Date of Award
2024
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Civil and Environmental Engineering
First Advisor
Eric Roy
Abstract
Circular economy strategies such as composting and anaerobic digestion (AD) recover energy and nutrients from organic waste streams and provide multiple benefits over conventional linear resource use models. However, these applications also present practical challenges and environmental tradeoffs that are poorly understood.
Repeated digestate application to soils near AD systems can potentially contribute to P runoff and associated eutrophication. However, it is cost-prohibitive to transport digestate to distant crop production areas due to high water content. In Chapter 2, I evaluate the potential to recover P from dairy manure digestate using dissolved air flotation (DAF). At full operational capability, the DAF system captured 85 ± 12% of P from the influent in the separated fine solids. These findings indicate that most P in dairy manure digestate can be recovered using DAF, while remaining effluent has an expanded range of use options due to a 3-fold increase in N:P ratio.
Diverting food waste from landfills to composting or AD can reduce greenhouse gas emissions, enable the recovery of energy in usable forms, and create nutrient-rich soil amendments. However, many food waste streams are mixed with plastic packaging, raising concerns that food waste-derived composts and digestates may inadvertently introduce microplastics into agricultural soils. In Chapter 3, I review what is known and what is not known about the abundance of microplastics in composts, digestates, and food wastes. I also suggest ways to harmonize microplastic abundance and ecotoxicity studies with the design of related policies.
Mechanical depackagers separate valuable organics from residual food packaging, creating new opportunities to recover energy and nutrients via AD. However, the possibility of imperfect separation has raised concerns that digestate derived from depackaged food waste may contain microplastics. In Chapter 4, I evaluate biochemical methane potential (BMP) and plastic content of two mechanically depackaged food waste streams and a derived digestate. The depackaged organics exhibited substantial potential for energy recovery via AD. However, plastic contamination, while low on a mass basis, could limit digestate reuse options.
In Chapter 5, I use mass balance modeling and life cycle assessment to compare the climate, eutrophication, and plastic pollution impacts of current landfill-dominant US food waste management to a circular economy approach. I estimate that greenhouse gas emissions from food waste management would decrease by 89% and 83% under AD-dominant and composting-dominant organics recycling scenarios, respectively. However, land application of food waste-derived composts and digestates would result in the accumulation of up to 1.5 ± 0.6 million t of plastic in US agricultural soils over the next 100 years and offset only ~1% of US mineral N and P fertilizer consumption. Transitioning to 100% biodegradable plastic packaging could decrease 100-year plastic accumulation by >99%. I conclude the dissertation with recommendations for future circular economy research.
Language
en
Number of Pages
187 p.
Recommended Citation
Porterfield, Katherine Keith, "Environmental Benefits And Burdens In Circular Economy Applications" (2024). Graduate College Dissertations and Theses. 1934.
https://scholarworks.uvm.edu/graddis/1934