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Table of Contents
EXECUTIVE SUMMARY
The goal of this white paper is to review research on the
environmental benefits and risks associated with the beneficial
reuse of biosolids. This work examines two major questions:
- Is the beneficial reuse of biosolids, as regulated and practiced
in Maine, sufficiently safe and protective of public health and the
environment, particularly soil and water quality?
- Maine public policy since 1988 favors the beneficial use of biosolids over disposal options such as incineration or landfilling;
is this beneficial use of biosolids supported by research?
Answers to these questions have been derived from numerous
information sources: peer-reviewed research reports; conference
proceedings; National Academy of Science Publications; and state and
federal rulemaking documents and regulations. This white paper is
intended to be a comprehensive and independent assessment of the
beneficial reuse of biosolids as practiced in the state of Maine.
Overview
In Maine, the reuse of sewage sludge is regulated as a solid waste
residual. The rules developed by the Maine Department of
Environmental Protection (DEP) as 06-096 CMR Chapter 419 were
authorized by the Legislature under statute 38 MRSA Sections
1304(1), (13), and (13-A). These statutes authorize the DEP to
regulate solid waste to minimize pollution of the environment. The
innovative reuse of wastes, explicitly the agronomic use of sludge
by land application and similar uses, is supported by these
statutes. Simplified, the Maine statutes direct the Maine DEP to
support the highest value use for sewage sludge, to assure
compliance with sludge quality standards, and to keep the public
informed of sludge utilization activities and uses through various
public informational processes.
The U.S. Environmental Protection Agency defines biosolids as, “The
primarily organic solid product yielded by municipal wastewater
treatment processes that can be beneficially recycled”. Biosolids
are derived from sewage sludge that meet all the standards for reuse
and that have been treated to reduce pathogen content. There are two
standards for pathogen reduction: Class B (significant reduction)
and Class A (reduction to background concentrations). Biosolids are
considered beneficial because they contain essential plant nutrients
such as nitrogen and phosphorous, as well as important trace metals.
Biosolids may also contain trace concentrations of potentially
harmful metals and organic compounds. The maximum acceptable
concentrations of potentially harmful constituents are regulated.
The majority of biosolids generated in Maine have metal and organic
compound concentrations well below the maximums allowed.
According to Maine DEP records there are 200 licensed wastewater
treatment facilities in the state. In 2002, approximately 154,923
cubic yards of sewage sludge were generated in the state. A majority
(>75%) of the sewage sludge is processed into Class A or Class B
biosolids each year for some form of beneficial reuse. This
beneficial reuse may be the land application of Class B biosolids as
a fertilizer on farmland, or as a Class A compost for use as a
landscaping mulch.
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Findings
Soil Quality. Biosolids are complex mixtures of organic matter that
have agronomic value, as well as containing trace metals, such as
cadmium, zinc, and copper. A substantial body of research indicates
that biosolids provide a greater benefit through improved soil
quality that exceeds the risks posed by added metals. Biosolids do
pose some degree of risk to grazing animals and humans due to either
plant incorporation of trace chemical constituents, or pathogen
exposure from accidental ingestion of solids. There is some
measurable transfer of metals from biosolids in soil to certain
crops, but research shows that the amount of transfer up the food
chain appears to be limited. Managing soil pH to be nearly neutral
(pH~ 7) minimizes the loss of metals to either plant uptake, or
leaching to groundwater. Epidemiological studies have not
demonstrated systematic harmful effects to healthy people from
direct or indirect exposure to soil at biosolids at land-application
sites. Data collected in Maine suggest that risks to soil quality
posed by trace metals at biosolids utilizations sites are
negligible.
The land application of biosolids presents the following potential
benefits and risks to soil quality.
Potential Benefits:
- Inexpensive source of nitrogen.
- Source of trace nutrients and phosphorous.
- Biosolids increase soil organic matter and improve moisture
regulation.
- Concentrations of heavy metals in Maine’s biosolids are well below
the US EPA exceptional quality standard.
- Transfer of metals to food crops is limited.
- Organic matter in biosolids binds with metals and lowers their
bioavailability.
Potential Risks:
- Biosolids contain some trace metals of concern, but nearly all in
Maine are below regulatory risk thresholds.
- A small fraction of nutrients and metals may leach from biosolids
into groundwater.
- Added metals may persist in soils for decades and slowly become bioavailable.
- Soil pH needs to be managed over long time periods to minimize
metal losses.
Overall, Maine’s Chapter 419, the agronomic use rules, provides
adequate protection of Maine’s soil quality. Long-term management of
soil pH is an important point needing emphasis because the rules do
not address future land uses. Since the rules include the importance
of maintaining proper soil chemistry, the long-term risks from
mismanagement are minimized.
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Water Quality. Biosolids are managed by regulation to prevent any
degradation of water quality. This is because biosolids have the
potential to affect water quality through the leaching of at least
two general kinds of contaminants: essential plant nutrients and
trace metals. Biosolids contain some water soluble compounds that
could affect water quality. It is important to stress that biosolids
are composed of the least water-soluble components of the waste
stream. Research shows that when utilized, biosolids decompose over
a period of years to release nutrients, organic carbon, and metals.
The slow release of these components of biosolids controls how both
the beneficial and potentially harmful constituents become available
to plants and animals. Under certain conditions water-soluble
components can be carried into surface or ground waters. The use of
good agricultural practices, including soil erosion control
measures, minimizes the impact of biosolids, and other nutrient
sources. The risks posed to surface and ground waters by spreading
biosolids are probably small when appropriate setbacks are utilized.
Uncovered stockpiles on bare ground will leach small volumes of
concentrated liquid that can affect groundwater quality with
elevated concentrations of nitrogen and trace metals.
The land application of biosolids presents the following potential
benefits and risks to water quality.
Potential Benefits:
- Required separation distances from surface water and biosolids
protect water quality.
- The thickness of soils and absorption onto soil particles protects
groundwater below fields approved for land application of Class B biosolids.
- Plant nutrients in biosolids are released slowly and are readily
consumed by plants.
- Metals contained in biosolids are retained by organic matter and
minerals in near-neutral soils.
Potential Risks:
- Nutrients from biosolids stockpiles can be leached to groundwater
or be too concentration for plant uptake.
- Soluble metals from biosolids may be transported to groundwater.
- Plants can incorporate potentially toxic metals from soil
solutions.
- Long-term management of soil pH is needed to minimize metal loss.
Overall, Maine’s Chapter 419 provides adequate protection of Maine’s
water quality. Land application rules, using good erosion-control
practices, provide sufficient protection of surface waters.
Groundwater quality may be impacted locally by allowing uncovered
and unlined stockpiles of Class B biosolids. Composted biosolids are
more stable and less likely to leach nutrients. Separation distances
to ground water for Class B biosolids may not be sufficient to
restrict movement of the most mobile components of biosolids into
groundwater.
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Pathogens and Odors. Biosolids are classified on the basis of
pathogen reduction. Pathogens may include viable enteric bacteria
and other organisms, such as bacteria, viruses, and parasites that
have survived the treatment process. The goal for Class A pathogen
reduction is to destroy or inactivate pathogens to a concentration
equivalent to natural background content in soils. Class B biosolids
must have a reduction in populations of pathogens by a factor of at
least 100 times. Maine recognizes nine methods to produce Class A
biosolids and six methods to produce Class 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 state and federal rules. There is
uncertainty in the understanding of how long pathogenic organisms
can survive in biosolids and the associated risk of infections. Some
of this uncertainty is addressed by state mandated site-specific
licenses, mandatory setbacks, and restrictive site suitability
criteria for the land-application of Class B biosolids.
Class B biosolids are processed to reduce significantly, but not
eliminate, pathogen content and thus present some risk to humans due
to direct 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 magnitude of an illness risk is supported by epidemiological
studies of waste treatment 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. The
current standards reduce risks to very small levels, but do not
eliminate them. The combination of biosolids processing standards
and site use restrictions appear to be effective at protecting
public health.
Sewage sludges and biosolids have odors, some of which are
considered offensive. Odors may also act as vector attractors (e.g.
flies and rodents). The association of odors with a substance that
may contain pathogens is a commonly cited trigger for community
response to biosolids utilization. Setbacks from storage or
application sites reduce the impact of odors. New methods of odor
management are needed to reduce objections to biosolids reuse.
The land application of biosolids presents the following potential
benefits and risks to public health.
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.
Overall, Maine’s Chapter 419 provides protection of Maine’s public
health. A better understanding of pathogens and risks of infection
is needed. Odor control continues to be a challenge and it is a
source of public concern. The detection of offensive odors is an
important factor underlying complaints about biosolids reuse.
Operational setbacks help to control odor complaints and reduce the
chances of accidental exposure to residual pathogens.
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Conclusions
Maine public policy since 1988 has favored the beneficial reuse of
biosolids over disposal options such as incineration or landfilling.
The reuse of biosolids, as practiced in Maine, is part of the
concept of sustainability. Sewage sludge will be produced if we are
to continue keeping untreated wastes out of our rivers and streams.
Agronomic use of the resulting biosolids is a method to capture and
reuse valuable nutrients and organic matter. Like animal manure,
biosolids contain comparable nutrients and metals; but untreated
animal manures contain more pathogens than biosolids. When managed
properly, biosolids serve a useful function in agriculture by
increasing soil fertility. Other uses of biosolids, such as forest
fertilization and landscaping using Class A composted biosolids,
have lessened the dependence on farm applications.
The removal of untreated wastes from our surface waters has resulted
in a significant reduction of risks to the environment and human
health. Massive outbreaks of diseases caused by polluted water are
now uncommon. Ecosystems that were once destroyed by untreated
wastes now show evidence of recovery. Relative to untreated wastes,
biosolids present a manageable risk to human health and the
environment. The continued and sustainable reuse of biosolids in
Maine must be supported by research to identify and reduce
potentially hazardous chemicals, odors, and pathogen. Regulations
should be updated when research demonstrates a need to reduce
unacceptable risks. Public policy needs to lead the process towards
the reduction or elimination of harmful constituents in the waste
stream so that they do not end up in biosolids. There is a need for
greater effort to provide the public with a better overview of the
benefits and risks of waste management and biosolids reuse in
particular.
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