In the 1990 movie Days of Thunder, NASCAR racing veteran Harry Hogge tells rookie driver Cole Trickle, “There ain’t nothing stock about stock cars,” meaning that while the name might imply the vehicles are uniform, they’re anything but standard. Similarly, many may view the waste going to landfills as homogenous, but that is hardly the case—the composition of waste sent to landfills represents an enormous diversity of materials.
In recent years, the array of materials going to landfills has been expanding, and there has been a growing trend for municipal solid waste (MSW) landfills to accept non-MSW materials. In fact, 48 states now allow MSW landfills to accept non-MSW material, commonly referred to as special waste. Work conducted by the Environmental Research & Education Foundation (EREF), Raleigh, North Carolina, to quantify waste acceptance suggests that, on average, one-third of all Subtitle D waste accepted at MSW landfills in those states is non-MSW special waste.
Special wastes discarded in MSW landfills include industrial wastes, construction and demolition (C&D) wastes, ash, non-hazardous contaminated soils, foundry wastes, dredged materials, auto shredder fluff and sludges.
The proliferation of landfills accepting these kinds of wastes is important on a number of fronts. First, acceptance of special wastes at MSW landfills has implications on landfill gas generation and emissions, since some special waste streams may contain significantly more or less organic material than MSW streams. The U.S. Environmental Protection Agency (EPA) creates estimates on the amount of greenhouse gas landfills emit based on their incoming composition of waste materials. Entities like the EPA and Intergovernmental Panel on Climate Control (IPCC) generate inventory reports, which rank the top industry emitters of greenhouse gases in the U.S. and the world. In many situations, these inventory reports are then used to guide the development of policies and regulations. Thus, it is important that these estimates are as accurate as possible and reflect the most recent scientific findings.
When it comes to estimating greenhouse gas emissions from landfills, the IPCC and EPA use equations that contain empirical values to describe landfill characteristics. Since the biogas generated from landfills is derived from the breakdown of organic materials, a factor used to estimate greenhouse gas emissions is degradable organic carbon (DOC). In a recent study conducted by EREF, it was found that the DOC guideline used to estimate greenhouse gas emissions for bulk waste at MSW landfills was 20 percent higher than the actual value computed based on sample and sort studies and waste acceptance data for MSW landfills across the U.S. The finding suggests that the inventory reports noted above may be overestimating greenhouse gas emissions in significant ways.
Understanding special waste disposal at MSW landfills is also important due to considerations for how these wastes are managed. Of special wastes going to MSW landfills, EREF estimates 37 percent are comprised of ash and/or industrial wastes. Previously EREF-funded research has demonstrated that some types of ash and special wastes may generate heat to a point where it becomes problematic for landfill operations. Because the mechanisms for how this occurs are poorly understood, EREF has recently funded additional projects to further explore the issue. One project underway at the University of Central Florida looks at the implications of MSW combustion ash being disposed in landfills and whether this practice can trigger elevated temperatures. Another project initiated by North Carolina State University entails laboratory and modeling work aimed at understanding how different wastes that can generate heat can be tested. Collectively, a key anticipated outcome of both projects is that guidance or best management strategies will be developed to guide facility owners on how to manage disposal of special wastes with these potential attributes.
Additionally, a significant fraction of special wastes are composed of industrial wastes that have high moisture content. While the increased moisture content that these wastes possess can accelerate the breakdown of organic materials and contribute to long-term stability (a desirable trait for post-closure care), it can also raise concerns regarding structural stability of the waste mass and increased leachate production. A recent report from EREF, “Utilization and Effectiveness of Waste Bulking and Stabilization Strategies,” delves into the types of aqueous wastes accepted by MSW landfills, strategies used to manage them and some of the key operational challenges.
Further, some special wastes may be chemically or biologically reactive and may release heat or materials that lead to increased complexity in managing leachate. Such situations may require landfill operators to process the materials in a way that stabilizes them (e.g., structurally or chemically) before placement into the waste mass. “Utilization and Effectiveness of Waste Bulking and Stabilization Strategies” also discusses common strategies used to manage aqueous wastes via bulking or stabilization/solidification (S/S) agents. Bulking agents, such as sawdust or soil, absorb the moisture in the waste, whereas S/S agents harden the waste through chemical reactions. Bulking agents are used in situations where moisture reduction is the ultimate goal. When using these agents, structural integrity can be easily compromised, and once the saturation point is reached, the agents will no longer absorb liquids. S/S agents, on the other hand, are cementitious, binding to the waste and hardening when mixed with water. A more detailed list of commonly used bulking and S/S materials is available in the report.
From data generated from this report, it is apparent that protocols and strategies to manage aqueous wastes can be enhanced. As a result, EREF has funded Colorado State University to do a deeper dive on how to evaluate and manage high-moisture wastes going to MSW landfills. Key aspects of the project include waste stability, gas management and implications on leachate production and treatment.
Over the past few decades, the composition of waste materials going to landfills has changed drastically, and it is expected that the acceptance of special wastes by MSW landfills will likely increase. For many of these special wastes, a landfill is the most appropriate means to manage them because they were generated in a way that does not make it possible to reuse, recycle or divert the materials. Given this fact, until manufacturing practices change, the solid waste field is left to develop innovative management practices to safely and effectively deal with them.
Collectively, the data and research efforts being undertaken in the industry can provide critical information needed to guide policy, inform landfill operating strategies and help enhance stewardship approaches for dealing with special wastes in the future.