PROGRAM | Civil Engineering

By: Rachel Burch Chair: Daniel Cha Co-Chair: Michael Chajes

ABSTRACT

Sustainable disposal of food waste is recognized by The National Academy of Engineering as one of the fourteen grand challenges facing engineers today (National Academy of Engineering, 2017). 35% of the food produced in the United States goes uneaten or unsold, and 56% of food waste – 35.3 million tons – is sent directly to landfills, causing a myriad of environmental, societal, and economic issues (EPA, 2020). Food waste sent to landfills forfeits the opportunity to repurpose or recycle organic materials to create energy, fertilizers, and other valuable products. The most common sustainable solutions used today are commercial composting and anaerobic digestion, biological degradation processes which recover biogas and inert biosolids. These disposal methods are beneficial in many ways; however, these sustainable technologies are underutilized due to challenges such as convenience and upkeep. As such, development of new biochemical and thermal technologies for the sustainable disposal of food waste is a growing field of research.

Aerobic bio-digesters are upcoming sustainable technologies that aim to address the issues faced by larger disposal facilities. They are designed to be installed directly into commercial kitchens to microbially process food waste on-site, effectively liquifying the food waste and diverting it from the landfill. Currently, the liquified food waste is being sent through the sewer system to the wastewater treatment plant, not making use of the valuable organic materials. However, preliminary studies of this aerobic bio-digester suggest that the liquefied food waste effluent is rich solids and organics and has potential to be collected and repurposed for food waste valorization. Therefore, the primary objective of this dissertation was to assess the potential of the aerobic bio-digester to be a sustainable food waste valorization technology to rival existing alternatives.  The research laid out here considers all aspects of sustainability in this evaluation, including the environmental, social, and economic impacts.

The first phase of this research aimed to determine the potential environmental benefit of the digester to be used for resource recovery by understanding the microbial degradation of food waste and the composition of the liquified effluent. A mass balance of solids and organics comparing a representative influent food waste sample to the collected digestate showed 100% recovery of solids in the effluent. Organics were 50-80% recovered in the effluent, indicating the microbial community degraded readily available organics while still preserving most solids and organics in the liquified digestate. Analysis of the breakdown of starch and proteins in the digester further supports these findings by showing microbial degradation of complex macromolecules to simpler monomers which are quickly taken up by microbes in the digester. Finally, increase in microbial enzymes solidifies the correlation between food waste breakdown with the activity of microorganisms. The findings from these experiments suggest that the microbes are working to break down food waste in the digester, but that solids and organics are preserved, making this digestate an attractive feedstock for subsequent valorization and resource recovery through anaerobic digestion and the production of biomethane.

The second objective of this research was to further assess the potential environmental benefit of the digester by using it to pretreat food waste feedstocks subsequently fed to anaerobic digesters for biogas production. Batch-scale anaerobic reactors were fed with two food waste feedstocks: one that was aerobically pretreated by the bio-digester, and one that was mechanically grinded to represent a raw food waste stream. Reactors that were fed with the pretreated feedstocks produced considerably more biomethane than the reactors fed with untreated feedstocks (up to 30 times more), and in the case of a high-protein lentil feedstock, the pretreatment helped prevent reactor failure from volatile fatty acid accumulation that was observed in raw-feed reactors.

Social benefits of the digester were explored through the evaluation of the digester as a teaching tool in K-12 classrooms. An aerobic bio-digester was installed at a K-12 school in Wilmington, Delaware, and a relationship was formed between high school teachers and researchers to develop lesson plans that utilize the digester technology to teach science, engineering, and sustainability using a hands-on approach. Results of this study show that students increased their knowledge of sustainability issues, food waste, and aerobic respiration. Students also reported increased interest in STEM-related careers as a result of the lesson.

Finally, the social and economic impacts of the digester were evaluated via surveys to digester users from a field installation of the digester at Grotto Pizza in Newark, Delaware, as well as a nationwide survey of longstanding digester users.  At the pizzeria, employee perception varied from positive to negative. Negative opinions were developed when the digester had some initial issues with smell after installation that were quickly resolved but had lasting impact on user perception of the digester. However, feedback received from the nationwide survey was overwhelmingly positive when considering user perception. Additionally, the digester has economically benefitted many of the surveyed digester owners, yet there were still others that reported concern about the price of the digester. To help economically incentivize the adoption of this digester technology, The Delaware Solid Waste Authority has rolled out a grant program to overcome economic hurdles to bio-digester implementation.

Through these studies, it is shown that the aerobic bio-digester can be considered a competitive sustainable food waste management strategy. The digester effectively pretreated a food waste feedstock for increased biogas production in subsequent anaerobic digestion, showing that it is a viable food waste valorization technology. Application of this digester at a school and collaboration with teachers showed that this technology can be socially beneficial in helping high school students learn about engineering and sustainability through hand-on lessons. Surveys of user perception of the digester suggest there may be some hurdles to wide-spread implementation that must be overcome, but most companies that have installed digesters are pleased with their decision. Finally, initiatives are underway to help financially incentivize the aerobic bio-digester, so more people are willing and able to utilize this convenient, sustainable food waste disposal solution.

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