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| Fig. 1: A collection of discarded fruits and vegetables. (Source: Wikimedia Commons). |
The issue of food waste continues to plague the world, with a significant portion of produce being discarded rather than consumed. In the state of California alone, 5.2 million tons of food waste is generated and sent to disposal facilities. [1] However, several innovative strategies have emerged to leverage food waste as a resource rather than simply disposing of it. One such method involves the utilization of anaerobic digestion, a process that decomposes organic material in the absence of oxygen, to produce biogas that can be used for energy production. This report examines the potential yearly energy output from the anaerobic digestion of food waste in California.
California is the most populated state in the United States and boasts one of the most productive agricultural industries globally, making it a significant contributor to food waste. Food waste makes up approximately 15% of municipal solid waste generated in the United States. [1] As such, considerable food waste is available as feedstock for anaerobic digestion processes (Fig. 1).
Anaerobic digestion processes involve microorganisms breaking down organic matter in an oxygen-free environment, producing a mixture of gases composed primarily of methane (70%) and carbon dioxide (30%), following the equation [2]
| 2 COH2 → CH4 + CO2 |
Only methane is used as an energy source, which can subsequently be combusted for heat and power or cleaned and used as a renewable natural gas. The biogas yield from food waste can vary depending on the composition and specific anaerobic digestion technology employed. However, generally, one ton of food waste can produce approximately 110 cubic meters of biogas. [3] The energy content of biogas composed of 70% methane is approximately 6.0kWh/m3. [4]
Applying these values to calculate the potential biogas yield from California's food waste gives a value
| 5.2 million tons/year × 110 m3 biogas produced/ton × 6000 Wh/m3 | = | 3.432 TWh/year |
Aside from the energy potential, effectively using food waste for biogas production also contributes significantly to waste management and greenhouse gas emissions reduction. However, challenges exist, including the need to separate food waste from general waste streams effectively and the high capital and operating costs associated with anaerobic digestion infrastructure (Fig. 2). The capital cost of an on-farm anaerobic digester ranges from approximately $2 million to $10 million depending upon the size of the operation and technology used. [5]
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| Fig. 2: An anaerobic digester biogas plant. (Source: Wikimedia Commons). |
Moreover, the variable composition of food waste can influence biogas yield and quality, necessitating advanced anaerobic digestion techniques and thorough waste characterization. Capturing biogas also demands dedicated storage and distribution systems, creating added complexity and costs.
Though the idea of transforming California's food waste into a valuable, renewable energy resource through anaerobic digestion is enticing, it also involves complex challenges. Despite the estimated potential of producing 3.4 TWh/year of energy, critical logistical and economic factors must be overcome for this to translate into reality. Nevertheless, continued research and advancements in anaerobic digestion technology hold the promise of rendering this concept increasingly viable.
© Raymond Zhen. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] H. M. Bruening et al., "Bioenergy Potential from Food Waste in California," Environ. Sci. Technol. 51, 1120 (2017).
[2] "Biogas: Converting Waste to Energy," Environmental and Energy Study Institute, October 2017.
[3] S. Abanaes et al., "A Critical Review of Biogas Production and Usage With Legislations Framework Across the Globe," Int. J. Environ. Sci. Technol. (Tehran) 19, 3377 (2022).
[4] D. Deublein and A. Steinhauser, Biogas from Waste and Renewable Resources: An Introduction (Wiley-VCH, 2011), Table 1.1, p. 50.
[5] "Report of Funded Projects (2015-2022)," California Department of Food and Agriculture, 2022.
