application site in Arizona, and (2) an error in the Dowd (2000) model that used an erroneous and higher virus infectivity constant. The authors further stated that the risks of viral infections were orders of magnitude smaller for those working with biosolids than for wastewater treatment plant workers. According to these studies, the presence of odors is not equivalent to increased pathogen exposure.

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4.7 Transport into Groundwater
Another possible exposure route to humans is through drinking groundwater. The risk posed by biosolids is much less than raw sludge, septage, or any type of fresh manure. The Maine rules require 300-foot setbacks from wells to minimize risks to drinking water supplies. The occurrence of pathogenic organisms in drinking water is a problem of global proportions, not from using biosolids, but mainly due to the lack of wholly inadequate sewage treatment (Gerba and Rose, 1990). This problem occurs in rural areas mainly because poorly functioning septic systems have been shown to add pathogens to groundwater; yet overall the frequency of contamination is believed to be low (Yates, 1985). In some cases, pathogens may occur but not in ample strength to be infectious. The limited data available suggest that pathogens are sorbed onto soil particles and do no persist in large quantities (Epstein, 1998; Vance, 2002). Field studies using septic systems found that bacterial and viral deactivation were dependent upon soil pH and water saturation (Scandura and Sobsey, 1997). This deactivation is considered to be very important and it provides much protection to water supplies (Macler, 1996).

If we examine viruses, their sorption is controlled by the type of virus, soil composition, water, soil moisture, pH, temperature, and soil-solution chemistry (Gerba, 1984; Jin and Yates, 2002). The degree of moisture saturation and ionic strength of the soil solution are especially important and there is less transport of viruses in more concentrated solutions (Jin and Yates, 2002).

Experiments have been conducted with viruses because they are believed to be more mobile in soils and groundwater than other pathogens, partly because of their small size. For example, Bitton et al. (1984) determined that viruses appeared to bind with sludge solids and that no viruses were transported through 33 centimeter of soil cores. Powelson and Gerba (1994) assessed virus removal in 100-centimeter soil columns and found that virus deactivation followed a first-order rate law. Also, unsaturated flow (wetting-drying) had more deactivation than water-saturated flow; conditions that are common in soils. These near-surface processes help to deactivate viruses and likely lower any viral risks associated with land-applied biosolids. Jin and Yates (2002) confirm the deactivation of certain viruses, but point out that column studies can produce very different results depending upon experimental conditions.

Virus deactivation and transport in groundwater is undergoing additional study because of the increasing interest in wastewater reuse (additional information can be found through the Water Environment Research Foundation and American Water Works Association). According to Straub et al. (1993) more specific microbiological testing is needed because some pathogens are infective in small doses and the current indicator species methods are not adequate to evaluate the risks. Improved predictive models of virus transport into groundwater are currently being developed by the US EPA (Faulkner et al., 2002).

Generalizations about bacteria are more difficult to make because they are ubiquitous in soils and they tend to be retained in the soil (Schafer et al., 1998). Populations of pathogenic bacteria may change in response to environmental conditions so that certain species may seem to disappear and reappear (Gibbs et al., 1997).

Research on the occurrence and movement of bacteria in the subsurface has increased since the late 1980’s (Chapelle, 1992). Considerable effort has been focused on the mechanisms that control bacteria in partially-saturated conditions (see summary in Shafer et al., 1998), such as would occur at a biosolids spreading site. Apparently bacteria become trapped along the air-water interface, thereby arresting or retarding transport (Wan et al., 1994; Shafer et al., 1998). In column studies, less than 15 per cent of test bacteria were carried through a 20-centimeter column (Shafer et al., 1998). These studies suggest the probability that bacteria occurring in biosolids could persist into groundwater is exceedingly small. In contrast, field studies conducted in Canada using manure showed that bacteria can be transported into groundwater via macropores (Unc and Goss, 2003). In this study the authors reported that clayey soils allow deeper transport with peak concentrations at 2 to 4 meters and potential transport exceeds 10 meters. The authors speculate that this deeper transport may be due to macropores that form from cracks or other types of soil disruptions.

There are other pathogens besides bacteria and viruses. A specific and more recent concern to drinking water quality is Cryptosporidium parvum. Oocytes (dormant stage) of this organism can survive in harsh environments and are able to be transported in groundwater. Although significant numbers of the oocytes can be removed by soil particles, the oocytes are not strongly held and a fraction can become remobilized (Harter et al., 2000). This means that the oocytes can remain dormant in the soil and remobilize when soil is disturbed at some later date. In fine-grained soils the oocytes are attenuated by three-orders of magnitude (1,000 times) over a distance of 10 centimeters (Harter et al., 2000); so loadings would need to be very high to generate a large risk. The oocytes can also be transported in surface flow along with soil particles. According to Atwill et al. (2002) on slopes less than 20 per cent, buffer strips wider than 3 meters (10 feet) are sufficient to remove 99.9 per cent of the oocytes present. Monitoring of raw surface water indicates that cryptosporidium is rare in Maine’s public water supplies (Maine Drinking Water Program, pers. com., 2004). The available research indicates that the likelihood of oocytes, which are very robust, surviving in biosolids from the wastewater treatment process, followed by exposure to air and UV radiation, and then transported down into groundwater as viable organisms is very slight.

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4.8 Summary
Class B biosolids are processed to reduce significantly, but not eliminate, pathogen content. Biosolids (Class B) thus present some risk to humans due to exposure from accidental ingestion, or via inhalation of bioaerosols. These two routes of exposure require very close physical contact to cause exposure that may lead to illness. The small size of the illness risk is substantiated by epidemiological studies of treatment plant and compost facility workers, and of healthy people having direct or indirect exposure to biosolids at land-application sites. Pathogen viability is affected by many environmental conditions; conditions that allow few organisms to persist for long. The current standards reduce risks to very small levels, but do not eliminate them. A study of microbiological risks from sewage sludges applied to food crops in the United Kingdom substantiates the protection offered by both sludge processing methods (equivalent to biosolids) and the use of suitable harvest intervals (Gale, 2003).

The NRC (2002) study recommended that future research goals include a better characterization of pathogens and associated health risks. This includes adding clostridium to the list of pathogens and improving overall testing methods. Anecdotal reports of exposure-related illness underscore the need for more rigorous complaint tracking so that we have a better database for epidemiological studies and risk assessments. Multi-pathway, multi-stressor analyses of risk are needed to reflect realistic conditions. The NRC study recommended improved quality assurance standards using performance-based monitoring, especially of bioaerosols. The Biosolids Summit (Dixon and Field, 2004) was essentially in agreement with the NRC recommendations. The key findings of the Summit also emphasized bioaerosols and odors as areas needing more research. Current regulations do not try to anticipate new or emerging pathogens. Odors need to be managed for the general perception of increased risk, and the reality of well-being

Following is a summary of the relationship between public health and the use of biosolids as a soil amendment.

Potential Benefits:

• Class B biosolids protocols significantly reduce pathogen content to concentrations lower than detected in untreated animal manures.
• Class A biosolids have a pathogen content equal to background soil concentrations.
• Epidemiological studies show that risks of infection to a healthy population adjacent to properly managed biosolids facilities or Class B application sites are low.
• Transport of viable pathogens to groundwater is strongly attenuated by soil processes.
• Regulatory controls minimize public exposure (risks) to biosolids.
• Class A biosolids have odors similar to organic soils.

Potential Risks:

• Class B biosolids contain some residual concentrations of viable pathogens.
• Pathogens may be infectious and mobile as bioaerosols close to Class B biosolids, but not Class A.
• Pathogenic organisms in Class B biosolids may remain dormant but potentially infectious in the soil (this is addressed by site access restrictions).
• Odors may act as irritants or trigger immune responses.
• Rapid identification of pathogenic organisms is not a mature technique and it is difficult to accurately document presence or absence.

Following is a relative assessment of how Maine’s rules protect people from pathogens in biosolids.

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