End of Life, Post-Closure Care, and the Sustainable Landfill

The modern technology of MSW landfilling results in anaerobic biodegradation processes occurring in the waste mass, producing leachate and landfill gas (LFG) that are contained and controlled by engineered collection and treatment systems. Approximately 54% of the 220 million tons of MSW generated in the US in 2005 was disposed of in landfills (USEPA, 2006), although reliance on landfill varies considerably between states. The dependence on landfills in some other countries is decreasing, notably within European Union member states with the implementation of the European Waste Directive (CEC, 1999), which limits landfilling of organic material, and in Japan, where historic scarcity of land for disposal and other factors has led to an MSW incineration rate of approximately 78%. In most industrialized countries, integrated solid waste management strategies now place great emphasis on waste minimization, recycling, and alternative treatment technologies. For example, recycling and composting now account for about 32% of MSW management in the US (cit. in Barlaz et al, 2007). Nevertheless, whatever the predominant approach selected for management of solid wastes, some material will always require final disposal, and this will almost certainly continue to be to sanitary landfills. Therefore, landfills are likely to play a significant role in MSW management for the foreseeable future, not only in the US but also worldwide.

The modern basis for the industry as a whole is integrated control of waste generation, storage, collection, transfer and transport, processing, and disposal in a manner according to best principles of public health, economics, engineering, conservation, aesthetics, and environmental performance, and that is responsive to public attitudes. However, even with the documented success of engineered control systems and their associated community benefits (e.g., LFG-to-energy plants, methane oxidation through alternative covers, leachate management via wetlands development), recent concerns about greenhouse gas (GHG) emissions, carbon footprint, and other environmental indicators have amplified focus on the short- and long-term value of applying sustainability concepts to MSW landfills. This article seeks to provide a useful definition of landfill sustainability based to end of life (EOL) considerations, post-closure care (PCC), and the intended end use of the facility once PCC has been completed. By relating sustainability to the “functional stability” of the landfill, that is, the landfill’s long-term non-impacting relationship with its receiving environment in the absence of some or all active PCC provisions (definition originally proposed by SWANA’s Bioreactor Committee, June 2004; cit. in EREF, 2006), it is argued that the path to sustainability can be reached through a combination of enhanced waste degradation (e.g., bioreactor technology), use of passive fail-safe design and engineering features (e.g., wetlands, phytocaps, biocovers, and other self-sustaining natural analog systems that mimic local ecosystems as closely as possible), and by defining PCC control systems and end-use conditions that emphasize environmental responsibility and engagement of the host community, and minimize the need for land-use restrictions and buffers after completion of active PCC. In this way, the landfill property can be a community asset that requires minimal long-term active maintenance while remaining protective of human health and the environment (HHE).

The USEPA’s working definition of sustainability is “the ability to achieve economic prosperity while protecting the natural systems of the planet, and providing a higher quality of life for its people.” According to the USEPA, sustainable development marries two important themes: that environmental protection does not preclude economic development and that economic development must be ecologically viable now and in the long run. Common use of the term “sustainability” began with the 1987 publication of the World Commission on Environment and Development report entitled “Our Common Future” (WCED, 1987). This document coined the well-known and widely referenced definition of sustainable development as “development that meets the needs of the present without compromising the ability of future generations to meet their own needs.” This concept of sustainability encompasses ideas, aspirations, and values that inspire better stewardship of the environment and promote positive economic growth and social objectives. The principles of sustainability can stimulate technological innovation, advance competitiveness, and improve our quality of life.

For the purpose of this discussion, and in the spirit of the USEPA’s position on sustainability, a sustainable MSW landfill is defined as a landfill whereby

• byproducts of waste degradation are managed so that outputs are controlled or released in an acceptable manner;
• residues will not pose an unacceptable human health or environmental threat to the surrounding natural systems;
• the time needed for active management is minimized (albeit that there might be a longer term monitoring period);
• costs for long-term active management and monitoring of a closed landfill are not passed onto future generations; and
• future uses of groundwater and/or other natural resources are not compromised.

In practical terms, any discussion on landfill sustainability must include goals for EOL and PCC, because realization of the above sustainability objectives requires a landfill owner/operator to focus on proactively reducing a landfill’s threat potential through optimizing design, operations, and management during the operating life of the facility (i.e., long before landfill closure and the onset of PCC). Performance objectives for landfill sustainability (i.e., defining EOL and PCC) are thus related to the following:

• Waste containment-the integrity of the containment systems is essential to control waste contact/emissions while a threat to HHE remains.
• Waste treatment-the extent of biodegradation and proactive efforts to reduce threats to HHE can be achieved through design and operational changes.
• Maintenance and monitoring-reliable data of waste byproduct quantity and quality as well as data on potentially affected media is needed to ensure that containment and treatment are ongoing while necessary and to confirm that no significant variations in predicted media conditions have occurred, particularly if changes to containment or treatment systems have been implemented.

Leachate recirculation and bioreactor operations are examples of proactive design and operational changes that positively affect waste treatment, reducing long-term reliance on containment, and can be followed by installation of long-term passive control systems such as wetlands for leachate treatment, bioactive covers for fugitive LFG control, or in situ aeration for enhanced waste degradation. Such innovations in landfill management are all likely to have higher initial design and capital costs and require additional monitoring and control during their operating life, but are expected to accelerate the time frame for assurance that the landfill, once closed, is secure and protective of HHE over the long term.

Based on the above, the link between functional stability and sustainability in the landfill context is clear. The time taken for MSW landfills to achieve functional stability is controlled by the landfill’s overall contaminant load and by the measures taken to minimize releases. The concentrations of constituents of concern within leachate and LFG emissions will decrease with time. However, the flux or magnitude of the total emission will be controlled by the concentration and the rate of release of gas or leachate. The degree of threat to HHE, and absence of harm caused to environmental media, will also be a function of the location of the emission in addition to its concentration and period of release. The expected interaction between containment, treatment, and emissions over the long term is illustrated conceptually in Figure 1. Considered together, the expected longevity of liner and cover system components, and demonstrated improvement in leachate and LFG quality over time means that closed landfills can rely on Subtitle D compliant cover and liner systems to limit generation and emission of leachate and LFG to levels that remain protective of HHE over the very long term. For the landfill to be sustainable the service life of the components of a liner and cover in an appropriately designed and constructed MSW landfill should be significantly in excess of the time period during which leachate and LFG production rates progressively decline to nominal levels. In other words, the “hump” on the flux emission curve should occur while active PCC at the site is ongoing. The hump also identifies the time from which discontinuation of relevant PCC systems and activities can be measured because, by definition, the emission flux must be reduced thereafter (i.e., if the hump did not cause an impact to HHE, then no impact should be expected in the future).

It is a commonly held misconception that after the assumed 30 year PCC period under Subtitle D has expired, an owner/operator would simply be allowed to stop providing any further care for the landfill. It should be understood that solid waste regulations require a demonstration that it is technically appropriate to end PCC. A number of alternatives for defining an appropriate PCC period exist. However, the two most widely merited approaches currently proffered are based on defining an inert endpoint (i.e., organic stabilization) or defining the need for PCC in terms of performance (i.e., functional stability).

Functional Stability Versus Waste Stabilization
In common with other EU countries, Germany is considering introduction of organic stability criteria for the release from PCC in a new integrated landfill ordinance (Stegmann, et al., 2003). The approach is based on a defining a list of limit values reflecting stabilized waste, leachate, and LFG. Similarly, a number of states in the US are implementing regulatory approaches to evaluating PCC in terms of waste stabilization (e.g., Florida Administrative Code, Chapter 62-701.620.1, Rule Workshop Draft 08/25/07; Wisconsin NR 514.07[9], WDNR 2006). According to some definitions, a landfill is not considered to have stabilized until maximum settlement has occurred (UK Environment Agency, 2003). Clearly, in order to meet the requirements of organic stability, the approach for landfill operations and management must shift focus towards promoting long-term threat reduction through enhanced waste degradation (i.e., enhanced organic stability) and away from the convention Subtitle D approach of reducing infiltration and leachate generation in the post-closure period. As previously discussed, the former approach will require proactive landfill operations (e.g., leachate recirculation, alternative covers) to optimize the moisture content necessary for enhanced waste degradation while effectively managing leachate and LFG generation until the landfill becomes stable (ITRC, 2003 and 2006a). Alternative covers are needed in this scenario to address slope stability concerns under conditions of gas pressure as well as potential gas to groundwater impacts. Increasingly, alternative covers are being constructed at many landfills for their many benefits over both geomembrane and compacted clay covers.

Although in some ways having such rules makes it easier for an owner/operator to make an informed decision on measures to accelerate stabilization of the waste, a significant disadvantage is that site-specific conditions cannot be readily included. Similarly, it must also be put forward that zero emission landfills are an illusion, regardless of how strictly organic stability criteria are set. Regulators will have to accept that there will always be some emissions-a more important consideration is thus whether such emissions represent a threat to HHE. Regulators will need to be convinced with data that the threat posed by a landfill is minimized (i.e., leachate quality and quantity from the perspective of HHE needs to represent an acceptable level of emission). This is in keeping with Subtitle D, which allows the state director to reduce or terminate PCC at MSW landfills once it is demonstrated that the landfill does not present a threat to HHE at the point of exposure, and to extend PCC if such a demonstration cannot be made (USEPA, 1993). This regulatory framework, coupled with the predictability of landfill systems and known longevity of Subtitle D containment systems (as previously discussed), serves to ensure that PCC will not be ended without protection of HHE.

Performance-based approaches to evaluating PCC focus on identifying and quantifying the potential for a landfill to pose a threat to HHE, and the duration over which such threat potential may occur. A performance-based approach thus focuses PCC obligations on actual landfill conditions and distinguishes when the end of regulatory PCC is appropriate given site conditions, potential threat to HHE, and intended future use of the property. This type of evaluation generally involves examining statistical trends in leachate, LFG generation, and/or groundwater quality, as well as other relevant biological, chemical, and/or physical data, to predict future performance based on current or past trends. A number of key reference tools for making statistically valid, site-specific, performance-based assessments of PCC at MSW landfills have recently been developed by multiyear studies of PCC aimed at the US landfill industry, including Gibbons & Bull (2006), ITRC (2006b), and EREF (2006). The latter approach, termed the evaluation of post-closure care [EPCC] methodology, involves a series of modular evaluations that help an owner/operator assess the potential for impacts after PCC is modified or terminated. Four modules were developed, consistent with the main categories of requirements for PCC under Subtitle D (i.e., leachate management, control of gas migration, groundwater monitoring, and cap maintenance). Consistent with Subtitle D guidance, threats to HHE are measured at the point-of-exposure (POE) and not at some arbitrary point within the landfill footprint.

If an evaluation shows that no impacts are expected, then monitoring is recommended to confirm the conclusion. If, on the other hand, impacts are expected, then the owner/operator continues PCC until such time that impacts are not expected after PCC is ended. In this way, rather than relying on a determination that PCC is either complete or must be continued at the same level of intensity, the EPCC Methodology follows a step-down approach, evaluating each potential exposure mechanism and allowing for the possibility that certain aspects of PCC could be discontinued while others are maintained. For example, it may be appropriate to reduce the level of or discontinue leachate management or groundwater monitoring while, at the same time, it may be entirely appropriate to continue cover inspections and maintenance. Further, where groundwater monitoring is required, it is generally not needed for all of the very large number of constituents that could potentially be contained in a landfill release; rather, monitoring is only needed to detect constituents that are uniquely indicative of a release.

Functional Stability
Performance-based approaches to evaluating the main landfill components focus on identifying and quantifying the potential for a landfill to pose a threat to HHE with functional stability as the ultimate goal. Demonstrating functional stability of a landfill requires that we understand site-specific characteristics of the landfill, among them:

• Side-slope, cover, and liner stability
• Site geology and hydrogeology
• Potential receiving bodies (i.e., receptors) and exposures to applicable ecosystems and human health to characterize the POE (clearly, for example, leachate leaking through a thick unsaturated zone into a minor or non-aquifer will have a lower environmental impact than leachate flowing to a ditch that is discharging into a high quality surface water system)
• Climate
• Leachate quantity and quality trends
• LFG composition and generation potential over time

Demonstrating that leachate is not a potential threat to HHE is a primary driver for functional stability and achieving a sustainable MSW landfill. The predictability of decreasing MSW leachate constituent trends allows the current and future threat potential of leachate to be determined (Gibbons, et al., 2007) and the landfill’s progress towards sustainability to be quantified in a meaningful way. Research indicates that about half the organic carbon of MSW is sequestered and that mobilization of metals in leachate over the long term is unlikely unless landfill conditions are allowed to deteriorate significantly. Furthermore, as the material in a landfill degrades, the bottommost MSW layers become well decomposed and act as a biofilter with a relatively inexhaustible attenuating capacity for consuming degradable organics and heavy metals in leachate (cit. in EREF, 2006).

Recognizing that landfill PCC will become increasingly difficult with the long-term degradation of engineering systems (liners, caps, and leachate management systems), then it would be sensible to design landfills that have certain fail-safe features. These features might include natural wetland treatment systems that are purpose built to deal with the low-level residual contaminant load that might seep from a landfill over a protracted period, but that would be highly amenable to such passive treatment. Alternately, in some instances, correct selection of liner and capping systems could result in a landfill that is designed to increase its basal leakage rate as the cap degrades, thus negating the probability of leachate accumulation in the waste mass (the so-called “bathtub effect”). Examples of modifications to a leachate management system, in order of increasing level of modification to original leachate collection and removal system (LCRS) operating conditions, might include the following:

• Modifying LCRS operation under “optimized” conditions (e.g., after resizing leachate storage facilities, realigning leachate transmission systems, or rescheduling monitoring events or leachate disposal to POTW)
• Implementing long-term lower maintenance or passive engineered leachate controls, (e.g., replacement of electric pumps with wind or solar powered models, gravity drainage of leachate into constructed wetlands or infiltration trenches)
• Ceasing LCRS operations and all other leachate control activities for which maintenance may be required

Similarly, LFG generation at very low levels may well continue long after it is economically or technically feasible to collect and/or utilize it. Passive treatment systems such as the utilization of waste-derived compost or organic-rich soils to actively encourage methane oxidation in biocovers or specially constructed bioventing systems can provide a means of passive low-maintenance emission control (Rachor, et al, 2008). Examples of modifications to an existing LFG management system that could be considered include the following:

• Scaling back from operating a fully active gas collection and control system (GCCS) across the entire landfill to an intermittently operated active GCCS, or changing from a LFG-to-energy facility to smaller utilization projects
• Scaling back from continuous or intermittent GCCS operation over the entire landfill to operation over some portions of the landfill only (e.g., emissions “hotspots”)
• Shutting-down an active GCCS and converting to a passive system using passive flares with solar self-ignition systems, biovents, bioactive capping, and/or cut-off trenches

Finally, if the outcome of an evaluation indicates that no further regulatory PCC is needed for any component of PCC, then all PCC activities for the landfill have been completed. In this case, PCC under the jurisdiction of the state agency would be ended, although some de minimus level of care would typically still be provided (mainly for the cap). This de minimus care is defined by the EPCC methodology as “custodial care” and includes meeting end-use obligations, satisfying institutional controls and local ordinances, and fulfilling other non-MSW applicable regulations.

In many ways, provision of custodial care is similar to the care provided at sites exiting post-remediation care under the EPA’s Brownfields Program. At this juncture, the landfill is considered to be functionally stable and, therefore, protective of HHE in the absence of regulated PCC. A conceptual illustration of a landfill’s step down progress from active PCC through custodial care is presented as Figure 2. Two case studies outlined below demonstrate the manner in which a site may embark upon the step-down reduction in care illustrated on the figure.

Case Study 1
The study site is a closed, 27-acre MSW landfill unit comprising two conjoined cells at an active facility. The site is located in Delaware and receives approximately 40 inches of rain annually with high summer temperatures typically around 90°F and winter lows generally below freezing. The cells, which accepted waste from 1980-1988, have a geomembrane liner and leachate collection system. The cells were closure capped with a 2-foot thick sandy soil cover in accordance with the regulations applicable at that time, and currently generate approximately 10,000 gallons per day of leachate. Leachate recirculation was used as the primary means of leachate management for over 10 years, a practice that, in conjunction with the permeable soil cover, has contributed significantly to the relatively mild leachate presently generated at the unit (Morris, et al., 2003), although it was eventually necessary to ship leachate to an offsite wastewater treatment plant in addition to recirculation (which ceased altogether in March 1995 because of a regulatory ban).

The overall goal of this project was to create a self-sustaining leachate management system using engineered wetlands to treat all leachate generated by a discrete closed landfill to arboreal standards and then irrigating a phytocap on the landfill cover to take up all the treated effluent, resulting in no discharge to the environment. Because pristine wetlands surround their rural property, the Delaware Solid Waste Authority (DSWA), the site owner/operator, required that neither leachate nor treated effluent be discharged to groundwater or surface water (although this may be considered in due course). In addition, no liquid should require removal from the closed loop system (i.e., offsite disposal) except where this cannot be avoided as a contingency measure. DSWA also required that the project not increase total leachate management costs above 5 cents per gallon, the then cost for offsite leachate disposal. To achieve these goals and maintain a perennial zero/negative water balance, a closed-loop system was designed, consisting of five main components: flow-balancing leachate storage tanks, constructed wetlands, treated effluent storage pond, irrigation system, and phytocap. A schematic layout of the design is shown on Figure 3. Further details on the project are discussed in Morris, et al. (2007).

The constructed wetland design was based on the 14-month operation of a pilot scale system (Pendleton, et al., 2005). Because the total volume of treated effluent often exceeds the amount of irrigation liquid needed to sustain the phytocap, some of the effluent volume is eliminated utilizing evaporation (E) and evapotranspiration (ET) processes. To maximize E/ET losses, irrigation occurs only during optimal dry, hot conditions during the warmest six to eight months of the year. A disused 4-million-gallon capacity leachate storage pond was upgraded to store treated effluent during the remainder of the year (incident precipitation is excluded using a floating geomembrane cover). Effluent from the storage pond is fed into an irrigation system consisting of an automated control system and network of zoned sprinkler heads that have low-efficiency, misting settings in order to maximize E losses during spraying. The phytocap was planted with tree species that are compatible with site conditions and suitable for maximizing ET losses. Irrigation of the phytocap is controlled by means of a weather station and soil moisture sensor control system. To prevent potential migration to groundwater, irrigation occurs over the lined cell area only. To minimize runoff and potential migration to surface water, irrigation does not occur during precipitation events or in a cover zone where the soil is already saturated. The entire flow process is automated using pressure and float switches to control liquid flow through the system and prevent overflow of any system component. Redundant systems and controls are also provided throughout. Construction commenced in 2006; the system became fully operational in 2008.

Case Study 2
The study site is an active MSW landfill operated by Allied Waste Industries (now Republic Services) and located in Alabama. The landfill covers 246 acres of land and has a total permitted disposal area of 134 acres, of which about 79 acres of landfill cells have been constructed. The overall intent for this project is to treat all leachate generated at the site for the remainder of its active life and after closure. Currently, approximately half of the daily leachate flow is reported to be managed through recirculation with the remaining half hauled off site to POTW. The designed treatment system consists of a three-cell surface flow wetland followed by two parallel four-cell vertical flow wetland biofilter system (WBS) units. Thereafter, commingled treated effluent from the WBS is to be either discharged to surrounding natural wetlands via an existing stormwater management pond, returned to an existing lift station for transfer to onsite leachate storage tanks via an existing leachate transmission line. Treated effluent transferred to the tanks in this way will continue to be used for recirculation back into the landfill for operational benefits. The design was based on a 15-month pilot test that showed that leachate can successfully be treated to meet stringent surface water quality criteria (Pendleton, et al, 2005).

A conceptual layout of the design is shown on Figure 4. Further details on the project, construction of which was recently completed, are discussed in Goldemund, et al. (2008). Currently about 15,000 gallons per day of leachate is generated (including the portion of the flow that is used for recirculation). Use of treated effluent rather than raw leachate for recirculation is expected to enhance overall leachate quality over time. This will stabilize the treatment performance of the constructed wetlands system early on. The approach will lead to a more sustainable leachate management system capable of improving the environmental and economic performance of the landfill. Although the WBS currently utilizes small electric pumps to transfer influent/effluent between cells, the future use of a solar-powered pumping system will be investigated.

The Path Toward Sustainability
This article has proffered some objectives for landfill sustainability and explored methods by which performance-based evaluations of PCC and proactive design, operation, and end use considerations can aid meeting such objectives. The demonstration of functional stability considered necessary to define PCC in terms of performance requires clear understanding of site-specific landfill characteristics, not least long-term trends in leachate generation and quality.

Innovative operational practices such as bioreactors are specifically designed to enhance the rate of degradation resulting in a greater rate of LFG production during the period of time the landfill is operating. Such LFG recovery enhancements provide for potential beneficial use LFG recovery projects whether the site is open or closed. Bioreactors can also be designed and operated to manage and reduce the long-term threat of organics and inorganics in the waste mass. For example, recent research provides evidence that landfills have significant capacity to convert nitrate to nitrogen gas that can be safely released to the atmosphere; thus, providing a viable alternative for long term management of nitrogen in landfills (Price et al, 2003).

The body of research knowledge on MSW landfill waste degradation provides a technically defensible basis to evaluate threat depending on the stage of waste degradation during post-closure. A critical element of the evaluation of threat is the end-use strategy for the closed landfill property. If the property end use allows for a potential exposure pathway to receptors on or near the landfill footprint, then any subsequent evaluation of threat to HHE needs to consider a proximal point-ofexposure.

In contrast, if the end-use strategy eliminates exposure pathways or covenants or deed restrictions are enforced which preclude exposure within or near the landfill footprint, than an evaluation of threat to HHE with the point-of-exposure at the property boundary or beyond would be appropriate.

Innovative management of landfill operations and robust evaluation of leachate and LFG trends over time are important examples of sustainability concepts that optimize decision-making and greatly improve the ability to effectively and safely manage MSW landfills. Furthermore, promoting proactive operational practices that enhance waste degradation and reduce long-term threat potential (e.g., bioreactor technology) provide a means to achieving a sustainable landfill faster and may provide justification of more varied land reuse options for the local community. The integration of long-term environmental objectives like attaining functional stability through optimizing site operations, PCC, and end-use strategies will result in landfills being modified from being just a disposal practice to facilities that promote sound environmental solutions and sustainability.

Proactively managing a landfill during the design and operating phases should reduce the duration of PCC. Such proactive management includes, for example:
• Planning for active management activities (e.g., operation of a LCRS) to eventually be transferred to semi-active or passive management
• Consulting stakeholders on the practicality
of conventional and innovative design options
• Implementation of anaerobic or aerobic bioreactor technology, leachate recirculation, in-situ aeration, or other processes that accelerate waste degradation and shorten the period of significant leachate and LFG generation
• Optimizing the gas collection and control system and other control systems to eliminate odors, gas migration, or other noncompliance issues that may hinder future flexibility
• Encouraging best management practices, including use of construction quality assurance (CQA), environmental management systems (EMS), and other planning or monitoring program records and documentation
• Encouraging collection and management of data many years before an evaluation to modify PCC is performed

Early collection of data is strongly recommended because evaluations are based on trends in data over a period of several years, which requires that several years of complete data be available. Note that some data may not be required under existing permit conditions, and historic data records may need to be converted to a useable format. The data requirements portion of the methodology should be an effective internal tool for a facility owner/operator to evaluate the effects of varying operating and end-use scenarios on their PCC obligations. This should also serve to reward facilities having data that demonstrate a history of protection of the environment. Such facilities are also more likely to be able to benefit from “retroactive” analyses and monitoring, where offered (e.g. using historical data to account for earlier time periods, even during the active life), that allow for reduced future PCC monitoring to demonstrate protection of HHE.

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