|
A summary
of the SWANA Applied Research Foundation’s findings
By
Jeremy O’Brien
In recent
years, there has been a growing movement to ban certain
products from disposal in MSW landfills because of a
concern for the potential release of heavy metals to
the environment.
In response,
the SWANA Applied Research Foundation’s Disposal
Group commissioned a study to summarize and document
what is known concerning the actual environmental releases
of heavy metals associated with the landfill disposal
of these products.
This article
summarizes the findings of the report that was developed
as a result of the SWANA research effort. The report,
which is available through SWANA and was published in
March 2004, provides up-to-date scientific and technical
information on this subject based on a comprehensive
review of the published literature and ongoing research.
Heavy
Metals in Municipal Solid Waste
Heavy metals are metallic elements with relatively
high atomic weights that are used in a variety of consumer
products and industrial processes. At trace levels,
many of these elements are necessary to support life.
However, at elevated levels they become toxic and become
a significant health hazard.
“RCRA
heavy metals” are those metals and metalloids for
which specific groundwater limits are established in
the Resource Conservation and Recovery Act (RCRA), which
was enacted by the US Congress in 1976 to address the
management and disposal of municipal and industrial
solid wastes. RCRA heavy metals, which include arsenic,
barium, cadmium, chromium, lead, mercury, selenium,
and silver, are the focus of the SWANA study.
As indicated
in Table 1, about 130,200 tpy of RCRA heavy metals were
disposed in MSW landfills in the US in the year 2000.
This tonnage represents 0.08% of the 162 million tons
of MSW that were disposed in that year. Lead represents
the major fraction (97.6%) by weight of the RCRA heavy
metals being disposed of in MSW landfills on an annual
basis, followed by cadmium (2.1%) and mercury (0.3%).
SWANA found
that available data indicate that despite the dramatic
increase in the disposal of discarded consumer electronics
in recent years, the tonnages of heavy metals being
disposed in MSW landfills have decreased over the last
15 years primarily as a result of lead-acid battery
recycling efforts.
Heavy
Metals in MSW Landfill Leachate
The concentrations of heavy metals in leachate
vary over a wide range depending on a number of factors
including waste composition, landfill age, and moisture
availability.
SWANA found
that, on average, heavy metal concentrations in leachate
have been reported in numerous recent studies to be
relatively low.
For example,
a draft database has been developed for the USEPA entitled
“Leach 2000” that includes leachate data from
over 200 MSW landfills. The database was compiled using
data from the following sources:
- Data
representing 60 MSW landfills owned by Browning Ferris
Inc. (BFI)
- A 1992
Chemical Waste Management study of leachate quality
that included data from 47 landfills, including a
number of MSW landfills
- Data
collected by the USEPA during the development of effluent
guidelines for landfills from 21 MSW landfills
- Data
from the State of Florida on leachate characteristics
for 65 MSW landfills
- Data
from the State of Wisconsin on leachate characteristics
for 39 MSW landfills
As shown
in Table 2, the mean concentrations of the RCRA heavy
metals are relatively low, averaging less than 1 milligram
per liter (or part per million) in all cases.
The Toxicity
Characteristic Leaching Procedure (TCLP) is a federally
prescribed test used to determine whether or not a solid
waste should be classified as hazardous. As indicated
in Table 2, the mean concentrations of RCRA heavy metals
reported in the Leach 2000 database for non–hazardous-waste
landfills are at least 10 times less than the TCLP regulatory
levels. In addition, the 90th percentile leachate values
for RCRA heavy metals (values for which 90% of the data
points are equal to or below) are all lower than the
TCLP regulatory levels.
A 1997 study
conducted at the University of Central Florida to characterize
MSW landfill leachate in Florida found the average concentrations
of the RCRA heavy metals to be low, “generally
on the order of micrograms per liter” (see Table
3).
In total,
five studies representing all recent published investigations
regarding leachate characteristics were reviewed in
the SWANA research effort and are summarized in the
project report. All of these studies concluded that
heavy metal concentrations in leachate are, on average,
relatively low.
The USEPA
confirmed the findings of recent studies regarding the
low levels of heavy metals in leachate in December 1999
when it published final effluent limitation guidelines,
pretreatment standards, and new source performance standards
for the landfill’s point-source category. In this
action, no limits were established for any of the RCRA
heavy metals for MSW landfill leachate that is directly
discharged to receiving waters following onsite treatment
at the landfill.
Under the
current regulatory framework for water pollution control,
a local government can establish pretreatment standards,
based on local conditions, for industrial wastewaters
such as landfill leachate that are discharged for treatment
to a local POTW. The objectives of local pretreatment
standards are to prevent pass-through of pollutants
to receiving water bodies, interference with treatment
plant operations, and to improve opportunities to recycle
and reclaim wastewater and sludges.
The pretreatment
standards for RCRA heavy metals established by four
counties located in different parts of the US are presented
in Table 4. As indicated, the average RCRA heavy metal
concentrations reported in the Leach 2000 database are
lower than the pretreatment standards established by
two of the four counties. However, in one county (Broward
County, FL) leachate pretreatment would be required
to meet local pretreatment standards for arsenic, while
in another county (Henrico County, VA) leachate pretreatment
would be required to meet local standards for mercury.
It is clear that, in these two cases, leachate pretreatment
would be required to meet the relatively stringent local
pretreatment standards set by these counties.
The SWANA
study found that attenuating mechanisms that occur naturally
during the latter phases of waste degradation in MSW
landfills (referred to as Phases IV and V) limit the
leaching of RCRA heavy metals. These mechanisms include
a neutral to high pH; the formation of relatively insoluble
heavy metal precipitates due to the presence of sulfide,
carbonate, and hydroxide ions; and the adsorption and/or
absorption of the heavy metals within the waste mass.
Natural
Processes That Limit the Leaching of Metals From Landfills
The characteristics of leachate generated during
Phase IV (the methanogenic phase) strongly favor the
removal of any soluble metals from the leachate through
precipitation processes. These characteristics include
the following:
- Neutral
to High pH—In Phase IV of the stabilization process,
anaerobic bacteria metabolize the organic acids produced
in Phase III, producing methane, carbon dioxide, and
ammonium (NH4+) ions as byproducts. As a result, the
prevailing pH is neutral or above. Generally, metals
are less soluble at higher pH levels.
- Availability
of Sulfide, Carbonate, and Hydroxide Ions—In oxidation-reduction
chemical reactions, elements either lose electrons (and
are oxidized) or gain electrons (i.e., are reduced).
In Phase IV of the stabilization process, reducing conditions
exist, meaning that elements and compounds tend to gain
electrons using organic matter as the electron donor.
For example, in this phase sulfate ions (SO4-3) are
reduced to sulfide ions (S-2). These sulfide ions, as
well as carbonate and hydroxide ions, are then available
during subsequent phases to react with many heavy metal
species to form insoluble compounds that effectively
remove these heavy metals from the leachate (Poland
et al. 2003).
- In Situ
Filtration—The landfilled waste effectively acts
as a filter, especially for landfills with leachate
recirculation. The cleansing effect of this in situ
filtration mechanism results in the removal of suspended
solids and other leachate constituents.
- Forms
of Heavy Metals in Disposed Products—The forms
in which heavy metals occur in the products that are
disposed are also likely to impact their leaching
characteristics (Kjeldsen et al., n.d.). One study
found that a major portion of the total metal content
in MSW existed in forms that were not likely to undergo
chemical reactions in landfills (Prudent et al. 1996).
Examples include the disposal of cadmium in plastics
and zinc in scrap metal.
Theoretically,
RCRA heavy metal concentrations in leachate could increase
over very long periods of time following the closure
of a landfill if the landfill liner systems are breached
and air re-enters the landfill, enabling aerobic decomposition
processes to be reinitiated. This scenario would require
the restoration and sustainment of a viable aerobic
microbial consortium, with continuing availability of
oxygen and nutrients. SWANA found that computer modeling
and limited laboratory investigations regarding this
long-term risk indicate that mobilization of heavy metals
from closed landfills, if it does occur, is not likely
to occur within a very long time frame.
Heavy
Metals in MSW Landfill Gas
Data from recent and historical studies of landfill
gas indicate that the quantities of heavy metals in
landfill gas are also relatively low. For example, as
indicated in Table 5, in a study conducted at the Central
Solid Waste Management Center Landfill of the Delaware
Solid Waste Authority, mercury concentrations were found
in the nanograms per cubic meter range (i.e., billionths
of grams per cubic meter).
The same
attenuating mechanisms that naturally limit the leaching
of heavy metals in landfills—including reducing
conditions, neutral to high pH, and presence of sulfides—also
limit the release of significant gas phase metals (including
metallic or methylated mercury). In addition, the low
vapor pressures for all metals except mercury are also
limiting factors.
The low quantities
of heavy metals contained in landfill gas (LFG) are
evidenced by the fact that, in its issuance of National
Emission Standards for Toxic Air Pollutants for MSW
landfills in January 2003, the USEPA did not establish
standards for any of the RCRA heavy metals.
The SWANA
review found that there is evidence of the existence
of gaseous mercury in LFG in the range of micrograms
per cubic meter. In addition, recent studies have identified
both monomethyl mercury and dimethyl mercury as being
constituents of the total gaseous mercury in LFG.
The relative
amount of mercury emitted into the air by MSW landfills
is also very low when compared to the amounts of mercury
emitted from other sources. In 1997, as required by
the Clean Air Act Amendments of 1990, the USEPA issued
a report to the US Congress referred to as the Mercury
Study. As reported in this study, the USEPA estimated
that, in 1994–1995, landfills emitted a total of
70 kilograms of mercury to the atmosphere. This quantity
represented less than 0.1% of the total amount emitted
from all source categories.
Effectiveness
of Landfill Pollution Control Systems
MSW landfills can be defined as land-based waste
management cells that contain MSW (USEPA 2002). To protect
the environment, MSW landfills are now constructed with
waste containment systems, which consist of (1) a liner
system that underlies the waste, and (2) a final cover
system constructed over the waste.
The landfill
liner system provides a relatively impermeable barrier
between the landfilled waste and the land on which the
landfill has been constructed. The primary purpose of
the liner system is to minimize the migration of waste
constituents out of the landfill. Another purpose of
the liner system is to enable the landfill leachate
and LFG to be collected and treated.
US federal
landfill regulations require that the liner system be
constructed as a “composite” liner. A composite
liner is an effective hydraulic barrier because it combines
the complementary properties of two different materials
(namely compacted soil and a synthetic geomembrane)
into one system.
US federal
regulations also require that MSW landfills be equipped
with a leachate collection and removal system that limits
the depth of leachate retained over the liner systems
to 12 inches (30 centimeters).
US federal
regulations require that the final cover must be placed
over the landfill within one year after the landfill
reaches its final permitted height. The final cover
system must provide the same maximum level of hydraulic
conductivity as the bottom liner system. With respect
to the long-term control of pollution from the landfill,
final cover systems are as important as, and in some
ways more important than, the liner system (Bonaparte
1995).
Landfill
liner systems substantially prevent the leaking of leachate
from the landfill to the land upon which the landfill
is constructed. Based on recent investigations, these
liners appear to have a “half life” (i.e.,
a time frame during which a 50% change in the material
properties of the liner occurs) of 970 years. Therefore,
the integrity of the liner system can be expected to
last through the time frame when significant quantities
of leachate are being generated.
Due to the
effectiveness of the landfill liner systems that have
been constructed with good quality assurance programs,
it appears that 99% or more of the leachate generated
in MSW landfills is collected and treated.
For landfills
equipped with LFG collection and control systems, the
combustion of gas in landfill flares or energy recovery
technologies enables the conversion of methylated mercury
(and other methylated metal compounds) to elemental
metal forms, which, at least in the case of mercury,
are much less hazardous.
Overall
Conclusions
Based on a review of recent studies and published
literature, the SWANA report concluded that MSW landfills
can provide for the safe, efficient, and long-term management
of disposed products containing RCRA heavy metals without
exceeding limits that have been established to protect
public health and the environment. It further concluded
that MSW landfills should contain the releases of RCRA
heavy metal pollutants at levels that protect public
health and the environment for extremely long periods
of time if not forever.
As is evident
from its organizational goals and policies, SWANA endorses
and actively promotes the implementation of economically
and environmentally sound waste reduction and recycling
programs for products containing heavy metals. However,
as evidenced in its report, SWANA concluded that modern
MSW landfills can provide an effective “safety
net,” as well as an environmentally sound means
of disposal, for those products containing heavy metals
that are not diverted through waste reduction and recycling
programs. These findings underscore the need to emphasize
other reasons—besides the avoidance of heavy metal
releases to the environment from disposed products—for
the institution of waste reduction and recycling programs
for products containing heavy metals. Such reasons include
natural resource conservation, and energy conservation
and pollutant minimization during the product manufacturing
process.
References
Bonaparte, R. 1995. Long-term performance of landfills.
Proceedings of the ASCE Specialty Conference Geoenvironment
2000 (ASCE Geotechnical Special Publication no. 46,
vol. 1), 415–553.
Kjeldsen, P., M.A. Barlaz, A.P. Rooker, A. Baun, A. Ldin, and T.H. Christensen. No date. Present and long-term composition of MSW landfill leachate—A review. Critical reviews in environmental science and technology, 32, 4.
Pohland,
F.G., A.B. Al-Yousfi, and D.R. Reinhart. 2003. Anaerobic
digestion of organic solid waste in bioreactor landfills.
Chap. 11 of Biomethanization of the organic fraction
of municipal solid wastes, ed. J. Mata-Alvarez.
London, England: IWA Publishing.
Prudent,
P., Domeizel, M., and C. Massiani. 1996. Chemical sequential
extraction as decision-making tool: Application to municipal
solid waste and its individual constituents. Sci
Total Environ, 178, 55.
US Environmental
Protection Agency. December 2002. Assessment and recommendations
for improving the performance of waste containment systems
(EPA/600/R-02/099). Cincinnati, OH: EPA National Risk
Management Research Laboratory.
Author
Jeremy O’Brien, P.E., is SWANA’s director
of applied research.
MSW
- May/June 2005
|
