Long-Living Landfills

In the context of urban development and growing land pressure, closed non-hazardous municipal solid waste (MSW) landfills offer significant potential for beneficial reuse. Parks and recreational open areas, golf courses, greenspace set-aside, and/or wildlife habitation are a few of the potential deveopments.

Waste to Energy Biological Treatment Landfill

A recreation park being built on the site of the closed 1.5 million tonne Jordan Valley landfill in Hong Kong
Image Credit: Shaded. B

The sustainable reuse of a former landfill site requires regulatory agreement as to what constitutes aftercare completion. Jeremy Morris and Marion Crest discuss the a methodology to help determine functional landfill stability within four primary aftercare elements.

In the context of urban development and growing land pressure, closed non-hazardous municipal solid waste (MSW) landfills offer significant potential for beneficial reuse. Parks and recreational open areas, golf courses, greenspace set-aside, and/or wildlife habitation are a few of the potential deveopments.

However, several factors may limit the reuse options at a landfill or even pose an ongoing threat to human health or the environment (HHE). Aside from site-specific hydrogeologic and ecological considerations, closed landfills generally contain significant quantities of undegraded organic material and other wastes. Awareness of this has led to regulations and operational practices becoming progressively more stringent over the last several decades. At modern landfills, containment, treatment, and control systems must be installed, monitored, and maintained to manage the waste and emissions.

To allow for stabilisation of the waste mass after the site has been filled and closed, most regulations require a 30 – 50 year period of aftercare which essentially consists in managing and monitoring leachate and gas emissions, monitoring potential receiving systems (groundwater, surface water, soil, and air), and maintaining the containment system (primarily the final cover).

Although controlled activities (e.g., livestock grazing, hunting, and composting) are sometimes permitted on closed landfills during the aftercare period, it is generally accepted that broader reuse of the landfill will only be possible once the landfill is stabilised and aftercare deemed complete.

Objectives for sustainable landfill reuse

A key goal of responsible waste management strategy must be that of sustainable final waste disposal using methods that minimise net depletion of limited energy and material resources, and do not compromise HHE. For MSW landfills, sustainable waste disposal can therefore be defined as the safe transfer of material from society to nature (Lagerkvist et al., 1997). In this article, a sustainable landfill is defined as one in which:

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

The emphasis is on the landfill's demonstrable attainment of performance criteria for protection of HHE through its life cycle, not simply on meeting prescriptive definitions or mandates at some arbitrary point in time. Performance objectives for landfill sustainability are thus related to:

- 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 potential threats to HHE through design and operational changes - Maintenance and monitoring – reliable data on geomechanical settlement, waste by-product quantity and quality, and potentially impacted 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.

Realising these sustainability objectives will incentivise the operator to focus on proactively reducing a landfill's threat potential. Leachate recirculation and bioreactor operations are examples of proactive design and operational changes that positively affect waste treatment, reducing long-term reliance on containment systems through enhanced degradation rates.

Defining aftercare completion

In most countries, the jurisdictional national or sub-national authority typically defines aftercare completion as a condition when it can be demonstrated that a landfill no longer poses an unacceptable threat to HHE. At this point it is assumed that aftercare will end and the operator released from financial provisions. Logically, this requires that the authority develops both technical criteria to define completion and administrative criteria to release the operator from regulatory aftercare obligations.

In practice, useful or proven criteria are often lacking. Approaches to defining the end of aftercare generally range from: (1) comparison of the landfill's source characteristics (i.e., properties of waste, leachate, and LFG) to absolute target values for organic stabilisation, without consideration of the landfill's long-term relationship with its receiving environment; to (2) site-specific impact or risk assessment in which the potential for waste, leachate, or LFG to impact HHE are considered.

The former might appear to simplify administrative procedures and help operators implementing measures to accelerate stabilisation of the waste within a given timeframe. However, a disadvantage of this approach is that the site-specific threat to HHE posed by the undisturbed waste under current and proposed land uses cannot be considered. Furthermore, given that effective emissions are unlikely to reach zero as long as some waste is buried, this approach raises the question as to what constitutes an acceptable emission.

Aftercare completion & functional stability

The Evaluation of Post-Closure Care (EPCC) Methodology relates to aftercare completion and the "functional stability" of the landfill. Once a landfill is considered to be functionally stable, regulated aftercare is completed, although some de minimus level of control (termed "custodial care") would typically still be provided to protect against disturbance of passive barriers (ITRC, 2006).

The time required for a landfill to achieve functional stability is controlled by site-specific factors, including: (1) waste composition; (2) manner of operation (especially actions taken to accelerate waste degradation); (3) measures taken to manage emissions; (4) climatic conditions; (5) the hydrogeologic and environmental setting in relation to potential human and ecological receptors; and (6) the property's intended end use.

Based on typical biochemical decomposition characteristics for organic wastes, the concentrations of constituents of potential concern within leachate and LFG emissions will decrease with time. However, the flux or magnitude of the total emission in mass per time will be controlled by the concentration and the rate of release of the carrier medium (i.e., leachate or LFG). This in turn is influenced by the containment system and its performance over time. The degree of threat to HHE, and absence of harm caused to environmental receptors, will also be a function of the location and significance of the emission (Morris & Barlaz, 2011).

The EPCC Methodology establishes a modular approach for demonstrating functional stability within the four primary aftercare elements (i.e., leachate management, landfill gas management, groundwater monitoring, and final cover system maintenance) and determining how and when active care and/or monitoring within each module can be optimised or terminated. This follows a step-down approach, evaluating each potential exposure mechanism and allowing for the possibility that certain aspects of care can be reduced or discontinued, but not others.

For example, it may be appropriate to reduce or discontinue leachate management or groundwater monitoring while, at the same time, continuing LFG management and cover inspections. Furthermore, where groundwater monitoring is required, it is only generally required for certain identified substances and not for the very large number of regulated constituents (i.e., monitoring should be focused on constituents that are uniquely indicative of a release). To evaluate future performance based on historical performance, application of the methodology involves examining statistical trends in leachate, LFG, and/or groundwater, as well as other relevant biological, chemical, and physical data with the objective of demonstrating that the flux of future emissions will only decrease from the current state.

Initially, the user would attempt to demonstrate that the landfill meets target values (i.e., universal stability criteria) at the source, thereby allowing for a "fast-track" conservative demonstration of completion without regard to the landfill condition. For elements failing that, the user would need to demonstrate that under defined long-term conditions for the cover, the landfill can pass a rigorous site-specific evaluation of threats posed to HHE, first at the point of compliance (POC) or, if necessary, at the landfill's point of exposure (POE). In this way, different aspects of landfill management and aftercare may be considered independently at different levels.

The path to sustainable landfill reuse

From above discussion, there is a clear link between assessment of aftercare in terms of functional stability and sustainable reuse in the landfill context. Such assessment allows care activities to be adapted to focus on providing a level of care consistent with an evaluation of worst-case environmental threats derived from the sensitivity of the surrounding environment. This enables aftercare activities to be progressively reduced, wherever possible, until under a best-case scenario no active care is necessary.

A conceptual illustration of a landfill's step-down progress from active aftercare through custodial care is presented in Figure 1.

Figure 1. Performance-based aftercare and functional stability
The decreased level of care and increased range of reuse options with time is dependent on decreased leachate volumes, improved leachate quality, decreased LFG generation, and reduced settlement and cover maintenance. The time required to proceed to post-completion custodial care is site-specific.

As shown, it is proposed that conditions for sustainable reuse of a former landfill could be met through a combination of enhanced waste degradation (e.g., through leachate recirculation or in-situ aeration), use of passive design systems such as gravity leachate drainage supplemented with wind/solar powered pumping systems (Zeiss, 2007) and natural analog engineered features such as constructed wetlands (Goldemund, et al., 2008), phyto-caps (Morris, et al., 2007), and biocovers (Barlaz, et al, 2004).

This is as well as defining aftercare control systems and reuse conditions that emphasise environmental responsibility, engagement of the host community, and minimise the need for land use restrictions and buffers after completion of active aftercare (Tippetts, 2011).

Closing

Innovative landfill management operations and robust evaluation of leachate and LFG trends over time are important examples of sustainable concepts that optimise decision-making and greatly improve the ability to manage landfills effectively and safely. Engineered containment systems (liners, covers, and leachate and biogas management systems) may degrade over the longer term. However, the residual threat associated with leachate and LFG emissions improves over the same timeframe. Therefore it's sensible to design or modify landfills with features to accommodate or even take advantage of such changes in the balance between necessary levels of containment, treatment, and maintenance and monitoring.

A good example is monolithic all-soil evapotranspirative (ET) cover system designs. These can be implemented as part of a sustainable landfill design and constructed to be naturally analogous and compatible with the local ecosystem (Dwyer & Bull, 2008). Such low maintenance or self-sustaining "natural analog" cover designs can provide protection of the environment equivalent to prescriptive geosynthetic covers, but with the added benefits of enhanced methane oxidation, reduced greenhouse gas emissions, and ability to allow controlled infiltration for enhanced waste degradation. Other features might include wetland treatment systems to deal with residual leachate seeps. In some instances, correct selection of liner and capping materials could allow a landfill to gradually 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").

Proactively managing a landfill should reduce the duration of aftercare and may provide a means of achieving a sustainable landfill reuse faster as well as providing justification of more varied land reuse options for the local community. Such proactive management strategies include, for example:

- Long-term planning for active management activities to eventually be transferred to semi-active or passive management - Incorporating redundancy in all control systems to the extent possible - Consulting stakeholders on the potential longer term practicality and benefits of innovative design options, such as anaerobic or aerobic bioreactor technology, or other processes that accelerate waste degradation and shorten the period of significant leachate and LFG generation - Encouraging best-management practices, environmental management systems (EMS), and other programs that optimise site operations and focus on eliminating litter, odors, gas migration, and other non-compliance issues and community concerns that may hinder future flexibility

Finally, comprehensive collection of performance data is strongly recommended because evaluations of functional stability are based on trends in behavior over a period of several years, which typically requires that several years of complete data be available.

Jeremy Morris is a senior engineer at Geosyntec Consultants based in Maryland, U.S. and Marion Crest is a research engineer at Suez Environnement.
Email: jmorris@geosyntec.com and marion.crest@suez-env.com

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