MICROBIAL ACTIVITY, SURVIVAL, AND TRANSPORT
IN SOILS AMENDED WITH BIOSOLIDS
INTRODUCTION
Biosolids consist of organic matter that can improve the aeration, water-holding, and
other physical properties of soil, and can supply nutrients to plants and animals. However,
biosolids also contain microbial populations, some of which are classified as pathogens, and
small quantities of trace metals and trace synthetic organics. As a result, the land application
of biosolids can alter the existing microbial populations in soil. Land application can increase
the soil’s productivity; frequently, increases in productivity correspond with increased
microbial populations and activities in soil. However, land application has also been known
to decrease microbial populations in soil; it has been suggested that such decreases may be a
result of the application of biosolids creating conditions that favor a particular microbial
population over others.
Due to the existence of pathogens in biosolids, the land application of biosolids can be a
public health concern. Although biosolids treatment processes greatly reduce the number of
pathogens, it is unlikely that pathogens can be entirely eliminated from biosolids using the
traditional treatment processes of anaerobic and aerobic digestion. As a result, biosolidsamended soil can potentially become a vector of transmission for diseases. However,
treatment processes (for example, thermophilic digestion, composting, heat drying, and lime
stabilization) exist that can further reduce or eliminate the pathogens remaining in biosolids.
The soil environment is never sterile and contains background levels of human pathogens
that, in some cases, may be relatively high. The land application of biosolids must be
evaluated according to the amount (if any) by which it raises pathogen levels above the
background level at the application site. Infectious doses of the various pathogens must also
be compared to the levels found in the biosolids and in the environment after land application
of biosolids. The potential for organisms to be carried away from the application site (for
example, in surface water, groundwater, and in air) and result in human exposure must also
be evaluated.
Even with the above mentioned concerns, the land application of biosolids is one of the
few biosolids management methods with benefits and has been used increasingly as concerns
have increased about environmental degradation and negative health impacts of other
biosolids management methods, such as incineration, landfilling, and ocean dumping.
The following is a summary of the content of various literature on the issues of concern
regarding land application of biosolids as they relate to microbial activity, survival, and
transport. Abstracts of the articles that are referenced in this summary are included and are
available for further review by contacting the Northwest Biosolids Management Association.
NBMA Summary—Microbes
IMPACT OF LAND APPLICATION OF BIOSOLIDS ON MICROBIAL ACTIVITY
Decomposition implies cycling of nutrients and, consequently, is important to soil
fertility. Associated with decomposition, the mineralization of organic matter releasing
essential plant nutrients and subsequent production of stable organic residues in soil are
important functions of the soil microbial biomass.
Generally, low-metal biosolids have a beneficial effect on microbial biomass carbon
(Cmic) and on microbial activity. High-metal contamination of soils results in a substantial
decrease in Cmic (Fließbach et al., 1994). Similarly, Chander and Brookes (1993) found that,
in biosolids-amended soils, zinc (Zn) and copper (Cu) at higher levels than the European
Community (EC) limits decreased the amounts of soil microbial biomass. Dar and Mishra
(1994) found that a biosolids amendment caused a 15 to 39 percent increase in microbial
respiration. Their work identified a maximum carbon (C) mineralization in sandy loam and a
minimum in loam soil. Additionally, DTPA-extractable cadmium (Cd) exhibited a significant
negative correlation with microbial biomass. Hattori (1991) found that the toxicity of trace
metals to colony formation by soil bacteria was high for Cd and Cu; both metals seemingly
inhibited carbon dioxide (CO2) evolution substantially. The decrease of CO2 evolution by
trace metals depended mainly on the degree of toxicity of the metals to bacterial growth and
the amount of water-soluble trace metals in the soil. In contrast, Cd at about twice the EC
limits did not affect the amount of biomass or soil organic matter. Metals (Zn, Cu, nickel
[Ni], and Cd) at or below EC limits did not decrease the amount of soil microbial biomass
(Chander and Brookes, 1991). Likewise, Brendecke et al. (1993) observed that the
application of biosolids at a rate of 24 Mg ha-1 yr-1 during 4 years had no significant adverse
effect on microbial populations or activity in soil.
Glucosidases catalyze the hydrolysis of glucosides, whose products are important energy
sources for microorganisms in soil. Eivazi and Zakaria (1993) attributed an increase of
glucosidase activity to the enhanced microbial activity that was stimulated by the higher
nutrients and organic matter content of biosolids-amended soils. Their study suggested that
binding with humic substances and clay minerals may have played a role in reducing the
adverse effects of trace metals.
Dehydrogenase is a key enzyme in the oxidation of organic matter in soil. Dehydrogenase
activity (DHA) was found to be a sensitive and precise assay for determining the effect of
trace metals on microbial biomass in biosolids-amended soils. Obbard et al. (1992) found that
the addition of relatively uncontaminated biosolids enhanced DHA. Chander and Brookes
(1991) found that the main reason why Cu-contaminated soils apparently decreased DHA
was an abiological reaction of Cu. They suggested that research on the effects of Cu on DHA
that did not consider this phenomenon was, therefore, probably invalid.
In addition to decomposition, another important factor in soil fertility is symbiosis, which
involves the direct exchange of inorganic nutrients for photosynthetic carbohydrates between
microorganisms and the host plant (Obbard et al., 1992). Obbard and Jones (1991) found
effective symbiosis and nitrogen (N)-fixation even though metal concentrations were high at
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biosolids-amended sites. Conversely, Martensson and Witter (1990) found that N-fixing
processes were retarded in soils amended with biosolids.
Leung and Young (1990) studied the growth of clover plants in soils with increased
metals resulting from the 10-year annual application of biosolids. The results of the study
showed that the biosolids amendment did not appear to affect surface morphology of the Nfixing nodules, cell structure, or form of bacteroids. Some studies have reported that rhizobial
numbers decreased in soils amended with biosolids; this decrease was attributed to trace
metals (Hirsch et al., 1993; Giller et al., 1993; Chaudri et al., 1993). However, Leung and
Chant (1990) found an increase in nodule weight following biosolids application. Madariaga
and Angle (1992) observed that biosolids-soluble salts, not trace metals, were primarily
responsible for short-term reductions in rhizobial populations. In general, microbial
population, chemical characteristics of the soil, concentration of the metals, biosolids type
and application rates, and length of time the microbes were exposed to the metals each
affected the response of the macro- and microsymbionts (Chaudri et al., 1992; Angle et al.,
1993; Obbard et al., 1993).
It has been suggested that free-living, N-fixing bacteria can be biological indicators of
metal toxicity in soil. Concentrations of trace metals in soil close to or below the United
Kingdom limits led to a strong inhibition of heterotrophic and a strong reduction of
cyanobacteria N-fixation activity (Lorenz et al., 1992).
Following long-term applications of biosolids to the Woburn experimental plots in
England, it was determined that (McGrath et al., 1988 in Martensson and Witter, 1990).
However, normal nodule formation and N-fixation occurred when plots were inoculated with
Rhizobium leguminosarum biovar trifolii, and persisted at least as long as clover was
regularly grown (Angle et al., 1993). The EPA concluded that, due to the uniqueness of these
findings, it was not appropriate to include data from this study to limit metal applications for
40 CFR 503. The EPA concluded that United States strains of Rhizobium leguminosarum
biovar trifolii were considerably less sensitive to Cd and Zn under similar conditions, and
numerous studies have shown no adverse effects on soil microbial activity associated with the
metals in biosolids (EPA, 1995).
OCCURRENCE OF PATHOGENS IN BIOSOLIDS
Potential human pathogens found in biosolids include bacteria, viruses, protozoa, and
helminths. Some diseases caused by these microbes are polio, hepatitis, and dysentery
(Munger, 1983a). Table 1 lists other human pathogens found in biosolids and the diseases
which may result from their transmission. Levels of pathogens found in biosolids vary
greatly, depending on the health and composition (for example, industrial or residential) of
the source community and, to a lesser extent, on the season. The total dose and concentration
of a pathogen required to cause disease in humans can also vary between types of pathogens.
Wide differences between types of pathogens and between species of pathogens make general
studies of pathogen occurrence and survival in biosolids difficult. The approach scientists
have taken to this problem has been to study pathogens that meet one of the following
criteria:
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•
The organisms appear to be indicators for a type of organism (for example, fecal
coliforms used as indicators of the presence of human waste and associated
organisms).
•
The organisms are extremely common in biosolids.
•
The organisms have been linked to waterborne disease outbreaks.
TABLE 1. Major Pathogens and Associated Diseases Found in Wastewater (Munger, 1983a)
Pathogen
Disease
Bacteria
Salmonella spp
Salmonellosis
Shigella spp
Shigellosis
Mycobacterium tuberculosis
Pulmonary tuberculosis
Protozoa
Entamoeba histolytica
Ameobic dysentery
Helminthic Parasites
Ascaris lumbricoides
Ascariasis
Ancylostoma duodenale
Hookworm infection
Necator americanus
Hookworm infection
Taenia saginata (Beef tapeworm)
Taeniasis
Trichuris trichiura (Whipworm)
Trichuriasis
Viruses
Poliovirus
Poliomyelitis
Coxsackie virus
Aseptic meningitis, gastroenteritisa
Echovirus
Aseptic meningitisb
Reovirus
Mild respiratory infection, gastroenteritis
Adenovirus
Acute respiratory infection, pharyngitis, infant
pneumonia
Hepatitis virus
Infectious hepatitis
a
These are two of the diseases caused by several serotypes of this virus. Diseases caused range from
trivial to lethal.
b
Diseases caused are similar to those of Coxsackie virus.
Salmonella has been the most widely studied bacteria in biosolids because it is relatively
common. The organism was found to be present in 100 percent of treatment plants serving
more than 100,000 people in the United States (McCoy, 1979). Salmonella is an easy bacteria
to study because it must usually be ingested in large numbers (105 organisms for a healthy
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adult) in order to produce symptoms of disease, and it is much easier to detect large numbers
of organisms than smaller numbers (Munger, 1983a).
The study of viruses in biosolids has until recently been thwarted by a lack of accurate
separation and enumeration methods (Hurst et al., 1978). Viruses tend to sorb very strongly
to biosolids and soil particles and, thus, often did not show up in virus counts in leachate.
With recent advances in analysis, however, accurate counts have been possible. Most studies
show that viruses occur in fairly large numbers in wastewater effluent (200 to 7,000 plaqueforming-units/100 ml). (See Munger [1983a], p. 5 to 36, for tables detailing the virus
concentrations found in biosolids in the Seattle, Washington area.) From 96 to 99 percent of
the total viruses are either inactivated or adsorbed to biosolids during the treatment process
(Munger, 1983a). Viruses sorbed to biosolids remain active and may survive longer than
viruses free in wastewater, but they are infectious only if they become separated from
biosolids particles (Gerba and Bitton, 1984). Viral transport in soil and in biosolids will be
discussed later in this paper.
Parasite ova tend to settle out with biosolids during primary wastewater treatment and are
able to survive many forms of biosolids treatment. These organisms are of particular concern,
as they can cause disease when present in small numbers. Infection has been noted in people
who ingested only 10 to 25 cysts of Giardia lamblia (Munger, 1983a). Although ova and
cysts are eventually killed by exposure to the environment, Hannan (1981) suggests that care
should be taken to avoid public contact with land recently applied with biosolids.
A large array of pathogens are present in biosolids. In evaluating and controlling land
application of biosolids, it has been suggested that the pathogen content of the biosolids
could be regulated. However, although biosolids come from a point source (the treatment
plant), regulation would be extremely difficult, because the pathogen content of biosolids
varies considerably between plants and seasons of the year. Furthermore, pathogens die at
different rates, depending on the environment and type of soil in which they are deposited.
Therefore, regulation would potentially require the creation of separate standards for each
climatic zone of the country. A more practical solution might be to examine the chemical and
physical mechanisms that regulate pathogen survival in biosolids and in the environment.
CLASSIFICATION OF PATHOGEN REDUCTION IN 40 CFR 503
Subpart D of 40 CFR 503 provides regulatory guidelines for the land application and
surface disposal of biosolids with regard to the reduction of pathogens and vector attraction
in biosolids. Subpart D also includes criteria for classifying biosolids as Class A or Class B
with respect to pathogens. These classifications are based on the level of pathogens present in
biosolids at the time of use or disposal. In general, Class A corresponds to the 40 CFR Part
257 Process to Further Reduce Pathogens (PFRP) designation and Class B roughly
corresponds to the 40 CFR Part 257 Process to Significantly Reduce Pathogens (PSRP)
designation. The main difference between Part 257 and the current Part 503 regulations
located in §503.32 (a) and (b) is that Part 257 requires the use of an accepted process and
assumes that pathogen reduction occurs, whereas Part 503 requires documentation of
achieving a level of pathogen reduction.
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NBMA Summary—Microbes
Biosolids are designated as Class A if the biosolids are virtually pathogen-free after
treatment. Biosolids are designated as Class B if pathogens are reduced to levels that do not
pose a threat to public health and the environment as long as actions are taken to prevent
exposure to the biosolids after disposal or use.
Part 503 relies heavily upon the processes accepted in Part 257 and requires only periodic
monitoring if a facility uses one of the accepted methods. The PFRP designation includes
processes such as composting, pasteurization, and treatment by electron beams or gamma ray
irradiation. Under Part 257, before a new process is accepted, it must be shown to achieve
pathogen die-off that is similar to these processes; they will expose biosolids to conditions
that would essentially eliminate pathogens of concern (Sorber and Moore, 1987). One study
found that aerobic thermophilic co-composting of biosolids mixed with a carbon source
resulted in the reduction of fecal coliforms, fecal streptococcus, and salmonella (Shuval et al.,
1991). The residual compost proved to be an effective soil amendment in subsequent plant
growth experiments.
For a biosolids treatment process to achieve PSRP designation, it must reduce pathogens
at least as effectively as good anaerobic digestion. The logic of this requirement rests on the
observation that agricultural use of anaerobically digested biosolids as a fertilizer has been
practiced for many years with no evidence that the practice causes human illness, provided
the digestion is adequate. Farming operations have limited public access and clearly defined
use, which reduces exposure; the same restriction applies to the application of PSRP
biosolids in other practices. Under Part 503, each batch of biosolids must be tested if an
accepted process (such as anaerobic digestion) is not used. Alternately, there are procedures
to certify a process produces Class A or B, then documentation reverts to periodic
monitoring.
Most biosolids applied to land in Washington State are treated to meet the PSRP
treatment standards. Several of the risk assessments referenced in this literature review
describe pathogen reductions achieved by various treatment processes (Ryan and Chaney,
1992; Shuval et al., 1991; Munger, 1983a; Kowal, 1985). It is important to note both the
pathogen content of biosolids and the physical and chemical conditions of the site to which
biosolids are applied, as the interaction between these two factors will determine the infection
hazard of the area.
EPIDEMIOLOGICAL IMPLICATIONS OF THE LAND APPLICATION OF
BIOSOLIDS
Given that biosolids can contain pathogens, it is important to examine the human health
risk of land application of biosolids. Evidence is scant on whether land application of
biosolids can be a vector in disease transmission. Possible methods of disease transmission
include all forms of water and air movement, consumption of products grown in biosolidsamended soils, and direct human contact with the biosolids/soil mixture. In addition, some
pathogens can become airborne and infect people who breath the resulting aerosols (Kowal,
1985; Foster and Engelbrecht, 1973). Pathogens can also be carried over or below the ground
surface to rivers, streams, and wells, thereby causing illness to persons who drink the
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contaminated water (Abu–Ashour et al., 1993; Brandt and Hendrickson, 1990; Kowal, 1985;
Foster and Engelbrecht, 1973; Munger, 1983a; Burge and Marsh, 1978).
Health risks must be carefully examined, although many authors have concluded that
properly digested biosolids applied to land pose little health risk to humans (Burge et al.,
1978; Alderslade, 1981; Munger, 1983a; Kowal, 1985). Most wastewater-related disease
outbreaks have been attributed to the use of raw wastewater, the use of raw human feces (or
“night soils”) on food crops that are consumed raw, contamination of drinking water from
septic tanks, or consumption of raw shellfish from wastewater-polluted water. Despite this
positive appraisal, the movement of pathogens through pathways of infection, the infectious
dose, and the probabilities of occurrence need to be far better defined before quantitative
exposure risk assessment analysis can be adequately performed.
Survival of Pathogens in the Biosolids/Soil Environment
Most human pathogens are adapted to the environment of the human gut and die quickly
when exposed to the environment. Pathogen survival in the environment is influenced by
several factors including soil temperature, moisture, pH, and the presence of antagonistic
indigenous microbes (Pepper et al., 1995; Abu–Ashour et al., 1993; Yeager, 1981; Watson,
1980). In addition, different pathogens have different survival times. For the land manager,
these factors should be understood to effectively manage a site to enhance pathogen
reduction. The following paragraphs describe some of the conditions that affect pathogen
survival.
Weather. In all field experiments, it was found that organisms died more quickly in warm
weather than in cold weather. In a study performed in England, 90 percent of salmonella
bacteria were inactivated after 17 days in the winter and after only 3.7 days in the summer
(Andrews et al., 1983). In another study, parasite ova stored at 4º C were still viable after 10
to 16 months, while ova stored at 25º C were rendered nonviable after the same length of
time (O’Donnel et al., 1984). Pepper et al. (1995) found that both a decrease in temperature
and an increase in clay content resulted in longer pathogen survival times.
Moisture. Moisture levels
have a large effect on pathogen survival (Pepper et al., 1995;
Soares et al., 1995). A study of long-term survival of bacteria in biosolids (Yeager, 1981)
found that bacterial inactivation was proportional to moisture loss up to a solids content of 90
percent. After a 90 percent solids content was reached in biosolids, bacterial populations
became extremely stable (they seemed to enter a dormant stage). Although it is unclear
whether these results can be extrapolated to field conditions, other researchers have noted
bacterial declines during dry periods (Dunigan et al., 1980; Edmonds, 1976). Moisture levels
in soils are influenced both by climate and by the physical properties of the soil. For example,
porous, sandy soils hold less water and thus dry out more quickly than do fine soils with large
quantities of clay.
Soil pH. Soil
pH also influences the survival of organisms in the soil environment. A
neutral pH favors growth of bacteria (Watson, 1980). Other organisms are directly and
indirectly affected by the pH. Funderburg (1981) correlated the retention time of viruses in
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NBMA Summary—Microbes
the soil with cation exchange sites in the soil and found that a lowering of the pH can lower
the number of exchange sites and can affect the availability of some nutrients. Soil acidity
can also substantially affect the movement of viruses in the soil environment.
Bacteria. Enteric bacteria survive for a limited time in soil. However, survival times are
strongly influenced by the above mentioned factors and vary greatly in different studies.
Table 2 provides a rough idea of the time periods involved.
TABLE 2. Survival Times of Bacteria in Soil (Kowal, 1985).
Bacteria days
Survival Time
Coliform
From 4 to 77 days
Fecal coliform
From 4 to 55 days
Fecal streptococci
From 8 to greater than 70 days
Leptospira
Less than 15 days
Mycobacterium
From 10 days to 15 months
Salmonella paratyphi
Greater than 259 days
Salmonella typhi
From 11 to greater than 280 days
Streptococcus faecalis
From 26 to 77 days
Viruses. Virus
survival in soil is similar in terms of time to that of bacteria. In contrast to
bacteria, however, viruses sorb strongly to soil particles. This protects the viruses, but also
renders them more susceptible to temperature and moisture extremes, as the viruses do not
move with percolating water through the soil profile (Bitton et al., 1984). Viruses have been
detected in biosolids-amended soils from 4 to 100 days after initial application (Sorber and
Moore, 1987).
Parasites. Parasites
survive for the longest time of any human pathogens in the soil
environment. Because these organisms can survive in cyst form for many months under
unfavorable conditions, it is hard to estimate exact survival times. Ascaris is considered to be
the most resistant to adverse conditions; it has been found to survive up to 15 years in some
soils (Munger, 1983a). Fortunately, this is the extreme case. Giardia lamblia remained viable
in water for 32 days at 12 to 22º C (Munger, 1983a). Other parasites vary around these
values.
Pathogen Movement in Soil, Surface Water, and Groundwater
Pathogen movement into and by surface and groundwater is a potential pathway of
disease transmission. Conversely, pathogens that are retained in the soil are subject to
environmental conditions that accelerate their die-off. In general, the movement and survival
of pathogens in soil depends on the physical characteristics of the soil, not on pathogen
concentrations (Ibiebele, 1986; Ibiebele, 1985). Soil moisture, texture, chemical composition
(for example, clay, sand, or silt), organic matter content, pH, permeability, and temperature
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all affect the movement of pathogens. In most cases, pathogens were retained in the upper 5
to 15 cm of soil (Gerba et al., 1975). Each group of pathogens moves differently and is
influenced by individual soil properties.
Larger bacteria can produce a mat at the soil surface that filters smaller organisms from
the water (Gerba et al., 1975). This filtering at the soil surface appears to be the main
limitation in the travel of bacteria through soil. Therefore, few bacteria have been detected in
groundwater from biosolids-amended sites.
Bacterial retention in the surface soil horizon is directly related to soil texture; porous,
sandy soils allow more bacteria to percolate than do finely textured loams and silts. Surface
water runoff has been found to contain some indicator bacteria, but bacterial contamination
of surface water seems unlikely (Metropolitan Sanitary District of Greater Chicago, 1979).
The survival time of enteric bacteria and viruses in soil is relatively short; therefore, the time
in which living organisms could move into water sources is also relatively short (Sorber and
Moore, 1987).
Viral retention is most closely linked to the clay content and resulting cation exchange
capacity (CEC) of the soil. Generally, a higher clay content in a soil leads to a higher CEC,
resulting in an increased viral retention. Viruses bind to negatively charged particles in the
upper layer of the soil (Stotzky et al., 1981). Although this binding action protects viruses
from inactivation, it also immobilizes them. Viral retention is seriously impacted by the
presence of humic acids (which are released by organic matter decomposition and tend to
decrease viral retention) and by the absence of clay in the soil (Scheuerman et al., 1979).
Additionally, drying and wetting cycles and the percolation of de-ionized water rather than
salt solution can, in some cases, cause viral de-adsorption. (Lance et al., 1976).
Parasites are strained out at the soil surface because they are larger and heavier than
bacteria and viruses, and the organisms frequently become trapped in the upper layer of soil
during land application (Sorber and Moore, 1987). Several studies have examined the vertical
movement of parasites. Most have concluded that movement through the soil profile is
minimal (Sorber and Moore, 1987). Few studies have been done on the horizontal movement
of parasites (in surface water or aerosols).
Pathogens in Aerosols
Pathogens can potentially be carried away from biosolids-amended sites by the wind.
Some airborne pathogens may cause infection if they are inhaled by humans in large enough
numbers to constitute an infectious dose. Since the microbes in biosolids are mostly of
intestinal origin, infection by aerosols occurs when people inhale and then swallow microbes,
thus allowing the microbes to become lodged in the gastrointestinal system (Munger, 1983b).
In order for an organism to become airborne at a biosolids application site, it must be both
light enough or a part of an aggregate that is light enough to be carried by the wind, and it
must be available at the soil surface. Most bacteria and viruses occurring in wastewater meet
these criteria; however, the potential for inhalation of at least some bacteria by humans
appears to be more of a concern at wastewater application sites than at liquid biosolids (or
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NBMA Summary—Microbes
biosolids) application sites, according to Kowal (1985). Kowal (1985) summarized the work
of Sorber et al., (1984) as finding that the levels of bacteria such as fecal coliforms, fecal
streptococci, and salmonella were significantly lower at liquid biosolids application sites than
those observed at wastewater spray application sites, and it was concluded that the spray
application of biosolids did not represent a serious health threat for individuals more than 100
m downwind of the application site.
The survival time of organisms in an aerosol depends on the application method (for
example, nozzle type and spray velocity) and on the relative humidity and wind speed during
application (Kowal, 1985). Organisms transported in an aerosol are much more susceptible to
inactivation than they would be in soil. Solar radiation, desiccation, and high temperature all
act to reduce pathogen levels. In addition, it has been suggested that there is an initial
“aerosol shock” or “impact factor,” which may reduce bacterial levels tenfold within seconds
of aerosolization (Kowal, 1985). Although little is known about the movement of pathogens
in aerosols, several researchers suggest that public access to biosolids-amended sites be
limited for a time following application of biosolids (Kowal, 1985).
SUMMARY
With proper site management, biosolids can be safely applied to land with minimal risk to
human health. Most studies have shown few, if any, negative impacts on microbial
populations following the land application of biosolids. The few studies that have suggested
negative impacts are typically related to rare conditions, such as the presence of very high
levels of metals in the soil or very specific microbial strains (for example, a particular
Rhizobium). Often, masses of microbes have been increased; such increases were presumably
due to the increase in substrate associated with the biosolids.
A small portion of the microbes contained in biosolids are pathogenic. Fortunately,
various types of wastewater and biosolids treatment processes are designed to significantly
reduce pathogen concentrations. Some biosolids treatment processes, such as composting,
virtually eliminate pathogens. Typically, pathogens remaining after treatment are inactivated
relatively rapidly; this usually occurs within days or a few months, but it may take years for
some of the parasites (for example, Ascaris) to become inactivated. The effective filtering of
the soil generally prevents the downward movement of pathogens through the soil; this
filtering is apparently the result of several physical properties of the soil.
The EPA (1995) has developed standards for the level of pathogens in biosolids. The
biosolids product is designated Class A if the treatment process results in a product that is
essentially pathogen-free. The Class B designation refers to biosolids that result from
treatments that reduce pathogens to levels that do not pose a threat to public health and the
environment—providing actions are taken to prevent exposure to the biosolids after their
disposal or use.
Reference: Henry, C., and R. Harrison. 1998. Environmental Effects of Biosolids
Management. Trace Metals: Potential for Movement and Toxicity from Biosolids
Application, Effects on Wildlife and Domestic Animals from Biosolids Application, Air
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Emissions and Ash Resulting from Incineration of Biosolids, Nitrogen Cycle and Nitrate
Leaching from Biosolids Application, Microbial Activity, Survival and Transport in Soils
Amended with Biosolids, The Fate of Trace Synthetic Organics in Biosolids Applied to
Soil, Runoff Water Quality from Biosolids Application, Effects of Organic Residuals on
Poplars. Northwest Biosolids Management Association.
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