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Monitoring of waste degradation processes for sustainable MSW landfills

 

Introduction

 

Across the world, the majority of MSW is landfilled, incinerated, placed in an open dump, or discarded into rivers or streets. Some cities have made enormous strides towards the ambitious goal of “zero waste”, particularly San Francisco, which has one of the highest recycle and reuse rates at 55% (Global Waste 2013). However, the reality remains that more than half of the U.S.'s MSW goes to landfills, and that this rate is similar or higher across the world. For the next few decades, while we work towards a “zero waste” economy, it is essential that we create landfill facilities that provide the highest environmental and economic benefits, as well as the lowest risk to contributing to climate change. Landfill facilities should strive to be sustainable - in essence leaving the world in equal or better shape to following generations. Next-generation and retrofitted bioreactor facilities appear to show great promise for providing these sustainable benefits.

Sustainable development requires MSW management that promotes public health, protects the environment, and is economically feasible. This is particularly true for low and middle-income countries that currently struggle to provide consistent waste management services. In Mumbai, open burning of waste is estimated to emit 10,000 grams toxin equivalents of carcinogenic dioxins/furans every year, causing 20% of the city’s air pollution (Annepu 2015).  In Nepal, “garbage is…piled high in empty lots, on the roadside and on the edges of the city’s sewage-filled rivers…[with] [a]crid smoke from burning plastic filled the air.” (Lorch 2015). Increasing urban populations, changing consumption habits, natural disasters, and human conflicts weaken waste management services and threaten public health. The U.S. has been a pioneer in both innovative sustainable technologies and in MSW management practices. Experimenting with innovative technologies is risky and resource-intense; however, the rewards can be enormous for addressing our energy and environmental challenges, both domestically and internationally. With greater understanding of waste degradation processes landfill facilities can transform MSW from a hazard to be contained to a renewable energy source in the form of collected methane.

Recent studies on U.S. municipal solid waste (MSW) management have indicated that the U.S. is sending significantly more waste to landfills than previously estimated. One study published in 2015, indicated that the actual MSW disposed of in landfills is double that of current EPA estimates. For example, in 2012, the study finds that Americans disposed of 262 million metric tons of waste in landfills; while the EPA estimated 122 million metric tons (Powell et al. 2015).  These findings are significant because MSW landfills, in their current state, represent a significant threat to the environment and climate change.

Modern MSW landfills were developed in response to the Resource Conservation and Recovery Act (RCRA) in 1976. For management of MSW, RCRA created Subtitle D landfills that are designed to minimize moisture addition to the landfill. Subtitle D landfills are also called sanitary or “dry-tomb” landfills, and operate to reduce the risk of leachate and gas emissions to the environment. Landfills emit methane in the form of biogas, which consists primarily of carbon dioxide and methane. Landfills represent the second largest anthropogenic source of methane in the U.S. at 18%; a significant contribution as methane’s impact on atmospheric pollution is considered to be 25 times greater than carbon dioxide over a 100-year period. Of 1754 landfills in the U.S., 558 (32%) have landfill gas collection systems that capture a portion of the biogas generated from landfills (EPA 2006). The gas collection systems operate sub-optimally, allowing anywhere from 50% to 90% of methane generated to be released into the atmosphere (Xunchang et al. 2015). In Subtitle D landfills the waste degrades slowly on the order of decades to centuries, however, the waste is isolated by containment systems with a significantly shorter design life.

The U.S. has experimented with improving MSW landfills through bioreactor landfills, which operate fundamentally as the opposite of Subtitle D landfills, with the encouragement of moisture addition to the waste. Bioreactor landfills recirculate either air or water to accelerate waste degradation and waste stabilization. Advantages of bioreactor landfills include accelerated waste degradation and stabilization in a matter of years rather than decades to centuries in “dry-tombs”, increased generation of biogas, lower waste toxicity, reduced leachate disposal costs, an estimated 15 to 30 percent gain in landfill space due to increased waste density, and reduced post-closure care. Despite these advantages, fewer than 2% of U.S. landfills are operated as bioreactors due to technological and scientific uncertainties (EPA 2006).

Bioreactor landfills lack robust data sets to demonstrate their benefits (Benson et al. 2006). Laboratory experiments and some field-scale experiments have demonstrated the above-mentioned advantages. Laboratory-scale experiments provide greater control and greater ability to measure parameters to describe the process of waste degradation. Waste degradation involves three interdependent physical processes, namely a fluid model of leachate and gas flows in the waste mass, a mechanical model of waste geotechnical properties, and a biochemical model describing aerobic and anaerobic degradation (Reddy et al. 2015). The field-scale demonstrations lack this control found in the laboratory, making it more challenging to obtain data that is indicative of the waste degradation process.

To address this challenge, researchers have explored the use of sensors and instrumentation at bioreactor landfills. These technologies provide a means to measure important parameters to describe the waste degradation processes. A better understanding of these processes can lead to optimization of bioreactor landfills to reach performance levels achieved in the laboratory.   This paper will focus on sensors and instrumentation for leachate monitoring, gas monitoring, and in-situ monitoring of waste degradation processes. 

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