Municipal solid waste (MSW) landfills have evolved during the past half century from uncontrolled dump sites to sophisticated systems that are environmentally sound and economically viable. Modern MSW landfills employ engineered barrier systems that have minimized environmental contamination concerns, such as leachate seeps into groundwater and greenhouse gas emissions into the atmosphere.

In addition to our ability to efficiently and effectively manage MSW in modern landfills, research during the past few decades has expanded our knowledge of the numerous advantages of operating MSW landfills as bioreactors. Whereas conventional landfills are operated with minimal waste decomposition due to waste sequestration as the primary objective, bioreactor landfills are operated with liquid, air, and/or nutrient (e.g., wastewater treatment biosolids) additions to enhance waste decomposition.

The most common approach to operating bioreactor landfills is to recirculate leachate, which enhances waste moisture content and distributes an active anaerobic microbial community throughout the waste mass. This anaerobic bioreactor approach has been widely documented to enhance waste decomposition, treat leachate in situ, increase methane generation, and accelerate waste settlement.

The issue of MSW settlement is significant to all stages of a landfill, from siting and regulation to post-closure monitoring. The amount of waste that can be placed in a given landfill is regulated in terms of airspace, i.e., the maximum volume waste can occupy. An increase in the amount of waste that can be placed within a given airspace increases potential landfill revenue via waste tipping fees. When the waste mass of a landfill fills the allotted airspace, a final cover is required to minimize long-term environmental contamination concerns. The integrity of the final cover is critical to maintaining effective waste sequestration from the environment; thus, minimizing post-closure waste settlement is directly related to maintaining final cover integrity and minimizing long-term maintenance.

The ability to enhance MSW settlement during filling operations allows for a greater amount of waste to be placed within a landfill’s given airspace. Additionally, long-term concerns of final cover integrity and post-closure care are reduced when the majority of MSW settlement occurs during filling and prior to landfill closure.

Operating MSW landfills as bioreactors provides an opportunity to enhance MSW settlement during filling. While numerous researchers have documented an increase in MSW settlement due to anaerobic biological activity in MSW bioreactor landfills, our ability to quantify the effects of enhanced MSW settlement and predict MSW compression behavior has been ad hoc at best.

A recent project focused on developing predictive tools for sustainable solid waste management in bioreactor landfills was completed through a partnership consisting of academia (University of Wisconsin-Madison and North Carolina State University), industry (regulatory agencies, consulting firms and waste industry leaders), and the National Science Foundation. In regards to MSW settlement, the following key developments evolved from this project: (1) a mechanistic understanding of abiotic and biotic contributions of MSW compression; (2) a comparison of MSW compression behavior and settlement model parameters between laboratory- and field-scale experiments; and (3) a systematic approach for generating a priori predictions of MSW settlement in bioreactor landfills.