Grantee Research Project Results
Final Report: Closing the Gaps in the Regulation of Municipal Solid Waste Landfills: Defining the End of the Post-Closure Monitoring Period and the Future Stability of Leachate Recirculation Landfills
EPA Grant Number: R827580Title: Closing the Gaps in the Regulation of Municipal Solid Waste Landfills: Defining the End of the Post-Closure Monitoring Period and the Future Stability of Leachate Recirculation Landfills
Investigators: Barlaz, Morton A. , Gabr, Mohammed A. , Mohammed, P. E.
Institution: North Carolina State University
EPA Project Officer: Chung, Serena
Project Period: September 13, 1999 through September 12, 2000
Project Amount: $145,213
RFA: Futures: Detecting the Early Signals (1999) RFA Text | Recipients Lists
Research Category: Water , Sustainable and Healthy Communities , Land and Waste Management , Aquatic Ecosystems , Ecological Indicators/Assessment/Restoration
Objective:
The objective of this research project was to develop and present a plan to establish the scientific basis for determining a stable landfill, including landfills that utilize leachate recirculation. This project's specific objectives were to:
· Review data on the composition of leachate associated with well-decomposed refuse and propose criteria for the identification of stable leachate.
· Estimate the quantity and quality of water in a landfill at the end of the postclosure monitoring period and subsequent leachate generation rates.
· Develop and evaluate strategies for managing methane emissions that will occur after the end of the postclosure monitoring period.
· Identify techniques for measuring geotechnical properties of MSW over time, in terms of compressibility and strength.
· Review models of material compressibility that include biological decomposition and provide a rational scenario of cover system configurations in landfills that utilize leachate recirculation.
In 1991, the U.S. Environmental Protection Agency (EPA) promulgated regulations governing the disposal of nonhazardous wastes in landfills under Subtitle D of the Resource Conservation and Recovery Act (RCRA) (CFR, 1991). These landfills typically receive municipal solid waste (MSW), construction and demolition waste, and a wide variety of nonhazardous industrial wastes. MSW includes waste generated from residential, commercial, and institutional sectors. Approximately 55 percent of the 220 million tons of MSW generated in 1998 were placed in landfills (EPA, 1999).
Once a landfill has reached its design capacity, a final cover must be installed. The site owner is required to monitor and maintain a closed landfill in the post-closure monitoring period. Post-closure monitoring includes leachate collection and treatment, groundwater monitoring, inspection of the final cover, and the required maintenance to ensure that methane does not migrate offsite. Regulations specify a 30-year post-closure monitoring period, unless this period is extended by the regulatory agency on a site-specific basis. The implication is that monitoring will be discontinued after 30 years because the landfill is stable and no longer represents a threat to the environment. However, the EPA lacks technical criteria from which to define this stability. This lack of criteria makes it difficult for regulators to decide whether to extend or reduce the post-closure monitoring period.
For landfills that operate in a traditional manner, with efforts to minimize water infiltration, it is unrealistic to expect that the landfill is stable after 30 years and to discontinue monitoring. However, interest has grown for a second type of landfill that is operated as a bioreactor to accelerate waste decomposition and gas production (Pacey, et al., 1999). There are numerous advantages to bioreactor landfills, including the expectation that the landfill will reach a stable state more rapidly, thus permitting a decrease in the post-closure monitoring period (Reinhart and Townsend, 1998).
Summary/Accomplishments (Outputs/Outcomes):
All work has been completed, including a critical analysis of leachate composition data, a review of data on leachate collection system efficiency, a review of methods to estimate the quantity and quality of leachate produced after final cover installation, and a review of the mechanics of waste compressibility and strength. In addition, work was conducted to evaluate the impact of a hypothetical release of landfill leachate on surface water. A limited testing program has been developed and completed to measure compressibility and strength properties of MSW while taking into account mechanistic and biological contributions.
Landfill Leachate Composition
There have been a number of reports on leachate composition in the past, so a majority of the work on leachate composition was to assimilate and analyze existing data. The major components of leachate include dissolved organic matter, inorganic ions such as ammonia, phosphate, and sulfate, and heavy metals (Christensen, et al., 2001). The concentrations of dissolved organics, ammonia, and heavy metals were reviewed with a focus on concentrations typically present in well-decomposed or stable refuse.
Organic Indicators of Waste Decomposition. Several parameters have been reviewed as indicators of the stage of waste decomposition, including: chemical oxygen demand (COD), biological oxygen demand (BOD), the BOD:COD ratio, and the sulfate:chlorine ratio (Pohland and Harper, 1985; Chian and DeWalle, 1977; Reinhart and Grosh, 1998). The BOD:COD ratio appears to be the most common indicator of well-decomposed refuse. A ratio of < 0.1 should be considered as a necessary but insufficient condition to ensure leachate stability, because: (1) refuse in an active state of methane production may have a low ratio; and (2) leachate from refuse in the acid phase that percolates through well-decomposed refuse will have a low BOD:COD ratio even though a portion of the waste mass is not stable.
Ammonia in Landfill Leachate. Another long-term pollution problem associated with landfill leachate is ammonia nitrogen. Although it is associated with biodegradable substrates, there is no mechanism for ammonia degradation under methanogenic conditions (Robinson, 1995; Burton and Watson-Craik, 1998). As such, strategies are required to reduce ammonia concentrations in leachate from well-decomposed refuse.
Heavy Metals in Landfill Leachate. Metal species are of concern when they are biologically available at toxic concentrations to organisms in the receiving ecosystem (Smith, et al., 1999). This includes not only free metal ions in solution, which are usually present in low concentrations, but also organic complexes, inorganic complexes, and metals bound to colloids (Christensen, et al., 2001). Information is summarized on heavy metal concentrations in the leachate produced from landfills that contain well-decomposed refuse. Existing data from ten landfills known to contain well-decomposed refuse indicate that, in general, heavy metal concentrations (Cd, Cu, Cr, Ni, Pb, and Zn) in leachate often meet U.S. drinking water standards. Of course, this is a rather strict standard for untreated leachate, but it serves to emphasize that heavy metal concentrations are rarely of concern in leachate from MSW landfills. One remaining issue is the behavior of metals under more oxidized conditions. Theoretically, as refuse approaches complete decomposition, the rate of oxygen infiltration into a landfill will exceed its rate of consumption and a landfill could become more oxidized. It is unknown whether such an increase in oxidation will occur over centuries order, or over geologic time. It is recommended that future research be conducted to evaluate the potential for increased metals mobilization in refuse that is so well decomposed that the oxidation-reduction potential of the refuse increases because of oxygen infiltration.
Leachate Production Rates
Estimates of both leachate quantity and composition are required to evaluate the potential impact of a leachate release to either groundwater or surface water. In this task, a number of methods were considered to evaluate potential flow rates from landfills after installation of the final cover. While there are no documented data on leachate production from bioreactor landfills, data on traditional landfills were reviewed. These data can be applied to a bioreactor landfill after all free water remaining in the waste has been drained. Leachate release rates can then be used in conjunction with leachate quality parameters such as BOD, COD, and ammonia to calculate mass release rates and to analyze potential environmental impacts of a leachate release during and after the post-closure monitoring period. Three methods are presented and used to estimate leachate quantities released to the environment. These methods are based on field data, a liner efficiency (Ea) calculation, and an assumed density of holes in a liner.
Flow Rates Through Landfill Liners. Actual measurements from landfills with Subtitle D liners show that flow rates of a typical leachate detection system (LDS) in the post-closure period range from 5 to 210 Lphd (0.5 to 22 gal/ac/d). Analyses conducted for single- and double-lined cells, utilizing average monthly leachate flow data, indicated that flow rates of the leachate collection system (LCS) decreased by a factor of 4, approximately 1 year after closure, and by 1 order of magnitude 2 to 4 years after closure. Nine years after closure, these flow rates were negligible. These values are for landfills with covers and are not specific to a certain liner type (Bonaparte, et al., accepted for publication).
Liner Efficiency Calculation. The amount of leachate reaching the LDS, or the environment for a landfill that contains a single liner only (no LDS), was calculated based on assumed liner efficiencies and typical parameters. For a landfill that receives precipitation of 40 inches per year with a 60 percent loss to runoff plus evapotranspiration, and assuming efficiencies of 99 percent for drainage through both the cover and leachate collection system, a leachate release rate of 1.1 Lphd (0.12 gal/ac/day) is calculated. If the concentrations of BOD, COD, and NH3-N are 100 mg/L, 1,000 mg/L, and 250 mg/L, respectively, then the mass released will be 45.4 mg BOD/ac/d, 454 mg COD/ac/d, and 113 mg NH3-N/ac/d, respectively.
Liner Defects. Murray, et al. (1995) estimated liner leakage rates through low permeability soil and composite liners. They also reported leak estimates found by other researchers and used them as input data to the HELP model. Based on modeling results, they estimated that flexible membrane liners (FML) reduce leakage when used in conjunction with compacted clay liners (CCL) by 62 to 73 percent with poor installation or 99 percent with good installation. Giroud, et al. (1992) described equations that estimate leakage rates through composite liners as a function of the number of defects, the head on the liner, and coefficients of permeability. The factors that most significantly affect flow through a composite liner are the size and number of defects, the hydraulic conductivity of the underlying clay, and the head of liquid on top of the geomembrane (Giroud, et al., 1992). Flow rates as a function of the number of 3-mm- and 10-mm-diameter holes with good and poor liner contact have been calculated based on Giroud's equations. Estimated leakage rates range from 3.9 to 27.1 Lphd (or 0.42 to 2.9 gal/ac/d) for the conditions analyzed assuming 9 holes/ha (3.6 holes/ac) and 0.3 m of head on the geomembrane.
Landfill Performance in New York State. New York is one of few states in which double liners are required for MSW landfills. The New York State Department of Environmental Conservation provided electronic copies of 1998 and 1999 Annual Reports for operating landfills. These reports contained data on flow rates in leachate collection and detection systems as well as the applicable acreage. These data made it possible to calculate liner efficiencies. The mean collection efficiency was found to be 99.1 and 99.2 percent for 1998 and 1999, respectively. This leachate collection efficiency was then used to calculate a hypothetical leachate flow rate in both the LCS and the LDS. The results of the liner efficiency calculation are reasonably consistent with the mean values reported above at 99 percent collection efficiency. The measured mean LCS flow rate was 10,252 Lphd (1097 gal/ac/d) and the actual mean LDS flow rate was 72.9 Lphd (7.8 gal/ac/d). A comparison of the liner efficiency calculation to the New York State landfill data shows that the efficiency method is fairly accurate when reasonable estimates of annual precipitation, runoff, and infiltration can be made. The New York State data also confirm that 99 percent collection efficiencies can be achieved across a broad cross section of full-scale landfills.
Surface Water Quality Impacts of Landfill Leachate
Two components of leachate, ammonia and some amount of dissolved organic carbon as biochemical oxygen demand (BOD), will persist even after the refuse mass has reached a state of near complete decomposition. A methodology that can be used to assess the potential impact of a hypothetical leachate release to surface water is discussed. Clearly, if a release of leachate to surface water at a known rate and composition does not threaten a surface water body (or an aquifer), then the landfill can be considered stable with respect to the threat of a leachate release.
The Streeter-Phelps model for dissolved oxygen depletion in a stream was modified to include the oxygen demand associated with ammonia. The model was then applied to a hypothetical stream that was assumed to receive leachate from a landfill. This type of analysis is quite specific to the quantity of leachate released and the characteristics of the receiving stream. However, this approach appeared to represent an effective way to estimate the effects of a leachate release to surface water. This approach also allows for different water quality criteria with respect to dissolved oxygen and nutrient concentrations.
Landfill Gas
There are several issues to consider with respect to landfill gas production prior to termination of postclosure monitoring and the deactivation of a landfill gas collection and control system. First, methane that is not collected can be released to the environment. As methane is a greenhouse gas, there should be specific mass (or volume) limits on acceptable releases. In conversations with EPA staff, it has been suggested that the EPA may lack the regulatory authority to specify an acceptable mass release rate. This could represent a regulatory gap that should be further investigated by the EPA. Second, when a gas collection system is discontinued, there is increased potential for both subsurface lateral migration and odor problems. To evaluate whether post-closure monitoring can be terminated, it may be necessary to monitor the vadose zone around a landfill after a gas collection system has been turned off. The presence of significant methane migration, or development of an odor problem, could indicate that gas control must be continued irrespective of the age of the landfill. Of course, such decisions must be made in the context of potential deleterious impacts on a site-specific basis.
Geotechnical Properties With Leachate Recycling
A literature review yielded a dearth of information on geotechnical properties of MSW during decomposition in the presence of leachate recycle. In addition to settlement, the decomposition of MSW over time and the increased moisture content of the buried waste will affect waste strength, which will impact slope stability as well as cover system components. It is apparent from the literature review that an evaluation of compressibility and strength properties and a comprehensive model for estimating the rate and magnitude of settlement in bioreactor landfills are missing.
Compressibility Parameters. A limited testing program was conducted to evaluate the impact of biological decomposition of MSW with leachate recycle on the measured compressibility parameters. Enhanced degradation of refuse caused an increase in the void ratio and weakened the structural matrix leading to the increase in initial settlement with degradation. Monitored settlement with time, under an applied constant stress of 95 kN/m2, indicated compressibility parameters (on the basis of strain versus time data) that ranged from 0.02 to 0.19, depending on the state of decomposition, with higher values measured during the period of rising methane production. Comparing measured properties using small-scale laboratory equipment to values backfigured from field tests indicated the feasibility of using the small-scale equipment for property evaluation once appropriate correction for the size effect is applied.
Strength Parameters. There is a limited understanding of the mechanisms affecting shear strength changes as a function of solids composition, moisture content, and waste density. In comparison to the literature on waste compressibility, the body of literature on strength contains no work that systematically documents shear strength as a function of waste decomposition, which is an issue of great importance in landfills with leachate recycle where changes in properties are accelerated with time. However, a number of studies have been conducted on the shear strength of traditional MSW. Similar to soils, the shear strength parameters normally used for waste characterization are cohesion and friction angle. The published literature yielded no data on the shear strength of waste from landfills operated under high moisture conditions, despite the significant influence of moisture on waste shear strength and, consequently, on the stability of slopes and cover systems.
Direct shear tests were performed on simulated bioreactor refuse specimens. Results showed the shearing angle to decrease with degradation of refuse with values on the order of 18o at the final stages of decomposition. This value is lower than values reported in the literature for traditional waste material. Values were obtained using 100-mm direct shear cells and lower friction angles can be anticipated for the same waste material as the dimension of the testing device is increased (Gabr and Valero, 1995). A possible explanation can be based on the composition of decomposed waste being different from that of fresh waste. As a waste sample reaches stability, the lignin concentration increases well above that in fresh refuse, while the cellulose and hemicellulose material significantly decrease (Wang, et al., 1994). Lightweight material such as paper and textiles contain high concentrations of cellulose and hemicellulose, which provide a reinforcement-like contribution to the shear strength that is significantly reduced as these compounds decompose.
Cover Configuration to Accommodate Leachate Recycling
At present, typical MSW landfill regulations for a cover configuration specify a soil layer, a flexible membrane, a granular drainage layer, and possibly a geotextile filtration layer for gas venting, separation, and drainage. As leachate recirculation is implemented, it is of paramount importance to re-think the final and interim cover design. Leachate recirculation will enhance waste decomposition. This decomposition may occur unevenly and lead to the imposition of unacceptable tensile strains on the cover system components. Current indications point to the stability of landfills with leachate recycle over perhaps as little as 10 to 15 years though better definition of this time period is needed. Therefore, one concept would be to delay placement of the final cover until stability is achieved. Leachate recirculation will induce large differential settlement, and the placement of a final cover immediately after landfill capacity is reached will likely result in a damaged final cover. Instead, simple interim covers for odor control and gas collection can be implemented. Possible scenarios for interim cover systems that can accommodate the bioreactor concept include the use of geosynthetic components with a soil cover. In this case, a layer of geotextile can be used as permeable barrier/separator on the top of a layer of genet for gas collection. Select fill with relatively high hydraulic conductivity can be place on the top of the geotextile layer for odor control. The select fill layer can be armored with geosynthetic erosion control measures given the relatively long time during which such a cover system will need to function. Once landfill stability is reached, the type of permanent cover to be placed on the top of the landfill should be selected on the basis of intended future land use.
Workshop on the Future of Landfills After Waste Stabilization
A workshop was held in conjunction with the 5th Solid Waste Association of North America Landfill Symposium in Austin, TX, in June 2000. The workshop was attended by approximately 150 people and included representatives of the regulated community of landfill owners, operators, and designers; state and federal regulators; and leading researchers. The workshop introduced participants to such issues as determining an appropriate end for the post-closure monitoring period and how leachate recirculation affects waste properties. After presentations by both Project Investigators, a panel of experts participated in a discussion and exchange of information. In addition, Dr. Barlaz has made presentations on landfill stability at two additional landfill meetings in the United States and abroad. Dr. Barlaz also chaired a session on landfill stability at the first Intercontinental Landfill Symposium in Lulea, Sweden. Leading researchers and practitioners from Europe attended the conference in Lulea.
Conclusions:
The objective of this research project was to develop and present a plan to establish the scientific basis for the appropriate regulation and operation of MSW landfills through the post-closure monitoring period. Regulators will be facing decisions to either extend the post-closure monitoring period or allow its termination over the next 20 to 30 years as landfills that were closed in the 1990s approach 30 years post closure. In future work, the conceptual approach presented here must be evaluated on a site-specific basis to determine its utility and to identify areas where further development is required. Such development could include the need for data acquisition and may identify areas where regulatory authority must be clarified through legislative or rulemaking activity.
The approach presented here is modular and flexible. For example, an alternative methodology was recently proposed for calculation of leakage through liners as a function of liner defects (Foose, et al., 2001), and this methodology could easily be substituted for the work of Giroud, et al. (1992). Similarly, alternate surface water quality and landfill gas production models could be used.
The concepts described here have been presented to groups of regulators, landfill owners and operators, and design engineers. There is widespread agreement on the need to develop technical criteria for landfill stability, though a range of views exist on how to define stability. At one extreme is the position that the landfill is only stable when its contents are no longer a threat, even under conditions of a massive release. Under these conditions, the presence of any ammonia or BOD could suggest that the landfill is not completely stable, a condition unlikely to be achieved for centuries without extensive flushing. At the other extreme is complete reliance on engineered containment systems in the form of the liner and cap. Between these two extremes is a mixed strategy, with reliance on engineered containment to minimize potential releases, an assessment of the decomposition state of the contained refuse, and an evaluation of the severity of impacts of potential leachate and gas releases. Ultimately, it will most likely be necessary to evaluate the first group of landfills that reach the 30-year post-closure state. These sites will require significant assessment of all available data to determine if post-closure can be terminated while protecting human health and the environment.
References:
Bonaparte R, Daniel DE, Koerner RM. Assessment and recommendations for optimal
performance of waste containment systems. U.S. Environmental Protection Agency,
Cincinnati, OH (accepted for publication).
Burton SAQ, Watson-Craik IA. Ammonia and nitrogen fluxes in landfill sites: applicability to sustainable landfilling. Waste Management and Research 1998;16(1):41-53.
Chian EWK, DeWalle FB. Evaluation of leachate treatment: volume I, characterization of leachate. U. S. Environmental Protection Agency, Report No. EPA-600/2-77-186a, Cincinnati, OH.
Christensen TH, Kjeldsen P, Bjerg PL, Jensen DL, Christensen JB, Baun A, Albrechtsen HJ, Heron G. Biogeochemistry of landfill leachate plumes. Applied Geochemistry 2001;16:659.
Foose GJ, Benson CH, Edil TB. Predicting leakage through composite landfill liners. Journal of Geotechnical and Geoenvironmental Engineering 2001;127(6):510.
Gabr MA, Valero SN. Geotechnical properties of solid waste. ASTM, Geotechnical Testing Journal 1995;18(2):241-251.
Giroud JP, Badu-Tweneboah K, Bonaparte R. Rate of leakage through a composite liner due to geomembrane defects. Geotextiles and Geomembranes 1992;11:1-28.
Murray GB, McBean EA, Sykes JF. Estimation of leakage rates through flexible membrane liners. Groundwater Monitoring and Remediation Fall 1995;148-154.
Pacey J, Augenstein D, Morck R, Reinhart D, Yazdani R. Bioreactive landfill. MSW Manage 1999;Sept/Oct:53-60.
Pohland FG, Harper SR. Critical review and summary of leachate and gas production from landfills, EPA/600/2-86/073, U.S. Environmental Protection Agency, Cincinnati, OH, 1985.
Robinson HD. A review of the composition of leachates from domestic wastes in landfill sites. U.K. Department of the Environment, Reference: DE0918A/FR1, 1995.
Reinhart DR, Grosh CJ. Analysis of Florida MSW landfill leachate quality. Florida Center for Solid and Hazardous Waste Management, Report # 97-3.
Reinhart DR, Townsend TG. Landfill bioreactor design and operation. Lewis Publishers: New York, 1998.
Smith DC, Sacks J, Senior E. Irrigation of soil with synthetic landfill leachate-speciation and distribution of selected pollutants. Environmental Pollution 1999;106;429-441.
United States Environmental Protection Agency. Characterization of municipal solid waste generation in the United States: 1998 update. EPA530-R-99-02, U.S. Environmental Protection Agency, 1999.
Wang YS, Byrd CS, Barlaz MA. Anaerobic biodegradability of cellulose and hemicellulose in excavated refuse samples. Journal of Industrial Microbiology 1994;13:147-153.
Journal Articles on this Report : 2 Displayed | Download in RIS Format
Other project views: | All 7 publications | 2 publications in selected types | All 2 journal articles |
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Barlaz MA, Rooker AP, Kjeldsen P, Gabr MA, Bordent RC. Critical evaluation of factors required to terminate the postclosure monitoring period at solid waste landfills. Environmental Science & Technology 2002;36(16):3457-3464. |
R827580 (Final) |
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Kjeldsen P, Barlaz MA, Rooker AP, Baun A, Ledin A, Christensen TH. Present and long-term composition of MSW landfill leachate: a review. Critical Reviews in Environmental Science and Technology 2002;32(4):297-336. |
R827580 (2000) R827580 (2001) R827580 (Final) |
Exit Exit |
Supplemental Keywords:
compressibility, landfills, leachate, liners, municipal solid waste, MSW, bioreactors, settlement, stability., RFA, Scientific Discipline, Waste, Municipal, Environmental Chemistry, Hazardous Waste, Ecological Risk Assessment, Hazardous, Exp. Research/future, geotechnical properties, emerging environmental problems, hazardous waste disposal, hazardous waste management, leachate recirculation landfills, municipal solid waste landfill regulations, landfill operation, methane emissions, municipal waste, decomposition, municipal solid waste landfills, post-closure monitoring period, environmental policy, landfills, solid waste landfills, municipal solid waste regulations, RCRA, water managementRelevant Websites:
http://www4.ncsu.edu/~barlaz/post_closure/ Exit
The perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.