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About 55% of all MSW generated in the US is currently being disposed of in approximately 2,300 municipal landfills (EPA, 1998a). In 1998, the US generated an estimated 220 million tons of MSW (EPA, 1998b.). Nationwide, MSW landfills are estimated to release approximately 9×105 Mg/yr. of methane and 1.3×104 Mg/yr. of NMOCs (Eklund et al., 1998). Methane and carbon dioxide emissions into the atmosphere contribute to the greenhouse effect, and the release of VOCs has been known to cause air-quality issues such as smog and ozone formation. VOCs comprise about 39% of NMOCs in a MSW landfill (EPA AP-42 , Section 2.4 1997).
Although in the US solid waste management only contributes 10% of the total VOC emission source (EPA, 1995), federal and state environmental agencies have developed stringent regulations for air emissions from municipal and industrial solid waste landfills. One such regulation is the NSPS issued on March 12, 1996. The main purpose of the NSPS is to reduce the emission of NMOCs, which are implicated for formation of atmospheric ozone. In addition, the NSPS also indirectly controls the release of methane, which contributes to global warming and other hazardous air pollutants (HAPs).
Based on the NSPS, any large landfill (capacity 2.75 million tons) that emits more than 55 tpy (50 Mg/yr.) of NMOCs is required to capture and reuse, or destroy, these gases (EPA, 1997). The required engineering control and gas management system can add significant cost to the operation of landfills. To assure compliance with the NSPS and to avoid Type I and Type II error , a feasible, accurate, efficient, and economical approach for estimating the potential emissions from MSWLFs is necessary.
To calculate the NMOCs emissions, one must determine LFG flux and its NMOCs concentrations. These two values allow one to calculate and determine if the NMOC emission is more than 55 tpy, in which case a landfill is required to implement gas recovery and management systems. A landfill facility may use Tier 1 calculations, which are based on the default input values; Tier 2 calculations, which require site-specific NMOC concentration measurements; or Tier 3 calculations, which are based on site-specific NMOC concentration and methane generation rate constants (EPA, 1999).
If the Tier 1 calculations show NMOC emission above 55 tpy (or 50 Mg/yr.) then the landfill facility can either implement the required NMOC control system, which adds extra capital and operating cost, or conduct Tier 2 or Tier 3 calculations to show that NMOC emission is less than 55 tpy. Conducting Tier 2 and 3 calculations requires field measurements, which could be expensive and less than accurate.
There are currently no established statistically reproducible standard methods for sampling and measuring the LFG emission from MSWLF. Furthermore, the macroscopic global empirical and/or semiempirical models need to be improved to better estimate LFG emissions into the atmosphere. MSW landfill science and process technology are still young; the process of compiling data and identifying trends as far as the process design, operation, and management systems, and their effect on the environment are concerned continues. With the promulgation and implementation of the NSPS regulations, database-building, for better emission estimate has accelerated.
Such database-building has been stimulated by the economic disincentives since errors of LFG determinations lead to increased compliance costs. To better direct future activity, there is a need to summarize the state of knowledge in NMOC/VOC measurements and their impacts on MSW LFG management and control.
This paper reviews and compiles information from the current literature regarding concentrations of NMOCs and VOCs from MSW LFG. Various potential techniques for VOC treatment with their advantages and disadvantages are described. In addition, a critical review of sample source, concentration, and flux measurement techniques is presented. Methods for field measurements of LFG flux and its NMOCs concentrations are recommended to provide representative data that, in turn, can be used to validate and/or modify the EPA model for Tier 1 calculations.
Composition of MSW LFG
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A recent literature review was conducted on the LFG generation, transport, characteristics, management, and control systems . The composition of LFG depends on the solid waste the landfill; the stage of decomposition, oxygen availability, moisture, rainfall infiltration, pH, amount of solid waste, organic quantities, and types in the solid waste; and available microbes. These are important factors that affect the type and rate of biochemical decomposition. The generation and transport of LFG and their subsequent emissions into the atmosphere are a complicated function of a number of variables. These variables are comprised of the nature and age of the solid waste, environment for biological activities, geographical conditions, landfill design, and management practices. The management system can influence LFG emissions by installing proper LFG recovery, collection and treatment facilities. Furthermore, management choices of solid waste shredding and particle size reduction, daily cover type, and leachate recirculation can influence the LFG generation rate (ICF, 1999).
LFG mainly consists of methane, carbon dioxide, water vapor, and trace amount of nonmethane organic compounds. It is estimated that solid waste containing 75% organics can generate up to 6.6 ft.3 of gas per pound of waste.
NMOCs
Gaseous emissions of NMOCs from landfills are dependent on solid waste characteristics, landfill age and construction techniques, climatological factors, the physical and biochemical properties of the soil cover, and other variables. NMOC gases are produced by the complex interaction of the physical, chemical, and biological processes occurring within the solid waste.
A total of 116 different trace organic compounds were identified in LFG samples in Great Britain (Herrera et al., 1988). NMOCs comprise less than 1% of LFG. A typical constituent of NMOCs could range from below detection limit to 1,780 ppm.
However, a recent survey (Huitric, 1999) has shown that typical VOC emissions from large MSW landfills have declined by 80% over the past 10 years for active landfills. In addition to improved analytical methods, the main reason for the decline was attributed to the implementation of EPA’s Resource Conservation and Recovery Act (RCRA) hazardous waste regulations of 1980, 1984, and 1991 (EPA 1980, 1984, 1991) These regulations have resulted in a reduction of hazardous wastes in MSW from a high of about 0.4% in 1981 to less than 0.04% in 1998. In addition, Huitric et al. have shown that landfill closures over 35 years have resulted in a significant reduction, approaching 100%, in typical VOC emissions.
The implication of the above findings is that the EPA AP-42 default values for MSW LFG VOCs, used for various regulatory purposes, tend to significantly overestimate the VOC emissions from MSWLF. This is because the AP-42 VOCs’ default values are derived from a database of analyses accumulated over the years, primarily from the 1980s through early 1991. This led Huitric et al. to conclude that these older data are not representative of current VOC emission from MSW landfill and need to be updated.
Since the number of NMOC constituents and their average concentrations are showing a declining trend (Table 5), it is reasonable to consider reducing the AP-42 default values as reflected by removal of older data from the database rather than the averaging technique as used in revising the AP-42 defaults in 1995.
VOCs
VOCs represent a subset of NMOCs that are known to react with sunlight to form ground-level ozone. Since VOCs in LFG cannot easily be measured separately, NMOCs are used as a surrogate. Table 2 also indicates which of the NMOCs are considered to be VOCs per EPA’s definition (EPA, 1992). The leads to the conclusion that VOC emissions from MSW landfills are also declining.
There are several definitions of VOCs by different authors. In general, VOCs are known as a class of substances in which organic carbon is bonded to hydrogen or other elements. As an approximate rule, most organic compounds with less than 12 carbon atoms are VOCs (Waldbott, 1973), and this includes most HAPs. De Nevers (1995) defined VOCs as organic liquids or solids that have vapor pressures greater than 0.0007 atm (0.532 mmHg) and boiling points less than 260?C (500?F). The World Health Organization defines any organic compound that has a vapor pressure larger than 0.0013 atm at standard temperature and pressure as a VOC.
EPA (40 CFR 51.100, 1992) defines VOCs as “Any compound of carbon, excluding carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates, and ammonium carbonate, which participates in atmospheric chemical reactions.”
Adverse Effects of VOCs
VOCs are very mobile in the environment. Because of their high vapor pressures, VOCs are found predominantly in the atmosphere.
VOCs in LFG can present issues associated with human health problems, the formation of ozone and urban aerosol precursors, and odor. The effects of VOCs on human health range from a simple nuisance to a serious hazard. Depending on dose and route of exposure, they can attack specific organs or the entire body. VOCs are lipid soluble and could bioaccumulate in lipid tissues in the body. The lung and gastrointestinal tract could readily absorb these compounds (AIHA, 1991; Ashley et al., 1996; Bloemen et al., 1993).
Other studies have assessed the health risks of populations near landfills. Findings show no difference in VOC levels between the exposed and control population (Hamar et al., 1996). It is believed that air dilution of LFG is normally sufficient to protect populations that might be exposed to LFG by living nearby landfills.
Emissions of VOCs adversely affect air quality. VOCs are known as precursors to photochemical smog and acid deposition. The reactions occur such that when VOCs are mixed with nitrogen oxides and irradiated by UV light, a complex chain of reactions converts them into products generally indicated as photochemical pollutants (Finlayson-Pitts et al., 1986). These reactions are mainly unstable and highly reactive, and the process ultimately leads to the production of ozone, aldehydes, hydrogen peroxide, peroxyacetyl nitrate, organic and inorganic acids, and fine particles (Batkinson et al., 1984). Among these, ozone is considered to be the most serious because of the high concentrations reached and the wide range of effects it may have on human health, plant growth, materials, and climatic change.
LFG TREATMENT
Flares are the most practiced control technology for LFG. A properly designed flare can achieve higher than 99% destruction of the total LFG. However, individual VOCs destruction by even the best of flares could range from 90% to 98% (Walsh, 2000). The NSPS/Emission Guidelines require LFG control devices to meet 98% control efficiency for NMOCs (Valis, 2000). The application of flaring is limited to situations in which the pollutants are not economically viable for energy recovery (US Army Corps of Engineers, 1995). In general, flaring is not ideal for halogenated compounds due to the long residence time required for complete combustion and also their corrosivity of burner nozzles. In addition, it is not economically feasible to treat the acid byproducts that result from burning halogenated compounds.
The choice of treatment technology for VOCs is dependent on concentration ranges, flow rates, temperature, and pressure of VOCs. Also, required control efficiency, labor, and capital help form the basis for a control technology choice. Table 10 summarizes existing VOC abatement and treatment technologies used in various industries. Typically, flaring is the treatment of choice for VOC abatement, but adsorption and absorption have also been used with limited success (Bogner 2000). The other technologies, besides flaring, listed in Table 10 may provide opportunities for developing economically viable and effective process for VOCs treatment/abatement from MSW LFG in the future.
Conclusions
In addition to methane, carbon dioxide, water vapor, nitrogen, hydrogen, and carbon monoxide a total of 116 different traces of NMOC have been identified in LFG from landfills around the globe. The detected concentration of NMOC gases from landfills have ranged from less than the lower limit of detection to 1,780 ppm. A recent survey from 146 landfills in the US and 1 from Puerto Rico (Sullivan et al., 2000) demonstrates that the average NMOC concentration from all these landfills is 454 ppmv (mixing ratio by volume, as hexane). This is significantly lower than the previous EPA-NSPS default value (4,000 ppm). This can be explained by improved analytical methods and the implementation of regulations, which have significantly reduced the amount of incidental hazardous waste deposited in MSWLF. It can also be attributed to the fact that today’s Subtitle D MSW landfills do not receive the same wastes as those that EPA used to determine its NSPS values.
Among the various available field measurement methods, the static chamber with FTIR method and the within cover concentration probe are recommended for the field measurements of MSW LFG flux and its composition. The main reason for their selection is that these methods provide for a more direct and representative determination of LFG emission into the atmosphere.
To persuade the removal of older data from the NSPS database, which are no longer representative of the current trend line on NMOC concentration in MSWL LFG, the NSPS database should be expanded by conducting a number of field measurements in various regions in the US. A combination of the static chamber, just-below cover concentration probe, and FTIR to determine the flux and NMOC concentrations should be used. This data can also be used to validate/modify the current NSPS model for Tier 1 calculations. Work is underway by EPA with funding from the EIA Research and Education Foundation at up to 12 landfills in the US to provide the needed data.
The best available VOC treatment in MSW landfill is flaring. However, there are a number of VOC treatment technologies, developed or being developed, in other industries that may find application in MSW landfills.
Acknowledgements
This project was supported through the Lanny & Kay Hickman internship, Delaware Solid waste Authority (DSWA), and Solid Waste Association of North America (SWANA).
I would like to thank my advisor, Professor C.P. Huang for introducing me to this interesting project and for his guidance throughout this work. Much gratitude is given to H. Lanier Hickman Jr. and N.C. Vasuki, CEO of DSWA, for their time, interest, and support during this project. I also would like to thank Jeanne Bogner, Ray Huitric, Mike Michels, James Walsh, Edwin Valis, Richard P. Watson, and Anne M. Germain for their useful suggestions, input, and contributions.
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