UC Berkeley

Most municipal solid waste in the United States is currently landfilled despite growing concerns of waste accumulation and associated greenhouse gas emissions. In response, significant efforts have been made to implement and pursue zero-waste goals and develop circular economy strategies to recover value from waste products. My research aims to inform decision-makers on the environmental tradeoffs between waste management strategies by evaluating the emissions implications of various novel and existing technology options for organic and plastic waste, two major components of municipal solid waste streams. This dissertation includes three studies: a life-cycle assessment of organic waste management strategies, a comprehensive and quantitative review of emissions from composting organic waste, and a life-cycle assessment of plastic waste management strategies. All of this work provides evidence of the benefits from landfill diversion and suggests more sustainable solutions for both organic and plastic waste management. A more detailed summary of these studies and their results are provided below.

Life-Cycle Greenhouse Gas Emissions and Human Health Tradeoffs of Organic Waste Management StrategiesWaste-to-energy systems can play an important role in diverting organic waste from landfills. However, real-world waste management can differ from idealized practices, and emissions driven by microbial communities and complex chemical processes are poorly understood. This study presents a comprehensive life-cycle assessment, using reported and measured data, of competing management alternatives for organic municipal solid waste including landfilling, composting, dry anaerobic digestion (AD) for the production of renewable natural gas (RNG), and dry AD with electricity generation. Landfilling is the most greenhouse gas (GHG)-intensive option, emitting nearly 400 kg CO2e per tonne of organic waste. Composting raw organics resulted in the lowest GHG emissions, at -41 kg CO2e per tonne of waste, while upgrading biogas to RNG after dry AD resulted in -36 to -2 kg CO2e per tonne. Monetizing the results based on social costs of carbon and other air pollutant emissions highlights the importance of ground-level NH3 emissions from composting nitrogen-rich organic waste or post-AD solids. However, better characterization of material-specific NH3 emissions from landfills and land-application of digestate is essential to fully understand the tradeoffs between alternatives.

Greenhouse Gas and Air Pollutant Emissions from Composting Composting can divert organic waste from landfills, reduce landfill methane emissions, and recycle nutrients back to soils. However, the composting process is also a source of greenhouse gas and air pollutant emissions. Researchers, regulators, and policy decision-makers all rely on emissions estimates to develop local emissions inventories and weigh competing waste diversion options, yet reported emission factors are difficult to interpret and highly variable. This study reviews the impacts of waste characteristics, pretreatment processes, and composting conditions on CO2, CH4, N2O, NH3 and VOC emissions by critically reviewing and analyzing 38 emission factors from 46 studies. The values reported to-date suggest that CH4 is the single largest contributor to 100-year global warming potential (GWP100) for yard waste composting, comprising approximately 80% of total GWP100. For nitrogen-rich wastes including manure, mixed municipal organic waste, and wastewater treatment sludge, N2O is the largest contributor to GWP100, accounting for half to as much as 90% of the total GWP100. If waste is anaerobically digested prior to composting, N2O, NH3 and VOC emissions tend to decrease relative to composting the untreated waste. Effective pile management and aeration are key to minimizing CH4 emissions. Increasing aeration of piles, while useful for minimizing CH4 emissions, can drive increases in NH3 emissions in some cases.

Complementary Roles for Mechanical and Solvent-Based Recycling in Low-Carbon, Circular Polypropylene Plastic recycling presents a vexing challenge. Mechanical recycling offers substantial GHG emissions savings relative to virgin plastic production but suffers from degraded aesthetic and mechanical properties. Polypropylene, one of the most widely used and lowest-cost plastics, is particularly susceptible to declining properties, performance, and aesthetics across a succession of mechanical recycles. Advanced processes, such as solvent-assisted recycling, promise near-virgin quality outputs at a greater energy and emissions foot-print. Mechanical and advanced recycling are often presented as competing options, but real-world plastic waste streams are likely to require preprocessing regardless of whether they are routed to an advanced process. This study quantifies the life-cycle GHG implications of multiple recycling strategies and proposes a system in which mechanical and solvent-assisted recycling can be leveraged together to boost recycling rates and satisfy demand for a wider range of product applications. Polypropylene can be recovered from mixed-plastic bales produced at material recovery facilities and processed through mechanical recycling, with a varying fraction sent for further upgrading via solvent-assisted recycling to produce material approved for food packaging and other higher-quality applications. The resulting mechanically recycled rigid polypropylene reduces life-cycle greenhouse gas emissions by 80% relative to the same quantity of virgin material, while the upgraded higher-quality material achieves GHG savings of 30%.